Prepubertal Changes in the Hypothalamic-Pituitary Axis of Holstein Bulls1

Abstract

We investigated the nature and sites of changes in the hypothalamic-pituitary axis associated with the onset of high-frequency, high-amplitude discharges of luteinizing hormone (LH) in young bulls during the transition from the infantile to the prepubertal phase of development. Blood serum and neuroendocrine tissues from bulls killed at 1, 6, 10, 14, or 18 wk of age were evaluated. Concentrations of LH in serum from bulls 1 or 6 wk old averaged less than 0.25 ng/ml and only one episodic discharge of LH was detected for 10 bulls. At 10, 14, or 18 wk, 14 of 15 bulls had episodic discharges of LH. Concentrations of testosterone in serum were progressively higher at 10, 14, and 18 wk, but the concentration of estradiol was maximal at 6 wk. The concentrations of gonadotropin-releasing hormone (GnRH) in the anterior hypothalamus, posterior hypothalamus, or median eminence were not influenced by age. However, concentration of GnRH receptors in the anterior pituitary gland increased 314% between 6 and 10 wk and the concentration of LH increased 67%. Between 6 and 10 wk, concentrations of estradiol receptors in the anterior and posterior hypothalamus declined by 68% and 46%, but the concentration of estradiol receptors in the anterior pituitary gland increased by 103%. For most characteristics, there was no major change between 10 and 18 wk. We postulate that between 6 and 10 wk of age, there is 1) removal of an estradiol-mediated block of GnRH secretion and 2) an estradiol-mediated, and possibly GnRH-mediated, increase in pituitary GnRH receptors. Together, these changes result in greatly increased stimulation of the anterior pituitary gland by GnRH between 6 and 10 wk of age and stimulation of the discharges of LH characteristic of bulls in the early prepubertal phase of development.

Some Andrological Parameters of Charolais Breeding Bulls in Self‐performance Test

“…According to the results of the second GnRH treatment, the bulls were ranked in the following categories: The testosterone concentrations (before and after the exogenous GnRH treatment and the increment within 120 min) of the blood serum determined at the end of the investigation period were significantly (p < 0.05) higher (+3.79 nmol/l, + 10.13 nmol/l, + 6.34 nmol/l) than the values of the first GnRH test. These results are in agreement with some previous findings (Oltner et al 1979;Dorst et al 1983;Amann et al 1986). which refer to positive correlation of the testosterone content of the blood plasma and the age of bulls.…”

Integrity of the plasma membrane, the acrosomal membrane, and the mitochondrial membrane potential of sperm in Nelore bulls from puberty to sexual maturity

“…LH also stimulates Sertoli cells to differentiate and synthetize growth factors, which promote Leydig cell maturation (Sharpe, 1994;Walker and Cheng, 2005). Mature Leydig cells respond to the increasing LH stimulus synthetizing primarily testosterone, which is essential for bull puberty (Amann and Schanbacher, 1983;Amann et al, 1986). With 18-19 mo of age, the elevated basal testosterone levels (Table 2) likely triggered a negative feedback, decreasing GnRH and gonadotrophin release and thus lowering Leydig cell stimulation and testotosterone production (Amann and Schanbacher, 1983).…”

Desenvolvimento ponderal e testicular, concentrações periféricas de testosterona e características de abate em touros da raça Nelore

“…A elevação nos níveis basais de testosterona é conseqüência da diferenciação das células de Leydig e está associada à proliferação das células germinativas, eventos essenciais para que o animal se torne púbere (Amann & Schanbacher, 1983;Amann et al, 1986). A diminuição na secreação de testosterona após os 18 meses é desencadeada pelos próprios níveis elevados deste esteróide, os quais reduzem a secreção de GnRH e gonadotrofinas; esta redução, conseqüentemente, é a causa do menor estímulo para as células de Leydig produzirem menos testosterona (Amann & Schanbacher, 1983).…”