Chick Optic Lobe Contains a Developmentally Regulated α2α5β2 Nicotinic Receptor Subtype

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Abstract
The most widely expressed neuronal nicotinic acetylcholine receptor subtype in chick brain is that containing the α4 and β2 subunits. However, immunoprecipitation and localization studies have shown that some brain areas also contain the α2 and/or α5 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 α2-containing receptors. Immunoprecipitation studies of these immunopurified α2-containing receptors labeled with the nicotinic agonist [3H]epibatidine showed that almost all of them contained the β2 subunit and that more than 66% contained the α5 subunit. Western blot analyses of the purified receptors confirmed the presence of the α2, α5, and β2 subunits and the absence of the α3, α4, α6, α7, α8, β3, and β4 subunits. The α2-containing receptors are developmentally regulated: their expression increases 25 times from embryonic day 7 to posthatching day 1 in the optic lobe, compared with an increase of only 5-fold in the forebrain. The α2-containing optic lobe receptors bind [3H]epibatidine (Kd = 29 pM) and a number of other nicotinic agonists with very high affinity and have a pharmacological profile very similar to that of the α4β2 subtype. They form functional cationic channels when reconstituted in lipid bilayers, with pharmacological and biophysical properties different from those of the α4β2 subtype. These channels are activated by nicotinic agonists in a dose-dependent manner and are blocked by the nicotinic antagonist d-tubocurarine.