Agonists binding nicotinic receptors elicit specific channel-opening patterns at ?? and ?? sites.

'Embryonic' muscle-type nicotinic acetylcholine receptor channels (nAChRs) bind ligands at interfaces of ?- and ?- or ?-subunits. ?? and ?? sites differ in affinity, but their contributions to opening the channel have remained elusive. We compared high-resolution patch clamp currents evoked by epibatidine (Ebd), carbamylcholine (CCh) and acetylcholine (ACh). Ebd binds with 75-fold higher affinity at ?? than at ?? sites, whereas CCh and ACh prefer ?? sites. Similar short (?O1), intermediate (?O2) and long (?O3) types of opening were observed with all three agonists. ?O2 openings were maximally prevalent at low Ebd concentrations, binding at ?? sites. By contrast, ?O1 openings appear to be generated at ?? sites. In addition, two types of burst appeared: short bursts of an average of 0.75 ms (?B1) that should arise from the ?? site, and long bursts of 12-25 ms (?B2) in duration arising from double liganded receptors. Limited by the temporal resolution, the closings within bursts were invariant at 3 ?s. Corrected for missed closings, in the case of ACh the openings within long bursts lasted 170 ?s and those in short bursts about 30 ?s. Blocking ?? sites with ?-conotoxin M1 (CTx) eliminated both ?O1 and ?B2 and left only ?O2 and the short ?B1 bursts, as expected. Furthermore we found desensitization when the receptors bound ACh only at the ?? site. When CTx was applied to 'embryonic' mouse endplates, monoquantal current rise times were increased, and amplitude and decay time constants were reduced, as expected. Thus the ?? and ?? sites of nAChRs elicit specific channel-opening patterns.     «

'Embryonic' muscle-type nicotinic acetylcholine receptor channels (nAChRs) bind ligands at interfaces of ?- and ?- or ?-subunits. ?? and ?? sites differ in affinity, but their contributions to opening the channel have remained elusive. We compared high-resolution patch clamp currents evoked by epibatidine (Ebd), carbamylcholine (CCh) and acetylcholine (ACh). Ebd binds with 75-fold higher affinity at ?? than at ?? sites, whereas CCh and ACh prefer ?? sites. Similar short (?O1), intermediate (?O2)...     »