Localization of 2-[125I]iodomelatonin binding sites in visual areas of the turtle brain.

The hormone melatonin is believed to play an important role in the regulation of both circadian and circannual rhythms. In mammalian vertebrates melatonin receptors are discretely localized, with broader distributions reported in avians and reptiles. To examine the sites at which melatonin may act in the turtle brain, 2-[125I]iodomelatonin binding sites were assessed using quantitative autoradiography. Specific binding sites were primarily restricted to forebrain structures with a wide distribution in visual recipient areas. The distribution of melatonin sensitive sites within the turtle visual system suggests that the ability to transduce received photoperiodic signals in the reptilian brain is broadly distributed within the central nervous system.