Cocaine-induced effects on pain behavior and c-fos expression in the spinal dorsal horn of the rat

Effects of systemically administered cocaine on formalin-induced pain behavior and c-fos-like immunoreactivity in the spinal dorsal horn were studied in rats. During the whole observation period of 90 min, formalin-induced pain behavior was completely suppressed by cocaine (30 mg/kg i.p. followed by two boosters of 15 mg/kg). Formalin-induced a marked c-fos-like immunoreactivity both in the superficial and deep laminae of the ipsilateral spinal dorsal horn in control rats. Cocaine did not significantly suppress the c-fos-like immunoreactivity induced by formalin, although according to behavioral criteria the animals were completely without pain. The results indicate that the antinociceptive effect of cocaine dissociates from the effect of cocaine on the noxious stimulation-induced expression of immediate early genes in the spinal cord.