Imaging cannabinoid effects on developing cortical circuits

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Project Summary/Abstract Marijuana (Cannabis sativa) is the most widely used illicit drug in the U.S. and use in adolescence is common. Onset of cannabis use in early adolescence is associated with temporary cognitive impairments, but the long- term neurobiological consequences are not well understood. In addition, medicinal use of cannabinoids in children and adolescents may be warranted for treating certain seizure-related disorders; therefore, an increased understanding of the costs vs. benefits of adolescent cannabinoid exposure are warranted to inform the ongoing development of policies regulating legal marijuana use. Rodent models provide the opportunity to perform controlled experiments on the timing, duration, and amount of cannabinoid exposure and eliminate potential confounds of human studies, such as pre-existing conditions and exposure to other drugs of abuse. In addition to determining the long-term behavioral consequences of cannabis exposure, it is also important to determine the effects on neural circuit activity that may produce more subtle effects on functional outcomes. Current technologies limit our ability to perform longitudinal recordings of neural activity in a cell-type specific manner across adolescent development, particularly in animals self-administering intravenous drugs of abuse and performing complex cognitive tasks. One solution is to perform in vivo imaging of ensemble neural activity using genetically-encoded calcium indicators (GECIs). In vivo Ca2+ imaging can resolve cellular activity in deep brain structures in awake behaving animals using integrated, head-mounted gradient refraction index (GRIN) lenses and miniaturized epifluorescent microscopes (microendoscopes). Currently, this technique has been primarily restricted to studies in adult mice. However, development of a transgenic rat expressing the latest generation GECI, GCaMP6f, will permit imaging of cortical activity over the course of adolescent development and make it feasible to compare neural circuit development in rodents that self-administer intravenous drugs of abuse, like cannabinoids, relative to controls. Thus, in this two-year project we propose to develop transgenic rats expressing GCaMP6f under the control of a neuronal promoter. We will assess neural activity patterns in prefrontal cortical regions responsible for working memory and cognitive performance, in rats that self-administer cannabinoids in adolescence. Adolescent self-administration groups will be compared to similarly trained adults and to food self-administering controls. We will assess working memory performance and associated cortical activity in these rats across development and in adulthood after a period of abstinence. The results of these studies will clarify the acute vs. long-term consequences of adolescent cannabinoid self- administration on working memory performance, and determine if the development of task appropriate neural activity patterns are affected by ongoing or prior cannabinoid self-administration. In addition, the GCaMP6f transgenic rat will be publicly available for broad use by the neuroscience community.