Evaluation of reproductive hormones levels in intact and castrated Ghezel lambs at pre-puberty period

Introduction: Biosynthesis and storage of gonadotropins in farm animals begin at the same embryonic stage. The hypothalamus-pituitary-testicle axis controls animal reproductive activity through complex neurological and endocrine pathways. Disturbance in any of these pathways results in the inadequacy of the reproductive system and as a result, lower reproductive performance. Puberty is the end point of a series of events affecting the development of the ‘hypothalamo–pituitary–gonadal’ axis leading to reproductive competence. The changes in GnRH pattern release during puberty are under steroid-dependent and steroid-independent control mechanisms (reference?). Steroid-dependent mechanisms involve changes in sensitivity to gonadal steroid negative-feedback regulation of GnRH neurons (reference?). Although the ability to release high frequency GnRH pulses is inherent from an early age in sexually immature sheep, because of the presence of gonadal steroids and the greater sensitivity of the GnRH system to these steroids. As the time of puberty approaches, sensitivity to gonadal steroids decreases, which allows GnRH pulse frequency to increase. This remarkable change in the ‘gonadostat’ clearly underlies with pubertal increasing in GnRH/LH pulse frequency, which appears to be the key determinant of reproductive function. However, there are sexual differences in the responsiveness of GnRH system to inhibit by steroids, which underlie the different timing of puberty onset in male and female animals. Before  puberty,  a functional negative feedback system exists between testosterone and LH, estradiol and LH, and between inhibin and FSH in male sheep. Male lambs have a similar degree of responsiveness to estradiol and testosterone with regard to the regulation of LH pulse frequency. In view of these observations we have tested the hypothesis that the differences in the patterns of gonadotrophin secretions might be related to the influence of the testosterone on changes in the castrated males and accumulation of LH and FSH during the postnatal transition to puberty in male lambs. The main objective of this study was to evaluate the pattern of changes in sex hormones in intact and castrated Ghezel lambs during pre-puberty at different ages.
 Material and methods:  In this study, 20 Ghezel lambs were considered in four age groups included: group 1 (one-month-old castrated lambs, n = 4), group 2 (two-months-old castrated lambs n = 4), group 3  (three-months-old castrated lambs n = 4), group 4  (four-months-old castrated lambs n = 4),   group 5:  (one-month-old non-castrated  lambs as the control group,  n = 4). Lambs of the five groups were fed with the same diet according to the   NRC 2007 until the age of nine months. Blood samples from lambs were taken monthly until the age of nine months. After serum isolation, serum levels of testosterone, LH, and FSH were measured by ELISA.
Results and discussion:  In intact lambs, testosterone concentrations increased at the age of four months (0.65 ng/ml) and peaked at nine months of age (2.33 ng/ml).  In intact male lambs, FSH concentrations were the highest levels after weaning and early puberty periods (P <0.05). The LH concentration increased significantly in intact males at three months of age, while the testosterone concentration in the one-month group showed a significant difference there were no other months (P> 0.05). In this study, two phases of surging for LH were observed. The first phase of the LH peak was observed at 1-4 months of age (2.4 ng/ml) and the second phase was observed at 7-8 months of age (1.576 ng/ml). Reducing LH secretion between the first and second phases had a high correlation with increasing in testosterone concentration (r = -0.96, p <0.05) Testosterone concentrations were not significantly different in the   one-month-old, two-months-old, and three-months-old castrated groups (P> 0.05).
Conclusion: Puberty related hormonal changes in the Ghezel breed begins at 2-4 months with a change in the secretion   of LH and FSH at the end of weaning and then, puberty achieved at 8-9 months of age. According to the effect of testosterone on aggressive behaviors, castration is recommended in the early stages of birth (1-3 months), when the testosterone level is lower and the animal is under a little stress. The outstanding point of the present study is observing the increase in gonadotropins at the age of 2-4 months, which indicates the beginning of puberty related hormonal changes in the Ghezel breed.