Hypothalamic-pituitary-gonadal endocrine system in the hagfish.

Masumi Nozaki

doi:10.3389/fendo.2013.00200

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.

Highlights

Reproduction in gnathostomes is controlled by a hierarchically organized endocrine system called the hypothalamic-pituitary-gonadal (HPG) axis [1]
Until recent identification of a functional GTH in the hagfish pituitary [11], it had not been established whether the hagfish pituitary gland contained adenohypophysial hormones of any kind [9]
The sea lamprey GTHβ groups with the GPHβs but appears to be one of the outgroups of the LHβs. These results clearly show that the glycoprotein hormones (GPH) identified in the hagfish acts as a functional gonadotropin, and hereafter it is referred as to GTH

Summary

Masumi Nozaki *

Sado Marine Biological Station, Faculty of Science, Niigata University, Sado, Japan. Reviewed by: Rosanna Chianese, Second University of Naples, Italy Rosaria Meccariello, University of Naples Parthenope, Italy. Hagfishes as one of the only two extant members of the class of agnathans are considered the most primitive vertebrates known, living or extinct. Studies on their reproduction are important for understanding the evolution and phylogenetic aspects of the vertebrate reproductive endocrine system. 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

INTRODUCTION

HPG axis in hagfish

GLYCOPROTEIN HORMONE FAMILY

Findings

CONCLUSION