Axons of retinal ganglion cells form the optic nerve and innervate areas of the brain important for visual processing, including the lateral geniculate nucleus and the superior colliculus. Development of eye-specific layers in these projection areas are dependant upon retinal waves, which are initially mediated by neuronal nicotinic acetylcholine receptors (nAChRs) (Feller et al., 1996; Penn et al., 1998; Bansal et al., 2000). Unilateral eye-enucleation studies in the rat indicate that nAChRs are on the terminals of optic nerve axons, where they may mediate influences of acetylcholine on visual pathways. Here, we use radioligand binding and immunoprecipitation with subunit-selective antibodies to investigate the subunit composition of nAChRs in the rat optic nerve. We found multiple nAChR subtypes in the optic nerve, all of which contain the beta2 subunit. Most of these receptors are mixed heteromeric subtypes, composed of at least three different subunits. Included among these subtypes is the highest percentage and density of alpha6- and beta3-containing nAChRs found in the CNS.
The rat visual system contains an abundance of nAChRs; however, there have been few studies of nAChRs in the human visual system. We have recently discovered that the human retinoblastoma cell line, WERI-Rb-1, contains a high density of nAChRs. In fact the density of nAChR binding sites in these cells is much greater than in other tumor-derived cell lines commonly used in nAChR research. Using radioligand binding and immunoprecipitation with subunit-selective antibodies, we determined the nAChR subunit composition expressed in WERI-Rb-1 cells. Interestingly, WERI-Rb-1 cells do not exhibit nicotine-induced up-regulation unlike nAChRs found in brain, in tumor-derived cell lines and in heterologously expressed cells. The discovery of nAChRs expressed by WERI-Rb-1 cells adds an important tool to the field and generates a very useful model for human nAChR research. We are just beginning to learn that there are some differences between human and rat nAChR subtypes, thus WERI-Rb-1 cells will be useful in expanding our understanding of these differences. In fact, they have already proven their value by revealing a difference in the affinity values obtained in competition binding assays compared to rat nAChRs.
