Differences in multiple forms of short-term plasticity between excitatory and inhibitory hippocampal neurons in culture

Synaptic transmission is highly dynamic, especially during periods of repetitive activity. This short-term synaptic plasticity, elicited by either pairs or short trains of action potentials at moderate frequencies (1-10 Hz), may give rise to either depression or facilitation of synaptic transmission. We analyzed these processes in isolated, synaptically coupled pairs of inhibitory or excitatory neurons grown in low-density cultures of hippocampal neurons. Most inhibitory and excitatory synapses in these cultures displayed paired pulse depression, although the responses of excitatory synapses were more variable and occasionally facilitation was seen. With tetanic stimuli, inhibitory synapses showed depression, but excitatory synapses showed a much richer repertoire of behaviors, including depression and facilitation. While many inhibitory synapses showed posttetanic depression following short trains of action potentials, excitatory synapses instead showed posttetanic facilitation. This facilitation is accompanied by an increase in paired pulse ratio, suggesting that it is the result of presynaptic mechanisms. Finally, excitatory synapses often displayed paired pulse and tetanic facilitation of asynchronous release, a process not seen in inhibitory synapses in these cultures. These similarities and differences in short-term plasticity exhibited by inhibitory and excitatory cells are likely to be critical for information processing and the control of neuronal excitability, under both normal and pathological conditions, such as epilepsy.     

Dynamic synapse: A new concept of neural representation and computation

Presynaptic mechanisms influencing the probability of neurotransmitter release from an axon terminal, such as facilitation, augmentation, and presynaptic feedback inhibition, are fundamental features of biological neurons and are cardinal physiological properties of synaptic connections in the hippocampus. The consequence of these presynaptic mechanisms is that the probability of release becomes a function of the temporal pattern of action potential occurrence, and hence, the strength of a given synapse varies upon the arrival of each action potential invading the terminal region. From the perspective of neural information processing, the capability of dynamically tuning the synaptic strength as a function of the level of neuronal activation gives rise to a significant representational and processing power of temporal spike patterns at the synaptic level. Furthermore, there is an exponential growth in such computational power when the specific dynamics of presynaptic mechanisms varies quantitatively across axon terminals of a single neuron, a recently established characteristic of hippocampal synapses. During learning, alterations in the presynaptic mechanisms lead to different pattern transformation functions, whereas changes in the postsynaptic mechanisms determine how the synaptic signals are to be combined. We demonstrate the computational capability of dynamic synapses by performing speech recognition from unprocessed, noisy raw waveforms of words spoken by multiple speakers with a simple neural network consisting of a small number of neurons connected with synapses incorporating dynamically determined probability of release. The dynamics included in the model are consistent with available experimental data on hippocampal neurons in that parameter values were chosen so as to be consistent with time constants of facilitative and inhibitory processes governing the dynamics of hippocampal synaptic transmission studied using nonlinear systems analytic procedures. © 1997 Wiley-Liss, Inc.     

Frequency-dependent synaptic potentiation, depression and spike timing induced by Hebbian pairing in cortical pyramidal neurons.

Experiments by Markram and Tsodyks (Nature, 382 (1996) 807-810) have suggested that Hebbian pairing in cortical pyramidal neurons potentiates or depresses the transmission of a subsequent pre-synaptic spike train at steady-state depending on whether the spike train is of low frequency or high frequency, respectively. The frequency above which pairing induced a significant decrease in steady-state synaptic efficacy was as low as about 20 Hz and this value depends on such synaptic properties as probability of release and time constant of recovery from short-term synaptic depression. These characteristics of cortical synapses have not yet been fully explained by neural models, notably the decreased steady-state synaptic efficacy at high pre-synaptic firing rates. This article suggests that this decrease in synaptic efficacy in cortical synapses was not observed at steady-state, but rather during a transition period preceding it whose duration is frequency-dependent. It is shown that the time taken to reach steady-state may be frequency-dependent, and may take considerably longer to occur at high than low frequencies. As a result, the pairing-induced decrease in synaptic efficacy at high pre-synaptic firing rates helps to localize the firing of the post-synaptic neuron to a short time interval following the onset of high-frequency pre-synaptic spike trains. This effect may "speed up the time scale" in response to high-frequency bursts of spikes, and may contribute to rapid synchronization of spike firing across cortical cells that are bound together by associatively learned connections.     

Chronic BDNF deficiency permanently modifies excitatory synapses in the piriform cortex

Brain-derived neurotrophic factor (BDNF), aside from its classic neurotrophic role in development and survival of neurons, has been shown to be involved in modification and plasticity of central synapses. In mice with BDNF gene deletion (BDNF+/-), deficits in synaptic transmission are often observed but are reversed readily by administration of BDNF, suggesting its acute effect. In support, blockade of BDNF signaling in wild-type hippocampal slices by TrkB-IgG closely reproduces synaptic alterations observed in BDNF+/- mice. We demonstrate that in BDNF+/- mice, lateral olfactory tract (LOT) synapses exhibit decreased release probability of glutamate, suggested by increased paired-pulse facilitation (PPF) of field excitatory postsynaptic potentials (fEPSPs), as well as by slower blocking rate of N-methyl-D-aspartate (NMDA) receptor-mediated excitatory postsynaptic currents (EPSCs) by MK-801 in the pyramidal neurons of the piriform cortex. The changes in PPF were not mimicked in wild-type mice by acute blockade of BDNF signaling by TkrB-IgG. These data imply that BDNF deficit during development might lead to chronic changes of excitatory transmission in LOT synapses. Modification of the LOT synapses in BDNF+/- mice was associated with altered inhibitory drive onto the mitral cells from the granule and glomerular neurons, which in turn exhibited decreased renewal rate compared to that in wild-type mice. Taken together, these data suggest that BDNF deficiency can have both acute and more permanent effects on synaptic function, particularly when BDNF signaling is compromised during the early stages of brain development. In the latter case, altered synaptic properties in BDNF+/- mice could be secondary to other complex changes in the brain, e.g., cell survival/proliferation.     

Differential regulation of spontaneous and evoked inhibitory synaptic transmission in somatosensory cortex by retinoic acid

Retinoic acid (RA), a developmental morphogen, has emerged in recent studies as a novel synaptic signaling molecule that acts in mature hippocampal neurons to modulate excitatory and inhibitory synaptic transmission in the context of homeostatic synaptic plasticity. However, it is unclear whether RA is capable of modulating neural circuits outside of the hippocampus, and if so, whether the mode of RA's action at synapses is similar to that within the hippocampal network. Here we explore for the first time RA's synaptic function outside the hippocampus and uncover a novel function of all‐trans retinoic acid at inhibitory synapses. Acute RA treatment increases spontaneous inhibitory synaptic transmission in L2/3 pyramidal neurons of the somatosensory cortex, and this effect requires expression of RA's receptor RARα both pre‐ and post‐synaptically. Intriguingly, RA does not seem to affect evoked inhibitory transmission assayed with either extracellular stimulation or direct activation of action potentials in presynaptic interneurons at connected pairs of interneurons and pyramidal neurons. Taken together, these results suggest that RA's action at synapses is not monotonous, but is diverse depending on the type of synaptic connection (excitatory versus inhibitory) and circuit (hippocampal versus cortical). Thus, synaptic signaling of RA may mediate multi‐faceted regulation of synaptic plasticity. In addition to its classic roles in brain development, retinoic acid (RA) has recently been shown to regulate excitatory and inhibitory transmission in the adult brain. Here, the authors show that in layer 2/3 (L2/3) of the somatosensory cortex (S1), acute RA induces increases in spontaneous but not action‐potential evoked transmission, and that this requires retinoic acid receptor (RARα) both in presynaptic PV‐positive interneurons and postsynaptic pyramidal (PN) neurons.     

Release probability is regulated by the size of the readily releasable vesicle pool at excitatory synapses in hippocampus

Synapses in the central nervous system can be very unreliable: stimulation of an individual synapse by an action potential often does not lead to release of neurotransmitter. The probability of transmitter release is not always the same, however, which enables the average strength of synaptic transmission to be regulated by modulation of release probability. Release probability is believed to be determined by the number of fusion competent vesicles (the readily releasable vesicle pool) and the release probability per vesicle. Studies from single synapses have shown that release probability correlates with the size of the readily releasable pool of vesicles across the population of excitatory CA3-CA1 synapses, both in hippocampal slices and in cultured cells. Here I present evidence that the same relationship exists between release probability and the size of the readily releasable vesicle pool within individual synapses, further suggesting that the size of the readily releasable pool helps determine release probability. In addition, using a simple model, I examine how both the number of readily releasable vesicles and the average release probability per vesicle change during trains of high frequency stimulation, and present evidence for non-uniformity of the release probability among vesicles.     

Coordination of size and number of excitatory and inhibitory synapses results in a balanced structural plasticity along mature hippocampal CA1 dendrites during LTP

Enlargement of dendritic spines and synapses correlates with enhanced synaptic strength during long‐term potentiation (LTP), especially in immature hippocampal neurons. Less clear is the nature of this structural synaptic plasticity on mature hippocampal neurons, and nothing is known about the structural plasticity of inhibitory synapses during LTP. Here the timing and extent of structural synaptic plasticity and changes in local protein synthesis evidenced by polyribosomes were systematically evaluated at both excitatory and inhibitory synapses on CA1 dendrites from mature rats following induction of LTP with theta‐burst stimulation (TBS). Recent work suggests dendritic segments can act as functional units of plasticity. To test whether structural synaptic plasticity is similarly coordinated, we reconstructed from serial section transmission electron microscopy all of the spines and synapses along representative dendritic segments receiving control stimulation or TBS‐LTP. At 5 min after TBS, polyribosomes were elevated in large spines suggesting an initial burst of local protein synthesis, and by 2 h only those spines with further enlarged synapses contained polyribosomes. Rapid induction of synaptogenesis was evidenced by an elevation in asymmetric shaft synapses and stubby spines at 5 min and more nonsynaptic filopodia at 30 min. By 2 h, the smallest synaptic spines were markedly reduced in number. This synapse loss was perfectly counterbalanced by enlargement of the remaining excitatory synapses such that the summed synaptic surface area per length of dendritic segment was constant across time and conditions. Remarkably, the inhibitory synapses showed a parallel synaptic plasticity, also demonstrating a decrease in number perfectly counterbalanced by an increase in synaptic surface area. Thus, TBS‐LTP triggered spinogenesis followed by loss of small excitatory and inhibitory synapses and a subsequent enlargement of the remaining synapses by 2 h. These data suggest that dendritic segments coordinate structural plasticity across multiple synapses and maintain a homeostatic balance of excitatory and inhibitory inputs through local protein‐synthesis and selective capture or redistribution of dendritic resources. ©2010 Wiley‐Liss, Inc.     

Long-term maintenance of mature hippocampal slices in vitro

Cultures of primary neurons or thin brain slices are typically prepared from immature animals. We introduce a method to prepare hippocampal slice cultures from mature rats aged 20-30 days. Mature slice cultures retain hippocampal cytoarchitecture and synaptic connections up to 3 months in vitro. Spontaneous epileptiform activity is rarely observed suggesting long-term retention of normal neuronal excitability and of excitatory and inhibitory synaptic networks. Picrotoxin, a GABAergic Cl(-) channel antagonist, induced characteristic interictal-like bursts that originated in the CA3 region, but not in the CA1 region. These data suggest that mature slice cultures displayed long-term retention of GABAergic inhibitory synapses that effectively suppressed synchronized burst activity via recurrent excitatory synapses of CA3 pyramidal cells. Mature slice cultures lack the reactive synaptogenesis, spontaneous epileptiform activity, and short life span that limit the use of slice cultures isolated from immature rats. Mature slice cultures are anticipated to be a useful addition for the in vitro study of normal and pathological hippocampal function.     

Interactions between behaviorally relevant rhythms and synaptic plasticity alter coding in the piriform cortex

Understanding how neural and behavioral timescales interact to influence cortical activity and stimulus coding is an important issue in sensory neuroscience. In air-breathing animals, voluntary changes in respiratory frequency alter the temporal patterning olfactory input. In the olfactory bulb, these behavioral timescales are reflected in the temporal properties of mitral/tufted (M/T) cell spike trains. As the odor information contained in these spike trains is relayed from the bulb to the cortex, interactions between presynaptic spike timing and short-term synaptic plasticity dictate how stimulus features are represented in cortical spike trains. Here, we demonstrate how the timescales associated with respiratory frequency, spike timing, and short-term synaptic plasticity interact to shape cortical responses. Specifically, we quantified the timescales of short-term synaptic facilitation and depression at excitatory synapses between bulbar M/T cells and cortical neurons in slices of mouse olfactory cortex. We then used these results to generate simulated M/T population synaptic currents that were injected into real cortical neurons. M/T population inputs were modulated at frequencies consistent with passive respiration or active sniffing. We show how the differential recruitment of short-term plasticity at breathing versus sniffing frequencies alters cortical spike responses. For inputs at sniffing frequencies, cortical neurons linearly encoded increases in presynaptic firing rates with increased phase-locked, firing rates. In contrast, at passive breathing frequencies, cortical responses saturated with changes in presynaptic rate. Our results suggest that changes in respiratory behavior can gate the transfer of stimulus information between the olfactory bulb and cortex.     

The role of the adenyl cyclase system in cholinergic modulation of synaptic transmission in the hippocampus

The adenylate cyclase system has been studied from the standpoint of its significance in cholinergic modulation of the synaptic transmission in the CA1 field of the rat hippocampal slices. Microionphoretic application of ACh as well as addition of either carbachol or tolbutamide (an inhibitor of cAMP-dependent protein kinase) blocked the transmission in synapses formed by the Schaffer collaterals and commissural fibres with dendrites of carbacholine both the number of releasing quanta of the neurotransmitter and the probability of their release decreased. Atropine eliminated the inhibitory effect of carbacholine on synaptic transmission. Dibutyryl cAMP and forskolin increased the amplitude of synaptic potentials and completely or partially prevented the inhibitory effect of cholinomimetics on synaptic potentials. The results obtained revealed opposite effects of cholinomimetics and activators of the adenylate cyclase system on neurotransmission in synapses formed by the Schaffer collaterals/commissural fibres and dendrites of pyramidal neurons of the hippocampal CA1 field.     

Activity-dependent maturation of excitatory synaptic connections in solitary neuron cultures of mouse neocortex

Activity plays important roles in the formation and maturation of synaptic connections. We examined these roles using solitary neocortical excitatory neurons, receiving only self-generated synaptic inputs, cultured in a microisland with and without spontaneous spike activity. The amplitude of excitatory postsynaptic currents (EPSCs), evoked by applying brief depolarizing voltage pulses to the cell soma, continued to increase from 7 to 14 days in culture. Short-term depression of EPSCs in response to paired-pulse or 10-train-pulse stimulation decreased with time in culture. These developmental changes were prevented when neurons were cultured in a solution containing tetrodotoxin (TTX). The number of functional synapses estimated by recycled synaptic vesicles with FM4-64 was significantly smaller in TTX-treated than control neurons. However, the miniature EPSC amplitude remained unchanged during development, irrespective of activity. Transmitter release probability, assessed by use-dependent blockade of N-methyl-D-aspartate receptor-mediated EPSCs with MK-801, was higher in TTX-treated than control neurons. Therefore, the activity-dependent increase in EPSC amplitude was mainly ascribed to the increase in synapse number, while activity-dependent alleviation of short-term depression was mostly ascribed to the decrease in release probability. The effect of activity blockade on short-term depression, but not EPSC amplitude, was reversed after 4 days of TTX removal, indicating that synapse number and release probability are controlled by activity in very different ways. These results demonstrate that activity regulates the conversion of immature synapses transmitting low-frequency input signals preferentially to mature synapses transmitting both low- and high-frequency signals effectively, which may be necessary for information processing in mature cortex.     

Temporal and spatial localization of nectin-1 and l-afadin during synaptogenesis in hippocampal neurons

Nectins are cell adhesion molecules that, together with the intracellular binding partner afadin, mediate adhesion and signaling at a variety of intercellular junctions. In this work we studied the distribution of nectin-1 and afadin during hippocampal synapse formation using cultured primary hippocampal neurons. Nectin-1 and afadin cluster at developing synapses between hippocampal neurons. These nectin-afadin clusters uniformly colocalize with N-cadherin-catenin pairs, suggesting that formation of developing synapses involves participation of both bimolecular systems. Nectin-1 is initially expressed at excitatory and inhibitory synapses but is progressively lost at inhibitory synapses during their maturation. Treatment of neurons with actin depolymerizing agents disrupts the synaptically localized nectin-1 and afadin cluster at an early stage and elicits nectin-1 ectodomain shedding. These data indicate that the synaptic localization of nectin-1 and l-afadin are F-actin-dependent and that the shedding of nectin-1 is a mechanism contributing to synaptic plasticity.     

The Anaphase-Promoting Complex (APC) ubiquitin ligase regulates GABA transmission at the C. elegans neuromuscular junction

Regulation of both excitatory and inhibitory synaptic transmission is critical for proper nervous system function. Aberrant synaptic signaling, including altered excitatory to inhibitory balance, is observed in numerous neurological diseases. The ubiquitin enzyme system controls the abundance of many synaptic proteins and thus plays a key role in regulating synaptic transmission. The Anaphase-Promoting Complex (APC) is a multi-subunit ubiquitin ligase that was originally discovered as a key regulator of protein turnover during the cell cycle. More recently, the APC has been shown to function in postmitotic neurons, where it regulates diverse processes such as synapse development and synaptic transmission at glutamatergic synapses. Here we report that the APC regulates synaptic GABA signaling by acting in motor neurons to control the balance of excitatory (acetylcholine) to inhibitory (GABA) transmission at the Caenorhabditis elegans neuromuscular junction (NMJ). Loss-of-function mutants in multiple APC subunits have increased muscle excitation at the NMJ; this phenotype is rescued by expression of the missing subunit in GABA neurons. Quantitative imaging and electrophysiological analyses indicate that APC mutants have decreased GABA release but normal cholinergic transmission. Consistent with this, APC mutants exhibit convulsions in a seizure assay sensitive to reductions in GABA signaling. Previous studies in other systems showed that the APC can negatively regulate the levels of the active zone protein SYD-2 Liprin-α. Similarly, we found that SYD-2 accumulates in APC mutants at GABAergic presynaptic sites. Finally, we found that the APC subunit EMB-27 CDC16 can localize to presynapses in GABA neurons. Together, our data suggest a model in which the APC acts at GABAergic presynapses to promote GABA release and inhibit muscle excitation. These findings are the first evidence that the APC regulates transmission at inhibitory synapses and have implications for understanding nervous system pathologies, such as epilepsy, that are characterized by misregulated GABA signaling.     

Presynaptic inhibition of Schaffer collateral synapses by stimulation of hippocampal cholinergic afferent fibres

It has been known for decades that muscarinic agonists presynaptically inhibit Schaffer collateral synapses contacting hippocampal CA1 pyramidal neurons. However, a demonstration of the inhibition of Schaffer collateral synapses induced by acetylcholine released by cholinergic hippocampal afferents is lacking. We present original results showing that electrical stimulation at the stratum oriens/alveus with brief stimulus trains inhibited excitatory postsynaptic currents evoked by stimulation of Schaffer collaterals in CA1 pyramidal neurons of rat hippocampal slices. The increased paired-pulse facilitation and the changes in the variance of excitatory postsynaptic current amplitude that paralleled the inhibition suggest that it was mediated presynaptically. The effects of oriens/alveus stimulation were inhibited by atropine, and blocking nicotinic receptors with methyllycaconitine was ineffective, suggesting that the inhibition was mediated via the activation of presynaptic muscarinic receptors. The results provide a novel demonstration of the presynaptic inhibition of glutamatergic neurotransmission by cholinergic fibres in the hippocampus, implying that afferent cholinergic fibres regulate the strength of excitatory synaptic transmission.     

Two differential frequency-dependent mechanisms regulating tonic firing of thalamic reticular neurons

Transmission through the thalamus activates circuits involving the GABAergic neurons of the thalamic reticular nucleus (TRN). TRN cells receive excitatory inputs from thalamocortical and corticothalamic cells and send inhibitory projections to thalamocortical cells. The inhibitory output of TRN neurons largely depends on the level of excitatory drive to these cells but may also be partly under the control of mechanisms intrinsic to the TRN. We examined two such possible mechanisms, short-term plasticity at glutamatergic synapses in the TRN and intra-TRN inhibition. In rat brain slices, responses of TRN neurons to brief trains of stimuli applied to glutamatergic inputs were recorded in voltage- or current-clamp mode. In voltage clamp, TRN cells showed no change in α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor-mediated excitatory postsynaptic current amplitudes to stimulation at non-gamma frequencies (< 30 Hz), simulating background activity, but exhibited short-term depression in these amplitudes to stimulation at gamma frequencies (> 30 Hz), simulating sensory transmission. In current clamp, TRN cells increased their spike outputs in burst and tonic firing modes to increasing stimulus-train frequencies. These increases in spike output were most likely due to temporal summation of excitatory postsynaptic potentials. However, the frequency-dependent increase in tonic firing was attenuated at gamma stimulus frequencies, indicating that the synaptic depression selectively observed in this frequency range acts to suppress TRN cell output. In contrast, intra-TRN inhibition reduced spike output selectively at non-gamma stimulus frequencies. Thus, our data indicate that two intrinsic mechanisms play a role in controlling the tonic spike output of TRN neurons and these mechanisms are differentially related to two physiologically meaningful stimulus frequency ranges.     

Phasic bursts in rat magnocellular neurosecretory cells are not intrinsically regenerative in vivo

Vasopressinergic hypothalamic magnocellular neurosecretory cells fire in phasic bursts. Burst initiation involves summation of postsynaptic potentials to generate action potentials. Action potentials are each followed by a nonsynaptic depolarizing after-potential that summates temporally to generate a plateau potential and so sustain activity throughout the burst. It is unknown whether this plateau potential exceeds spike threshold in vivo to cause intrinsic regenerative firing or simply approaches threshold to increase the probability that excitatory postsynaptic potentials will trigger further action potentials. Here we show that pharmacological blockade of ionotropic glutamatergic transmission by microdialysis application of kynurenic acid into the supraoptic nucleus of anaesthetized rats prevents spontaneous bursts and bursts (after-discharge) evoked by short trains of antidromically stimulated action potentials in magnocellular neurosecretory cells. Even during prolonged depolarization induced by 1 m NaCl infusion, kynurenic acid microdialysis application still blocked after-discharge. The ability of kynurenic acid to block after-discharge during osmotic stimulation was not caused by an unmasking of inhibitory postsynaptic potentials as kynurenic acid was equally effective in the presence of the ionotropic gamma-aminobutyric acid receptor antagonist, bicuculline, nor did it result from inhibition of plateau potential amplitude as this was unaffected by kynurenic acid and bicuculline in vitro, as was after-discharge evoked in vitro. We conclude that phasic bursts are nonregenerative in vivo but rather require continued excitatory synaptic input activity superimposed upon a subthreshold plateau potential to sustain burst activity.     

Activity-induced decrease in early and late inhibitory synaptic conductances in hippocampus

The use dependence of inhibitory postsynaptic potentials (IPSPs) and their underlying conductances was studied in area CA1 of the hippocampal brain slice preparation, using a two-pulse paradigm in which paired activation of two separate synaptic inputs resulted in changes in the second, or "primed" response. In intracellular current-clamp recordings, the "primed" response, normally triphasic, exhibited a larger, wider excitatory PSP (EPSP) component and greatly reduced or absent IPSP components. Maximal widening occurred when the interval between synaptic stimuli was between 200 and 250 msec. Hyperpolarization of the postsynaptic cell reversed both the early IPSP and the direction of change of the width of the "primed" EPSP response, suggesting that the changes in the "primed" waveform were not due to the addition of an unidentified inward current(s). Furthermore, the reduction of the IPSPs during the "primed" response could not be accounted for by the fact that the membrane potential of the postsynaptic cell was hyperpolarized and therefore closer to IPSP reversal potential. Using single-electrode voltage-clamp techniques, we found that the early inhibitory conductance generally decreased by approximately 50%, with little if any change in reversal potential. The late inhibitory conductance also showed a priming-induced decrease of approximately 95%. Finally, "primed" four-pulse bursts of stimuli induced a larger depolarization in the postsynaptic cell than did unprimed bursts, also with an optimal interval of about 250 msec. We conclude that activation of certain synaptic pathways in the hippocampus results in a temporal window of 200-300 msec during which inhibitory synaptic activity is depressed and excitatory synaptic transmission is maximally effective, especially if the excitation occurs in short bursts. Such a mechanism would endow the inhibitory synaptic components of the hippocampus with a "gating" function to control long-term synaptic modification at excitatory synapses in the same region.     

Dynamics of glutamatergic synapses in the medial vestibular nucleus of the mouse

During sinusoidal rotation or translation, primary vestibular afferents modulate their discharge rates at the frequency of motion, effectively transmitting frequency-modulated (FM) signals. This study indicates a possible role for excitatory synapses in the processing of FM signals by vestibular brainstem pathways. Inputs to medial vestibular neurons were activated with FM pulse trains, while inhibitory transmission was blocked. The relationship between the presynaptic pulse rate and the postsynaptic membrane potential was found to be linear within a range of pulse rates. Short-term plasticity was a factor contributing to sensitivity at higher modulating frequencies. The amount of low-pass filtering was correlated with excitatory postsynaptic potential (EPSP) shape, which affected temporal summation during the train. Although the NMDA component of glutamatergic transmission affected EPSP shape, it made only a minor contribution to the dynamics of synaptic transmission. Most responses showed low-pass filtering over the entire 1–16 Hz range. Overall, excitatory synapses in the medial vestibular nucleus contribute a low-pass filter to central vestibular processing and complement the high-pass filtering that is introduced both by peripheral vestibular dynamics and by the intrinsic dynamics of secondary vestibular neurons.     

Enhanced Synaptic Inhibition Disrupts the Efferent Code of Cerebellar Purkinje Neurons in Leaner Cav2.1 Ca2+ Channel Mutant Mice

Cerebellar Purkinje cells (PCs) encode afferent information in the rate and temporal structure of their spike trains. Both spontaneous firing in these neurons and its modulation by synaptic inputs depend on Ca²⁺ current carried by Ca_v2.1 (P/Q) type channels. Previous studies have described how loss-of-function Ca_v2.1 mutations affect intrinsic excitability and excitatory transmission in PCs. This study examines the effects of the leaner mutation on fast GABAergic transmission and its modulation of spontaneous firing in PCs. The leaner mutation enhances spontaneous synaptic inhibition of PCs, leading to transitory reductions in PC firing rate and increased spike rate variability. Enhanced inhibition is paralleled by an increase in the frequency and amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs) measured under voltage clamp. These differences are abolished by tetrodotoxin, implicating effects of the mutation on spike-induced GABA release. Elevated sIPSC frequency in leaner PCs is not accompanied by increased mean firing rate in molecular layer interneurons, but IPSCs evoked in PCs by direct stimulation of these neurons exhibit larger amplitude, slower decay rate, and a higher burst probability compared to wild-type PCs. Ca²⁺ release from internal stores appears to be required for enhanced inhibition since differences in sIPSC frequency and amplitude in leaner and wild-type PCs are abolished by thapsigargin, an ER Ca²⁺ pump inhibitor. These findings represent the first account of the functional consequences of a loss-of-function P/Q channel mutation on PC firing properties through altered GABAergic transmission. Gain in synaptic inhibition shown here would compromise the fidelity of information coding in these neurons and may contribute to impaired cerebellar function resulting from loss-of function mutations in the Ca_V2.1 channel gene.