Desensitization of embryonic nicotinic acetylcholine receptors expressed in Xenopus oocytes

Xenopus oocytes where injected with mRNA coding for the embryonic type of the bovine nicotinic acetylcholine receptor (nAChR). Pulses of actylcholine (ACh) were applied to outside-out patches of the injected oocytes with the liquid filament switch. With high concentrations of ACh (10^-3 M), the ACh response desensitized rapidly; the decay could be fitted with a single exponential with a time constant of about 50 ms. In double pulse experiments, recovery from desensitization was tested. Complete recovery was observed after 5 s. The affinity of ACh to the desensitized receptor was measured by preincubating the outside-out patch with different concentrations of ACh. Preincubation with 10^-6 M ACh led to complete desensitization of the nAChRs on the patch. The results can be fitted with the molecular scheme describing activation and desensitization of the nAChR which was developed recently for embryonic nAChRs of mouse muscle.