S1/S2 Amacrine Cells Are Coupled via Connexin 45 in the Rabbit Retina

Purpose: Neuronal coupling via gap junctions is abundant in major cell types including amacrine cells in the retina. Previously, we and other groups have shown that connexin 36 is localized on AII amacrine cells in the mammalian retina and plays a critical role in the rod pathway (Feigenspan et al., 2001; Mills et al., 2001; Guldenagel et al., 2001; Deans et al., 2002). S1/S2 amacrine cells are stratified at the same level as AII amacrine cells, yet they do not use connexin 36 (Mills et al., 2001, Zhang et al., 2002). Therefore, we examined the distribution of connexin 45 in the rabbit retina and tested whether it is expressed in S1/S2 amacrine cells.

Methods: Rabbit retinas were dissected and incubated in Ames solution containing serotonin. After fixation, the retinas were labeled with antibodies against connexin 45, connexin 36, serotonin, protein kinase C (PKC), calretinin or calbindin for confocal analysis.

Results: In vertical sections of the rabbit retina, connexin 45 immunoreactivitiy was found as small plaques in the inner plexiform layer (IPL). Labeled plaques were distributed over the IPL, but concentrated in stratum 5. In wholemounts, numerous connexin 45–labeled plaques were found close to AII dendrites in sublamina b of the IPL but connexin 45 did not colocalize with connexin 36. Instead, in stratum 5 of the IPL, connexin 45 plaques were clearly seen in the dense matrix of the S1/S2 dendrites surrounding rod bipolar terminals. In addition, some connexin 45 plaques occurred at the dendritic crossings of S1/S2 dendrites in stratum 1 of the IPL.

Conclusions: These results demonstrate that S1/S2 amacrine cells, which provide negative feedback at rod bipolar terminals, are coupled via connexin 45 in the rabbit retina. This is a good example that various retinal neurons use different neuronal connexins.