Pituitary and hypothalamic concentration of met-enkephalin in young and old rats

Anterior pituitary and hypothalamic concentration of met-enkephalin was measured by a specific radioimmunoassay in old (22-24 months) and young (4-6 months) rats. The old rats were reproductively abnormal and based on their vaginal cytology, they were further classified under pseudopregnant, irregular cycle and constant estrus groups. All old rats had significantly higher concentration of met-enkephalin in the anterior pituitary gland when compared with young rats in estrus. Pseudopregnant rats had the highest concentration of met-enkephalin (7.38 ± 1.72 ng met-enkephalin/mg protein, mean ± SEM) followed by rats with irregular cycles (3.07 ± 1.54) and old rats exhibiting constant estrus (2.22 ± 0.47). Young rats in estrus had a mean concentration of 0.13 ± 0.08 ng met-enkephalin/mg protein in the anterior pituitary gland. However, there was no significant difference in the hypothalamic concentration of met-enkephalin of old animals when compared with young animals.