The gonadotropin-releasing hormones: Lessons from fish

Area of Science:

• Comparative neuroendocrinology
• Fish reproductive physiology
• Evolution of hormone systems

Background:

• Fish are crucial models for understanding vertebrate neuroendocrinology and hormone evolution.
• The gonadotropin-releasing hormone (GnRH) system in fish is complex, featuring multiple isoforms and receptors.
• Existing knowledge on GnRH functions, especially non-hypothalamic roles, requires further elucidation.

Purpose of the Study:

• To review and integrate current knowledge on the fish GnRH system.
• To highlight the diversity of GnRH isoforms and receptors in teleosts.
• To discuss the differential roles of GnRH forms in fish reproduction and the impact of physiological conditions.

Main Methods:

• Literature review integrating past and recent findings.
• Analysis of data from transgenesis and mutagenesis studies in fish models.
• Incorporation of findings from neuronal electrophysiology and genome editing studies.

Main Results:

• Teleosts possess multiple GnRH isoforms (2-3) and receptors (up to 5) with distinct localizations and functions.
• The primary hypophysiotropic GnRH differs between teleost groups (Gnrh3 in some, Gnrh1 in others).
• Non-hypothalamic GnRH isoforms, like Gnrh2, may play roles in reproduction under stress (e.g., fasting).
• Transgenic fish models and electrophysiology have clarified GnRH neuron innervation and modulation of Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH).

Conclusions:

• The fish GnRH system is highly complex, with distinct roles for different isoforms in reproduction.
• While genome editing studies aimed to clarify GnRH functions, they have sparked debate and opened new research directions.
• Further research is essential to fully understand the multifaceted roles of the GnRH system in fish physiology and evolution.