G protein-linked cannabinoid receptors are present in high density in cerebellum, where they inhibit adenylyl cyclase. This study explored whether cannabinoid receptors are co-localized with GABAB receptors on cerebellar granule cells. In rat cerebellar membranes, receptor-coupled G protein function was assayed by agonist stimulation of low Km GTPase as well as agonist-inhibited adenylyl cyclase. Addition of cannabinoid agonists together with the GABAB agonist, baclofen, produced additive responses with stimulation of low Km GTPase but only partially additive responses with inhibition of adenylyl cyclase. In Weaver and Staggerer but not Nervous mutant mice, cannabinoid-inhibited adenylyl cyclase was significantly decreased in cerebellar but not striatal membranes compared to littermate controls. In primary cultures of rat cerebellar granule cells, cannabinoids inhibited forskolin-stimulated cAMP levels, with IC50 values ranging from 0.1 to 2.0 microM. Cannabinoid inhibition of intracellular cAMP levels was blocked by pretreatment of cell cultures with pertussis toxin. Addition of baclofen and cannabinoid agonists together in cultured granule cells produced no additivity in response for inhibition of intracellular cAMP levels. These data confirm that G protein-linked cannabinoid receptors are present in cerebellar granule cells and may share adenylyl cyclase catalytic units with GABAB receptors.