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.     

Endocannabinoid signaling and synaptic plasticity in the brain

Repetitive firing neuron or activation of synaptic transmission plays an important role in the modulation of synaptic efficacy, such as long-term potentiation (LTP) and long-term depression (LTD). These activity-dependent changes in synaptic efficacy are thought to be critical to learning and memory; however, the underlying mechanisms remain to be defined. Endogenous cannabinoids (eCBs) are diffusible modulators that are released from depolarized postsynaptic neurons and act on presynaptic terminals. Persistent release of eCBs can lead to long-term modulation of synaptic plasticity in the brain. Given a broad distribution of eCB receptors in the brain, the eCB signaling system could contribute to use-dependent modification of brain functions.     

Presynaptic CB1 receptors regulate synaptic plasticity at cerebellar parallel fiber synapses

Endocannabinoids are potent regulators of synaptic strength. They are generally thought to modify neurotransmitter release through retrograde activation of presynaptic type 1 cannabinoid receptors (CB1Rs). In the cerebellar cortex, CB1Rs regulate several forms of synaptic plasticity at synapses onto Purkinje cells, including presynaptically expressed short-term plasticity and, somewhat paradoxically, a postsynaptic form of long-term depression (LTD). Here we have generated mice in which CB1Rs were selectively eliminated from cerebellar granule cells, whose axons form parallel fibers. We find that in these mice, endocannabinoid-dependent short-term plasticity is eliminated at parallel fiber, but not inhibitory interneuron, synapses onto Purkinje cells. Further, parallel fiber LTD is not observed in these mice, indicating that presynaptic CB1Rs regulate long-term plasticity at this synapse.     

Endocannabinoid-dependent LTD in a nociceptive synapse requires activation of a presynaptic TRPV-like receptor

Recent studies have found that some forms of endocannabinoid-dependent synaptic plasticity in the hippocampus are mediated through activation of transient potential receptor vanilloid (TRPV) receptors instead of cannabinoid receptors CB1 or CB2. The potential role for synaptic localization of TRPV receptors during endocannabinoid modulation of nociceptive synapses was examined in the leech CNS where it is possible to record from the same pair of neurons from one preparation to the next. Long-term depression (LTD) in the monosynaptic connection between the nociceptive (N) sensory neuron and the longitudinal (L) motor neuron was found to be endocannabinoid-dependent given that this depression was blocked by RHC-80267, an inhibitor of DAG lipase that is required for 2-arachidonoyl glycerol (2AG) synthesis. Intracellular injection of a second DAG lipase inhibitor, tetrahyrdolipstatin (THL) was also able to block this endocannabinoid-dependent LTD (ecLTD) when injected postsynaptically but not presynaptically. N-to-L ecLTD was also inhibited by the TRPV1 antagonists capsazepine and SB 366791. Bath application of 2AG or the TRPV1 agonists capsaicin and resiniferatoxin mimicked LTD and both capsaicin- and 2AG-induced depression were blocked by capsazepine. In addition, pretreatment with 2AG or capsaicin occluded subsequent expression of LTD induced by repetitive activity. Presynaptic, but not postsynaptic, intracellular injection of capsazepine blocked both activity- and 2AG-induced ecLTD, suggesting that a presynaptic TRPV-like receptor in the leech mediated this form of synaptic plasticity. These findings potentially extend the role ecLTD to nociceptive synapses and suggest that invertebrate synapses, which are thought to lack CB1/CB2 receptor orthologues, utilize a TRPV-like protein as an endocannabinoid receptor.     

Postsynaptic endocannabinoid release is critical to long-term depression in the striatum

The striatum functions critically in movement control and habit formation. The development and function of cortical input to the striatum are thought to be regulated by activity-dependent plasticity of corticostriatal glutamatergic synapses. Here we show that the induction of a form of striatal synaptic plasticity, long-term depression (LTD), is dependent on activation of the CB1 cannabinoid receptor. LTD was facilitated by blocking cellular endocannabinoid uptake, and postsynaptic loading of anandamide (AEA) produced presynaptic depression. The endocannabinoid necessary for striatal LTD is thus likely to be released postsynaptically as a retrograde messenger. These findings demonstrate a new role for endocannabinoids in the induction of long-term synaptic plasticity in a circuit necessary for habit formation and motor control.     

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.     

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

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

Depolarization-induced suppression of excitation in murine autaptic hippocampal neurones

Depolarization-induced suppression of excitation and inhibition (DSE and DSI) appear to be important forms of short-term retrograde neuronal plasticity involving endocannabinoids (eCB) and the activation of presynaptic cannabinoid CB1 receptors. We report here that CB1-dependent DSE can be elicited from autaptic cultures of excitatory mouse hippocampal neurones. DSE in autaptic cultures is both more robust and elicited with a more physiologically relevant stimulus than has been thus far reported for conventional hippocampal cultures. An additional requirement for autaptic DSE is filled internal calcium stores. Pharmacological experiments favour a role for 2-arachidonyl glycerol (2-AG) rather than arachidonyl ethanolamide (AEA) or noladin ether as the relevant endocannabinoid to elicit DSE. In particular, the latter two compounds fail to reversibly inhibit EPSCs, a quality inconsistent with the role of bona fide eCB mediating DSE. Δ⁹-Tetrahydrocannabinol (Δ⁹-THC) fails to inhibit EPSCs, yet readily occludes both DSE and EPSC inhibition by a synthetic CB1 agonist, WIN 55212-2. With long-term exposure (∼18 h), Δ⁹-THC also desensitizes CB1 receptors. Lastly, a functional endocannabinoid transporter is necessary for the expression of DSE.     

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) 210, 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 α/β-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.     

Anandamide inhibits metabolism and physiological actions of 2-arachidonoylglycerol in the striatum

Of the endocannabinoids (eCBs), anandamide (AEA) and 2-arachidonoylglycerol (2-AG) have received the most study. A functional interaction between these molecules has never been described. Using mouse brain slices, we found that stimulation of metabotropic glutamate 5 receptors by 3,5-dihydroxyphenylglycine (DHPG) depressed inhibitory transmission in the striatum through selective involvement of 2-AG metabolism and stimulation of presynaptic CB1 receptors. Elevation of AEA concentrations by pharmacological or genetic inhibition of AEA degradation reduced the levels, metabolism and physiological effects of 2-AG. Exogenous AEA and the stable AEA analog methanandamide inhibited basal and DHPG-stimulated 2-AG production, confirming that AEA is responsible for the downregulation of the other eCB. AEA is an endovanilloid substance, and the stimulation of transient receptor potential vanilloid 1 (TRPV1) channels mimicked the effects of endogenous AEA on 2-AG metabolism through a previously unknown glutathione-dependent pathway. Consistently, the interaction between AEA and 2-AG was lost after pharmacological and genetic inactivation of TRPV1 channels.     

Depolarization-induced retrograde synaptic inhibition in the mouse cerebellar cortex is mediated by 2-arachidonoylglycerol

Endocannabinoids acting on CB₁ cannabinoid receptors are involved in short- and long-term depression of synaptic transmission. The aim of the present study was to determine which endocannabinoid, anandamide or 2-arachidonoylglycerol (2-AG), is involved in depolarization-induced suppression of inhibition (DSI) in the cerebellar cortex, which is the most widely studied form of short-term depression. Depolarization of Purkinje cells in the mouse cerebellum led to an increase in intracellular calcium concentration and to suppression of the inhibitory input to these neurons (i.e. DSI occurred). Orlistat and RHC80267, two blockers of sn -1-diacylglycerol lipase, the enzyme catalysing 2-AG formation, abolished DSI by acting downstream of calcium influx. In contrast, DSI occurred also in the presence of a phospholipase C inhibitor. Intact operation of the calcium-dependent messengers calmodulin and Ca²⁺–calmodulin-dependent protein kinase II were necessary for DSI. DSI was potentiated by an inhibitor of the main 2-AG-degrading enzyme, monoacylglycerol lipase. Interference with the anandamide metabolizing enzyme, fatty acid amide hydrolase, did not modify DSI. Thus, three kinds of observations identified 2-AG as the endocannabinoid involved in DSI in the mouse cerebellum: DSI was abolished by diacylglycerol lipase inhibitors; DSI was potentiated by a monoglyceride lipase inhibitor; and DSI was not changed by an inhibitor of fatty acid amide hydrolase. Further experiments indicated that 2-AG is the endocannabinoid mediating short-term retrograde signalling also at other synapses: orlistat abolished DSI in the rat cerebellum, DSI in the mouse substantia nigra pars reticulata and depolarization-induced suppression of excitation in the mouse cerebellum.     

The serine hydrolases MAGL, ABHD6 and ABHD12 as guardians of 2-arachidonoylglycerol signalling through cannabinoid receptors

The endocannabinoid 2-arachidonoylglycerol (2-AG) is a lipid mediator involved in various physiological processes. In response to neural activity, 2-AG is synthesized post-synaptically, then activates pre-synaptic cannabinoid CB1 receptors (CB1Rs) in a retrograde manner, resulting in transient and long-lasting reduction of neurotransmitter release. The signalling competence of 2-AG is tightly regulated by the balanced action between 'on demand' biosynthesis and degradation. We review recent research on monoacylglycerol lipase (MAGL), ABHD6 and ABHD12, three serine hydrolases that together account for approx. 99% of brain 2-AG hydrolase activity. MAGL is responsible for approx. 85% of 2-AG hydrolysis and colocalizes with CB1R in axon terminals. It is therefore ideally positioned to terminate 2-AG-CB1R signalling regardless of the source of this endocannabinoid. Its acute pharmacological inhibition leads to 2-AG accumulation and CB1R-mediated behavioural responses. Chronic MAGL inactivation results in 2-AG overload, desensitization of CB1R signalling and behavioural tolerance. ABHD6 accounts for approx. 4% of brain 2-AG hydrolase activity but in neurones it rivals MAGL in efficacy. Neuronal ABHD6 resides post-synaptically, often juxtaposed with CB1Rs, and its acute inhibition leads to activity-dependent accumulation of 2-AG. In cortical slices, selective ABHD6 blockade facilitates CB1R-dependent long-term synaptic depression. ABHD6 is therefore positioned to guard intracellular pools of 2-AG at the site of generation. ABHD12 is highly expressed in microglia and accounts for approx. 9% of total brain 2-AG hydrolysis. Mutations in ABHD12 gene are causally linked to a neurodegenerative disease called PHARC. Whether ABHD12 qualifies as a bona fide member to the endocannabinoid system remains to be established.     

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

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

Modular competition driven by NMDA receptor subtypes in spike-timing-dependent plasticity

N-methyl-d-aspartate receptors (NMDARs) play a critical role in transducing neuronal activity patterns into changes in synaptic strength. However, how they mediate this transduction in response to physiological stimuli has remained elusive. In particular, it has been debated whether different NMDAR subtypes play opposing signaling roles in synaptic plasticity. Using perforated patch-clamp recordings from pairs of synaptically connected glutamatergic neurons in dissociated hippocampal culture, we found that spike-timing-dependent potentiation induced by pairing pre- and postsynaptic spikes required the activation of a fast component of NMDAR current that is likely to be mediated by NR2A-containing NMDARs (NR2A-NRs). In contrast, spike-timing-dependent depression required a slow component of NMDAR current carried by NR2B-containing NMDARs (NR2B-NRs). CV analysis showed that the locus of this depression was primarily presynaptic in pairs of cells making strong synaptic connections, whereas weaker synapses showed no clear preference for pre- or postsynaptic expression. This depression was not significantly reduced by antagonism of the CB1 receptor, in contrast to spike-timing-dependent depression in the neocortex that requires presynaptic CB1 signaling. With blockade of NR2B-NRs, spike triplets that contained both potentiating and depressing spike-timing components induced net potentiation. However, when the putative NR2A-NR population is inhibited, these spike triplets resulted in either depression or no net change, depending on the temporal order of the spike-timing components. These results imply a dynamic competition between signaling modules that can be biased by differentially antagonizing NMDAR subtypes during the induction of spike-timing-dependent plasticity. Using a simple model, we show that such a modular competition recapitulates our observations.     

Activity-dependent release and actions of endocannabinoids in the rat hypothalamic supraoptic nucleus

Exogenous cannabinoids have been shown to significantly alter neuroendocrine output, presaging the emergence of endogenous cannabinoids as important signalling molecules in the neuroendocrine control of homeostatic and reproductive functions, including the stress response, energy metabolism and gonadal regulation. We showed recently that magnocellular and parvocellular neuroendocrine cells of the hypothalamic paraventricular nucleus and supraoptic nucleus (SON) respond to glucocorticoids by releasing endocannabinoids as retrograde messengers to modulate the synaptic release of glutamate. Here we show directly for the first time that both of the main endocannabinoids, anandamide (AEA) and 2-arachidonoyl glycerol (2-AG), are released in an activity-dependent fashion from the soma/dendrites of SON magnocellular neurones and suppress synaptic glutamate release and postsynaptic spiking. Cannabinoid reuptake blockade increases activity-dependent endocannabinoid levels in the region of the SON, and results in the inhibition of synaptically driven spiking activity in magnocellular neurones. Together, these findings demonstrate an activity-dependent release of AEA and 2-AG that leads to the suppression of glutamate release and that is capable of shaping spiking activity in magnocellular neurones. This activity-dependent regulation of excitatory synaptic input by endocannabinoids may play a role in determining spiking patterns characteristic of magnocellular neurones under stimulated conditions.     

2-arachidonoylglycerol, an endogenous cannabinoid receptor ligand, induces the migration of EoL-1 human eosinophilic leukemia cells and human peripheral blood eosinophils

2-arachidonoylglycerol (2-AG) is an endogenous cannabinoid receptor ligand. To date, two types of cannabinoid receptors have been identified: the CB1 receptor, abundantly expressed in the brain, and the CB2 receptor, expressed in various lymphoid tissues such as the spleen. The CB1 receptor has been assumed to play an important role in the regulation of synaptic transmission, whereas the physiological roles of the CB2 receptor remain obscure. In this study, we examined whether the CB2 receptor is present in human eosinophils and found that the CB2 receptor is expressed in human peripheral blood eosinophils. In contrast, human neutrophils do not contain a significant amount of the CB2 receptor. We then examined the effect of 2-AG on the motility of eosinophils. We found that 2-AG induces the migration of human eosinophilic leukemia EoL-1 cells. The migration evoked by 2-AG was abolished in the presence of SR144528, a CB2 receptor antagonist, or by pretreatment of the cells with pertussis toxin, suggesting that the CB2 receptor and Gi/o are involved in the 2-AG-induced migration. The migration of EoL-1 cells induced by 2-AG was suggested to be a result of chemotaxis. In contrast to 2-AG, neither anandamide nor free arachidonic acid elicited the migration. Finally, we examined the effect of 2-AG on human peripheral blood eosinophils and neutrophils and found that 2-AG induces migration of eosinophils but not neutrophils. These results suggest that the CB2 receptor and its endogenous ligand 2-AG may be closely involved in allergic inflammation accompanied by the infiltration of eosinophils.     

Synapse-specific plasticity and compartmentalized signaling in cerebellar stellate cells

Here we demonstrate that cerebellar stellate cells diffusionally isolate synaptically evoked signals in dendrites and are capable of input-specific synaptic plasticity. Sustained activity of parallel fibers induces a form of long-term depression that requires opening of calcium (Ca(2+))-permeable AMPA-type glutamate receptors (CP-AMPARs) and signaling through class 1 metabotropic glutamate receptors (mGluR1) and CB1 receptors. This depression is induced by postsynaptic increases in Ca(2+) concentration ([Ca(2+)]) and is limited to activated synapses. To understand how synapse-specific plasticity is induced by diffusible second messengers in aspiny dendrites, we examined diffusion of Ca(2+) and small molecules within stellate cell dendrites. Activation of a single parallel fiber opened CP-AMPARs, generating long-lived Ca(2+) transients that were confined to submicron dendritic stretches. The diffusion of Ca(2+) was severely retarded due to interactions with parvalbumin and a general restriction of small molecule mobility. Thus stellate cell dendrites spatially restrict signaling cascades that lead from CP-AMPAR activation to endocannabinoid production and trigger the selective regulation of active synapses.     

A single in-vivo exposure to delta 9THC blocks endocannabinoid-mediated synaptic plasticity

Endogenous cannabinoids (eCB) mediate synaptic plasticity in brain regions involved in learning and reward. Here we show that in mice, a single in-vivo exposure to Delta 9-tetrahydrocannabinol (THC) abolishes the retrograde signaling that underlies eCB-mediated synaptic plasticity in both nucleus accumbens (NAc) and hippocampus in vitro. This effect is reversible within 3 days and is associated with a transient modification in the functional properties of cannabinoid receptors.     

Novel cannabinoid-sensitive receptor mediates inhibition of glutamatergic synaptic transmission in the hippocampus

Psychoactive effects of cannabinoids are thought to be mediated, at least in part, by suppression of both glutamate and GABA release via CB1 cannabinoid receptor. Two types of cannabinoid receptor (CB1 and CB2) have been cloned so far. The CB1 receptors are abundantly expressed in the nervous system, whereas CB2 receptors are limited to lymphoid organs (Matsuda et al., 1990; Munro et al., 1993). Immunocytochemical and electrophysiological studies revealed that in the hippocampus CB1 receptors are expressed on axon terminals of GABAergic inhibitory interneurons (Tsou et al., 1999; Katona et al., 1999) and activation of these receptors decreases GABA release (Hájos et al., 2000). Other physiological studies pointed out the involvement of CB1 receptors in the modulation of hippocampal glutamatergic synaptic transmission and long-term potentiation (Stella et al., 1997; Misner and Sullivan, 1999), but anatomical studies could not confirm the existence of CB1 receptors on glutamatergic terminals. Here we examined cannabinoid actions on both glutamatergic and GABAergic synaptic transmission in the hippocampus of wild type (CB1+/+) and CB1 receptor knockout mice (CB1-/-). The synthetic cannabinoid agonist WIN55,212-2 reduced the amplitudes of excitatory postsynaptic currents in both wild type and CB1-/- mice, while inhibitory postsynaptic currents were decreased only in wild type mice, but not in CB1-/- animals. Our findings are consistent with a CB1 cannabinoid receptor-dependent modulation of GABAergic postsynaptic currents, but a novel cannabinoid-sensitive receptor must be responsible for the inhibition of glutamatergic neurotransmission.     

Cannabinoids suppress synaptic input to neurones of the rat dorsal motor nucleus of the vagus nerve

Cannabinoids bind central type 1 receptors (CB1R) and modify autonomic functions, including feeding and anti-emetic behaviours, when administered peripherally or into the dorsal vagal complex. Western blots and immunohistochemistry indicated the expression of CB1R in the rat dorsal vagal complex, and tissue polymerase chain reaction confirmed that CB1R message was made within the region. To identify a cellular substrate for the central autonomic effects of cannabinoids, whole-cell patch-clamp recordings were made in brainstem slices to determine the effects of CB1R activation on synaptic transmission to neurones of the dorsal motor nucleus of the vagus (DMV). A subset of these neurones was identified as gastric related after being labelled retrogradely from the stomach. The CB1R agonists WIN55,212-2 and anandamide decreased the frequency of spontaneous excitatory or inhibitory postsynaptic currents in a concentration-related fashion, an effect that persisted in the presence of tetrodotoxin. Paired pulse ratios of electrically evoked postsynaptic currents were also increased by WIN55,212-2. The effects of  WIN55,212-2 were sensitive to the selective CB1R antagonist AM251. Cannabinoid agonist effects on synaptic input originating from neurones in the nucleus tractus solitarius (NTS) were determined by evoking activity in the NTS with local glutamate application. Excitatory and inhibitory synaptic inputs arising from the NTS were attenuated by WIN55,212-2. Our results indicate that cannabinoids inhibit transfer of synaptic information to the DMV, including that arising from the NTS, in part by acting at receptors located on presynaptic terminals contacting DMV neurones. Inhibition of synaptic input to DMV neurones is likely to contribute to the suppression of visceral motor responses by cannabinoids.