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Marijuana affects neural functions through the binding of its active component ([Δ.sup.9]-THC) to cannabinoid receptors in the CNS. Recent studies have elucidated that endogenous ligands for cannabinoid receptors, endocannabinoids, serve [...]

Type B [gamma]-aminobutyric acid receptor (GAB[A.sub.B]R) is a G protein-coupled receptor that regulates neurotransmitter release and neuronal excitability throughout the brain. In various neurons, GAB[A.sub.B]RS are concentrated at excitatory synapses. Although these receptors are assumed to respond to GABA spillover from neighboring inhibitory synapses, their function is not fully understood. Here we show a previously undescribed function of GAB[A.sub.B]R exerted independent of GABA. In cerebellar Purkinje cells, interaction of GAB[A.sub.B]R with extracellular [Ca.sup.2+] ([Ca.sup.2+]) leads to a constitutive increase in the glutamate sensitivity of metabotropic glutamate receptor 1 (mGluR1). mGluR1 sensitization is clearly mediated by GA[B.sub.A]BR because it is absent in GAB[A.sub.B]R1 subunit-knockout cells. However, the mGluR1 sensitization does not require [G.sub.i/o] proteins that mediate the GAB[A.sub.B]R'S classical functions. Moreover, coimmunoprecipitation reveals complex formation between GAB[A.sub.B]R and mGluR1 in the cerebellum. These findings demonstrate that GAB[A.sub.B]R can act as [Ca.sup.2+.sub.o]-dependent cofactors to enhance neuronal metabotropic glutamate signaling. calcium | cerebellum | G protein-coupled receptor | oligomerization | Purkinje cell

Since the first reports of endocannabinoid-mediated retrograde signaling in 2001, great advances have been made toward understanding the molecular basis and functions of the endocannabinoid system. Electrophysiological studies have revealed that the endocannabinoid system is functional at various types of synapses throughout the brain. Basic mechanisms have been clarified as to how endocannabinoids are produced and released from postsynaptic neurons and regulate neurotransmitter release through activating presynaptic cannabinoid CB1 receptors, although there remain unsolved questions and some discrepancies. In addition to this major function, recent studies suggest diverse functions of endocannabinoids, including control of other endocannabinoid-independent forms of synaptic plasticity, regulation of neuronal excitability, stimulation of glia-neuron interaction, and induction of CB1R-independent plasticity. Using recently developed pharmacological and genetic tools, behavioral studies have elucidated the roles of the endocannabinoid system in various aspects of neural functions. In this review, we make a brief overview of molecular mechanisms underlying the endocannabinoid-mediated synaptic modulation and also summarize recent findings, which shed new light on a diversity of functional roles of endocannabinoids. [ABSTRACT FROM PUBLISHER]

Endogenous cannabinoids (endocannabinoids) serve as retrograde messengers at synapses in various regions of the brain. They are released from postsynaptic neurons and cause transient and long-lasting reduction of neurotransmitter release through activation of presynaptic cannabinoid receptors. Endocannabinoid release is induced either by increased postsynaptic Ca2+ levels or by activation of Gq/11-coupled receptors. When these two stimuli coincide, endocannabinoid release is markedly enhanced, which is attributed to the Ca2+ dependency of phospholipase Cβ (PLCβ). This Ca2+-assisted receptor-driven endocannabinoid release is suggested to participate in various forms of synaptic plasticity, including short-term associative plasticity in the cerebellum and spike-timing-dependent long-term depression in the somatosensory cortex. In these forms of plasticity, PLCβ seems to function as a coincident detector of presynaptic and postsynaptic activities. [Copyright &y& Elsevier]

Endocannabinoids mediate retrograde signaling and modulate synaptic transmission in various regions of the CNS. Depolarization-induced elevation of intracellular Ca2+ concentration causes endocannabinoid- mediated suppression of excitatory/inhibitory synaptic transmission. Activation of Gq/11-coupled receptors including group I metabotropic glutamate receptors (mGluRs) also causes endocannabinoid-mediated suppression of synaptic transmission. However, precise mechanisms of endocannabinoid production initiated by physiologically relevant synaptic activity remain to be determined. To address this problem, we made whole-cell recordings from Purkinje cells (PCs) in mouse cerebellar slices and examined their excitatory synapses arising from climbing fibers (CFs) and parallel fibers (PFs). We first characterized three distinct modes to induce endocannabinoid release by analyzing CF to PC synapses. The first mode is strong activation of mGluR subtype 1 (mGluR1)-phospholipase C (PLC)β4 cascade without detectable Ca2+ elevation. The second mode is Ca2+ elevation to a micromolar range without activation of the mGluR1-PLCβ4 cascade. The third mode is the Ca2+-assisted mGluR1-PLCβ4 cascade that requires weak mGluR1 activation and Ca2+ elevation to a submicromolar range. By analyzing PF to PC synapses, we show that the third mode is essential for effective endocannabinoid release from PCs by excitatory synaptic activity. Furthermore, our biochemical analysis demonstrates that combined weak mGluR1 activation and mild depolarization in PCs effectively produces 2-arachidonoylglycerol (2-AG), a candidate of endocannabinoid, whereas either stimulus alone did not produce detectable 2-AG. Our results strongly suggest that under physiological conditions, excitatory synaptic inputs to PCs activate the Ca2+-assisted mGluR1-PLCβ4 cascade, and thereby produce 2-AG, which retrogradely modulates synaptic transmission to PCs. [ABSTRACT FROM AUTHOR]

Endocannabinoids mediate retrograde signal and modulate transmission efficacy at various central synapses. Although endocannabinoid release is induced by either depolarization or activation of Gq/11-coupled receptors, it is markedly enhanced by the coincidence of depolarization and receptor activation. Here we report that this coincidence is detected by phospholipase Cβ1 (PLCβ1) in hippocampal neurons. By measuring cannabinoid-sensitive synaptic currents, we found that the receptor-driven endocannabinoid release was dependent on physiological levels of intracellular Ca2+ concentration ([Ca2+]i), and markedly enhanced by depolarization-induced [Ca2+]i elevation. Furthermore, we measured PLC activity in intact neurons by using exogenous TRPC6 channel as a biosensor for the PLC product diacylglycerol and found that the receptor-driven PLC activation exhibited similar [Ca2+]i dependence to that of endocannabinoid release. Neither endocannabinoid release nor PLC activation was induced by receptor activation in PLCβ1 knockout mice. We therefore conclude that PLCβ1 serves as a coincidence detector through its Ca2+ dependency for endocannabinoid release in hippocampal neurons. [Copyright &y& Elsevier]

Type B γ-aminobutyric acid receptor (GABABR) is a G protein-coupled receptor that regulates neurotransmitter release and neuronal excitability throughout the brain. In various neurons, GABABRs are concentrated at excitatory synapses. Although these receptors are assumed to respond to GABA spillover from neigh- boring inhibitory synapses, their function is not fully understood. Here we show a previously undescribed function of GABABR exerted independent of GABA. In cerebellar Purkinje cells, inter- action of GABABR with extracellular Ca2+ (Car) leads to a constitutive increase in the glutamate sensitivity of metabotropic glutamate receptor I (mGluR1). mGluR1 sensitization is clearly mediated by GABABR because it is absent in GABABR1 subunit- knockout cells. However, the mGluR1 sensitization does not require Gi/o proteins that mediate the GABABR's classical functions. Moreover, coimmunoprecipitation reveals complex formation between GABABR and mGluR1 in the cerebellum. These findings demonstrate that GABABR can act as Cao2+-dependent cofactors to enhance neuronal metabotropic glutamate signaling. [ABSTRACT FROM AUTHOR]

Marijuana smoking elicits various psychoactive effects through type 1 cannabinoid receptors (CB(1)Rs) in the brain. CB(1)R is a seven-transmembrane domain. G(i/o)-protein coupled receptors, and is expressed throughout the central nervous system including the hippocampus, cerebellum, striatum and cerebral cortex. Endogenous ligands for CB(1)R (endocannabinoids) are lipid in nature, and anandamide and 2-arachidonoylglycerol (2-AG) are considered to be the two major endocannabinoids. Endocannabinoids are known to function as retrograde messengers at synapses. Endocannabinoids are released from postsynaptic neurons in activity-dependent manners, and retrogradely activate presynaptic CB(1)Rs, resulting in short-term or long-term suppression of synaptic transmission. Endocannabinoid-mediated retrograde signaling is observed at various brain regions and considered as a general mechanism of synaptic modulation in the brain. Endocannabinoid release is triggered by postsynaptic Ca2+ elevation or activation of G(q/11)-protein coupled receptors. Recent studies have demonstrated that 2-AG mediates retrograde signaling at synapses in the brain. Endocannabinoid-mediated retrograde signaling is involved in long-term synaptic plasticity in several brain regions. At behavioral level, endocannabinoid signaling is known to be involved in hippocampus-, amygdala- and cerebellum-dependent learning and memory.

Endocannabinoids are released from postsynaptic neurons and cause retrograde suppression of synaptic transmission. Anandamide and 2-arachidonoylglycerol (2-AG) are regarded as two major endocannabinoids. To determine to what extent 2-AG contributes to retrograde signaling, we generated and analyzed mutant mice lacking either of the two 2-AG synthesizing enzymes diacylglycerol lipase alpha (DGLalpha) and beta (DGLbeta). We found that endocannabinoid-mediated retrograde synaptic suppression was totally absent in the cerebellum, hippocampus, and striatum of DGLalpha knockout mice, whereas the retrograde suppression was intact in DGLbeta knockout brains. The basal 2-AG content was markedly reduced and stimulus-induced elevation of 2-AG was absent in DGLalpha knockout brains, whereas the 2-AG content was normal in DGLbeta knockout brains. Morphology of the brain and expression of molecules required for 2-AG production other than DGLs were normal in the two knockout mice. We conclude that 2-AG produced by DGLalpha, but not by DGLbeta, mediates retrograde suppression at central synapses., (Copyright 2010 Elsevier Inc. All rights reserved.)