Two parallel experiments in rats 2-21 days of age investigated the onset and characteristics of morphine-induced antinociception. One measure of reactivity to pain, limb retraction from a hotplate, was utilized for three different limbs (forepaw, hindpaw, and tail) to chart the development of opioid sensitivity. Morphine-induced antinociception, even in 2-day-old rats, was obtained for all limbs, in a dose-related fashion, and reached peak sensitivity at 6-7 days of age. Naltrexone did not affect limb retraction latencies in nonmorphine treated rats at any age. These studies demonstrate early antinociception to low doses of an opiate and establish that the pain system, like positive reinforcement systems, is opiate sensitive.
