Cannabinoid receptor and WIN-55,212-2-stimulated [35S]GTP gamma S binding and cannabinoid receptor mRNA levels in the basal ganglia and the cerebellum of adult male rats chronically exposed to delta 9-tetrahydrocannabinol.

Abstract

The inhibition of motor behavior in rodents caused by the exposure to plant or synthetic cannabinoids has been reported to develop tolerance after repeated exposure. This tolerance seems to have a pharmacodynamic basis, since downregulation of cannabinoid receptors in motor areas, basal ganglia and cerebellum, has been demonstrated in cannabinoid-tolerant rats. The present study was designed to further explore this previous evidence by analyzing simultaneously in several motor areas of delta 9-tetrahydrocannabinol- (delta 9-THC)-tolerant rats: 1. Cannabinoid receptor binding, by using [3H]WIN-55,212-2 autoradiography; 2. Cannabinoid receptor activation of signal transduction mechanisms, by using WIN-55,212-2-stimulated [35S]-guanylyl-5'-O-(gamma-thio)-triphosphate ([35S]-GTP gamma S) autoradiography; 3. Cannabinoid receptor mRNA expression, quantitated by in situ hybridization. Results were as follows. As expected, the exposure to delta 9-THC for 5 d resulted in a decrease of cannabinoid receptor binding in the molecular layer of the cerebellum, medial, and lateral caudate-putamen and, in particular, entopeduncular nucleus. We also found decreased cannabinoid receptor binding in the superficial and deep layers of the cerebral cortex, two regions used as a reference to test the specificity of changes observed in motor areas. There were only two brain regions, the globus pallidus and the substantia nigra, where the specific binding for cannabinoid receptors was unaltered after 5 d of a daily delta 9-THC administration. However, in the substantia nigra, the magnitude of WIN-55,212-2-stimulated [35S]-GTP gamma S binding was lesser in delta 9-THC-tolerant rats than controls, thus suggesting a possible specific change at the level of receptor coupling to GTP-binding proteins. This was not seen neither in the globus pallidus nor in the lateral caudate-putamen, where agonist stimulation produced similar [35S]-GTP gamma S binding levels in delta 9-THC-tolerant rats and controls. Finally, animals chronically exposed to delta 9-THC also exhibited a decrease in the levels of cannabinoid receptor mRNA in the medial and lateral caudate-putamen, but there were no changes in the cerebellum (granular layer) and cerebral cortex. In summary, the chronic exposure to delta 9-THC resulted in a decrease in cannabinoid receptor binding and mRNA levels in the caudate-putamen, where cell bodies of cannabinoid receptor-containing neurons in the basal ganglia are located. However, this decrease particularly affected the receptor binding levels in those neurons projecting to the entopeduncular nucleus, but not in those projecting to the globus pallidus and substantia nigra, although, in this last region, a specific decrease in the efficiency of receptor activation of signal transduction mechanisms was seen in delta 9-THC-tolerant rats. The chronic exposure to delta 9-THC also resulted in decreased cannabinoid receptor binding in the cerebellum, although without affecting mRNA expression.