Subtype-specific expression of group III metabotropic glutamate receptors and Ca2+ channels in single nerve terminals

The release properties of glutamatergic nerve terminals are influenced by a number of factors, including the subtype of voltage-dependent calcium channel and the presence of presynaptic autoreceptors. Group III metabotropic glutamate receptors (mGluRs) mediate feedback inhibition of glutamate release by inhibiting Ca(2+) channel activity. By imaging Ca(2+) in preparations of cerebrocortical nerve terminals, we show that voltage-dependent Ca(2+) channels are distributed in a heterogeneous manner in individual nerve terminals. Presynaptic terminals contained only N-type (47.5%; conotoxin GVIA-sensitive), P/Q-type (3.9%; agatoxin IVA-sensitive), or both N- and P/Q-type (42.6%) Ca(2+) channels, although the remainder of the terminals (6.1%) were insensitive to these two toxins. In this preparation, two mGluRs with high and low affinity for l(+)-2-amino-4-phosphonobutyrate were identified by immunocytochemistry as mGluR4 and mGluR7, respectively. These receptors were responsible for 22.2 and 24.1% reduction of glutamate release, and they reduced the Ca(2+) response in 24.4 and 30.3% of the nerve terminals, respectively. Interestingly, mGluR4 was largely (73.7%) located in nerve terminals expressing both N- and P/Q-type Ca(2+) channels, whereas mGluR7 was predominantly (69.9%) located in N-type Ca(2+) channel-expressing terminals. This specific coexpression of different group III mGluRs and Ca(2+) channels may endow synaptic terminals with distinct release properties and reveals the existence of a high degree of presynaptic heterogeneity.     

Facilitatory effect of aspirin on glutamate release from rat hippocampal nerve terminals: involvement of protein kinase C pathway

The effect of aspirin on glutamate release from isolated nerve terminals (synaptosomes) from rat hippocampus was examined. The Ca(2+)-dependent release of glutamate evoked by 4-aminopyridine (4AP) was facilitated by aspirin in a concentration-dependent manner, but the 4AP-evoked Ca(2+)-independent release was not modified. Also, aspirin-mediated facilitation of glutamate release was completely inhibited by bafilomycin A1, which depletes vesicle content by inhibiting the synaptic vesicle H(+)-ATPase that drives glutamate uptake, not by l-trans-pyrrolidine-2,4-dicarboxylic acid (l-trans-PDC), a excitatory amino acid (EAA) transporter inhibitor, suggesting that the facilitation of glutamate release produced by aspirin originates from synaptic vesicle exocytosis rather than reversal of the plasma membrane glutamate transporter. In addition, aspirin did not alter either 4AP-evoked depolarization of the synaptosomal plasma membrane potential or Ca(2+) ionophore ionomycin-induced glutamate release, but significantly increased in 4AP-evoked Ca(2+) influx. A possible effect of aspirin on synaptosomal Ca(2+) channels was confirmed in experiments where synaptosomes pretreated with a combination of the N- and P/Q-type Ca(2+) channel blockers, which abolished the aspirin-mediated facilitation of glutamate release. The facilitatory action by aspirin observed in glutamate release was mimicked and occluded by arachidonic acid (AA) and eicosatetraynoic acid (ETYA), an analogue of AA that mimics the effect of AA but cannot be metabolized. Furthermore, this aspirin-mediated facilitation of glutamate release may depend on activation of protein kinase C (PKC), because PKC activator and PKC inhibitor, respectively, superseding or suppressing the facilitatory effect of aspirin. Together, these results suggest that aspirin exerts their presynaptic facilitatory effect, likely through AA directly to induce the activation of PKC, which subsequently enhances the Ca(2+) influx through voltage-dependent N- and P/Q-type Ca(2+) channels to cause an increase in evoked glutamate release from rat hippocampal nerve terminals.     

Regulation of presynaptic Ca(V)2.1 channels by Ca2+ sensor proteins mediates short-term synaptic plasticity

Short-term synaptic plasticity shapes the postsynaptic response to bursts of impulses and is crucial for encoding information in neurons, but the molecular mechanisms are unknown. Here we show that activity-dependent modulation of presynaptic Ca(V)2.1 channels mediated by neuronal Ca(2+) sensor proteins (CaS) induces synaptic plasticity in cultured superior cervical ganglion (SCG) neurons. A mutation of the IQ-like motif in the C terminus that blocks Ca(2+)/CaS-dependent facilitation of the P/Q-type Ca(2+) current markedly reduces facilitation of synaptic transmission. Deletion of the nearby calmodulin-binding domain, which inhibits CaS-dependent inactivation, substantially reduces depression of synaptic transmission. These results demonstrate that residual Ca(2+) in presynaptic terminals can act through CaS-dependent regulation of Ca(V)2.1 channels to induce short-term synaptic facilitation and rapid synaptic depression. Activity-dependent regulation of presynaptic Ca(V)2.1 channels by CaS proteins may therefore be a primary determinant of short-term synaptic plasticity and information-processing in the nervous system.     

Calmidazolium selectively inhibits exocytotic glutamate release evoked by P2X7 receptor activation

We previously observed that activation of presynaptic P2X7 receptors located on rat cerebrocortical nerve terminals induced the release of glutamate through different modes: the channel conformation allowing Ca(2+) entry triggered exocytotic release, while the receptor itself functioned as a permeation pathway for the non-exocytotic glutamate release. Considering that exocytotic and non-exocytotic glutamate release evoked by the activation of P2X7 receptors might play a role in the control of glutamatergic synapses, we investigated whether calmidazolium (which has been found to inhibit small cation currents through recombinant P2X7 receptors, but not organic molecule permeation) could distinguish between P2X7-related exocytotic and non-exocytotic modes of glutamate release. We found that calmidazolium inhibited the intrasynaptosomal Ca(2+) response to P2X7 receptor activation and the Ca(2+)-dependent exocytotic glutamate release from rat cerebrocortical nerve terminals, but was ineffective against the Ca(2+)-independent glutamate release. The P2X7 competitive antagonist A-438079 eliminated both exocytotic and non-exocytotic P2X7 receptor-evoked glutamate release. Selective inhibition of exocytotic glutamate release indicates that calmidazolium inhibits events dependent on the function of native rat P2X7 receptors as Ca(2+) channels, and suggests that it can be used as a tool to dissociate P2X7-evoked exocytotic from non-exocytotic glutamate release.     

Regulation of neurotransmitter release by metabotropic glutamate receptors

The G protein-coupled metabotropic glutamate (mGlu) receptors are differentially localized at various synapses throughout the brain. Depending on the receptor subtype, they appear to be localized at presynaptic and/or postsynaptic sites, including glial as well as neuronal elements. The heterogeneous distribution of these receptors on glutamate and nonglutamate neurons/cells thus allows modulation of synaptic transmission by a number of different mechanisms. Electrophysiological studies have demonstrated that the activation of mGlu receptors can modulate the activity of Ca(2+) or K(+) channels, or interfere with release processes downstream of Ca(2+) entry, and consequently regulate neuronal synaptic activity. Such changes evoked by mGlu receptors can ultimately regulate transmitter release at both glutamatergic and nonglutamatergic synapses. Increasing neurochemical evidence has emerged, obtained from in vitro and in vivo studies, showing modulation of the release of a variety of transmitters by mGlu receptors. This review addresses the neurochemical evidence for mGlu receptor-mediated regulation of neurotransmitters, such as excitatory and inhibitory amino acids, monoamines, and neuropeptides.     

Enzyme-linked sensitive fluorometric imaging of glutamate release from cerebral neurons of chick embryos

This paper describes a method for imaging the endogenous release of glutamate from cerebral neurons. This method is based on the reactions of glutamate oxidase and peroxidase, and on the detection of hydrogen peroxide by a fluorescent substrate of peroxidase. Glutamate has been sensitively measured in vitro in the range of 20 nM to 1 microM. We used two types of Ca(2+) channel inhibitors, MK-801 and omega-Conotoxin GVIA, which act to suppress Ca(2+) transport at postsynaptic and presynaptic neurons, respectively. MK-801 did not inhibit the increase in glutamate release after KCl stimulation, while there was no increase in glutamate release after KCl stimulation when omega-Conotoxin GVIA was used, probably due to the inhibition of voltage-activated Ca(2+) channels in the presynapse. Glutamate release and Ca(2+) flow in the synaptic regions were imaged using a laser confocal fluorescence microscope. KCl-evoked glutamate release was localized around cell bodies linked to axon terminals. This procedure allows imaging that can be sensitively detected by the fluorometric enzymatic assay of endogenous glutamate release in synapses.     

Inhibition of N- and P/Q-type Ca2+ channels by cannabinoid receptors in single cerebrocortical nerve terminals

Since cannabinoid receptors inhibit excitatory synaptic transmission by reducing glutamate release, we have examined whether this might occur through the direct inhibition of presynaptic Ca2+ channels. In cerebrocortical nerve terminals, activation of cannabinoid receptors with WIN55,212-2 reduces the KCl-evoked release of glutamate. However, this inhibition is attenuated when N- and P/Q-type Ca2+ channels are blocked. Through Ca2+ imaging in single nerve terminals, we found that WIN55,212-2 reduced the influx of Ca2+ both in nerve terminals that contain N-type Ca2+ channels and those that contain P/Q-type Ca2+ channels. Thus, cannabinoid receptors modulate the two major Ca2+ channels coupled to glutamate release in the cerebral cortex.     

Long-term potentiation of excitatory inputs to brain reward areas by nicotine

Nicotine reinforces smoking behavior by activating nicotinic acetylcholine receptors (nAChRs) in the midbrain dopaminergic (DA) reward centers, including the ventral tegmental area (VTA). Although nicotine induces prolonged excitation of the VTA in vivo, the nAChRs on the DA neurons desensitize in seconds. Here, we show that activation of nAChRs on presynaptic terminals in the VTA enhances glutamatergic inputs to DA neurons. Under conditions where the released glutamate can activate NMDA receptors, long-term potentiation (LTP) of the excitatory inputs is induced. Both the short- and the long-term effects of nicotine required activation of presynaptic alpha7 subunit-containing nAChRs. These results can explain the long-term excitation of brain reward areas induced by a brief nicotine exposure. They also show that nicotine alters synaptic function through mechanisms that are linked to learning and memory.     

TRPV1 marks synaptic segregation of multiple convergent afferents at the rat medial solitary tract nucleus

TRPV1 receptors are expressed on most but not all central terminals of cranial visceral afferents in the caudal solitary tract nucleus (NTS). TRPV1 is associated with unmyelinated C-fiber afferents. Both TRPV1+ and TRPV1- afferents enter NTS but their precise organization remains poorly understood. In horizontal brainstem slices, we activated solitary tract (ST) afferents and recorded ST-evoked glutamatergic excitatory synaptic currents (ST-EPSCs) under whole cell voltage clamp conditions from neurons of the medial subnucleus. Electrical shocks to the ST produced fixed latency EPSCs (jitter<200 μs) that identified direct ST afferent innervation. Graded increases in shock intensity often recruited more than one ST afferent and ST-EPSCs had consistent threshold intensity, latency to onset, and unique EPSC waveforms that characterized each unitary ST afferent contact. The TRPV1 agonist capsaicin (100 nM) blocked the evoked TRPV1+ ST-EPSCs and defined them as either TRPV1+ or TRPV1- inputs. No partial responses to capsaicin were observed so that in NTS neurons that received one or multiple (2-5) direct ST afferent inputs--all were either blocked by capsaicin or were unaltered. Since TRPV1 mediates asynchronous release following TRPV1+ ST-evoked EPSCs, we likewise found that recruiting more than one ST afferent further augmented the asynchronous response and was eliminated by capsaicin. Thus, TRPV1+ and TRPV1- afferents are completely segregated to separate NTS neurons. As a result, the TRPV1 receptor augments glutamate release only within unmyelinated afferent pathways in caudal medial NTS and our work indicates a complete separation of C-type from A-type afferent information at these first central neurons.     

The physiological role of kainate receptors in the amygdala

The kainate subtype of glutamate receptors has received considerable attention in recent years, and a wealth of knowledge has been obtained regarding the function of these receptors. Kainate receptors have been shown to mediate synaptic transmission in some brain regions, modulate presynaptic release of glutamate and gamma-aminobutyric acid (GABA), and mediate synaptic plasticity or the development of seizure activity. This article focuses on the function of kainate receptors in the amygdala, a brain region that plays a central role in emotional behavior and certain psychiatric illnesses. Evidence is reviewed indicating that postsynaptic kainate receptors containing the glutamate receptor 5 kainate receptor (GLUk5) subunit are present on interneurons and pyramidal cells in the basolateral amygdala and mediate a component of the synaptic responses of these neurons to glutamatergic input. In addition, GLUk5-containing kainate receptors are present on presynaptic terminals of GABAergic neurons, where they modulate the release of GABA in an agonist concentration-dependent, bidirectional manner. GLUk5-containing kainate receptors also mediate a longlasting synaptic facilitation induced by low-frequency stimulation in the external capsule to the basolateral nucleus pathway, and they appear to be partly responsible for the susceptibility of the amygdala to epileptogenesis. Taken together, these findings have suggested a prominent role of GLUk5-containing kainate receptors in the regulation of neuronal excitability in the amygdala.     

Glucose increases synaptic transmission from vagal afferent central nerve terminals via modulation of 5-HT3 receptors

Acute hyperglycemia has profound effects on vagally mediated gastrointestinal functions. We have reported recently that the release of glutamate from the central terminals of vagal afferent neurons is correlated directly with the extracellular glucose concentration. The present study was designed to test the hypothesis that 5-HT(3) receptors present on vagal afferent nerve terminals are involved in this glucose-dependent modulation of glutamatergic synaptic transmission. Whole-cell patch-clamp recordings were made from neurons of the nucleus tractus solitarius (NTS) in thin rat brainstem slices. Spontaneous and evoked glutamate release was decreased in a concentration-dependent manner by the 5-HT(3) receptor selective antagonist, ondansetron. Alterations in the extracellular glucose concentration induced parallel shifts in the ondansetron-mediated inhibition of glutamate release. The changes in excitatory synaptic transmission induced by extracellular glucose concentration were mimicked by the serotonin uptake inhibitor, fenfluramine. These data suggest that glucose alters excitatory synaptic transmission within the rat brainstem via actions on tonically active 5-HT(3) receptors, and the number of 5-HT(3) receptors on vagal afferent nerve terminals is positively correlated with the extracellular glucose concentration. These data indicate that the 5-HT(3) receptors present on synaptic connections between vagal afferent nerve terminals and NTS neurons are a strong candidate for consideration as one of the sites where glucose acts to modulate vagovagal reflexes.     

A metabotropic glutamate receptor regulates transmitter release from cone presynaptic terminals in carp retinal slices.

The role of group III metabotropic glutamate receptors (mGluRs) in photoreceptor-H1 horizontal cell (HC) synaptic transmission was investigated by analyzing the rate of occurrence and amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) in H1 HCs uncoupled by dopamine in carp retinal slices. Red light steps or the application of 100 microM cobalt reduced the sEPSC rate without affecting their peak amplitude, which is consistent with hyperpolarization or the suppression of Ca(2+) entry into cone synaptic terminals reducing vesicular transmitter release. Conversely, postsynaptic blockade of H1 HC AMPA receptors by 500 nM CNQX reduced the amplitude of sEPSCs without affecting their rate. This analysis of sEPSCs represents a novel methodology for distinguishing between presynaptic and postsynaptic sites of action. The selective agonist for group III mGluRs, l-2-amino-4-phosphonobutyrate (L-APB or L-AP4; 20 microM), reduced the sEPSC rate with a slight reduction in amplitude, which is consistent with a presynaptic action on cone synaptic terminals to reduce transmitter release. During L-APB application, recovery of sEPSC rate occurred with 500 microM (s)-2-methyl-2-amino-4-phosphonobutyrate (MAP4), a selective antagonist of group III mGluR, and with 200 microM 4-aminopyridine (4-AP), a blocker of voltage-dependent potassium channels. Whole-cell recordings from cones in the retinal slice showed no effect of L-APB on voltage-activated Ca(2+) conductance. These results suggest that the activation of group III mGluRs suppresses transmitter release from cone presynaptic terminals via a 4-AP-sensitive pathway. Negative feedback, operating via mGluR autoreceptors, may limit excessive glutamate release from cone synaptic terminals.     

Ca2+-dependent binding of calcium-binding protein 1 to presynaptic group III metabotropic glutamate receptors and blockage by phosphorylation of the receptors

Presynaptic group III metabotropic glutamate receptors (mGluRs) and Ca(2+) channels are the main neuronal activity-dependent regulators of synaptic vesicle release, and they use common molecules in their signaling cascades. Among these, calmodulin (CaM) and the related EF-hand Ca(2+)-binding proteins are of particular importance as sensors of presynaptic Ca(2+), and a multiple of them are indeed utilized in the signaling of Ca(2+) channels. However, despite its conserved structure, CaM is the only known EF-hand Ca(2+)-binding protein for signaling by presynaptic group III mGluRs. Because the mGluRs and Ca(2+) channels reciprocally regulate each other and functionally converge on the regulation of synaptic vesicle release, the mGluRs would be expected to utilize more EF-hand Ca(2+)-binding proteins in their signaling. Here I show that calcium-binding protein 1 (CaBP1) bound to presynaptic group III mGluRs competitively with CaM in a Ca(2+)-dependent manner and that this binding was blocked by protein kinase C (PKC)-mediated phosphorylation of these receptors. As previously shown for CaM, these results indicate the importance of CaBP1 in signal cross talk at presynaptic group III mGluRs, which includes many molecules such as cAMP, Ca(2+), PKC, G protein, and Munc18-1. However, because the functional diversity of EF-hand calcium-binding proteins is extraordinary, as exemplified by the regulation of Ca(2+) channels, CaBP1 would provide a distinct way by which presynaptic group III mGluRs fine-tune synaptic transmission.     

Presynaptic mechanisms underlying the alpha-lipoic acid facilitation of glutamate exocytosis in rat cerebral cortex nerve terminals

The antioxidant alpha-lipoic acid has been reported to prevent and reverse age-related impairments in learning and memory. However, it is unclear how alpha-lipoic acid improves cognitive function. In this study, the effect of alpha-lipoic acid on the release of endogenous glutamate from rat cerebrocortical nerve terminals (synaptosomes) was examined. We found that alpha-lipoic acid potently facilitated 4-aminopyridine (4AP)-evoked glutamate release, and this release facilitation results from an enhancement of vesicular exocytosis and not from an increase of non-vesicular release. Examination of the effect of alpha-lipoic acid on cytosolic [Ca(2+)] revealed that the facilitation of glutamate release was associated with an increase in voltage-dependent Ca(2+) influx. Consistent with this, alpha-lipoic acid-mediated facilitation of glutamate release was completely prevented in synaptosomes pretreated with a wide spectrum blocker of the N- and P/Q-type Ca(2+) channels, omega-conotoxin MVIIC. The facilitatory effect of alpha-lipoic acid on Ca(2+) influx was not due to an increase of synaptosomal excitability because alpha-lipoic acid did not alter the 4AP-evoked depolarization of the synaptosomal plasma membrane potential. In addition, both ionomycin and hypertonic sucrose-induced glutamate release were enhanced by alpha-lipoic acid. Furthermore, disruption of cytoskeleton organization with cytochalasin D occluded the facilitatory effect of alpha-lipoic acid on 4AP or ionomycin-evoked glutamate release. These results suggest that the antioxidant alpha-lipoic acid enhances the Ca(2+) entry through presynaptic N- and P/Q-type Ca(2+) channels as well as the vesicular release machinery to cause an increase in evoked glutamate release from rat cerebrocortical synaptosomes. Also, activation of PKA and PKC may underlie, at least in part, the alpha-lipoic acid-mediated facilitation of glutamate release observed here as alpha-lipoic acid-enhanced 4AP and ionomycin-evoked glutamate release were significantly attenuated by PKA and PKC inhibitors. This finding may provide some information regarding the mechanism of action of alpha-lipoic acid in the central nervous system (CNS).     

Enhanced NMDA Receptor-Mediated Modulation of Excitatory Neurotransmission in the Dorsal Vagal Complex of Streptozotocin-Treated,Chronically Hyperglycemic Mice

A variety of metabolic disorders, including complications experienced by diabetic patients, have been linked to altered neural activity in the dorsal vagal complex. This study tested the hypothesis that augmentation of N-Methyl-D-Aspartate (NMDA) receptor-mediated responses in the vagal complex contributes to increased glutamate release in the dorsal motor nucleus of the vagus nerve (DMV) in mice with streptozotocin-induced chronic hyperglycemia (i.e., hyperglycemic mice), a model of type 1 diabetes. Antagonism of NMDA receptors with AP-5 (100 μM) suppressed sEPSC frequency in vagal motor neurons recorded in vitro, confirming that constitutively active NMDA receptors regulate glutamate release in the DMV. There was a greater relative effect of NMDA receptor antagonism in hyperglycemic mice, suggesting that augmented NMDA effects occur in neurons presynaptic to the DMV. Effects of NMDA receptor blockade on mEPSC frequency were equivalent in control and diabetic mice, suggesting that differential effects on glutamate release were due to altered NMDA function in the soma-dendritic membrane of intact afferent neurons. Application of NMDA (300 μM) resulted in greater inward current and current density in NTS neurons recorded from hyperglycemic than control mice, particularly in glutamatergic NTS neurons identified by single-cell RT-PCR for VGLUT2. Overall expression of NR1 protein and message in the dorsal vagal complex were not different between the two groups. Enhanced postsynaptic NMDA responsiveness of glutamatergic NTS neurons is consistent with tonically-increased glutamate release in the DMV in mice with chronic hyperglycemia. Functional augmentation of NMDA-mediated responses may serve as a physiological counter-regulatory mechanism to control pathological disturbances of homeostatic autonomic function in type 1 diabetes.     

Boosting of synaptic potentials and spine Ca transients by the peptide toxin SNX-482 requires alpha-1E-encoded voltage-gated Ca channels

The majority of glutamatergic synapses formed onto principal neurons of the mammalian central nervous system are associated with dendritic spines. Spines are tiny protuberances that house the proteins that mediate the response of the postsynaptic cell to the presynaptic release of glutamate. Postsynaptic signals are regulated by an ion channel signaling cascade that is active in individual dendritic spines and involves voltage-gated calcium (Ca) channels, small conductance (SK)-type Ca-activated potassium channels, and NMDA-type glutamate receptors. Pharmacological studies using the toxin SNX-482 indicated that the voltage-gated Ca channels that signal within spines to open SK channels belong to the class Ca(V)2.3, which is encoded by the Alpha-1E pore-forming subunit. In order to specifically test this conclusion, we examined the effects of SNX-482 on synaptic signals in acute hippocampal slices from knock-out mice lacking the Alpha-1E gene. We find that in these mice, application of SNX-482 has no effect on glutamate-uncaging evoked synaptic potentials and Ca influx, indicating that that SNX-482 indeed acts via the Alpha-1E-encoded Ca(V)2.3 channel.     

Role of Ca(2+) channels in short-term synaptic plasticity

Repetitive nerve activity induces various forms of short-term synaptic plasticity that have important computational roles in neuronal networks. Several forms of short-term plasticity are caused largely by changes in transmitter release, but the mechanisms that underlie these changes in the release process have been difficult to address. Recent studies of a giant synapse - the calyx of Held - have shed new light on this issue. Recordings of Ca(2+) currents or Ca(2+) concentrations at nerve terminals reveal that regulation of presynaptic Ca(2+) channels has a significant role in three important forms of short-term plasticity: short-term depression, facilitation and post-tetanic potentiation.     

Brain-derived neurotrophic factor-induced potentiation of Ca(2+) oscillations in developing cortical neurons.

Brain-derived neurotrophic factor (BDNF) has been reported to exert an acute potentiation of synaptic activity. Here we examined the action of BDNF on synchronous spontaneous Ca(2+) oscillations in cultured cerebral cortical neurons prepared from postnatal 2-3-day-old rats. The synchronous spontaneous Ca(2+) oscillations began at approximately DIV 5. It was revealed that voltage-dependent Ca(2+) channels and ionotropic glutamate receptors were involved in the synchronous spontaneous oscillatory activity. BDNF potentiated the frequency of these oscillations. The BDNF-potentiated activity reached 207 +/- 20.1% of basal oscillatory activity. NT-3 and NT-4/5 also induced the potentiation. However, nerve growth factor did not. We examined the correlation between BDNF-induced glutamate release and the BDNF-potentiated oscillatory activity. Both up-regulation of phospholipase C-gamma (PLC-gamma) expression and the BDNF-induced glutamate release occurred at approximately DIV 5 when the BDNF-potentiated oscillations appeared. We confirmed that the BDNF-induced glutamate release occurred through a glutamate transporter that was dependent on the PLC-gamma/IP(3)/Ca(2+) pathway. Transporter inhibitors blocked the BDNF-potentiated oscillations, demonstrating that BDNF enhanced the glutamatergic transmissions in the developing cortical network by inducing glutamate release via a glutamate transporter.     

Non-additive potentiation of glutamate release by phorbol esters and metabotropic mGlu7 receptor in cerebrocortical nerve terminals

We recently showed that prolonged activation of metabotropic glutamate receptor 7 (mGlu7) potentiates glutamate release. This signalling involves phospholipase C activation via a pertussis toxin insensitive G protein and the subsequent hydrolysis of phosphatidylinositol (4,5)-bisphosphate. Release potentiation is independent of protein kinase C activation but it is dependent on the downstream release machinery, as reflected by the concomitant translocation of active zone Munc13-1 protein from the soluble to particulate fractions. Here we show that phorbol ester and mGlu7 receptor-dependent facilitation of neurotransmitter release is not additive, suggesting they share a common signalling mechanism. However, release potentiation is restricted to release sites that express N-type Ca(2+) channels, because phorbol ester and mGlu7 receptor-mediated release potentiation are absent in nerve terminals from mice lacking N-type Ca(2+) channels. In addition, phorbol esters but not mGlu7 receptors potentiate release at nerve terminals with P/Q-type Ca(2+) channels, although only under restricted conditions of Ca(2+) influx. The differential effect of phorbol esters at nerve terminals with either N- or P/Q-type Ca(2+) channels seems to be unrelated to the type Munc13 isoform expressed, and it is more likely dependent on other properties of the release machinery.     

M1 muscarinic receptors boost synaptic potentials and calcium influx in dendritic spines by inhibiting postsynaptic SK channels

Acetylcholine release and activation of muscarinic cholinergic receptors (mAChRs) enhance synaptic plasticity in?vitro and cognition and memory in?vivo. Within the hippocampus, mAChRs promote NMDA-type glutamate receptor-dependent forms of long-term potentiation. Here, we use calcium (Ca) imaging combined with two-photon laser glutamate uncaging at apical spines of CA1 pyramidal neurons to examine postsynaptic mechanisms of muscarinic modulation of glutamatergic transmission. Uncaging-evoked excitatory postsynaptic potentials and Ca transients are increased by muscarinic stimulation; however, this is not due to direct modulation of glutamate receptors. Instead, mAChRs modulate a negative feedback loop in spines that normally suppresses synaptic signals. mAChR activation reduces the Ca sensitivity of small conductance Ca-activated potassium (SK) channels that are found in the spine, resulting in increased synaptic potentials and Ca transients. These effects are mediated by M1-type muscarinic receptors and occur in a casein kinase-2-dependent manner. Thus, muscarinic modulation regulates synaptic transmission by tuning the activity of nonglutamatergic postsynaptic ion channels.