Effect of various partial separations of the litters from their mother on plasma prolactin levels of lactating rats.

Removal of the pups results in an abrupt and marked depression in plasma prolactin (PRL) level of the lactating mother. The present studies were undertaken to investigate what kind of sensory input (smell, sound, visual, touch etc.) from the pups is essential for the mother to avoid the pituitary PRL response to pup-removal. Therefore, various partial separations were made and their effect on plasma PRL levels tested: a. The pups were placed into a small glass having holes on its cover; b. they were put into a long measuring tube not covered; c. the pups were placed into the feeding trough made of a wireframe; d. a dividing wall made of glass or metal was slowly let down when the mother spontaneously went away from her pups; e. the nipples were covered by a cotton plaster. Pituitary PRL responses were almost identical after all these separations and similar to that one obtained after removal of the pups from the cage. In addition, separation of the mother resulted in a rise in plasma corticosterone concentrations. The findings suggest that the pup-removal induced inhibition of PRL secretion is a very complex event for the mother and cannot be prevented by partial separations when the mother can see, smell her pups, or hear them or even can touch them with her nose. We assume that separation of the pups is a stress for the mother and cannot simply be due to the lack of just one kind of sensory input from the pups. This assumption is in line with our recent observations indicating that in lactating rat stress causes a decrease in plasma PRL level.