Dopamine Signaling Differences in the Nucleus Accumbens and Dorsal Striatum Exploited by Nicotine

The dorsal striatum and the nucleus accumbens (NAc) shell of the ventral striatum have similar cellular components and are both richly innervated by dopamine neurons. Despite similarities that extend throughout the striatum, only the NAc shell has a conspicuous increase in basal dopamine upon the initial administration of psychostimulant drugs such as nicotine. As measured by microdialysis, the elevated dopamine in the NAc shell is considered an identifying functional characteristic of addictive drugs. To examine this general functional difference between nicotine's action on the dorsolateral striatum and NAc shell, we directly monitored dopamine release in rat striatal slices using fast-scan cyclic voltammetry. In addition, we separately monitored the in vivo unit firing activity of putative midbrain dopamine neurons from freely moving rats using chronic multiple tetrodes. Nicotine administration increased the firing frequency of dopamine neurons and specifically increased the number and the length of phasic burst firing. The frequency dependence for dopamine release in the dorsolateral striatum and NAc shell is fundamentally different, enabling mainly the NAc shell to capitalize on the nicotine-induced phasic burst firing by dopamine neurons. Although nicotine decreased low-frequency (tonic) dopamine release in both areas, the increased ratio of phasic bursts relative to tonic firing caused by nicotine boosted the basal dopamine concentration predominantly in the NAc shell. By favoring release from bursts while depressing release from tonic signals, nicotine spreads the range of dopamine signaling and effectively increases the signal-to-noise relationship along dopamine afferents.

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title: Nucleus accumbens shell and core dopamine: differential role in behavior and addiction
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title: Striatonigrostriatal Pathways in Primates Form an Ascending Spiral from the Shell to the Dorsolateral Striatum
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title: The nucleus accumbens as a complex of functionally distinct neuronal ensembles: An integration of behavioural, electrophysiological and anatomical data
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title: Basal forebrain in the context of schizophrenia
abstract: The human basal forebrain has been notoriously difficult to analyze, and it was only in the last part of the twentieth Century that its various components came into sharper focus. It has now been demonstrated that the main parts of what was previously referred to as the 'substantia innominata' (a neurological equivalent of the geographer's 'terra incognita') belong to nearby and better defined anatomical systems. These include the ventral aspects of the basal ganglia, i.e. the ventral striatopallidal system, extensions of the centromedial amygdala that links it via subpallidal cell columns to the bed nucleus of stria terminalis, i.e. the extended amygdala, and a more or less continuous collection of aggregated and non-aggregated, predominantly large, hyperchromatic projections neurons referred to as the basal nucleus of Meynert. Following a pictorial survey of the basal forebrain, the anatomy of these three systems are described with special emphasize on clinically relevant details. Sections devoted to clinical-anatomical correlations emphasize the potential significance of the basal forebrain in the context of schizophrenia. The functional-pathological importance of the cortico-subcortical re-entrant circuits through the ventral striatopallidal system is well recognized in the field of neuropsychiatry. Less appreciated is the fact that both the ventral striatum and the extended amygdala contain prominent collections of islands with small neurons, which have collectively been referred to as 'interface islands'. The abundance of neuroactive substances in the interface islands, and the potential for significant postnatal development of the neurons in these islands make them especially intriguing in the context of the developmental hypothesis of schizophrenia. The basal nucleus of Meynert is also of special interest. Involvement of the basal nucleus of Meynert and its related circuits may well be one of the main reasons for attentional dysfunction and cognitive symptoms in this complex disorder. Finally, we emphasize that changes in the neuronal circuits related to the ventral striatopallidal system, extended amygdala and basal nucleus of Meynert in all likelihood provide the anatomical substrate through which pathologic activities in the medial temporal lobe and prefrontal-orbitofrontal structures are translated into disruption of a number of functions ranging from motor activities and basic drives, to personality changes involving stress, mood and higher cognitive functions.

title: From motivation to action: Functional interface between the limbic system and the motor system
abstract: Limbic forebrain structures and the hypothalamus are essential in the initiation of food-seeking, escape from predators and other behaviors essential for adaptation and survival. Neural integrative activities subserving these behaviors initiate motor responses but the neural interface between limbic and motor systems has received relatively little attention. This neglect has been in part because of the emphasis on the motor control of the movements and on the contributions of the cerebral cortex, cerebellum, spinal cord and other components of the motor system, but more importantly, because of a lack of relevant anatomical evidence of connections. Anatomical findings obtained in recent years now make it possible to investigate the neural interface between limbic and motor systems—neural mechanisms by which “motivation” gets translated into “action”. It has been proposed that the nucleus accumbens is a key component of this neural interface since it receives inputs from limbic forebrain structures, either directly or indirectly via the ventral tegmental area of Tsai, and sends signals to the motor system via the globus pallidus. 
 
The nucleus accumbens has been implicated in locomotion, a fundamental component of attack, feeding and other behaviors utilized in adaptation and survival. It has also been implicated in oral motor responses, utilized in feeding, drinking, vocalization and other adaptive responses. The role of the nucleus accumbens and its functional relationship with the ventral tegmental area and globus pallidus has been investigated using neuropharmacological-behavioral techniques to initiate and disrupt locomotor and ingestive responses and using electrophysiological recording techniques. The results of these investigations are interpreted in relation to a proposed model of the limbic-motor interface and further experiments are suggested. This model for the initiation of actions by limbic forebrain structures (e.g. the “emotice brain”) is considered in relation to what is known about the initiation of actions by cognitive processes involving previous experience and learning, which include research is how the “emotive brain” and the “cognitive brain” operate together in response initiation.

title: Effects of nicotine on the nucleus accumbens and similarity to those of addictive drugs
abstract: THE question of whether nicotine, the neuroactive compound of tobacco, is addictive has been open to considerable scientific and public discussion. Although it can serve as a positive reinforcer in several animal species, including man, nicotine is thought to be a weak reinforcer in comparison with addictive drugs such as cocaine and heroin1,2, and has been argued to be habit forming but not addictive3,4. Here we report that intravenous nicotine in the rat, at doses known to maintain self-administration, stimulates local energy metabolism, as measured by 2-deoxyglucose autoradiography, and dopamine transmission, as estimated by brain microdialysis, in the shell of the nucleus accumbens. These neurochemical and metabolic effects are qualitatively similar to those of other drugs, such as cocaine, amphetamine and morphine, which have strong addictive properties5–7. Our results provide functional and neurochemical evidence that there are specific neurobiological commonalities between nicotine and addictive drugs.

title: Drug addiction as dopamine-dependent associative learning disorder.
abstract: Natural rewards preferentially stimulate dopamine transmission in the nucleus accumbens shell. This effect undergoes adaptive changes (one-trial habituation, inhibition by appetitive stimuli) that are consistent with a role of nucleus accumbens shell dopamine in associative reward-related learning. Experimental studies with a variety of paradigms confirm this role. A role in associative stimulus-reward learning can provide an explanation for the extinction-like impairment of primary reinforcement that led Wise to propose the 'anhedonia hypothesis'. Addictive drugs share with natural rewards the property of stimulating dopamine transmission preferentially in the nucleus accumbens shell. This response, however, in contrast to that to natural rewards, is not subjected to one-trial habituation. Resistance to habituation allows drugs to activate dopamine transmission in the shell non-decrementally upon repeated self-administration. It is hypothesized that this process abnormally strengthens stimulus-drug associations thus resulting in the attribution of excessive motivational value to discrete stimuli or contexts predictive of drug availability. Addiction is therefore the expression of the excessive control over behaviour acquired by drug-related stimuli as a result of abnormal associative learning following repeated stimulation of dopamine transmission in the nucleus accumbens shell.

title: Nicotinic cholinergic synaptic mechanisms in the ventral tegmental area contribute to nicotine addiction.
abstract: Tobacco use is a major health problem that is estimated to cause 4 million deaths a year worldwide. Nicotine is the main addictive component of tobacco. It acts as an agonist to activate and desensitize nicotinic acetylcholine receptors (nAChRs). A component of nicotine's addictive power is attributable to actions on the mesolimbic dopaminergic system, which serves a fundamental role in the acquisition of behaviors that are inappropriately reinforced by addictive drugs. Here we show that nicotine, in the same concentration and time ranges as obtained from tobacco, has three main actions that regulate the activity of midbrain dopamine (DA) neurons. Nicotine first activates and then desensitizes nAChRs on the DA neurons. This process directly excites the DA neurons for a short period of time before the nAChRs desensitize. Nicotine also enhances glutamatergic excitation and decreases GABAergic inhibition onto DA neurons. These events increase the probability for synaptic plasticity, such as long-term potentiation. The short-lived direct excitation of the DA neurons coupled with the enhanced glutamatergic afferent activity provides the presynaptic and postsynaptic coincidence necessary to initiate synaptic potentiation. In total, these synaptic events lead to a relatively long-lasting heightened activity of midbrain DA neurons. Consistent with other summarized studies, this work indicates that the synaptic changes normally associated with learning and memory can be influenced and commandeered during the nicotine addiction process.

title: Nicotinic effects on the firing pattern of midbrain dopamine neurons.
abstract: The effects of systemic administration of nicotine or the nicotinic antagonist mecamylamine on the midbrain dopamine (DA) systems of the rat were studied with single cell recording techniques. Dopamine cells of the zona compacta, substantia nigra (ZC-SN) and the ventral tegmental area (VTA) were identified by their characteristic action potentials, antidromic stimulation methodology and conventional histological procedures. Firing rates as well as firing patterns were determined from computer-generated interspike interval histograms describing burst-firing in relation to single-spike firing. A larger proportion of burst-firing DA cells was found in the VTA when compared with the ZC-SN area. (-)-Nicotine bitartrate (0.5 mg kg-1 i.p.) not only increased the firing rate of ZC-SN neurons but also the amount of burst firing of the ZC-SN neurons and VTA neurons, respectively. Mecamylamine HCl (4.0 mg kg-1 i.p.) decreased the firing rate of VTA cells which, in principle, indicates a tonic nicotinic input in this area. The increase in firing rate of central DA neurons following nicotine administration was found to be associated with increased bursting of the burst firing cells whereas, in contrast, the non-bursting neurons did not respond with burst-firing. Generally, the correlation between nicotine induced changes in firing rate and in burst-firing activity, respectively, was found to be low for midbrain DA neurons. These observations, in conjunction with the previous demonstration of nicotinic receptors in these areas, indicate the existence of a nicotinic input, specifically regulating the firing pattern of these central DA cells. This neuromodulatory effect of nicotine may be significant for its behavioural stimulant action.(ABSTRACT TRUNCATED AT 250 WORDS)

title: Nicotine activates and desensitizes midbrain dopamine neurons
abstract: Tobacco use in developed countries is estimated to be the single largest cause of premature death. Nicotine is the primary component of tobacco that drives use, and like other addictive drugs, nicotine reinforces self-administration and place preference in animal studies. Midbrain dopamine neurons normally help toshape behaviour by reinforcing biologically rewarding events, but addictive drugs such as cocaine can inappropriately exert a reinforcing influence by acting upon the mesolimbic dopamine system. Here we show that the same concentration of nicotine achieved by smokers activates and desensitizes multiple nicotinic receptors thereby regulating the activity of mesolimbic dopamine neurons. Initial application of nicotine can increase the activity of the dopamine neurons, which could mediate the rewarding aspects of tobacco use. Prolonged exposure to even these low concentrations of nicotine, however, can cause desensitization of the nicotinic receptors, which helps to explain acute tolerance to nicotine's effects. The effects suggest a cellular basis for reports that the first cigarette of the day is the most pleasurable, whereas the effect of subsequent cigarettes may depend on the interplay between activation and desensitization of multiple nicotinic receptors.

title: Dual effects of nicotine on dopamine neurons mediated by different nicotinic receptor subtypes.
abstract: Burst firing of dopaminergic neurons has been found to represent a particularly effective means of increasing dopamine release in terminal areas as well as activating immediate early genes in dopaminoceptive cells. Spontaneous burst firing is largely controlled by the level of activation of NMDA receptors in the ventral tegmental area (VTA) as a consequence of glutamate released from afferents arising mainly in the prefrontal cortex. Nicotine has been found to effectively increase burst firing of dopaminergic cells. This effect of nicotine may be due to an alpha 7 nicotinic receptor-mediated presynaptic facilitation of glutamate release in the VTA. By the use of in-vivo single-cell recordings and immunohistochemistry we here evaluated the role of alpha 7 nicotinic receptors in nicotine-induced burst firing of dopamine cells in the VTA and the subsequent activation of immediate early genes in dopaminoceptive target areas. Nicotine (0.5 mg/kg s.c.) was found to increase firing rate and burst firing of dopaminergic neurons. In the presence of methyllycaconitine (MLA, 6.0 mg/kg i.p.) nicotine only increased firing rate. Moreover, in the presence of dihydro-beta-erythroidine (DH beta E, 1.0 mg/kg i.p.), an antagonist at non-alpha 7 nicotinic receptors, nicotine produced an increase in burst firing without increasing the firing rate. Nicotine also increased Fos-like immunoreactivity in dopamine target areas, an effect that was antagonized with MLA but not with DH beta E. Our data suggest that nicotine's augmenting effect on burst firing is, indeed, due to stimulation of alpha 7 nicotinic receptors whereas other nicotinic receptors seem to induce an increase in firing frequency.

title: Variable Dopamine Release Probability and Short-Term Plasticity between Functional Domains of the Primate Striatum
abstract: Release of the neuromodulator dopamine (DA) is critical to the control of locomotion, motivation, and reward. However, the probability of DA release is not well understood. Current understanding of neurotransmitter release probability in the CNS is limited to the conventional synaptic amino acid transmitters (e.g., glutamate and GABA). These fast neurotransmitters are released with a repertoire of probabilities according to synapse type, and these probabilities show activity-dependent plasticity according to synapse use. Synapses for neuromodulators such as DA, however, are designed for signaling that diverges temporally and spatially from that for fast neurotransmitters: DA receptors are exclusively metabotropic and at sites that extend to extrasynaptic locations and neighboring synapses. In this study, the release probability of DA was explored in real time in limbicversus motor-associated functional domains of the striatum of a primate (marmoset; Callithrix jacchus) using fast-scan voltammetry at a carbon-fiber microelectrode. We show that the probability of axonal DA release varies with striatal domain. Furthermore, release probability exhibits a short-term, activity-dependent plasticity that ranges from depression to facilitation in motor-through limbic-associated regions, respectively. Rapid plasticity does not result from metabotropic D2-like DA receptor activation or ionotropic GABAA receptor effects but is dependent on Ca2+ availability. These data reveal that rapid dynamics in DA release probability will participate in the transmission of the patterns and frequencies encoded by DA neuron action potential discharge. Furthermore, the regional variation in these features indicates that limbic-versus motor-associated DA neurons are permitted to generate diverse DA signals in response to a given firing pattern.

title: Nicotine amplifies reward-related dopamine signals in striatum
abstract: Reward-seeking behaviors depend critically on dopamine signaling—dopamine neurons encode reward-related information by switching from tonic to phasic (burst-like) activity. Using guinea pig brain slices, we show that nicotine, like cocaine and amphetamine, acts directly in striatum where it enhances dopamine release during phasic but not tonic activity. This amplification provides a mechanism for nicotine facilitation of reward-related dopamine signals, including responses to other primary reinforcers that govern nicotine dependence in smokers.

title: Frequency-dependent modulation of dopamine release by nicotine
abstract: Although nicotine activation of dopamine release is implicated in addiction, it also desensitizes nicotinic acetylcholine receptors (nAChRs), leading to a prolonged depression of evoked dopamine release. Here we show that nicotine's effects depend on the firing pattern of dopamine neurons, so that while desensitization of nAChRs indeed curbs dopamine released by stimuli emulating tonic firing, it allows a rapid rise in dopamine from stimuli emulating phasic firing patterns associated with incentive/salience paradigms. Nicotine may thus enhance the contrast of dopamine signals associated with behavioral cues.

title: Uniform Inhibition of Dopamine Neurons in the Ventral Tegmental Area by Aversive Stimuli
abstract: Dopamine neurons play a key role in reward-related behaviors. Reward coding theories predict that dopamine neurons will be inhibited by or will not respond to aversive stimuli. Paradoxically, between 3 and 49% of presumed dopamine neurons are excited by aversive stimuli. We found that, in the ventral tegmental area of anesthetized rats, the population of presumed dopamine neurons that are excited by aversive stimuli is actually not dopaminergic. The identified dopamine neurons were inhibited by the aversive stimulus. These findings suggest that dopamine neurons are specifically excited by reward and that a population of nondopamine neurons is excited by aversive stimuli.

title: Restraint Increases Dopaminergic Burst Firing in Awake Rats
abstract: In anesthetized animals, dopamine neurons fire in tonic and phasic firing modes hypothesized to be regulated by dissociable circuit mechanisms. Salient events critical to learning, reward processing, and attentional selection elicit transient phasic bursts. It is unclear, however, how burst activity contributes to sustained firing patterns in awake animals and if behavioral conditions known to affect dopaminergic neurotransmission change impulse activity levels. Acute stress is known to increase extracellular dopamine in the striatum and the prefrontal cortex. In this study, we have used multiunit recording to define and follow activity patterns in single dopaminergic neurons across days and to determine how restraint, a model of acute stress, changes tonic and phasic firing patterns. Long-term recording shows that a population of 23 putative dopamine neurons has heterogeneous firing profiles under baseline conditions. In all, 62% showed significant burst activity under resting conditions, while others showed predominantly regular (17%) or random (21%) activity patterns. Restraint increased mean firing rate in all dopamine neurons, but preferentially increased burst firing in neurons with higher burst rates under resting conditions. Finally, we show that increased burst firing can persist 24 h after a single exposure to stress. These data indicate that subsets of dopamine neurons may be sensitive to circuit mechanisms activated by stress and that persistent changes in burst firing may be evidence of synaptic plasticity. Furthermore, increased burst firing may be a mechanism through which stress augments extracellular dopamine in selected terminal regions.

title: Dopaminergic neurons in the rat ventral tegmental area. I. Identification and characterization
abstract: By use of various histochemical techniques, it was shown that both DA and non-DA cells in the VTA project to the NAc. Of these VTA-NAc output cells, the great majority were DA-containing cells. A small number of non-DA cells were encountered most frequently in the lateral part of the VTA. Correspondingly, two distinct groups of neurons, types I and II, could be identified by antidromic stimulation of the NAc. Several lines of evidence suggest that type I cells are DA-containing neurons. The evidence may be summarized as follows: 
1. 
(1) type I cells had a slow-bursting or regular firing pattern, slow discharge rate and wide spike duration which appears to be identical to the characteristics of DA neurons originally described by Bunney et al.16; 
 
2. 
(2) the great majority of these cells could be activated antidromically by stimulation of the NAc; 
 
3. 
(3) the conduction velocity and absolute refractory period of type I cells are consistent with unmyelinated fine DA fibers; 
 
4. 
(4) injection of 6-OHDA, but not 5,7-DHT directly in the MFB blocked antidromic responses of these cells; 
 
5. 
(5) they were extremely sensitive to intravenously administered DA agonist apomorphine (ID50 = 7 μg/kg); and 
 
6. 
(6) direct fluorescence histochemical examination of serial sections from brains of animals in which type I cells have been identified by antidromic stimulation of the NAc showed that type I cells are most likely catecholamine-containi ng neurons. By contrast, type II cells possessed an entirely different spectrum of physiological characteristics; in addition, they showed no consistent response to apomorphine and their antidromic responses to stimulation of the NAc were not affected by 6-OHDA. It is concluded that (1) VTA output neurons consist of both DA and nonDA neurons, and (2) identified types I and II neurons in the VTA by antidromic stimulation of the NAc are DA and non-DA cells, respectively.

title: Firing modes of midbrain dopamine cells in the freely moving rat
abstract: There is a large body of data on the firing properties of dopamine cells in anaesthetised rats or rat brain slices. However, the extent to which these data relate to more natural conditions is uncertain, as there is little quantitative information available on the firing properties of these cells in freely moving rats. We examined this by recording from the midbrain dopamine cell fields using chronically implanted microwire electrodes. (1) In most cases, slowly firing cells with broad action potentials were profoundly inhibited by the dopamine agonist apomorphine, consistent with previously accepted criteria. However, a small group of cells was found that were difficult to classify because of ambiguous combinations of properties. (2) Presumed dopamine cells could be divided into low and high bursting (>40% of their spikes in bursts) groups, with the majority having low bursting rates. The distribution of burst incidence was similar to that previously reported with chloral hydrate anaesthesia, but the average intraburst frequency was higher in the conscious animal at rest and was higher again in bursts triggered by salient stimuli. (3) There was no evidence for spike frequency adaptation within bursts on average, consistent with the hypothesis that afterhyperpolarisation currents may be disabled during behaviourally induced bursting. (4) Presumed dopamine cells responded to reward-related stimuli with increased bursting rates and significantly higher intraburst frequencies compared to bursts emitted outside task context, indicating that modulation of afferent activity might not only trigger bursting, but may also regulate burst intensity. (5) In addition to the irregular single spike and bursting modes we found that extremely regular (clock-like) firing, previously only described for dopamine cells in reduced preparations, can also be expressed in the freely moving animal. (6) Cross-correlation analysis of activity recorded from simultaneously recorded neurones revealed coordinated activity in a quarter of dopamine cell pairs consistent with at least "functional" connectivity. On the other hand, most dopamine cell pairs showed no correlation, leaving open the possibility of functional sub-groupings within the dopamine cell fields. Taken together, the data suggest that the basic firing modes described for dopamine cells in reduced or anaesthetised preparations do reflect natural patterns of activity for these neurones, but also that the details of this activity are dependent upon modulation of afferent inputs by behavioural stimuli.

title: Properties and Opioid Inhibition of Mesolimbic Dopamine Neurons Vary according to Target Location
abstract: The mesolimbic dopamine system, which mediates the rewarding properties of nearly all drugs of abuse, originates in the ventral tegmental area (VTA) and sends major projections to both the nucleus accumbens (NAc) and the basolateral amygdala (BLA). To address whether differences occur between neurons that project to these separate areas, retrograde microspheres were injected to either the BLA or the NAc of DBA/2J mice. Whole-cell recordings were made from labeled VTA dopamine neurons. We found that identified neurons that projected to the BLA and NAc originated within different quadrants of the VTA with neither group exhibiting large-amplitude h-currents. Neurons that projected to the NAc exhibited a greater outward current in response to the κ-opioid agonist (5α,7α,8α)-(+)-N-methyl-N-[7-(pyrrolidinyl)-1-oxaspiro [4,5]dec-8-yl]-benzeneacetamide (U69593; 200 nm), whereas neurons that projected to the BLA exhibited greater inhibition to the μ/δ opioid agonist [Met5] enkephalin (ME; 3 μm). In addition, we found that the presynaptic inhibition of GABAergic transmission at both GABAA and GABAB receptors was differentially regulated by U69593 between the two groups. When dopamine IPSCs were examined, U69593 caused a greater inhibition in NAc- than BLA-projecting neurons. ME had no effect on either. Finally, the regulation of extracellular dopamine by dopamine uptake transporters was equal across the VTA. These results suggest that opioids differentially inhibit mesolimbic neurons depending on their target projections. Identifying the properties of projecting mesolimbic VTA dopamine neurons is crucial to understanding the action of drugs of abuse.

title: Unique Properties of Mesoprefrontal Neurons within a Dual Mesocorticolimbic Dopamine System
abstract: The mesocorticolimbic dopamine system is essential for cognitive and emotive brain functions and is thus an important target in major brain diseases like schizophrenia, drug addiction, and attention deficit hyperactivity disorder. However, the cellular basis for the diversity in behavioral functions and associated dopamine-release pattern within the mesocorticolimbic system has remained unclear. Here, we report the identification of a type of dopaminergic neuron within the mesocorticolimbic dopamine system with unconventional fast-firing properties and small DAT/TH mRNA expression ratios that selectively projects to prefrontal cortex and nucleus accumbens core and medial shell as well as to basolateral amygdala. In contrast, well-described conventional slow-firing dopamine midbrain neurons only project to the lateral shell of the nucleus accumbens and the dorsolateral striatum. Among this dual dopamine midbrain system defined in this study by converging anatomical, electrophysiological, and molecular properties, mesoprefrontal dopaminergic neurons are unique, as only they do not possess functional somatodendritic Girk2-coupled dopamine D2 autoreceptors.

title: The control of firing pattern in nigral dopamine neurons: burst firing
abstract: In addition to firing in a single spiking mode, dopamine (DA) cells have been observed to fire in a bursting pattern with consecutive spikes in a burst displaying progressively decreasing amplitude and increasing duration. In vivo intracellular recording demonstrated the bursts to typically ride on a depolarizing wave (5 to 15 mV amplitude). Although the burst-firing frequency of DA cells showed little correlation with the base line firing rate, increases in firing rate were usually associated with an increase in burst firing. Increases in burst firing could also be elicited by intracellular calcium injection and could be prevented by intracellular injection of EGTA, suggesting a calcium involvement in bursting. Blockade of potassium conductances with extracellular iontophoresis of barium or intracellular injection of tetraethylammonium bromide could also trigger an increased degree of burst firing in DA cells. These data suggest that the increased calcium influx accompanying an increased firing rate triggers burst firing, possibly by inactivating a potassium conductance. A switch from a single spiking mode to a burst-firing mode may be important in modulating striatal DA release, as shown for burst firing in other preparations.

title: Arteriovenous differences in plasma concentration of nicotine and catecholamines and related cardiovascular effects after smoking, nicotine nasal spray, and intravenous nicotine
abstract: Delivery of a high concentration bolus of nicotine through the arterial circulation is believed to be an important determinant of the addictive, behavioral, and physiologic effects of nicotine. To better understand the pharmacologic features of nicotine with different routes of administration, we measured arterial and venous plasma concentrations of nicotine, cotinine, epinephrine, and norepinephrine after tobacco smoking, intravenous nicotine infusion, and use of a nicotine nasal spray.

title: Reactivation of hippocampal ensemble memories during sleep.
abstract: Simultaneous recordings were made from large ensembles of hippocampal "place cells" in three rats during spatial behavioral tasks and in slow-wave sleep preceding and following these behaviors. Cells that fired together when the animal occupied particular locations in the environment exhibited an increased tendency to fire together during subsequent sleep, in comparison to sleep episodes preceding the behavioral tasks. Cells that were inactive during behavior, or that were active but had non-overlapping spatial firing, did not show this increase. This effect, which declined gradually during each post-behavior sleep session, may result from synaptic modification during waking experience. Information acquired during active behavior is thus re-expressed in hippocampal circuits during sleep, as postulated by some theories of memory consolidation.

title: Low doses of apomorphine elicit two opposing influences on dopamine cell electrophysiology
abstract: Dopamine (DA) cells are known to be very sensitive to direct acting DA agonists, and inhibition of DA cell firing by low doses of DA agonists is generally considered to be an action of these agonists on DA cell autoreceptors. During intracellular recording from spontaneously discharging DA cells in vivo, intravenous administration of apomorphine (20 micrograms/kg i.v.) elicited a hyperpolarization and an increase in input resistance. The calculated reversal potential of the apomorphine effect was approximately -40 mV. However, in non-firing DA cells the reversal potential of these effects was significantly different (P less than 0.01), being close to the reversal potential of responses induced by stimulation of striatonigral pathways (i.e. -67 mV). In addition, haloperidol (0.01 mg/kg i.v.) reversed the hyperpolarization produced by apomorphine but not the increase in input resistance. Transection of striatonigral pathways eliminated most of the increase in input resistance accompanying apomorphine administration, and shifted the apomorphine reversal potential to a value positive to 0 mV. Low doses of apomorphine were also found to affect a class of zona reticulata (ZR) interneurons. Apomorphine caused decreases in ZR cell firing rate, which were abolished by striatonigral pathway transection. Thus, the following mechanism is proposed for the electrophysiological actions of autoreceptor-selective doses of apomorphine on DA cells: (1) apomorphine directly inhibits spontaneous DA cell discharge by inhibiting the slow depolarization preceding action potentials and thereby hyperpolarizes the DA cell, (2) decreased DA cell firing disinhibits GABAergic striatal cells, whose increased firing preferentially (3) inhibits GABAergic ZR interneurons, and thus (4) removes an inhibitory input to DA cells, resulting in an increase in input resistance.

title: Dopamine D2 Receptor-Mediated Inhibition of Dopaminergic Neurons in Mice Lacking D2L Receptors
abstract: Two isoforms of the dopamine (DA) D2 receptor are generated from the same gene by alternative splicing, D2L and D2S. To identify which isoform is involved in the autoregulation of midbrain DA neuron activity, intracellular electrophysiological recordings were performed from substantia nigra and ventral tegmental area neurons of mice lacking either D2L(D2L−/−) or both D2L and D2S receptors (D2−/−). In midbrain DA neurons from wild-type mice, DA and quinpirole, a DA D2-like receptor agonist, produced a significant somatic membrane hyperpolarization, which led to a reversible inhibition of firing activity. Interestingly, this effect was fully abolished in D2−/− neurons but still present in D2L−/− DA neurons. These data clearly show that D2S receptors are the main somatodendritic autoreceptors of central DA neurons. Thus, pharmacological compounds able to interfere selectively with presynaptic D2S receptors might constitute effective therapeutic strategies in neuropsychiatric disorders, by causing negligible side-effects.

title: α6-Containing Nicotinic Acetylcholine Receptors Dominate the Nicotine Control of Dopamine Neurotransmission in Nucleus Accumbens
abstract: Modulation of striatal dopamine (DA) neurotransmission plays a fundamental role in the reinforcing and ultimately addictive effects of nicotine. Nicotine, by desensitizing β2 subunit-containing (β2*) nicotinic acetylcholine receptors (nAChRs) on striatal DA axons, significantly enhances how DA is released by reward-related burst activity compared to nonreward-related tonic activity. This action provides a synaptic mechanism for nicotine to facilitate the DA-dependent reinforcement. The subfamily of β2*-nAChRs responsible for these potent synaptic effects could offer a molecular target for therapeutic strategies in nicotine addiction. We explored the role of α6β2*-nAChRs in the nucleus accumbens (NAc) and caudate-putamen (CPu) by observing action potential-dependent DA release from synapses in real-time using fast-scan cyclic voltammetry at carbon-fiber microelectrodes in mouse striatal slices. The α6-specific antagonist α-conotoxin-MII suppressed DA release evoked by single and low-frequency action potentials and concurrently enhanced release by high-frequency bursts in a manner similar to the β2*-selective antagonist dihydro-β-erythroidine (DHβE) in NAc, but less so in CPu. The greater role for α6*-nAChRs in NAc was not due to any confounding regional difference in ACh tone since elevated ACh levels (after the acetylcholinesterase inhibitor ambenonium) had similar outcomes in NAc and CPu. Rather, there appear to be underlying differences in nAChR subtype function in NAc and CPu. In summary, we reveal that α6β2*-nAChRs dominate the effects of nicotine on DA release in NAc, whereas in CPu their role is minor alongside other β2*-nAChRs (eg α4*), These data offer new insights to suggest striatal α6*-nAChRs as a molecular target for a therapeutic strategy for nicotine addiction.

title: Postnatal development of the dopamine transporter: a quantitative autoradiographic study.
abstract: The dopamine transporter performs an important role in regulatinf neurochemical transmission at dopaminergic synapses, as well as dopamine synthetic activity in dopaminergic neurons. Certain drugs and toxins exert effects at the transportted, especially cocaine, a common drug of abuse. We studied the development of these sites in the rat postnatal ages day 0, 5, 10, 15, 21 and adult using quantitative autoradiography with the cocaine analogue [125I]RTI-55. At birth, certain structures such as the substantia nigra, interstitial nucleus of the medial longitudinal fasciculus, frontal and parietal cortex, and substantia inominata had [125I]RTI-55 binding levels that were already near he adult value. The striatum developed later, showing earlier growth in the anterior and dorsolateral regions, with early localization in both striosomes and a subcallosal streak. Anterio-to-posterior and lateral-to-medial gradients were present at day 0. The anterior striatum, ventral tegmental region, substantia nigra compacta and bed nucleus of the stria terminals showed transient peaks in binding levels that were higher than the adult values. Structures showing relatively late development included the prefrontal cortex, nucleus accumbens shell, olfactory tubercle and subthalamic nucleus. Knowledge of the differential developmental patterns of the dopamine transporter in different brain regions may have implications for understanding the neurodevelopmental effects of prenatal cocaine exposure.

title: The putative role of extra-synaptic mesolimbic dopamine in the neurobiology of nicotine dependence
abstract: A majority of habitual tobacco smokers find it very difficult to quit the habit because they become addicted to the nicotine present in tobacco smoke. Nicotine, like other psychostimulant drugs of abuse, increases dopamine release in the principal terminal field of the mesolimbic system, the nucleus accumbens, and there is evidence that this mediates the 'rewarding' properties of the drug, which reinforce its self-administration. This review focuses on the working hypothesis that addiction to nicotine, and other psychostimulant drugs, depends upon their ability to evoke a sustained increase in dopamine release directly into the extracellular space which lies between the cells in the nucleus accumbens where it stimulates extra-synaptic dopamine receptors. It is suggested that increased stimulation of these receptors is associated with increased incentive learning or the attribution of increased incentive salience to the cues associated with acquisition and delivery of the drug. The hypothesis proposes that these cues can become conditioned reinforcers of drug-taking behaviour. The receptors, which mediate the effects of nicotine on mesoaccumbens dopamine neurones, are desensitised by sustained exposure to nicotine at concentrations commonly found in the plasma of habitual smokers. It is proposed that, at times when the plasma nicotine concentration is sufficiently high to cause desensitisation of the receptors, tobacco smoking is maintained by the conditioned reinforcers present in the tobacco smoke. The hypothesis predicts, therefore, that conditioned reinforcement may play a more important role in the addiction to tobacco than for most other addictive behaviours. As a result, studies with nicotine have the potential to contribute to our understanding of the neurobiology of addiction which cannot easily be explored using drugs, such as cocaine and amphetamine, which invariably increase dopamine overflow in the forebrain.

title: Nicotine addiction and comorbidity with alcohol abuse and mental illness
abstract: The World Health Organization estimates that one-third of the global adult population smokes. Because tobacco use is on the rise in developing countries, death resulting from tobacco use continues to rise. Nicotine, the main addictive component of tobacco, initiates synaptic and cellular changes that underlie the motivational and behavioral alterations that culminate in addiction. Nicotine addiction progresses rapidly in adolescents and is most highly expressed in vulnerable people who have psychiatric illness or other substance abuse problems.

title: Nicotine use in schizophrenia: The self medication hypotheses
abstract: The behavioural and cognitive effects of nicotine in schizophrenia have received much interest in recent years. The rate of smoking in patients with schizophrenia is estimated to be two- to four-fold the rate seen in the general population. Furthermore such patients favour stronger cigarettes and may also extract more nicotine from their cigarettes than other smokers. The question has been raised whether the widespread smoking behaviour seen in this patient group is in fact a manifestation of a common underlying physiology, and that these patients smoke in an attempt to self-medicate. We present an overview of the explanations for elevated rates of smoking in schizophrenia, with particular emphasis on the theories relating this behaviour to sensory gating and cognitive deficits in this disorder that have been viewed as major support for the self-medication hypotheses.

title: Clinical Pharmacology of Nicotine: Implications for Understanding, Preventing, and Treating Tobacco Addiction
abstract: Understanding the basic and clinical pharmacology of nicotine provides a basis for improved prevention and treatment of tobacco addiction. Nicotine acts on nicotinic cholinergic receptors in the brain to release dopamine and other neurotransmitters that sustain addiction. Neuroadaptation and tolerance involve changes in both nicotinic receptors and neural plasticity. Nicotine addiction can occur in the context of physical dependence characterized by self‐medication to modulate negative affect and/or to relieve withdrawal symptoms, as well as, in light or occasional smokers, primarily for positive reinforcement in specific situations. Nicotine is metabolized primarily by CYP2A6. Its clearance exhibits considerable individual variability that is determined by genetic, racial, and hormonal (sex) factors. Genetically slow metabolism of nicotine appears to be associated with a lower level of dependence. Nicotine dependence is highly heritable and appears to be influenced by genes coding for some nicotine receptor subtypes, some neurotransmitter genes, and genes involved in neural connectivity. Novel pharmacotherapies for nicotine dependence include partial agonists for nicotinic receptors and nicotine vaccines. Pharmacogenetic studies suggest various candidate genes and a nicotine metabolism phenotype that influence outcome. Human pharmacology studies of nicotine and smoking behavior also provide a basis for assessing the benefits and risks of long‐term nicotine use for harm reduction and for a potential cigarette regulatory strategy that includes reducing nicotine content of cigarettes to nonaddictive levels.

title: Acetylcholine receptors containing the β2 subunit are involved in the reinforcing properties of nicotine
abstract: Release of the neurotransmitter dopamine in the mesolimbic system of the brain mediates the reinforcing properties of several drugs of abuse, including nicotine. Here we investigate the contribution of the high-affinity neuronal nicotinic acetylcholine receptor to the effects of nicotine on the mesolimbic dopamine system in mice lacking the β2 subunit of this receptor. We found that nicotine stimulates dopamine release in the ventral striatum of wild-type mice but not in the ventral striatum of β2-mutant mice. Using patch-clamp recording, we show that mesencephalic dopaminergic neurons from mice without the β2 subunit no longer respond to nicotine, and that self-administration of nicotine is attenuated in these mutant mice. Our results strongly support the idea that the β2-containing neuronal nicotinic acetylcholine receptor is involved in mediating the reinforcing properties of nicotine.

title: Presynaptic localisation of the nicotinic acetylcholine receptor β2 subunit immunoreactivity in rat nigrostriatal dopaminergic neurones
abstract: Nicotinic acetylcholine receptors (nAChR) are widely distributed in the central nervous system, where they exert a modulatory influence on synaptic transmission. For the striatum, pharmacological evidence supports the presence of presynaptic α3β2* and α4β2* nAChR that modulate dopamine release from nigrostriatal terminals. The objective of this study was to examine the precise subcellular distribution of the nAChR β2 subunit in these neurones and its localisation at presynaptic sites. Double immunolabelling with tyrosine hydroxylase (TH) at the confocal level revealed that the cell bodies and axon terminals (synaptosomes) of nigrostriatal neurones were also immunoreactive for the nAChR β2 subunit. Double‐preembedding electron microscopy confirmed that β2‐immunogold labelling was enriched in TH‐positive terminals in the dorsal striatum. Quantitative analysis of doubly immunogold‐labelled sections in postembedding electron microscopy showed that 86% of TH‐positive axonal boutons are also labelled for the nAChR β2 subunit, whereas 45% of β2 subunit‐immunolabelled boutons do not contain TH. Thus the β2 subunit is localised within at least two populations of axon terminals in the dorsal striatum. In these structures, 15% of β2 subunit immunoreactvity was at the plasma membrane but was rarely associated with synapses. These findings are compatible with functional presynaptic β2‐containing nAChR that may be stimulated physiologically by acetylcholine that diffuses from synaptic or nonsynaptic sites of acetylcholine release. These results demonstrate the presynaptic localisation of an nAChR subunit in nigrostriatal dopaminergic neurones, providing morphological evidence for the presynaptic nicotinic modulation of dopamine release.  2001.

title: Long-Term Potentiation of Excitatory Inputs to Brain Reward Areas by Nicotine
abstract: Nicotine reinforces smoking behavior by activating nicotinic acetylcholine receptors (nAChRs) in the midbrain dopaminergic (DA) reward centers, including the ventral tegmental area (VTA). Although nicotine induces prolonged excitation of the VTA in vivo, the nAChRs on the DA neurons desensitize in seconds. Here, we show that activation of nAChRs on presynaptic terminals in the VTA enhances glutamatergic inputs to DA neurons. Under conditions where the released glutamate can activate NMDA receptors, long-term potentiation (LTP) of the excitatory inputs is induced. Both the short- and the long-term effects of nicotine required activation of presynaptic alpha7 subunit-containing nAChRs. These results can explain the long-term excitation of brain reward areas induced by a brief nicotine exposure. They also show that nicotine alters synaptic function through mechanisms that are linked to learning and memory.

title: Nicotine reinforcement and cognition restored by targeted expression of nicotinic receptors
abstract: Worldwide, 100 million people are expected to die this century from the consequences of nicotine addiction, but nicotine is also known to enhance cognitive performance. Identifying the molecular mechanisms involved in nicotine reinforcement and cognition is a priority and requires the development of new in vivo experimental paradigms. The ventral tegmental area (VTA) of the midbrain is thought to mediate the reinforcement properties of many drugs of abuse. Here we specifically re-expressed the β2-subunit of the nicotinic acetylcholine receptor (nAChR) by stereotaxically injecting a lentiviral vector into the VTA of mice carrying β2-subunit deletions. We demonstrate the efficient re-expression of electrophysiologically responsive, ligand-binding nicotinic acetylcholine receptors in dopamine-containing neurons of the VTA, together with the recovery of nicotine-elicited dopamine release and nicotine self-administration. We also quantified exploratory behaviours of the mice, and showed that β2-subunit re-expression restored slow exploratory behaviour (a measure of cognitive function) to wild-type levels, but did not affect fast navigation behaviour. We thus demonstrate the sufficient role of the VTA in both nicotine reinforcement and endogenous cholinergic regulation of cognitive functions.

title: Nonlinear relationship between impulse flow, dopamine release and dopamine elimination in the rat brainin vivo

abstract: Central dopaminergic neurons exhibit two kinds of discharge activity: single spikes and bursts of two to six action potentials. Since these neurons can switch from one discharge pattern to the other whereas the mean discharge rate remains little affected, bursts may be more potent in triggering the release of their neurotransmitter, dopamine. Electrical stimulations mimicking the bursting pattern were actually twice as potent as regularly spaced stimulations to enhance the dopamine extracellular concentration. This suggested that dopamine release might be facilitated by increasing the impulse flow frequency. The high extracellular overflow evoked by a burst might also be due to accumulation of the released dopamine whereas, at lower frequencies, dopamine might be readily eliminated between every action potential. In the present study the dopamine overflow evoked by electrical stimulation of the dopaminergic pathway was measured in vivo by carbon fibre electrodes combined with continuous amperometry. We observed a small facilitation of the release per pulse during stimulations mimicking a burst but only in mesolimbic areas. The high extra-cellular dopamine level evoked by a burst was mainly due to accumulation of the released dopamine.

title: Functional Domains in Dorsal Striatum of the Nonhuman Primate Are Defined by the Dynamic Behavior of Dopamine
abstract: The dorsal striatum comprises a continuum of distinct functional domains, limbic, associative, and sensorimotor. In the primate it exclusively subdivides further into two nuclei, the putamen and caudate. Dopamine (DA) transmission is differentially affected between these nuclei in neurodegenerative diseases such as Parkinson's and by psychostimulants such as cocaine. Because rodent systems can offer only limited insight into DA systems of the human brain, a fuller appreciation of DA transmission and its role in dysfunction requires direct study in primates. DA behavior was explored in the major functional domains of the caudate nucleus and compared with the putamen, using fast-scan cyclic voltammetry in striatal sections from the marmoset (Callithrix jacchus). There was domain-specific variation in extracellular DA transients [i.e., concentration ([DA]o) released by a single stimulus and the rate maximum of DA uptake,Vmax]. Across nuclei, functional rather than anatomical regions were differentiated by these dynamics. The largest, fastest DA transients were at motor-associated loci. Evoked [DA]o at physiological frequencies was differently frequency-sensitive between functional domains but not between anatomical nuclei. In contrast, presynaptic depression was not an index of regional differentiation, recovering with similar kinetics at all loci. Within a given functional domain of dorsal striatum, the dynamics of DA release and uptake are similar for the putamen and the caudate nucleus. Conversely, distinct functional domains are defined by these DA dynamics, in a manner more marked in primates than in rodents. These data from the primate brain highlight differences in DA availability that may be central to DA function and dysfunction in the human.

title: Synaptology of Dopamine Neurons
abstract: Central dopamine (DA) neurons comprise several cell groups whose long range and local projections modulate neuronal systems mediating voluntary motor control, autonomic function, reproductive behavior, light/dark adaptation, and cognitive processes such as reward learning and working memory. The synaptology of DA cells reflects the richness of these diverse functions by exhibiting a continuum of morphological specializations that ranges from complete absence of synaptic connections to highly specific synapses onto selective dendritic compartments of target neurons. In addition to this diversity, certain common themes emerge from an examination of DA synaptology in various brain regions. For example, one of the most common synapses reported for DA nerve fibers is onto distal targets, such as spines or small-caliber dendrites in the immediate vicinity of convergent synaptic inputs from non-DA terminals. Such convergence suggests an anatomical substrate for DA modulation of afferent drive, as documented in electrophysiological studies of DA systems (Mantz et al. 1988; Cepeda and Levine 1998). Another common observation is the absence of identifiable DA synapses at particular sites where physiological studies suggest that DA mediates important regulatory control, or even at sites known to express DA receptors. Such findings have argued for the mediation of DA actions through non-synaptic mechanisms, sometimes referred to as volume transmission (Grace 1991; Gonon 1997; Zoli et al. 1998).

title: Electron-microscopic study of dopaminergic structures in the medial subdivision of the monkey nucleus accumbens
abstract: The medial subdivision of the monkey nucleus accumbens (NAC) is rich in dopamine (DA) and peptides. In the present investigation the mode of DA transmission in the medial subdivision was studied morphologically by light- and electron-microscopic immunocytochemistry using a monoclonal antibody raised against dopamine. The medial subdivision showed extremely dense accumulation of thick DA-immunoreactive varicose fibers. Electron-microscopic observation of single sections revealed that DA afferents had a relatively high incidence (33.2%) of asymmetric junctions in this area. Approximately 50% of the targets were dendritic shafts, 44.2% dendritic spines, and 5.1% somata. Some DA axons showed terminal profiles en passant within the synaptic complex, some of which showed synaptic triads. The unique ultrastructural features of DA terminals in the medial NAC indicate the existence of specific styles of DA transmission in the limbic structure.

title: Subsets of midbrain dopaminergic neurons in monkeys are distinguished by different levels of mRNA for the dopamine transporter: Comparison with the mRNA for the D2 receptor, tyrosine hydroxylase and calbindin immunoreactivity
abstract: The midbrain dopamine system can be divided into two groups of cells based on chemical characteristics and connectivity. The dorsal tier neurons, which include the dorsal pars compacta and the ventral tegmental area, are calbindin positive, and project to the shell of the nucleus accumbens. The ventral tier neurons are calbindin‐negative and project to the sensonmotor striatum. This study examined the distribution of the mRNAs for the dopamine transporter molecule (DAT) and the D2 receptor in the midbrain of monkeys by using in situ hybridization. The distribution patterns were compared to that of tyrosine hydroxylase and calbindin immunohistochemistry. The results show that high levels of hybridization for DAT and the D2 receptor mRNA are found in the ventral tier, calbindin‐negative neurons and relatively low levels are found in the dorsal, calbindin‐positive tier. Within the dorsal tier, the dorsal substantia nigra pars compacta has the least amount of both messages. These results show that in monkeys, the ventral tegmental area and the dorsal pars compacta form a dorsal continuum of dopamine neurons which express lower levels of mRNA for DAT and D2 receptor than the ventral tier. DAT has been shown to be involved in the selective neurotoxicity of N‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP). Different levels of DAT mRNA and calbindin may explain the differential effects of MPTP neurotoxicity.

title: Calcium Dependence and Recovery Kinetics of Presynaptic Depression at the Climbing Fiber to Purkinje Cell Synapse
abstract: Short-term depression is a widespread form of use-dependent plasticity found in the peripheral and central nervous systems of invertebrates and vertebrates. The mechanism behind this transient decrease in synaptic strength is thought to be primarily the result of presynaptic “depletion” of a readily releasable neurotransmitter pool, which typically recovers with a time constant of a few seconds. We studied the mechanism and dynamics of recovery from depression at the climbing fiber to Purkinje cell synapse, where marked presynaptic depression has been described previously. Climbing fibers are well suited to studies of recovery from depression because they display little, if any, facilitation (even under conditions of low-release probability), which can obscure rapid recovery from depression for hundreds of milliseconds after release. We found that recovery from depression occurred in three kinetic phases. The fast and intermediate components could be approximated by exponentials with time constants of 100 msec and 3 sec at 24° C. A much slower recovery phase was also present, but it was only prominent during prolonged stimulus trains. The fast component was enhanced by raising extracellular calcium and was eliminated by lowering presynaptic calcium, suggesting that, on short time scales, recovery from depression is driven by residual calcium. During regular and Poisson stimulus trains, recovery from depression was dramatically accelerated by accumulation of presynaptic residual calcium, maintaining synaptic efficacy under conditions that would otherwise deplete the available transmitter pool. This represents a novel form of presynaptic plasticity in that high levels of activity modulate the rate of recovery as well as the magnitude of depression.

title: Interplay between Facilitation, Depression, and Residual Calcium at Three Presynaptic Terminals
abstract: Synapses display remarkable alterations in strength during repetitive use. Different types of synapses exhibit distinctive synaptic plasticity, but the factors giving rise to such diversity are not fully understood. To provide the experimental basis for a general model of short-term plasticity, we studied three synapses in rat brain slices at 34°C: the climbing fiber to Purkinje cell synapse, the parallel fiber to Purkinje cell synapse, and the Schaffer collateral to CA1 pyramidal cell synapse. These synapses exhibited a broad range of responses to regular and Poisson stimulus trains. Depression dominated at the climbing fiber synapse, facilitation was prominent at the parallel fiber synapse, and both depression and facilitation were apparent in the Schaffer collateral synapse. These synapses were modeled by incorporating mechanisms of short-term plasticity that are known to be driven by residual presynaptic calcium (Cares). In our model, release is the product of two factors: facilitation and refractory depression. Facilitation is caused by a calcium-dependent increase in the probability of release. Refractory depression is a consequence of release sites becoming transiently ineffective after release. These sites recover with a time course that is accelerated by elevations of Cares. Facilitation and refractory depression are coupled by their common dependence on Cares and because increased transmitter release leads to greater synaptic depression. This model captures the behavior of three different synapses for various stimulus conditions. The interplay of facilitation and depression dictates synaptic strength and variability during repetitive activation. The resulting synaptic plasticity transforms the timing of presynaptic spikes into varying postsynaptic response amplitudes.

title: Neurotransmission: Chemical and electrical interneuron coupling
abstract: Recent studies have described the coupling between pairs of neocortical interneurons involving both electrical and chemical transmission; these new results may have important implications for the mechanisms underlying neuronal synchrony and rhythmic activity in the brain.

title: Spontaneous Activity of Neostriatal Cholinergic Interneurons In Vitro
abstract: Neostriatal cholinergic interneurons fire irregularly but tonicallyin vivo. The summation of relatively few depolarizing potentials and their temporal sequence are thought to underlie spike triggering and the irregularity of action potential timing, respectively. In these experiments we used whole-cell, cell-attached, and extracellular recording techniques to investigate the role of spontaneous synaptic inputs in the generation and patterning of action potentials in cholinergic interneurons in vitro. Cholinergic cells were spontaneously active in vitro at 25 ± 1°C during whole-cell recording from 2 to 3 week postnatal slices and at 35 ± 2°C during cell-attached and extracellular recording from 3 to 4 week postnatal slices. A range of firing frequencies and patterns was observed including regular, irregular, and burst firing. Blockade of AMPA and NMDA receptors altered neither the firing rate nor the pattern, and accordingly, voltage-clamp data revealed a very low incidence of spontaneous EPSCs. GABAAreceptor antagonists were also ineffective in altering the spiking frequency or pattern owing to minimal inhibitory input in vitro. Functional excitatory and inhibitory inputs to cholinergic cells were disclosed after application of 4-aminopyridine (100 μm), indicating that these synapses are present but not active in vitro. Blockade of D1 or D2 dopamine receptors or muscarinic receptors also failed to influence tonic activity in cholinergic cells. Together these data indicate that cholinergic interneurons are endogenously active and generate action potentials in the absence of any synaptic input. Interspike interval histograms and autocorrelograms generated from unit recordings of cholinergic cells in vitro were indistinguishable from those of tonically active neurons recorded in vivo. Irregular spiking is therefore embedded in the mechanism responsible for endogenous activity.

title: Pharmacological comparison of transient and persistent [3H]dopamine release from mouse striatal synaptosomes and response to chronic L-nicotine treatment.
abstract: L-Nicotine stimulates a biphasic release of [3H]dopamine from mouse striatal synaptosomes which does not persist after agonist is removed. Approximately 80% of the initial release is transient and disappears with a half-time of less than 1 min; the other 20% persists for several minutes (t(1/2), 5-10 min). Both the transient and persistent phases were investigated by 10-min exposures to agonists with an in vitro perfusion technique. A series of nicotinic agonists and antagonists were used to determine the pharmacological relationship of the two phases. Parameters measured included EC50 and Vmax values and desensitization rates for both phases for agonists, Ki values for antagonists and Ki values for low concentrations of agonists. The results are consistent with both phases being mediated by a single type of receptor. In addition, the effects of chronic nicotine treatment on transient and persistent [3H]DA release were measured. For both phases, release was decreased approximately 15% by chronic infusion of 4.0 mg/kg/hr L-nicotine. Correlation of the results with inactivation of a portion of the receptors rather than a reversible desensitization is discussed.

title: Presynaptic nicotinic receptors modulating dopamine release in the rat striatum.
abstract: The modulation of striatal dopamine release by presynaptic nicotinic acetylcholine receptors is well documented for both synaptosomes and slices. Because the latter retain local anatomical integrity, we have compared [3H]dopamine release evoked by the nicotinic receptor agonists (-)-nicotine and (+/-)-anatoxin-a from striatal synaptosome and slice preparations in parallel. At higher agonist concentrations, mecamylamine-sensitive [3H]dopamine release was greater from slices, indicative of an additional component, and this increase was abolished by glutamate receptor antagonists. To begin to examine the localisation of specific nicotinic acetylcholine receptor subtypes in the striatum, immunogold electron microscopy was undertaken with the beta2-specific monoclonal antibody 270. In striatal sections, gold particles were associated with symmetric synapses (dopaminergic) but were absent from asymmetric synapses (glutamatergic). Surface labelling of striatal synaptosomes with gold particles was also demonstrated. Taken together, these results are consistent with dopamine release mediated by beta2-containing nicotinic acetylcholine receptors on dopamine terminals, while non-beta2-containing nicotinic acetylcholine receptors may enhance dopamine release indirectly by releasing glutamate from neighbouring terminals.

title: Synaptic-type acetylcholine receptors raise intracellular calcium levels in neurons by two mechanisms
abstract: Nicotinic acetylcholine receptors (AChRs) located in the postsynaptic membrane on neurons are responsible for mediating fast, excitatory synaptic transmission. If synaptic AChRs are also highly permeable to calcium as reported recently for several kinds of neuronal AChRs, the synaptic receptors could regulate calcium-dependent events in the neurons in concert with normal transmission. Chick ciliary ganglion neurons have two classes of AChRs, one located predominantly in the synaptic membrane and responsible for synaptic signaling through the ganglion and the other located almost exclusively in nonsynaptic membrane and having no known function. The nonsynaptic receptors can readily elevate intracellular calcium concentrations. The experiments reported here indicate that synaptic-type receptors can raise intracellular calcium levels to the same extent as the nonsynaptic receptors and that they do so not only by being permeable to calcium themselves but also by activating voltage-dependent calcium channels (VDCCs). Currents of equivalent amplitude are obtained through the synaptic-type receptors when neurons are bathed in solutions containing either sodium or calcium as the sole extracellular cation. Measuring the effect of ion substitutions on the reversal potential of the receptors and applying the Goldman-Hodgkin-Katz constant field equation indicates the receptors are at least as permeable to calcium as to sodium. When neurons are loaded with the calcium-sensitive dye fluo-3 and challenged with nicotine, both the synaptic-type and nonsynaptic AChRs substantially elevate intracellular calcium levels under physiological conditions, and do so largely by activating VDCCs. Confirmation that synaptic-type AChRs can elevate intracellular calcium levels in the absence of contributions from VDCCs was obtained from voltage-clamp experiments on neurons loaded with fluo-3. The fluorescence signals indicate that the nicotine-induced calcium increases in neurons voltage clamped at rest are nearly as great as those induced in the same neurons when VDCCs are maximally activated by a voltage step. Calcium flux through AChRs may be particularly important for mediating local changes in calcium concentrations near the plasma membrane, which, in turn, could regulate specific membrane- associated calcium-dependent events.