Human neuronal nicotinic acetylcholine receptors (nAChRs) h alpha 2 beta 2, h alpha 2 beta 4, h alpha 3 beta 2, h alpha 3 beta 4, h alpha 4 beta 2, h alpha 4 beta 4 and h alpha 7 were expressed in Xenopus oocytes and tested for their sensitivities to the nicotinic agonists acetylcholine (ACh), nicotine, cytisine (CYT) and 1,1-dimethyl-4-phenylpiperazinium (DMPP) and the nAChR, antagonists mecamylamine (MEC), d-tubocurarine and dihydro-beta-erythroidine. CYT was the least efficacious agonist at hnAChRs containing beta 2 subunits, but it displayed significant activity at h alpha 2 beta 4, h alpha 3 beta 4, h alpha 4 beta 4 and h alpha 7 nAChRs. ACh was one of the most efficacious agonists at ali hnAChRs, except at h alpha 3 beta 2, where DMPP was markedly more efficacious than ACh. ACh was among the least potent agonists at all hnAChRs. The rank order of potency displayed by h alpha 3 beta 2 and h alpha 3 beta 4 nAChRs (DMPP approximate to CYT approximate to nicotine>ACh and DMPP > CYT approximate to nicotine>ACh, respectively), differs from that reported for their rat homologs (Luetje and Patrick, 1991; Covernton et al., 1994). The agonist profile observed in h alpha 7 also differs from that reported for its rat homolog (Seguela et al., 1993). Human alpha 4 beta 2 and h alpha 4 beta 4 nAChRs were more sensitive to dihydro-beta-erythroidine than d-tubocurarine, whereas h alpha 7 and h alpha 3 beta 4 were more sensitive to d-tubocurarine than dihydro-p-erythroidine. These antagonists were equipotent at h alpha 2 beta 2, h alpha 3 beta 2 and h alpha 2 beta 4 nAChRs. MEC (3 mu M) inhibited h alpha 2 beta 4 and h alpha 4 beta 4 nAChRs by > 80%, whereas h alpha 2 beta 2, h alpha 4 beta 2 and h alpha 7 nAChRs were inhibited by approximately 50%. Taken together, the differential sensitivities observed at various recombinant hnAChR subtypes indicate that both alpha and beta subunits contribute to the pharmacology of these ligand-gated channels. The unique selectivity profiles displayed by human nAChRs constitute a valuable tool for the development of selective nicotinic analogs as potential therapeutic drugs.
