Electrophysiological characterization of a nicotinic acetylcholine receptor on leech neuropile glial cells

Abstract

Abstract Ion‐selective double‐barrelled microelcetrodes were used to measure the activities of intracellular K + , Na + , Cl − , and H + (a , a , a , pH i ) and membrane potential (E m ) in neuropile glial cells as well as extracellular K + activity (a ) in the neuropile of the leech, Hirudo medicinalis , during bath application of carbachol. As measured with conventional single‐barrelled microelectrodes, acetylcholine (ACh), nicotine, carbachol, tetramethylammonium (TMA), and choline elicited concentration‐dependent (10 −6 –5 × 10 −3 M) transient membrane depolarizations of up to 60 mV amplitude whereas muscarine (10 −6 –10 −3 M) did not affect E m . α‐Bungarotoxin (10 −7 M), decamethonium (10 −5 M), d‐tubocurarine (5 × 10 −5 M), and strychnine (5 × 10 −5 M) blocked the carbachol depolarization by about 90%. Atropine (5 × 10 −5 M) blocked the response by about 75%, whereas hexaethonium was only effective at millimolar concentrations. Average baseline levels of a in the neuropile and of a , a , and a in the neuropile glial cells were about 3, 70, 10, and 7 mM, respectively. During the carbachol depolarization a and a transiently increased, whereas a decreased. In contrast, a rise of a and a fall of a were observed during glial depolarizations in solutions with elevated K + concentration. A increased during both the carbachol‐ and the K + ‐induced depolarization. During carbachol, pH i transiently fell by about 0.2 units from its average baseline level of 6.9, whereas an alkalinization of small amplitude was observed in high‐K + solutions. Bath‐applied choline, TMA, and decamethonium rapidly accumulated in the neuropile glial cells as intracellulary monitored with double‐barrelled microelectrodes filled with Corning K + exchanger resin, which is highly selective for these agents. The results suggest that leech neuropile glial cells have a nicotinic ACh receptor coupled to a cation channel. It is hypothesized that this channel might also be permeable to choline, TMA, and decamethonium.