Effects of neurogenic inflammation and morphine on operant reward licking behavior in hairless SKH1-Hrhr mice. Hairless SKH1-Hrhr mice treated with capsaicin cream (0.035%, topical, 5') had significantly lower reward licking events (*P < 0.05) as compared to baseline (A) at 47°C. Morphine (0.5 mg/kg) given concurrently with the capsaicin-treatment provided a significant antihyperalgesic effect under similar testing conditions, as the reward licking counts were significantly higher than the capsaicin-alone group (A). A different set of animals (N = 10) was tested with either morphine (0.5 mg/kg, s.c.) or water (100 μl, s.c.) at 37°C in order to assess the effects of morphine on feeding and reward behavior in the absence of a painful stimulus (B). There were no significant differences between the two treatment groups at this neutral stimulus temperature.
Neubert et al. Molecular Pain 2008 4:43 doi:10.1186/1744-8069-4-43