Peptidergic neurons of the macaque monkey retina

Until recently, peptides were thought to act as neuromodulators in the retina, and the localizations of peptides in wide field amacrine cells, associational cells and interplexiform cells seemed to support this hypothesis. Anatomical studies in the macaque monkey retina, however, found that some types of peptidergic amacrine cells made extensive contacts with bipolar cell axons and retinal ganglion cell dendrites. The most striking exception to the earlier generalizations about retinal peptide function was the localization of immunoreactive cholecystokinin in bipolar cells that contacted short wavelength cones selectively. These results suggested that peptides were not only interacting with the most direct pathway for visual information; they also appeared to be used as transmitters by the neurons that comprise that pathway. Taken with the localizations of peptides in retinal ganglion cells and recent electrophysiological evidence, these findings suggest that peptides can also act more like conventional neurotransmitters.