THE PRESENCE OF FUNCTIONAL MELATONIN RECEPTORS IN THE IRIS-CILIARY PROCESSES OF THE RABBIT EYE

There is evidence in the literature to suggest that melatonin has a fu nctional role in the iris-ciliary body (ICB). In this study we provide results which argue for the occurrence of melatonin receptors in this region of the eye. Autoradiography showed that specific binding of I- 125-iodomelatonin is associated with the ICB although the resolution d id not allow for discrimination of binding between the ciliary epithel ium and the iris musculature. The binding of I-125-iodomelatonin to IC B homogenates is saturable with a single population of binding sites a nd with K-D and B-max values of 428 pM and 170 fmol (g wet weight)(-1) respectively. The order of potency of various substances to compete w ith the I-125-iodomelatonin binding sites is chloromelatonin = melaton in > 6-OH-melatonin > N-acetylserotonin > 5-methoxytryptamine > seroto nin. Neither iodomelatonin nor serotonin affected basal levels of cAMP , but both decreased the forskolin elevated cAMP levels. However, the iodomelatonin effect, unlike that of serotonin, was not antagonised by propranolol. The combined data argue for the presence of melatonin re ceptors in the ICB of the rabbit. These receptors are negatively linke d to the stimulation of cAMP and are distinguishable from serotonin re ceptors.