Differential desensitization properties of rat neuronal nicotinic acetylcholine receptor subunit combinations expressed inXenopus laevis oocytes

Abstract

1. Chronic administration of nicotine up-regulates mammalian neuronal nicotinic acetylcholine receptors (nAChRs). A key hypothesis that explains up-regulation assumes that nicotine induces desensitization of receptor function. This is correlated with behaviorally expressed tolerance to the drug. 2. The present experiments were conducted to: (a) obtain information on the nicotine-induced desensitization of neuronal nAChR function, a less understood phenomenon as compared to that of the muscle and electric fish receptor counterparts; (b) test the hypothesis that different receptor subunit combinations exhibit distinct desensitization patterns. 3.Xenopus laevis oocytes were injected with mRNAs encoding rat receptor subunitsα2,α3, orα4 in pairwise combination with theβ2 subunit. The responses to various concentrations of acetylcholine (ACh) or nicotine were analyzed by the two electrode voltage clamp technique. 4. Concentration-effect curves showed that nicotine was more potent than ACh for all the receptor subunit combinations tested. Only theα4β2 combination exhibited a depression of the maximum effect at concentrations higher than 20µM nicotine. 5. After a single nicotine pulse, receptor desensitization (calculated as a single exponential decay) was significantly slower forα4β2 than for eitherα3β2 orα2β2. 6. Concentrations of nicotine that attained a near maximum effect were applied, washed, and re-applied in four minute cycles. The responses were calculated as percentages of the current evoked by the initial application. Following 16 minutes of this protocol, theα4β2 combination showed a greater reduction of the original response as compared to theα2β2 andα3β2 subunit combinations. Taking points 5 and 6 together, these experiments suggest that theα4β2 receptor subtype desensitizes at a slower rate and remains longer in the desensitized state. 7. Becauseα4β2 is the main receptor subunit combination within the brain and is up-regulated by nicotine, our data may be important for understanding the molecular basis of tolerance to this drug.