Age-related changes in the hypothalamic-pituitary-testicular function of the rat

The activity of the hypothalamic-pituitary-testicular axis was investigated in 3-4 months (young) and 24 months (old) rats. The results clearly demonstrate that aging reduces (p less than 0.01) the hypothalamic content of gonadotrophin releasing hormone (GnRH), decreases the capacity of the pituitary (p less than 0.01) to synthesize and or release follicle stimulating hormone and luteinizing hormone (LH) following a single stimulation of GnRH (50 ng/100 g body weight), lowers the capacity of the testes to produce testosterone (p less than 0.01) following multiple subcutaneous injections of human chorionic gonadotrophin (hCG 3IU/100 g body weight for 3 consecutive days), decreases the number of Leydig cell LH receptors and decreases the in vitro responsiveness of the hCG-challenged Leydig cell to synthesize testosterone (p less than 0.01). These phenomena are independent of a major alteration in the capacity of the hCG challenged Leydig cell to produce adenosine 3',5'-monophosphate. It is concluded that the decline in testicular activity accompanying senescence is not inherent to the testes only but is also associated with alteration in the function of the hypothalamus and pituitary which eventually lead to the loss of fecundity.