bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrical Coupling Between Model Midbrain Dopamine Neurons: Effects on Firing Pattern and Synchrony
Authors: Komendantov Alexander O. , Canavier Carmen C.
Published: May 2017

The role of gap junctions between midbrain dopamine (DA) neurons in mechanisms of firing pattern generation and synchronization has not been well characterized experimentally. We modified a multi-compartment model of DA neuron by adding a spike-generating mechanism and electrically coupling the dendrites of two such neurons through gap junctions. The burst-generating mechanism in the model neuron results from the interaction of a N-methyl-d-aspartate (NMDA)-induced current and the sodium pump. The firing patterns exhibited by the two model neurons included low frequency (2–7 Hz) spiking, high-frequency (13–20 Hz) spiking, irregular spiking, regular bursting, irregular bursting, and leader/follower bursting, depending on the parameter values used for the permeability for NMDA-induced current and the conductance for electrical coupling. All of these firing patterns have been observed in physiological neurons, but a systematic dependence of the firing pattern on the covariation of these two parameters has not been established experimentally. Our simulations indicate that electrical coupling facilitates NMDA-induced burst firing via two mechanisms. The first can be observed in a pair of identical cells. At low frequencies (low NMDA), as coupling strength was increased, only a transition from asynchronous to synchronous single-spike firing was observed. At high frequencies (high NMDA), increasing the strength of the electrical coupling in an identical pair resulted in a transition from high-frequency single-spike firing to burst firing, and further increases led to synchronous high-frequency spiking. Weak electrical coupling destabilizes the synchronous solution of the fast spiking subsystems, and in the presence of a slowly varying sodium concentration, the desynchronized spiking solution leads to bursts that are approximately in phase with spikes that are not in phase. Thus this transitional mechanism depends critically on action potential dynamics. The second mechanism for the induction of burst firing requires a heterogeneous pair that is, respectively, too depolarized and too hyperpolarized to burst. The net effect of the coupling is to bias at least one cell into an endogenously burst firing regime. In this case, action potential dynamics are not critical to the transitional mechanism. If electrical coupling is indeed more prominent in vivo due to basal level of modulation of gap junctions in vivo, these results may indicate why NMDA-induced burst firing is easier to observe in vivo as compared in vitro.
Synchronous and Asynchronous Exocytosis Induced by Subthreshold High K+ at Cs+-Loaded Terminals of Rat Hippocampal Neurons
Authors: Lu Fang-Min , Kuba Kenji
Published: May 2017

Transmitter release at Cs+-loaded autaptic terminals was selectively activated by the subthreshold concentration of external K+, and Ca2+ channel types and transmitter pools involved in synchronous and asynchronous exocytosis were studied. When a neuron was depolarized to +30 mV by applying a current through a pipette containing Cs+ for >30 s, a rapid external K+ jump to 3.75–10 mM, otherwise ineffective, produced an outward current (K10 response). K10 responses were initially graded (type-1) and then became a spike and plateau-shape with (type-2) or without a latency (type-3). On repolarization to –60 mV, a high K+ jump induced inward currents (called also K10 response) similar to those at +30 mV, whose shape changed from that of type-3, then type-2 and finally type-1 over 30 min. During a period favorable for inducing a type-3 response, a current similar to this response was generated by a voltage pulse (+ 80 or 90 mV, 20 or 30 ms) to the cell soma. Currents similar to K10 responses were rarely induced by a high K+ jump without a conditioning depolarization except for some cells, but consistently produced when 3 mM Cs+ and 50 μM 4-aminopyridine were externally applied for tens of minutes. Picrotoxin, 6-cyano-7-nitroquinoxaline-2,3-dione with 3-[(RS)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid or Cd2+ in, or Ca2+ removal from, a high-K+ solution blocked all the K10 responses, while a plateau remaining after a high K+ jump was not blocked by Ca2+ removal immediately after the K+ jump. Thus Cs+ loading and decreased K+ concentration in autaptic terminals by a conditioning depolarizing current selectively sensitize the terminals to a subthreshold high K+ jump for depolarization to activate synchronous or asynchronous transmitter release. Nicardipine (5–10 μM) blocked type-1 and -2 responses but not type-3 responses, while ω-conotoxin (10 μM) blocked all the types of K10 response in the presence of nicardipine. Increasing the interval of high K+ jumps biphasically increased the magnitude of K10 response, preferentially in the postjump fraction reflecting purely the asynchronous activation of exocytotic machinery, and decreased the reduction of miniature postsynaptic current frequency after a K10 response. These results suggest the roles of N(P/Q)-type Ca2+ channels in synchronous exocytosis at the terminals, L-type Ca2+ channels in initiating a Ca2+ action potential at the parent axon and both types in asynchronous exocytosis and also suggest the different releasable pools of transmitter for two modes of exocytosis in cultured hippocampal neurons.
Phase Relationships Between Segmentally Organized Oscillators in the Leech Heartbeat Pattern Generating Network
Authors: Masino Mark A. , Calabrese Ronald L.
Published: May 2017

Motor pattern generating networks that produce segmentally distributed motor outflow are often portrayed as a series of coupled segmental oscillators that produce a regular progression (constant phase differences) in their rhythmic activity. The leech heartbeat central pattern generator is paced by a core timing network, which consists of two coupled segmental oscillators in segmental ganglia 3 and 4. The segmental oscillators comprise paired mutually inhibitory oscillator interneurons and the processes of intersegmental coordinating interneurons. As a first step in understanding the coordination of segmental motor outflow by this pattern generator, we describe the functional synaptic interactions, and activity and phase relationships of the heart interneurons of the timing network, in isolated nerve cord preparations. In the timing network, most (∼75%) of the coordinating interneuron action potentials were generated at a primary spike initiation site located in ganglion 4 (G4). A secondary spike initiation site in ganglion 3 (G3) became active in the absence of activity at the primary site. Generally, the secondary site was characterized by a reluctance to burst and a lower spike frequency, when compared with the primary site. Oscillator interneurons in G3 inhibited spike activity at both initiation sites, whereas oscillator interneurons in G4 inhibited spike activity only at the primary initiation site. This asymmetry in the control of spike activity in the coordinating interneurons may account for the observation that the phase of the coordinating interneurons is more tightly linked to the G3 than G4 oscillator interneurons. The cycle period of the timing network and the phase difference between the ipsilateral G3 and G4 oscillator interneurons were regular within individual preparations, but varied among preparations. This variation in phase differences observed across preparations implies that modulated intrinsic membrane and synaptic properties, rather than the pattern of synaptic connections, are instrumental in determining phase within the timing network.
Period Differences Between Segmental Oscillators Produce Intersegmental Phase Differences in the Leech Heartbeat Timing Network
Authors: Masino Mark A. , Calabrese Ronald L.
Published: May 2017

Considerable experimental and theoretical effort has been exerted to understand how constant intersegmental phase relationships are produced between oscillators in segmentally organized pattern generating networks. The phase relationship between the segmental oscillators in the isolated timing network of the leech heartbeat central pattern generator is quite regular within individual preparations. However, it varies considerably among different preparations. Our goal is to determine how the phase relationships in this network are established. Here we assess whether inherent period differences, as suggested by the excitability-gradient hypothesis, play a role in establishing the phase relationships between the two coupled segmental oscillators of the leech heartbeat timing network. To do this we developed methods for reversibly uncoupling the segmental oscillators (sucrose knife) and pharmacological manipulation of the individual oscillators (split bath). Differences in inherent cycle periods between the third and fourth segmental oscillators (G3 and G4) were present in most (20 of 26) preparations. These period differences correlated with the phase differences observed between the segmental oscillators in the recoupled timing network, such that the oscillator with the faster cycle period, regardless of the segment in which it was located, led in phase in proportion to its period difference with the other oscillator. The phase differences between the original (coupled) and recoupled states of individual preparations were similar. Thus application and removal of the sucrose knife did not alter the period difference between the segmental oscillators in the timing network. Pharmacological manipulation of the uncoupled oscillators to alter the period difference between the oscillators led to similar correlated phase differences in the recoupled timing network. Across all experiments the uncoupled segmental oscillator with the faster cycle period established the cycle period of the timing network when recoupled. In conclusion, our findings indicate that an excitability-gradient plays a role in establishing the phase relationship between the segmental oscillators of the leech heartbeat central pattern generator since inherent period differences present between the oscillators are correlated to the phase relationships of the coupled/recoupled timing network.
Pharmacological Characterization of Glycine-Gated Chloride Currents Recorded in Rat Hippocampal Slices
Authors: Chattipakorn Siriporn C. , McMahon Lori L.
Published: May 2017

An inhibitory role for strychnine-sensitive glycine-gated chloride channels (GlyRs) in mature hippocampus has been overlooked, largely due to the misconception that GlyR expression ceases early during development and to few functional studies demonstrating their presence. As a result, little is known regarding the physiological and pharmacological properties of native GlyRs expressed by hippocampal neurons. In this study, we used pharmacological tools and whole cell patch-clamp recordings of CA1 pyramidal cells and interneurons in acutely prepared hippocampal slices from 3- to 4-wk old rats to characterize these understudied receptors. We show that glycine application to recorded pyramidal cells and interneurons elicited strychnine-sensitive chloride-mediated currents ( I gly) that did not completely desensitize in the continued presence of agonist but reached a steady state at 45–60% of the peak amplitude. Additionally, the inhibitory amino acid, taurine, which has been shown to activate GlyRs in other systems, activated GlyRs expressed by both pyramidal cells and interneurons, although with much less potency than glycine, having an EC50 10-fold higher. To examine the potential subunit composition of hippocampal GlyRs, we tested the effect of the GABAA receptor antagonist, picrotoxin, on I gly recorded from both cell types. At low micromolar concentrations of picrotoxin (≤100 μM), which selectively block α homomeric GlyRs, I gly was partially attenuated in both cell types, indicating that α homomeric receptors are expressed by pyramidal cells and interneurons. At picrotoxin concentrations ≤1 mM, ∼10–20% of the whole cell current remained, suggesting that αβ heteromeric GlyRs are also expressed because this subtype of GlyR is relatively resistant to picrotoxin antagonism. Finally, we examined whether hippocampal GlyRs are modulated by zinc. Consistent with previous reports in other preparations, zinc elicited a bidirectional modulation of GlyRs, with physiological zinc concentrations (1–100 μM) increasing whole cell currents and concentrations >100 μM depressing them. Furthermore, the same concentration of zinc that potentiates I gly suppressed currents mediated by the N-methyl-d-aspartate subtype of the glutamate receptor. Thus we provide a pharmacological characterization of native GlyRs expressed by both major neuron types in hippocampus and show that these receptors can be activated by taurine, an amino acid that is highly concentrated in hippocampus. Furthermore, our data suggest that at least two GlyR subtypes are present in hippocampus and that GlyR-mediated currents can be potentiated by zinc at concentrations that suppress glutamate-mediated excitability.
Position Sensitivity of Human Muscle Spindles: Single Afferent and Population Representations
Authors: Cordo Paul J. , Flores-Vieira Carmen , Verschueren Sabine M. P. , Inglis J. Timothy , Gurfinkel Victor
Published: May 2017

The representation of joint position at rest and during movement was investigated in 44 muscle spindle primary afferents originating from the extensor carpi radialis brevis (ECRb) and extensor digitorum (ED) of normal human subjects. Position sensitivity was estimated for each afferent, and 43 of 44 were position sensitive. In each trial, six sequential ramp-and-hold movements (2–6°, 2°/s, total 24°) flexed the relaxed wrist, beginning from the angle at which the afferent was just recruited. Joint position was represented by three specific features of afferent firing patterns: the steady-state firing rate during the 4-s hold period between ramps, the initial burst at the beginning of each ramp, and the ramp increase in firing rate later in the movement. The position sensitivity of the initial burst (1.27 ± 0.90 pps/°, mean ± SD) was several times higher than that of the hold period (0.40 ± 0.30 pps/°) and not different from that of the ramp increase in firing rate (1.36 ± 0.68 pps/°). The wrist position sensitivities of ECRb and ED afferents were equivalent, as were their recruitment angles and angular ranges of position sensitivity. Muscle spindle afferents, both individually and as a population, were shown to represent static joint position via the hold rate and the initial burst. Afferents were recruited over the entire 110° range of wrist positions investigated; however, the angular range over which each feature represented joint position was extremely limited (≈15°). The population response, based on the summed activity of the 43 afferents, was monotonically related to joint position, and it was strongly influenced by the afferent recruitment pattern, but less so by the position sensitivities of the individual afferents.
GABA Application to Hippocampal CA3 or CA1 Stratum Lacunosum-Moleculare Excites an Interneuron Network
Authors: Perkins Katherine L.
Published: May 2017

Whole cell voltage-clamp recording and focal application of the neurotransmitter γ-aminobutyric acid (GABA) were used to investigate the ability of exogenous GABA applied to different locations within the guinea pig hippocampal slice to trigger a giant GABA-mediated postsynaptic current (GPSC) in pyramidal cells. A GPSC reflects the synchronous release of GABA from a group of interneurons. Recordings were done in the presence of 4-aminopyridine (4-AP) and blockers of ionotropic glutamatergic synaptic transmission. Spontaneous GPSCs occurred rhythmically in pyramidal cells under these conditions. Brief focal pressure application of GABA (500 μM; 30–200 ms) to CA3 stratum lacunosum-moleculare (SLM) or to the border between CA3 s. radiatum (SR) and SLM triggered an “all-or-none” GPSC in CA3 and CA1 pyramidal cells that looked like the spontaneous GPSCs. During the refractory period following a spontaneous GPSC, application of GABA could not trigger a GPSC. Both spontaneous GPSCs and GPSCs triggered by exogenous GABA were blocked by suppressing synaptic transmission with high Mg2+/low Ca2+bath solution. On the other hand, focal application of GABA to CA3 s. oriens (SO) or to proximal SR did not trigger a GPSC in the CA3 pyramidal cell; instead it produced a graded response. Focal application of GABA to regions other than CA3 was also tested. Focal application of GABA to CA1 SLM always triggered a GPSC in the CA3 pyramidal cell. Focal application of GABA within the outer two-thirds of the dentate molecular layer often elicited a GPSC in the CA3 pyramidal cell. In contrast, focal application of GABA to CA1 SO, to CA1 SR, or to the hilus elicited no current response in the CA3 pyramidal cell. These data indicate that the GPSC recorded in pyramidal cells that was triggered by focal GABA application resulted from the synchronous synaptic release of GABA from activated interneurons rather than from the binding of exogenous GABA to receptors on the pyramidal cell. Furthermore, the “all-or-none” nature of the response to SLM GABA applications of different durations indicates that the exogenous GABA was exciting (directly or indirectly) some members of a network of interneurons, which in turn recruited the rest of the network, rather than individually activating each interneuron that contributed to the GPSC. Interestingly, the effective sites of GABA application—CA3 SLM, CA1 SLM, and the outer two-thirds of the dentate molecular layer—are also the sites which receive direct innervation from the entorhinal cortex in an intact animal.
What Triggers Catch-Up Saccades During Visual Tracking?
Authors: de Brouwer Sophie , Yuksel Demet , Blohm Gunnar , Missal Marcus , Lefèvre Philippe
Published: May 2017

When tracking moving visual stimuli, primates orient their visual axis by combining two kinds of eye movements, smooth pursuit and saccades, that have very different dynamics. Yet, the mechanisms that govern the decision to switch from one type of eye movement to the other are still poorly understood, even though they could bring a significant contribution to the understanding of how the CNS combines different kinds of control strategies to achieve a common motor and sensory goal. In this study, we investigated the oculomotor responses to a large range of different combinations of position error and velocity error during visual tracking of moving stimuli in humans. We found that the oculomotor system uses a prediction of the time at which the eye trajectory will cross the target, defined as the “eye crossing time” ( T XE). The eye crossing time, which depends on both position error and velocity error, is the criterion used to switch between smooth and saccadic pursuit, i.e., to trigger catch-up saccades. On average, for T XEbetween 40 and 180 ms, no saccade is triggered and target tracking remains purely smooth. Conversely, when T XEbecomes smaller than 40 ms or larger than 180 ms, a saccade is triggered after a short latency (around 125 ms).
Simulation of Different Firing Patterns in Paired Spider Mechanoreceptor Neurons: The Role of Na+Channel Inactivation
Authors: Torkkeli Päivi H. , French Andrew S.
Published: May 2017

The spider VS-3 slit-sense organ contains two types of primary mechanoreceptor neurons that are morphologically similar but have different electrical behavior. Type A neurons fire only one or two action potentials in response to a mechanical or electrical step of any amplitude above the threshold, whereas type B neurons fire prolonged bursts of action potentials in response to similar stimuli. Voltage-clamp studies have shown that two voltage-activated ion currents, a noninactivating potassium current and an inactivating sodium current, dominate the firing behavior. We simulated the electrical behavior of the two neuron types, using a simplified form of Hodgkin-Huxley model based on published voltage-clamp and current-clamp recordings. Changing only two parameters of sodium inactivation, the slope of the h∞curve and the time constant of recovery from inactivation, allowed a complete switch between the two firing patterns. Our simulations support previous evidence that sodium inactivation controls the firing properties of these neurons and indicate that two parameter changes are needed to achieve complete transformation between the two neuron types.
Age-Dependent Effects of Peripheral Inflammation on the Electrophysiological Properties of Neonatal Rat Dorsal Horn Neurons
Authors: Torsney Carole , Fitzgerald Maria
Published: May 2017

The aim of this study was to investigate the postnatal development of spinal cord neurophysiological mechanisms of inflammatory pain. The effect of hindpaw inflammation on the properties of neonatal spinal dorsal horn cells was investigated in urethane-anesthetized newborn rats using in vivo single-unit extracellular recordings. Responses to cutaneous mechanical and electrical A and C fiber stimulation were recorded at postnatal day (P) 3, 10, and 21 in pups that had received a unilateral intraplantar carageenan injection (1%, 1 μl/g body wt) 2–5 h earlier and compared with age-matched controls. At all three ages, carageenan inflammation increased A fiber evoked sensitization, spontaneous activity, and the suprathreshold response magnitude of dorsal horn cells. Receptive field size, which normally decreases with postnatal age, was unaffected by inflammation in P3 and P10 pups but significantly increased at P21 so that the size distribution closely resembled that in control P3 pups. Mechanical thresholds of individual dorsal horn neurons were not altered by carageenan inflammation at any age. The results show that some dorsal horn cell properties that are likely to underlie inflammatory hypersensitivity such as increased spontaneous activity and response magnitude are observed from the earliest postnatal age examined (P3). However inflammation induced expansion of mechanical receptive field size is not observed until at least the second postnatal week. These results have implications for the postnatal processing of inflammatory pain.
Dopamine Modulates Exocytosis Independent of Ca2+Entry in Melanotropic Cells
Authors: Mansvelder Huibert D. , Lodder Johannes C. , Sons Michèle S. , Kits Karel S.
Published: May 2017

Dopamine is a known inhibitor of pituitary melanotropic cells. It reduces Ca2+influx by hyperpolarizing the cell membrane and by modulating high- and low-voltage-activated (HVA and LVA) Ca2+channels. As a result, dopamine reduces the hormonal output of the cell. However, it is unknown how dopamine affects each of the four different HVA Ca2+channel types individually. Moreover, it is unknown whether dopamine interacts with exocytosis independent of Ca2+channels. Here we show that dopamine differentially modulates the HVA Ca2+channels and that it affects the stimulus-secretion coupling through a direct effect on the exocytotic machinery. Sustained L- and P-type Ba2+currents are reduced in amplitude and inactivating N- and Q-type currents acquire different activation and inactivation kinetics in the presence of dopamine. The Q-type current shows slow activation, which is a hallmark for direct G-protein modulation. We used membrane capacitance measurements to monitor exocytosis. Surprisingly, we find that the amount of exocytosis per step depolarization is not diminished by dopamine despite the reduction in Ca2+current. To test whether dopamine affects the release machinery downstream of Ca2+entry, we stimulated exocytosis by dialyzing cells with buffered high-Ca2+solutions. Dopamine increased the amount and the rate of exocytosis. In the first 90 s, the rate of secretion was increased two- to threefold, but it was normalized again at 180 s, suggesting that predominantly vesicles that fuse early in the exocytotic phase are modulated by dopamine. Thus while Ca2+channels are inhibited by dopamine, the exocytotic machinery downstream of Ca2+influx is sensitized. As a result, release is more effectively stimulated by Ca2+influx during dopamine inhibition.
Increase in Adenosine Sensitivity in the Nucleus Accumbens Following Chronic Morphine Treatment
Authors: Brundege James M. , Williams John T.
Published: May 2017

There is a growing body of evidence suggesting that the neuromodulator adenosine is involved in drug addiction and withdrawal and that adenosine signaling pathways may offer new targets for therapeutic treatments of addiction. Recent studies have suggested that chronic exposure to drugs of abuse may alter adenosine metabolism in the nucleus accumbens, a brain region critically involved in drug addiction and withdrawal. The present study examined the effects of chronic morphine treatment on the ability of adenosine to inhibit excitatory postsynaptic currents in nucleus accumbens medium spiny neurons. It was found that chronic morphine treatment via subcutaneous implantation of morphine pellets in rats for 1 wk did not alter the level of adenosine-mediated tonic inhibition of nucleus accumbens excitatory synapses. However, chronic morphine treatment did induce a leftward shift in the adenosine dose-response curve, indicating an increase in the sensitivity of synaptic currents to exogenously applied adenosine. This shift was not due to a change in adenosine receptors or their effectors, because chronic morphine treatment had no effect on the dose-response relationship of a nonmetabolized adenosine receptor agonist. When adenosine transport was blocked, the ability of chronic morphine to shift the adenosine dose-response curve was eliminated. These experiments suggest that the increase in the sensitivity of nucleus accumbens synapses to the inhibitory effects of adenosine may be due to a decrease in adenosine transport. The identification of these changes in the adenosine system after chronic drug exposure may help identify new therapeutic strategies aimed at easing withdrawal from opioids.
A Non-Excitatory Paradigm of Glutamate Toxicity
Authors: Shen Wen , Slaughter Malcolm M.
Published: May 2017

Retinal ganglion cells are driven by glutamatergic synapses, but they are also very susceptible to glutamate toxicity. Whereas the conventional excitotoxicity model of glutamate-induced cell death requires membrane depolarization, we have found that glutamate toxicity need not be linked with excitation. A large subset of ganglion cells possesses high-affinity kainate receptors that are calcium permeable. At 1–5 μM, kainate produced elevation of internal calcium but did not significantly depolarize ganglion cells. This low concentration of kainate caused ganglion cell death, which could be inhibited by specific kainate receptor antagonists. The toxic effect of kainate may be associated with calcium influx, because toxicity was reduced by polyamines that suppress calcium influx and by an inhibitor of calcium phosphatase. Thus activation of ionotropic glutamate receptors can produce neurotoxicity uncoupled from neuroexcitation.
Divergent Movement of Adjacent Whiskers
Authors: Sachdev Robert N. S. , Sato Takashi , Ebner Ford F.
Published: May 2017

The current view of whisker movement is that ∼25 whiskers on each side of the face move in synchrony. To determine whether whiskers are constrained to move together, we trained rats to use two whiskers on the same side of the face in simple behavioral tasks and videotaped the whiskers during the task. Here we report that the movement of adjacent whiskers is usually synchronous but can diverge: 1) the distance between whiskers can vary dramatically during movement; 2) one whisker can move while the second one remains stationary; 3) two whiskers can simultaneously move in opposite directions; and 4) one whisker can be maintained in contact with an object while the other is retracted and protracted. The frequency of whisker movement during the task falls within the previously reported range for rats whisking freely into air or performing roughness discrimination with their whiskers. Our data also suggest that whisker movement can be divided into three distinct phases: protraction, retraction, and a measurable delay between these movements. We conclude that, although whiskers often move in concert, adjacent caudal whiskers can be moved independently of each other.
Network Interactions Mediated by New Excitatory Connections Between CA1 Pyramidal Cells in Rats With Kainate-Induced Epilepsy
Authors: Smith Bret N. , Dudek F. Edward
Published: May 2017

Axon sprouting and synaptic reorganization in the hippocampus are associated with the development of seizures in temporal lobe epilepsy. Synaptic interactions among CA1 pyramidal cells were examined in fragments of hippocampal slices containing only the CA1 area from saline- and kainate-treated rats. Glutamate microapplication to the pyramidal cell layer increased excitatory postsynaptic current (EPSC) frequency, but only in rats with kainate-induced epilepsy. In bicuculline, action potentials evoked in single pyramidal cells increased the frequency of network bursts only in slices from rats with kainate-induced epilepsy. These data further support the hypothesis that excitatory connections between CA1 pyramidal cells increase after kainate-induced status epilepticus.
Regulation of the Timing and Pattern of Action Potential Generation in Rat Subthalamic Neurons In Vitro by GABA-A IPSPs
Authors: Bevan M. D. , Magill P. J. , Hallworth N. E. , Bolam J. P. , Wilson C. J.
Published: May 2017

The regulation of activity in the subthalamic nucleus (STN) by GABAergic inhibition from the reciprocally connected globus pallidus (GP) plays an important role in normal movement and disorders of movement. To determine the precise manner in which GABAergic synaptic input, acting at A-type receptors, influences the firing of STN neurons, we recorded the response of STN neurons to GABA-A inhibitory postsynaptic potentials (IPSPs) that were evoked by supramaximal electrical stimulation of the internal capsule using the perforated-patch technique in slices at 37°C. The mean equilibrium potential of the GABA-A IPSP (EGABA-A IPSP) was −79.4 ± 7.0 mV. Single IPSPs disrupted the spontaneous oscillation that underlies rhythmic single-spike firing in STN neurons. As the magnitude of IPSPs increased, the effectiveness of prolonging the interspike interval was related more strongly to the phase of the oscillation at which the IPSP was evoked. Thus the largest IPSPs tended to reset the oscillatory cycle, whereas the smallest IPSPs tended to produce relatively phase-independent delays in firing. Multiple IPSPs were evoked at various frequencies and over different periods and their impact was studied on STN neurons held at different levels of polarization. Multiple IPSPs reduced and/or prevented action potential generation and/or produced sufficient hyperpolarization to activate a rebound depolarization, which generated a single spike or restored rhythmic spiking and/or generated a burst of activity. The pattern of IPSPs and the level of polarization of STN neurons were critical in determining the nature of the response. The duration of bursts varied from 20 ms to several hundred milliseconds, depending on the intrinsic rebound properties of the postsynaptic neuron. These data demonstrate that inhibitory input from the GP can produce a range of firing patterns in STN neurons, depending on the number and frequencies of IPSPs and the membrane properties and voltage of the postsynaptic neuron.
Selective Suppression of Late Laryngeal Adductor Responses by N-Methyl-d-Aspartate Receptor Blockade in the Cat
Authors: Ambalavanar Ranjinidevi , Purcell Laura , Miranda Marcia , Evans Frank , Ludlow Christy L.
Published: May 2017

Laryngeal adductor responses to afferent stimulation play a key role in airway protection. Although vital for protection during cough and swallow, these responses also must be centrally controlled to prevent airway obstruction by laryngospasm during prolonged stimulation. Our purpose was to determine the role of N-methyl-d-aspartate (NMDA) receptors in modulating early R1 responses (at 9 ms) and/or later more prolonged R2 responses (at 36 ms) during electrical stimulation of the laryngeal afferent fibers contained in the internal branch of the superior laryngeal nerve in the cat. The percent occurrence, amplitude, and conditioning of muscle responses to single superior laryngeal nerve (SLN) stimuli presented in pairs at interstimulus intervals of 250 ms were measured in three experiments: 1) animals that had ketamine as anesthetic premedication were compared with those who did not, when both were maintained under alpha-chloralose anesthesia. 2) The effects of administering ketamine in one group of animals were compared with increasing the depth of alpha-chloralose anesthesia without NMDA receptor blockade in another group of animals. 3) The effects of dextromethorphan (without anesthetic effects) were examined in another group of animals. In the first experiment, the occurrence of R2 responses were reduced from 95% in animals without ketamine premedication to 25% in animals with ketamine premedication ( P = 0.015). No differences occurred in the occurrence, amplitude, latency, or conditioning effects on R1 responses between these groups. In the second experiment, the occurrence of R2 responses was reduced from 96 to 79% after an increase in the depth of anesthesia with alpha-chloralose in contrast with reductions in R2 occurrence from 98 to 19% following the administration of ketamine to induce NMDA receptor blockade along with increased anesthesia ( P = 0.025). In the third experiment, R2 occurrence was reduced from 89 to 27% ( P = 0.017) with administration of dextromethorphan while R1 response occurrence and amplitude did not change. In each of these experiments, NMDA receptor blockade did not have significant effects on cardiac or respiratory rates in any of the animals. The results demonstrate that NMDA receptors play an essential role in long latency R2 laryngeal responses to laryngeal afferent stimulation. On the other hand, early R1 laryngeal adductor responses are likely to involve non-NMDA receptor activation.
Sodium Channel Blocking Actions of the κ-Opioid Receptor Agonist U50,488 Contribute to Its Visceral Antinociceptive Effects
Authors: Su X. , Joshi S. K. , Kardos S. , Gebhart G. F.
Published: May 2017

The goal of the present study was to determine whether the κ-opioid receptor agonist (ORA) U50,488 attenuates behavioral and primary afferent nerve responses to noxious colorectal distension (CRD) by sodium channel blockade. We tested the analgesic κ-ORA (±)- trans U50,488, its enantiomers (−)- trans(1S,2S)-U50,488 and non κ-ORA (+)- trans (1R,2R)-U50,488, and/or its diastereomer (−)- cis (1S,2R)-U50,488 for their ability to attenuate visceromotor and pelvic nerve afferent fiber responses to noxious CRD in vivo and voltage-activated sodium current in colon sensory neurons in vitro. In unanesthetized rats, subcutaneous administration of U50,488, (1S,2S)-U50,488, and (1R,2R)-U50,488 attenuated the behavioral visceromotor response to noxious CRD; the rank order of potency was: (1S,2S)-U50,488 > U50,488 ≫ (1R,2R)-U50,488. U50,488 and its stereoisomers also inhibited responses of decentralized pelvic nerve afferent fibers to noxious CRD in a dose-dependent manner. Cumulative doses of 16 mg/kg of (1S,2S)-U50,488, (1S,2R)-U50,488, and (1R,2R)-U50,488 reduced responses to a mean 29, 30, and 47% of control, respectively. The mean inhibitory doses of these drugs were not different (range: 6.6–10.8 mg/kg). Sodium channel blockers mexiletine and carbamazepine mimicked the effect of U50,488. In contrast, the κ-ORAs dynorphin (1–13) and ICI 204,488 were ineffective in attenuating pelvic nerve activity. Perfusion of (1S,2S)-U50,488, (1S,2R)-U50,488, or (1R,2R)-U50,488 on colon sensory neurons in vitro decreased voltage-activated sodium currents. This inhibition by U50,488 and its stereoisomers was not opioid receptor–mediated because it could not be reversed by the opioid receptor antagonist naloxone and was also not a G protein–mediated effect. The results reported here suggest that the visceral antinociceptive effects of U50,488 and its stereoisomers are contributed to by their peripheral sodium channel blocking actions.
Calcium Transients in the Garter Snake Vomeronasal Organ
Authors: Cinelli Angel R. , Wang Dalton , Chen Ping , Liu Weimin , Halpern Mimi
Published: May 2017

The signaling cascade involved in chemosensory transduction in the VN organ is incompletely understood. In snakes, the response to nonvolatile prey chemicals is mediated by the vomeronasal (VN) system. Using optical techniques and fluorescent Ca2+indicators, we found that prey-derived chemoattractants produce initially a transient cytosolic accumulation of [Ca2+]iin the dendritic regions of VN neurons via two pathways: Ca2+release from IP3-sensitive intracellular stores and, to a lesser extent, Ca2+influx through the plasma membrane. Both components seem to be dependent on IP3production. Chemoattractants evoke a short-latency Ca2+elevation even in the absence of extracellular Ca2+, suggesting that in snake VN neurons, Ca2+release from intracellular stores is independent of a preceding Ca2+influx, and both components are activated in parallel during early stages of chemosensory transduction. Once the response develops in apical dendritic segments, other mechanisms can also contribute to the amplification and modulation of these chemoattractant-mediated cytosolic Ca2+transients. In regions close to the cell bodies of the VN neurons, the activation of voltage-sensitive Ca2+channels and a Ca2+-induced Ca2+release from intracellular ryanodine-sensitive stores secondarily boost initial cytosolic Ca2+elevations increasing their magnitude and durations. Return of intracellular Ca2+to prestimulation levels appears to involve a Ca2+extrusion mediated by a Na+/Ca2+exchanger mechanism that probably plays an important role in limiting the magnitude and duration of the stimulation-induced Ca2+transients.
Fornix Transection Impairs Conditional Visuomotor Learning in Tasks Involving Nonspatially Differentiated Responses
Authors: Brasted Peter J. , Bussey Timothy J. , Murray Elisabeth A. , Wise Steven P.
Published: May 2017

Rhesus monkeys learned a series of conditional visuomotor associations involving two-dimensional “objects” that instructed one of three responses: tapping a touch screen, steady contact with the screen for a brief period, or steady contact for a longer period. Relative to controls, fornix-transected monkeys were impaired in the acquisition of new associations and in the retention of preoperatively learned ones. These findings challenge the view that the hippocampal system participates in associative learning only when spatial information is relevant to either the stimulus or the response.
Activation of Embryonic Red and White Muscle Fibers During Fictive Swimming in the Developing Zebrafish
Authors: Buss Robert R. , Drapeau Pierre
Published: May 2017

Sub-threshold, motoneuron-evoked synaptic activity was observed in zebrafish embryonic red (ER) and white (EW) muscle fibers paralyzed with a dose of d-tubocurarine insufficient to abolish synaptic activity to determine whether muscle activation was coordinated to produce the undulating body movements required for locomotion. Paired whole-cell recordings revealed a synaptic drive that alternated between ipsilateral and contralateral myotomes and exhibited a rostral-caudal delay in timing appropriate for swimming. Both ER and EW muscle were activated during fictive swimming. However, at the fastest fictive swimming rates, ER fibers were de-recruited, whereas they could be active in isolation of EW fibers at the slowest fictive swimming rates. Prior to hatching, fictive swimming was preceded by a lower frequency, more robust and rhythmic synaptic drive resembling the “coiling” behavior of fish embryos. The motor activity observed in paralyzed zebrafish closely resembled the swimming and coiling behaviors observed in these developing fishes. At the early developmental stages examined in this study, myotomal muscle recruitment and coordination were similar to that observed in adult fishes during swimming. Our results indicate that the patterned activation of myotomal muscle is set from the onset of development.
A Non-α7 Nicotinic Acetylcholine Receptor Modulates Excitatory Input to Hippocampal CA1 Interneurons
Authors: Alkondon Manickavasagom , Albuquerque Edson X.
Published: May 2017

The nicotinic acetylcholine receptor (nAChR), particularly the α7 subtype, has received profound attention for its role in modifying excitatory postsynaptic currents (EPSCs) in hippocampal pyramidal neurons as well as in neurons from other brain regions. Here, we tested the possibility that an nAChR could affect EPSCs in the interneurons of rat hippocampal slices. Using whole-cell patch-clamp technique on CA1 stratum radiatum interneurons and U-tube application of agents, we show that nicotinic agonists enhance EPSC frequency in interneurons. Among the agents tested, cytisine and mecamylamine were the most effective agonist and antagonist, respectively, suggesting a role for α3β4-containing nAChRs in the modulation of interneuron EPSCs. Ligands selective for the α7 nAChR had very little or no effect on interneuron EPSCs. Low concentrations of nicotine also enhanced EPSC frequency, implicating the involvement of non-α7 nAChRs in controlling interneuron excitability in smokers. We conclude that nAChR-dependent EPSC modulation in the hippocampus is both subtype- and neuron-specific and that a non-α7 nAChR, presumably α3β4, controls glutamate transmission to CA1 interneurons.
Inactivation of Semicircular Canals Causes Adaptive Increases in Otolith-Driven Tilt Responses
Authors: Angelaki Dora E. , Newlands Shawn D. , Dickman J. David
Published: May 2017

Growing experimental and theoretical evidence suggests a functional synergy in the processing of otolith and semicircular canal signals for the generation of the vestibulo-ocular reflexes (VORs). In this study we have further tested this functional interaction by quantifying the adaptive changes in the otolith-ocular system during both rotational and translational movements after surgical inactivation of the semicircular canals. For 0.1–0.5 Hz (stimuli for which there is no recovery of responses from the plugged canals), pitch and roll VOR gains recovered during earth-horizontal (but not earth-vertical) axis rotations. Corresponding changes were also observed in eye movements elicited by translational motion (0.1–5 Hz). Specifically, torsional eye movements increased during lateral motion, whereas vertical eye movements increased during fore-aft motion. The findings indicate that otolith signals can be adapted according to a compromised strategy that leads to improved gaze stabilization during motion. Because canal-plugged animals permanently lose the ability to discriminate gravitoinertial accelerations, adapted animals can use the presence of gravity through otolith-driven tilt responses to assist gaze stabilization during earth-horizontal axis rotations.
Conjugate and Vergence Oscillations During Saccades and Gaze Shifts: Implications for Integrated Control of Binocular Movement
Authors: Sylvestre Pierre A. , Galiana Henrietta L. , Cullen Kathleen E.
Published: May 2017

Saccades made between targets at optical infinity require both eyes to rotate by the same angle. Nevertheless, these saccades are consistently accompanied by transient vergence eye movements. Here we have investigated whether the dynamics of these vergence movements depend on the trajectory of the coincident conjugate movement, and whether moving the head during eye-head gaze shifts modifies vergence dynamics. In agreement with previous reports, saccades with more symmetric (i.e., “bell-shaped”) conjugate velocity profiles were accompanied by stereotyped biphasic vergence transients (i.e., a divergence phase immediately followed by a convergence phase). However, we found that saccades with more asymmetric, oscillatory-like dynamics (characterized by a typical conjugate reacceleration of the eyes following the initial peak velocity) were systematically accompanied by more complex vergence movements that also exhibited oscillatory-like dynamics. These findings could be extended to conditions where the head was free to move: comparable conjugate and vergence oscillations were observed during head-restrained saccades and combined eye-head gaze shifts. The duration of the vergence oscillation increased with gaze shift amplitude, such that as many as four vergence phases (divergence-convergence-divergence-convergence) were recorded during 55° gaze shifts (≈240 ms). To quantify these observations, we first determined whether conjugate and vergence peak velocities were systematically correlated. Conjugate peak velocity was linearly related to the peak velocity of the initial divergence phase for saccades and gaze shifts of all amplitudes, regardless of their dynamics. However, for more asymmetric saccades and gaze shifts, the subsequent convergence and divergence peak velocities were not correlated with either the initial peak conjugate velocity or the peak velocity of the conjugate reacceleration. Next, we determined that the duration of the different conjugate and vergence oscillation phases remained relatively constant across all saccades and gaze shifts, and that the conjugate and vergence profiles oscillated together at approximately 7.5–10 Hz. Using computer simulations, we show that a classic feed-forward model is unable to reproduce vergence oscillations based solely on peripheral mechanisms. Furthermore, we demonstrate that small modifications to the gain and delay of a simple feedback model for saccade generation can generate conjugate oscillations, and propose that such changes reflect the influence of lowered alertness on the tecto-reticular pathways. We conclude that peripheral mechanisms can only account for the initial divergence that accompanies all saccades, and that the conjugate and vergence oscillations observed during asymmetric movements arise centrally from an integrative binocular controller.
TAM Receptor Tyrosine Kinases in Cancer Drug Resistance
Authors: Vouri Mikaella , Hafizi Sassan
Published: May 2017

Receptor tyrosine kinases (RTK) are major regulators of key biological processes, including cell growth, survival, and differentiation, and were established early on as proto-oncogenes, with aberrant expression linked to tumor progression in many cancers. Therefore, RTKs have emerged as major targets for selective therapy with small-molecule inhibitors. However, despite improvements in survival rates, it is now apparent that the targeting of RTKs with selective inhibitors is only transiently effective, as the majority of patients eventually become resistant to therapy. As chemoresistance is the leading cause of cancer spread, progression, and mortality, there is an increasing need for understanding the mechanisms by which cancer cells can evade therapy-induced cell death. The TAM (Tyro3, Axl, Mer) subfamily of RTKs in particular feature in a variety of cancer types that have developed resistance to a broad range of therapeutic agents, including both targeted as well as conventional chemotherapeutics. This article reviews the roles of TAMs as tumor drivers and as mediators of chemoresistance, and the potential effectiveness of targeting them as part of therapeutic strategies to delay or combat resistance. Cancer Res; 77(11); 2775–8. ©2017 AACR.