Time Course of the Development and Loss of Delta-9-tetrahydrocannabinol Tolerance: Effects on Hypothermia and Spontaneous Locomotor Activity in Mice

Deregulation of cannabis use is gradually expanding in Europe and the United States. However, the biological processes driving tolerance to delta-9-tetrahydrocannabinol (Δ9-THC), the main psychoactive component of cannabis, remain unclear. Thus, this study aimed to investigate the mechanisms and time course of tolerance development and loss to Δ9-THC in mice. Male ICR mice (7 weeks old) were administered Δ9-THC once daily for 3 days and then divided into three groups according to the washout period (3-, 10-, and 17-day washout groups). After each washout, changes in body temperature and locomotor activity were measured following re-exposure to Δ9-THC. Furthermore, the mRNA expression levels of CB1 and CB2 receptors in the brain were evaluated using real-time PCR. On day 1, significant hypothermia and reduced spontaneous locomotor activity were observed in the Δ9-THC-treated mice compared with the vehicle-treated mice. Tolerance to the hypothermic and locomotor-suppressing effects of Δ9-THC developed on days 2 and 3, respectively, and dissipated after 3 and 11 days of washout, respectively. These differences in the rates of tolerance development and recovery may reflect distinct underlying mechanisms. No significant changes in receptor mRNA expression were observed. These findings highlight the complexity of Δ9-THC tolerance and its potential implications for long-term cannabis use.