The nature of the membrane potential of glial cells associated with the medial giant axon of the crayfish

Abstract

Electrophysiological, pharmacological and electron microscopic methods were used to characterize the satellite glial (Schwann-like) cells associated with the medial giant axon of the crayfish, Procambarus clarkii. The satellite glial cell layer surrounding the axon is formed by 15–20 cells deeply interdigitated into each other, forming a vast system of intercellular channels which communicate the axo-glial space with the external medium. The satellite cell layer varies from 0.2–3 μm in thickness. Membrane potentials of the giant axon and the satellite glial cells were monitored before, during and after treatment with ouabain and a variety of cholinergic agonists and antagonists. The membrane potentials of 63 control satellite cells averaged−42.6 ± 0.6 mV. The intracellular localization of the glial cell potential difference was corroborated by lithium carminate marking of the microelectrode tip recording site. Superfusion of satellite cells with 10 −7 m carbachol, nicotine or acetylcholine caused a 15 to 20 mV hyperpolarization from resting level. Muscarine (10 −6 m) had no effect on the glial cell potential. The effect of nicotine was prevented or reversed by-tubocurarine (10 −9 m). The effects of cholinomimetics were reversed by washing the cells in drug-free solution. None of these agents had an obvious effect on axon membrane potential or action potential generation at the concentration used in this study. Ouabain (10 −3 m) also caused a rapid hyperpolarization of the glial cells. The effect lasted 15–18 min after which the membrane rapidly depolarized. The results suggest that (1) the satellite glial cell of the crayfish giant axon system may be studied simultaneously with the axon using electrophysiological techniques; (2) the satellite glial cell membrane appears to have typical acetylcholine receptors of the nicotinic type; (3) the membrane potential of the glial cell is sensitive to ouabain and (4) properties of the glial cell associated with the giant axon of the crayfish are similar to those of the Schwann cell of the tropical squid Sepioteuthis sepioidea.