Abstract
Purpose: :
To identify the bipolar cell type(s) providing input to inner stratifying melanopsin-expressing ganglion cells (inner melanopsin cells).
Methods: :
Whole retinas or cryostat sections of marmoset and macaque retinas were double or triple labelled using antibodies to melanopsin (gifts from Drs. J. Hannibal and K.-W. Yau), CD15 (to identify diffuse bipolar DB6) cells, antibodies against synaptic proteins C-terminal binding protein (CtBP2) and piccolo (gifts from Drs Gundelfinger and Altrock), and antibodies against the glutamate receptor subunits (GluR3 and GluR4) in combination with fluorescent secondary antibodies. The preparations were analysed using deconvolution and confocal microscopy.
Results: :
Triple labelling in 40 samples from vertical sections showed that over a total dendritic length of about 400 µm, inner melanopsin dendrites had 148 co-localizations with DB6 axon terminals and 92 co-localizations with synaptic markers. A total of 26 (18%) of the co-localized areas between DB6 terminals and melanopsin dendrites were also co-localized with a synaptic marker. This number is equivalent to about 6 synapses per 100 µm dendritic length. For one inner melanopsin cell all 276 DB6 axon terminals in the area directly above its dendritic tree were drawn. Only about half of these axon terminals overlapped with the dendritic branches, but nearly all of these axon terminals had regions that were colocalized with the dendritic branches. In total 275 potential contact areas between DB6 cells and this inner cell were determined.
Conclusions: :
These results support the idea that DB6 cells provide input to inner melanopsin cells. However, the majority of synaptic markers on inner melanopsin cells were not colocalized with DB6 terminals indicating that other bipolar cell types also contribute.
Keywords: retina: proximal (bipolar, amacrine, and ganglion cells) • color vision • immunohistochemistry