THE ENDOGENOUS CANNABINOID RECEPTOR-LIGAND, ANANDAMIDE, INHIBITS THE MOTOR BEHAVIOR - ROLE OF NIGROSTRIATAL DOPAMINERGIC-NEURONS

The present study has been designed to test whether the recently described endogenous ligand for the cannabinoid receptor, arachidonylethanolamide, termed anandamide, can mimic the effects produced by exogenous cannabinoids on motor behavior and to test possible neurochemical substrates for this potential effect. To this end, adult male rats were submitted to an acute i.p. injection of anandamide, Delta(9)-tetrahydrocannabinol (THC) or vehicle. Animals were behaviorally tested ten minutes after injection of the drug and, then, sacrificed and their brains used for dopaminergic analyses. Ambulation was not significantly affected by the treatment with either THC or anandamide, but a very pronounced increase was observed in the time spent in inactivity in rats treated with either THC or anandamide. This was accompanied by a marked decrease in the frequency of spontaneous non-ambulatory activities, such as grooming and rearing, although only the administration of THC decreased shaking behavior. The anandamide-induced decrease in grooming was dose-dependent, but the decrease in rearing was higher with the dose of 3 mg/kg than with the dose of 10 mg/kg. The administration of anandamide also caused a dose-dependent decrease in the activity of tyrosine hydroxylase and in the ratio between the number of D1 and D2 receptors in the striatum. Moreover, the administration of 3 mg/kg of anandamide significantly decreased the contents of dopamine and L-3,4-dihydroxyphenylacetic acid in the striatum although lesser and higher doses were less effective. THC only tended to decrease these parameters. No changes were seen in dopaminergic activity in the limbic forebrain after either cannabimimetics. In summary, anandamide, as well as THC, decreases motor behavior. This effect was paralleled by a reduction in the activity of nigrostriatal dopaminergic neurons. However, subtle differences in the behavioral and neurochemical effects between anandamide and THC could be observed.