β3 subunit is present in different nicotinic receptor subtypes in chick retina

Abstract

Although the neuronal nicotinic β3 subunit was cloned several years ago, it has only recently been shown to form heteromeric channels when associated with other nicotinic subunits, and very little information is available concerning its assembly in the native nicotinic receptors of the nervous system. Using subunit-specific antibodies and immunoprecipitation experiments, we have identified the retina as being the chick central nervous system (CNS) area that expresses the highest level of the β3 subunit. Sequential immunopurification experiments showed that there are at least two populations of β3-containing receptors in chick retina: in one, the β3 subunit is associated with the α6 and β4 subunits; in the other more heterogeneous population, the β3 subunit is associated with the α2, α3, α4, β2 and β4 subunits. Both of these receptor populations bind [ 3H]epibatidine and a number of nicotinic receptor agonists with high affinity (nM) and nicotinic receptor antagonists with a lower affinity (μM). The greatest pharmacological difference between the two populations is the affinity for the α-conotoxin MII, which inhibits binding to α6-containing receptors and not that to β3-containing receptors. We also searched for the presence of the β3 subunit associated with the α-bungarotoxin binding subunits α7 and/or α8 in retina and chick brain. Immunoprecipitation studies using anti-β3 antibodies did not detect any specific α-bungarotoxin labeled receptors, thus, indicating that the β3 subunit is not present in the α-bungarotoxin receptors of these areas.