Determination of Enkephalins in Rat Cerebrospinal Fluid with High‐Performance Liquid Chromatography‐Electrochemical Detection: Increased Concentrations During Lactation are Controlled by Sucking

Methionine- and leucine-enkephalin were measured in the cerebrospinal fluid of lactating rats by high-performance liquid chromatography and electrochemical detection. The concentration of both peptides was high while the rats were nursing their litter. The concentration of methionine-enkephalin decreased rapidly when the mother left her litter and increased equally rapidly after mother-young reunion, provided the pups were allowed direct contact with the nipples of the mother. The level of leucine-enkephalin did not change during the period of time the lactating rat normally stayed away from its litter but decreased after prolonged (12 h) mother-pup separation. These results show that the concentration of methionine-, but not leucine-enkephalin in the cerebrospinal fluid fluctuates as the lactating rat interacts with its litter and is directly dependent upon the suckling stimulus. Although methionine-enkephalin may contribute to the inhibition of sexual behaviour which occurs during lactation, the role of the enkephalins in the other behavioural and endocrine adaptations of lactation is unknown.