Question 2 Flashcards
Describe the hypothalamic-hypophyseal-ovarian axis
Starting at the hypothalamus, Gonadotropin Releasing hormone (GnRH) is released through two centers: surge or tonic. The tonic center releases GnRH (a neuropeptide) in small, constant concentrations. This GnRH travels to the anterior pituitary (gonadotroph cells) and causes the release of follicle stimulating hormone (FSH) and luteinizing hormone (LH) (both glycoproteins), which both act on the ovaries. FSH causes follicle growth and therefore synthesis of estradiol (steroid hormone) and inhibin (protein hormone). Estradiol has positive feedback on GnRH production by the hypothalamus, and causes GnRH to be released in large amounts by the surge center. Estradiol is also important in the expression of estrus behaviour. This GnRH surge causes an LH surge by the anterior pituitary. Meanwhile, inhibin has negative feedback on FSH production by the anterior pituitary. This high concentration of LH and low concentration of FSH acting on the ovaries stimulates ovulation. This ovulated follicle becomes the corpus luteum which begins to produce progesterone (steroid hormone). Progesterone has negative feedback on the hypothalamus so that no surge secretion of GnRH occurs.
Describe the seasonality endocrine effect on long-day breeders (female)
Seasonal breeders experience a period of hypothalamic “dormancy” during their off-season. In long day breeders, an increase of light (in spring/summer) causes an increase in firing of the retinal nerves, which excites the superior cervical ganglion. This inhibits the pineal gland. The pineal gland is sensitive to environmental light and can sense changes in photoperiod. It releases melatonin in response to darkness, which can alter physiological processes like reproduction. It regulates breeding activity in seasonal breeders. When inhibited, melatonin production by the pineal gland decreases. In long day breeders, this will cause an increase in GnRH production, causing release of FSH and LH, and then cyclicity through the axis discussed above.
Describe the hypothalamic-hypophyseal-testicular axis
In the male, the hypothalamus has no surge center (it was defeminized by estradiol during embryogenesis). GnRH is produced in frequent pulses or bursts, which last for a few moments and occur several times a day. These pulses induce LH and FSH surges which act on the testes. FSH acts on sertoli cells, which produce inhibin and androgen binding protein (ABP) (protein). Like in the female tract, inhibin inhibits FSH secretion. LH acts on the leydig cells of the testes, which produce testosterone (T4) (steroid hormone). Testosterone and ABP will bind in the seminiferous tubules and stimulate spermatogenesis. Testosterone acts has negative feedback effects on GnRH production by the hypothalamus. As T4 concentrations increase, this negative feedback stops GnRH which in turn lowers T4 levels. Once T4 is low enough, the hypothalamus once again begins to release GnRH. If T4 concentrations get too high, this will reduce spermatogenesis/fertility of the male.
Describe the seasonality endocrine effect on long-day breeders (male)
Seasonality affects males by changing testosterone levels and testes size. Long day breeders will have higher blood T4 levels and larger testes during the spring and summer.
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