Neurotransmitter systems in the outer plexiform layer of mammalian retina.

Melatonin represents a second type of chemical signal released from photoreceptors in response to increased darkness, one with characteristics which are significantly different from those of glutamate. Concise spatial and temporal aspects of the photoreceptor signal are conserved through discrete glutamatergic synapses. Different classes of post-synaptic neurons each have appropriate subclasses of glutamate receptors which transmit sign conserving or sign inverting images of the visual mosaic. In contrast, melatonin, because of its highly lipophilic nature is not released by stimulus-coupled secretion mechanisms, but rather by simple diffusion. Thus control of melatonin "release" may be less concise than glutamate. In addition, melatonin may diffuse beyond the confines of the synaptic area to target cells throughout the retina. Effects of melatonin in retina are not well understood; however, current hypotheses suggest that, perhaps via its control of dopamine systems in the inner retina, melatonin plays an important role in dark adaptation and in various retinal processes which exhibit a circadian rhythm. Melatonin and glutamate may represent "co-transmitters" which provide the visual pathway with two types of signals, with melatonin providing widespread modulatory influences on the discrete visual information conveyed via glutamatergic circuits.