Teleost isotocin receptor: structure, functional expression, mRNA distribution and phylogeny

Area of Science:

• Neuroendocrinology
• Comparative Physiology
• Molecular Biology

Background:

• The oxytocin system plays crucial roles in mammalian physiology.
• Isotocin is an oxytocin-related peptide found in non-mammalian vertebrates.
• The specific receptor for isotocin and its functions in fish remained largely uncharacterized.

Purpose of the Study:

• To identify and characterize the cDNA encoding the isotocin receptor (ITR) in the teleost Catostomus commersoni.
• To investigate the functional properties and signaling pathway of the cloned ITR.
• To determine the tissue distribution of ITR-encoding mRNA in fish.

Main Methods:

• Screening of a lambda gt11 cDNA library from teleost hypothalamus using PCR-amplified probes.
• cDNA sequencing to determine the full-length receptor sequence.
• Oocyte expression and electrophysiological recordings to assess receptor function.
• RT-PCR to analyze tissue distribution of ITR mRNA.

Main Results:

• A full-length cDNA encoding a 390-amino acid polypeptide with seven transmembrane domains was identified.
• The deduced amino acid sequence showed highest similarity to mammalian oxytocin receptors.
• Expressed oocytes responded to isotocin with chloride currents, indicating coupling to the inositol phosphate/calcium pathway.
• The ITR was activated by isotocin, vasotocin, mesotocin, oxytocin, and Arg-vasopressin, with varying potencies.
• ITR mRNA was detected in multiple tissues, including brain, intestine, bladder, skeletal muscle, lateral line, gills, and kidney.

Conclusions:

• The cloned cDNA represents a functional isotocin receptor (ITR) in fish.
• The ITR is coupled to the inositol phosphate/calcium signaling pathway.
• The widespread distribution of ITR suggests diverse physiological roles in fish, including osmoregulation and neuronal functions.