Abstract
Purpose: :
To identify the melanopsin ganglion cells of the rabbit retina and determine their dye coupling properties.
Methods: :
Currently available melanopsin antibodies failed to label rabbit cells. Therefore, we sequenced rabbit melanopsin by PCR and found that the peptides sequences used to raise antibodies were hypervariable (8 changes in 17 amino acids). We raised a rabbit melanopsin antibody in mice, which labeled ganglion cells with the morphological characteristics of melanopsin ganglion cells, thus confirming the specificity of the antibody.
Results: :
The melanopsin labeled ganglion cells consisted of two subpopulations; one conventionally placed in the ganglion cell layer and one displaced ganglion cell type located in the inner nuclear layer. Both dendritic trees were sparsely branched and stratified both in stratum 1, distal to the cholinergic band in sublamina a, and at the border between stratum 4 and 5, proximal to the cholinergic band in sublamina b. Dendrites could be traced from one sublamina to the other. These morphological characteristics are similar to previous reports of melanopsin ganglion cells in other mammalian species. We stained every melanopsin ganglion cell in one wholemount rabbit retina, and mapped a total of 1170 cells; 726 conventional ganglion cells and 444 displaced ganglion cells. They were distributed along a central to peripheral density gradient. Interestingly, tracing a dendrite from one soma often led directly to another labeled soma, suggesting dendro-dendritic gap junctions in the network of melanopsin ganglion cells. Finally, we were able to label melanopsin ganglion cells in live retina, because the antibody was raised against an extracellular epitope. When selected melanopsin ganglion cells were filled with Neurobiotin, several other melanopsin ganglion cells and many amacrine cells were also stained.
Conclusions: :
We have identified the melanopsin ganglion cells of the rabbit retina. They are bistratified with a sparsely branched pattern and they account for less that 1% of rabbit ganglion cells. The presence of dye coupling indicates that melanopsin ganglion cells are coupled to other melanopsin ganglion cells as well as a population of amacrine cells. This network may support synchronized firing among melanopsin ganglion cells.
Keywords: ganglion cells • retina: proximal (bipolar, amacrine, and ganglion cells) • retina