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Single-Channel Kinetic Analysis of Chimeric 7–5HT_3A Receptors

Instituto de Investigaciones Bioquimicas, Universidad Nacional del Sur–Consejo Nacional de Investigaciones Científicas y Técnicas, Bahia Blanca, Argentina (D.R., G.S., C.B.); and Receptor Biology Laboratory, Department of Physiology and Biomedical Engineering, Mayo Clinic College of Medicine, Rochester, Minnesota (S.M.S.)

The receptor chimera 7–5HT_3A has served as a prototype for understanding the pharmacology of 7 neuronal nicotinic receptors, yet its low single channel conductance has prevented studies of the activation kinetics of single receptor channels. In this study, we show that introducing mutations in the M3–M4 cytoplasmic linker of the chimera alters neither the apparent affinity for the agonist nor the EC₅₀ but increases the amplitude of agonist-evoked single channel currents to enable kinetic analysis. Channel events appear as single brief openings flanked by long closings or as bursts of several openings in quick succession. Both the open and closed time distributions are described as the sum of multiple exponential components, but these do not change over a wide range of acetylcholine (ACh), nicotine, or choline concentrations. Bursts elicited by a saturating concentration of ACh contain brief and long openings and closings, and a cyclic scheme containing two open and two closed states is found to adequately describe the data. The analysis indicates that once fully occupied, the receptor opens rapidly and efficiently, and closes and reopens several times before it desensitizes. Channel closing and desensitization occur at similar rates and account for the invariant open and closed time distributions.

Address correspondence to: Cecilia Bouzat, Instituto de Investigaciones Bioquímicas, Camino La Carrindanga Km 7, 8000 Bahía Blanca, Argentina. E-mail: inbouzat{at}criba.edu.ar

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