CA2-CELLS MEDIATED BY NICOTINIC ACETYLCHOLINE-RECEPTORS( INFLUX INTO LEECH NEUROPIL GLIAL)

The effect of cholinergic agonists and antagonists on the intracellula r free Ca2+ concentration ([Ca2+](i)) of leech neuropile glial cells w as investigated by use of iontophoretically injected fura-2. In neurop ile glial cells, cholinergic agonists induced a marked increase in [Ca 2+](i) that was inhibited by d-tubocurarine, alpha-bungarotoxin, stryc hnine, and atropine. The efficacy of the various agonists and antagoni sts indicates that the [Ca2+](i) increase is mediated by the nicotinic acetylcholine (ACh) receptors that have been characterized previously in these cells by using electrophysiological methods. In the presence of high agonist concentrations, [Ca2+](i) partly recovered, suggestin g that the ACh receptors desensitize. The [Ca2+](i) increase induced b y cholinergic agonists was abolished in Ca2+-free solution, which indi cates that it is caused by Ca2+ influx from the external medium. The a gonist-induced [Ca2+](i) increase was partly preserved in Na+-free sol ution, whereas the agonist-induced membrane depolarization was strongl y suppressed. The agonist-induced [Ca2+](i) increase was also partly p reserved in the presence of 5 mM Ni2+, which almost abolished the K+-i nduced [Ca2+](i) increase mediated by voltage-dependent Ca2+ channels. It is concluded that at low agonist concentrations the [Ca2+](i) incr ease in leech neuropile glial cells is mediated exclusively by the ion channels associated with the nicotinic ACh receptors. At high agonist concentrations, voltage-dependent Ca2+ channels activated by the conc omitant membrane depolarization also contribute to the agonist-induced Ca2+ influx. (C) 1995 Wiley-Liss, Inc.