The Gonadotropins Subunits, GNRH and GNRH Receptor Gene Expression and Role of Carbon Monoxide in Seasonal Breeding Animals
Seasonality in reproduction occurs mainly in wild species and it is the result of natural selection. Signals to start or finish the period of reproductive activity, both environmental and hormonal depend on the neuroendocrine pathway - synthesis and secretion of pituitary hormones, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), under the control of the hypothalamic gonadotropin-releasing hormone (GnRH) neurons. Variable frequency of GnRH pulses is not only the main factor governing primary and preovulatory release of gonadotropins, but it can also play a role in the specific transcriptional activity of gonadotropin subunit genes (LHβ, FSHβ and Cga). However, changes in release of GnRH pulse pattern do not explain the preferential stimulation of the synthesis and secretion of gonadotropins in the annual reproductive cycle. In this regulation also a GnRH independent mechanism participates. It seems that the main factor responsible for the occurrence of the seasonal modulation of reproduction in sheep and other mammals, is significant changes in response of GnRH systems to gonadal steroids. The effect of carbon monoxide on regulation of the hypothalamic-pituitary-gonadal axis has not been studied to date. There is substantial evidence to suggest that it may play a role in the transduction of information on day length. The presence of heme oxygenase-2 in hypothalamic areas important for regulation of pituitary secretory function and in the pituitary itself suggests that carbon monoxide, the product of this enzyme, may participate in the regulation of hormone secretion by the pineal gland.
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