Mechanisms of Cannabinoid CB2 Receptor-Mediated Reduction of Dopamine Neuronal Excitability in Mouse Ventral Tegmental Area

We have recently reported that activation of cannabinoid type 2 receptors (CB2Rs) reduces dopamine (DA) neuron excitability in mouse ventral tegmental area (VTA). Here, we elucidate the underlying mechanisms. Using cell-attached recording in VTA slices, bath-application of  CB2R agonists (JWH133 or five other CB2R agonists) significantly reduced VTA DA neuron action potential (AP) firing rate. Under patch-clamp whole-cell recording model, JWH133 (10 µM) mildly reduced the frequency of miniature excitatory postsynaptic currents (mEPSCs) but not miniature inhibitory postsynaptic currents (mIPSCs). JWH133 also did not alter evoked EPSCs or IPSCs. In freshly dissociated VTA DA neurons, JWH133 reduced AP firing rate, delayed AP initiation and enhanced AP after-hyperpolarization. In voltage-clamp recordings, JWH133 enhanced M-type K+ currents and this effect was absent in CB2-/- mice and abolished by co-administration of a selective CB2R antagonist (AM630). CB2R-mediated inhibition in VTA DA neuron firing can be mimicked by M-current opener and blocked by M-current blocker. In addition, enhancement of neuronal cAMP by forskolin reduced M-current and increased DA neuron firing rate. Finally, pharmacological block of synaptic transmission by NBQX, D-APV and picrotoxin in VTA slices failed to prevent CB2R-mediated inhibition, while intracellular infusion of guanosine 5'-O-2-thiodiphosphate (GDP-β-S) through recording electrode to block postsynaptic G-protein function prevented JWH133-induced reduction in AP firing. Collectively, our results suggest that CB2Rs modulate VTA DA neuron excitability mainly through an intrinsic mechanism, including a CB2R-mediated reduction of intracellular cAMP, and in turn enhancement of M-type K+ currents. Funding Statement: This research was supported by the Barrow Neuroscience Foundation, the BNI-BMS Seed Fund, the CNSF (81771437), and the National Institute on Drug Abuse, Intramural Research Program. Declaration of Interests: Dr. Ma, ZG reports no disclosures, Dr. Gao, FF reports no disclosures, Dr. Larsen, B. reports no disclosures, Dr. Gao M reports no disclosures, D. Chen, DJ reports no disclosures, Xiaokuang Ma reports no disclosures, Dr. Qiu, SF reports no disclosures, Dr. Zhou Y reports no disclosures, Dr. Xie, JX reports no disclosures, Dr. Xi, ZX reports no disclosures, Dr. Wu J reports no disclosures. Ethics Approval Statement: All experimental procedures were approved by the Institutional Animal Care and Use Committee at the Barrow Neurological Institute.