Dynamic Change in Circulating Pituitary and Gonadal Hormones from Birth to Puberty in the Thoroughbred Colts and Fillies.

Abstract

The equine fetal gonads of both sexes have attracted much interest because of remarkable enlargement of gonadal size resulting from increased number of interstitial cells, but the size of gonad reduces at birth and re-starts development for puberty. The present study was designed to elucidate the changes in circulating gonadotropins and gonadal hormones from birth to puberty and thus explicate relationship between gonadal seasonality and hormonal secretion in both sexes of thoroughbred horses. In the first experiment (1st Exp), daily point sampling of jugular blood was done using spring born thoroughbred colts (n=636) and fillies (n=694) from birth to 30 weeks of age. To further expand, in the second experiment (2nd Exp), spring-born colts (n=6) and fillies (n=9) were used for continuous weekly jugular blood samples from birth to 15 months of age. To confirm the gonadal seasonality, in the third experiment (3rd Exp), fully adult stallions (n=10) were used for monthly jugular blood samples for two years. To clarify the relationship between the photoperiod and reproductive endocrinology, in the fourth experiment (4th Exp), geldings males (n=10) were used for continuous monthly jugular blood samples for one year. In the final experiment (5th Exp), for determination of effect of extended photoperiod on reproductive endocrinology, yearlings (colts; 31, fillies; 31) were used. Horses (n=30) were subjected to an artificial photoperiod (14.5 h daylight and 9.5 h of night from December 20th to April 10th). Rest received natural light as the control group (n=32). In both sexes, the 1st Exp showed that levels of steroid hormones (estradiol, testosterone, progesterone) were remarkably high at birth, followed by rapid drop within 48 hr, and remained at the lower level until six months of age. The level of ir-inhibin began to decrease after 3 days, dropping by more than half in the first 20 days after birth, and then declined slowly until six months of age. The 2nd Exp showed a clear increase in serum levels of testosterone, estradiol, and ir-inhibin in colts from around 44 weeks later (April) and remained high until 60 weeks later (August). In fillies, serum levels of progesterone and estradiol increased at around 48 weeks later (April and May), and remained high until 60 weeks. The increase in ir-inhibin was less clear in fillies. In colts, levels of LH, FSH, and PRL showed the first increase by around 16 weeks of age (June to September) and the second large increase at around 50 weeks of age (April to July in the next year). Prolactin exhibited most clear seasonal changes in both sexes. In both sexes, the seasonal increase in circulating PRL was remarkable even in the first year. In the 3rd Exp, clear seasonal changes were observed in gonadotropins and testicular hormones of the adult stallions. The highest concentration of LH, FSH, PRL, ir-inhibin, testosterone and estradiol were noticed in the breeding season (March to July), and the lowest being noted in the non-breeding season (October to February). In the 4th Exp, a similar seasonal change was observed in circulating PRL of gelding males. In the 5th Exp, the light treatment to yearlings stimulated secretion of LH, FSH, and prolactin in both the sexes. The light treatment also induced early ovulation in fillies and early secretion of testosterone and estradiol in colts. These results clearly demonstrated that the hypothalamo-pituitary axis is already responsive to changes in photoperiod and secrete PRL similar to adult horses, but pituitary gonadotrophs for LH and FSH secretion is less sensitive. In addition, the responsiveness of gonads, especially testes of weanling are less sensitive to LH and FSH for testosterone and estradiol secretion. (poster)