Nicotinic Receptor Subtypes in the Developing Chick Brain: Appearance of a Species Containing the α4, β2, and α5 Gene Products

Abstract

Increasing evidence suggests nicotinic receptors regulate developmental events in the nervous system. We used [³H]epibatidine and¹²⁵I-α-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 α4 and β2 subunits, representing receptors previously distinguished by high affinity nicotine binding. A fraction of these also contain the α5 gene product, which is consistent with studies on transfected cells showing that the α4, β2, and α5 gene products coassemble to produce epibatidine-binding receptors. A small portion of the receptors contain α3 and β4 subunits, assembled in part with either α4 or β2 subunits. The most abundant nicotinic receptors, however, at both early and late embryonic stages are those having high affinity for α-bungarotoxin rather than epibatidine. Most contain α7 subunits, whereas about half contain α8 subunits as well. The sharpest developmental increase between embryonic days 8 and 17/18 occurs with receptors containing α5 subunits, whereas receptors containing α3 or β4 subunits undergo no specific increase. The three major receptor species (containing α4 and β2 but not α5 subunits; α7 subunits; or α7 and α8 subunits) each increase ≈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 α5 subunits in late development.