Melatonin receptors in brain areas and ocular tissues of the teleost Tinca tinca: Characterization and effect of temperature

The aim of the present study was to characterize the central melatonin receptors in brain areas and ocular tissues of the teleost Tinca tinca. We investigated the temperature-dependence of 2-iodo-melatonin ([125I]Mel) binding in the optic tectum-tegmentum area and the neural retina. The binding of [125I]Mel showed a widespread distribution in brain and ocular tissues, with the highest density in the optic tectum-thalamus and the lowest in hindbrain. The [125I]Mel affinity was similar in all the studied tissues, and it was on the order of the low pM range. Saturation, kinetic and pharmacological studies showed the presence of a unique MT1-like melatonin binding site. In addition, the non-hydrolysable GTP analog, the GTPγS, and sodium cations induced a specific binding decrease in the optic tectum and neural retina, suggesting that such melatonin binding sites in the tench are coupled to G protein. Thus, these melatonin binding sites in optic tectum and neural retina fulfil the requirements of a real hormone receptor, the specific binding is rapid, saturable, and reversible, and is inhibited by GTP analogs. Additionally, a clear effect of temperature on such central melatonin receptors was found. Temperature did not modify the Bmax and Kd, but the kinetics of [125I]Mel binding resulted in a highly thermosensitive process in both tissues. Both association and dissociation rates (K+1 and K−1) significantly increased with assay temperature (15–30 °C), but the Kd constant (estimated as K−1/K+1) remained unaltered. In conclusion, this high thermal dependence of the melatonin binding to its receptors in the tench central nervous system supports the conclusion that temperature plays a key role in melatonin signal transduction in fish.