Current knowledge of ghrelin in amphibians

Area of Science:

• Comparative physiology
• Endocrinology
• Amphibian biology

Background:

• The ghrelin system plays vital roles in vertebrate physiology.
• Current understanding of the ghrelin system in amphibians is limited.
• This review synthesizes existing knowledge on amphibian ghrelin systems.

Purpose of the Study:

• To explore the physiological importance of the ghrelin system in amphibians.
• To investigate structural variations in the ghrelin precursor among amphibian species.
• To analyze the functional characteristics of the ghrelin receptor (GHS-R1a) in different amphibian species.

Main Methods:

• Analysis of ghrelin precursor sequences in various amphibians.
• Functional assays of ghrelin receptors in HEK293 cells to measure intracellular calcium.
• Quantitative analysis of GHS-R1a mRNA expression in different tissues under various conditions (fasting, dehydration).
• In vitro studies on gastrointestinal motility using smooth muscle strips.

Main Results:

• A unique Thr-3 modification in the ghrelin precursor was identified in the frog genus Rana.
• Ghrelin and GHS-R1a agonists increased intracellular calcium in cells expressing amphibian receptors.
• Ligand selectivity of ghrelin with Ser-3 and Thr-3 was observed in newt receptors but not in frog receptors.
• GHS-R1a mRNA was highly expressed in the brain, gastrointestinal tract, kidney, and gonad.
• Fasting and ghrelin injection did not significantly affect food intake.
• Dehydration increased GHS-R1a mRNA expression in the tree frog, but ghrelin did not affect water absorption.
• Ghrelin did not influence gastrointestinal motility in vitro.

Conclusions:

• The ghrelin system is conserved across various amphibian species.
• Significant variations exist in ghrelin precursor structure and receptor ligand selectivity.
• The physiological functions of the amphibian ghrelin system are not fully elucidated, with limited roles observed beyond hypophyseal hormone secretion.