Developmental expression of heteromeric nicotinic receptor subtypes in chick retina

Acting through nicotinic acetylcholine receptors (nAChRs), acetylcholine plays an important role in retinal development and the formation of retinal connections to target tissues, but very little is known about the nAChR subtypes expressed in vertebrate retina during neuronal development. We used immunoprecipitation and [3H]epibatidine binding to study the expression of chick retina alpha-bungarotoxin-insensitive heteromeric nAChRs during development and adulthood, and found that it is strictly developmentally regulated, reaching a peak on postnatal day 1. The increase in [3H]epibatidine receptors is caused mainly by an increase in the receptors containing the alpha2, alpha6, beta3, and beta4 subunits. The contribution of beta subunits to [3H]epibatidine receptors significantly changes during development: the beta2 subunit is contained in the majority (84%) of receptors on embryonic day (E) 7 but in only 32% on postnatal day (P) 1, whereas the beta4-containing receptors increase from 22% to 78% during the same period. Using a sequential immunodepletion procedure, we purified the beta2- and beta4-containing subtypes and found that they coassemble with alpha4 and/or alpha3 on E11, and also with the alpha2, alpha6, and beta3 on P1. After the immunodepletion of alpha6-containing receptors, the beta2- and beta4-containing receptors have a very similar pharmacological profile on P1. Parallel immunoprecipitation experiments in other brain areas showed that the developmentally regulated receptors in optic lobe are those containing the alpha2, alpha5, and beta2 subunits and those containing the alpha4 and beta2 subunits, whereas the receptors in forebrain-cerebellum contain the alpha4 and beta2 subunits with or without the alpha5 subunit. These results indicate that there is an increase in receptor heterogeneity and complexity in chick retina during development that is also maintained in adulthood.