Marijuana, or Cannabis sativa, is the most commonly used illicit drug in the United States. Increased levels of use occur during adolescence and young adulthood, which is of concern from a public health perspective, since these are also critical periods of neural development. What is particularly disturbing is the fact that there are over 6000 first-time cannabis users added per day, 62.2 percent of which are under the age of eighteen. This fear is further underscored by the fact that cannabis may act as a gateway drug, since its use may predispose individuals to abuse other illicit drugs. Further, the potency of cannabis (concentration of delta-9- tetrahydrocannabinol (?9-THC)), is now exceeding 10 percent in the U.S. (compared to 4 percent in 1983), which could have unforeseen consequences for normal brain function. Given the vast number of individuals who consume cannabis on a regular basis, a thorough understanding of the neural mechanisms associated with its behavioral and physiological effects are of considerable relevance. The cerebellum arguably contains the highest density of CB1 receptors in the brain. While it is well known that ?9-THC causes abnormalities in cognitive functions such as short-term memory and attention, there are a paucity of data examining the effect of exogenous cannabinoids on associative learning, particularly as it relates to cerebellar versus forebrain-dependent classical eyeblink conditioning (EBC). Earlier work from our group has shown that in humans, chronic cannabis use alters conditioned response (CR) acquisition and timing in cerebellar-dependent delay EBC, but not in the forebrain-dependent trace EBC task. However, it remains unclear whether the deficits observed in chronic cannabis users are due to the residual effects of ?9- THC, some other cannabinoid present in cannabis, CB1 downregulation, or premorbid differences in drug- seeking individuals. Therefore, the overall aim of the current application is to investigate whether acute, i.v. ?9- THC administration mediates cerebellum- versus forebrain-dependent associative learning in humans as assessed with delay and trace EBC, respectively. The hypothesized outcome, based on known CB1 actions in the cerebellum, is that ?9-THC (as compared to placebo) will induce impairments in cerebellar dependent delay EBC in a dose-dependent manner (decreased percent CRs and altered CR latency). It is also expected that ?9-THC will have little or no effect on forebrain-dependent trace EBC. Taken together, it is hoped that data from these studies will further our understanding of the cannabinoid system, particularly in the context of associative learning, which will help elucidate the mechanism of action of one of the most commonly used drugs of abuse.
The public health relevance of this application can be summarized as follows: 1) Cannabis exposure likely produces neural changes in the endogenous cannabinoid system, and the effect of such changes on information processing within the brain is unclear. 2) Understanding how the active ingredient in cannabis (Delta-9-THC) interacts with the neural substrates underlying information processing will improve care and prevention of cannabis abuse/dependence. 3) The proposed study has a high potential for future translational research, as the eyeblink conditioning task is widely employed in animal studies, and the neural circuitry mediating this task is well conserved across species (i.e. result from the current studies will be testable in future animal models of cannabinoid function).