Coactivation of CB1 and GPR55 promotes GABA release and motor behavior at striatonigral terminals through increased dimerization induced by CB1 activation

CB1 and GPR55 receptors form heteromers in striatal neurons; however, the effects of these heteromers on GABA release at their terminals and their impact on motor behavior remain unknown. In this study, we investigate the presence of CB1-GPR55 heteromers on striatonigral neurons and their axon terminals, and also assess their impact on cAMP accumulation, GABA release, and motor behavior. Furthermore, we explore the effects of sequential receptor activation to examine the phenomenon of increased dimerization induced by receptor activation. A PLA assay combined with Substance P immunofluorescence demonstrated the presence of CB1-GPR55 heteromers in the dorsal striatum and substantia nigra of rats. The kainic acid lesion in the striatum leads to a decrease in PLA dots in both regions. Sequential activation of CB1R, followed by GPR55 activation (CB1→GPR55), increased cAMP accumulation and GABA release at the nigral terminals more compared to GPR55 alone activation. In contrast, simultaneous activation (CB1 + GPR55) or the reverse (GPR55→CB1) did not affect the stimulation effects of GPR55 on cAMP accumulation or GABA release. Additionally, CB1/GPR55 immunoprecipitation in synaptosomes revealed an increase during the sequential activation of CB1→GPR55. Treatments with PTx or ChTx did not alter the effects of CB1→GPR55 sequential activation on GABA release. Finally, intranigral injections of a CB1→GPR55 agonist induced more contralateral turns than GPR55 activation alone. These findings indicate that the sequential activation of CB1→GPR55 within CB1/GPR55 heteromers in striatonigral neurons enhances cAMP accumulation, GABA release, and motor behavior by increasing heteromerization via CB1 activation.