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

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Abstract
In order to study the voltage-dependent ion channels in microglia, and their possible modulation by proinflammatory substances like lipopolysaccharide (LPS) and interferon-γ (IFN-γ) 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-% 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-γ (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-γ 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-y share some electrophysiological effects, but differ in others.