Effect of β-endorphin imprinting during late pregnancy on the brain serotonin and plasma nocistatin levels of adult male rats

Female rats were treated with 10 mu g of beta-endorphin on the 19th day of pregnancy. Offspring were studied when five months old. Serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) content in four brain regions were determined by HPLC-EC and the nocistatin levels of blood plasma using RIA methods. In each brain region studied, the 5-HT levels were highly significantly reduced and that of 5-HIAA in three regions was highly significantly increased. When 5HIAA/5HT ratios, as a measure of serotonin turnover, were calculated, imprinted animals showed extremely high values. Plasma nocistatin level was also significantly elevated. The results call attention to the effect of perinatal endorphin imprinting and its long-term consequences (e.g., setting of aggressiveness, pain tolerance).