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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.