bluuebunny/crossref_metadata_2025 huggingface dataset:
The molecular basis of NMDA receptor subtypes: native receptor diversity is predicted by subunit composition
Authors: Buller AL , Larson HC , Schneider BE , Beaton JA , Morrisett RA , Monaghan DT
Published: April 2018

The relationship between four pharmacologically distinct NMDA receptor subtypes, identified in radioligand binding studies, and the recently identified NMDA receptor subunits (NR1a-g, NR2A-D) has not been determined. In this report, we demonstrate that the anatomical distribution of the four NMDA receptor subtypes strikingly parallels the distribution of mRNA encoding NR2A-D subunits. The distribution of NR2A mRNA was very similar to that of “antagonist-preferring” NMDA receptors [defined by high-affinity 3H-2-carboxypiperazine-4-yl-propyl- 1-phosphonic (3H-CPP) binding sites; correlation coefficient = 0.88]. Agonist-preferring NMDA receptors localized to brain regions expressing both NR2B mRNA and NR1- mRNA (NR1 splice variant lacking insert 1). NR2C mRNA was largely restricted to the cerebellar granule cell layer, a region that displays a unique pharmacological profile. NR2D mRNA localized exclusively to those diencephalic nuclei that have a fourth, distinct pharmacological profile (typified by the midline thalamic nuclei). The pharmacology of native NMDA receptors was compared to that of heteromeric NMDA receptors expressed in Xenopus oocytes (NR1/NR2A, NR1/NR2B, NR1/NR2C). The oocyte-expressed NR1/NR2A receptor displayed a higher affinity for antagonists and a slightly lower affinity for agonists than the NR1/NR2B receptor. These patterns are analogous to those found for radioligand binding to native receptors in the lateral thalamus and medial striatum, respectively. NMDA receptors in the lateral thalamus (with a high density of NR2A subunit mRNA) displayed higher affinity for antagonists and a lower affinity for agonists than did NMDA receptors of the medial striatum (a region rich in NR2B mRNA). Relative to the NR1/NR2A and NR1/NR2B receptors, oocyte-expressed NR1/NR2C receptors had a lower affinity specifically for both D-3-(2- carboxypiperazin-4-yl)-1-propenyl-1-phosphonic acid (D-CPPene) and homoquinolinate (HQ). This pattern was identical to that observed for cerebellar (NR2C-containing) versus forebrain (NR2A- and NR2B- containing) NMDA receptors. Taken together, the data in this report suggest that the four previously identified native NMDA receptor subtypes differ in their NR2 composition. Furthermore, the NR2 subunits significantly contribute to the anatomical and pharmacological diversity of NMDA receptor subtypes.
Kainate receptor gene expression in the developing rat brain
Authors: Bahn S , Volk B , Wisden W
Published: April 2018

Kainate-preferring receptors are a subclass of ionotropic glutamate receptors that might play a role in brain development. The expression of the five known genes encoding kainate receptor subunits (GluR-5, -6, -7, KA-1, and KA-2) was studied by in situ hybridization during pre- and postnatal development of the rat brain. We compared the combined expression patterns of these genes with autoradiography using 3H- kainate in the developing brain from embryonic day 12 (E12) through to adult. Although mRNAs for the receptor subunits (except KA-1) can be detected at stage E12, 3H-kainic acid binding (as an index of receptor protein) is not found at this stage. However, by E14 high-affinity kainate sites are found throughout the gray matter, but particularly in spinal cord, primordial cerebellum, and ventral forebrain structures. All genes undergo a peak in their expression in the late embryonic/early postnatal period. GluR-5 expression during development shows the most interesting features because the changes are qualitative. The GluR-5 gene shows peaks of expression around the period of birth in the sensory cortex (layers II, III, and IV), in CA1 hippocampal interneurons in the stratum oriens, in the septum, and in the thalamus. GluR-6 shows a prenatal expression peak in the cingulate gyrus of the neocortex. KA-1 transcripts appear with the development of the hippocampus and remain largely confined to discrete areas such as the CA3 region, the dentate gyrus, and subiculum. KA-2 transcripts are found throughout the CNS from as early as E12 and remain constant until adulthood. The GluR-5 and GluR-6 genes are coexpressed in multiple peripheral ganglia (e.g., cranial nerve ganglia, dorsal root ganglia, and mural ganglia) at E14.
Nerve growth factor rapidly prolongs the action potential of mature sensory ganglion neurons in culture, and this effect requires activation of Gs-coupled excitatory kappa-opioid receptors on these cells
Authors: Shen KF , Crain SM
Published: April 2018

Application of low concentrations (pM-nM) of NGF to mouse dorsal root ganglion (DRG)-spinal cord explants in long-term organotypic cultures rapidly prolongs the duration of the Ca(2+)-dependent component of the action potential (APD) in a major subset of DRG neurons that were previously shown to have characteristic responsiveness to exogenous opioids. These NGF-elicited excitatory modulating effects are blocked by pretreatment of the DRG neurons with monoclonal antibodies to rodent NGF receptors. NGF-induced APD prolongation is also prevented by the opioid receptor antagonist naloxone and the specific kappa-opioid antagonist nor-binaltorphimine (but not by specific mu- and delta- opioid antagonists). The results suggest that NGF stimulates the release of endogenous opioids (e.g., dynorphin) from DRG neurons and that prolongation of the APD occurs secondarily by activation of excitatory kappa-opioid receptor functions on these same or nearby cells. NGF-induced release of small quantities of opioids by DRG neurons would be expected to prolong the APD in view of the remarkable sensitivity of these neurons to the excitatory effects of extremely low (fM-nM) concentrations of exogenous opioid agonists. NGF-induced APD prolongation is blocked by the same cholera toxin A or B subunit treatments previously shown to block Gs coupling and GM1 ganglioside regulation of excitatory opioid receptors, respectively. These in vitro studies suggest that excitatory opioid receptor-mediated functions may play a role in mediating some types of rapid NGF-induced hyperalgesic and other physiologic effects on the nervous system.
Region-specific regulation of transforming growth factor alpha (TGF alpha) gene expression in astrocytes of the neuroendocrine brain
Authors: Ma YJ , Berg-von der Emde K , Moholt-Siebert M , Hill DF , Ojeda SR
Published: April 2018

Certain glial cells of the hypothalamus have been implicated in the neuroendocrine control of reproductive development. Hypothalamic astrocytes appear to exert this function via a cell-cell interactive mechanism that involves the production of transforming growth factor alpha (TGF alpha), a polypeptide able to affect both glial and neuronal functions in the CNS. In the hypothalamus, TGF alpha stimulates neuronal secretion of luteinizing hormone-releasing hormone (LHRH), the neuropeptide controlling sexual development, via activation of epidermal growth factor receptors (EGFR). Since astrocytes but not LHRH neurons express EGFR, it has been postulated that the stimulatory effect of TGF alpha on LHRH release is not exerted directly on LHRH neurons, but rather via glial intermediacy. The present experiments were undertaken to define whether TGF alpha is able to exert paracrine/autocrine effects on isolated hypothalamic astrocytes, and to determine if estradiol-previously shown to increase TGF alpha mRNA levels in the hypothalamus of immature animals--can act directly on hypothalamic astrocytes to upregulate TGF alpha gene expression. Treatment with either TGF alpha or its structural homolog, epidermal growth factor (EGF), increased TGF alpha mRNA levels within 8 hr of exposure; the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA) was similarly effective. Blockade of EGFR with either tyrphostin RG- 50864, an inhibitor of tyrosine kinase activity, or a monoclonal antibody that prevents ligand binding abolished the upregulatory effect of TGF alpha on TGF alpha mRNA levels. In contrast to hypothalamic astrocytes, cerebellar astrocytes did not respond to either TGF alpha or EGF with changes in TGF alpha mRNA abundance.(ABSTRACT TRUNCATED AT 250 WORDS)
The effects of L-glutamate, AMPA, quisqualate, and kainate on retinal horizontal cells depend on adaptational state: implications for rod- cone interactions
Authors: Krizaj D , Akopian A , Witkovsky P
Published: April 2018

We studied the responses of isolated and intact luminosity-type horizontal cells (L-HC) in the Xenopus retina to L-glutamate (L-glu) and its analogs. Isolated L-HCs studied with whole-cell patch clamp responded to L-glu, kainate (KA), AMPA, or quisqualate (quis) with inward currents from a holding potential of -60 mV, associated with a conductance increase. The current elicited by KA was relatively large and sustained, whereas AMPA or quis evoked a desensitizing current. Coapplication of quis and KA resulted in a smaller current and conductance change than that evoked by a pulse of either alone at the same concentration. This finding suggests that the L-HC has a single subtype of glutamate receptor that responds to both quis and KA. Prior exposure to dopamine enhanced the KA-evoked current about twofold. In the superfused eyecup we found that L-HC responses to quinoxalinediones (CNQX or DNQX) and to L-glu, KA, AMPA, and quis varied as a function of adaptational state. When driven exclusively by either cones or by rods, CNQX/DNQX hyperpolarized the L-HC and reduced its light response, without altering response kinetics, indicating that both rods and cones communicate with L-HCs at ionotropic glutamatergic synapses. Under mesopic conditions, however, as CNQX or DNQX reduced cone input, the rod input to the L-HC increased up to fivefold in magnitude and had slowed kinetics. The depolarizing response of the L-HC to L-glu, AMPA, or quis was relatively small and transient under photopic conditions, but was much larger and sustained when the eyecup was dark adapted. The D1 dopamine antagonist SCH 23390 potentiated the response to quis. In contrast, responses to KA were largest in light-adapted eyecups, were potentiated by a D1 dopamine agonist, SKF 38393, and were reduced by SCH 23390. We hypothesize that the segregated populations of glutamate receptors in the L-HC opposite cone and rod synaptic endings can be separately modulated to respond differentially to the native transmitter, glutamate. In photopic and mesopic states the dominant cone input tonically inhibits rod to L-HC communication. This inhibition appears to occur at the postsynaptic membrane and may be mediated by second messengers.
Functional properties and substrate specificity of the cloned L- glutamate/L-aspartate transporter GLAST-1 from rat brain expressed in Xenopus oocytes
Authors: Klockner U , Storck T , Conradt M , Stoffel W
Published: April 2018

The rat brain L-glutamate/L-aspartate transporter GLAST-1 is a member of a family of Na(+)-dependent high-affinity L-glutamate transporters proposed to be involved in the termination and modulation of excitatory neurotransmitter signals. Application of electrophysiological and radiotracer techniques on Xenopus oocytes expressing cloned GLAST-1 revealed that the apparent Km value of the transporter for L-glutamate and Na+ ions did not depend on voltage while the maximal transport rate increased with more negative potentials, indicative of a low-field access channel. The apparent Km value of the transporter for L- glutamate depends on the Na+ concentration, suggesting that substrate and ions are transported by GLAST-1 in a simultaneous manner. All of the L-glutamate uptake blockers tested either were substrates or did not affect the current induced by L-glutamate. The changes in the amplitude of the current induced by simultaneous application of two substrates can be interpreted by a competition for one binding site.
The grasshopper median neuroblast is a multipotent progenitor cell that generates glia and neurons in distinct temporal phases
Authors: Condron BG , Zinn K
Published: April 2018

The midline of the insect CNS provides a good system for studying the control of neuronal and glial cell fates, because it contains a small number of distinct cell types that arise from a unique set of precursors. In this report we analyze the development of the CNS midline in the grasshopper embryo, focusing on the median neuroblast (MNB) cluster, which contains the majority of midline neurons. We have directly traced cell lineage in the MNB cluster by injecting the MNB with tracer dyes and following the development of its progeny in whole embryo cultures. As least three types of neuronal progeny are labeled with tracer dye in these experiments. In addition, we find that all of the midline glia are also derived from the MNB. Thus, this neuroblast is actually a multipotent progenitor cell. We also examined the temporal and spatial patterns of midline development by staining embryos with antibodies to neuronal and glial markers and to the protein product of the engrailed gene, which is transiently expressed by all MNB progeny. Our data show that neuronal and glial progeny are generated from the MNB in distinct temporal phases. A change in the orientation of the MNB's mitotic spindle correlates with the transition between two of the phases of progeny production.
Neurotensin promotes oscillatory bursting behavior and is internalized in basal forebrain cholinergic neurons
Authors: Alonso A , Faure MP , Beaudet A
Published: April 2018

Cholinergic neurons of the basal forebrain magnocellular complex (BF) constitute the primary source of ACh to the cerebral cortex and are thought to be instrumental in mediating cortical activation and plasticity. Recent light and electron microscopic studies have revealed a selective association of receptors for the neuropeptide neurotensin (NT) with BF cholinergic neurons, suggesting that this peptide may be playing a key role in the control of BF cholinergic function. In the present study, we have investigated by means of intracellular recording in guinea pig brain slices the neuromodulatory actions of NT on the intrinsic excitability of BF cholinergic neurons that were identified electrophysiologically by their low-threshold discharge, slow afterhyperpolarization, and transient outward rectification (TOR). In all cholinergic neurons tested (n = 39), bath application of NT (20–200 nM for 1–4 min) produced, via a direct mechanism, a membrane potential depolarization associated with a decrease in apparent input conductance. Most significantly, NT led to the emergence of a very prominent slow rhythmic bursting pattern that could shape into complex spindle-like sequences that were intrinsically generated by the cholinergic cells. These NT actions were also accompanied by a reduction of both the slow afterhyperpolarization and TOR. Bursting oscillations relied on the activation of Ca2+ conductances as opposed to Na+ conductances, since they were absent during Ca(2+)-conductance block with Mn2+, but still occurred in the presence of the Na(+)- channel blocker TTX. NT actions were specific, since they could be reproduced by application of the active (NT 8–13) but not of the inactive (NT 1–8) fragment of the peptide. Identification of the BF cholinergic neurons as direct NT targets was further provided by confocal laser scanning microscopic demonstration of internalization of a fluoresceinylated derivative of NT (fluo-NT) within biocytin-filled, electrophysiologically identified cholinergic neurons. The results demonstrate the electrophysiological functionality of NT receptors on BF cholinergic neurons and the existence of a receptor-mediated internalization of NT in these cells. They also suggest that the peptide is an important player in the control of BF function and, in particular, in the generation of forebrain network oscillations.
LTP saturation and spatial learning disruption: effects of task variables and saturation levels
Authors: Barnes CA , Jung MW , McNaughton BL , Korol DL , Andreasson K , Worley PF
Published: April 2018

The prediction that “saturation” of LTP/LTE at hippocampal synapses should impair spatial learning was reinvestigated in the light of a more specific consideration of the theory of Hebbian associative networks, which predicts a nonlinear relationship between LTP “saturation” and memory impairment. This nonlinearity may explain the variable results of studies that have addressed the effects of LTP “saturation” on behavior. The extent of LTP “saturation” in fascia dentata produced by the standard chronic LTP stimulation protocol was assessed both electrophysiologically and through the use of an anatomical marker (activation of the immediate-early gene zif268). Both methods point to the conclusion that the standard protocols used to induce LTP do not “saturate” the process at any dorsoventral level, and leave the ventral half of the hippocampus virtually unaffected. LTP- inducing, bilateral perforant path stimulation led to a significant deficit in the reversal of a well-learned spatial response on the Barnes circular platform task as reported previously, yet in the same animals produced no deficit in learning the Morris water task (for which previous results have been conflicting). The behavioral deficit was not a consequence of any after-discharge in the hippocampal EEG. In contrast, administration of maximal electroconvulsive shock led to robust zif268 activation throughout the hippocampus, enhancement of synaptic responses, occlusion of LTP produced by discrete high- frequency stimulation, and spatial learning deficits in the water task. These data provide further support for the involvement of LTP-like synaptic enhancement in spatial learning.
A rhodopsin gene mutation responsible for autosomal dominant retinitis pigmentosa results in a protein that is defective in localization to the photoreceptor outer segment
Authors: Sung CH , Makino C , Baylor D , Nathans J
Published: April 2018

Over 45 mutations in the rhodopsin gene have been identified in patients with autosomal dominant retinitis pigmentosa, including a cluster near the extreme carboxy-terminus, a region of the protein for which no function has yet been assigned. To elucidate the biochemical defect(s) in this group of mutants, we have studied a naturally occurring stop codon mutation that removes the last five amino acids of rhodopsin (Q344ter). When produced in transfected tissue culture cells, the mutant protein is indistinguishable from the wild type in light- dependent activation of the photoreceptor G-protein (transducin), and in serving as a light-dependent substrate for rhodopskin kinase. Mice that express a Q344ter transgene in rod photoreceptors show nearly normal light responses as determined by suction electrode recordings of the membrane current from single rod outer segments; the main difference between transgenic and nontransgenic responses is a 15% longer time-to-peak in the response of transgenic rods. In the Q344ter transgenic retina, direct immunofluorescent staining with antibodies specific for either wild-type or Q344ter rhodopsin shows abnormal accumulation of the Q344ter, but not the endogenous rhodopsin, in the plasma membrane of the photoreceptor cell body. These data indicate that rhodopsin's carboxy-terminus is required for efficient transportation to or retention in the outer segment.
Metallothionein III is expressed in neurons that sequester zinc in synaptic vesicles
Authors: Masters BA , Quaife CJ , Erickson JC , Kelly EJ , Froelick GJ , Zambrowicz BP , Brinster RL , Palmiter RD
Published: April 2018

MT-III, a brain-specific member of the metallothionein gene family, binds zinc and may facilitate the storage of zinc in neurons. The distribution of MT-III mRNA within the adult brain was determined by solution and in situ hybridization and compared to that of MT-I mRNA. MT-III mRNA is particularly abundant within the cerebral cortex, hippocampus, amygdala, and nuclei at base of the cerebellum. Transgenic mice generated using 11.5 kb of the mouse MT-III 5′ flanking region fused to the E. coli lacZ gene express beta-galactosidase in many of the same regions identified by in situ hybridization. MT-III mRNA was present in readily identifiable neurons within the olfactory bulb, hippocampus, and cerebellum, and beta-galactosidase activity was localized to neurons throughout the brain, but not to glia, as determined by costaining with X-Gal and neural- and glia-specific antibodies. There is marked correspondence between the neurons that are rich in MT-III mRNA and those neurons that store zinc in their terminal vesicles. MT-III is found complexed with zinc in vivo and its expression in cultured cells leads to the intracellular accumulation of zinc and enhanced histochemical detection of zinc. These results are discussed in light of the possibility that MT-III may participate in the utilization of zinc as a neuromodulator.
Microtubule fragmentation and partitioning in the axon during collateral branch formation
Authors: Yu W , Ahmad FJ , Baas PW
Published: April 2018

Axons within the brain branch principally by the formation of collaterals rather than by bifurcation of the terminal growth cone (O'Leary and Terashima, 1988). This same behavior is recapitulated in cultures of embryonic hippocampal neurons (Dotti et al., 1988), rendering them ideal for studies on the cell biological mechanisms underlying collateral branch formation. In the present study, we focused on changes in the microtubule (MT) array that occur as these axons branch. In particular, we explored the mechanism by which MT number is locally increased to accommodate the need for more MTs during collateral branch formation. Serial reconstruction analyses indicate that MT number increases by severalfold and that MT length decreases correspondingly within the parent axon in the discrete region giving rise to the branch. These observations strongly suggest that MTs within the parent axon undergo a local fragmentation in this region, and hence raise the possibility that a portion of these new MTs might be destined for transport into the branch. To address this latter issue, we used quantitative immunofluorescence to compare the proportion of newly assembled to total MT polymer in different regions of the axon. As previously reported (Brown et al., 1992), the region of the axon contiguous with the terminal growth cone is particularly rich in newly assembled polymer. In contrast, there was no distinguishable difference in the proportion of newly assembled polymer in the newly formed collateral branches compared to the shaft region of the parent axon. These results indicate that the MTs within the newly formed collateral branches are on average assembled at the same time as those within the parent axon, and thus strongly suggest that the MTs in the collateral branch were assembled in the parent axon and then translocated into the branch. We conclude on the basis of these observations that collateral branch formation requires a local fragmentation of MTs within the parent axon, followed by the partitioning of a portion of the MT fragments into the branch. These short MTs presumably then resume their movement and elongation down the collateral branch as well as down the parent axon for the steady and orderly increase of both MT arrays.
EPSPs of dentate gyrus granule cells during epileptiform bursts of dentate hilar "mossy" cells and area CA3 pyramidal cells in disinhibited rat hippocampal slices
Authors: Scharfman HE
Published: April 2018

When hippocampal slices are exposed to GABAA antagonists, area CA3 pyramidal cells and dentate hilar “mossy” cells discharge in synchronized epileptiform bursts (Muller and Misgeld, 1991; Scharfman, 1994b). Dentate interneurons are excited simultaneously, but the degree of discharge varies (Scharfman, 1994b). This study primarily examined the activity of dentate granule cells simultaneous to the epileptiform bursts of pyramidal cells and mossy cells. EPSPs followed by large GABAB receptor-mediated IPSPs were generated in granule cells during all epileptiform bursts of pyramidal cells and mossy cells, regardless of whether they were evoked or spontaneous. By simultaneous recording it was determined that granule cell EPSPs began several milliseconds after the start of pyramidal cell bursts (n = 48 simultaneous recordings) and immediately after the first action potential of a mossy cell burst (n = 77). Interneurons were similar to granule cells in the timing of their depolarizations relative to the onset of pyramidal cell (n = 24; Scharfman, 1994b) and mossy cell (n = 9) bursts. All excitatory activity was blocked by bath application of the glutamatergic AMPA/kainate receptor antagonist CNQX (5 microM, n = 5), but not the NMDA receptor antagonist D-APV (25–50 microM, n = 9). Granule cell EPSPs were decreased after focal application of CNQX to the molecular layer at a site close to the impaled granule cell (n = 5), whereas D-APV had no effect (n = 3). EPSPs also decreased after focal application of CNQX to the hilus, in two of four slices tested. The extracellularly recorded EPSP of granule cells was maximal in the inner molecular layer (n = 33), the site of the mossy cell axonal plexus. Severing the junction of the dentate gyrus and area CA3 blocked all spontaneous and evoked activity of dentate neurons without affecting burst discharges in area CA3a and CA3b (n = 6). None of the excitatory activity of any cell type was affected by cholinergic antagonists (atropine and mecamylamine, 25 microM each, n = 5; pirenzipine and dihydro-beta-erythroidine, 25 microM each, n = 5). The results suggest that there is a glutamatergic, AMPA/kainate receptor- mediated, excitatory pathway from area CA3 to the dentate gyrus in disinhibited slices. The pharmacological results, analyses of latency, as well as the known axonal projections of the sampled cells, suggest that the excitatory pathway begins within area CA3 and leads to granule cells via mossy cells. The data also suggest that dentate interneurons are excited by mossy cells, and possibly by pyramidal cells as well.(ABSTRACT TRUNCATED AT 400 WORDS)
Influence of efferent stimulation on acoustically responsive vestibular afferents in the cat
Authors: McCue MP , Guinan JJ
Published: April 2018

In the preceding article (McCue and Guinan, 1994) we described a class of vestibular primary afferent fibers in the cat that responds vigorously to sounds at moderately high sound levels. Like their cochlear homologs, vestibular afferents and their associated hair cells receive efferent projections from brainstem neurons. In this report, we explore efferent influences on the background activity and tone-burst responses of the acoustically responsive vestibular afferents. Shock- burst stimulation of efferents excited acoustically responsive vestibular afferents; no inhibition was seen. A fast excitatory component built up within 100–200 msec of shock-burst onset and decayed with a similar time course at the end of each shock burst. During repeated 400 msec shock bursts at 1.5 sec intervals, a slow excitatory component grew over 20–40 sec and then decayed, even though the shock bursts continued. Efferent stimulation excited acoustically responsive vestibular afferents without appreciably changing an afferent's sound threshold or its average sound-evoked response. This evidence supports the hypothesis that excitation is due to efferent synapses on afferent fibers rather than on hair cells. Efferent stimulation enhanced the within-cycle modulation of afferent discharges evoked by a tone; that is, it increased the “AC gain.” No appreciable change was noted in the degree of phase locking to low-frequency tones as measured by the synchronization index. Little or no improvement in the bidirectionality (linearity) of transduction was seen. Vestibular afferent responses to tones normally had one peak per cycle; however, during efferent stimulation, two peaks per cycle were sometimes seen. We hypothesize that this is caused by two driving components acting at different sound phases with the components differentially affected by efferent activity. We discuss the relationship of our findings to efferent influences on acoustic responses in cochlear afferent fibers. The acoustically responsive vestibular afferents provide a mammalian model for studying purely excitatory efferent effects in a hair cell system.
Efflux of dopamine from the synaptic cleft in the nucleus accumbens of the rat brain
Authors: Garris PA , Ciolkowski EL , Pastore P , Wightman RM
Published: April 2018

Synaptic release of dopamine in the nucleus accumbens of the intact rat brain elicited by a single electrical impulse applied to ascending dopaminergic fibers results in extracellular concentrations sufficient to bind the known dopamine receptors. The dopamine concentration observed after four rapid, sequential pulses is exactly four times greater and is unaffected by pharmacological antagonism of dopamine uptake and receptor sites at supramaximal concentrations. Thus, dopamine efflux from the synaptic cleft is not restricted by binding to intrasynaptic proteins on the time scale of the measurements (50–100 msec). The extracellular concentration, as a result of a single stimulus pulse, is 0.25 microM and is rapidly removed by extrasynaptic uptake. This maximal, transient concentration of dopamine is 60 times higher than steady-state concentrations reported previously using dialysis techniques, illustrating that dopamine extracellular concentrations are spatially and temporally heterogenous. In contrast to ACh transmission at the neuromuscular junction, the dopamine synapse in the telencephalon is designed for the effective efflux of dopamine from the synaptic cleft to the extrasynaptic compartment during neurotransmission.
Dopamine enhances a glutamate-gated ionic current in OFF bipolar cells of the tiger salamander retina
Authors: Maguire G , Werblin F
Published: April 2018

The transmitter glutamate is thought to be used by all vertebrate photoreceptors to drive the second-order neurons of the retina, horizontal and bipolar neurons. Dopamine, an endogenous retinal neurotransmitter localized to amacrine and interplexiform cells, has previously been shown to enhance glutamate-gated currents in retinal horizontal cells. In the present study we demonstrate that bipolar cells, like horizontal cells, possess glutamate receptors that are modulated by dopamine. We then identify some components of the pathway through which dopamine acts. We used whole-cell patch recording to measure how bath-applied dopamine modulated the currents elicited by puffs of transmitter solutions at bipolar cell dendrites. Excitatory amino acid-gated currents were evoked by pressure ejecting 1 mM glutamate or 10 microM kainate for 40 msec through a micropipette positioned at the dendrites of bipolar cells. Bath-applied dopamine (20 microM) enhanced the response to glutamate in OFF bipolar cells in the retinal slice by 40% and in isolated OFF bipolar cells by 65%. We also explored the components of the intracellular pathway mediating this modulation. Response enhancement was blocked by the D1 receptor antagonist SCH23390, but not by the D2 receptor antagonist spiperone, suggesting that the enhancement by dopamine is mediated by a D1 receptor. GDP-beta-S, a G-protein inactivator, blocked the enhancing action of dopamine, suggesting that the D1 receptor activated a G- protein to enhance the glutamate-gated current. Both 8-(4- chlorophenylthio)adenosine, a cAMP analog, and the addition of the catalytic subunit of protein kinase A (PKA) to the recording pipette enhanced glutamate-gated currents, while H-7, a PK inactivator, and PKI20amide, a PKA-specific inhibitor, blocked the enhancing action of dopamine. These data suggest that dopamine acts at D1 receptors in the dendrites of bipolar cells to activate adenyl cyclase, which through cAMP enhances a glutamate-gated current in bipolar cell dendrites. Thus, dopamine may modulate synaptic transmission from photoreceptors to OFF bipolar cells.
Selective activation of deep layer (V-VI) retrohippocampal cortical neurons during hippocampal sharp waves in the behaving rat
Authors: Chrobak JJ , Buzsaki G
Published: April 2018

The coordinated activity of hippocampal neurons is reflected by macroscopic patterns, theta and sharp waves (SPW), evident in extracellular field recordings. The importance of these patterns is underscored by the ordered relation of specific neuronal populations to each pattern as well as the relation of each pattern to distinct behavioral states. During awake immobility, consummatory behavior, and slow wave sleep, CA3 and CA1 neurons participate in organized population bursts during SPW. In contrast, during theta-associated exploratory activity, the majority of principle cells are silent. Considerably less is known about the discharge properties of retrohippocampal neurons during theta, and particularly during SPW. These retrohippocampal neurons (entorhinal cortical, parasubicular, presubicular, and subicular) process and transmit information between the neocortex and the hippocampus. The present study examined the activity of these neurons in freely behaving rats during SPW (awake immobility) as well as theta (locomotion and REM sleep). A qualitative distinction between the activity of deep (V-VI) and superficial (II- III) layer retrohippocampal neurons was observed in relation to SPW as compared to theta. Deep layer retrohippocampal neurons exhibited a concurrent increase in activity during hippocampal SPW. In contrast, deep layer neurons were not modulated by the prominent theta oscillations observed throughout the hippocampus and entorhinal cortex. On the other hand, superficial layer retrohippocampal neurons were often phase-related to theta oscillations, but were surprisingly indifferent to the SPW-associated population bursting occurring within the deep layers. These findings indicate a concerted discharge of the hippocampal and retrohippocampal cortices during SPW that includes neurons within CA3, CA1, and subiculum as well as neurons in layers V- VI of the presubiculum, parasubiculum, and entorhinal cortex. Further, they suggest a temporal discontinuity in the input/output relations between the hippocampus and retrohippocampal structures. We suggest that SPW-associated population bursts in hippocampal and retrohippocampal cortices exert a powerful depolarizing effect on their postsynaptic neocortical targets and may represent a physiological mechanism for memory trace transfer from the hippocampus to the neocortex.
Responses of neurons in the parietal and temporal visual pathways during a motion task
Authors: Ferrera VP , Rudolph KK , Maunsell JH
Published: April 2018

The visual cortex of macaque monkeys has been divided into two functional streams that have been characterized in terms of sensory processing (color/form vs motion) and in terms of behavioral goals (object recognition vs spatial orientation). As a step toward unifying these two views of cortical processing, we compared the behavioral modulation of sensory signals across the two streams in monkeys trained to do a visual short-term memory task. We recorded from individual neurons in areas MT, MST, 7a, and V4 while monkeys performed a delayed match-to-sample task using direction of motion as the matching criterion. This task allowed us to determine if sensory responses were modulated by extraretinal signals related to the direction of the remembered sample. We sorted neuronal responses as a function of the remembered direction and calculated a modulation index, MI = (maximum response--minimum response)/(maximum response + minimum response). In the motion pathway, we found virtually no extraretinal signals in MT (average MI = 0.11 +/- 0.01 SE, 66 cells), but progressively stronger extraretinal signals in later stages, that is, MST (average MI = 0.17 +/- 0.01 SE, 57 cells) and 7a (average MI = 0.23 +/- 0.02 SE, 46 cells). In contrast to MT, strong extraretinal signals for direction matching were found in V4 (average MI = 0.28 +/- 0.02 SE, 94 cells), a relatively early stage of the color/form pathway, even though this pathway is not generally viewed as playing a major role in motion processing. Some cells in V4 were also tested while the animals performed a color matching task. These cells showed memory-related modulation of their response when either color or direction was used as the matching criterion. We conclude that extraretinal signals related to the match-to-sample task may be stronger in the temporal (color/form) pathway than in the parietal (motion) pathway, regardless of the stimulus dimension involved. Furthermore, our results indicate that the temporal pathway is capable of making a significant contribution to motion processing in tasks where motion can be considered as a cue for the identification of object attributes.
Insulin receptor substrate-1 (IRS-1) distribution in the rat central nervous system
Authors: Folli F , Bonfanti L , Renard E , Kahn CR , Merighi A
Published: April 2018

Insulin receptor substrate 1 (IRS-1) is the primary cytosolic substrate of the insulin and insulin-like growth factor-I (IGF-I) receptors. Following tyrosine phosphorylation IRS-1 binds to and activates specific proteins containing SH2 domains. Using biochemical and immunocytochemical techniques, we have mapped the distribution of IRS-1 in the CNS of the adult rat and compared it with that of insulin and IGF-I receptors and phosphatidylinositol 3-kinase (PI-3 kinase), a signaling molecule functionally related to IRS-1. Immunoprecipitation and Western blotting experiments demonstrate the presence of substantial amounts of IRS-1, insulin receptor, and PI-3 kinase in the brain. IRS-1 immunoreactivity is widely distributed in neurons from several areas of the brain and spinal cord. The cerebral cortex, the hippocampus, many hypothalamic and thalamic nuclei, the basal ganglia, the cerebellar cortex, the brainstem nuclei, and the lamina X of the spinal cord are particularly rich of immunopositive nerve cells. In these areas most of the neurons immunoreactive for IRS-1 are also stained by either anti-insulin receptor or anti-IGF-I receptor antibodies as well as PI-3 kinase antiserum. IRS-1 immunostaining was very weak or totally absent in neurons of the olfactory bulb, the supraoptic and paraventricular nuclei, the mesencephalic trigeminal nucleus, and the granule cell layer of the cerebellum, despite the fact that these areas were immunolabeled with antibodies against insulin or IGF-I receptors and/or PI-3 kinase. These results show that neurons in the adult rat CNS are endowed with some of the components of the early signaling pathway for growth factors of the insulin/IGF-I family, although IRS-1 has a distribution distinct from that of the two receptors.
NMDA receptor blockade rescues Clarke's and red nucleus neurons after spinal hemisection
Authors: Sanner CA , Cunningham TJ , Goldberger ME
Published: April 2018

Hemisection of the adult rat spinal cord at T9 transects the ascending ipsilateral axons of Clarke's nucleus (CN) neurons and the descending contralateral axons of red nucleus (RN) neurons. Eight weeks following axotomy, 30% of CN neurons and 22% of RN neurons die. Since both nuclei receive glutamatergic input, we wished to examine the possibility that glutamatergic excitotoxicity contributes to axotomy-induced neuronal death in these nuclei. To test this we studied the effects of administration of the NMDA receptor antagonist MK-801 on cell survival after axotomy. When 1 mg/kg body weight MK-801 is administered subcutaneously every day for 1–8 weeks to hemisected rats, cell death is prevented. Treatment with 0.5 mg/kg body weight MK-801 over the same time periods results in only partial rescue of axotomized neurons. Paradoxically, when 1 mg/kg MK-801 administration is restricted to the first week of an 8 week survival period, cell death in both the RN and CN is greatly exaggerated over the cell loss found in saline-treated animals. Withdrawal of 1 mg/kg MK-801 after 1 week of administration induces the loss of 92% of CN neurons, which is 63% greater than that occurring after axotomy alone. If, however, 1 mg/kg MK-801 is withdrawn after 2 weeks post-axotomy in the RN and 3 weeks postaxotomy in CN, all axotomized neurons survive. This rescue is found at 6 months postsurgery, the longest survival period studied, and therefore appears to be permanent. These results suggest that glutamatergic afferent input contributes significantly to the death of axotomized red nucleus and Clarke's nucleus neurons via NMDA receptors located on these neurons.
Topographic organization of respiratory responses to glutamate microstimulation of the parabrachial nucleus in the rat
Authors: Chamberlin NL , Saper CB
Published: April 2018

The parabrachial complex, also known as the pneumotaxic center or pontine respiratory group, has long been recognized as an important participant in respiratory control. One line of evidence supporting this idea is the demonstration of changes in breathing pattern following injection of neuroactive substances into or near the parabrachial complex. However, it is not yet known exactly which cell groups and projections mediate those responses. In order to address this issue, we explored the topographic organization of respiratory responses to chemical stimulation of the parabrachial complex of the rat and examined the descending projections of the most sensitive sites. Injection of glutamate (5–100 pmol) at specific sites in or near the parabrachial nucleus produced three distinct site-specific response patterns. First, hyperpnea followed glutamate injection into far rostral and midcaudal areas of the Kolliker-Fuse nucleus and most of the lateral parabrachial nucleus, including the external lateral, central lateral, dorsal lateral, and superior lateral subnuclei. Threshold hyperpneic effects were manifested as single, deepened breaths of premature onset. Suprathreshold doses of glutamate at these locations produced tachypnea. Neurons in these sites projected to the ventral respiratory group in the ventrolateral medulla. Second, the most intense inspiratory facilitatory responses were seen at mid to rostral levels of the Kolliker-Fuse nucleus, near the ventrolateral tip of the superior cerebellar peduncle. Even at threshold doses of glutamate, exhalation was incomplete, resulting in a breathing pattern that resembled apneusis (an inspiratory cramp). This site contained an especially dense cluster of neurons that projected either to the ventrolateral medulla or to the dorsal respiratory group in the nucleus of the solitary tract, but not to both areas. The third type of response, decreases in respiratory rate, occurred following glutamate injection at the most lateral and ventral boundaries of the Kolliker- Fuse nucleus. The most sensitive apneic sites were not found in the parabrachial nucleus but along the dorsal and medial edge of the principal sensory trigeminal nucleus and extending ventrally between the sensory and motor trigeminal nuclei. Scattered neurons in these sites were retrogradely labeled from the ventral but not the dorsal respiratory group. These results indicate that there are anatomically and functionally distinct cell populations in and near the parabrachial complex that, when chemically stimulated, can produce specific and sometimes opposing effects on respiration. The predominant effect of lateral parabrachial stimulation is respiratory facilitation, while inhibitory effects are elicited by trigeminal injections of glutamate.
Neurons responding to whole-body motion in the primate hippocampus
Authors: O'Mara SM , Rolls ET , Berthoz A , Kesner RP
Published: April 2018

We describe here hippocampal cells that respond during whole-body motion when a monkey is moved on a remote-controlled robot-mounted platform in a cue-controlled test chamber (2 x 2 x 2 m). Some of these cells responded to linear motion, and others to axial rotation. Some of these cells responded when the same motion occurred without a view of the visual field. Such cells appeared to be driven by vestibular inputs. Other cells required a view of the visual field for their response, and these cells appeared to be driven by the visual motion relative to the monkey of the test chamber. Further evidence that this was the case was that some of the cells responded to rotation and linear motion of the test chamber while the monkey remained stationary. Other cells responded to combinations of whole-body motion and a view of the environment. These findings show that information about whole- body motion, as well as about where the animal is looking in an environment, is represented in the primate hippocampus. We suggest that this information is important in spatial memory and thus in spatial navigation.
A novel gamma-aminobutyric acid receptor subunit (rho 2) cloned from human retina forms bicuculline-insensitive homooligomeric receptors in Xenopus oocytes
Authors: Wang TL , Guggino WB , Cutting GR
Published: April 2018

The rho 2 subunit, a novel GABA receptor subunit, has been cloned from a human retinal complementary DNA library. This subunit shares 74% amino acid sequence identity with the rho 1 subunit that forms homooligomeric bicuculline-, barbiturate-, and benzodiazepine- insensitive GABA receptors. The rho 2 subunit also forms homooligomeric GABA-activated chloride channels when expressed in Xenopus oocytes. The amplitudes of the whole-cell currents of rho 2 receptors are always smaller than those of rho 1 receptors, but the affinity and cooperativity of GABA are very similar. Like the rho 1 subunit, the currents generated by rho 2 are insensitive to GABAA receptor modulators including bicuculline, hexobarbital, and diazepam and can be reversibly inhibited by ZnCl2. Homooligomeric rho 2 and rho 1 receptors are less sensitive to muscimol and picrotoxin, and desensitize slower than GABAA receptors. These data demonstrate that homooligomeric receptors formed by rho 2 and rho 1 subunits have a number of electrophysiologic and pharmacologic characteristics that differ from receptors formed by GABAA receptor subunits. The distinctive properties of rho receptors are very similar to those of bicuculline-insensitive GABA-gated chloride channels identified in retina, suggesting a molecular basis for this form of GABA receptor in visual pathways.
Neurotrophin-3 induces neural crest-derived cells from fetal rat gut to develop in vitro as neurons or glia
Authors: Chalazonitis A , Rothman TP , Chen J , Lamballe F , Barbacid M , Gershon MD
Published: April 2018

The precursor cells that form the enteric nervous system (ENS) are multipotent when they arrive in the gut from the neural crest. Their differentiation thus depends on signals from the enteric microenvironment. Crest-derived cells were isolated from the fetal rat bowel by immunoselection at E14 with NC-1/HNK-1 antibodies and secondary antibodies coupled to magnetic beads. NC-1/HNK-1- immunoreactive cells were enriched approximately 36-fold. The NC-1/HNK- 1-selected population and the residual population were plated at equal cell density and maintained in a defined medium for 6–7 d. The total number of cells found in the cultures of the residual cells was three- to fourfold that in cultures of immunoselected cells. Neurotrophin-3 (NT-3), but not nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), or neurotrophin-4/5 (NT-4/5), was found to increase the proportion of neurons (neurofilament-immunoreactive or neuron-specific enolase-immunoreactive) or glia (S-100-immunoreactive) (from 6.6 +/- 0.9% to 15.2 +/- 1.4%; p < 0.001). This effect was concentration dependent (from 1 to 40 ng/ml) and observed only in the cultures of immunoselected cells. NT-3 also enhanced neurite outgrowth. NT-3 increased neither cell number nor bromodeoxyuridine incorporation and thus was not mitogenic. Exposure of immunoselected cells to NT-3 rapidly and transiently induced the appearance of nuclear Fos immunoreactivity. Transcripts coding for TrkC, the transducing receptor for NT-3, were identified in the fetal rat gut (E14-E16) and in the immunoselected population of cells using reverse transcriptase and the polymerase chain reaction. It is concluded that NT-3 specifically promotes the differentiation of enteric crest-derived cells as neurons or glia and may thus play a role in the development and/or maintenance of the ENS.