Localization of intravenously administered [ 3H]-testosterone and its metabolites in the brain of the male rat: The absence of a major effect related to the time of day of the injection

Abstract

Different groups of three castrated male rats were injected with 40μCi [ 3H]-testosterone at each of six time points at 4 h intervals throughout the light-dark cycle. One half hour after the administration of the hormone, testosterone and its metabolites were extracted from several brain regions and isolated by t.l.c. Particular attention was given to testosterone and 5α-dihydrotestosterone (DHT). Testosterone was concentrated to a greater extent in the hypothalamus and cerebral peduncles than in the cortex and plasma. This effect was not altered significantly by the time of day of the injection. More DHT was found in the posterior hypothalamus and cerebral peduncles than in the cortex. There was more testosterone than DHT in all brain samples examined. There was a small time of day effect in the amount of DHT present but not in the ratio of DHT to testosterone. The DHT to testosterone ratio was greater in the posterior hypothalamus, cerebral peduncles, and cerebellum than in the cerebral cortex, the preoptic and anterior hypothalamic areas. Although too small to be statistically significant, changes in the amount of testosterone found in the brain appeared to parallel the fluctuations in DHT. It was concluded that changes in the amount of substrate present may have been responsible for the observed differences in DHT. The time of day effect seemed most obvious as a decrease in DHT in the posterior hypothalamus two to six hours after the onset of the dark phase of the light-dark cycle.