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Brain and pituitary beta-endorphin levels at different developmental stages of the rat.

beta-Endorphin-like immunoreactivity in whole brains of Sprague-Dawley rat fetuses of different gestational ages was measured by radioimmunoassay and found to increase throughout the gestational period studied. The immunoreactivity in various brain parts (forebrain, midbrain, hindbrain, hypothalamus and pituitary) of late prenatal, early postnatal, young mature and retired breeder rats was also determined. In all the brain parts studied, a maximum in the content and concentration of beta-endorphin-like immunoreactivity was attained when the rats were about 3-4 months old.