A Cannabinoid Receptor 1 Agonist Reduces Light-induced Phase Delays in Male But Not Female Mice.

Animals adapt to a changing environment by synchronizing their circadian rhythms to different stimuli, the strongest and most reliable being the daily light-dark cycle. Photic information reaches the central circadian pacemaker, the suprachiasmatic nucleus (SCN), which drives rhythms in physiology and behavior throughout the brain and body. The endocannabinoid system (ECS) is a neuromodulatory system that is present within the SCN, including the primary receptor, cannabinoid receptor 1 (CB1). Exogenous cannabinoids that target CB1 inhibit the phase-shifting effects of light in hamsters, mice, and rats. Furthermore, there is evidence in cultured microglial cells that cannabidiol (CBD), a constituent of Cannabis sativa, alters core circadian clock genes, while the CB1 agonist delta-9-tetrahydrocannabinol (THC) does not. The CB1 agonist studies were conducted using male animals only, but cannabinoids exhibit sex-dependent effects in various aspects of physiology and behavior. In addition, the effects of CBD on circadian behavioral rhythms have yet to be investigated. Therefore, we decided to test the effects of acute injections of CBD or the CB1 agonist CP 55,940 on light-induced phase delays in male and female C57BL/6J mice. Animals received a single injection at circadian time (CT) 15.5, followed by a 10-min light or dark (sham) pulse at CT 16. Running-wheel activity was monitored to determine activity levels and the behavioral phase shifts from different treatments. We observed a sex difference in the magnitude of phase delay size in response to CP 55,940 administration. Males had attenuated phase delays with increasing doses of CP 55,940, while females did not differ from control. Various doses of CBD had no effect on the phase-delaying effects of light in either sex. Our results show a sex difference in the gating of photic phase shifts by CB1 activation.