DIVERSITY OF NICOTINIC ACETYLCHOLINE-RECEPTORS IN RAT HIPPOCAMPAL-NEURONS .4. REGULATION BY EXTERNAL CA-BUNGAROTOXIN-SENSITIVE RECEPTOR FUNCTION AND OF RECTIFICATION INDUCED BY INTERNAL MG++(+ OF ALPHA)

Applying the whole-cell mode of the patch-clamp technique to cultured hippocampal neurons, we demonstrated that extracellular Ca++ modulates the activation and inactivation of type IA nicotinic currents, i.e., the currents subserved by cr-bungarotoxin (alpha-BGT)-sensitive, alpha -7-containing nicotinic acetylcholine receptors (nAChRs). The rundown profile of acetylcholine (ACh)-induced type IA currents that were obta ined using patch pipettes filled with F--based internal solution had t wo components: the first component of rundown was counteracted by a mo re physiological internal solution containing an organic anion (malate or aspartate), suggesting energy dependence; the second component exh ibited dependence on concentration of CaCl2 added to the external solu tion ([Ca++](o)), with rundown minimized at 0.32 mM. The inward rectif ication of ACh-elicited type IA currents, induced by intracellular Mg+ was augmented by lowering [Ca++](o) (from 2 to 0.32 mM). Moreover, e xtracellular Ca++ (0.01-10 mM) acted in a concentration-dependent mann er (IC50 = 0.26 mM) to decrease the cooperativity induced by ACh (n(H) was reduced from 2.7 to 1). Extracellular Ca++ (EC(50) = 0.1 mM) also increased the efficacy of ACh, but exposure to [Ca++](o) from 1 to 32 mM decreased the efficacy of ACh and inactivated the alpha-BGT-sensit ive nAChRs (IC50 = 11 mM). In conclusion, Ca++ regulates agonist effic acy and cooperativity at the alpha-BGT-sensitive neuronal nAChR, modul ates rundown and counteracts Mg++-dependent inward rectification of ty pe IA currents, It is suggested that the regulation by Ca++ of the alp ha-BGT-sensitive nAChR activity could modulate many neuronal functions .