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

Ion-selective double-barrelled microelectrodes were used to measure the activities of intracellular K+, Na+, Cl-, and H+ (aiK, aiNa, aiCl, pHi) and membrane potential (Em) in neuropile glial cells as well as extracellular K+ activity (aeK) 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 X 10(-3) M) transient membrane depolarizations of up to 60 mV amplitude whereas muscarine (10(-6)-10(-3) M) did not affect Em. alpha-Bungarotoxin (10(-7) M), decamethonium (10(-5) M), d-tubocurarine (5 X 10(-5) M), and strychnine (5 X 10(-5) M) blocked the carbachol depolarization by about 90%. Atropine (5 X 10(-5) M) blocked the response by about 75%, whereas hexamethonium was only effective at millimolar concentrations. Average baseline levels of aeK in the neuropile and of aiK, aiNa, and aiCl in the neuropile glial cells were about 3, 70, 10, and 7 mM, respectively. During the carbachol depolarization aeK and aiNa transiently increased, whereas aiK decreased. In contrast, a rise of aiK and a fall of aiNa were observed during glial depolarizations in solutions with elevated K+ concentration. aiCl increased during both the carbachol- and the K+-induced depolarization. During carbachol, pHi 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 intracellularly 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.