Fig. 1.

Repeated morphine treatment exacerbates gp120-induced hyperalgesia. A. Temporal diagram of drug administration. Morphine (20 mg/Kg/day) and gp120 (100 ng/2 days) were administered alone or in combination for 7 days. For co-administration, morphine was injected immediately after gp120 injection. Animals were sacrificed one day after the last time of drug administration to allow the expression of drug effect. B. Von Frey testing was performed 1 hour prior to gp120 or morphine injection to measure mechanical withdrawal threshold at hind paws to evaluate the expression of mechanical hyperalgesia. Gp120- and morphine-induced mechanical hyperalgesia was detected at day 1 and 3, respectively. Morphine exacerbation of the hyperalgesia in the gp120 model was observed after day 4 (*, p<0.05; **, p<0.01; ***, p<0.001; Two-Way ANOVA with Tukey Test for Multiple Comparisons, n=10). Control animals were administered with heat-inactivated gp120 (i.t.) and saline (i.p.).