The capsaicin sensitivity of the preoptic region is preserved in adult rats pretreated as neonates, but lost in rats, pretreated as adults

Summary

• 1. Two days old rats were pretreated with subcutaneous injection of 50 mg/kg, and adult animals with either 20, 50 or 300 mg/kg capsaicin. The responsiveness of these and naive animals to microinjection into the preoptic region of capsaicin (10 μg) and to subcutaneously injected capsaicin (2 mg/kg) was studied at the age of 3–4 months by recording the tail skin vasodilatation and colon temperature, respectively.

• 2. On preoptic injection of capsaicin, the reaction of neonatally-pretreated adult rats was similar to that of naive animals, while in all groups pretreated as adults the tail skin vasodilatation was abolished.

• 3. In response to 2 mg/kg capsaicin administered subcutaneously, the group pretreated neonatally and the adults pretreated with 20 mg/kg capsaicin produced significantly less hypothermia than the naive animals. Rats pretreated as adults with 50 and 300 mg/kg capsaicin failed to show a hypothermic reaction.

• 4. It is concluded that the sensitivity of the preoptic region to capsaicin is preserved when 2 days old rats are treated with the drug, but lost when adults are injected with capsaicin. These features of capsaicin sensitivity indicate a functioning preoptic and an impaired extrapreoptic thermoregulation in rats pretreated with capsaicin as neonates.