Retroinhibition of presynaptic Ca2+ currents by endocannabinoids released via postsynaptic mGluR activation at a calyx synapse.
ABSTRACT: We investigated the mechanisms by which activation of group I metabotropic glutamate receptors (mGluRs) and CB1 cannabinoid receptors (CB1Rs) leads to inhibition of synaptic currents at the calyx of Held synapse in the medial nucleus of the trapezoid body (MNTB) of the rat auditory brainstem. In approximately 50% of the MNTB neurons tested, activation of group I mGluRs by the specific agonist (s)-3,5-dihydroxyphenylglycine (DHPG) reversibly inhibited AMPA receptor- and NMDA receptor-mediated EPSCs to a similar extent and reduced paired-pulse depression, suggestive of an inhibition of glutamate release. Presynaptic voltage-clamp experiments revealed a reversible reduction of Ca2+ currents by DHPG, with no significant modification of the presynaptic action potential waveform. Likewise, in approximately 50% of the tested cells, the CB1 receptor agonist (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone (WIN) reversibly inhibited EPSCs, presynaptic Ca2+ currents, and exocytosis. For a given cell, the amount of inhibition by DHPG correlated with that by WIN. Moreover, the inhibitory action of DHPG was blocked by the CB1R antagonist N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251) and occluded by WIN, indicating that DHPG and WIN operate via a common pathway. The inhibition of EPSCs by DHPG, but not by WIN, was abolished after dialyzing 40 mm BAPTA into the postsynaptic cell, suggesting that DHPG activated postsynaptic mGluRs. Light and electron microscopy immunolabeling indicated a presynaptic expression of CB1Rs and postsynaptic localization of mGluR1a. Our data suggest that activation of postsynaptic mGluRs triggers the Ca2+-dependent release of endocannabinoids that activate CB1 receptors on the calyx terminal, which leads to a reduction of presynaptic Ca2+ current and glutamate release.
Project description:<h4>Background</h4>Hippocampal CA1 pyramidal neurons receive two excitatory glutamatergic synaptic inputs: their most distal dendritic regions in the stratum lacunosum-moleculare (SLM) are innervated by the perforant path (PP), originating from layer III of the entorhinal cortex, while their more proximal regions of the apical dendrites in the stratum radiatum (SR) are innervated by the Schaffer-collaterals (SC), originating from hippocampal CA3 neurons. Endocannabinoids (eCBs) are naturally occurring mediators capable of modulating both GABAergic and glutamatergic synaptic transmission and plasticity via the CB1 receptor. Previous work on eCB modulation of excitatory synapses in the CA1 region largely focuses on the SC pathway. However, little information is available on whether and how eCBs modulate glutamatergic synaptic transmission and plasticity at PP synapses.<h4>Methodology/principal findings</h4>By employing somatic and dendritic patch-clamp recordings, Ca(2+) uncaging, and immunostaining, we demonstrate that there are significant differences in low-frequency stimulation (LFS)- or DHPG-, an agonist of group I metabotropic glutamate receptors (mGluRs), induced long-term depression (LTD) of excitatory synaptic transmission between SC and PP synapses in the same pyramidal neurons. These differences are eliminated by pharmacological inhibition with selective CB1 receptor antagonists or genetic deletion of the CB1 receptor, indicating that these differences likely result from differential modulation via a CB1 receptor-dependent mechanism. We also revealed that depolarization-induced suppression of excitation (DSE), a form of short-term synaptic plasticity, and photolysis of caged Ca(2+)-induced suppression of Excitatory postsynaptic currents (EPSCs) were less at the PP than that at the SC. In addition, application of WIN55212 (WIN) induced a more pronounced inhibition of EPSCs at the SC when compared to that at the PP.<h4>Conclusions/significance</h4>Our results suggest that CB1 dependent LTD and DSE are differentially expressed at the PP versus SC synapses in the same neurons, which may have an impact on synaptic scaling, integration and plasticity of hippocampal CA1 pyramidal neurons.
Project description:Actions of endocannabinoids in the cerebellum can be demonstrated following distinct stimulation protocols in Purkinje cells. First, depolarization-induced elevations of intracellular Ca2+ lead to the suppression of neurotransmitter release from both inhibitory and excitatory afferents. In another case, postsynaptic group I metabotropic glutamate receptors (mGluRs) trigger a strong inhibition of the glutamatergic inputs from parallel and climbing fibers. Both pathways involve endocannabinoids retrogradely acting on type 1 cannabinoid receptors (CB1Rs) at presynaptic terminals. Here, we show that group I mGluR activation also depresses GABAergic transmission at the synapses between molecular layer interneurons and Purkinje cells. Using paired recordings, we found that application of the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine reduced the evoked IPSCs in Purkinje cells. This effect was independent of postsynaptic Ca2+ increases and was completely blocked by a CB1R antagonist. Experiments performed with the GTP-analogues GDP-betaS and GTP-gammaS provided evidence that endocannabinoids released after G-protein activation can also inhibit GABAergic inputs onto nearby, unstimulated Purkinje cells. Block of the enzymes DAG lipase or phospholipase C reduced the group I mGluR-dependent inhibition, suggesting that 2-arachidonyl glycerol could act as retrograde messenger. Finally, group I mGluR activation by brief bursts of activity of the parallel fibers induced a short-lived depression of spontaneous IPSCs via presynaptic CB1Rs. Our results reveal a mechanism with potential physiological importance, by which glutamatergic synapses induce an endocannabinoid-mediated inhibition of the GABAergic inputs onto Purkinje cells.
Project description:To investigate the role of M1 muscarininc acetylcholine receptors (m1 receptors) in metabotropic glutamate receptor (mGluR)-mediated long-term depression (LTD), we produced mouse lines in which deletion of the m1 gene is restricted to the forebrain (FB-m1KO) or hippocampal CA3 pyramidal neurons (CA3-m1KO). Stimulation in FB-m1KO hippocampal slices resulted in excitatory postsynaptic potentials and long-term synaptic plasticity (long-term potentiation and LTD) similar to controls. The mice were deficient in (S)-3,5-dihydroxyphenylglycine hydrate (DHPG)-induced mGluR LTD, which correlated with a presynaptic increase in the release of neurotransmitters. Protein kinase C (PKC) activity, which is downstream from both mGluRs and m1 receptors, was reduced in CA3 but not in CA1. The presynaptic requirement of m1 receptors was confirmed by the lack of DHPG-induced mGluR LTD in the CA1 of slices from CA3-m1KO mice. mGluR LTD was rescued by stimulating PKC activity pharmacologically in CA3-m1KO mice. These data confirm a role for PKC activation in presynaptic induction of mGluR LTD and distinguish between the roles of mGluRs and m1 receptors.
Project description:A central principle of synaptic transmission is that action potential-induced presynaptic neurotransmitter release occurs exclusively via Ca2+-dependent secretion (CDS). The discovery and mechanistic investigations of Ca2+-independent but voltage-dependent secretion (CiVDS) have demonstrated that the action potential per se is sufficient to trigger neurotransmission in the somata of primary sensory and sympathetic neurons in mammals. One key question remains, however, whether CiVDS contributes to central synaptic transmission. Here we report, in the central transmission from presynaptic (dorsal root ganglion) to postsynaptic (spinal dorsal horn) neurons in vitro, (1) excitatory postsynaptic currents (EPSCs) are mediated by glutamate transmission through both CiVDS (up to 87%) and CDS; (2) CiVDS-mediated EPSCs are independent of extracellular and intracellular Ca2+; (3) CiVDS is faster than CDS in vesicle recycling with much less short-term depression; (4) the fusion machinery of CiVDS includes Cav2.2 (voltage sensor) and SNARE (fusion pore). Together, an essential component of activity-induced EPSCs is mediated by CiVDS in a central synapse.
Project description:Calcium plays a crucial role as a ubiquitous second messenger and has a key influence in many forms of synaptic plasticity in neurons. The spatiotemporal properties of dendritic Ca2+ signals in hippocampal interneurons are relatively unexplored. Here we use two-photon calcium imaging and whole-cell recordings to study properties of dendritic Ca2+ signals mediated by different glutamate receptors and their regulation by synaptic activity in oriens/alveus (O/A) interneurons of rat hippocampus. We demonstrate that O/A interneurons express Ca2+-permeable AMPA receptors (CP-AMPARs) providing fast Ca2+ signals. O/A cells can also coexpress CP-AMPARs, Ca2+-impermeable AMPARs (CI-AMPARs), and group I/II metabotropic glutamate receptors (mGluRs) (including mGluR1a), in the same cell. CI-AMPARs are often associated with mGluRs, resulting in longer-lasting Ca2+ signals than CP-AMPAR-mediated responses. Finally, CP-AMPAR- and mGluR-mediated Ca2+ signals demonstrate distinct voltage dependence and are differentially regulated by presynaptic and postsynaptic activity: weak synaptic stimulation produces Ca2+ signals mediated by CP-AMPARs, whereas stronger stimulation, or weak stimulation coupled with postsynaptic depolarization, recruits Ca2+ signals mediated by mGluRs. Our results suggest that differential activation of specific glutamate receptor-mediated Ca2+ signals within spatially restricted dendritic microdomains may serve distinct signaling functions and endow oriens/alveus interneurons with multiple forms of Ca2+-mediated synaptic plasticity. Specific activation of mGluR-mediated Ca2+ signals by coincident presynaptic and postsynaptic activity fulfills the conditions for Hebbian pairing and likely underlies their important role in long-term potentiation induction at O/A interneuron synapses.
Project description:The N-palmitoylethanolamine (PEA) is an endogenous member of the endocannabinoid system (ECS) with several biological functions, including a neuromodulatory activity in the central nervous system. To shed light on the neuronal function of PEA, we investigated its involvement in the control of both excitatory and inhibitory transmission in the murine striatum, a brain region strongly modulated by the ECS. By means of electrophysiological recordings, we showed that PEA modulates inhibitory synaptic transmission, through activation of GPR55 receptors, promoting a transient increase of GABAergic spontaneous inhibitory postsynaptic current (sIPSC) frequency. The subsequently rundown effect on sIPSC frequency was secondary to the delayed stimulation of presynaptic cannabinoid CB1 receptors (CB1Rs) by the endocannabinoid 2-AG, whose synthesis was stimulated by PEA on postsynaptic neurons. Our results indicate that PEA, acting on GPR55, enhances GABA transmission in the striatum, and triggers a parallel synthesis of 2-AG at the postsynaptic site, that in turn acts in a retrograde manner to inhibit GABA release through the stimulation of presynaptic CB1Rs. This electrophysiological study identifies a previously unrecognized function of PEA and of GPR55, demonstrating that GABAergic transmission is under the control of this compound and revealing that PEA modulates the release of the endocannabinoid 2-AG.
Project description:EPSCs at the synapses of sensory receptors and of some CNS neurons include large events thought to represent the synchronous release of the neurotransmitter contained in several synaptic vesicles by a process known as multiquantal release. However, determination of the unitary, quantal size underlying such putatively multiquantal events has proven difficult at hair cell synapses, hindering confirmation that large EPSCs are in fact multiquantal. Here, we address this issue by performing presynaptic membrane capacitance measurements together with paired recordings at the ribbon synapses of adult hair cells. These simultaneous presynaptic and postsynaptic assays of exocytosis, together with electron microscopic estimates of single vesicle capacitance, allow us to estimate a single vesicle EPSC charge of approximately -45 fC, a value in close agreement with the mean postsynaptic charge transfer of uniformly small EPSCs recorded during periods of presynaptic hyperpolarization. By thus establishing the magnitude of the fundamental quantal event at this peripheral sensory synapse, we provide evidence that the majority of spontaneous and evoked EPSCs are multiquantal. Furthermore, we show that the prevalence of uniquantal versus multiquantal events is Ca2+ dependent. Paired recordings also reveal a tight correlation between membrane capacitance increase and evoked EPSC charge, indicating that glutamate release during prolonged hair cell depolarization does not significantly saturate or desensitize postsynaptic AMPA receptors. We propose that the large EPSCs reflect the highly synchronized release of multiple vesicles at single presynaptic ribbon-type active zones through a compound or coordinated vesicle fusion mechanism.
Project description:Delta(9)-tetrahydrocannabinol (THC), the principal psychoactive ingredient in marijuana, acts as a partial agonist on presynaptic cannabinoid type 1 (CB1) receptors to inhibit neurotransmitter release. Here, we report that THC inhibits excitatory neurotransmission between cultured rat hippocampal neurons in a manner highly sensitive to stimulus rate. THC (1 microM) inhibited excitatory postsynaptic currents (EPSCs) and whole-cell I(Ca) evoked at 0.1 Hz but at 0.5 Hz THC had little effect. The cannabinoid receptor full agonists [(R)-(+)-[2,3-dihydro-5-methyl-3[(4-morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl)methanone mesylate salt] (Win55212-2) (100 nM) and 2-arachidonylglycerol (1 microM) inhibited EPSCs independent of stimulation at 0.1 or 0.5 Hz. THC occupied CB1 receptors at 0.5 Hz, but the receptors failed to couple to presynaptic Ca(2+) channels. Consequently, 1 microM THC blocked the inhibition of EPSC amplitude by Win55212-2 when EPSCs were evoked at 0.5 Hz. A depolarizing prepulse to 0 mV reversed THC inhibition of I(Ca), but reversal of the inhibition produced by Win55212-2 required a pulse to +80 mV, suggesting that the voltage-dependent reversal of Gbetagamma inhibition of voltage-gated Ca(2+) channels accounts for the frequency-dependence of cannabinoid action. THC blocked depolarization-induced suppression of EPSCs evoked at 0.5 Hz, indicating that it inhibited retrograde endocannabinoid signaling in a frequency-dependent manner. Thus, THC displayed a state-dependent switching from agonist to antagonist that may account for its complex actions in vivo.
Project description:Fibroblast growth factor homologous factors (FHFs) are not growth factors, but instead bind to voltage-gated Na+ channels (NaV) and regulate their function. Mutations in FGF14, an FHF that is the locus for spinocerebellar ataxia 27 (SCA27), are believed to be pathogenic because of a dominant-negative reduction of NaV currents in cerebellar granule cells. Here, we demonstrate that FGF14 also regulates members of the presynaptic CaV2 Ca2+ channel family. Knockdown of FGF14 in granule cells reduced Ca2+ currents and diminished vesicular recycling, a marker for presynaptic Ca2+ influx. As a consequence, excitatory postsynaptic currents (EPSCs) at the granule cell to Purkinje cell synapse were markedly diminished. Expression of the SCA27-causing FGF14 mutant in granule cells exerted a dominant-negative reduction in Ca2+ currents, vesicular recycling, and the resultant EPSCs in Purkinje cells. Thus, FHFs are multimodal, regulating several discrete neuronal signaling events. SCA27 most likely results at least in part from dysregulation of Ca2+ channel function.
Project description:<h4>Background and purpose</h4>While arachidonyl ethanolamine (anandamide) produces pharmacological effects mediated by cannabinoid CB1 receptors, it is also an agonist at the transient receptor potential vanilloid type 1 (TRPV1) ion channel. This study examined the cellular actions of anandamide in the midbrain periaqueductal grey (PAG), a region implicated in the analgesic actions of cannabinoids, and which expresses both CB1 receptors and TRPV1.<h4>Experimental approach</h4>In vitro whole cell patch clamp recordings of glutamatergic excitatory postsynaptic currents (EPSCs) were made from rat and mouse PAG slices.<h4>Key results</h4>Capsaicin (1?µM) increased the rate, but not the amplitude of miniature EPSCs in subpopulations of neurons throughout the rat and mouse PAG. Capsaicin had no effect on miniature EPSCs in PAG neurons from TRPV1 knock-out mice. In mouse PAG neurons, anandamide (30?µM) had no effect on the rate of miniature EPSCs alone, or in the presence of either the CB1 antagonist AM251 (3?µM) or the TRPV1 antagonist iodoresiniferatoxin (300?nM). Anandamide produced a decrease in miniature EPSC rate in the presence of the fatty acid amide hydrolase (FAAH) inhibitor URB597 (1?µM). By contrast, anandamide produced an increase in miniature EPSC rate in the presence of both URB597 and AM251, which was absent in TRPV1 knock-out mice.<h4>Conclusions and implications</h4>These results suggest that the actions of anandamide within PAG are limited by enzymatic degradation by FAAH. FAAH blockade unmasks both presynaptic inhibition and excitation of glutamatergic synaptic transmission which are mediated via CB1 receptors and TRPV1 respectively.