The relationship between serum hormone levels and reproductive ability in aging male rats

The relation between fertility and hormonal levels was studied in aging male rats. Reproductive ability was inspected at 30, 31, 59, 60, 93 and 94 weeks of age, and the serum hormone levels were determined by the radioimmunoassay. Reproductive ability decreased with aging (mating rates: 15/20, 11/20, 3/20 and 6/20 at 30, 31, 59 and 60 weeks, respectively) and no male mated at 93 and 94 weeks. Serum testosterone levels in males without reproductive activity were significantly lower than those in males with reproductive activity. LH and FSH showed the lowest level in 95 weeks. The highest value of LH was found at 60 weeks in males without reproductive activity having low testosterone levels. The highest value of prolactin appeared in 95 weeks which was three times higher than those in the other groups. The results suggest that the loss in the reproductive activity in aging rats starts with a decrease in libido, which is followed by a high gonadotropin and low testosterone condition, and accomplished under a low gonadotropin and high prolactin circumstance.