Figure 3

Operant reward licking events for three different strains of mice. Hairless mice significantly increased their reward licking events (A) when fasted overnight (N = 13–27/session). Unfasted animals had significantly lower (+ P < 0.05) licking events in session 1 as compared to session 2, but there was no difference between session 2 and 3, and overall the fasted sessions produced significantly higher performance as compared to the unfasted sessions. Hairless (B) and C57/BL6 (C) mice demonstrated a stimulus response such that increasing temperatures significantly reduced the number of reward licking events, indicating aversion to the more noxious stimuli. The TRPV1 knock-out mice (D) were relatively insensitive to temperatures ≤ 52°C, as their responses in the noxious heat range of 46–52°C produced responses similar to baseline 37°C testing conditions. These knock-out mice only demonstrated a significant decrease in reward licking events when they were tested at the highest stimulus temperature of 55°C. A total of N = 10 hairless, N = 5 CB57BL6, and N = 5 TRPV1 knock-out mice were used to generate these data. N.T. = not tested. *Significantly higher (P < 0.05). Note that there were no significant differences between male and female mice when comparing the number of licking events during the unfasted and fasted sessions, therefore their data was pooled.