Permeant but not impermeant divalent cations enhance activation of nondesensitizing alpha 7 nicotinic receptors

Departments of 1 Pharmacology and 2 Cell and Molecular Physiology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599; and 3 Division of Infectious Diseases and International Health, Duke University, Durham, North Carolina 27710 Neuronal 7 nicotinic acetylcholine receptors (nAChRs) are permeable to Ca 2+ and other divalent cations. We characterized the modulation of the pharmacological properties of nondesensitizing mutant (L 247 T and S 240 T/L 247 T) 7 nAChRs by permeant (Ca 2+ , Ba 2+ , and Sr 2+ ) and impermeant (Cd 2+ and Zn 2+ ) divalent cations. 7 receptors were expressed in Xenopus oocytes and studied with two-electrode voltage clamp. Extracellular permeant divalent cations increased the potency and maximal efficacy of ACh, whereas impermeant divalent cations decreased potency and maximal efficacy. The antagonist dihydro- -erythroidine (DH E) was a strong partial agonist of L 247 T and S 240 T/L 247 T 7 receptors in the presence of divalent cations but was a weak partial agonist in the presence of impermeant divalent cations. Mutation of the "intermediate ring" glutamates (E 237 A) in L 247 T 7 nAChRs eliminated Ca 2+ conductance but did not alter the Ca 2+ -dependent increase in ACh potency, suggesting that site(s) required for modulation are on the extracellular side of the intermediate ring. The difference between permeant and impermeant divalent cations suggests that sites within the pore are important for modulation by divalent cations. acetylcholine receptor; calcium; M2 domain; potency; permeation