(d-Met2,Pro5)Enkephalinamide causes a decrease in plasma prolactin levels of lactating rats continuously suckled and an increase in those deprived of their litter

The effect of an enkephalin analogue, (d-Met²,Pro⁵)enkephalinamide (EKNH₂) on prolactin (PRL) secretion of lactating rats continuously suckled or separated from their pups was investigated. In rats together with their pups 0.5 mg/kg EKNH₂ caused a dramatic decrease, 0.25 mg/kg a mild and short-lasting reduction in plasma PRL levels. In contrast, in lactating rats separated for 4 h from their pups 0.5 mg/kg of the drug induced a slight and 1.0 mg/kg a considerable increase in plasma PRL levels. The data indicate that in lactating rats depending on the circumstances the enkephalin analogue causes opposite effects on PRL secretion.