Two Modes of Prolactin Secretion Induced by Young in Maternally Behaving Virgin Rats*

Publisher Website
Google Scholar
Abstract
Virgin rats exposed for several days to foster young develop a typical pattern of maternal behavior. After the establishment of maternal behavior, the majority of virgins fail to maintain regular estrous cycles in the presence of young and enter a state of prolonged diestrus. The present study investigated whether maternal virgin rats are capable of secreting PRL in two modes which are known to be controlled differentially: 1) secretion in a circadian pattern, which is regulated autonomously, and 2) acute release, which is governed by stimulation from young. To check for circadian secretion of PRL in maternal virgins, we measured daytime (1200 h) and nigh time (0230 h) concentrations of PRL during pup-induced diestrus and throughout the estrous cycle. In maternal virgins which entered prolonged diestrus through exposure to young, PRL was secreted autonomously in a rhythmic pattern of nightly surges. In this respect, the pupinduced diestrus is similar to pseudopregnancy. In contrast, maternally behaving virgins that maintained their estrous cyclicity failed to show any elevation in PRL. To examine acute PRL release induced by pup stimulation, PRL concentrations were measured in cycling and pseudopregnant maternal virgins 4 h after separation from young (at 1300 h) and again 30 min after reunion with young. Only cycling maternal virgins exhibited acute PRL elevation in response to reunion with pups. This response was most pronounced on proestrus and estrus and was moderate or absent on diestrus. During pup-induced pseudopregnancy, however, reunion with young failed to induce any elevation in PRL levels. These results indicate that maternal virgin rats are capable of secreting PRL in two distinct modes. However, the two modes cannot occur simultaneously. Autonomous secretion of PRL in a pattern of daily nocturnal surges occurred only during pupinduced pseudopregnancy, whereas acute PRL release in response to reunion with young occurred only during the estrous cycle. It is possible that in maternal virgin rats, the acute release of PRL induced by pup stimulation is 1) dependent upon a background of estrogen which fluctuates during the estrous cycle and/or 2) suppressed by the mechanism underlying the rhythmic nightly secretion of PRL during pup-induced pseudopregnancy.