The effects of pup presentation on the PRL responses in parental male rats were measured and compared with those in parental virgin and lactating female rats. Blood samples were collected from rats through indwelling intraatrial cannulas after a suckling challenge, i.e. presentation of rat young. Lactating rats showed full parental behavior and characteristic large surges in plasma PRL levels within the first 5-10 min on each day that rat young were presented (days 4, 8, and 12 of lactation). When pups were not presented, PRL rises did not occur. In contrast to the pattern of PRL responses shown by lactating mothers, parental ovariectomized nulliparous female and parental intact male rats failed to show specific increases in PRL in response to pup presentation. Plasma PRL levels in these groups, as in nonparental female and male rats, occasionally rose in response to blood collection rather than to pup presentation alone. Treatment of nulliparous female as well as male rats with estradiol and progesterone Silastic implants for 21 days before the initiation of behavioral testing significantly reduced the latencies of both nulliparous females and males to respond to foster young from about 5 to 2 days. The PRL responses of these steroid-primed groups were quite different. The steroid-primed females exhibited a pattern of PRL responses to pups identical to that found in lactating rats. The steroid-primed parental males, in contrast, failed to show specific increases in plasma PRL levels in response to young. These data demonstrate a sex difference in the hormonal, but not behavioral, responses of male and female rats to young and are suggestive of possible sex differences in the hypothalamic and/or peripheral regulation of pup-induced PRL secretion.
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