DIFFERENTIAL REINNERVATION OF RETINAL BIPOLAR CELL DENDRITES AND AXONTERMINALS BY DOPAMINE INTERPLEXIFORM CELLS FOLLOWING DOPAMINE DEPLETION WITH 6-OHDA

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 bipo lar cells (ON BC) by dopaminergic interplexiform cells (DA-IPCs) norma lly and during the period of neogeneration of new DA-IPCs at the margi nal zone following DA-IPC destruction. DA-IPCs were destroyed via intr aocular injection of B-hydroxydopamine over 2 successive days. Control s and 1 year post-injection retinas were double labeled for protein ki nase C and tyrosine hydroxylase (TH) immunocytochemistry to identify t he ON-BCs and the DA-IPCs, respectively. Double-labeled 25 mu m tissue sections were examined on a confocal laser scanning microscope by usi ng dual channel immunofluorescence acquisition. Image stacks were anal yzed for DA-IPC/ON-BC contacts in the distal inner nuclear layer (INL) and inner plexiform layer (IPL). Image stacks were rotated 180 degree s with respect to each other and reanalyzed to determine potential ran domness of the contacts. For control retinas there mere 1.8 contacts/a xon 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-immunore active boutons in the retina recovered to 16% of control in the IPL bu t only 10% in the distal INL. Establishment of DA-IPC/ON-BC contacts r ecovered to 36% of control for ON-BC axon terminals (n = 103), whereas there was no recovery of contacts in the distal INL (n = 30). Reinner vation 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 followi ng retinal dopamine depletion. Thus, dopamine input to ON-BCs in the o uter retina maybe involved in setting background sensitivity under pho topic conditions. (C) 1997 Wiley-Liss, inc.