Repeated cocaine administration upregulates CB2 receptor expression in striatal medium-spiny neurons that express dopamine D1 receptors in mice

Cannabinoid CB 2 receptors (CB 2 R) are importantly involved in drug reward and addiction. However, the cellular mechanisms underlying CB 2 R action remain unclear. We have previously reported that cocaine self-administration upregulates CB 2 R expression in midbrain dopamine (DA) neurons. In the present study, we investigated whether cocaine or heroin also alters CB 2 R expression in striatal medium-spiny neurons that express dopamine D 1 or D 2 receptors (D 1 -MSNs, D 2 -MSNs) and microglia. Due to the concern of CB 2 R antibody specificity, we developed three mouse CB 2 -specific probes to detect CB 2 R mRNA using quantitative RT-PCR and RNAscope in situ hybridization (ISH) assays. We found that a single injection of cocaine failed to alter, while repeated cocaine injections or self-administration dose-dependently upregulated CB 2 R gene expression in both brain (cortex and striatum) and periphery (spleen). In contrast, repeated administration of heroin produced a dose-dependent reduction in striatal CB 2 mRNA expression. RNAscope ISH assays detected CB 2 R mRNA in striatal D 1 - and D 2 -MSNs, not in microglia. We then used transgenic CX3CR1 eGFP/+ microglia reporter mice and D 1 - or D 2 -Cre-RiboTag mice to purify striatal microglia or ribosome-associated mRNAs from CX3CR1 eGFP/+ , D 1 -MSNs, or D 2 -MSNs, respectively. We found that CB 2 R upregulation occurred mainly in D 1 -MSNs, not in D 2 -MSNs or microglia, in the nucleus accumbens rather than the dorsal striatum. These findings indicate that repeated cocaine exposure may upregulate CB 2 R expression in both brain and spleen, with regional and cell type-specific profiles. In the striatum, CB 2 R upregulation occurs mainly in D 1 -MSNs in the nucleus accumbens. Given the important role of D 1 -MSNs in brain reward function, the present findings provide new insight into mechanisms by which brain CB 2 Rs modulate cocaine action.