Differential desensitization properties of rat neuronal nicotinic acetylcholine receptor subunit combinations expressed in Xenopus laevis oocytes.
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 subunits alpha 2, alpha 3, or alpha 4 in pairwise combination with the beta 2 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 the alpha 4 beta 2 combination exhibited a depression of the maximum effect at concentrations higher than 20 microM nicotine. 5. After a single nicotine pulse, receptor desensitization (calculated as a single exponential decay) was significantly slower for alpha 4 beta 2 than for either alpha 3 beta 2 or alpha 2 beta 2. 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, the alpha 4 beta 2 combination showed a greater reduction of the original response as compared to the alpha 2 beta 2 and alpha 3 beta 2 subunit combinations. Taking points 5 and 6 together, these experiments suggest that the alpha 4 beta 2 receptor subtype desensitizes at a slower rate and remains longer in the desensitized state. 7. Because alpha 4 beta 2 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.
Vibat
C R
CR
Section of Molecular and Cellular Biology, University of California at Davis 95616, USA.
Lasalde
J A
JA
McNamee
M G
MG
Ochoa
E L
EL
D000109:Acetylcholine / Q000494:pharmacology; D000818:Animals; D004361:Drug Tolerance / Q000502:physiology; D004533:Egtazic Acid; D004594:Electrophysiology; D006531:HEPES; D007552:Isotonic Solutions; D009474:Neurons / Q000648:ultrastructure; D009538:Nicotine / Q000494:pharmacology; D009865:Oocytes / Q000502:physiology; D051381:Rats; D011978:Receptors, Nicotinic / Q000502:physiology* / Q000648:ultrastructure; D000077331:Ringer's Solution; D012680:Sensitivity and Specificity; D015854:Up-Regulation / Q000502:physiology; D014982:Xenopus laevis
D007552:Isotonic Solutions; D011978:Receptors, Nicotinic; D004533:Egtazic Acid; D009538:Nicotine; D000077331:Ringer's Solution; D000109:Acetylcholine; D006531:HEPES