Chick optic lobe contains a developmentally regulated α2α5β2 nicotinic receptor subtype

The most widely expressed neuronal nicotinic acetylcholine receptor subtype in chick brain is that containing the alpha 4 and beta 2 subunits. However, immunoprecipitation and localization studies have shown that some brain areas also contain the alpha 2 and/or alpha 5 subunits, whose role in the definition of receptor properties is still intriguing. Using subunit-specific polyclonal antibodies, we found that the optic lobe is the chick central nervous system region that expresses the highest level of alpha 2-containing receptors. Immunoprecipitation studies of these immunopurified alpha 2-containing receptors labeled with the nicotinic agonist [H-3]epibatidine showed that almost all of them contained the beta 2 subunit and that more than 66% contained the alpha 5 subunit. Western blot analyses of the purified receptors confirmed the presence of the alpha 2, alpha 5, and beta 2 subunits and the absence of the alpha 3, alpha 4, alpha 6, alpha 7, alpha 8, beta 3, and beta 4 subunits. The alpha 2-containing receptors are developmentally regulated: their expression increases 25 times from embryonic day 7 to post-hatching day 1 in the optic lobe, compared with an increase of only 5-fold in the forebrain. The alpha 2-containing optic lobe receptors bind [H-3]epibatidine (K-d = 29 pM) and a number of other nicotinic agonists with very high affinity and have a pharmacological profile very similar to that of the alpha 4 beta 2 subtype. They form functional cationic channels when reconstituted in lipid bilayers, with pharmacological and biophysical properties different from those of the alpha 4 beta 2 subtype. These channels are activated by nicotinic agonists in a dose-dependent manner and are blocked by the nicotinic antagonist d-tubocurarine.