Catecholamine‐ and indoleamine‐ containing neurons in the turtle retina

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Abstract
We identified a population of presumed dopaminergic amacrine cells and populations of presumed serotonergic bipolar and amacrine cells in the retina of the turtle Pseudemys scripta elegans by a combination of autoradiographic, fluorescence, and immunocytochemical techniques. Antisera directed against the dopamine‐synthesizing enzyme, tyrosine hydroxylase (TOH), stained perikarya located at the border of inner nuclear (INL) and inner plexiform (IPL) layers. Processes emitted by these cells arborized in sublaminae 1, 3, and 5 of the IPL. Incubation of retinas in 10−6 M 3H‐dopamine yielded a labeling pattern identical to the staining pattern achieved with TOH antisera, but when the concentration of 3H‐dopamine was increased 25‐fold, both amacrine and bipolar cells are labeled. Following intraocular injection of dopamine, fluorescence micrography revealed both stained amacrine and bipolar cells. The bipolar cells had Landolt's clubs, pyriform perikarya located in the distal portion of the INL, and axons that coursed horizontally in the INL, then entered the IPL, and ramified in both its superficial and deeper layers. Although no fluorescent neuronal profiles were revealed following injection of serotonin (5HT), bipolar cells identical to those described were visualized with 5HT antisera. The intensity of bipolar cell staining with 5HT antisera was improved by preinjection of the eye with exogenous 5HT. We suggest that the bipolar cell is serotonergic, but that it also can actively accumulate dopamine. The 5HT antisera also stained a population of large amacrine cells whose processes ramified in IPL sublaminae 1, 4, and 5. The same populations of presumed serotonergic bipolar and amacrine cells were labeled following incubation of the eyecup in 10−6 M 3H‐5HT.