Role of Central Monoamines in Regulation of Gonadotropin-II Secretion

Abstract

Two types of gonadotropin: gonadotropin I (GTH-I) and gonadotropin II (GTH-II) have been demonstrated in many teleosts (Kawauchi et al, 1989, (Swanson and Dittman, 1997). (Prat et al. (1996) and (Swanson and Dittman (1997) suggested designating them as follicle stimulating hormone (FSH) and luteinizing hormone (LH), respectively in line with the tetrapod nomenclature. However, in African catfish (Clarias gariepinus), only a single GTH corresponding to GTH-II or LH has been identified (Koide et.al, 1992, (Schulz et al. 1995). The latter authors have suggested three possibilities to explain the absence of FSH in the catfish. They are, 1) FSH is synthesized from the same gonadotropes as does LH and is biochemically, immunologically and biologically (steriodogenic activity) indistinguishable from LH, 2) FSH is devoid of any steroidogenic activity and is synthesized from a cell type that does not show ultrastructural features of the gonadotropes, and 3) the gene encoding for 0-subunit of FSH, if present, is “silent”. Most studies on the regulation of GTH in teleosts are related to GTH-II secretion and demonstrate the participation of multiple brain regulatory/modulatory systems, as has been detailed in goldfish (Peter et al, 1986, (1991, (Trudeau and Peter, 1995, (Peter and Yu, 1997) and African catfish ((de Leeuw et al, 1987). The dominant brain mechanism controlling GTH-II is stimulatory by gonadotropin-releasing hormone (GnRH). The activity of GnRH (synthesis and release) is influenced by a number of factors originating from both intrinsic and extrinsic environments. The well known variables, among them, are the circulating levels of sex steroids, seasonally varying day length, temperature, rainfall, sex pheromones, etc. Deciphering the mechanisms by which these factors influence the GnRH activity and GTH-II secretion is an exciting task. To date, a number of neurohormones have been demonstrated to modulate positively (stimulatory) or negatively (inhibitory) GnRH activity and consequently GTH-II secretion. They include a wide range of substances like monoamines, neuropeptides, amino acids, etc. (see Trudeau and Peter, 1995 for details).