Rodent Habenulo–Interpeduncular Pathway Expresses a Large Variety of Uncommon nAChR Subtypes, But Only the α3β4 and α3β3β4 Subtypes Mediate Acetylcholine Release

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Abstract
Recent studies suggest that the neuronal nicotinic receptors (nAChRs) present in the habenulo–interpeduncular (Hb–IPn) system can modulate the reinforcing effect of addictive drugs and the anxiolytic effect of nicotine. Hb and IPn neurons express mRNAs for most nAChR subunits, thus making it difficult to establish the subunit composition of functional receptors. We used immunoprecipitation and immunopurification studies performed in rat and wild-type (+/+) and β2 knock-out (−/−) mice to establish that the Hb and IPn contain significant β2* and β4* populations of nAChR receptors (each of which is heterogeneous). The β4* nAChR are more highly expressed in the IPn. We also identified novel native subtypes (α2β2*, α4β3β2*, α3β3β4*, α6β3β4*). Our studies on IPn synaptosomes obtained from +/+ and α2, α4, α5, α6, α7, β2, β3, and β4−/−mice show that only the α3β4 and α3β3β4 subtypes facilitate acetylcholine (ACh) release. Ligand binding, immunoprecipitation, and Western blotting studies in β3−/−mice showed that, in the IPn of these mice, there is a concomitant reduction of ACh release and α3β4* receptors, whereas the receptor number remains the same in the Hb. We suggest that, in habenular cholinergic neurons, the β3 subunit may be important for transporting the α3β4* subtype from the medial habenula to the IPn. Overall, these studies highlight the presence of a wealth of uncommon nAChR subtypes in the Hb–IPn system and identify α3β4 and α3β3β4, transported from the Hb and highly enriched in the IPn, as the subtypes modulating ACh release in the IPn.

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