Effect of delta 9-tetrahydrocannabinol on circling in rats induced by intranigral muscimol administration

Unilateral injection of 25 ng of muscimol into the rat substantia nigra pars reticulata induced contralateral circling. Circling induced by 25 ng of muscimol was increased by simultaneous administration of 1 microgram of delta 9-tetrahydrocannabinol and abolished by 10 micrograms. Previous experiments have shown that delta 9-tetrahydrocannabinol enhances muscimol-induced catalepsy in the globus pallidus. These results identify another brain area in which synergism between delta 9-tetrahydrocannabinol and muscimol can occur and show that delta 9-tetrahydrocannabinol can enhance an excitatory as well as inhibitory GABAergic motor effect. The data support the hypothesis that cannabinoids can facilitate transmission along central gamma-aminobutyric acid (GABA) releasing pathways.