Gap junction particle density of horizontal cells in goldfish retinas lesioned with 6‐OHDA
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abstract
AbstractThe I1 dopaminergic interplexiform cells of the fish retina are believed to modulate horizontal cell coupling by increasing gap junction resistance. Dopamine also modulates the morphology of horizontal cell gap junctions and mimics the effects of light adaptation. To determine whether the light‐dependent changes in gap junction morphology are due to endogenous dopamine release, horizontal cell gap junctions were studied in goldfish retinas lacking dopaminergic neurons.Dopaminergic interplexiform cells were destroyed by intraocular injections of 6‐hydroxydopamine in both eyes. After lesioning, fish were treated in one of four ways: (1) light‐adapted, (2) dark‐adapted (1 hour), (3) light‐adapted and given and intraocular injection of dopamine, or (4) dark‐adapted (1 hour) and injected with dopamine. The effectiveness of lesioning was evaluated by autoradiographic detection of [3H]‐dopamine uptake in the retina of one eye. Retinas in which lesioning of the contralateral eye was deemed effective were processed for freeze‐fracture electron microscopy and the particle density of horizontal cell gap junctions determined.Lesioned retinas, whether light‐ or dark‐adapted, had elevated horizontal cell soma gap junction particle densities compared to lesioned retinas treated with dopamine. These results demonstrate that high soma gap junction particle densities can be correlated with the absence of dopamine and low densities associated with the presence of dopamine. The differences in gap junction particle density between lesioned and lesioned + dopamine‐treatment were similar to differences between nonlesioned dark‐adapted (1 hour) and light‐adapted retinas, respectively. Therefore, the particle density of light‐ and dark‐adapted soma gap junctions suggests a greater release of dopamine in light‐adapted fish than in 1 hour dark‐adapted fish.Gap junctions on axon terminals of horizontal cells in lesioned light‐ and dark‐adapted retinas had reduced particle densities compared to nonlesioned light‐adapted but not to nonlesioned dark‐adapted retinas. However, intraocular injections of dopamine into lesioned eyes did not alter axon terminal gap junction particle densities. This suggests that coupling at the horizontal cell soma and axon terminal may be regulated by different mechanisms.
