Enkephalin-containing amacrine cells in the avian retina: immunohistochemical localization

The distribution of enkephalin-like immunoreactivity within the avian retina was studied by immunofluorescence and immunoperoxidase techniques with antiserum to [Met5]enkephalin and [Leu5]enkephalin. Formaldehyde-fixed retinae were sectioned and incubated in antiserum to either [Met5]- or [Leu5]enkephalin. Specificity to the antiserum was established by absorption of the antiserum with synthetic [Met5]- or [Leu5]enkephalin at 1 mM. Positive immunohistochemical staining for enkephalin was observed in the somata of amacrine cells and their processes within the inner plexiform layer. A large number of enkephalin-containing amacrine cells were distributed throughout the retina, and their density appeared to be greatest within central retinal regions. The majority of labeled amacrine cells were about 7.5 micrometers in diameter although, occasionally, amacrine cells were observed that were 12--15 micrometers in diameter. Amacrine cells had a cell-to-cell spacing of approximately 40 micrometers within central retinal regions. Labeled processes of the amacrine cells were observed to project into the inner plexiform layer where they arborized as a fine plexus, within laminae 1, 3--5 of the inner plexiform layer. These observations demonstrate the existence of opioid peptides in seemingly select populations of amacrine cells within the retina. The localization of enkephalin-like immunoreactivity within the retina suggests that opioid peptides play a specific and unique functional role in retinal processing.