Licking and c-fos expression in the dorsal horn of the spinal cord after the formalin test.

We investigated whether c-fos expression in the dorsal horn is affected by licking in the formalin test. Thirty adult Sprague-Dawley rats were divided into 5 groups of 6 rats each: a free condition control (Free Cont) group, formalin test under free condition (Free F-test) group, scrub stimulation under free condition (Free Scrub) group, restrained condition control (Restricted Cont) group, and formalin test under restrained condition (Restricted F-test) group. Animals in the three free condition groups and two restricted groups were put in a clear plastic chamber and a restraining chamber, respectively. Ten percent formalin was injected into the left rear paw in the Free and Restricted F-test groups. Animals in the Free Scrub group were scrubbed on the left rear paw with a wet cotton swab. The Free Cont, Restricted Cont, and Free Scrub groups showed little c-fos expression. The number of c-fos positive cells in the ipsilateral surface dorsal horn of the Restricted F-test group was significantly less than that of the Free F-test group (P < 0.05). The results indicated that the licking action increased c-fos expression of the lumbar dorsal horn in the formalin test.