Functional consequences of the repeated administration of Delta9-tetrahydrocannabinol in the rat.

The repeated administration of Delta(9)-tetrahydrocannabinol (THC) results in tolerance to many of its behavioral and physiological effects. It also produces changes in the functionality of cannabinoid receptors. What is not completely understood is how these cellular events translate into the behavioral and physiological changes that are associated with repeated cannabinoid agonist treatment. The purpose of these studies was to determine the development of changes in the patterns of functional activity, as measured by the 2-[14C]deoxyglucose method (2DG), associated with repeated THC exposure. Male Sprague-Dawley rats (n=4-5) were administered THC (vehicle or 10 mg/kg, intraperitoneally), daily for 7 or 21 days. Fifteen minutes following the final THC treatment the 2DG procedure was initiated. In separate sets of rats similarly treated with THC, locomotor activity and core body temperature were measured at corresponding time points in order to establish the behavioral profile of repeated THC administration. The acute administration of THC following 7 or 21 days of drug exposure resulted in a significant attenuation of changes in rates of glucose utilization throughout the majority of brain regions analyzed when compared to the large global decreases observed following a single administration of THC. After 7 and 21 days of treatment, cerebral metabolic rates were no longer different from vehicle-treated controls in most cortical, thalamic and basal ganglia regions. This attenuation closely paralleled the development of tolerance to the effects of THC on locomotor activity and core body temperature. However, glucose utilization remained altered in the nucleus accumbens, mediodorsal thalamus, basolateral amygdala, portions of the hippocampus and median raphe. These data suggest that the development of tolerance to the cerebral metabolic effects of THC is regionally specific and temporally distinct. The persistence of effects in limbic areas as well as portions of the hippocampal complex, however, suggests that processes such as stress, reward, and aspects of memory mediated by these brain regions may continue to be affected by THC even after prolonged THC exposure.