Hypothalamic-Pituitary-Gonadal Endocrine System in the Hagfish

Abstract

The hypothalamic-pituitary system is considered to be a seminal event that emerged prior to or during the differentiation of the ancestral agnathans (jawless vertebrates). Hagfishes as one of the only two extant members of the class of agnathans are considered the most primitive vertebrates known, living or extinct. Accordingly, studies on their reproduction are important for understanding the evolution and phylogenetic aspects of the vertebrate reproductive endocrine system. In gnathostomes (jawed vertebrates), the hormones of the hypothalamus and pituitary have been extensively studied and shown to have well-defined roles in the control of reproduction. In hagfish, it was thought that they did not have the same neuroendocrine control of reproduction as gnathostomes, since it was not clear whether the hagfish pituitary gland contained tropic hormones of any kind. This review highlights the recent findings of the hypothalamic-pituitary-gonadal endocrine system in the hagfish. In contrast to gnathostomes that have two gonadotropins (GTH: luteinizing hormone and follicle-stimulating hormone), only one pituitary GTH has been identified in the hagfish. Immunohistochemical and functional studies confirmed that this hagfish GTH was significantly correlated with the developmental stages of the gonads and showed the presence of a steroid (estradiol) feedback system at the hypothalamic-pituitary levels. Moreover, while the identity of hypothalamic gonadotropin-releasing hormone (GnRH) has not been determined, immunoreactive (ir) GnRH has been shown in the hagfish brain including seasonal changes of ir-GnRH corresponding to gonadal reproductive stages. In addition, a hagfish PQRFamide peptide was identified and shown to stimulate the expression of hagfish GTHβ mRNA in the hagfish pituitary. These findings provide evidence that there are neuroendocrine-pituitary hormones that share common structure and functional features compared to later evolved vertebrates.

Marine Peptides: Bioactivities and Applications

“…The gonadotropin-releasing hormone, which increases the secretion of pituitary gonadotropin has been detected in hagfish [ 110 ] and Japanese anchovy [ 111 ] brains and in gonads of the prosobranch Patella caerulea [ 112 ]. Proopiomelanocortin-derived hormones ( i.e.…”

A short evolutionary history of FSH-stimulated spermatogenesis

“…51 These subunits have given rise, through another whole-genome duplication, to another GPH, which is first detected as the single gonadotrophin in the most primitive ancestral agnathan (jawless) vertebrates. [52][53][54] Hagfish (or the slime eel) and lampreys are the two remaining members of this group and considered the most primitive vertebrates known, living or extinct. Their single pituitary GPH has no specific LH-or FSH-like function.…”

Molecular cloning of cytochrome P450 side-chain cleavage and changes in its mRNA expression during gonadal development of brown hagfish, Paramyxine atami

“…Accordingly, studies on the hagfish reproduction are indispensable for understanding phylogenetic aspects of vertebrate reproduction. However, our knowledge of endocrine regulation of reproductive processes is poorly understood in the hagfish (for reviews, see Gorbman, 1983;Nozaki, 2008Nozaki, , 2013. For example, it was thought that they did not have the same neuroendocrine control of reproduction as gnathostomes (jawed vertebrates), since it was not clear whether the hagfish pituitary gland contained tropic hormones of any kind (Matty et al, 1976;Hardisty, 1979;Gorbman, 1983).…”