FIG. 4.

Antinociceptive response to THC. (A) Tail withdrawal latency in response to a thermal stimulus is shown for B6 and D2 females (top panel) and males (bottom panel) 60 min post-injection of THC (10 mg/kg) or VEH. Trait data for every individual have been transformed relative to baseline (day 0) by subtracting trait values for each day by the trait values for day 0 and averaging data by strain, sex, treatment, day, and post-injection time. Data represented as mean±SEM. A significant increase in tail flick latency following THC treatment is evident in both strains and sexes. Rapid tolerance upon second exposure is more prominent in the D strain and desensitization is nearly complete in both strains by the fifth treatment, especially in females. (B) Initial response to THC is shown as the difference between day 1 and 0 (baseline) for the THC treatment group (average of every individual's difference score). The D2 strain (both sexes combined) demonstrates significantly greater analgesia after a single exposure to THC relative to the B6 strain. (C) Rapid desensitization to THC is shown as the difference between day 2 and 1 (THC treatment group only). Significant rapid desensitization to the analgesic effects of THC upon second exposure was observed for the B6 strain relative to the D2 strain.