Deletion of the α7, β2, or β4 Nicotinic Receptor Subunit Genes Identifies Highly Expressed Subtypes with Relatively Low Affinity for [3H]Epibatidine

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Abstract
Diversity of neuronal nicotinic acetylcholine receptor binding was measured using [3H]epibatidine after deletion of α7, β2, or β4 subunits. [3H]Epibatidine binding is distinctly biphasic. Densities of higher (Kd ≈ 0.02 nM) and lower (Kd ≈ 5 nM) affinity sites in whole brains of wild-type mice are very similar. Relative sensitivity to inhibition by cytisine or α-bungarotoxin was used to evaluate pharmacological subsets of the higher- and loweraffinity sites, respectively. Deletion of each subunit had distinct effects on the binding sites. Deletion of α7 did not affect higher-affinity sites but reduced the numbers of lower-affinity sites. This reduction was confined to the [3H]epibatidine binding sites sensitive to inhibition by α-bungarotoxin. Deletion of the β2 subunit had the largest effect. Higher-affinity sites sensitive to inhibition by cytisine were eliminated, and cytisine-resistant sites were reduced. Deletion of the β2 subunit also significantly reduced the number of lower-affinity sites insensitive to α-bungarotoxin. β4 Gene deletion partially reduced cytisine-resistant and α-bungarotoxin-resistant sites with lower and higher affinity for [3H]epibatidine, respectively. Gene deletion in four brain regions (thalamus, hippocampus, superior colliculus, and inferior colliculus) elicited changes generally similar to whole brain. However, relative expression of the binding sites differed among the regions. [3H]Cytisine and 125I-α-bungarotoxin binding sites were eliminated by β2 and α7 gene deletion, respectively. These studies establish that the lower-affinity sites represent a structurally diverse set of sites that require expression of either α7, β2, or β4 subunits and extend and confirm previous classifications of the higher-affinity [3H]epibatidine binding sites.

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