Serotonin systems in the inner and outer plexiform layers of the vertebrate retina

In retinas of certain nonmammalian vertebrate species such as frog, pigeon, and chick, serotonin appears to function as the neurotransmitter of a specific population of amacrine cells. Neurochemical and morphological studies have demonstrated high endogenous levels of 5-hydroxytryptamine (5-HT) as well as uptake, release, and receptor-binding activity restricted to the inner plexiform layer. In retinas from most mammalian species, uptake, release, and receptor-binding activity have also been localized to amacrine cell terminals in the inner plexiform layer. However, serotonin content in mammalian retinas is low, and attempts to localize the endogenous store of 5-HT have failed. Thus the status of serotonin as a candidate in mammalian retina is still open to question. Our more recent studies have revealed a light-sensitive serotonin system associated with photoreceptor terminals in retinas of Long-Evans rats. Uptake, synthesis, and release of [3H]serotonin have been demonstrated. Endogenous levels of 5-HT decrease in the dark and increase in the light. Electrophysiological studies are needed to illucidate the functional role(s) of serotonin within retinas of different species.