Effects of formalin-induced pain on exploratory behaviour in rabbits

We have used formalin to assess the long-term behavioural and hormonal effects of persistent pain in rabbits. Behaviour has been recorded over 10 min sessions both before and after 10 min, 4 and 24 hours after injection of formalin. A low dose of formalin (0.1 ml at 5%) elicited licking of the injected paw which lasted only a few minutes. Exploratory behaviour was greater than in the control (sham-injected) animals in all post-injection sessions. Following a larger dose of formalin (0.5 ml at 8%) licking occurred in all post-injection sessions, while exploration was lower than the controls only immediately after injection. Twenty-four hours after the formalin injection, testosterone levels were unaffected. Corticosterone levels increased over the course of the experiment but this increase was equal for the experimental and the control groups. Results suggest that different pain intensities and durations have opposite effects on exploratory behaviour in rabbits.