The effects of pyrilamine and cimetidine on mRNA c-fos expression and nociceptive flinching behavior in rats

C-fos and Fos expression, frequently used as a neural nociceptive marker, is altered by many drugs. The effects of histamine receptor antagonists m c-fos messenger (m)RNA expression are unknown. We examined the effect of local and systemic administration of pyrilamine (H, receptor antagonist) and cimetidine (H, receptor antagonist) on the nociceptive flinching behavior elicited by injection of 50 muL of 1% formalin into the dorsal region of the hind paw of rats. Nociceptive flinching behavior was observed for 45 min, and the rats were then killed and lumbar spinal cord obtained for c-fos mRNA expression, measured using the Northern blot hybridization technique. Systemic administration of pyrilamine and cimetidine did not elicit response in nociceptive behavior or in c-fos mRNA expression. When the drugs were locally administered, they affected behavior and c-fos mRNA expression in different patterns. Pyrilamine decreased the number of flinches in a dose dependent manner in both phases, whereas cimetidine did not affect Phase I and decreased the number of flinches in Phase 11, but only partially. Pyrilamine 5 and 20 mM decreased c-fos mRNA expression, and cimetidine decreased the expression only at 100 mM. The systemic use of the drugs had no effect on c-fos mRNA expression.