Evidence that the Depletion and Release Phases of Prolactin Secretion in the Lactating Rat Have Different Activation Thresholds in Response to Exteroceptive Stimulation from Rat Pups*

Abstract

Lactating rats with indwelling intraatrial catheters were exposed for 30 min to the exteroceptive stimuli (ECS) from their pups, which were placed in open-top containers under the mothers' cages. The wire mesh construction of the cages allowed each mother to see, smell, and hear her pups, but she could not make physical contact with them. The litters (six pups each) were removed from the mothers' cages 5–6 h beforehand; all experiments were done on days 14–16 of lactation. Exposure of the mother to the ECS from two of her six pups did not release PRL into the circulation during any part of the 30-min exposure period. Since releasable (into the circulation) PRL derives or is transformed from depleted PRL, it was not unexpected that the pituitary stores of PRL also were unaffected by this stimulus. Increasing the stimulus strength by exposing the mother to her entire litter of six pups for 30 min, however, caused both PRL depletion and a rapid and sustained elevation in the plasma PRL concentration comparable to that seen from suckling by six pups. ECS from two pups caused a rapid sustained elevation in the concentration of PRL in the plasma of previously depleted (by suckling) rats comparable to that seen after exposure to six pups. Thus, it appears that less stimulus strength is required to activate the release than is required to activate the depletion-transformation phase of PRL secretion in the lactating rat. In other experiments, exposure of lactating rats to strong nonphysiological ECS (loud vocalizations and movement) from two female laboratory personnel neither depleted nor released PRL in the nondepleted rat. A large sustained release of PRL into the circulation occurred, however, provided depletion-transformation had been effected by a previous 10-min period of suckling. These data suggest that in addition to having a lower excitability threshold, the release phase of PRL secretion may also respond to stimulus modalities that do not cause PRL depletion. (Endocrinology108: 820, 1981)