Ca 2+ influx into leech neuropile glial cells mediated by nicotinic acetylcholine receptors

The effect of cholinergic agonists and antagonists on the intracellular free Ca 2+ concentration ([Ca 2+ ] i ) of leech neuropile glial cells was investigated by use of iontophoretically injected fura‐2. In neuropile glial cells, cholinergic agonists induced a marked increase in [Ca 2+ ] i that was inhibited by d‐tubocurarine, α‐bungarotoxin, strychnine, and atropine. The efficacy of the various agonists and antagonists indicates that the [Ca 2+ ] 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, [Ca 2+ ] i partly recovered, suggesting that the ACh receptors desensitize. The [Ca 2+ ] i increase induced by cholinergic agonists was abolished in Ca 2+ ‐free solution, which indicates that it is caused by Ca 2+ influx from the external medium. The agonist‐induced [Ca 2+ ] i increase was partly preserved in Na + ‐free solution, whereas the agonist‐induced membrane depolarization was strongly suppressed. The agonist‐induced [Ca 2+ ] i increase was also partly preserved in the presence of 5 mM Ni 2+ , which almost abolished the K + ‐induced [Ca 2+ ] i increase mediated by voltage‐dependent Ca 2+ channels. It is concluded that at low agonist concentrations the [Ca 2+ ] i increase in leech neuropile glial cells is mediated exclusively by the ion channels associated with the nicotinic ACh receptors. At high agonist concentrations, voltage‐dependent Ca 2+ channels activated by the concomitant membrane depolarization also contribute to the agonist‐induced Ca 2+ influx. © 1995 Wiley‐Liss, Inc.