Gonadotropin releasing hormone systems
Gonadotropin releasing hormone (GnRH) is a neuroendocrine hormone regulating the reproductive functions in vertebrates. It is concerned with the regulation of two gonadotropins called Follicle stimulating hormone or FSH and Luteinizing hormone or LH in fish as well as mammals. In some species of fish (for example, Gold fish Carassius auratus), GTH-I and GTH-II, which are structurally similar to FSH and LH, are the gonadotrophic hormones released from the pituitary (Trudeau, 1997). The release of gonadotrophins in mammals is regulated by the release of gonadotropin releasing hormone from the hypothalamus.
Structure of GnRH
In mammals & fish, GnRH is a decapeptide containing ten aminoacids in its structure. Till now, thirteen varieties of GnRH (Millar et al., 2004) are discovered from vertebrates, out of which eleven types of GnRH are found in fish. Out of these eleven types, the most important ones are the mammalian GnRH (m-GnRH), the chicken GnRH (c-GnRH) and the Salmon GnRH (s-GnRH) seen in salmonids. All these GnRH molecules are decapeptides even though they differ in the aminoacid sequence. The only GnRH which has twelve aminoacids in its structure is seen in Octopus vulgaris(Iwakoshi et al., 2002). Chicken GnRH is an important form of gonadotropin releasing hormone found in birds.
There are three types of GnRH, viz., GnRH-I, GnRH-II and GnRH-III. GnRH-I and GnRH-II are found in mammals and in birds while GnRH-III is found initially in Lamprey, a jawless marine organism.
Release of Gonadotropin releasing hormone
In mammals, GnRH is released from the hypothalamus in response to the control mechanisms exerted by sex steroids. Androgens, estrogens and progesterone cause a feedback inhibition of the release of GnRH from the hypothalamus. The release of GnRH from the hypothalamus results in the release of gonadotropins from the pituitary, which in turn acts on the gonads to release sex steroids. The neural pulse generator located in the arcuate nucleus of hypothalamus is concerned with the frequency and dose of GnRH release from the hypothalamus. The released GnRH is transmitted through the hypothalamo hypophyseal portal tract to the pituitary where it acts to produce the effects.
The hypophyseal portal system is absent in fish. In fish, GnRH will be reaching the pituitary through the hypothalamic neurons directly from the hypothalamus(Levavi-Sivan, 2004). GnRH, stimulatory neurohormones and the dopamine neurons in the preoptic region of hypothalamus regulate the release of gonadotropins in fish. The stimulatory neurotransmitters include GABA (gamma-aminobutyric acid, which enhances the release of GnRH and neuropeptide Y, which stimulates the release of gonadotropins directly and through the increased release of GnRH. The dopamine neurons act as the inhibitory control over the release of gonadotropin releasing hormone in fish (Trudeau, 1997).
In birds, there are two types of GnRH called chicken GnRH-I and chicken GnRH-II released from the hypothalamus. These are released from the preoptic nucleus and from the mediobasal hypothalamus. It has been shown that there exists a photoperiodic control over the release of GnRH in birds like hen (Sharp, 1993) and quail (Urbanski & Follett, 1982). GnRH release is also increased during the spring breeding season in birds. The seasonal variation in GnRH release facilitates breeding of birds as the GnRH releases gonadotropins from the anterior pituitary which in turn helps in the maturation of gonads and facilitates reproduction (Moore, 2006). The photoreceptors present in the hypothalamus interact with the GnRH releasing neurons to release GnRH.
Inhibitory control over GnRH release
It was discovered that there exists an inhibitory neuropeptide in the hypothalamus that inhibits the release of GnRH (Bentley et al., 2006). This inhibitory peptide is called gonadotropin inhibitory hormone (or GnIH). There are reports suggesting its presence in mammals also. It is clear that GnIH interacts with GnRH to time the release of gonadotropin releasing hormone and there by acts as a control of gonadotropin release in birds and mammals. There are other chemical mediators like opioids, steroids and dopamine, which inhibits the release of GnRH.
GnRH is present in all vertebrates and in some protochordates. Even though the functionality of GnRH exists the same across species, the regulatory role of GnRH varies slightly from one species to another. The regulation of GnRH release also varies substantially. Higher up the evolution series, GnRH release goes from a seasonal release or a photoperiodic pattern to a pulsatile order. The forms of GnRH are also becoming more evolved as we climb up the evolution ladder. We can conclude that GnRH release system has transformed during the line of evolution to suit the reproductive needs of a particular species.
Bentley, G. E., Kriegsfeld, L. J., Osugi, T., Ukena, K., O'Brien, S., Perfito, N., et al. (2006). Interactions of gonadotropin-releasing hormone (GnRH) and gonadotropin-inhibitory hormone (GnIH) in birds and mammals. J Exp Zoolog A Comp Exp Biol, 305(9), 807-814.
Iwakoshi, E., Takuwa-Kuroda, K., Fujisawa, Y., Hisada, M., Ukena, K., Tsutsui, K., et al. (2002). Isolation and characterization of a GnRH-like peptide from Octopus vulgaris. Biochem Biophys Res Commun, 291(5), 1187-1193.
Levavi-Sivan, B., Safarian, H., Rosenfeld, H., Elizur, A., & Avitan, A. (2004). Regulation of Gonadotropin-Releasing Hormone (GnRH)-Receptor Gene Expression in Tilapia: Effect of GnRH and Dopamine. Biol Reprod, 70(6), 1545-1551.
Millar, R. P., Lu, Z.-L., Pawson, A. J., Flanagan, C. A., Morgan, K., & Maudsley, S. R. (2004). Gonadotropin-Releasing Hormone Receptors. Endocr Rev, 25(2), 235-275.
Moore, I. T., Bentley, G. E., Wotus, C., & Wingfield, J. C. (2006). Photoperiod-independent changes in immunoreactive brain gonadotropin-releasing hormone (GnRH) in a free-living, tropical bird. Brain Behav Evol, 68(1), 37-44.
Sharp, P. J. (1993). Photoperiodic control of reproduction in the domestic hen. Poult Sci, 72(5), 897-905.
Trudeau, V. L. (1997). Neuroendocrine regulation of gonadotrophin II release and gonadal growth in the goldfish, Carassius auratus. Rev Reprod, 2(1), 55-68.
Urbanski, H. F., & Follett, B. K. (1982). Photoperiodic modulation of gonadotrophin secretion in castrated Japanese quail. J Endocrinol, 92(1), 73-83.