Age-related differences in Δ9-tetrahydrocannabinol-induced antinociception in female and male rats

Given the use of cannabis as an analgesic by a broadening age range of patients, the aim of this study was to determine whether the antinociceptive effects of Δ9-tetrahydrocannabinol (THC) differ by age. The antinociceptive potency and efficacy of THC (1.0-18 mg/kg ip) was compared in male and female rats aged postnatal day 35-40 (adolescent), 60-70 (young adult), and 291-325 (middle-aged adult), using warm water tail withdrawal and paw pressure tests. Motoric effects of THC were assessed using a locomotor activity test. On the tail withdrawal test, THC was significantly more effective in middleaged adult than in young adult rats and significantly less effective in adolescent than in young adult rats. Similar but smaller age-related differences were observed on the paw pressure test. Sex differences in THC's antinociceptive effects were consistent across the 3 ages examined, with greater THC effects observed in females than males of each age. Age-related differences in THC's locomotor-suppressing effect were also observed, with the greatest effect in young adult female rats. Serum THC levels were slightly higher in adolescent than in young adult rats, and levels of the active metabolites 11-OH-THC and cannabinol, as well as the inactive metabolite 11-nor-9-carboxy-THC, did not differ between adolescent and young adult rats. These results suggest that the pain-relieving effects of THC may be more limited in adolescents than in adults and that these age-related differences in THC effect are not attributable to differential absorption or metabolism of THC.