Synaptic analysis of amacrine cells in the turtle retina which contain tyrosine hydroxylase-like immunoreactivity

This study examined amacrine cells in the retina of the turtle Pseudemys scripta elegans, which were labelled using an antiserum directed against tyrosine hydroxylase (an enzyme participating in catecholamine synthesis). These cells were investigated using both light and electron microscopy. Labelled somata were located in the inner nuclear layer near the border of the inner plexiform layer. The dendritic arborizations of these neurons were tristratified and arborized in strata 1 and 3 and near the border between strata 4 and 5. Serial tangential sections taken through the entire inner plexiform layer of a 1 mm-2 region in mid-peripheral retina were examined. All of the synapses associated with labelled profiles were counted and classified. The majority (84%) of the synapses involving labelled processes represented output, while the remaining 16% represented synaptic input. The synaptic output of the labelled processes was as follows: 87% onto unlabelled amacrine cells, 4% onto ganglion cells, 9% onto unidentified cell processes. None of the synaptic output from labelled processes was onto bipolar cells. The synaptic input to these labelled cells was from bipolar cells (29%) and from unlabelled amacrine cells (71%). A well labelled amacrine cell was serially sectioned and examined at the ultrastructural level to analyze its synaptic connectivity. Immunoreaction product was located diffusely throughout the cytoplasm and in large vesicles. The synaptic organization of the cell was directed primarily toward output. The labelled processes were postsynaptic and presynaptic to unlabelled amacrine cell processes in strata 1 and 3 and at the border between strata 4 and 5. Synaptic input from bipolar cells was seen exclusively near the border between strata 4 and 5. Labelled processes were presynaptic to ganglion cell processes in stratum 1 and at the border between strata 4 and 5, but not in stratum 3. Quantitative studies suggested that amacrine cell inputs and outputs were evenly distributed across the dendritic arborization, while bipolar cell inputs and outputs to ganglion cells were concentrated on the distal parts of the dendritic arborization. No labelled processes were seen in the outer plexiform layer, indicating that the cells with tyrosine hydroxylase-like immunoreactivity in the turtle retina were true amacrine cells and not interplexiform cells.