Neonatal capsaicin treatment in rats results in scratching behavior with skin damage: potential model of non-painful dysesthesia.

We administered capsaicin or vehicle in 2-day-old rat pups, and for over 6 months examined the rats for damaged skin and for the behaviors of scratching, gnawing and biting their skin. By 35 days of age, all rats in the capsaicin group (n = 10) had damaged skin (i.e., lesions, hair loss and red skin) on the rostral half of their bodies. Skin damage remained prevalent over 6 months, whereas vehicle-treated rats (n = 8) had virtually no skin damage. Gnawing and biting behaviors were rarely observed, however, rats in the capsaicin group frequently scratched themselves. There was a significant positive correlation between the frequency at which rats scratched themselves and the total area of skin damage. Morphine (3.0 mg/kg, i.p.) greatly increased scratching behavior in only the capsaicin-treated rats and naloxone (0.5 mg/kg, i.p.) significantly reduced scratching in these rats. Thus, neonatal capsaicin, in its destruction of the majority of primary afferent C-fibers, is capable of inducing opioid-sensitive scratching behavior.