Uptake of potassium by nonmyelinating Schwann cells induced by axonal activity

Abstract

1. The electrophysiological properties of the rabbit vagus nerve (membrane potential, compound action potentials, and afterpotentials) and potassium accumulation were measured simultaneously during low-frequency stimulation (LFS) (0.5 and 1 Hz) by using a modified sucrose-gap apparatus and potassium-sensitive microelectrodes (KSM). 2. During LFS at 0.5 and 1 Hz, the concentration of K+ in the extracellular space ([K+]c) increased in approximately 30 s to a maximal level that was 0.6 and 1.5 mM, respectively, above the resting concentration. Concomitantly the preparation developed an ouabain-sensitive hyperpolarization. 3. The compound action potential (CAP) was followed by a fast hyperpolarizing afterpotential (fHAP), a depolarizing afterpotential (DAP), and a slow hyperpolarizing afterpotential (sHAP). During LFS the characteristics of all these afterpotentials were profoundly modified. In parallel to the increase in [K+]e, the fHAP was decreased and the amplitude of the DAP was dramatically enhanced. Furthermore, the sHAP which had a duration of < 1 s when it followed a single CAP, turned into a ouabain-sensitive hyperpolarization (indicating that it was generated by the electrogenic Na(+)-K+ pump) that lasted several minutes. 4. The application of external Ba2+ produced a hyperpolarizing sag on the sHAP following a single isolated CAP. During LFS, Ba2+ enhanced the build-up of the DAP, raised the maximal level of [K+]e, and increased the activity-induced ouabain-sensitive hyperpolarization. 5. The increase by Ba2+ of the activity-induced hyperpolarization shifted the spikes from both myelinated and nonmyelinated fibers toward a more negative potential but did not increase their amplitude, indicating that this Ba(2+)-induced hyperpolarization originated from an extra-axonal source, presumably the Schwann cells. 6. It is proposed that the electrogenic activity of the Na(+)-K+ pump was enhanced in Schwann cells situated near active axons. This hyperpolarization was, however, not recorded in normal conditions because it was fully short-circuited by a K+ influx through Ba(2+)-sensitive channels. 7. Our results lead to the hypothesis that the Na(+)-K+ pump of the nonmyelinating Schwann cells is important in the mechanisms maintaining the homeostasis of K+ in the axonal microenvironment. They show that the Na(+)-K+ pump contributes to the K+ buffering not only by actively pumping K+ but also by generating a hyperpolarization that drives a passive K+ influx through Ba(2+)-sensitive K+ channels.

Modular double sucrose gap apparatus for improved recording of compound action potentials from rat and mouse spinal cord white matter preparations

“…Typically, the inter-diffusion between compartments is minimized by applying grease around the preparation at points which traverse between the compartments; however, the quality of this insulation remains low (discussed in Mert, 2007), and the grease insulation is not compact enough to suit short WM strips. Rubber membranes with holes closely matching the preparations proved highly efficient in earlier sucrose gap experiments (Berger, 1963;Boev and Golenhofen, 1974;Cleemann and Suenson, 1984;Hoyle, 1987), including studies on most challenging C-fibers in peripheral nerves, which require higher stimulation intensities and shorter conduction distances (Jirounek et al, 1991;Gaumann et al, 1992;Robert and Jirounek, 1994;Erne-Brand et al, 1999;Dalle et al, 2001). In this paper we designed and tested a DSG apparatus with rubber membrane separation of the compartments that enhances the conditions for recording and analysis of CAPs and by virtue of the compactness of the compartments is ideally designed to accommodate relatively short preparations such as those derived from rat and mouse spinal cords.…”

“…Studying slowly conducting non-myelinated axons, like C-fibers of peripheral nerves, by CAP recording (Jirounek et al, 1991;Gaumann et al, 1992;Robert and Jirounek, 1994;Erne-Brand et al, 1999;Dalle et al, 2001), requires short conduction distances because of increased temporal dispersion of action potential arrival times in individual non-myelinated axons with increased distance. The use of 3.5 mm-wide ACSF module in our experiments on Shiverer mice presented here provided sufficient synchrony to detect the slow multi-component C-wave reflecting action potential propagation in dysmyelinated axons.…”

“…Compound action potential (CAP) recording is a powerful tool for characterizing axonal properties in multi-axonal populations (Erlanger and Gasser, 1937), which has been widely used in the past decades on CNS white matter and PNS nerves using wire electrodes (Rasband et al, 1998), extracellular microelectrode field recording (Foster et al, 1982;Agrawal and Fehlings, 1996;Tekkok and Goldberg, 2001;Akiyama et al, 2002), suction electrodes (Kocsis et al, 1986;Stys et al, 1991;Devaux et al, 2003;Ouardouz et al, 2006;Tekkok et al, 2007) and grease gap (Stys et al, 1993) or sucrose gap (Kocsis and Waxman, 1983;Robert and Jirounek, 1994;Fehlings and Nashmi, 1997;Nashmi et al, 2000;Mert, 2007;Devaux and Gow, 2008) techniques.…”

Expression and Localization of Endothelin Receptors: Implications for the Involvement of Peripheral Glia in Nociception

“…This proximity between ESCs and C fibers provides a structural basis by which cellular events occurring in ESCs could modify the excitability of these nociceptors. This may occur in part by ESCs buffering extracellular K ϩ in the axonal environment (Robert and Jirounek, 1994) and regulating lipid metabolism (Fullerton et al, 1998).…”

Potassium homeostasis and glial energy metabolism

“…The pump has a lower affinity for intracellu- Robert and Jirounek, 1994 lar Na ϩ so that it is stimulated by increases in Na i in the physiological range. In contrast, studies on the functioning of the Na ϩ pump on glial cells have shown that the functional affinity for K e is much lower so that the pump is stimulated by increases in K e in the physiological and pathological ranges (Ballanyi et al, 1987;Ballanyi and Kettenmann, 1990;Coles et al, 1986;Reichenbach et al, 1992;Walz et al, 1983).…”