Morphology and synaptic connectivity of nitric oxide synthase-immunoreactive neurons in the guinea pig retina

Immunocytochemical methods with an antiserum against neuronal nit-ic oxide synthase (NOS) were applied to identify the morphology and synaptic connect ivity of NOS-like immunoreactive neurons in the guinea pig retina. In the p resent study, two types of amacrine cells were labeled with anti-NOS antise ra. Type I cells had large somata located in the inner nuclear layer (INL) with long, sparsely branched processes ramifying mainly in stratum 3 of the inner plexiform layer (IPL). The somata of type 2 cells (smaller diameters ) were located in the INL. Some displaced amacrine cells in the ganglion ce ll layer were labeled. The soma size of the displaced amacrine cells was si milar to that of the type 2 amacrine cells. However, processes originating from type 2 amacrine cells and displaced amacrine cells stratified mainly i n strata 1 and 5, respectively. Some cone bipolar cells were weakly NOS-imm unoreactive. The synaptic connectivity of NOS-like immunoreactive amacrine cells was identified in the IPL by electron microscopy. NOS-labeled amacrin e cell processes received synaptic input from other amacrine cell processes and bipolar cell axon terminals in all strata of the IPL. The most frequen t postsynaptic targets of NOS-immunoreactive amacrine cells were other amac rine cell processes. Cone bipolar cells were postsynaptic to NOS-labeled am acrine cells in all strata of the IPL. Labeled amacrine cells synapsing ont o ganglion cells were found only in sublamina b. A few synaptic contacts we re observed between labeled cell processes. In the outer plexiform layer, d endrites of labeled bipolar cells made basal contact with cone pedicles or formed a synaptic triad opposed to a synaptic ribbon of cone pedicles.