In subject area: Agricultural and Biological Sciences
The hypothalamic pituitary gonadal (HPG) axis is defined as the hormonal system that regulates testosterone production and spermatogenesis through the release of gonadotropin-releasing hormone (GnRH) from the hypothalamus, which stimulates the pituitary gland to produce luteinizing hormone (LH) and follicle-stimulating hormone (FSH). This axis is critical for maintaining hormonal balance and reproductive function throughout adulthood.
AI generated definition based on: Molecular Aspects of Medicine, 2024
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2016, Progress in Molecular Biology and Translational Science S.L. Asa, S. Ezzat
The hypothalamic-pituitary-gonadal axis represents one of the most complex and intricately networked systems in mammals. The perpetual demands placed on this system during health and disease requires highly coordinated functions. It appears likely that stressful demands engage epigenetic mechanisms that, within the adenohypophysis, frequently involve gonadotropes. This positions gonadotrope tumors at the center stage as the most common form of pituitary neoplasia. Greater insights into the machinery of the supply and demand neuroendocrine network of reproduction will prove essential for the next wave of progress in this field.
2020, Obesity and Gynecology (Second Edition) Man-wa Lui, ... Richard A. Anderson
Endocrine regulation of the human hypothalamic–pituitary–gonadal axis has been studied extensively in the last three decades since the Nobel Prize–winning experiments involving the extraction of gonadotropin-releasing hormone (GnRH) from porcine hypothalami. It is now well established that two hormones secreted by the pituitary, luteinizing hormone (LH) and follicular-stimulating hormone (FSH), regulate the functions of Leydig and germ cells, respectively. Secretion of these pituitary hormones, in turn, is regulated by the modulation of frequency and/or amplitude of pulsatile hypothalamic secretion of GnRH. A range of environmental, metabolic, and endocrine factors regulates hypothalamic–pituitary function. In particular, testosterone and estradiol (formed by the aromatization of testosterone in adipose tissue) exert negative feedback inhibition on the reproductive endocrine axis at the level of the hypothalamus and the pituitary. Inhibin B, secreted by the Sertoli cells of the testes, also plays an inhibitory role in gonadotropin secretion.
2024, Encyclopedia of Fish Physiology (Second Edition) Carolyn R. Wheeler, ... Cynthia Awruch
Reproduction in chondrichthyan fishes, like other vertebrates, is regulated by the hypothalamic–pituitary–gonadal axis (HPG axis). This axis consists of a cascade of hormones from regions of the brain stimulating the gonads via circulation, where in turn, the gonads produce sex steroid hormones that provide a negative feedback loop on the brain. At the top/beginning of the HPG axis, a tropic peptide hormone called gonadotropin-releasing hormone (GnRH) is produced in the hypothalamus. In mammals, GnRH travels via the hypophyseal portal system to the anterior lobe of the pituitary gland to stimulate the release of gonadotropins. However, it remains unclear how GnRH travels in chondrichthyan fishes. While, teleost fishes do have neuronal fiber projections connecting the hypothalamus with the anterior pituitary, early work in chondrichthyans showed clear evidence of a pituitary portal system. However, a direct link between the hypothalamus and the ventral lobe (i.e., in elasmobranchs) or buccal lobe (i.e., in Holocephali), where gonadotropic activity is located, is conspicuously absent. This absence has led researchers to hypothesize that high levels of GnRH in the bloodstream may reach gonadotropes through systemic circulation. Further work is needed to confirm the possible connection in chondrichthyan species, however. Three forms of GnRH have been categorized in chondrichthyans; specifically, the genes for three forms (GnRH1, 2, 3) have been documented in elasmobranchs (i.e., sharks and rays) and GnRH1(a,b) and GnRH2 have been documented in holocephalans (i.e., chimaeras). Beyond basic characterization, there is minimal experimental research focused on understanding the complete physiological role of GnRH in chondrichthyans. Fasano et al. (1989) reported that a GnRH agonist analog (GnRHA, HOE766) increased plasma androgen activity after intratesticular injections in two electric ray species. Furthermore, studies based in public aquaria have aimed to use intramuscular implants or GnRH agonist and antagonist injections to control female elasmobranch breeding and to assess changes in sex steroid hormones and dominance hierarchies in males with varying degrees of success. These cases indicate that GnRH function in this taxon is similar to that of other vertebrates, but the area is still greatly understudied, especially compared to the multitude of work on GnRH function in teleost fishes.
After GnRH stimulates gonadotropes in the ventral lobe (i.e., elasmobranchs) or buccal lobe (i.e., holocephalans) region of the pituitary gland, gonadotrophic hormones GTH I and GTH II, which are functional analogs of luteinizing hormone (LH) and follicle stimulating hormones (FSH), are released.
