Light- and electron-microscopic analysis of neuropeptide Y-immunoreactive amacrine cells in the guinea pig retina

We investigated the morphology and synaptic connections of neuropeptide Y ( NPY)-containing neurons in the guinea pig retina by immunocytochemistry, us ing antisera against NPY. Specific NPY immunoreactivity was localized to a population of wide-field and regularly spaced amacrine cells with processes ramifying mainly in stratum I of the inner plexiform layer (IPL). Double-l abel immunohistochemistry demonstrated that all NPY-immunoreactive cells po ssessed glutamic acid decarboxylase 65 immunoreactivity. The synaptic conne ctivity of NPY-immunoreactive amacrine cells was identified in the IPL by e lectron microscopy. The NPY-labeled amacrine cell processes received synapt ic input from other amacrine cell processes and bipolar cell axon terminals in stratum I of the IPL. The most frequent postsynaptic targets of NPY-imm unoreactive amacrine cells were other amacrine cell processes. Synaptic out puts to bipolar cells were also observed in a small number of cases. This f inding suggests that NPY-containing amacrine cells may influence inner reti nal circuitry in stratum I of the IPL, thus mediating visual processing.