F. Mena

A study of factors involved in the development of the exteroceptive release of prolactin in the lactating rat

Abstract Thirty minutes of suckling after 8 hr of isolation of mother and pups induced a significant fall in the prolactin concentration of the primiparous mothers pituitary on postpartum Day 7. Thirty minutes of exposure without contact of the mother to her pups, however, did not alter the pituitary prolactin concentration from that found for nonexposed controls, whereas both suckling and exposure to their pups were effective in lowering prolactin concentration on postpartum Day 7 in rats undergoing their second lactation. Pituitary prolactin was reduced significantly in primiparous mothers on postpartum Day 7 after suckling or exposure to cross-fostered pups which were 13–14 days of age (inserted when 7–8 days old on postpartum Day 2). No reduction occurred on postpartum Day 7 when primiparous mothers were exposed to 14-day-old pups instead of their own 7-day-old pups. Exposure of primiparous lactating rats on postpartum Day 14 to their own pups for 30 min resulted in a significant fall in pituitary prolactin concentration in comparison to nonexposed controls. A fall in prolactin also occurred after 30 min of exposure to either live or recently killed 14-day-old pups from other mothers but not in response to 7–8- or to 20–21-day-old live pups. The fall in prolactin stores induced by exteroceptive stimulation was not discernibly associated with any qualitative aspect of the mothers behavior, with the duration of the mothers behavior or with the activity of the pups. The duration of the mothers behavior, however, appeared to be directly correlated with the extent of the pups activity. Our data suggest that the suckling stimulus constitutes the inherent (and primary) means of effecting prolactin release in the primiparous rat in early lactation, but from postpartum Day 7 to 14 the suckling stimulus becomes conditioned by stimuli emanating from the pups and associated with suckling to where the associative exteroceptive stimuli alone can effect the release of prolactin. The relationship of various factors to the onset of function of the exteroceptive mechanism is discussed.

Evidence that suckling pups, through an exteroceptive mechanism, inhibit the milk stimulatory effects of prolactin in the rat during late lactation ☆

Abstract Lactating rats were removed from the animal room on postpartum Day 20 and placed in a room where there were no other rats. On postpartum Day 21, these rats released prolactin either in response to exteroceptive signals from their own pups which were placed underneath or in response to those from 25 to 30 other lactating rats which along with their litters were placed in a rack 3 ft in front of the mothers cage. Milk secretion was stimulated in the isolated rats on Day 21 by the prolactin released in response to exposure to the rack of lactators, but paradoxically was not stimulated by that released in response to exposure of the mother to her own pups. In fact, the stimulatory effects upon milk secretion, resulting from exposure to the rack of lactators was totally prevented if the mother was exposed to her pups 3–4 min before exposure to the rack of lactators. The blocking effect of the pups, however, did not occur when the pups were placed in cages alongside the mothers cage. From subsequent experiments, it was concluded that sensory cues, from the pups, appeared to activate the sympathetico-adrenal system of the mother to release catecholamines which then blocked the milk stimulatory effects of prolactin which already had been discharged into the circulation. The pups did not inhibit milk secretion on Day 14 which indicates that the inhibiting mechanism becomes established sometime between Days 14 and 21 of lactation. These data suggest that a peripheral mechanism may operate to reduce milk secretion in the rat during late lactation, and thus, may be involved in the normal weaning process.

Sensory stimuli from pups involved in inhibition of milk secretion in rats during late lactation

Abstract Lactating rats of two strains, Holtzman and Sprague-Dawley-Rolfsmeyer, were removed from the animal room on postpartum Day 20 and were placed in a testing room where there were no other rats. Milk secretion was stimulated in mothers of each strain 24 hr later in response to 30 min of exposure to a rack of lactators and their pups, placed 3 ft in front of the test mothers; this stimulation to milk secretion was prevented, provided their own pups were placed under, but not when they were placed alongside, the mothers 3–4 min beforehand. The milk secretion responses on the whole were quantitatively less in the Holtzman rats. The sensory signals emanating from the pups which were responsible for the inhibitory effect upon milk secretion were analyzed in Holtzman rats. Sound and odor each proved to be inhibitory; sight, however, was not. The function of the exteroceptive inhibition of milk secretion in the control of milk secretion in late lactation is discussed.

Failure of self-licking of nipples to alter pituitary prolactin concentration in lactating rats☆

Abstract Primiparous lactating rats were separated from their litters of 6 pups each for 8 hr on postpartum Day 14. The pups then were replaced for 30 min under their mother so they could be seen, smelled, and heard but not touched. In this situation, the mother licked her nonnipple areas more often, but was not stimulated to lick her nipples more often or to lick a greater number of nipples. Prolactin concentration in the pituitary glands of the mothers was significantly reduced by 30 min exposure to their pups in respect to that in pituitaries of mothers not exposed to their pups but killed at the end of the 8-hr isolation period. The painting of the nipples of another group of rats with a nonalcoholic orange-flavored syrup at the end of the 8-hr isolation period markedly increased self-licking of the nipples and the number of nipples licked. The intensified licking of the nipples, however, did not provoke any alteration in the pituitary concentration of prolactin. We conclude that the fall in pituitary prolactin concentration which occurs in response to the presence of the pups and without suckling is not the result of self-licking of the nipples, but is due to exteroceptive cues emanating from the pups.