Rapid decrease of plasma prolactin levels in lactating rats separated from their pups.

The time course of the effect of separation of the pups on plasma prolactin (PRL) of the mother and the mechanism of this effect were investigated in conscious primiparous lactating rats. Within the first 15 min the separation of the mother from her litter caused a very rapid decrease in the concentration of plasma PRL. There was already a very marked depression by the end of the first 3 min period. During the first 3 min the disappearance rate of PRL from the blood was identical to that occurring after hypophysectomy and thus similar to the biological half-life of plasma PRL in lactating rats. The observed marked decrease in PRL could be completely blocked by alpha-methyl-p-tyrosine pretreatment (250 mg kg-1 i.p.) or by haloperidol (2 mg kg-1 i.p.) at 90 min prior to the start of separation. It was demonstrated that the marked depression in plasma PRL concentration develops within the first hour and particularly in the first few minutes. Thus, the separation of lactating rats apparently results in an immediate complete blockade of PRL release from the pituitary. Finally, the data suggest that a dopaminergic mechanism plays an important role in the inhibition of PRL release induced by separation of the mother.