alpha 3 beta 4 subunit-containing nicotinic receptors dominate function inrat medial habenula neurons

Regional-specific differences in nicotinic acetylcholine receptors (nAChRs) were examined using the whole-cell patch clamp technique in rat medial hab enula (MHb) slices. The majority of cells in the ventral two thirds of the MHb responded robustly to local pressure application of nAChR agonists. Mea n agonist potency profiles in the middle and ventral thirds of the MHb were similar: cytisine was the most potent agonist and DMPP the weakest, consis tent with a significant contribution of the beta 4 subunit to functional nA ChRs in all areas of the MHb. In acutely isolated MHb neurons, the alpha 3 beta 4-selective toxin alpha-CTx-AuIB (1 mu M) reversibly blocked approxima te to 75% of the nicotine-induced currents, as expected for cells solely ex pressing alpha 3 beta 4 nAChRs. However, the alpha 3 beta 2-selective toxin , alpha-CTx-MII (100 nM), blocked a variable fraction (0-90%) of the MHb ni cotinic response implying that beta 2 subunits may contribute to some funct ional receptors. We suggest that the effects of alpha-CTx-MII may arise fro m interaction with alpha 3 beta 2 beta 4 subunit-containing nAChRs. This id ea is supported by the findings (1) that alpha-CTx-MII antagonizes receptor s comprised of alpha 3, beta 2 and beta 4 subunits in Xenopus oocytes, and (2) that a mutant alpha-CTx-MII toxin[H12A], which blocks alpha 3 beta 2 be ta 4 receptors but not alpha 3 beta 2 or alpha 3 beta 4 nAChRs, also reduce s nicotinic currents in some MHb neurons. Overall these data imply that mos t functional nAChRs on MHb cells contain at least alpha 3 and beta 4 subuni ts, and that a variable subpopulation additionally contains the beta 2 subu nit. (C) 1999 Elsevier Science Ltd. All rights reserved.