Neuroendocrine regulation of GnRH release in induced ovulators.

GnRH signaling, the gonadotrope and endocrine control of fertility.

Puberty is the transitional period between the juvenile state and adulthood during which the adolescent growth spurt occurs, secondary sexual charac­ teristics appear, fertility is achieved, and profound psychological changes take place. We propose that the events characterizing pubertal maturation of the reproductive endocrine system can be viewed as part of a continuum extending from sexual differentiation and the ontogeny of the hypothalam­ ic-pituitary gonadotropin-gonadal system in the fetus to the attainment of full sexual maturation and fertility, and then ultimately to senescence (20, 21). Two independent processes, controlled by different mechanisms but closely linked temporally, are involved in the increase of sex steroid secre­ tion in the peripubertal and pubertal periods. One process, adrenarche, involves the increase in adrenal androgen secretion, which precedes by about two years the second event, gonadarche, the activation of the hypo­ thalamic-pituitary gonadotropin-gonadal apparatus that had been active at a low level during childhood. The regulatory systems that control human male and female reproduc­ tion comprise the following fundamental components:

Neuroendocrine Control Mechanisms and the Onset of Puberty

Alterations in follicular estradiol and gonadotropin receptors during development of bovine antral follicles.

Thirty-two nutritionally anestrous cows were used to determine the effect of the frequency of exogenous GnRH pulses on ovarian follicular growth, serum concentrations of LH and FSH, and concentrations of LH, FSH, GnRH receptors (GnRH-R), messenger RNA (mRNA) for GnRH-R, and mRNA for gonadotropin subunits in the pituitary. Cows were randomly assigned to one of four treatments: 2 μg GnRH infused (iv) continuously during 1 h, 2 μg GnRH infused during 5 min once every hour, 2 μg GnRH infused during 5 min once every fourth hour, or saline (control) for 13 days. Infusion of GnRH every hour increased LH concentrations in serum (P < 0.05), but FSH concentrations were not affected by GnRH infusion. Luteal activity (LA) was assessed by the presence of corpora lutea and/or serum progesterone greater than 1 ng/ml. Six of eight cows infused with GnRH every hour had LA by day 13, whereas only 25% of cows infused either continuously or with a pulse every fourth hour had LA by day 13. None of the control cows had LA duri...

/pdf/effect-of-gonadotropin-releasing-hormone-gnrh-pulse-c57tvgw7hg.pdf

Effect of gonadotropin-releasing hormone (GnRH) pulse frequency on serum and pituitary concentrations of luteinizing hormone and follicle-stimulating hormone, GnRH receptors, and messenger ribonucleic acid for gonadotropin subunits in cows

GnRH-receptor interaction. V. Down-regulation of pituitary receptors for GnRH in ovariectomized ewes by infusion of homologous hormone.

The effects of estradiol and progesterone on 1) hypophyseal and serum concentrations of luteinizing hormone (LH) and follicle-stimulating hormone (FSH); 2) hypothalamic content of gonadotropin releasing hormone (GnRH); and 3) numbers of receptors for GnRH were examined in ovariectomized ewes. Three days after ovariectomy, ewes were given 16 mg progesterone (n 5), 0.7 mg estradiol (n 5) or 16 mg progesterone plus 0.7 mg estradiol (n 5) s.c. in ethanol twice daily for 3 weeks prior to slaughter. Ten control ewes were given injections of ethanol according to the same schedule. Five intact ewes were killed 12 days after an induced ovulation. Blood samples were collected daily for 4 days prior to slaughter, and the serum was analyzed for concentrations of progesterone, estradiol, LH, and FSH. Hypothalami were removed after sacrifice and the amount of GnRH present in the preoptic area, median eminence, and hypothalamus proper was determined. In addition, the pituitary gland from each animal was removed and the number of receptors for GnRH and concentrations of LH, FSH, and prolactin were measured. Steroid treatment produced concentrations of estradiol and progesterone similar to those observed during late pregnancy in ewes. Administration of either progesterone or estradiol, suppressed (P 0.05) as a result of treatment. Likewise, neither hypophyseal weight nor prolactin content was affected by steroid treatment, suggesting that the effects of the steroids were limited to the gonadotrophs. These data indicate that estradiol can increase the number of receptors for GnRH, while the quantity of LH and FSH released into the circulation or contained in the pituitary decreases. Thus, the effects of estradiol appear to be mediated at the level of the pituitary gland. In contrast, the inhibitory effect of progesterone on secretion of LH is not mediated via receptors for GnRH or pituitary concentrations of LH, suggesting an influence at higher neural centers.

/pdf/gnrh-receptor-interaction-vi-effect-of-progesterone-and-4vqo7h8fcx.pdf

GnRH-Receptor Interaction. VI. Effect of Progesterone and Estradiol on Hypophyseal Receptors for GnRH, and Serum and Hypophyseal Concentrations of Gonadotropins in Ovariectomized Ewes

Pituitary content of gonadotropins and GnRH-receptors in pregnant, postpartum and steroid-treated OVX ewes.

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GnRH-receptor interaction. V. Down-regulation of pituitary receptors for GnRH in ovariectomized ewes by infusion of homologous hormone.

GnRH-Receptor Interaction. VI. Effect of Progesterone and Estradiol on Hypophyseal Receptors for GnRH, and Serum and Hypophyseal Concentrations of Gonadotropins in Ovariectomized Ewes

Pituitary content of gonadotropins and GnRH-receptors in pregnant, postpartum and steroid-treated OVX ewes.