Reelin deficiency contributes to long-term behavioral abnormalities induced by chronic adolescent exposure to Δ9-tetrahydrocannabinol in mice

Heavy and frequent use of cannabis during adolescence increases the risk of developing psychiatric disorders. However, the neurobiological mechanisms underlying this vulnerability remain largely unknown. Here, we explore whether adolescent vulnerability to long-term behavioral effects of cannabis is modulated by Reelin, a gene implicated in the development of the brain and of psychiatric disorders. To this aim, heterozygous Reeler (HR) mice, that express reduced level of Reelin, were chronically exposed during adolescence to high doses (10mg/kg) of {Delta}9-tetrahydrocannabinol (THC), a major psychoactive component of cannabis. Mice were tested in early adulthood with multiple behavioral assays, including working memory, social interaction, locomotor activity, anxiety-like responses, stress reactivity, and pre-pulse inhibition. Compared to wild-type (WT), HR mice treated with THC showed impaired social behaviors, elevated disinhibitory phenotypes and increased responsiveness to aversive situations, in a sex-specific manner. Independent of THC exposure, HR mice also spent more time exploring unfamiliar objects, indicating that Reelin modulates novelty seeking behavior. To identify the neuronal ensemble underlying this elevated novelty seeking in HR mice, we mapped the regional brain expression of the immediate early gene, Fos, in mice exposed to novel objects. HR mice exhibited reduced neuronal activation in the lateral septum, a subcortical brain structure implicated in emotions, cognition and reward processes. Overall, these findings show that (1) Reelin deficiency influences behavioral abnormalities caused by heavy consumption of THC during adolescence, and (2) that Reelin plays a role in the neurobiological mechanisms underlying disinhibitory behaviors, such as novelty seeking. Significant StatementThe link between cannabis abuse and the development psychiatric disorders, especially in adolescents, makes understanding the neurobiological mechanisms underlying cannabis effects on the brain a significant biomedical problem. Reelin is a key signaling molecule in the development of the adolescent brain and of psychiatric disorders, but its role in modulating the behavioral changes induced by cannabis remain unknown. Here, we report an interaction between Reelin deficiency and chronic adolescent exposure to THC, a major psychoactive component of cannabis. This interaction led to cognitive deficits, disinhibitory behaviors and altered emotional reactivity in mice, in a sex-specific manner. These experiments are the first to establish a link between Reelin signaling and the endocannabinoid system targeted by THC.