Nicotinic receptor subtypes in the developing chick brain: Appearance of a species containing the alpha 4, beta 2, and alpha 5 gene products

Increasing evidence suggests nicotinic receptors regulate developmental events in the nervous system. We used [ super(3)H]epibatidine and super(125)I- alpha -bungarotoxin, together with subunit-specific monoclonal antibodies, to distinguish and quantify nicotinic receptor subtypes in developing chick brain. The results show that more than three fourths of the epibatidine-binding receptors at both early and late embryonic stages contain alpha 4 and beta 2 subunits, representing receptors previously distinguished by high affinity nicotine binding. A fraction of these also contain the alpha 5 gene product, which is consistent with studies on transfected cells showing that the alpha 4, beta 2, and alpha 5 gene products coassemble to produce epibatidine-binding receptors. A small portion of the receptors contain alpha 3 and beta 4 subunits, assembled in part with either alpha 4 or beta 2 subunits. The most abundant nicotinic receptors, however, at both early and late embryonic stages are those having high affinity for alpha -bungarotoxin rather than epibatidine. Most contain alpha 7 subunits, whereas about half contain alpha 8 subunits as well. The sharpest developmental increase between embryonic days 8 and 17/18 occurs with receptors containing alpha 5 subunits, whereas receptors containing alpha 3 or beta 4 subunits undergo no specific increase. The three major receptor species (containing alpha 4 and beta 2 but not alpha 5 subunits; alpha 7 subunits; or alpha 7 and alpha 8 subunits) each increase approximately 3-fold during the same period. The results indicate greater receptor complexity than appreciated previously; they provide information about the rules governing subunit assembly in neuronal nicotinic receptors and draw attention to the role of alpha 5 subunits in late development.