Cooperative endocannabinoid production by neuronal depolarization and group I metabotropic glutamate receptor activation

Endocannabinoid-mediated synaptic plasticity and addiction-related behavior

Endogenous cannabinoids (eCBs) are retrograde messengers that provide feedback inhibition of both excitatory and inhibitory transmission in brain through the activation of presynaptic CB₁ receptors. Substantial evidence indicates that eCBs mediate various forms of short- and long-term plasticity in brain regions involved in the etiology of addiction. The present review provides an overview of the mechanisms through which eCBs mediate various forms of synaptic plasticity and discusses evidence that eCB-mediated plasticity is disrupted following exposure to a variety of abused substances that differ substantially in pharmacodynamic mechanism including alcohol, psychostimulants and cannabinoids. The possible involvement of dysregulated eCB signaling in maladaptive behaviors that evolve over long-term drug exposure is also discussed, with a particular focus on altered behavioral responses to drug exposure, deficient extinction of drug-related memories, increased drug craving and relapse, heightened stress sensitivity and persistent affective disruption (anxiety and depression).

Mechanisms underlying input-specific expression of endocannabinoid-mediated synaptic plasticity in the dorsal cochlear nucleus

A hallmark of brain organization is the integration of primary and modulatory pathways by principal neurons. Primary sensory inputs are usually not plastic, while modulatory inputs converging to the same principal neuron can be plastic. However, the mechanisms determining this input-specific expression of synaptic plasticity remain unknown. We investigated this problem in the dorsal cochlear nucleus (DCN), where principal cells integrate primary auditory nerve input with plastic, parallel fiber input. Our previous DCN studies have shown that parallel fiber inputs exhibit short- and long-term plasticities mediated by endocannabinoid signaling. Here we show that auditory nerve inputs to principal cells do not show short- or long-term endocannabinoid-mediated synaptic plasticity. Electrophysiological and electron microscopy studies indicate that input specificity arises from selective expression of presynaptic cannabinoid (CB1) receptors in parallel fiber terminals, but not in auditory nerve terminals. However, pairing of parallel fiber activity with auditory nerve activity elicits plasticity in parallel fiber inputs, thus suggesting a role for synaptic plasticity in multisensory integration.

Endocannabinoids mediate synaptic plasticity at glutamatergic synapses on spiny neurons within a basal ganglia nucleus necessary for song learning

Activation of type 1 cannabinoid receptors (CB(1)R) in many central nervous system structures induces both short- and long-term changes in synaptic transmission. Within mammalian striatum, endocannabinoids (eCB) are one of several mechanisms that induce synaptic plasticity at glutamatergic terminals onto medium spiny neurons. Striatal synaptic plasticity may contribute a critical component of adaptive motor coordination and procedural learning. Songbirds are advantageous for studying the neural mechanisms of motor learning because they possess a neural pathway necessary for song learning and adult song plasticity that includes a striato-pallidal nucleus, area X (homologous to a portion of mammalian basal ganglia). Recent findings suggest that eCBs contribute to vocal development. For example, dense CB(1)R expression in song control nuclei peaks around the closure of the sensori-motor integration phase of song development. Also, systemic administration of a CB(1)R agonist during vocal development impairs song learning. Here we test whether activation of CB(1)R alters excitatory synaptic input on spiny neurons in area X of adult male zebra finches. Application of the CB(1)R agonist WIN55212-2 decreased excitatory postsynaptic current (EPSC) amplitude; that decrease was blocked by the CB(1)R antagonist AM251. Guided by eCB experiments in mammalian striatum, we tested and verified that at least two mechanisms indirectly activate CB(1)Rs through eCBs in area X. First, activation of group I metabotropic glutamate receptors with the agonist 3,5-dihydroxyphenylglycine (DHPG) induced a CB(1)R-mediated reduction in EPSC amplitude. Second, we observed that a 10 s postsynaptic depolarization induced a calcium-mediated, eCB-dependent decrease in synaptic strength that resisted rescue with late CB(1)R blockade. Together, these results show that eCB modulation occurs at inputs to area X spiny neurons and could influence motor learning and production.

Alterations in metabotropic glutamate receptor 1α and regulator of G protein signaling 4 in the prefrontal cortex in schizophrenia

OBJECTIVE

Certain cognitive deficits in individuals with schizophrenia have been linked to disturbed gamma-aminobutyric acid (GABA) and glutamate neurotrans-mission in the prefrontal cortex. Thus, it is important to understand how the mechanisms that regulate GABA and glutamate neurotransmission are altered in schizophrenia. For example, group I metabo-tropic glutamate receptors (mGluR1α, mGluR5) modulate both GABA and gluta-mate systems. In addition, regulator of G protein signaling 4 (RGS4) reduces intra-cellular signaling through several different G protein-coupled receptors, including group I mGluRs. Finally, the endocannabinoid system plays an important role in regulating GABA and glutamate neurotrans-mission. The status of endocannabinoid ligands, such as 2-arachidonoylglycerol, can be inferred in part through measures of diacylglycerol lipase and monoglyceride lipase, which synthesize and degrade 2-arachidonoylglycerol, respectively.

METHOD

Quantitative polymerase chain reaction was used to measure mRNA levels for group I mGluRs, RGS4, and markers of the endocannabinoid system in the prefrontal cortex Brodmann's area 9 of 42 schizophrenia subjects and matched normal comparison subjects. Similar analyses in monkeys chronically exposed to haloperidol, olanzapine, or placebo were also conducted.

RESULTS

Schizophrenia subjects had higher mRNA levels for mGluR1α and lower mRNA levels for RGS4, and these differences did not appear to be attributable to antipsychotic medications or other potential confounds. In contrast, no differences between subject groups were found in mRNA levels for endocannabinoid synthesizing and metabolizing enzymes.

CONCLUSIONS

Together, higher mGluR1α and lower RGS4 mRNA levels may represent a disturbed "molecular hub" in schizophrenia that may disrupt the function of prefrontal cortical networks, including both GABA and glutamate systems.

A novel non-CB1/TRPV1 endocannabinoid-mediated mechanism depresses excitatory synapses on hippocampal CA1 interneurons

Endocannabinoids (eCBs) mediate various forms of synaptic plasticity at excitatory and inhibitory synapses in the brain. The eCB anandamide binds to several receptors including the transient receptor potential vanilloid 1 (TRPV1) and cannabinoid receptor 1 (CB1). We recently identified that TRPV1 is required for long-term depression at excitatory synapses on CA1 hippocampal stratum radiatum interneurons. Here we performed whole-cell patch clamp recordings from CA1 stratum radiatum interneurons in rat brain slices to investigate the effect of the eCB anandamide on excitatory synapses as well as the involvement of Group I metabotropic glutamate receptors (mGluRs), which have been reported to produce eCBs endogenously. Application of the nonhydrolysable anandamide analog R-methanandamide depressed excitatory transmission to CA1 stratum radiatum interneurons by ∼50%. The Group I mGluR agonist DHPG also depressed excitatory glutamatergic transmission onto interneurons to a similar degree, and this depression was blocked by the mGluR5 antagonist MPEP (10 μM) but not by the mGluR1 antagonist CPCCOEt (50 μM). Interestingly, however, neither DHPG-mediated nor R-methanandamide-mediated depression was blocked by the TRPV1 antagonist capsazepine (10 μM), the CB1 antagonist AM-251 (2 μM) or a combination of both, suggesting the presence of a novel eCB receptor or anandamide target at excitatory hippocampal synapses. DHPG also occluded R-methanandamide depression, suggesting the possibility that the two drugs elicit synaptic depression via a shared signaling mechanism. Collectively, this study illustrates a novel CB1/TRPV1-independent eCB pathway present in the hippocampus that mediates depression at excitatory synapses on CA1 stratum radiatum interneurons.

Distinct endocannabinoid control of GABA release at perisomatic and dendritic synapses in the hippocampus

Endocannabinoid-mediated retrograde synaptic signaling is a key regulator of GABA release at synapses formed on the perisomatic region of pyramidal cells by basket cells that coexpress the cannabinoid type 1 receptor (CB(1)R) and cholecystokinin (CCK). However, CB(1)R and CCK-positive GABAergic terminals are present on pyramidal cell dendrites as well, but the principles of endocannabinoid control of GABA release in dendrites are not understood. We performed paired recordings from CCK-positive perisomatically (basket cells) or dendritically projecting (Schaffer collateral-associated cells) interneurons and postsynaptic CA1 pyramidal cells to determine the properties of endocannabinoid signaling at GABAergic synapses along the somato-dendritic axis. Although several key elements of the currently known molecular machinery for endocannabinoid synthesis are thought be primarily localized in dendrites, our results revealed that the depolarization-induced suppression of inhibition, the endocannabinoid-mediated tonic inhibition of GABA release, and the metabotropic glutamate receptor activation-induced, CB(1)R-mediated depression of GABA release were all significantly less effective at dendritic compared with perisomatic synapses. In addition, low concentration of exogenous CB(1) receptor agonist inhibited GABA release to a lesser extent at dendritic compared with perisomatic synapses, indicating that presynaptic differences are partly responsible for the differential control of GABA release by endocannabinoids in dendrites. Together, these data demonstrate a novel domain-specific regulation of GABA release by endocannabinoid signaling in the hippocampus.

Endogenous cannabinoid signaling is required for voluntary exercise-induced enhancement of progenitor cell proliferation in the hippocampus

Voluntary exercise and endogenous cannabinoid activity have independently been shown to regulate hippocampal plasticity. The aim of the current study was to determine whether the endocannabinoid system is regulated by voluntary exercise and if these changes contribute to exercise-induced enhancement of cell proliferation. In Experiment 1, 8 days of free access to a running wheel increased the agonist binding site density of the cannabinoid CB(1) receptor; CB(1) receptor-mediated GTPgammaS binding; and the tissue content of the endocannabinoid anandamide in the hippocampus but not in the prefrontal cortex. In Experiment 2, the CB(1) receptor antagonist AM251 (1 mg kg(-1)) was administered daily to animals given free access to a running wheel for 8 days, after which cell proliferation in the hippocampus was examined through immunohistochemical analysis of the cell cycle protein Ki-67. Voluntary exercise increased proliferation of progenitor cells, as evidenced by the increase in the number of Ki-67 positive cells in the granule cell layer of the dentate gyrus (DG) in the hippocampus. However, this effect was abrogated by concurrent treatment with AM251, indicating that the increase in endocannabinoid signaling in the hippocampus is required for the exercise-induced increase in cell proliferation. These data demonstrate that the endocannabinoid system in the hippocampus is sensitive to environmental change and suggest that it is a mediator of experience-induced plasticity.

Metabotropic glutamate mGlu1 receptor stimulation and blockade: therapeutic opportunities in psychiatric illness

Metabotropic glutamate mGlu(1) receptors play a modulatory role in the nervous system. They enhance cell excitability, modulate synaptic neurotransmission and are involved in synaptic plasticity. During the last 10 years, several selective metabotropic glutamate mGlu(1) receptor competitive antagonists and potentiators have been discovered. These pharmacological tools, together with early and later work in metabotropic glutamate mGlu(1) receptor mutant mice have allowed studying the role of the receptor in various aspects of psychiatric illnesses such as anxiety, depression and schizophrenia. We here review the data on selective metabotropic glutamate mGlu(1) receptor antagonists in support of their potential as anxiolytic and antidepressant treatments. We propose a rationale for the development of metabotropic glutamate mGlu(1) receptor positive allosteric modulators for the treatment of schizophrenia. Potential side effects of blockade and activation of metabotropic glutamate mGlu(1) receptors are addressed, with special focus on the differential effects of metabotropic glutamate mGlu(1) receptor antagonists in cognition models with positive reinforcement versus those that use aversive learning procedures. Further development of negative allosteric modulators and more drug-like positive allosteric modulators will be required in order to decipher the therapeutic efficacy and safety margin of these compounds in the clinic.

Control of cannabinoid CB1 receptor function on glutamate axon terminals by endogenous adenosine acting at A1 receptors

Marijuana is a widely used drug that impairs memory through interaction between its psychoactive constituent, Delta-9-tetrahydrocannabinol (Delta(9)-THC), and CB(1) receptors (CB1Rs) in the hippocampus. CB1Rs are located on Schaffer collateral (Sc) axon terminals in the hippocampus, where they inhibit glutamate release onto CA1 pyramidal neurons. This action is shared by adenosine A(1) receptors (A1Rs), which are also located on Sc terminals. Furthermore, A1Rs are tonically activated by endogenous adenosine (eADO), leading to suppressed glutamate release under basal conditions. Colocalization of A1Rs and CB1Rs, and their coupling to shared components of signal transduction, suggest that these receptors may interact. We examined the roles of A1Rs and eADO in regulating CB1R inhibition of glutamatergic synaptic transmission in the rodent hippocampus. We found that A1R activation by basal or experimentally increased levels of eADO reduced or eliminated CB1R inhibition of glutamate release, and that blockade of A1Rs with caffeine or other antagonists reversed this effect. The CB1R-A1R interaction was observed with the agonists WIN55,212-2 and Delta(9)-THC and during endocannabinoid-mediated depolarization-induced suppression of excitation. A1R control of CB1Rs was stronger in the C57BL/6J mouse hippocampus, in which eADO levels were higher than in Sprague Dawley rats, and the eADO modulation of CB1R effects was absent in A1R knock-out mice. Since eADO levels and A1R activation are regulated by homeostatic, metabolic, and pathological factors, these data identify a mechanism in which CB1R function can be controlled by the brain adenosine system. Additionally, our data imply that caffeine may potentiate the effects of marijuana on hippocampal function.

mGluR-mediated and endocannabinoid-dependent long-term depression in the hilar region of the rat dentate gyrus

We report that bath application of the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) causes acute inhibition of evoked IPSCs recorded from hilar mossy cells, and that significant long-term depression (LTD) of synaptic transmission remains following washout of DHPG. Subsequent experiments using minimal stimulation techniques revealed that expression of both acute and long-term effects of DHPG are restricted to a subset of GABAergic afferents that are also sensitive to depolarization-induced suppression of inhibition (DSI). Experiments with a selective CB1 antagonist and with transgenic animals lacking CB1 receptors indicate that all effects of DHPG, like DSI, depend on activation of CB1 receptors. Further work with selective mGluR antagonists suggests a direct involvement of mGluR1 receptors. Interestingly, we also report that induction of LTD under our experimental conditions does not require prior direct somatic depolarization via the patch pipette and does not appear to depend critically on the level of activity in incoming GABAergic afferents. Collectively, these results represent the first characterization of mGluR-mediated and endocannabinoid-dependent LTD in the hilar region of the dentate gyrus. The dentate gyrus is thus one of relatively few areas where this mechanism has clearly been demonstrated to induce long-term modulation of inhibitory synaptic transmission.

Developmental alteration of endocannabinoid retrograde signaling in the hippocampus

Endocannabinoids are lipid derivatives that mediate paracrine and juxtacrine signaling between cells. In the hippocampal CA1 region, a retrograde endocannabinoid signal suppresses GABA release by acting on presynaptic cannabinoid receptor-1 (CB1) and can be functionally manifested as depolarization-induced suppression of inhibition (DSI). In the present study, whole cell patch-clamp recordings in hippocampal slices were made to examine DSI in rats from P7-P21. Robust DSI develops in rat hippocampus at postnatal ages greater than two weeks, but only modest DSI is observed in P7-9 rat. DSI in neonatal rats can be enhanced by activation of group I metabotropic glutamate receptors (mGluRs) or muscarinic acetylcholine receptors in those neonatal rats. The DSI is also enhanced by sustained low-frequency (1 Hz) stimulation (5 min). This stimulus-enhanced DSI was prevented in the presence of 6-methyl-2-(phenylethynyl)-pyridine (10 microM), a group I mGluR antagonist. WIN55212-2, a synthetic CB1 agonist, produced a similar level of inhibition of GABAergic synaptic transmission at different postnatal time points. Therefore postsynaptic mechanisms appear to be mainly responsible for developmental changes in DSI, although presynaptic mechanisms cannot be ruled out entirely. We have also obtained evidence that tonic endocannabinoid release suppresses GABAergic transmission in the mature but not the neonatal hippocampus. The differential DSI magnitude at different stages of maturation could alter synaptic plasticity and learning and memory during hippocampal development.

Endocannabinoid-dependent homeostatic regulation of inhibitory synapses by miniature excitatory synaptic activities

Homeostatic regulation of synaptic strength in response to persistent changes of neuronal activity plays an important role in maintaining the overall level of circuit activity within a normal range. Absence of miniature EPSCs (mEPSCs) for a few hours is known to cause upregulation of excitatory synaptic strength, suggesting that mEPSCs contribute to the maintenance of excitatory synaptic functions. In the present study, we found that the absence of mEPSCs for 1-3 h also resulted in homeostatic suppression of presynaptic functions of inhibitory synapses in acute cortical slices from juvenile rats, as suggested by the reduced frequency (but not amplitude) of miniature IPSCs (mIPSCs) as well as the reduced amplitude of IPSCs. This homeostatic regulation depended on endocannabinoid (eCB) signaling, because blockade of either the activation of cannabinoid type-1 receptors (CB1Rs) or the synthesis of its endogenous ligand 2-arachidonoylglycerol (2-AG) abolished the suppression of inhibitory synapses caused by the absence of mEPSCs. Blockade of group I metabotropic glutamate receptors (mGluR-I) also abolished the suppression of inhibitory synapses, consistent with the mGluR-I requirement for eCB synthesis and release in cortical synapses. Furthermore, this homeostatic regulation also required eukaryotic elongation factor-2 (eEF2)-dependent protein synthesis, but not gene transcription. Activation of eEF2 alone was sufficient to suppress the mIPSC frequency, an effect abolished by inhibiting CB1Rs. Thus, mEPSCs contribute to the maintenance of inhibitory synaptic function and the absence of mEPSCs results in presynaptic suppression of inhibitory synapses via protein synthesis-dependent elevation of eCB signaling.

Endocannabinoid signaling and long-term synaptic plasticity

Endocannabinoids (eCBs) are key activity-dependent signals regulating synaptic transmission throughout the central nervous system. Accordingly, eCBs are involved in neural functions ranging from feeding homeostasis to cognition. There is great interest in understanding how exogenous (e.g., cannabis) and endogenous cannabinoids affect behavior. Because behavioral adaptations are widely considered to rely on changes in synaptic strength, the prevalence of eCB-mediated long-term depression (eCB-LTD) at synapses throughout the brain merits close attention. The induction and expression of eCB-LTD, although remarkably similar at various synapses, are controlled by an array of regulatory influences that we are just beginning to uncover. This complexity endows eCB-LTD with important computational properties, such as coincidence detection and input specificity, critical for higher CNS functions like learning and memory. In this article, we review the major molecular and cellular mechanisms underlying eCB-LTD, as well as the potential physiological relevance of this widespread form of synaptic plasticity.

Somatic spikes regulate dendritic signaling in small neurons in the absence of backpropagating action potentials

Somatic spiking is known to regulate dendritic signaling and associative synaptic plasticity in many types of large neurons, but it is unclear whether somatic action potentials play similar roles in small neurons. Here we ask whether somatic action potentials can also influence dendritic signaling in an electrically compact neuron, the cerebellar stellate cell (SC). Experiments were conducted in rat brain slices using a combination of imaging and electrophysiology. We find that somatic action potentials elevate dendritic calcium levels in SCs. There was little attenuation of calcium signals with distance from the soma in SCs from postnatal day 17 (P17)-P19 rats, which had dendrites that averaged 60 microm in length, and in short SC dendrites from P30-P33 rats. Somatic action potentials evoke dendritic calcium increases that are not affected by blocking dendritic sodium channels. This indicates that dendritic signals in SCs do not rely on dendritic sodium channels, which differs from many types of large neurons, in which dendritic sodium channels and backpropagating action potentials allow somatic spikes to control dendritic calcium signaling. Despite the lack of active backpropagating action potentials, we find that trains of somatic action potentials elevate dendritic calcium sufficiently to release endocannabinoids and retrogradely suppress parallel fiber to SC synapses in P17-P19 rats. Prolonged SC firing at physiologically realistic frequencies produces retrograde suppression when combined with low-level group I metabotropic glutamate receptor activation. Somatic spiking also interacts with synaptic stimulation to promote associative plasticity. These findings indicate that in small neurons the passive spread of potential within dendrites can allow somatic spiking to regulate dendritic calcium signaling and synaptic plasticity.

Substance P drives endocannabinoid-mediated disinhibition in a midbrain descending analgesic pathway

Substance P is thought to play an essential role in several forms of supraspinally mediated analgesia. The actions of substance P on synaptic transmission within descending analgesic pathways, however, are largely unknown. Here, we used whole-cell recordings from rat midbrain slices to examine the effects of substance P on GABAergic and glutamatergic transmission within the periaqueductal gray (PAG), a key component of a descending analgesic pathway that projects via the rostral ventromedial medulla (RVM) to the spinal cord dorsal horn. We found that substance P reversibly decreased the amplitude and increased the paired-pulse ratio of evoked IPSCs recorded from identified PAG-RVM projection neurons and from unidentified PAG neurons. Substance P had no effect on miniature IPSCs, implying an indirect mode of action. The effects of substance P were abolished by metabotropic glutamate type 5 and cannabinoid CB1 receptor antagonists, but unaltered by NMDA, GABA(B), mu,delta-opioid, adenosine A(1), and 5HT(1A) receptor antagonists. Consistent with a role for endogenous glutamate in this process, substance P increased the frequency of action potential-dependent spontaneous EPSCs. Moreover, the effect of substance P on evoked IPSCs was mimicked and occluded by a glutamate transport inhibitor. Finally, these effects were dependent on postsynaptic G-protein activation and diacylglycerol lipase activity, suggesting the requirement for retrograde signaling by the endocannabinoid 2-arachidonoylglycerol. Thus, substance P may facilitate descending analgesia in part by enhancing glutamate-mediated excitation and endocannabinoid-mediated disinhibition of PAG-RVM projection neurons.

Cannabinoid signaling in inhibitory autaptic hippocampal neurons

Depolarization-induced suppression of excitation and inhibition (DSE/DSI) appears to be an important form of short-term retrograde neuronal plasticity involving endocannabinoids (eCBs), the activation of presynaptic cannabinoid CB1 receptors, and the suppression of neurotransmitter release. Using murine autaptic hippocampal cultures, we have distinguished five populations of autaptic inhibitory neurons that exhibit differential cannabinoid responses, including three temporally distinct forms of DSI. One remaining population responded to cannabinoids but did not have DSI while a fifth had neither DSI nor cannabinoid responses. Of the two chief candidate eCBs, 2-AG reversibly inhibited inhibitory post synaptic currents (IPSCs) while anandamide did so irreversibly, the latter's action inconsistent with a role as a bona fide eCB mediator of DSI. The duration of depolarization necessary to elicit the two most prominent forms of DSI (effective dose (ED-50) approximately 210, approximately 280 ms) was far less than for autaptic DSE. However the nearly identical concentration response for 2-AG to inhibit excitatory postsynaptic currents (EPSCs) and IPSCs indicates that this difference is not due to differential cannabinoid receptor sensitivity. Interestingly, of the two populations exhibiting prominent DSI, one had a substantially faster recovery time course both after DSI and 2-AG, this despite being cultured under identical conditions. Several enzymes have been proposed to play a role in 2-AG breakdown, presumably determining the time course of DSI: fatty acid amide hydrolase (FAAH), cyclooxygenase-2 (COX-2), monoacyl glycerol lipase (MGL), and alpha/beta-hydrolase domains 6 and 12 (ABHD6 and ABHD12). We tested the impact on DSI duration by blockers of FAAH, COX-2, MGL and ABHD6. Notably, the population with slow DSI was regulated only by MGL, whereas the fast DSI population was regulated by both MGL and COX-2. This suggests that the faster DSI time course may occur as a result of the concerted action of multiple enzymes, which may represent a more general mechanism for regulation of the duration of different forms of DSI and DSE.

Endocannabinoid-mediated control of synaptic transmission

The discovery of cannabinoid receptors and subsequent identification of their endogenous ligands (endocannabinoids) in early 1990s have greatly accelerated research on cannabinoid actions in the brain. Then, the discovery in 2001 that endocannabinoids mediate retrograde synaptic signaling has opened up a new era for cannabinoid research and also established a new concept how diffusible messengers modulate synaptic efficacy and neural activity. The last 7 years have witnessed remarkable advances in our understanding of the endocannabinoid system. It is now well accepted that endocannabinoids are released from postsynaptic neurons, activate presynaptic cannabinoid CB(1) receptors, and cause transient and long-lasting reduction of neurotransmitter release. In this review, we aim to integrate our current understanding of functions of the endocannabinoid system, especially focusing on the control of synaptic transmission in the brain. We summarize recent electrophysiological studies carried out on synapses of various brain regions and discuss how synaptic transmission is regulated by endocannabinoid signaling. Then we refer to recent anatomical studies on subcellular distribution of the molecules involved in endocannabinoid signaling and discuss how these signaling molecules are arranged around synapses. In addition, we make a brief overview of studies on cannabinoid receptors and their intracellular signaling, biochemical studies on endocannabinoid metabolism, and behavioral studies on the roles of the endocannabinoid system in various aspects of neural functions.

Involvement of endocannabinoid signaling in the neuroprotective effects of subtype 1 metabotropic glutamate receptor antagonists in models of cerebral ischemia

Experimental evidence indicates that metabotropic glutamate (mGlu) receptors of the mGlu1 and mGlu5 subtypes play a differential role in models of cerebral ischemia and that only mGlu1 receptors are implicated in the pathways leading to postischemic neuronal injury. The localization of mGlu1 receptors in GABA-containing interneurons rather than in hippocampal CA1 pyramidal cells that are vulnerable to ischemia has prompted experimental studies that have demonstrated mGlu1 receptor antagonist agents attenuate postischemic injury by enhancing GABA-mediated neurotransmission, thus providing a new viewpoint on the neuroprotective mechanism of these pharmacological agents. In view of the recent discovery of a functional interaction between group I mGlu receptors and the cannabinoid system in the modulation of synaptic transmission, we propose a novel mechanism that predicts that the neuroprotective effects of mGlu1 receptor antagonists on CA1 pyramidal cells are mediated by a mechanism that overcomes the "synaptic circuit break" operated by endocannabinoids on GABAergic transmission.

Type-1 metabotropic glutamate receptor in cerebellar Purkinje cells: a key molecule responsible for long-term depression, endocannabinoid signalling and synapse elimination

The cerebellum is a brain structure involved in the coordination, control and learning of movements, and elucidation of its function is an important issue. Japanese scholars have made seminal contributions in this field of neuroscience. Electrophysiological studies of the cerebellum have a long history in Japan since the pioneering works by Ito and Sasaki. Elucidation of the basic circuit diagram of the cerebellum in the 1960s was followed by the construction of cerebellar network theories and finding of their neural correlates in the 1970s. A theoretically predicted synaptic plasticity, long-term depression (LTD) at parallel fibre to Purkinje cell synapse, was demonstrated experimentally in 1982 by Ito and co-workers. Since then, Japanese neuroscientists from various disciplines participated in this field and have made major contributions to elucidate molecular mechanisms underlying LTD. An important pathway for LTD induction is type-1 metabotropic glutamate receptor (mGluR1) and its downstream signal transduction in Purkinje cells. Sugiyama and co-workers demonstrated the presence of mGluRs and Nakanishi and his pupils identified the molecular structures and functions of the mGluR family. Moreover, the authors contributed to the discovery and elucidation of several novel functions of mGluR1 in cerebellar Purkinje cells. mGluR1 turned out to be crucial for the release of endocannabinoid from Purkinje cells and the resultant retrograde suppression of transmitter release. It was also found that mGluR1 and its downstream signal transduction in Purkinje cells are indispensable for the elimination of redundant synapses during post-natal cerebellar development. This article overviews the seminal works by Japanese neuroscientists, focusing on mGluR1 signalling in cerebellar Purkinje cells.

Endocannabinoid- and mGluR5-dependent short-term synaptic depression in an isolated neuron/bouton preparation from the hippocampal CA1 region

Endocannabinoids released from the postsynaptic neuronal membrane can activate presynaptic CB1 receptors and inhibit neurotransmitter release. In hippocampal slices, depolarization of the CA1 pyramidal neurons elicits an endocannabinoid-mediated inhibition of gamma-aminobutyric acid release known as depolarization-induced suppression of inhibition (DSI). Using the highly reduced neuron/synaptic bouton preparation from the CA1 region of hippocampus, we have begun to examine endocannabinoid-dependent short-term depression (STD) of inhibitory synaptic transmission under well-controlled physiological and pharmacological conditions in an environment free of other cells. Application of the CB1 synthetic agonist WIN55212-2 and endogenous cannabinoids 2-AG and anandamide produced a decrease in spontaneous inhibitory postsynaptic current (sIPSC) frequency and amplitude, indicating the presence of CB1 receptors at synapses in this preparation. Endocannabinoid-dependent STD is different from DSI found in hippocampal slices and the neuron/bouton preparation from basolateral amygdala (BLA) since depolarization alone was not sufficient to induce suppression of sIPSCs. However, concurrent application of the metabotropic glutamate receptor (mGluR) agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) and postsynaptic depolarization resulted in a transient (30-50 s) decrease in sIPSC frequency and amplitude. Application of DHPG alone had no effect on sIPSCs. The depolarization/DHPG-induced STD was blocked by the CB1 antagonist SR141716A and the mGluR5 antagonist MPEP and was sensitive to intracellular calcium concentration. Comparing the present findings with earlier work in hippocampal slices and BLA, it appears that endocannabinoid release is less robust in isolated hippocampal neurons.

Cannabinoid CB(1) receptor antagonist rimonabant attenuates reinstatement of ketamine conditioned place preference in rats

Recent evidence suggests that cannabinoid CB(1) receptors may represent effective targets for therapeutic agents used to treat cocaine and heroin relapse. However, the role of cannabinoid CB(1) receptors in the potential treatment for other drugs of abuse is still largely unknown. The present study was conducted to determine whether cannabinoid CB(1) receptors play a similar role in relapse to ketamine abuse. To establish a ketamine reinstatement model in the conditioned place preference paradigm, rats were trained to develop place preference conditioned by ketamine, which was subsequently extinguished through daily exposure to the test chambers in the absence of ketamine. On the day following the last extinction session, four groups of rats were injected with ketamine (1, 5, 10 and 15 mg/kg, i.p.) to reinstate previously extinguished conditioned place preference. To investigate the effects of rimonabant, a cannabinoid CB(1) receptor antagonist, on reinstatement of ketamine-induced place preference, different doses of rimonabant (0.1, 0.5 and 3 mg/kg, i.p) were injected 30 min prior to the ketamine (5 and 15 mg/kg, i.p.) priming injection in a separate group of rats. To determine whether rimonabant itself produces conditioned place preference or conditioned place aversion, rats were trained for conditioned place preference or place aversion with rimonabant (0, 0.1, 0.5, 3.0 mg/kg, i.p.). While ketamine priming injections reinstated extinguished place preference, rimonabant administration significantly attenuated the reinstatement of ketamine-induced place preference in a dose-dependent manner. Importantly, rimonabant itself did not produce conditioned place preference or place aversion. Since the reinstatement effects of ketamine administration were inhibited by rimonabant, these findings suggest that a cannabinoid CB(1) receptor antagonist may be useful in preventing relapse to ketamine abuse.

Potentiation of electrical and chemical synaptic transmission mediated by endocannabinoids

Endocannabinoids are well established as inhibitors of chemical synaptic transmission via presynaptic activation of the cannabinoid type 1 receptor (CB1R). Contrasting this notion, we show that dendritic release of endocannabinoids mediates potentiation of synaptic transmission at mixed (electrical and chemical) synaptic contacts on the goldfish Mauthner cell. Remarkably, the observed enhancement was not restricted to the glutamatergic component of the synaptic response but also included a parallel increase in electrical transmission. This effect involved the activation of CB1 receptors and was indirectly mediated via the release of dopamine from nearby varicosities, which in turn led to potentiation of the synaptic response via a cAMP-dependent protein kinase-mediated postsynaptic mechanism. Thus, endocannabinoid release can potentiate synaptic transmission, and its functional roles include the regulation of gap junction-mediated electrical synapses. Similar interactions between endocannabinoid and dopaminergic systems may be widespread and potentially relevant for the motor and rewarding effects of cannabis derivatives.

Glutamate spillover modulates GABAergic synaptic transmission in the rat midbrain periaqueductal grey via metabotropic glutamate receptors and endocannabinoid signaling

Glutamate spillover regulates GABAergic synaptic transmission at several CNS synapses via presynaptic ionotropic and metabotropic glutamate receptors (mGluRs). We have previously demonstrated that activation of group I-III mGluRs inhibits GABAergic transmission in the midbrain periaqueductal gray (PAG), a region involved in organizing behavioral responses to threat, stress, and pain. Here, we examined the role of glutamate spillover in the modulation of GABAergic transmission in the PAG. Using whole-cell recordings from rat PAG slices, we found that evoked IPSCs were reduced by the nonspecific glutamate transport blockers DL-threo-beta-benzyloxyaspartic acid (TBOA) and L-trans-pyrrolidine-2,4-dicarboxylic acid, but not by the glial GLT1-specific blocker dihydrokainate. In contrast, TBOA had no effect on evoked IPSCs when glutamate uptake into the postsynaptic neuron was selectively impaired. TBOA increased the paired-pulse ratio of evoked IPSCs and reduced the rate but not the amplitude of spontaneous miniature IPSCs. The effect of TBOA on evoked IPSCs was abolished by the broad-spectrum mGluR antagonist (2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid (100 microM), reduced by the mGluR5-specific antagonist 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP) and mimicked by the mGluR1/5 agonist (RS)-3,5-dihydroxyphenylglycine (DHPG). Furthermore, the effects of both TBOA and DHPG were reduced by the cannabinoid CB1 receptor antagonist 1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-1-piperidinyl-1H-pyrazole-3-carboxamide (AM251). Finally, although MPEP and AM251 had no effect on single evoked IPSCs, they increased evoked IPSCs during repetitive stimulation. These results indicate that neuronal glutamate transporters limit mGluR5 activation and endocannabinoid signaling, but may be overwhelmed during conditions of elevated glutamate release. Thus, neuronal glutamate transporters play a key role in regulating endocannabinoid-mediated cross talk between glutamatergic and GABAergic synapses within the PAG.

Developmental Decrease in Short-Term Facilitation at Schaffer Collateral Synapses in Hippocampus Is mGluR1 Sensitive

Developmental changes can occur in the dynamic properties of synapses, known as short-term plasticity. Using rat acute hippocampal slices at room temperature, we have previously shown a decrease in short-term facilitation at Schaffer collateral synapses from young adults compared with juveniles in response to temporally complex natural stimulus patterns such as synapses receive in vivo. Here we show that this developmental decrease in facilitation is also seen at 32°C and investigate the underlying mechanism. Addition of the mGluR1 antagonist LY367385 increases short-term facilitation in response to the natural stimulus pattern, showing that mGluR1 is activated by synaptically released glutamate. Although synaptic activation of mGluR1 occurs at both ages, the effect is larger in young adults. Furthermore, blocking mGluR1 eliminates most of the developmental decrease in short-term facilitation during the natural stimulus pattern. We investigated possible retrograde/downstream messengers involved after synaptic activation of mGluR1. Blocking cannabinoid receptors has no effect on the response during the natural stimulus pattern, indicating that the reduction in facilitation during synaptic activation of mGluR1 does not occur through release of endocannabinoids. We find that blocking GABAB receptors increases facilitation during the natural stimulus pattern and occludes the effect of the mGluR1 antagonist, indicating a role for the modulation of GABA release from inhibitory interneurons by mGluR1 activation. These data suggest a model where synaptic activation of mGluR1 on inhibitory interneurons causes an increase in GABA release by inhibitory interneurons, which activates GABAB receptors on Schaffer collateral synapses and inhibits short-term facilitation during the natural stimulus pattern.

Dopamine and corticotropin-releasing factor synergistically alter basolateral amygdala-to-medial prefrontal cortex synaptic transmission: functional switch after chronic cocaine administration

Basolateral amygdala (BLA) neurons provide a major excitatory input to medial prefrontal cortex (mPFC)-layer V pyramidal neurons. Under stressful conditions, commonly associated with chronic cocaine abuse, altered BLA-to-mPFC synaptic transmission could lead to defective emotional information processing and decision making within the mPFC and result in misguided and inappropriate behaviors. We examined the effects of cocaine administered chronically in vivo on EPSCs recorded from a putative BLA-mPFC pathway in vitro and their modulation by dopamine (DA), corticotropin-releasing factor (CRF), and their combination (DA plus CRF). In saline-treated animals, activation of D(1/5) receptors depressed BLA-mPFC EPSCs, whereas CRF1 receptor activation alone had no effect on EPSCs. Activating D(1/5) and CRF1 receptors in combination, however, worked synergistically through presynaptic and postsynaptic mechanisms to depress EPSCs to levels greater than D(1/5) receptor activation alone. After chronic cocaine administration, the function of DA(1/5) and CRF receptors switched from inhibitory to excitatory. In slices from cocaine-treated animals, putative BLA-mPFC EPSCs were depressed through a presynaptic mechanism. Now, activation of either D(1/5) or CRF2 receptors increased the cocaine-induced, depressed EPSCs. Additionally, simultaneous activation of presynaptic D(1/5) and CRF2 receptors led to further enhancement of EPSCs. These data indicate that CRF acting synergistically with DA normally potentiates D(1/5)-induced synaptic depression. However, after chronic cocaine, the combined synergistic actions of DA and CRF switched polarity to enhance facilitation of BLA-mPFC glutamatergic transmission. Also unmasked after acute withdrawal from chronic cocaine are endogenous, tonic-inhibitory D2-like and tonic-facilitatory CRF2 receptor actions. These multiple functional and receptor changes may underlie the altered, possibly aberrant, decision-making process after chronic cocaine.

Presynaptic modulation by endocannabinoids

Modulation of neurotransmitter release by G-protein-coupled receptors (GPCRs) is a prominent presynaptic mechanism for regulation of synaptic transmission. Activation of GPCRs located at the presynaptic terminal can decrease the probability of neurotransmitter release. This presynaptic depression involves activation of Gi/o-type G-proteins that mediate different inhibitory mechanisms, including inhibition of voltage-gated calcium channels, activation of potassium channels, and direct inhibition of the vesicle fusion process. A variety of neurotransmitters and modulatory agents can activate GPCRs that produce presynaptic depression. Among these are lipid metabolites that serve as agonists for GPCRs. The discovery of endocannabinoids and their cognate receptors, including the CB1 receptor, has stimulated intense investigation into the neurophysiological roles of these lipid metabolites. It is now clear that presynaptic depression is the major physiological role for the CB1 receptor. Endocannabinoids activate this receptor mainly via a retrograde signaling process in which these compounds are synthesized in and released from postsynaptic neuronal elements, and travel back to the presynaptic terminal to act on the CB1 receptor. This retrograde endocannabinoid modulation has been implicated in short-term synaptic depression, including suppression of excitatory or inhibitory transmission induced by postsynaptic depolarization and transient synaptic depression induced by activation of postsynaptic GPCRs during agonist treatment or synaptic activation. Endocannabinoids and the CB1 receptor also play a key role in one form of long-term synaptic depression (LTD) that involves a longlasting decrease in neurotransmitter release.

Somatodendritic release of glutamate regulates synaptic inhibition in cerebellar Purkinje cells via autocrine mGluR1 activation

In the cerebellum, the process of retrograde signaling via presynaptic receptors is important for the induction of short- and long-term changes in inhibitory synaptic transmission at interneuron-Purkinje cell (PC) synapses. Endocannabinoids, by activating presynaptic CB1 receptors, mediate a short-term decrease in inhibitory synaptic efficacy, whereas glutamate, acting on presynaptic NMDA receptors, induces a longer-latency sustained increase in GABA release. We now demonstrate that either low-frequency climbing fiber stimulation or direct somatic depolarization of Purkinje cells results in SNARE-dependent vesicular release of glutamate from the soma and dendrites of PCs. The activity-dependent release of glutamate caused the activation of postsynaptic metabotropic glutamate receptor 1 (mGluR1) on PC somatodendritic membranes, resulting in the cooperative release of endocannabinoids and an mGluR1-mediated slow membrane conductance. The activity of excitatory amino acid transporters regulated the spatial spread of glutamate and thus the extent of PC mGluR1 activation. We propose that activity-dependent somatodendritic glutamate release and autocrine activation of mGluR1 on PCs provides a powerful homeostatic mechanism to dynamically regulate inhibitory synaptic transmission in the cerebellar cortex.

Input-specific plasticity at excitatory synapses mediated by endocannabinoids in the dentate gyrus

Endocannabinoids (eCBs) mediate transient and long-lasting synaptic plasticity in several brain structures. In the dentate gyrus, activation of the type 1 cannabinoid receptor (CB1R) by exogenous ligands reportedly depresses excitatory synaptic transmission. However, direct evidence of eCB signaling at excitatory synapses in this region has been lacking. Here, we demonstrate that eCB release can be induced by a brief postsynaptic depolarization of dentate granule cells (DGCs), which potently and transiently suppresses glutamatergic inputs from mossy cell interneurons (MCs) but not from entorhinal cortex via the lateral and medial perforant paths. This input-specific depolarization-induced suppression of excitation (DSE) is calcium-dependent and can be modulated by agonists of cholinergic and group I metabotropic glutamate receptors. Inhibiting the synthesis of 2-arachidonoyl glycerol (2-AG), one of the most abundant eCBs in the brain, by diacyglycerol lipase (DGL) does not abolish DSE. Moreover, preventing the breakdown of anandamide, the other main eCB, does not potentiate DSE. Thus, eCB signaling underlying DSE in the dentate does not require DGL activity and is unlikely to be mediated by anandamide. Finally, we find that manipulations known to induce eCB-LTD at other central synapses do not trigger LTD at MCF-DGC synapses.

Roles of phospholipase Cbeta and NMDA receptor in activity-dependent endocannabinoid release

Endocannabinoids are released from postsynaptic neurons, activate presynaptic cannabinoid receptors and cause various forms of short-term and long-term synaptic plasticity throughout the brain. Using hippocampal and cerebellar neurons, we have revealed that endocannabinoid release can be induced through two different pathways. One is independent of phospholipase Cbeta (PLCbeta) and driven by Ca(2+) elevation alone (Ca(2+)-driven endocannabinoid release, CaER), and the other is PLCbeta-dependent and driven by activation of G(q/11)-coupled receptors (receptor-driven endocannabinoid release, RER). CaER is induced by activation of either voltage-gated Ca(2+) channels or NMDA receptors. RER is functional even at resting Ca(2+) levels (basal RER), but markedly enhanced by a small Ca(2+) elevation (Ca(2+)-assisted RER). In Ca(2+)-assisted RER, PLCbeta serves as a coincidence detector of receptor activation and Ca(2+) elevation. We have also demonstrated that Ca(2+)-assisted RER is essential for the endocannabinoid release triggered by synaptic activity. Our anatomical data show that a set of receptors and enzymes required for RER are well organized so that the excitatory input can trigger RER effectively. Certain forms of spike-timing-dependent plasticity (STDP) are reported to depend on endocannabinoid signalling. The NMDA receptor and PLCbeta might play key roles in the endocannabinoid-dependent forms of STDP as coincidence detectors with different timing dependences.

Endocannabinoid signalling triggered by NMDA receptor-mediated calcium entry into rat hippocampal neurons

Endocannabinoids are released from neurons in activity-dependent manners, act retrogradely on presynaptic CB(1) cannabinoid receptors, and induce short-term or long-term suppression of transmitter release. The endocannabinoid release is triggered by postsynaptic activation of voltage-gated Ca(2+) channels and/or G(q)-coupled receptors such as group I metabotropic glutamate receptors (I-mGluRs) and M(1)/M(3) muscarinic receptors. However, the roles of NMDA receptors, which provide another pathway for Ca(2+) entry into neurons, in endocannabinoid signalling have been poorly understood. In the present study, we investigated the possible contribution of NMDA receptors in endocannabinoid production by recording IPSCs in cultured hippocampal neurons. Under the conditions minimizing the activation of voltage-gated Ca(2+) channels, local application of NMDA (200 microm) transiently suppressed cannabinoid-sensitive IPSCs, but not cannabinoid-insensitive IPSCs. This NMDA-induced suppression was abolished by blocking NMDA receptors, CB(1) receptors and diacylglycerol lipase, but not by inhibiting voltage-gated Ca(2+) channels. When the postsynaptic neuron was dialysed with 30 mm BAPTA, the NMDA-induced suppression was reduced significantly. A lower dose of NMDA (20 microm) exerted little effect when applied alone, but markedly enhanced the cannabinoid-dependent suppression driven by muscarinic receptors or I-mGluRs. These data clearly indicate that the activation of NMDA receptors facilitates the endocannabinoid release either alone or in concert with the G(q)-coupled receptors.

Retrograde endocannabinoid inhibition of goldfish retinal cones is mediated by 2-arachidonoyl glycerol

A functional role for retinal endocannabinoids has not been determined. We characterized retrograde suppression of membrane currents of goldfish cones in a retinal slice. Whole-cell recordings were obtained from cone inner segments under voltage clamp. I(K(V)) was elicited by a depolarizing pulse to +54 mV from a holding potential of -70 mV. A fifty-millisecond puff of saline with 70 mM KCl or Group I mGluR agonist DHPG was applied through a pipette directly at a mixed rod/cone (Mb) bipolar cell body. The amplitude of I(K(V)) decreased 25% compared to the pre-puff control. Retrograde suppression of I(K(V)) was blocked by CB1 receptor antagonist, SR141716A. The FAAH inhibitor URB597 had no effect on the suppression of I(K(V)), whereas nimesulide, a COX-2 inhibitor, prolonged the effects of the K+ puff 10-fold. Orlistat, a blocker of 2-AG synthesis, blocked the effect of the K+ puff. Group I mGluR activation of Gq/11 was demonstrated in that a puff with DHPG decreased I(K(V)) of cones by 32%, an effect blocked by SR141716A. The effect of DHPG was not blocked by the mGluR5 antagonist MPEP, indicating involvement of mGluR1. The suppressive effect of the K+ puff vanished in a Ca2+-free, 2 mM Co2+ saline. TMB-8 or ryanodine, blocked the effect of DHPG, but not that of the K+ puff, showing that calcium influx or release from intracellular stores could mediate retrograde release. We suggest that retrograde suppression of cone I(K(V)) is mediated by Ca2+-dependent release of 2-AG from Mb bipolar cell dendrites by separate mechanisms: (1) voltage-dependent, mimicked by the K+ puff, that may be activated by the depolarizing ON response to light; (2) voltage-independent, occurring under ambient illumination, mediated by tonic mGluR1 activation. The negative feedback of this latter mechanism could regulate tonic glutamate release from cones within narrow limits, regardless of ambient illumination.

Endocannabinoid Liberation from Neurons in Transsynaptic Signaling

Endocannabinoids are fatty acid derivatives that have a variety of biological actions, most notably via activation of the cannabinoid receptors. These receptors are also targets for drugs derived from Cannabis sativa. In the nervous system, endocannabinoids act as neuromodulators that depress neurotransmitter release at the presynaptic terminal. In most instances of neural endocannabinoid signaling, the compounds appear to be released from the postsynaptic neuron to act on the presynaptic terminal in a "retrograde" manner. Several common mechanisms involved in postsynaptic endocannabinoid production and presynaptic depression produced via activation of the CB1 cannabinoid receptor have been identified. However, significant problems remain in defining the mechanisms underlying endocannabinoid production, release, and movement across the membrane. These issues are discussed in the present review.

Ca2+-assisted receptor-driven endocannabinoid release: mechanisms that associate presynaptic and postsynaptic activities

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 Ca(2+) levels or by activation of G(q/11)-coupled receptors. When these two stimuli coincide, endocannabinoid release is markedly enhanced, which is attributed to the Ca(2+) dependency of phospholipase Cbeta (PLCbeta). This Ca(2+)-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, PLCbeta seems to function as a coincident detector of presynaptic and postsynaptic activities.

Subcellular arrangement of molecules for 2-arachidonoyl-glycerol-mediated retrograde signaling and its physiological contribution to synaptic modulation in the striatum

Endogenous cannabinoids (endocannabinoids) mediate retrograde signals for short- and long-term suppression of transmitter release at synapses of striatal medium spiny (MS) neurons. An endocannabinoid, 2-arachidonoyl-glycerol (2-AG), is synthesized from diacylglycerol (DAG) after membrane depolarization and Gq-coupled receptor activation. To understand 2-AG-mediated retrograde signaling in the striatum, we determined precise subcellular distributions of the synthetic enzyme of 2-AG, DAG lipase-alpha (DAGLalpha), and its upstream metabotropic glutamate receptor 5 (mGluR5) and muscarinic acetylcholine receptor 1 (M1). DAGLalpha, mGluR5, and M1 were all richly distributed on the somatodendritic surface of MS neurons, but their subcellular distributions were different. Although mGluR5 and DAGLalpha levels were highest in spines and accumulated in the perisynaptic region, M1 level was lowest in spines and was rather excluded from the mGluR5-rich perisynaptic region. These subcellular arrangements suggest that mGluR5 and M1 might differentially affect endocannabinoid-mediated, depolarization-induced suppression of inhibition (DSI) and depolarization-induced suppression of excitation (DSE) in MS neurons. Indeed, mGluR5 activation enhanced both DSI and DSE, whereas M1 activation enhanced DSI only. Importantly, DSI, DSE, and receptor-driven endocannabinoid-mediated suppression were all abolished by the DAG lipase inhibitor tetrahydrolipstatin, indicating 2-AG as the major endocannabinoid mediating retrograde suppression at excitatory and inhibitory synapses of MS neurons. Accordingly, CB1 cannabinoid receptor, the main target of 2-AG, was present at high levels on GABAergic axon terminals of MS neurons and parvalbumin-positive interneurons and at low levels on excitatory corticostriatal afferents. Thus, endocannabinoid signaling molecules are arranged to modulate the excitability of the MS neuron effectively depending on cortical activity and cholinergic tone as measured by mGluR5 and M1 receptors, respectively.

Endocannabinoids in the dentate gyrus

Recent years have produced rapid and enormous growth in our understanding of endocannabinoid-mediated signaling in the CNS. While much of the recent progress has focused on other areas of the brain, a significant body of evidence has developed that indicates the presence of a robust system for endocannabinoid-mediated signaling in the dentate gyrus. This chapter will provide an overview of our current understanding of that system based on available anatomical and physiological data.

Endogenous mGluR activity suppresses GABAergic transmission in avian cochlear nucleus magnocellularis neurons

GABAergic transmission in the avian cochlear nucleus magnocellularis (NM) of the chick is subject to modulation by gamma-aminobutyric acid type B (GABA(B)) autoreceptors. Here, I investigated modulation of GABAergic transmission in NM by metabotropic glutamate receptors (mGluRs) with whole cell recordings in brain slice preparations. I found that tACPD, a nonspecific mGluR agonist, exerted dose-dependent suppression on evoked inhibitory postsynaptic currents (eIPSCs) in NM neurons. At concentrations of 100 or 200 microM, tACPD increased the failure rate of GABAergic transmission. Agonists for group I (3,5-DHPG, 200 microM), group II (DCG-IV, 2 microM), and group III (L-AP4, 10 microM) mGluRs produced a significant reduction in the amplitude of eIPSCs and a significant increase in failure rate, indicating the involvement of multiple mGluRs in this modulation. The frequency, but not the amplitude, of miniature IPSCs (mIPSCs) was decreased significantly by 3,5-DHPG or DCG-IV. Neither frequency nor amplitude of mIPSCs was affected by L-AP4. mGluR antagonists LY341495 (20 microM) plus CPPG (10 microM) significantly increased the amplitude of eIPSCs, indicating that endogenous mGluR activity suppresses GABA release to NM neurons. Furthermore, blockage of mGluRs increased GABA-evoked discharges recorded under physiological Cl(-) concentrations, whereas tACPD (100 microM) eliminated them. The results indicate that mGluRs play important roles in achieving balanced excitation and inhibition in NM and preserving fidelity of temporal information encoded by NM neurons.

Metabotropic suppression of excitation in murine autaptic hippocampal neurons

Depolarization-induced suppression of excitation (DSE) and inhibition (DSI) are forms of short-term neuronal plasticity involving postsynaptic release of an endocannabinoid and the activation of presynaptic cannabinoid CB1 receptors. We have recently reported that CB1-dependent DSE can be elicited in autaptic cultures of excitatory hippocampal neurons of the mouse. We now report that the same preparation exhibits a parallel G(q)-coupled receptor-dependent production of endocannabinoids causing retrograde inhibition, also via CB1 receptors, which we will refer to as metabotropic suppression of excitation (MSE). We tested a spectrum of G(q)-coupled receptor agonists and found that both muscarinic and metabotropic glutamate receptors (group I) mediate retrograde inhibition via CB1 receptors in autaptic hippocampal neurons. Thus these neurons possess not only the pre- and postsynaptic machinery necessary for DSE but also that for MSE. This permitted a closer examination of MSE and its interaction with other aspects of the endocannabinoid retrograde signalling machinery: MSE mimics and occludes DSE and is itself occluded by the endocannabinoid 2-arachidonoyl glycerol (2-AG), consistent with 2-AG as a likely mediator of MSE. In contrast to DSE, MSE undergoes heterologous desensitization over the time course of minutes. In keeping with data reported for metabotropic suppression of inhibition (MSI) and DSI in the hippocampus, subthreshold MSE and DSE act synergistically. We additionally found that Delta9-tetrahydrocannabinol, which has been shown to attenuate DSE, antagonizes MSE. Finally, we have distinguished a neuronal subpopulation that exhibits DSE and a differential complement of MSE-mediating Gq-coupled receptors, making possible contrasting studies of MSE. Autaptic endocannabinoid signalling is rich, robust and complex in a deceptively simple package, including a previously unreported postsynaptic mechanism of adaptation in addition to known presynaptic CB1 desensitization. These adaptive sites offer novel targets for modulation of endogenous cannabinoid signalling.

Depolarization-induced suppression of inhibition mediated by endocannabinoids at synapses from fast-spiking interneurons to medium spiny neurons in the striatum

Endogenous cannabinoids (endocannabinoids) act as retrograde inhibitory messengers in various regions of the brain. We have recently reported that endocannabinoids mediate short-term retrograde suppression of excitatory synaptic transmission from the neocortex to medium spiny (MS) neurons, the major projection neurons from the striatum. However, it remains unclear whether endocannabinoids modulate inhibitory transmission in the striatum. Here we show that depolarization of MS neurons induces transient suppression of inhibition that is mediated by retrograde endocannabinoid signalling. By paired recording from a fast-spiking (FS) interneuron and an MS neuron, we demonstrated that FS-MS inhibitory synapses undergo endocannabinoid-mediated retrograde suppression. We verified that GABAergic inhibitory terminals immunopositive for parvalbumin (PV), a marker for FS interneurons, expressed CB1 receptors. These PV-CB1 double-positive terminals surrounded dopamine D1 receptor-positive and D2 receptor-positive MS neurons; these constitute direct and indirect pathways, respectively. These results suggest that endocannabinoid-mediated retrograde suppression of inhibition influences information flow along both direct and indirect pathways, depending on the activity of MS neurons.

Forebrain-specific inactivation of Gq/G11 family G proteins results in age-dependent epilepsy and impaired endocannabinoid formation

Metabotropic receptors coupled to Gq/G11 family G proteins critically contribute to nervous system functions by modulating synaptic transmission, often facilitating excitation. We investigated the role of Gq/G11 family G proteins in the regulation of neuronal excitability in mice that selectively lack the alpha-subunits of Gq and G11, G alpha q and G alpha 11, respectively, in forebrain principal neurons. Surprisingly, mutant mice exhibited increased seizure susceptibility, and the activation of neuroprotective mechanisms was impaired. We found that endocannabinoid levels were reduced under both basal and excitotoxic conditions and that increased susceptibility to kainic acid could be normalized by the enhancement of endocannabinoid levels with an endocannabinoid reuptake inhibitor, while the competitive cannabinoid type 1 receptor antagonist SR141716A did not cause further aggravation. These findings indicate that Gq/G11 family G proteins negatively regulate neuronal excitability in vivo and suggest that impaired endocannabinoid formation in the absence of Gq/G11 contributes to this phenotype.

Endocannabinoid-mediated synaptic plasticity in the CNS

Changes in synaptic efficacy are thought to be crucial to experience-dependent modifications of neural function. The diversity of mechanisms underlying these changes is far greater than previously expected. In the last five years, a new class of use-dependent synaptic plasticity that requires retrograde signaling by endocannabinoids (eCB) and presynaptic CB1 receptor activation has been identified in several brain structures. eCB-mediated plasticity encompasses many forms of transient and long-lasting synaptic depression and is found at both excitatory and inhibitory synapses. In addition, eCBs can modify the inducibility of non-eCB-mediated forms of plasticity. Thus, the eCB system is emerging as a major player in synaptic plasticity. Given the wide distribution of CB1 receptors in the CNS, the list of brain structures and synapses expressing eCB-mediated plasticity is likely to expand.

Metabotropic glutamate receptors

Metabotropic glutamate receptors (mGlus) are a family of G-protein-coupled receptors activated by the neurotransmitter glutamate. Molecular cloning has revealed eight different subtypes (mGlu1-8) with distinct molecular and pharmacological properties. Multiplicity in this receptor family is further generated through alternative splicing. mGlus activate a multitude of signalling pathways important for modulating neuronal excitability, synaptic plasticity and feedback regulation of neurotransmitter release. In this review, we summarize anatomical findings (from our work and that of other laboratories) describing their distribution in the central nervous system. Recent evidence regarding the localization of these receptors in peripheral tissues will also be examined. The distinct regional, cellular and subcellular distribution of mGlus in the brain will be discussed in view of their relationship to neurotransmitter release sites and of possible functional implications.

Molecular composition of the endocannabinoid system at glutamatergic synapses

Endocannabinoids play central roles in retrograde signaling at a wide variety of synapses throughout the CNS. Although several molecular components of the endocannabinoid system have been identified recently, their precise location and contribution to retrograde synaptic signaling is essentially unknown. Here we show, by using two independent riboprobes, that principal cell populations of the hippocampus express high levels of diacylglycerol lipase alpha (DGL-alpha), the enzyme involved in generation of the endocannabinoid 2-arachidonoyl-glycerol (2-AG). Immunostaining with two independent antibodies against DGL-alpha revealed that this lipase was concentrated in heads of dendritic spines throughout the hippocampal formation. Furthermore, quantification of high-resolution immunoelectron microscopic data showed that this enzyme was highly compartmentalized into a wide perisynaptic annulus around the postsynaptic density of axospinous contacts but did not occur intrasynaptically. On the opposite side of the synapse, the axon terminals forming these excitatory contacts were found to be equipped with presynaptic CB1 cannabinoid receptors. This precise anatomical positioning suggests that 2-AG produced by DGL-alpha on spine heads may be involved in retrograde synaptic signaling at glutamatergic synapses, whereas CB1 receptors located on the afferent terminals are in an ideal position to bind 2-AG and thereby adjust presynaptic glutamate release as a function of postsynaptic activity. We propose that this molecular composition of the endocannabinoid system may be a general feature of most glutamatergic synapses throughout the brain and may contribute to homosynaptic plasticity of excitatory synapses and to heterosynaptic plasticity between excitatory and inhibitory contacts.

Dendritic calcium spikes are tunable triggers of cannabinoid release and short-term synaptic plasticity in cerebellar Purkinje neurons

Understanding the relationship between dendritic excitability and synaptic plasticity is vital for determining how dendrites regulate the input-output function of the neuron. Dendritic calcium spikes have been associated with the induction of long-term changes in synaptic efficacy. Here we use direct recordings from cerebellar Purkinje cell dendrites to show that synaptically activated local dendritic calcium spikes are potent triggers of cannabinoid release, producing a profound and short-term reduction in synaptic efficacy at parallel fiber synapses. Enhancing dendritic excitability by modulating dendritic large-conductance calcium-activated potassium (BK) channels improves the spread of dendritic calcium spikes and enhances cannabinoid release at the expense of spatial specificity. Our findings reveal that dendritic calcium spikes provide a local and tunable coincidence detection mechanism that readjusts synaptic gain when synchronous activity reaches a threshold, and they reveal a tight link between the regulation of dendritic excitability and the induction of synaptic plasticity.

Two coincidence detectors for spike timing-dependent plasticity in somatosensory cortex

Many cortical synapses exhibit spike timing-dependent plasticity (STDP) in which the precise timing of presynaptic and postsynaptic spikes induces synaptic strengthening [long-term potentiation (LTP)] or weakening [long-term depression (LTD)]. Standard models posit a single, postsynaptic, NMDA receptor-based coincidence detector for LTP and LTD components of STDP. We show instead that STDP at layer 4 to layer 2/3 synapses in somatosensory (S1) cortex involves separate calcium sources and coincidence detection mechanisms for LTP and LTD. LTP showed classical NMDA receptor dependence. LTD was independent of postsynaptic NMDA receptors and instead required group I metabotropic glutamate receptors and calcium from voltage-sensitive channels and IP3 receptor-gated stores. Downstream of postsynaptic calcium, LTD required retrograde endocannabinoid signaling, leading to presynaptic LTD expression, and also required activation of apparently presynaptic NMDA receptors. These LTP and LTD mechanisms detected firing coincidence on approximately 25 and approximately 125 ms time scales, respectively, and combined to implement the overall STDP rule. These findings indicate that STDP is not a unitary process and suggest that endocannabinoid-dependent LTD may be relevant to cortical map plasticity.