Drugs that increase GABA levels in the brain reduce cocaine seeking in rodents and humans, suggesting that GABAergic inhibition regulates cocaine-seeking behavior. We previously reported that repeated cocaine exposurein vivofacilitates long-term potentiation by reducing the strength of GABAergic inhibition in dopamine neurons of the ventral tegmental area (VTA). Selective blockade of cocaine-induced reduction of GABAergic inhibition in the VTA might diminish cocaine-induced aberrant synaptic plasticity and addictive behavior. Here, we investigated the mechanism for cocaine-induced reduction of GABAergic inhibition. We show that a pathophysiologically relevant concentration of cocaine enables a normally ineffective stimulus to induce long-term depression (LTD) of IPSCs (I-LTD) in VTA dopamine neurons of midbrain slices. Activation of D₂dopamine receptors and group I metabotropic glutamate receptors and subsequent recruitment of endocannabinoid signaling are required for I-LTD induction. We further demonstrate thatin vivopretreatment with antagonists to these receptors blocks cocaine-induced reduction of GABAergic inhibition and that repeated cocaine exposurein vivooccludes the subsequent induction of I-LTDex vivo. Together, these results suggest that repeated cocaine exposure reduces the strength of GABAergic inhibition in dopamine neurons by inducing I-LTD-like modificationin vivo.

Endocannabinoid (eCB) signaling mediates depolarization-induced suppression of excitation (DSE) and inhibition (DSI), two prominent forms of retrograde synaptic depression. N-Arachidonoylethanolamine (AEA) and 2-arachidonoylglycerol (2-AG), two known eCBs, are degraded by fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL), respectively. Selective blockade of FAAH and MAGL is critical for determining the roles of the eCBs in DSE/DSI and understanding how their action is regulated. 4-Nitrophenyl 4-(dibenzo[d][1,3]dioxol-5-yl(hydroxy)methyl)piperidine-1-carboxylate (JZL184) is a recently developed, highly selective, and potent MAGL inhibitor that increases 2-AG but not AEA concentrations in mouse brain. Here, we report that JZL184 prolongs DSE in Purkinje neurons in cerebellar slices and DSI in CA1 pyramidal neurons in hippocampal slices. The effect of JZL184 on DSE/DSI is mimicked by the nonselective MAGL inhibitor methyl arachidonyl fluorophosphonate. In contrast, neither the selective FAAH inhibitor cyclohexylcarbamic acid 3′-carbomoylbiphenyl-3-yl ester (URB597) nor FAAH knockout has a significant effect on DSE/DSI. JZL184 produces greater enhancement of DSE/DSI in mouse neurons than that in rat neurons. The latter finding is consistent with biochemical studies showing that JZL184 is more potent in inhibiting mouse MAGL than rat MAGL. These results indicate that the degradation of 2-AG by MAGL is the rate-limiting step that determines the time course of DSE/DSI and that JZL184 is a useful tool for the study of 2-AG-mediated signaling.

The endocannabinoid (eCB) system regulates mood, emotion, and stress coping, and dysregulation of the eCB system is critically involved in pathophysiology of depression. The eCB ligand 2-arachidonoylglycerol (2-AG) is inactivated by monoacylglycerol lipase (MAGL). Using chronic unpredictable mild stress (CUS) as a mouse model of depression, we examined how 2-AG signaling in the hippocampus was altered in depressive-like states and how this alteration contributed to depressive-like behavior. We report that CUS led to impairment of depolarization-induced suppression of inhibition (DSI) in mouse hippocampal CA1 pyramidal neurons, and this deficiency in 2-AG-mediated retrograde synaptic depression was rescued by MAGL inhibitor JZL184. CUS induced depressive-like behaviors and decreased mammalian target of rapamycin (mTOR) activation in the hippocampus, and these biochemical and behavioral abnormalities were ameliorated by chronic JZL184 treatments. The effects of JZL184 were mediated by cannabinoid CB₁ receptors. Genetic deletion of mTOR with adeno-associated viral (AAV) vector carrying the Cre recombinase in the hippocampus of mTORf/f mice recapitulated depressive-like behaviors induced by CUS and abrogated the antidepressant-like effects of chronic JZL184 treatments. Our results suggest that CUS decreases eCB-mTOR signaling in the hippocampus, leading to depressive-like behaviors, whereas MAGL inhibitor JZL184 produces antidepressant-like effects through enhancement of eCB-mTOR signaling.

Endocannabinoid (eCB) signaling is tightly regulated by eCB biosynthetic and degradative enzymes. The eCB 2-arachidonoylglycerol (2-AG) is hydrolyzed primarily by monoacylglycerol lipase (MAGL). Here, we investigated whether eCB signaling, synaptic function, and learning behavior were altered in MAGL knock-out mice. We report that MAGL^−/−mice exhibited prolonged depolarization-induced suppression of inhibition (DSI) in hippocampal CA1 pyramidal neurons, providing genetic evidence that the inactivation of 2-AG by MAGL determines the time course of the eCB-mediated retrograde synaptic depression. CB₁receptor antagonists enhanced basal IPSCs in CA1 pyramidal neurons in MAGL^−/−mice, while the magnitude of DSI or CB₁receptor agonist-induced depression of IPSCs was decreased in MAGL^−/−mice. These results suggest that 2-AG elevations in MAGL^−/−mice cause tonic activation and partial desensitization of CB₁receptors. Genetic deletion of MAGL selectively enhanced theta burst stimulation (TBS)-induced long-term potentiation (LTP) in the CA1 region of hippocampal slices but had no significant effect on LTP induced by high-frequency stimulation or long-term depression induced by low-frequency stimulation. The enhancement of TBS-LTP in MAGL^−/−mice appears to be mediated by 2-AG-induced suppression of GABA_Areceptor-mediated inhibition. MAGL^−/−mice exhibited enhanced learning as shown by improved performance in novel object recognition and Morris water maze. These results indicate that genetic deletion of MAGL causes profound changes in eCB signaling, long-term synaptic plasticity, and learning behavior.

Endocannabinoid (eCB) signaling mediates short-term and long-term synaptic depression (LTD) in many brain areas. In the ventral tegmental area (VTA) and striatum, D₂ dopamine receptors cooperate with group I metabotropic glutamate receptors (mGluRs) to induce eCB-mediated LTD of glutamatergic excitatory and GABAergic inhibitory (I-LTD) synaptic transmission. Because D₂ receptors and group I mGluR agonists are capable of inducing the release of eCBs, the predominant hypothesis is that the cooperation between these receptors to induce eCB-mediated synaptic depression results from the combined activation of type I cannabinoid (CB₁) receptors by the eCBs. By determining the downstream effectors for D₂ receptor and group I mGluR activation in VTA dopamine neurons, we show that group I mGluR activation contributes to I-LTD induction by enhancing eCB release and CB₁ receptor activation. However, D₂ receptor activation does not enhance CB₁ receptor activation, but facilitates I-LTD induction via direct inhibition of cAMP-dependent protein kinase A (PKA) signaling. We further demonstrate that cAMP/PKA signaling pathway is the downstream effector for CB₁ receptors and is required for eCB-mediated I-LTD induction. Our results suggest that D₂ receptors and CB₁ receptors target the same downstream effector cAMP/PKA signaling pathway to induce I-LTD and D₂ receptor activation facilitates eCB-mediated I-LTD in dopamine neurons not by enhancing CB₁ receptor activation, but by enhancing its downstream effects.