Ion channels in rat microglia and their different sensitivity to lipopolysaccharide and interferon‐γ

Abstract

In order to study the voltage-dependent ion channels in microglia, and their possible modulation by pro-inflammatory substances like lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma) we employed the patch-clamp technique on purified rat microglial cell subcultures grown for 1 - 5 days in control condition or after a 24 hour treatment with those agents. Regardless of the culture condition, almost 100% of the cells presented inward-rectifying (IR) K+ currents identified by the following features: (a) extracellular K(+)-dependence of Vrev and whole-cell conductance; (b) inward-rectifying property; (c) channel blocking mechanism by Cs+; and (d) single channel conductance of 27 pS. A 'n' type outward-rectifying (OR) K+ current was present in 30% of the cells during the first 2 days of subcultivation. Its occurrence was strongly dependent on the preparation, varying from 0% to almost 80%, and it decreased to 13% of the cells after three days in culture. It showed the following features: (i) threshold of activation close to -30 mV; (ii) sigmoid current onset; (iii) voltage-dependent kinetics; and (iv) sensitivity to 4-aminopyridine (4-AP) and tetraethylammonium (TEA). Furthermore, we detected two ion currents not previously described in microglia: (i) a slowly activating outward current which appeared at potentials more positive than +20 mV and with a reversal potential close to 0 mV, tentatively identified as a proton current; and (ii) a Cl- conductance identified in ion substitution experiments as the current sensitive to the Cl- channel blocker SITS. The two agents, LPS (20 - 2,000 ng/ml) and IFN-gamma (10 - 100 u/ml), shared the following effects: (a) enhancement of membrane capacitance, and (b) increase of OR current amplitude and frequency of occurrence. Moreover, IFN-gamma was also able to increase IR current density, especially in cells with ameboid morphology, while LPS was ineffective. We conclude that the voltage-dependent ion channel pattern of microglia is more complex than previously thought and that activating agents such as LPS and IFN-gamma share some electrophysiological effects, but differ in others.

Physiology of Microglia

“…They are expressed in all microglial cells with the exception of resting microglia. The current was first described in cultured rat microglia, and a single-channel conduc-tance of 30 pS was reported (442); subsequently, similar unitary currents were found in other in vitro microglial preparations (234,572,812,965). A second inward rectifier channel with unitary conductance of 43 pS was also found in rat microglia cocultured with fibroblasts (812).…”

“…237 for review). In microglia, H ϩ currents were discovered in primary cultures from mice, rats, and humans (239,460,461,574,965). Similarly to other cell types, microglial H ϩ current have an exceptionally high selectivity to protons (238).…”

Chloride/anion channels in glial cell membranes

“…The gene encoding this channel is unknown. A chloride current with similar (not identical) biophysical and pharmacological properties was found in cultured microglia cells (Visentin et al, 1995;Schlichter et al, 1996;Eder, 1998). In these cells, it was found to be spontaneously active, although cell swelling, application of physical stretch, or the ramification process all increased its expression.…”

A Kv1.5 to Kv1.3 Switch in Endogenous Hippocampal Microglia and a Role in Proliferation

“…The hypothesis that Kv current is absent from resting cells (Kettenmann et al, 1990;Brockhaus et al, 1993) and upregulated by microglial activation (Norenberg et al, 1994;Fischer et al, 1995;Visentin et al, 1995;Gebicke-Haerter et al, 1996) has been challenged because ramified microglia, widely believed to represent resting cells, express a prominent Kv current when exposed to astrocytes or astrocyte-conditioned medium (Schmidtmayer et al, 1994;Eder et al, 1997). One group reported stronger Kv1.5 and weaker Figure 10.…”