Abstract
Melatonin is present within various eye tissues, in particular the retina, of many species and evidence exists for a local synthetic capacity. Melatonin is also reported to have several actions within the eye. The human ocular melatonin system, however, has previously been little studied. In these investigations, melatonin was quantified in human neural retina (NR) and choroid-retinal pigment epithelium (C-RPE) tissues of 46 post-mortem human eyes, using an existing radioimmunoassay which was fully validated for use in these tissues. Melatonin was present in 26 specimens in at least one tissue (range 10 to 486. 8 pg/g wet weight tissue), but levels were not related to any donor parameter (sex; age; time, month and cause of death; specimen age; and post-mortem interval). Additional studies investigated 2-[125I]iodomelatonin binding sites within the eye. The quail displays a robust retinal melatonin rhythm under the control of an intra-ocular clock. Membrane radioreceptor assays indicated a single class of high affinity bindings sites in both NR and C-RPE tissues. Pharmacologically the sites in the two tissues were very similar. Characterisation studies indicated that binding was saturable, reversible and specific. Photoperiod duration had little effect on either binding site density or affinity in both tissues. Autoradiographic studies confirmed the existence of a binding site in both tissues. Using the methodology developed in the quail, 2-[125I]iodomelatonin binding sites were subsequently investigated in post-mortem human NR and C-RPE membranes. Saturable, specific binding which could be Scatchard transformed was demonstrated in NR and/or C-RPE membranes of only 3/18 specimens. Autoradiography confirmed this low occurrence of binding (1/11). Preliminary evidence was also obtained for a melatonin binding site in human iris sphincter muscle. This study has detected melatonin and its binding sites in a proportion of human eyes. It is likely that the data were influenced by the post-mortem nature of the specimens.