Immunoreactive β-endorphin in the plasma, pituitary and hypothalamus of young and old male rats

Immunoreactive beta-endorphin (IR-β-ENDO) was measured in the plasma, pituitary, and hypothalamus of young (3–5 mo.) and old (19–23 mo.) male Sprague-Dawley rats, using a specific radioimmunoassay. Plasma IR-β-ENDO in old male rats (3.44±10.54 ng/ml) was more than three times higher than values observed in young male rats (1.00±0.10 ng/ml). Pituitary content and concentration of IR-β-ENDO also were significantly greater in the old (5.85±0.51 μg/gland and 1.17±0.10 μg/mg protein) than in the young (3.53±0.29 μg/gland and 0.78±0.06 μg/mg protein) male rats. The content of IR-β-ENDO in the hypothalamus of old and young rats was nearly the same (43.45±2.47 and 49.88±6.35 ng/hypothalamus, respectively), whereas the concentration of IR-β-ENDO in the hypothalamus of the old male rats (3.89±0.25 ng/mg protein) was approximately 50% lower than that observed in the young male rats (7.80±0.85 ng/mg protein). These changes in plasma, pituitary, and hypothalamic IR-β-ENDO may contribute to the increase in prolactin and decrease in gonadotropins observed in old male rats, since β-ENDO administration is known to produce these effects on prolactin and gonadotropin secretion.