Changes in uptake of 3H-progesterone by female rat brain and pituitary from birth to sexual maturity.

3H-progesterone uptake by various parts of the brain, pituitary and skeletal muscle was compared in newborn, 5-, 10-, 15-, 20-, 25- and 50-day old female rats at 1 hr after a single intraperitoneal injection of 50 muCi/100g body weight. High uptake values in newborn animals and in those aged 5 days were found in all tissues investigated. A sharp decrease in accumulation was observed from birth and/or 5th day of life. The uptake by pituitary was persistantly higher than those by other tissues investigated. The ratio of the concentration of radioactivity between the tissues and the cerebellar cortex increased significantly only in the posterior hypothalamus of adult females (at the age of 50 days). In the pituitary the ratio tissue/cortex was already significantly higher in newborns. The high level of brain radioactivity in the youngest animals is probably a manifestation of high plasma concentrations of tritiated progesterone. The striking decrease in uptake of radioactivity by the brain and pituitary during the first two weeks of life most likely reflects the decrease in level of plasma radioactivity, as shown indirectly by the concomitant decrease in labelled progesterone uptake by the skeletal muscle. The increase in tissue/cortex ratio in the posterior hypothalamus with attainment of sexual maturity suggests the first appearance of specific binding capacity for progesterone which is assumed to be present in the pituitary already from birth.