Feedback Regulation and Male Puberty: Testosterone-Luteinizing Hormone Relationships in the Developing Rat1

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Abstract
Studies were performed with the primary objective of evaluating the hypothesis that the onset of puberty in males is related to a change in the sensitivity of the negative feedback mechanisms which mediate the androgen suppression of LH secretion. Unlike previous studies, stabilized testosterone (T) levels were produced by sc implantation of Silastic capsules containing T, and both plasma T and LH were directly measured by RIA. Male rats were castrated and simultaneously received capsule implants at 26, 55 and 115 days of age, and autopsied 5 days later. A given level of plasma T produced a significantly greater suppression of plasma LH in the prepuberal than in the two older groups. It was concluded that the sensitivity of the testosterone-LH feedback mechanism is higher before than after puberty. A clear trend to increases of LH over the castrate control levels was found in castrated puberal rats treated with low T doses, but this stimulatory effect was not replicated in subsequent experiments. In addition, neither intact nor castrated puberal males showed diurnal variations in plasma LH levels. With increasing age, Silastic capsules of smaller surface area (adjusted for body weight) were required to produce a given T level, suggesting a progressive, age-related decrease in clearance of T from the circulation. When the mean plasma T level in groups of intact rats was reproduced by Silastic capsules following castration, it was found that the same plasma T level was more effective in suppressing LH and stimulating sexual accessory glands. One interpretation of this finding is that the natural pulsatile form of testicular androgen secretion results in a reduced biological effectiveness.