Increase In Release Probability Research Articles

A Novel Form of Low-Frequency Hippocampal Mossy Fiber Plasticity Induced by Bimodal mGlu1 Receptor Signaling

Mossy fiber synapses act as the critical mediators of highly dynamic communication between hippocampal granule cells in the dentate gyrus and CA3 pyramidal neurons. Excitatory synaptic strength at mossy fiber to CA3 pyramidal cell synapses is potentiated rapidly and reversibly by brief trains of low-frequency stimulation of mossy fiber axons. We show that slight modifications to the pattern of stimulation convert this short-term potentiation into prolonged synaptic strengthening lasting tens of minutes in rodent hippocampal slices. This low-frequency potentiation of mossy fiber EPSCs requires postsynaptic mGlu1 receptors for induction but is expressed presynaptically as an increased release probability and therefore impacts both AMPA and NMDA components of the mossy fiber EPSC. A nonconventional signaling pathway initiated by mGlu1 receptors contributes to induction of plasticity, because EPSC potentiation was prevented by a tyrosine kinase inhibitor and only partially reduced by guanosine 5'-O-(2-thiodiphosphate). A slowly reversible state of enhanced synaptic efficacy could serve as a mechanism for altering the integrative properties of this synapse within a relatively broad temporal window.

Optical quantal analysis of synaptic transmission in wild-type and rab3-mutant Drosophila motor axons

Synaptic transmission from a neuron to its target cells occurs via neurotransmitter release from dozens to thousands of presynaptic release sites whose strength and plasticity can vary considerably. We report an in vivo imaging method that monitors real-time synaptic transmission simultaneously at many release sites with quantal resolution. We applied this method to the model glutamatergic system of the Drosophila melanogaster larval neuromuscular junction. We find that, under basal conditions, about half of release sites have a very low release probability, but these are interspersed with sites with as much as a 50-fold higher probability. Paired-pulse stimulation depresses high-probability sites, facilitates low-probability sites, and recruits previously silent sites. Mutation of the small GTPase Rab3 substantially increases release probability but still leaves about half of the sites silent. Our findings suggest that basal synaptic strength and short-term plasticity are regulated at the level of release probability at individual sites.

Receptor saturation controls short-term synaptic plasticity at corticothalamic synapses

Glutamatergic synapses of layer 6 corticothalamic (CT) neurons form a major excitatory input onto thalamic relay cells, allowing neocortex to continuously control sensory information processing in thalamic circuits. CT synapses display both short- and long-term forms of use-dependent synaptic enhancement, mediated at least in part by increases in the probability of transmitter release. At some synapses, such increases in release probability are accompanied by a higher degree of multivesicular release (MVR) and larger glutamate transients at individual release sites, resulting in the saturation of postsynaptic receptors. The extent to which MVR and postsynaptic saturation interact and control short-term plasticity at CT synapses is not known. Here we examined two distinct presynaptic forms of short-term enhancement, facilitation and augmentation, at CT synapses contacting relay neurons in the ventrobasal nucleus of the mouse thalamus. We found that, in the presence of the low-affinity antagonist γ-D-glutamylglycine, to relieve postsynaptic DL-α-amino-3-hydroxy-5-methylisox azole-propionic acid (AMPA) receptor saturation, the magnitude of facilitation and augmentation increased. Whereas receptor saturation was prominent for both AMPA and N-methyl-D-aspartate receptors, desensitization of AMPA receptors did not significantly alter short-term plasticity. Our results suggest that at CT synapses the activity-dependent increase in synaptic strength is controlled by postsynaptic receptor saturation.

Axonal GABAAReceptors Increase Cerebellar Granule Cell Excitability and Synaptic Activity

We report that activation of GABA(A) receptors on cerebellar granule cell axons modulates both transmitter release and the excitability of the axon and soma. Axonal GABA(A) receptors depolarize the axon, increasing its excitability and causing calcium influx at axonal varicosities. GABA-mediated subthreshold depolarizations in the axon spread electrotonically to the soma, promoting orthodromic action potential initiation. When chloride concentrations are unperturbed, GABA iontophoresis elicits spikes and increases excitability of parallel fibers, indicating that GABA(A) receptor-mediated responses are normally depolarizing. GABA release from molecular layer interneurons activates parallel fiber GABA(A) receptors, and this, in turn, increases release probability at synapses between parallel fibers and molecular layer interneurons. These results describe a positive feedback mechanism whereby transmission from granule cells to Purkinje cells and molecular layer interneurons will be strengthened during granule cell spike bursts evoked by sensory stimulation.

Local Presynaptic Activity Gates Homeostatic Changes in Presynaptic Function Driven by Dendritic BDNF Synthesis

Homeostatic synaptic plasticity is important for maintaining stability of neuronal function, but heterogeneous expression mechanisms suggest that distinct facets of neuronal activity may shape the manner in which compensatory synaptic changes are implemented. Here, we demonstrate that local presynaptic activity gates a retrograde form of homeostatic plasticity induced by blockade of AMPA receptors (AMPARs) in cultured hippocampal neurons. We show that AMPAR blockade produces rapid (<3 hr) protein synthesis-dependent increases in both presynaptic and postsynaptic function and that the induction of presynaptic, but not postsynaptic, changes requires coincident local activity in presynaptic terminals. This "state-dependent" modulation of presynaptic function requires postsynaptic release of brain-derived neurotrophic factor (BDNF) as a retrograde messenger, which is locally synthesized in dendrites in response to AMPAR blockade. Taken together, our results reveal a local crosstalk between active presynaptic terminals and postsynaptic signaling that dictates the manner by which homeostatic plasticity is implemented at synapses.

Synaptic changes underlying the strengthening of GABA/glycinergic connections in the developing lateral superior olive

Before hearing onset, the topographic organization of the auditory GABA/glycinergic pathway from the medial nucleus of the trapezoid body (MNTB) to the lateral superior olive (LSO) is refined by synaptic silencing and strengthening. The synaptic mechanisms underlying the developmental strengthening of maintained MNTB-LSO connections are unknown. Here we address this question using whole-cell recordings from LSO neurons in slices prepared from prehearing mice. Minimal and maximal stimulation techniques demonstrated that during the first two postnatal weeks, individual LSO neurons lose about 55% of their initial presynaptic MNTB partners while maintained single-fiber connections become about 14-fold stronger. Analysis of MNTB-evoked miniature events indicates that this strengthening is accompanied by a 2-fold increase in quantal amplitude. Strengthening is not caused by an increase in the probability of release because paired pulse ratios (PPRs) increased from 0.7 in newborn animals to 0.9 around hearing onset, indicating a developmental decrease rather than increase in release probability. In addition, a possible soma-dendritic relocation of MNTB input seems unlikely to underlie their strengthening as indicated by analysis of the rise times of synaptic currents. Taken together, we conclude that the developmental strengthening of MNTB-LSO connections is achieved by a 2-fold increase in quantal size and an 8-fold increase in quantal content.

Essential cooperation of N-cadherin and neuroligin-1 in the transsynaptic control of vesicle accumulation

Cell adhesion molecules are key players in transsynaptic communication, precisely coordinating presynaptic differentiation with postsynaptic specialization. At glutamatergic synapses, their retrograde signaling has been proposed to control presynaptic vesicle clustering at active zones. However, how the different types of cell adhesion molecules act together during this decisive step of synapse maturation is largely unexplored. Using a knockout approach, we show that two synaptic adhesion systems, N-cadherin and neuroligin-1, cooperate to control vesicle clustering at nascent synapses. Live cell imaging and fluorescence recovery after photobleaching experiments at individual synaptic boutons revealed a strong impairment of vesicle accumulation in the absence of N-cadherin, whereas the formation of active zones was largely unaffected. Strikingly, also the clustering of synaptic vesicles triggered by neuroligin-1 overexpression required the presence of N-cadherin in cultured neurons. Mechanistically, we found that N-cadherin acts by postsynaptically accumulating neuroligin-1 and activating its function via the scaffolding molecule S-SCAM, leading, in turn, to presynaptic vesicle clustering. A similar cooperation of N-cadherin and neuroligin-1 was observed in immature CA3 pyramidal neurons in an organotypic hippocampal network. Moreover, at mature synapses, N-cadherin was required for the increase in release probability and miniature EPSC frequency induced by expressed neuroligin-1. This cooperation of two cell adhesion systems provides a mechanism for coupling bidirectional synapse maturation mediated by neuroligin-1 to cell type recognition processes mediated by classical cadherins.

Serotoninergic control of glycinergic inhibitory postsynaptic currents in rat hypoglossal motoneurons

This report presents the results of a study of the frequency potentiation of inhibitory postsynaptic currents (IPSCs) in hypoglossal motoneurons and its modulation by serotonin. A release-site model of synaptic plasticity was used to characterize the frequency-related potentiation of evoked IPSCs. Data were obtained to determine if the frequency potentiation of IPSCs occurs as a consequence of a low baseline quantal content of evoked IPSCs using whole cell patch-clamp recordings from hypoglossal motoneurons in the neonatal rat brainstem slice preparation. In these motoneurons, EPSCs and GABAergic IPSCs were blocked by the application of CNQX, AP-5 and bicuculline. Glycinergic IPSCs were evoked by threshold stimulation of inhibitory neurons in the nucleus of Roller, which is located ventro-lateral to the hypoglossal nucleus. IPSC responses to trains of stimuli were recorded in control solutions and in solutions containing serotonin, which is known to reduce IPSPs in this preparation. The amplitude of non-potentiated IPSCs was reduced and their frequency potentiation was enhanced when serotonin was added to the bath. These data were examined using a release-site model of synaptic plasticity in which facilitation is attributed to a time-dependent increase in the probability of transmitter release; depression is attributed to a time-dependent decrease in the number of sites available for release. Using this model, the effect of serotonin on frequency potentiation was explained by a combination of a reduction in the baseline probability of transmitter release and an increase in the time constant of decay of the increase in probability of release that follows a stimulus.

Properties of glutamatergic synapses in immature layer Vb pyramidal neurons: coupling of pre- and postsynaptic maturational states

Following initial contact formation, glutamatergic synapses in cortical neurons undergo pronounced functional maturation. These maturational events, occurring both pre- and postsynaptically, have been well described in the developing hippocampus. In this paper, we characterized glutamatergic synapses in immature layer Vb pyramidal neurons of the mouse somatosensory cortex during early postnatal development. At postnatal day 7, a significant subpopulation of glutamatergic synapses exhibited a low release probability that was accompanied by strong paired-pulse facilitation of AMPA EPSCs (paired-pulse ratio C > or = 2). Increasing extracellular Ca(2+) concentration increased release probability and led to paired-pulse depression. During further postnatal development, these functionally immature synapses disappeared. As shown pharmacologically,these synapses expressed postsynaptic NMDA receptors containing NR2B subunits, while NMDA receptors with NR2A subunits were lacking. Taken together, a low release probability presynaptically was coupled to postsynaptic NR2B signaling. This subpopulation of neocortical synapses thus differed from the majority of synapses in the developing hippocampus, where high release probability is coupled to NR2B signaling. The novel type of functionally immature glutamatergic synapse described here might play an important role in early developmental synapse elimination and in the activity-dependent refinement of the neocortical synaptic microcircuitry.

Brain-Derived Neurotrophic Factor Controls Activity-Dependent Maturation of CA1 Synapses by Downregulating Tonic Activationof Presynaptic Kainate Receptors

Immature hippocampal synapses express presynaptic kainate receptors (KARs), which tonically inhibit glutamate release. Presynaptic maturation involves activity-dependent downregulation of the tonic KAR activity and consequent increase in release probability; however, the molecular mechanisms underlying this developmental process are unknown. Here, we have investigated whether brain derived neurotrophic factor (BDNF), a secreted protein implicated in developmental plasticity in several areas of the brain, controls presynaptic maturation by regulating KARs. Application of BDNF in neonate hippocampal slices resulted in increase in synaptic transmission that fully occluded the immature-type KAR activity in area CA1. Conversely, genetic ablation of BDNF was associated with delayed synaptic maturation and persistent presynaptic KAR activity, suggesting a role for endogenous BDNF in the developmental regulation of KAR function. In addition, our data suggests a critical role for BDNF TrkB signaling in fast activity-dependent regulation of KARs. Selective acute inhibition of TrkB receptors using a chemical-genetic approach prevented rapid change in synapse dynamics and loss of tonic KAR activity that is typically seen in response to induction of LTP at immature synapses. Together, these data show that BDNF-TrkB-dependent maturation of glutamatergic synapses is tightly associated with a loss of endogenous KAR activity. The coordinated action of these two receptor mechanisms has immediate physiological relevance in controlling presynaptic efficacy and transmission dynamics at CA3-CA1 synapses at a stage of development when functional contact already exists but transmission is weak.

Autapses and Networks of Hippocampal Neurons Exhibit Distinct Synaptic Transmission Phenotypes in the Absence of Synaptotagmin I

Synaptotagmin-I (syt-I) is required for rapid neurotransmitter release in mouse hippocampal neurons. However, contradictory results have been reported regarding evoked and spontaneous secretion from syt-I knock-out (KO) neurons. Here, we compared synaptic transmission in two different hippocampal neuron preparations: autaptic cultures in which a single isolated cell innervates itself, and dissociated mass cultures in which individual cells are innervated by neighboring cells. In autaptic cultures, the total extent of evoked release, size of readily releasable pool of synaptic vesicles, and release probability were unchanged in syt-I KO neurons. In contrast, in cultures containing multiple interconnected neurons, total evoked release, the number of docked vesicles, and release probability, were significantly reduced in syt-I KO neurons. Using a micronetwork system in which we varied the number of cells on an island, we found that the frequency of spontaneous synaptic vesicle fusion events (minis) was unchanged in syt-I KO neurons when two or fewer cells were present on an island. However, in micronetworks composed of three or more neurons, mini frequency was increased threefold to fivefold in syt-I KO neurons compared with wild type. Moreover, interneuronal synapses exhibited higher rates of spontaneous release than autaptic synapses. This higher rate was attributable to an increase in release probability because excitatory hippocampal neurons in micronetworks formed a set number of synapses per cell regardless of the number of connected neurons. Thus, aspects of synaptic transmission differ between autaptic and dissociated cultures, and the synaptic transmission phenotype, resulting from loss of syt-I, is dictated by the connectivity of neurons.

Presynaptic Release Probability and Readily Releasable Pool Size Are Regulated by Two Independent Mechanisms during Posttetanic Potentiation at the Calyx of Held Synapse

At the immature calyx of Held, the fast decay phase of a Ca(2+) transient induced by tetanic stimulation (TS) was followed by a period of elevated [Ca(2+)](i) for tens of seconds, referred to as posttetanic residual calcium (Ca(res)). We investigated the source of Ca(res) and its contribution to posttetanic potentiation (PTP). After TS (100 Hz for 4 s), posttetanic Ca(res) at the calyx of Held was largely abolished by tetraphenylphosphonium (TPP(+)) or Ru360, which inhibit mitochondrial Na(+)-dependent Ca(2+) efflux and Ca(2+) uniporter, respectively. Whereas the control PTP lasted longer than Ca(res), inhibition of Ca(res) by TPP(+) resulted in preferential suppression of the early phase of PTP, the decay time course of which well matched with that of Ca(res). TS induced significant increases in release probability (P(r)) and the size of the readily releasable pool (RRP), which were estimated from plots of cumulative EPSC amplitudes. TPP(+) or Ru360 suppressed the posttetanic increase in P(r), whereas it had little effect on the increase in RRP size. Moreover, the posttetanic increase in P(r), but not in RRP size, showed a linear correlation with the amount of Ca(res). In contrast, myosin light chain kinase (MLCK) inhibitors and blebbistatin reduced the posttetanic increase in RRP size with no effect on the increase in P(r). Application of TPP(+) in the presence of MLCK inhibitor peptide caused further suppression of PTP. These findings suggest that Ca(res) released from mitochondria and activation of MLCK are primarily responsible for the increase in P(r) and that in the RRP size, respectively.

5-HT4 receptor activation facilitates recovery from synaptic rundown and increases transmitter release from single varicosities of myenteric neurons

5-HT(4) receptor agonists facilitate synaptic transmission in the enteric nervous system, and these drugs are used to treat constipation. In the present study, we investigated the effects of the 5-HT(4) receptor agonist, renzapride, on rundown and recovery of fast excitatory postsynaptic potentials (fEPSPs) during and after trains of stimulation and on transmitter release from individual myenteric neuronal varicosities. Intracellular electrophysiological methods were used to record fEPSPs from neurons in longitudinal muscle myenteric plexus preparations of guinea pig ileum in vitro. During trains of supramaximal electrical stimulation (10 Hz, 2 s), fEPSP amplitude declined (time constant = 0.6 +/- 0.1 s) from 17 +/- 2 mV to 0.7 +/- 0.3 mV. Renzapride (0.1 microM) did not change the time constant for fEPSP rundown, but it decreased the time constant for recovery of fEPSP amplitude after the stimulus train from 7 +/- 2 s to 1.6 +/- 0.2 s (P < 0.05). 5-HT (0.1 microM) also increased fEPSPs and facilitated recovery from rundown. The adenylate cyclase activator, forskolin (1 muM), mimicked the actions of renzapride and 5-HT, whereas H-89, a protein kinase A (PKA) inhibitor, blocked the effects of renzapride. We used nicotinic acetylcholine receptor containing outside-out patches obtained from myenteric neurons maintained in primary culture to detect acetylcholine release from single varicosities. Renzapride (0.1 microM) increased release probability twofold. We conclude that 5-HT(4) receptors activate the adenylyl cyclase-PKA pathway to increase acetylcholine release from single varicosities and to accelerate recovery from synaptic rundown. These responses may contribute to the prokinetic actions of 5-HT(4) receptor agonists.

Increased striatal dopamine release and hyperdopaminergic‐like behaviour in mice lacking both alpha‐synuclein and gamma‐synuclein

Alpha-synuclein is intimately involved in the pathogenesis of Parkinson's disease, and has been implicated in the regulation of synthesis, release and reuptake of dopamine (DA). However, mice lacking members of the synuclein family have been reported to display no overt behavioural phenotype. This may be a result of compensatory upregulation of other synucleins during development. Here we report on behaviour and DA synapse function of alpha-synuclein null, gamma-synuclein null, and alpha-gamma-synuclein double-null knockout mice. Double-null mice were hyperactive in a novel environment and alternated at a lower rate in a T-maze spontaneous alternation task, a phenotype reminiscent of mice expressing reduced levels of the DA transporter. To investigate a possible hyperdopaminergic phenotype in alpha-gamma-synuclein double-null mice, we used fast-scan cyclic voltammetry at carbon-fibre microelectrodes to assess DA release and reuptake in striatal slices from wild-type, alpha-null, gamma-null and double-null mice in real time. Double-null mice were found to have a twofold increase in the extracellular concentration of DA detected after discrete electrical stimuli in the striatum. By measuring the rate of reuptake of DA and tissue DA content in these animals, we showed that the observed increase in size of striatal DA transients was not attributable to a decrease in reuptake of DA via the DA transporter, and can not be attributed to an increase in tissue DA levels in the striatum. Rather, we propose that loss of both alpha- and gamma-synuclein causes an increase in release probability from dopaminergic synapses.

Quantitative Analysis of Calcium-Dependent Vesicle Recruitment and Its Functional Role at the Calyx of Held Synapse

Recruitment of release-ready vesicles at synapses is one of the important factors, which determine dynamic properties of signaling between neurons in the brain. It has been shown that the rate of vesicle recruitment is accelerated by strong synaptic activity. An elevated concentration of calcium ions in the presynaptic terminal ([Ca2+]i) has been proposed to be responsible for this effect. However, the precise relationship between [Ca2+]i and recruitment has not been established yet, and the functional consequences of accelerated recruitment during synaptic activity have not been quantified experimentally. To probe the intracellular Ca2+ dependence of vesicle recruitment and to examine its functional role during trains of action potential (AP)-like stimuli, we monitored [Ca2+]i and synaptic responses simultaneously with paired recordings at the calyx of Held synapse. We found that a distinct, rapidly releasing vesicle pool is replenished with a rate that increases linearly with [Ca2+]i, without any apparent cooperativity. The slope factor for this increase is approximately 1 pool/(microM x s). Blocking Ca2+-dependent recruitment specifically with a calmodulin binding peptide revealed that the steady-state EPSCs during 100 Hz AP-like trains were maintained through this Ca2+-dependent recruitment mechanism. Using a simple model of vesicle dynamics, we estimated that the recruitment rate accelerated 10-fold during the steady-state compared with the rate at resting [Ca2+]i. We could also demonstrate an approximate sixfold increase in release probability (facilitation) during the initial 5-15 AP-like stimuli of such trains in our experimental condition, regardless of EPSC depression.

Time course and calcium dependence of transmitter release at a single ribbon synapse

At the first synapse in the auditory pathway, the receptor potential of mechanosensory hair cells is converted into a firing pattern in auditory nerve fibers. For the accurate coding of timing and intensity of sound signals, transmitter release at this synapse must occur with the highest precision. To measure directly the transfer characteristics of the hair cell afferent synapse, we implemented simultaneous whole-cell recordings from mammalian inner hair cells (IHCs) and auditory nerve fiber terminals that typically receive input from a single ribbon synapse. During a 1-s IHC depolarization, the synaptic response depressed >90%, representing the main source for adaptation in the auditory nerve. Synaptic depression was slightly affected by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor desensitization; however, it was mostly caused by reduced vesicular release. When the transfer function between transmitter release and Ca(2+) influx was tested at constant open probability for Ca(2+) channels (potentials >0 mV), a super linear relation was found. This relation is presumed to result from the cooperative binding of three to four Ca(2+) ions at the Ca(2+) sensor. However, in the physiological range for receptor potentials (-50 to -30 mV), the relation between Ca(2+) influx and afferent activity was linear, assuring minimal distortion in the coding of sound intensity. Changes in Ca(2+) influx caused an increase in release probability, but not in the average size of multivesicular synaptic events. By varying Ca(2+) buffering in the IHC, we further investigate how Ca(2+) channel and Ca(2+) sensor at this synapse might relate.

Fast changes in the functional status of release sites during short‐term plasticity: involvement of a frequency‐dependent bypass of Rac at Aplysia synapses

Synaptic transmission can be described as a stochastic quantal process defined by three main parameters: N, the number of functional release sites; P, the release probability; and Q, the quantum of response. Many changes in synaptic strength that are observed during expression of short term plasticity rely on modifications in P. Regulation of N has been also suggested. We have investigated at identified cholinergic inhibitory Aplysia synapses the cellular mechanism of post-tetanic potentiation (PTP) expressed under control conditions or after N has been depressed by applying lethal toxin (LT) from Clostridium sordellii or tetanus toxin (TeNT). The analysis of the Ca(2+) dependency, paired-pulse ratio and variance to mean amplitude relationship of the postsynaptic responses elicited at distinct extracellular [Ca(2+)]/[Mg(2+)] elicited during control post-tetanic potentiation (PTP(cont)) indicated that PTP(cont) is mainly driven by an increase in release probability, P. The PTP expressed at TeNT-treated synapses (PTP(TeNT)) was found to be similar to PTP(cont), but scaled to the extent of reduction in N produced by TeNT. Despite LT inducing a decrease in N as TeNT does, the PTP expressed at LT-treated synapses (PTP(LT)) was characterized by exceptionally large amplitude and bi-exponential time course, as compared to PTP(cont) or the PTP(TeNT). Analysis of the Ca(2+) dependency of PTP(LT), paired-pulse ratio and fluctuations in amplitude of the postsynaptic responses elicited during PTP(LT) or the variance to mean amplitude relationship of time-locked postsynaptic responses in a series of subsequent PTP(LT) indicated that an N-driven change is involved in the early phase (1 s time scale) of PTP(LT), while at a later stage PTP(LT) is composed of both N and P increases. Our results suggest that fast switching on of the functional status of the release sites occurs also during the early events of PTP(cont). The early N-driven phase of PTP(LT) is likely to be a functional recovery of the release sites silenced by Rac inactivation. This effect did not appear to result from reversion of LT inhibitory action but from bypassing the step regulated by Rac. Altogether the data suggest that Rac and the intracellular pathway which allows the bypassing of Rac are key players in new forms of short-term plasticity that rely on fast, activity-dependent changes in the functional status of the release sites.

A delayed response enhancement during hippocampal presynaptic plasticity in mice

High frequency afferent stimulation of chemical synapses often induces short-term increases in synaptic efficacy, due to increased release probability and/or increased supply of readily releasable synaptic vesicles. This may be followed by synaptic depression, often caused by vesicle depletion. We here describe an additional, novel type of delayed and transient response enhancement phase which occurred during prolonged stimulation at 5-20 Hz frequency of excitatory glutamatergic synapses in slices from the adult mouse CA1 hippocampal region. This second enhancement phase, which was most clearly defined at physiological temperatures and essentially absent at 24 degrees C, was dependent on the presence of F-actin filaments and synapsins I and/or II, and could not be ascribed to changes in presynaptic action potentials, inhibitory neurotransmission or glutamate receptor desensitization. Time course studies showed that the delayed response phase interrupted the synaptic decay 3-4 s after stimulus train initiation and continued, when examined at 5-10 Hz frequencies, for approximately 75 stimuli before decay. The novel response enhancement, probably deriving from a restricted pool of synaptic vesicles, may allow maintenance of synaptic efficacy during prolonged periods of excitatory synaptic activity.

Dynamics of the readily releasable pool during post‐tetanic potentiation in the rat calyx of Held synapse

The size of the readily releasable pool (RRP) of vesicles was measured in control conditions and during post-tetanic potentiation (PTP) in a large glutamatergic terminal called the calyx of Held. We measured excitatory postsynaptic currents evoked by a high frequency train of action potentials in slices of 4-11-day-old rats. After a tetanus the cumulative release during such a train was enlarged by approximately 50%, indicating that the size of the RRP was increased. The amount of enhancement depended on the duration and frequency of the tetanus and on the age of the rat. After the tetanus, the size of the RRP decayed more slowly (t(1/2)=10 versus 3 min) back to control values than the release probability. This difference was mainly due to a very fast initial decay of the release probability, which had a time constant compatible with an augmentation phase (tau approximately 30 s). The overall decay of PTP at physiological temperature was not different from room temperature, but the increase in release probability (P(r)) was restricted to the first minute after the tetanus. Thereafter PTP was dominated by an increase in the size of the RRP. We conclude that due to the short lifetime of the increase in release probability, the contribution of the increase in RRP size during post-tetanic potentiation is more significant at physiological temperature.

Post-tetanic potentiation of GABAergic IPSCs in cultured hippocampal neurons is exclusively time-dependent

We have previously shown that post-tetanic potentiation (PTP) of GABAergic IPSCs in cultured hippocampal neurons involves activation of L-type Ca 2+ channels. Although there is little Ca 2+ entry by this route, it is possible that L-type Ca 2+ channels mediate an increase in probability of release (Pr) by a mechanism that remains dormant in the absence of stimulation. We have tested this hypothesis in the present study using dual whole-cell patch clamp recordings. IPSCs were evoked by low-frequency stimulation (LFS; 0.2 Hz) of presynaptic GABAergic neurons. Run-down was corrected by linear regression. Following tetanic stimulation (80 pulses at 40 Hz), the presence of PTP was probed by resuming LFS after various post-tetanic intervals (PTI). To control for possible effects associated with LFS, the train and PTI were replaced by corresponding pauses. Following pauses ≥ 16 s, the first IPSC was significantly increased by 20–25% ( P < 0.01, paired t-test). These post-pause responses were subtracted from IPSCs following tetanic stimulation. Following correction, PTP was greatest (∼ 50%) after the shortest PTI (4 s) and IPSC amplitudes declined back to the baseline value over 1–2 min. With a PTI of 16 s, the first IPSC was potentiated to the same level as that to which PTP with a PTI of 4 s had decayed with continued LFS. There was no significant PTP with PTIs of 64 and 128 s. Since PTP decays entirely in the absence of stimulation, it is concluded that the process(es) mediating the increase in vesicular Pr appear to be time-dependent, but not use-dependent.