Opiate disruption of maternal behavior: morphine reduces, and naloxone restores, c-fos activity in the medial preoptic area of lactating rats.

Morphine significantly impairs maternal behavior; naloxone, an opiate antagonist, restores it. Maternal behavior is associated with c-fos expression, an immediate early gene product, in the medial preoptic area (mPOA) of females. In two experiments, the effects of morphine-alone and morphine plus naloxone on the expression of c-fos were examined. On postpartum day 5, females were injected with morphine or saline (experiment 1), and morphine + naloxone or morphine + saline (experiment 2) and placed back in the home-cage, separated from their pups by a wire-mesh partition. A separate group in experiment 1 was injected but not exposed to pups. Processing for c-fos immunohistochemistry commenced, and c-fos positive cells in a proscribed portion of mPOA were counted. Morphine-treated females had fewer c-fos cells in mPOA compared to saline-treated females, and the presence of pups accounted for a significant increase in c-fos-expressing neurons, whereas in females not exposed to pups, morphine treatment did not significantly reduce baseline c-fos expression (experiment 1). Furthermore, naloxone mitigated the effect as morphine + naloxone-treated females expressed more c-fos cells compared to morphine + saline females (experiment 2). Morphine-treated females, therefore, may exhibit reductions in maternal behavior because of relative opiate-induced inactivation of areas of the brain devoted to the regulation of maternal behavior.