Expression of μ-opioid receptor mRNA in the medial preoptic area of juvenile rats

Previous studies indicate that the latency to initiate parental behavior in both male and female rats increases with age; weanling (21 days old) rats display parental behavior 0-2 days after exposure to newborn pups, while older juveniles (30 days old) require 5-6 days of pup exposure before they express the behavior. Furthermore, activation of μ-opioid receptors inhibits parental behavior in juvenile and adult rats. We hypothesized that the age- related increase in behavioral latency could be modulated by the induction of μ-receptor expression in the medial preoptic area (MPOA), a region in which μ-receptors regulate parental behavior. In situ hybridization histochemistry was used to measure μ-receptor mRNA expression in the MPOA of male and female Sprague-Dawley rats that were 21, 24, 27, 30, or 33 days old. Using autoradiographic film analysis, we observed that neurons within part of the MPOA expressed very dense μ-receptor mRNA. Comparison of mRNA distribution with histological boundaries indicated that neurons within the medial preoptic nucleus (MPN), excluding the central part, exhibited the highest density of μ-receptor mRNA within the MPOA. High densities of μ-receptor mRNA extended dorsolaterally and caudally from the MPN toward the bed nucleus of the stria terminalis. MPN μ-receptor mRNA expression was not altered with age and no sex difference was observed. The dense presence of μ-receptor mRNA in the MPN suggests that ample substrate exists on which μ-receptor ligands could modulate the latency to begin parental behavior in juvenile rats, but such behavioral expression apparently is not mediated by a change in μ-receptor mRNA production. (C) 2000 Elsevier Science B.V.