Endocannabinoid-mediated short-term suppression of excitatory synaptic transmission to medium spiny neurons in the striatum

Medium spiny neurons in the dorsal striatum receive glutamatergic excitatory synaptic inputs from the cerebral cortex. These synapses undergo long-term depression that requires release of endocannabinoids from medium spiny neurons and activation of cannabinoid CB1 receptors. However, it remains unclear how cortico-striatal synapses exhibit endocannabinoid-mediated short-term suppression, which has been found in various brain regions including the hippocampus and cerebellum. Endocannabinoids are released from postsynaptic neurons by strong depolarization and resultant Ca2+ elevation or activation of postsynaptic Gq/11-coupled receptors such as group I metabotropic glutamate receptors (mGluRs) and M1/M3 muscarinic acetylcholine receptors. Moreover, endocannabioids are effectively released when weak depolarization is combined with Gq/11-coupled receptor activation. We found that muscarinic activation induced transient suppression of excitatory synaptic transmission to medium spiny neurons, which was independent of retrograde endocannabinoid signaling but was mediated directly by presynaptic muscarinic receptors. Neither postsynaptic depolarization alone nor depolarization and muscarinic activation caused suppression of cortico-striatal synapses. In contrast, activation of group I mGluRs readily suppressed cortico-striatal excitatory synaptic transmission. Furthermore, postsynaptic depolarization induced clear suppression when combined with group I mGluR activation. These results indicate that group I mGluRs but not muscarinic receptors contribute to endocannabinoid-mediated short-term suppression of cortico-striatal excitatory synaptic transmission.     

Cannabinoids modulate the P-type high-voltage-activated calcium currents in purkinje neurons

Endocannabinoids released by postsynaptic cells inhibit neurotransmitter release in many central synapses by activating presynaptic cannabinoid CB1 receptors. In particular, in the cerebellum, endocannabinoids inhibit synaptic transmission at granule cell to Purkinje cell synapses by modulating presynaptic calcium influx via N-, P/Q-, and R-type calcium channels. Using whole cell patch-clamp techniques, we show that in addition to this presynaptic action, both synthetic and endogenous cannabinoids inhibit P-type calcium currents in isolated rat Purkinje neurons independent of CB1 receptor activation. The IC50 for the anandamide (AEA)-induced inhibition of P-current peak amplitude was 1.04 +/- 0.04 microM. In addition, we demonstrate that all the tested cannabinoids in a physiologically relevant range of concentrations strongly accelerate inactivation of P currents. The effects of AEA cannot be attributed to the metabolism of AEA because a nonhydrolyzing analogue of AEA, methanandamide inhibited P-type currents with a similar efficacy. All effects of cannabinoids on P-type Ca2+ currents were insensitive to antagonists of CB1 cannabinoid or vanilloid TRPV1 receptors. In cerebellar slices, WIN 55,212-2 significantly affected spontaneous firing of Purkinje neurons in the presence of CB1 receptor antagonist, in a manner similar to that of a specific P-type channel antagonist, indicating a possible functional implication of the direct effects of cannabinoids on P current. Taken together these findings demonstrate a functionally important direct action of cannabinoids on P-type calcium currents.     

Endocannabinoids contribute to metabotropic glutamate receptor-mediated inhibition of GABA release onto hippocampal CA3 pyramidal neurons in an isolated neuron/bouton preparation

Retrograde synaptic signaling by endogenous cannabinoids (endocannabinoids) is a recently discovered form of neuromodulation in various brain regions. In hippocampus, it is well known that endocannabinoids suppress presynaptic inhibitory neurotransmitter release in CA1 region. However, endocannabinoid signaling in CA3 region remains to be examined. Here we investigated whether presynaptic inhibition can be caused by activation of postsynaptic group I metabotropic glutamate receptors (mGluRs) and following presynaptic cannabinoid receptor type 1 (CB1 receptor) using mechanically dissociated rat hippocampal CA3 pyramidal neurons with adherent functional synaptic boutons. Application of group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) reversibly suppressed spontaneous inhibitory postsynaptic currents (IPSCs). In the presence of tetrodotoxin (TTX), frequency of miniature IPSCs was significantly reduced by DHPG, while there were no significant changes in minimum quantal size and sensitivity of postsynaptic GABA_A receptors to the GABA_A receptor agonist muscimol, indicating that this suppression was caused by a decrease in GABA release from presynaptic nerve terminals. Application of CB1 synthetic agonist WIN55212-2 (mesylate(R)-(+)-[2,3-dihydro-5-methyl-3-[4-morpholino)methyl]pyrrolo-[1,2,3-de]-1,4-benzoxazin-6-yl](1-naphthyl)methanone) or endocannabinoid 2-arachidonoylglycerol also suppressed the spontaneous IPSC. The inhibitory effect of DHPG on spontaneous IPSCs was abolished by SR-141716 (5-(4-chlorophenyl)-1-(2,4-dichloro-phenyl)-4-methyl-N-(piperidin-1-yl)-1H-pyrazole-3-carboxamide), a CB1 receptor antagonist. Furthermore, postsynaptic application of GDP-βS blocked the DHPG-induced inhibition of spontaneous IPSCs, indicating the involvement of endcannabinoid-mediated retrograde synaptic signaling. These results provide solid evidence for retrograde signaling from postsynaptic group I mGluRs to presynaptic CB1 receptors, which induces presynaptic inhibition of GABA release in rat hippocampal CA3 region.     

Brief trains of action potentials enhance pyramidal neuron excitability via endocannabinoid-mediated suppression of inhibition

Depolarization-induced suppression of inhibition (DSI) is a form of retrograde signaling at GABAergic synapses that is initiated by the calcium- and depolarization-dependent release of endocannabinoids from postsynaptic neurons. In the neocortex, pyramidal neurons (PNs) appear to use DSI as a mechanism for regulating somatic inhibition from a subpopulation of GABAergic inputs that express the type 1 cannabinoid receptor. Although postsynaptic control of afferent inhibition may directly influence the integrative properties of neocortical PNs, little is known about the patterns of activity that evoke endocannabinoid release and the impact such disinhibition may have on the excitability of PNs. Here we provide the first systematic survey of action potential (AP)-induced DSI in the neocortex. The magnitude and time course of DSI was directly related to the number and frequency of postsynaptic APs with significant suppression induced by a 20-Hz train containing as few as three APs. This AP-induced DSI was mediated by endocannabinoids as it was prevented by the cannabinoid receptor antagonist AM251 and potentiated by the endocannabinoid transport inhibitor AM404. We also explored the effects of endocannabinoid-mediated DSI on PN excitability. We found that single AP trains markedly increased PN responsiveness to excitatory synaptic inputs and promoted AP discharge by suppressing GABAergic inhibition. The time course of this effect paralleled DSI expression and was completely blocked by AM251. Taken together, our data suggest a role for endocannabinoids in regulating the output of cortical PNs.     

CB1 modulation of temporally distinct synaptic facilitation among local circuit interneurons mediated by N-type calcium channels in CA1

One of the critical factors in determining network behavior of neurons is the influence of local circuit connections among interneurons. The short-term synaptic plasticity and the subtype of presynaptic calcium channels used at local circuit connections of inhibitory interneurons in CA1 were investigated using dual whole-cell recordings combined with biocytin and double immunofluorescence labeling in acute slices of P18- to 21-day-old rat stratum radiatum (SR) and stratum lacunosum molecular (SLM). Two forms of temporally distinct synaptic facilitation were observed among interneuron connections involving presynaptic cholecystokinin (CCK)-positive cells in SR, frequency-dependent facilitation, and a delayed onset of release (45-80 ms) with subsequent facilitation (DORF). Inhibition at both these synapses was under tonic cannabinoid-type 1 (CB1) receptor activity. DORF synapses did not display conventional release-dependent properties; however, blocking CB1 receptors with antagonist AM-251 (10 μM) altered the synaptic transmission to frequency-dependent depression with a fast onset of release (2-4 ms). Presynaptic CCK-negative interneurons in SLM elicited inhibitory postsynaptic potentials (IPSPs) insensitive to CB1 receptor pharmacology displayed frequency-dependent depression. Release of GABA at facilitating synapses was solely mediated via N-type presynaptic calcium channels, whereas depressing synapses utilized P/Q-type channels. These data reveal two distinct models of neurotransmitter release patterns among interneuron circuits that correlate with the subtype of presynaptic calcium channel. These data suggest that endocannabinoids act via CB1 receptors to selectively modulate N-type calcium channels to alter signal transmission.     

Role of endogenous cannabinoids in synaptic signaling

Research of cannabinoid actions was boosted in the 1990s by remarkable discoveries including identification of endogenous compounds with cannabimimetic activity (endocannabinoids) and the cloning of their molecular targets, the CB1 and CB2 receptors. Although the existence of an endogenous cannabinoid signaling system has been established for a decade, its physiological roles have just begun to unfold. In addition, the behavioral effects of exogenous cannabinoids such as delta-9-tetrahydrocannabinol, the major active compound of hashish and marijuana, await explanation at the cellular and network levels. Recent physiological, pharmacological, and high-resolution anatomical studies provided evidence that the major physiological effect of cannabinoids is the regulation of neurotransmitter release via activation of presynaptic CB1 receptors located on distinct types of axon terminals throughout the brain. Subsequent discoveries shed light on the functional consequences of this localization by demonstrating the involvement of endocannabinoids in retrograde signaling at GABAergic and glutamatergic synapses. In this review, we aim to synthesize recent progress in our understanding of the physiological roles of endocannabinoids in the brain. First, the synthetic pathways of endocannabinoids are discussed, along with the putative mechanisms of their release, uptake, and degradation. The fine-grain anatomical distribution of the neuronal cannabinoid receptor CB1 is described in most brain areas, emphasizing its general presynaptic localization and role in controlling neurotransmitter release. Finally, the possible functions of endocannabinoids as retrograde synaptic signal molecules are discussed in relation to synaptic plasticity and network activity patterns.     

Synaptic regulation of proopiomelanocortin neurons can occur distal to the arcuate nucleus

Energy homeostasis is controlled to a large extent by various signals that are integrated in the hypothalamus. It is generally considered that neurons in each of the hypothalamic nuclei are regulated by afferent projections that terminate within the cell body region of the nucleus. However, here it is shown that hypothalamic proopiomelanocortin (POMC) neurons receive synaptic inputs onto distal dendrites that reside outside of the cell body region in the arcuate nucleus. Previous studies using whole cell recordings from identified neurons in brain slices have shown that cannabinoids reduce GABA release from inhibitory synapses onto the POMC cells. Here it was found that endocannabinoids inhibited GABAergic inhibitory postsynaptic currents in POMC neurons only in intact sagittal brain slices, but not coronal, horizontal, or sagittal slices that were truncated rostrally at the level of the optic chiasm. Thus endocannabinoids inhibited presynaptic GABA release only at an anatomically distinct subset of POMC-neuron dendrites that extends rostrally beyond the arcuate nucleus into preoptic hypothalamic regions. There are two key results. First, the activity of POMC neurons can be regulated by afferent input at sites much farther from the soma than previously recognized. Second, endocannabinoids can act to inhibit inputs only at selective dendrites. POMC neurons play a critical role in the maintenance of body weight. Therefore these data suggest that energy balance may be regulated, in part, by modulation of POMC neuron activity at sites outside of the arcuate nucleus.     

Endocannabinoid signaling depends on the spatial pattern of synapse activation

The brain's endocannabinoid retrograde messenger system decreases presynaptic transmitter release, but its physiological function is uncertain. We show that endocannabinoid signaling is absent when spatially dispersed synapses are activated on rodent cerebellar Purkinje cells but that it reduces presynaptic glutamate release when nearby synapses are active. This switching of signaling according to the spatial pattern of activity is controlled by postsynaptic type I metabotropic glutamate receptors, which are activated disproportionately when glutamate spillover between synapses produces synaptic crosstalk. When spatially distributed synapses are activated, endocannabinoid inhibition of transmitter release can be rescued by inhibiting glutamate uptake to increase glutamate spillover. Endocannabinoid signaling initiated by type I metabotropic glutamate receptors is a homeostatic mechanism that detects synaptic crosstalk and downregulates glutamate release in order to promote synaptic independence.     

Analysis of synaptic transmission at single identified boutons on rat spinal neurons in culture

The spatial organization of receptor channels has a major influence on the speed and possible plasticity of synaptic signal transmission. We have studied glutamatergic synapses on neurons in organotypic cultures of rat spinal cord. In order to avoid the problems related to the analysis of currents of unknown origin within a neuron, we chose to examine the functional properties of single identified synapses. Iontophoretic mapping of the cell surface revealed hot spots of high glutamate sensitivity coincident with presynaptic boutons stained with the dye FM 1–43. Local application of KCl to these sites caused bursts of synaptic release. Hot spots typically consisted of 330 receptors with an average single-channel conductance of 8.3 pS. Evoked synaptic currents involved only about 40–50 receptors and nevertheless showed characteristics of saturation. This suggests that glutamate receptor clusters at sites of presynaptic terminals are organized into well separated subclusters opposite release sites.This award-winning article is published as received and has not been subjected to the normal peer review process     

Retrograde activation of presynaptic NMDA receptors enhances GABA release at cerebellar interneuron-Purkinje cell synapses

Synaptic inhibition is a vital component in the control of cell excitability within the brain. Here we report a newly identified form of inhibitory synaptic plasticity, termed depolarization-induced potentiation of inhibition, in rodents. This mechanism strongly potentiated synaptic transmission from interneurons to Purkinje cells after the termination of depolarization-induced suppression of inhibition. It was triggered by an elevation of Ca(2+) in Purkinje cells and the subsequent retrograde activation of presynaptic NMDA receptors. These glutamate receptors promoted the spontaneous release of Ca(2+) from presynaptic ryanodine-sensitive Ca(2+) stores. Thus, NMDA receptor-mediated facilitation of transmission at this synapse provides a regulatory mechanism that can dynamically alter the synaptic efficacy at inhibitory synapses.     

Presynaptic transmitter release is specified by postsynaptic neurons of Aplysia buccal ganglia.

1. In Aplysia buccal ganglia, in which dual presynaptic neurons innervate multiple postsynaptic cells, strengths of the same identified synapses differ from animal to animal, consistent with developmental or plastic modulation. Synaptic strengths are specified by the postsynaptic neuron, so that synaptic current amplitudes are similar for inputs from different presynaptic cells converging on a postsynaptic cell but different for branches of the same neuron diverging onto different targets. 2. The coefficient of variation method of quantal analysis reveals that differences in synaptic strength, although specified postsynaptically, result partially from differences in the number of quanta released by presynaptic terminals. 3. This quantization is consistent with classical presynaptic models and suggests retrograde modulation of quantal release as postulated for hippocampal long-term potentiation.     

Developmental changes in retrograde messengers involved in depolarization-induced suppression of excitation at parallel fiber-Purkinje cell synapses in rodents

At parallel fiber (PF) to Purkinje cell (PC) synapses, depolarization-induced suppression of excitation (DSE) and suppression of PF-excitatory postsynaptic currents (EPSCs) by activation of postsynaptic mGluR1 glutamate (Glu) receptors involve retrograde release of endocannabinoids. However, Levenes et al. suggested instead that Glu was the retrograde messenger in this latter case. Because the study by Levenes et al. was performed in nearly mature rats, whereas most others were performed in juvenile animals, DSE was re-investigated in juvenile versus nearly mature rats and mice. Indeed, DSE was preferred here to agonist-induced suppression of PF-EPSCs, to avoid possible indirect effects in this latter case. In 10- to 12-day-old rats, DSE of PF-EPSCs was entirely mediated through retrograde release of endocannabinoids. In 18- to 22-day-old-rats, DSE was partly resistant to CB1 cannabinoid receptor antagonists. The remaining component was potentiated by the Glu uptake inhibitor d-threo-beta-benzyloxyaspartate (d-TBOA) and blocked by the desensitizing kainate (KA) receptor agonist (2S,4R)-4-methylglutamic acid (SYM 2081). This SYM-2081-sensitive component of DSE was accompanied by a paired-pulse facilitation increase that was also potentiated by d-TBOA and blocked by SYM 2081. In nearly mature wild-type and GluR6 -/- mice, results fully confirmed the presence of an endocannabinoid-independent component of DSE that involves retrograde release of Glu and activation of presynaptic KA receptors including GluR6 receptor subunits. Therefore retrograde release of Glu by PCs participates to DSE at PF-PC synapses in nearly mature rodents but not in juvenile ones, and Glu probably operates through activation of presynaptic KA receptors that include GluR6 receptor subunits.     

Pathway-specific use-dependent dynamics of excitatory synaptic transmission in rat intracortical circuits

Information processing in neuronal networks is determined by the use-dependent dynamics of synaptic transmission. Here we characterize the dynamic properties of excitatory synaptic transmission in two major intracortical pathways that target the output neurons of the neocortex, by recording unitary EPSPs from layer 5 pyramidal neurons evoked in response to action potential trains of increasing complexity in presynaptic layer 2/3 or layer 5 pyramidal neurons. We find that layer 2/3 to layer 5 synaptic transmission is dominated by frequency-dependent depression when generated at fixed frequencies of > 10 Hz. Synaptic depression evolved on a spike-by-spike basis in response to action potential trains that possessed a broad range of interspike intervals, but a low mean frequency (10 Hz). Layer 2/3 to layer 2/3 and layer 2/3 to layer 5 synapses were incapable of sustained release during prolonged, complex trains of presynaptic action potential firing (mean frequency, 48 Hz). By contrast, layer 5 to layer 5 synapses operated effectively across a wide range of frequencies, exhibiting increased efficacy at frequencies > 10 Hz. Furthermore, layer 5 to layer 5 synapses sustained release throughout the duration of prolonged, complex spike trains. The use-dependent properties of synaptic transmission could be modulated by pharmacologically changing the probability of release and by induction of long-term depression. The dynamic properties of intracortical excitatory synapses are therefore pathway-specific. We suggest that the synaptic output of layer 5 pyramidal neurons is ideally suited to control the neocortical network across a wide range of frequencies and for sustained periods of time, a behaviour that helps to explain the pivotal role played by layer 5 neurons in the genesis of periods of network 'up' states and epileptiform activity in the neocortex.     

Retrograde signaling in the regulation of synaptic transmission: focus on endocannabinoids

This review covers recent developments in the cellular neurophysiology of retrograde signaling in the mammalian central nervous system. Normally at a chemical synapse a neurotransmitter is released from the presynaptic element and diffuses to the postsynaptic element, where it binds to and activates receptors. In retrograde signaling a diffusible messenger is liberated from the postsynaptic element, and travels "backwards" across the synaptic cleft, where it activates receptors on the presynaptic cell. Receptors for retrograde messengers are usually located on or near the presynaptic nerve terminals, and their activation causes an alteration in synaptic transmitter release. Although often considered in the context of long-term synaptic plasticity, retrograde messengers have numerous roles on the short-term regulation of synaptic transmission. The focus of this review will be on a group of molecules from different chemical classes that appear to act as retrograde messengers. The evidence supporting their candidacy as retrograde messengers is considered and evaluated. Endocannabinoids have recently emerged as one of the most thoroughly investigated, and widely accepted, classes of retrograde messenger in the brain. The study of the endocannabinoids can therefore serve as a model for the investigation of other putative messengers, and most attention is devoted to a discussion of systems that use these new messenger molecules.     

大鼠脊髓背角的GABA能神经末梢来源及其突触联系──HRP追踪与免疫细胞化学结合法研究及免疫电镜观察

本文用ＨＲＰ追踪与免疫细胞化学结合法和免疫电镜技术研究了脊髓背角的ＧＡＢＡ神经元的分布、ＧＡＢＡ能末梢的来源及其超微结构联系。结果表明：在脊髓背角Ⅰ～Ⅵ层内均有ＧＡＢＡ神经元胞体和纤维分布，其中Ⅰ～Ⅲ层较为密集，在后外侧束内也存在ＧＡＢＡ能纤维及胞体。脊髓背角的ＧＡＢＡ能神经末梢有３个来源：①延髓的大缝核、隐缝核、苍白缝核及腹侧网状结构的ＧＡＢＡ能神经元；②脊髓固有的ＧＡＢＡ能神经元；③脊神经节的ＧＡＢＡ能神经元。ＧＡＢＡ能末梢可作为突触前成分或突触后成分与未标记末梢形成轴－树突触，也可同时作为突触前、后成分而形成轴－树型自调节突触。结果提示突触前的ＧＡＢＡ能末梢可能对脊髓背角内的其它神经元起抑制和脱抑制作用；同时背角内ＧＡＢＡ能神经元还接受其它神经元的调控。     

Heterogeneity of Postsynaptic Receptor Occupancy Fluctuations among Glycinergic Inhibitory Synapses in the Zebrafish Hindbrain

The amplitude of glycinergic miniature inhibitory postsynaptic currents (mIPSCs) varies considerably in neurons recorded in the isolated hindbrain of 50-h-old zebrafish larvae. At this age, glycinergic synapses are functionally mature. In order to measure the occupancy level of postsynaptic glycine receptors (GlyRs) and to determine the pre- and/or postsynaptic origin of its variability, we analysed mIPSCs within bursts evoked by α-latrotoxin (0.1–1 n m ). Two types of burst were observed according to their mIPSC frequencies: 'slow' bursts with clearly spaced mIPSCs and 'fast' bursts characterised by superimposed events. Non-stationary noise analysis of mIPSCs in some 'slow' bursts recorded in the presence or in the absence of Ca²⁺ denoted that mIPSC amplitude variance did not depend on the quantity of neurotransmitters released (presynaptic origin), but rather on intrinsic stochastic behaviour of the same group of GlyRs (postsynaptic origin). In these bursts, the open probability measured at the peak of the mIPSCs was close to 0.5 while the maximum open probability is close to 0.9 for the synaptic isoform of GlyRs (heteromeric α1/β GlyRs). In 'fast' bursts with superimposed events, a correlation was found between the amplitude of mIPSCs and the basal current level measured at their onset, which could suggest that the same group of GlyRs is activated during such bursts. Altogether, our results indicate that glycine synapses can display different release modes in the presence of α-latrotoxin. They also indicate that, in our model, postsynaptic GlyRs cannot be saturated by the release of a single vesicle.     

GABA(B) receptors couple to potassium and calcium channels on identified lateral perforant pathway projection neurons

Activation of presynaptic GABA(B) receptors inhibits neurotransmitter release at most cortical synapses, at least in part because of inhibition of voltage-gated calcium channels. One synapse where this is not the case is the lateral perforant pathway synapse onto dentate granule cells in the hippocampus. The current study was conducted to determine whether the neurons that make these synapses express GABA(B) receptors that can couple to ion channels. Perforant pathway projection neurons were labeled by injecting retrograde tracer into the dorsal hippocampus. The GABA(B) receptor agonist baclofen (10 microM) activated inwardly rectifying potassium channels and inhibited currents mediated by voltage-gated calcium channels in retrogradely labeled neurons in layer II of the lateral entorhinal cortex. These effects were reversed by coapplication of the selective GABA(B) receptor antagonist CGP 55845A (1 microM). Equivalent effects were produced by 100 microM adenosine, which inhibits neurotransmitter release at lateral perforant pathway synapses. The effects of baclofen and adenosine on inward currents were largely occlusive. These results suggest that the absence of GABA(B) receptor-mediated presynaptic inhibition at lateral perforant pathway synapses is not simply due to a failure to express these receptors and imply that GABA(B) receptors can either be selectively localized or regulated at terminal versus somatodendritic domains.     

Developmental changes in agonist-induced retrograde signaling at parallel fiber-Purkinje cell synapses: role of calcium-induced calcium release

In cerebellar Purkinje cells (PCs), activation of postsynaptic mGluR1 receptors inhibits parallel fiber (PF) to PC synaptic transmission by retrograde signaling. However, results were conflicting with respect to whether endocannabinoids or glutamate (Glu) is the retrograde messenger involved. Experiments in cerebellar slices from 10- to 12-day-old rats and mice confirmed that suppression of PF-excitatory postsynaptic currents (EPSCs) by mGluR1 agonists was entirely blocked by cannabinoid receptor antagonists at this early developmental stage. In contrast, suppression of PF-EPSCs by mGluR1 agonists was only partly blocked by cannabinoid receptor antagonists in 18- to 22-day-old rats, and the remaining suppression was accompanied by an increase in paired-pulse facilitation. This endocannnabinoidindependent suppression of PF-EPSCs was potentiated by the Glu uptake inhibitor D-threo-beta-benzyloxyaspartate (D-TBOA) and blocked by the desensitizing kainate (KA) receptors agonist SYM 2081, by nonsaturating concentrations of 6-cyano-7-nitroquinoxaline-2-3-dione (CNQX) [but not by GYKI 52466 hydrochloride (GYKI)] and by dialyzing PCs with guanosine 5'-[beta-thio]diphosphate (GDP-betaS). An endocannnabinoid-independent suppression of PF-EPSCs was also present in nearly mature wild-type mice but was absent in GluR6(-/-) mice. The endocannnabinoid-independent suppression of PF-EPSCs induced by mGluR1 agonists and the KA-dependent component of depolarization-induced suppression of excitation (DSE) were blocked by ryanodine acting at a presynaptic level. We conclude that retrograde release of Glu by PCs participates in mGluR1 agonist-induced suppression of PF-EPSCs at nearly mature PF-PC synapses and that Glu operates through activation of presynaptic KA receptors located on PFs and prolonged release of calcium from presynaptic internal calcium stores.     

Frequency-dependent synaptic depression and the balance of excitation and inhibition in the neocortex

The stability of cortical neuron activity in vivo suggests that the firing rates of both excitatory and inhibitory neurons are dynamically adjusted. Using dual recordings from excitatory pyramidal neurons and inhibitory fast-spiking neurons in neocortical slices, we report that sustained activation by trains of several hundred presynaptic spikes resulted in much stronger depression of synaptic currents at excitatory synapses than at inhibitory ones. The steady-state synaptic depression was frequency dependent and reflected presynaptic function. These results suggest that inhibitory terminals of fast-spiking cells are better equipped to support prolonged transmitter release at a high frequency compared with excitatory ones. This difference in frequency-dependent depression could produce a relative increase in the impact of inhibition during periods of high global activity and promote the stability of cortical circuits.     

NMDA receptors induce somatodendritic secretion in hypothalamic neurones of lactating female rats

Many neurones in the mammalian brain are known to release the content of their vesicles from somatodendritic locations. These vesicles usually contain retrograde messengers that modulate network properties. The back-propagating action potential is thought to be the principal physiological stimulus that evokes somatodendritic release. In contrast, here we show that calcium influx through NMDA receptor (NMDAR) channels, in the absence of postsynaptic cell firing, is also able to induce vesicle fusion from non-synaptic sites in nucleated outside-out patches of dorsomedial supraoptic nucleus (SON) neurones of adult female rats, in particular during their reproductive stages. The physiological significance of this mechanism was characterized in intact brain slices, where NMDAR-mediated release of oxytocin was shown to retrogradely inhibit presynaptic GABA release, in the absence of postsynaptic cell firing. This implies that glutamatergic synaptic input in itself is sufficient to elicit the release of oxytocin, which in turn acts as a retrograde messenger leading to the depression of nearby GABA synapses. In addition, we found that during lactation, when oxytocin demand is high, NMDA-induced oxytocin release is up-regulated compared to that in non-reproductive rats. Thus, in the hypothalamus, local signalling back and forth between pre- and postsynaptic compartments and between different synapses may occur independently of the firing activity of the postsynaptic neurone.