Nicotinic Acetylcholine Receptor Subtypes Expression during Rat Retina Development and Their Regulation by Visual Experience

Abstract

By acting through retinal nicotinic acetylcholine receptors (nAChRs), acetylcholine plays an important role in the development of both the retina and central visual pathways. Ligand binding and immunoprecipitation studies with subunit-specific antibodies showed that the expression of ␣Bungarotoxin (␣Bgtx) and high-affinity epibatidine (Epi) receptors is regulated developmentally and increases until postnatal day 21 (P21). The increase in Epi receptors is caused by a selective increase in the subtypes containing the ␣2, ␣4, ␣6, ␤2, and ␤3 subunits. Immunopurification studies revealed three major populations of Epi receptors on P21: ␣6* receptors (26%), which contain the ␣6␤3␤2, ␣6␣4␤3␤2, and ␣6␣3/␣2␤3␤2 subtypes; ␣4(non-␣6)* receptors (60%), which contain the ␣2␣4␤2 and ␣4␤2 subtypes; and (non-␣4/non-␣6)* receptors (14%), which contain the ␣2␤2/␤4 and ␣3␤2/␤4 subtypes. These three populations can be pharmacologically discriminated using ␣conotoxin MII, which binds the ␣6* population with high affinity. In situ hybridization showed that the transcripts for all of the subunits are heterogeneously distributed throughout retinal neurons at P21, with ␣3, ␣6, and ␤3 transcripts preferentially concentrated in the ganglion cell layer, ␣5 in the inner nuclear layer, and ␣4 and ␤2 distributed rather homogeneously. To investigate whether nAChR expression is affected by visual experience, we also studied dark-reared P21 rats. Visual deprivation had no effect on the expression of ␣Bgtx receptors or the developmentally regulated Epi receptors containing the ␣2, ␣6, and/or ␤3 subunits but significantly increased the expression of the Epi receptors containing the ␣4 and ␤2 subunits. Overall, this study demonstrates that the retina is the rat neural region that expresses the widest array of nAChR subtypes. These receptors have a specific distribution, and their expression is finely regulated during development and by visual experience.The nicotinic acetylcholine receptors (nAChRs) in vertebrate retina play a role in signaling at the earliest stages of development (long before there is any evidence of synaptic transmission) and also later during neuronal growth and synaptogenesis (Zhou, 2001;Feller, 2002). Neuronal nAChRs are cationic channels whose opening is physiologically controlled by acetylcholine (ACh) neurotransmitter. They form a heterogeneous family of pentameric oligomers made up of combinations of subunits encoded by at least 12 different genes. Although there are many subtypes consisting of different subunits, depending on their phylogenetic, functional, and pharmacological properties (Gotti et al., 1997a;Corringer et al., 2000;Hogg et al., 2003), two main classes have been identified: the ␣Bungarotoxin (␣Bgtx)-sensitive receptors made up of the ␣7, ␣8, ␣9, and/or ␣10 subunits, which can form homomeric or heteromeric receptors; and the ␣Bgtx-insensitive receptors consisting of the ␣2 to ␣6 and ␤2 to ␤4 subunits, which only form heteromeric receptors that bind epibatidine (Epi) with a high affinity. The number ...

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