Nicotinic α5 subunit deletion locally reduces high‐affinity agonist activation without altering nicotinic receptor numbers

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Abstract
Neuronal nicotinic acetylcholine receptor subunit α5 mRNA is widely expressed in the CNS. An α5 gene polymorphism has been implicated in behavioral differences between mouse strains, and α5‐null mutation induces profound changes in mouse acute responses to nicotine. In this study, we have examined the distribution and prevalence of α5* nicotinic acetylcholine receptor in mouse brain, and quantified the effects of α5‐null mutation on pre‐synaptic nicotinic acetylcholine receptor function (measured using synaptosomal86Rb+efflux) and overall [125I]epibatidine binding site expression. α5* nicotinic acetylcholine receptor expression was found in nine of fifteen regions examined, although < 20% of the total nicotinic acetylcholine receptor population in any region contained α5. Deletion of the α5 subunit gene resulted in localized loss of function (thalamus, striatum), which was itself confined to the DHβE‐sensitive receptor population. No changes in receptor expression were seen. Consequently, functional changes must occur as a result of altered function per unit of receptor. The selective depletion of high agonist activation affinity sites results in overall nicotinic function being reduced, and increases the overall agonist activation affinity. Together, these results describe the receptor‐level changes underlying altered behavioral responses to nicotine in nicotinic acetylcholine receptor α5 subunit‐null mutants.