Repeated cocaine administration upregulates CB receptor expression in striatal medium-spiny neurons that express dopamine D receptors in mice.

Cannabinoid CB receptors (CBR) are importantly involved in drug reward and addiction. However, the cellular mechanisms underlying CBR action remain unclear. We have previously reported that cocaine self-administration upregulates CBR expression in midbrain dopamine (DA) neurons. In the present study, we investigated whether cocaine or heroin also alters CBR expression in striatal medium-spiny neurons that express dopamine D or D receptors (D-MSNs, D-MSNs) and microglia. Due to the concern of CBR antibody specificity, we developed three mouse CB-specific probes to detect CBR 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 CBR 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 mRNA expression. RNAscope ISH assays detected CBR mRNA in striatal D- and D-MSNs, not in microglia. We then used transgenic CX3CR1 microglia reporter mice and D- or D-Cre-RiboTag mice to purify striatal microglia or ribosome-associated mRNAs from CX3CR1, D-MSNs, or D-MSNs, respectively. We found that CBR upregulation occurred mainly in D-MSNs, not in D-MSNs or microglia, in the nucleus accumbens rather than the dorsal striatum. These findings indicate that repeated cocaine exposure may upregulate CBR expression in both brain and spleen, with regional and cell type-specific profiles. In the striatum, CBR upregulation occurs mainly in D-MSNs in the nucleus accumbens. Given the important role of D-MSNs in brain reward function, the present findings provide new insight into mechanisms by which brain CBRs modulate cocaine action.