Effects of temperature on 2‐[ 125 I]‐iodomelatonin binding to melatonin receptors in the neural retina of the frog Rana perezi

  The present study analyzes the effect of temperature‐dependent modifications on the binding of the analog 2‐[ 125 I]‐melatonin to melatonin receptors in isolated neural retina membranes from the greenfrog Rana perezi . Association and dissociation rate constants ( K +1 , K −1 ) were exponentially increased by the assay temperature. At 15°C, association and dissociation required several hours; meanwhile, at 35°C, rate constants were 100‐ and 34‐fold faster, respectively. However, the K d constant calculated as K −1 / K +1 was unmodified by the assay temperature. When frogs were acclimated at either 5 or 22°C for 1 month, K +1 , and K −1 constants determined at 15 and 25°C were identical in both cold‐ and warm‐acclimated groups. Thus, the binding kinetics of melatonin receptors in frog retinas did not shown any thermal compensation. Results from saturation curves and pharmacological profiles of melatonin binding sites support a lack of effect of assay temperature on the affinity of melatonin receptors in the frog retina. The inhibition of [ 125 I]Mel binding by GTP γ S showed clearly that the coupling of melatonin receptors to G proteins is temperature‐dependent. Higher concentrations of the GTP analog were needed to inhibit specific binding when temperature decreased. The temperature effect on binding kinetics and on the G protein coupling to melatonin receptors suggests that the melatonin signal could be transduced distinctly depending on the temperature. Thus, temperature plays a major role, not only on melatonin synthesis, but also in the transduction of melatonin signal in ectotherms.