α3β4 subunit-containing nicotinic receptors dominate function in rat medial habenula neurons

Regional-specific differences in nicotinic acetylcholine receptors (nAChRs) were examined using the whole-cell patch clamp technique in rat medial habenula (MHb) slices. The majority of cells in the ventral two thirds of the MHb responded robustly to local pressure application of nAChR agonists. Mean agonist potency profiles in the middle and ventral thirds of the MHb were similar: cytisine was the most potent agonist and DMPP the weakest, consistent with a significant contribution of the beta 4 subunit to functional nAChRs 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 approximate to 75% of the nicotine-induced currents, as expected for cells solely expressing 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 nicotinic response implying that beta 2 subunits may contribute to some functional receptors. We suggest that the effects of alpha-CTx-MII may arise from interaction with alpha 3 beta 2 beta 4 subunit-containing nAChRs. This idea is supported by the findings (1) that alpha-CTx-MII antagonizes receptors 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 beta 4 receptors but not alpha 3 beta 2 or alpha 3 beta 4 nAChRs, also reduces nicotinic currents in some MHb neurons. Overall these data imply that most functional nAChRs on MHb cells contain at least alpha 3 and beta 4 subunits, and that a variable subpopulation additionally contains the beta 2 subunit. (C) 1999 Elsevier Science Ltd. All rights reserved.