(1) Atropine, a classical muscarinic antagonist, has been reported previously to inhibit neuronal nicotinic acetylcholine receptors (nAChRs). In the present study, the action of atropine has been examined on alpha3beta4 receptors expressed heterologously in Xenopus oocytes and native nAChRs in medial habenula neurons. (2) At concentrations of atropine often used to inhibit muscarinic receptors (1 micro M), responses induced by near-maximal nicotine concentrations (100 micro M) at negative holding potentials (-65 mV) are inhibited (14-30%) in a reversible manner in both alpha4 and alpha3 subunit-containing heteromeric nAChRs. Half-maximal effective concentrations (IC(50) values) for atropine inhibition are similar for the four classes of heteromeric receptors studied (4-13 micro M). (3) For alpha3beta4 nAChRs in oocytes, inhibition by atropine (10 micro M) is not overcome at higher concentrations of agonist, and is increased with membrane hyperpolarization. These results are consistent with non-competitive antagonism--possibly ion channel block. (4) At low concentrations of both nicotine (10 micro M) and atropine (<10 micro M), potentiation ( approximately 25%) of alpha3beta4 nAChR responses in oocytes is observed. The relative balance between potentiation and inhibition is dependent upon membrane potential. (5) In rat medial habenula (MHb) neurons, atropine (0.3-3.0 micro M) inhibited nicotine-induced responses in both a concentration and membrane potential-dependent manner (at -40 mV, IC(50)=4 micro M), similar to the effects on alpha3beta4-nAChRs in oocytes. However, unlike heterologously expressed receptors, potentiation was barely detectable at depolarized membrane potentials using low concentrations of nicotine (3-10 micro M). Conversely, the weak agonist, choline (1-3 mM) was observed to augment responses of MHb nAChRs.