Gonadotropin-inhibitory hormone (GnIH) is a hypothalamic neuropeptide that belongs to the RFamide peptide family and was first identified in the quail brain. From the discovery of avian GnIH, orthologous GnIH peptides have been reported in a variety of vertebrates, including mammals, amphibians, teleosts and agnathans, but also in protochordates. It has been clearly established that GnIH suppresses reproduction in avian and mammalian species through its inhibitory actions on brain GnRH and pituitary gonadotropins. In addition, GnIH also appears to be involved in the regulation of feeding, growth, stress response, heart function and social behavior. These actions are mediated via G protein-coupled GnIH receptors (GnIH-Rs), of which two different subtypes, GPR147 and GPR74, have been described to date. With around 30,000 species, fish represent more than one-half of the total number of recognized living vertebrate species. In addition to this impressive biological diversity, fish are relevant because they include model species with scientific and clinical interest as well as many exploited species with economic importance. In spite of this, the study of GnIH and its physiological effects on reproduction and other physiological processes has only been approached in a few fish species, and results obtained are in some cases conflicting. In this review, we summarize the information available in the literature on GnIH sequences identified in fish, the distribution of GnIH and GnIH-Rs in central and peripheral tissues, the physiological actions of GnIH on the reproductive brain-pituitary-gonadal axis, as well as other reported effects of this neuropeptide, and existing knowledge on the regulatory mechanisms of GnIH in fish.
Subcutaneous body fat and Quetelet's Indices (QI) of 52, 18-29 year old normal female volunteers were determined. These body mass indices were then grouped according to the phase of each subject's menstrual cycle, early or late follicular and early or late luteal phase. The subcutaneous body fat is 27.07 +or- 1.0% in the early follicular but drops to 24.68 +or- 1.84% in the late follicular phase. The value then rises significantly higher than that in the late follicular phase to 30.14 +or- 1.15% (P0.02) in the early luteal drops to 27.17 +or- 0.55% towards the level of the early follicular phase (P0.05). Variations in the values of QI during each menstrual cycle exactly mirror those for subcutaneous body fat. The fall in the 2 body mass indices during the late follicular phase coincides somewhat with the established preovulatory LH and FSH surges as well as the high levels of estrogen of this period. On the other hand the significant rise in the 2 parameters during the early luteal phase coincides with the marked rise in the ratio of progesterone to estrogen. Clearly, increased levels of progesterone relative to estrogen appear to cause an increase in the body fat during each menstrual cycle. The implication of this finding for women on contraceptive pills which are predominantly progesterone and those whose normal menstrual cycle is "interrupted" at the early luteal phase by a successful fertilization raises very interesting questions with regards to prediction of ovulation.