Rodent habenulo-interpeduncular pathway expresses a large variety of uncommon nAChR subtypes, but only the {alpha}3{beta}4 and {alpha}3{beta}3{beta}4 subtypes mediate acetylcholine release

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 wildtype (+/+) and beta2 knockout (-/-) mice to establish that the Hb and IPn contain significant beta2* and beta4* populations of nAChR receptors (each of which is heterogeneous). The beta4* nAChR are more highly expressed in the IPn. We also identified novel native subtypes (alpha2beta2*, alpha4beta3beta2* alpha3beta3beta4*, alpha6beta3beta4*). Our studies on IPn synaptosomes obtained from +/+ and alpha2 alpha4, alpha5, alpha6, alpha7, beta2, beta3 and beta4-/- mice, show that only the alpha3beta4 and alpha3beta3beta4 subtypes facilitate acetylcholine (ACh) release. Ligand binding, immunoprecipitation and Western blotting studies in beta3-/- mice showed that in the IPn of these mice there is a concomitant reduction of ACh release and alpha3beta4* receptors, while the receptor number remains the same in the Hb. We suggest that in habenular cholinergic neurons the beta3 subunit may be important for transporting the alpha3beta4* subtype from the medial habenula (MHb) to the IPn. Overall, these studies highlight the presence of a wealth of uncommon nAChR subtypes in the Hb-IPn system and identify alpha3beta4 and alpha3beta3beta4, transported from the Hb and highly enriched in the IPn, as the subtypes modulating ACh release in the IPn.