Prolactin (PRL) is considered to induce maternal behavior toward foster young in female rats. In the present study, we studied the relationship between pup contact-induced maternal behavior and serum PRL concentrations and brain PRL receptor (PRL-R) mRNA expression in male rats. Both intact and castrated male rats exposed to foster pups gradually developed caretaking behavior such as crouching and licking, but their exhibitions of other maternal behavior components, retrieval/grouping and nest building, were incomplete. However, in the male rats displaying crouching and licking, the concomitant increases in serum PRL concentration and brain mRNA expression for long-form PRL-R were observed. The expression of short-form PRL-R mRNA in the brain was not stimulated by pup contact. Administration of PRL remarkably promoted the onset of those maternal responses in male rats. On the other hand, when an intact male rat was housed in a cage where a lactating female rat and her pups were living, his scores in maternal behavior tests toward pups were lowered. And, concomitantly, increases in serum PRL concentration and brain expression of long-form PRL-R mRNA were reduced. In castrated male rats, however, the ratings of maternal behavior toward foster young, serum PRL concentration increase, or brain long-form PRL-R mRNA expression were not reduced at all by cohabitation with a female and her pups. These findings indicated that maternal behavior was triggered and maintained in pup-contacted male rats through elevated serum PRL levels and induced brain long-form PRL-R.
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