IDENTIFICATION OF 4 CLASSES OF BRAIN NICOTINIC RECEPTORS USING BETA-2MUTANT MICE

Although the expression patterns of the neuronal nicotinic acetylcholi ne receptor (nAChR) subunits thus far described are known, the subunit composition of functional receptors in different brain areas is an on going question. Mice lacking the beta 2 subunit of the nAChR were used for receptor autoradiography studies and patch-clamp recording in thi n brain slices. Four distinct types of nAChRs were identified, expandi ng on an existing classification [Alkondon M, Albuquerque EX (1993) Di versity of nicotinic acetylcholine receptors in rat hippocampal neuron s. I. Pharmacological and functional evidence for distinct structural subtypes. J Pharmacol Exp Ther 265:1455-1473.], and tentatively identi fying the subunit composition of nAChRs in different brain regions. Ty pe 1 nAChRs bind alpha-bungarotoxin, are not altered in beta 2 -/- mic e, and contain the alpha 7 subunit, Type 2 nAChRs contain the beta 2 s ubunit because they are absent in beta 2 -/- mice, bind all nicotinic agonists used with high affinity (excluding alpha-bungarotoxin), have an order of potency for nicotine >> cytisine in electrophysiological e xperiments, and are likely to be composed of alpha 4 beta 2 in most br ain regions, with other alpha subunits contributing in specific areas. Type 3 nAChRs bind epibatidine with high affinity in equilibrium bind ing experiments and show that cytisine is as effective as nicotine in electrophysiological experiments; their distribution and persistence i n beta 2 -/- mice strongly suggest a subunit composition of alpha 3 be ta 4. Type 4 nAChRs bind cytisine and epibatidine with high affinity i n equilibrium binding experiments and persist in beta 2 -/- mice; cyti sine = nicotine in electrophysiological experiments. Type 4 nAChRs als o exhibit faster desensitization than type 3 nAChRs at high doses of n icotine. Knock-out animals lacking individual alpha subunits should al low a further dissection of nAChR subclasses.