Abstract
Depletion of retinal dopamine in goldfish increases light sensitivity at photopic backgrounds. As horizontal cells appear not to be involved with this effect (Yazulla and Studholme [1995] Vis. Neurosci. 12:827-837), we investigated the innervation patterns of the ON rod/cone bipolar cells (ON-BC) by dopaminergic interplexiform cells (DA-IPCs) normally and during the period of neogeneration of new DA-IPCs at the marginal zone following DA-IPC destruction. DA-IPCs were destroyed via intraocular injection of 6-hydroxydopamine over 2 successive days. Controls and 1 year post-injection retinas were double labeled for protein kinase C and tyrosine hydroxylase (TH) immunocytochemistry to identify the ON-BCs and the DA-IPCs, respectively. Double-labeled 25 microns tissue sections were examined on a confocal laser scanning microscope by using dual channel immunofluorescence acquisition. Image stacks were analyzed for DA-IPC/ON-BC contacts in the distal inner nuclear layer (INL) and inner plexiform layer (IPL). Image stacks were rotated 180 degrees with respect to each other and reanalyzed to determine potential randomness of the contacts. For control retinas there were 1.8 contacts/axon terminal in the IPL (n = 165) and 9.4 contacts/ON-BC in the distal INL (n = 28). At 1 year after injection, reinnervation of TH-immunoreactive boutons in the retina recovered to 16% of control in the IPL but only 10% in the distal INL. Establishment of DA-IPC/ON-BC contacts recovered to 36% of control for ON-BC axon terminals (n = 103), whereas there was no recovery of contacts in the distal INL (n = 30). Reinnervation of ON-BC by DA-IPCs preferentially targets the axon terminals. The absence of reinnervation of bipolar cell dendrites by DA-IPCs may account for the persistence of the increased light sensitivity following retinal dopamine depletion. Thus, dopamine input to ON-BCs in the outer retina maybe involved in setting background sensitivity under photopic conditions.
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