Influence of month of birth on the serum hormone concentrations and weights of the accessory sex organs and testes during maturation of the male laboratory rat.

Abstract

The serum hormone concentrations of testosterone, LH and prolactin were studied in cohorts of male rats born one group/month over 20 consecutive months to determine the influence of month of birth on serum hormone concentrations observed at given ages during maturation. There was a significant effect of both age and month of birth on the concentrations of the three hormones studied. Serum levels of testosterone increased with age and reached maximal levels at 50–70 days of age in rats born in the spring and at 80–90 days of age in rats born in the fall. Serum LH levels were generally elevated at 30–40 days of age and then declined to adult levels by 60 days of age. However, secondary rises in serum LH levels were noted in 70–90-day-old rats born in the spring. Serum prolactin levels increased with age from 30 to 80 days of age. There was a further elevation of serum levels of prolactin in mature animals (120 or 150 day old) in all months of birth except in those born during the spring and early summer. The changes in weights of testes and accessory sex organs (ventral prostate, seminal vesicle, coagulating gland) of maturing male rats were studied in cohorts of animals born during different consecutive months for the last 14 months of the above study. The weights of the accessory sex organs were significantly affected by the month in which the animals were born, with rats born in the spring (March-May) exhibiting accelerated growth during maturation as compared to rats born at other times of the year. Changes in the weights of the testes during maturation were not influenced by month of birth. These results indicate that serum hormone concentrations and weights of accessory sex organs of sexually maturing male rats are influenced by the month of birth, even though the animals are maintained under controlled environmental conditions.