Abstract

Effects of agonists on rat α3β4 nicotinic acetylcholine receptors expressed in KXα3β4R2 cells [human embryonic kidney 293-derived cells] were studied. The potencies of seven agonists varied over a 7000-fold range, with a rank order of epibatidine ≫A85380 > cytisine ≈ 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP) ≈ nicotine > acetylcholine > carbachol. The efficacies of all of the agonists studied here were similar except for DMPP, which seemed to be a partial agonist compared with nicotine and acetylcholine. Nicotine and carbachol desensitized the receptors in a time- and concentration-dependent manner. The EC₅₀ values for nicotine and carbachol to desensitize the receptors during a 60-min exposure were 3 and 51 μM, respectively, indicating that these agonists are more potent at desensitizing the receptors than at activating them. The function of the receptors recovered from agonist-induced desensitization rapidly and almost completely. The half-time for recovery of function from desensitization after a 60-min treatment with nicotine increased with the concentration of nicotine used to desensitize the receptors. In contrast, no such concentration dependence for time to recovery of function was found when carbachol was used to desensitize the receptors. We propose that this difference may be due to the cell permeability of nicotine, allowing it to enter and be sequestered inside of cells and then slowly diffuse out to maintain receptor desensitization. After a 5-day exposure to 100 μM nicotine, the receptors were completely desensitized, but receptor function recovered to 83% of control values with a half-time of about 10.5 min. Although the number of nicotinic receptor binding sites measured with (±)-[³H]epibatidine was increased during the chronic treatment with nicotine, no increase in function was detected.