Nicotinic acetylcholine receptor gene expression in the goldfish retina and visual pathway.

Abstract

Nicotinic acetylcholine receptors (nAChRs) are expressed in the vertebrate retina. The numbers of different types of receptors, their cellular distribution, and their function within the retina, is not known. We have begun addressing these issues by characterizing cDNA clones encoding neural nAChRs from a goldfish retinal cDNA library. We have identified and characterized three cDNAs corresponding to three different subunits of neural nAChRs expressed in retina. Two of these cDNAs encode novel members of the neural nAChR gene family. These novel polypeptides bear an 88% amino acid sequence similarity to each other, both lacking the two cysteines characteristic of ligand-binding subunits of nAChRs. For this reason these clones have been designated Goldfish non-alpha (GFn$\alpha$) subunits. Since the rat $\beta$-2 cDNA (Deneris et al., Neuron 1: 45-54) represents the first vertebrate neural non-alpha nAChR subunit, we have designated these two novel cDNAs GFn$\alpha$-2 and GFn$\alpha$-3. GFn$\alpha$-2 and GFn$\alpha$-3 have 82% and 85% amino acid sequence similarity to the rat $\beta$-3 polypeptide published in 1989 (Deneris et al., J. Biol. Chem., 264: 6268-6272); $\beta$-3 may represent the rat homolog of either one of these genes. The third clone encodes a protein with 89% amino acid sequence similarity to the rat $\alpha$-3 subunit (Boulter et al., Nature 319: 368-374, 1986), and likely represents the goldfish homolog of the rat $\alpha$-3 cDNA. In situ hybridization shows that the GFn$\alpha$-2 gene is expressed predominantly by ganglion cells of the retina (Cauley et al., J. Cell Biol. 108: 637-645, 1989). GF$\alpha$-3 shows a similar pattern of expression in the retina. GF$\alpha$-3 is expressed by retinal ganglion cells, but also by cells of the retina's inner nuclear layer. In tectum, which receives synaptic input from retinal ganglion cells, GFn$\alpha$-2 gene expression is not detectable. GFn$\alpha$-3 and GF$\alpha$-3 are expressed by cells of the deepest layers of the tectum, the periventricular zone (PVZ), and also by cells of more superficial tectal layers. These data indicate that multiple types of neural nAChRs are expressed in retina and tectum.