Neonatal capsaicin modifies morphine withdrawal signs in the rat

Rats received injections of either capsaicin (50 mg/kg, s.c.) or the capsaicin vehicle at two days of age. When the animals were 90-120 days of age they were implanted with morphine or placebo pellets (s.c.) for 3 (one pellet) or 6 (3 pellets) days. Naloxone (0.4 mg/kg, s.c.) produced no effects in the placebo-pelleted groups but elicited salivation, lacrimation and rhinorrhea in the morphine-treated animals. These abstinence signs were less severe in the morphine-pelleted rats (3- and 6-day groups) that were treated as neonates with capsaicin. However, the neonatal capsaicin treatment increased the number of naloxone-precipitated wet-dog shakes in morphine-dependent rats. The increase was statistically significant in rats treated with morphine for 3 but not 6 days. Since capsaicin induces a long-lasting depletion of substance P and other peptides in peripheral and central neurons, it was concluded that substance P or other related peptides may be involved in the expression of some signs of opioid withdrawal.