The presence of interplexiform cells in primate retina has been disputed, with the
dopaminergic interplexiform cell in the New World monkey being the most fully understood.
We have examined interplexiform cells in the Old World monkey using immunocytochemistry
with the peroxidase-antiperoxidase method of visualization. In several species of
macaque retina, two types of interplexiform cells are found. One stains with antisera
to tyrosine hydroxylase, a biosynthetic enzyme for dopamine, and the other stains
with antisera to gamma aminobutyric acid (GABA). The cell bodies of these two populations
of interplexiform cells are located among the amacrine cells in the inner nuclear
layer, and they send processes into both the inner and outer plexiform layers. GABA-positive
interplexiform processes to the outer plexiform layer arise from the cell body while
tyrosine hydroxylase-positive interplexiform processes most often originate from the
heavily tyrosine hydroxylase-stained sublamina one of the inner plexiform layer. Cell-body
diameter measurements and morphology suggest that these are different neuronal populations.