A QUANTITATIVE STUDY ON FEEDBACK CONTROL OF LH BY TESTOSTERONE IN YOUNG ADULT AND OLD MALE RATS

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Abstract
The hypothalamic-pituitary gonadal axis was studied in young adult (3 mo. old) and old (24-27 mo. old) male Wistar rats. Plasma testosterone decreased significantly in old animals (.hivin.x [mean]: 262 ng/100 ml (n = 35); versus .hivin.x: 110 ng/100 ml (n = 30)). The fall in LH [lutropin] was less pronounced but still significant (54.5 ng LH-RP[rat pituitary]-1/ml in young versus 39.5 ng/ml in old rats). Groups of 6 to 8 animals of both ages were castrated and implanted with silastic capsules continuously releasing testosterone. The length of the capsules was directly proportional to the plasma testosterone levels achieved (between 63-350 ng/100 ml). After 1 wk young castrated rats not substituted with testosterone showed LH values 3 times higher (.hivin.x: 351 ng/ml) than old rats treated in the same way (.hivin.x = 126 ng/ml). LH values in the animals substituted with testosterone indicate that the sensitivity of the negative testosterone-LH feedback is greatly increased in old rats. Testosterone can be depressed to 60 ng/100 ml before an increase in LH occurs. In young rats no increase in LH was observed when testosterone values were higher than 170 ng/100 ml. In the 170-100 ng/100 ml range, .apprx. 1/2 the young animals reacted with increased LH secretion. An increase was observed in all young animals when testosterone dropped below 100 ng/100 ml.