ABSTRACT
OBJECTIVE Cannabinoids modulate do-pamine(DA)transmission and DA-related behavior,which has been thought to be mediated initially by acti-vation of cannabinoid CB1 receptors(CB1Rs)on GABA neurons.However,the cellular and receptor mechanisms underlying cannabinoids' psychoactive effects are not fully understood.The present study is to explore the pos-sible expression character of CB1Rs and elucidated the underlying mechanism of them.METHODS We took advantage of RNAscope in situ hybridization(ISH)assays and triple-staining assays to detect the CB1R-expressing neurons.We established an optical intracranial self-stimulation(OICSS)behavioral model by using opto-genetics to study dopaminergic reinforcement function.Natural and synthetic cannabinoids were used to study the function of CB1Rs.Conditional genetic depletion of CB1Rs and behavioral assay were performed to study the modulatory role of CB1Rs in DA-related behaviors.RESULTS We found that CB1Rs are also expressed in a subset of DA neurons and functionally underlie cannabi-noid action in male and female mice.ISH assays demon-strated CB1 mRNA in tyrosine hydroxylase(TH)-posi-tive DA neurons in the ventral tegmental area(VTA)and glutamate decarboxylase 1(GAD1)-positive GABA neu-rons.The CB1R expressing DA neurons were located mainly in the middle portion of the VTA with the number of CB1-TH colocalization progressively decreasing from the medial to the lateral VTA.Triple-staining assays indi-cated CB1R mRNA colocalization with both TH and vesicular glutamate transporter 2(VgluT2,a glutamate neuronal marker)in the medial VTA close to the midline of the brain.Optogenetic activation of this population of DA neurons was rewarding as assessed by OICSS.D9-tetrahydrocannabinol(D9-THC)or ACEA(a selective CB1R agonist)dose-dependently inhibited optical intra-cranial self-stimulation in DAT-Cre control mice,but not in conditional knockout mice with the CB1R gene absent in DA neurons.In addition,deletion of CB1Rs from DA neurons attenuated D9-THC-induced reduction in DA release in the NAc,locomotion,and anxiety.CONCLU-SION Our results indicated that CB1Rs are expressed in a subset of DA neurons that corelease DA and gluta-mate,and functionally underlie cannabinoid modulation of DA release and DA-related behavior.