ION CHANNELS IN RAT MICROGLIA AND THEIR DIFFERENT SENSITIVITY TO LIPOPOLYSACCHARIDE AND INTERFERON-GAMMA

In order to study the voltage-dependent ion channels in microglia, and their possible modulation by proinflammatory substances like lipopoly saccharide (LPS) and interferon-gamma (IFN-gamma) we employed the patc h-clamp technique on purified rat microglial cell subcultures grown fo r 1-5 days in control condition or after a 24 hour treatment with thos e agents. Regardless of the culture condition, almost 100% of the cell s presented inward-rectifying (IR) K+ currents identified by the follo wing features: (a) extracellular K+-dependence of V-rev and whole-cell conductance; (b) inward-rectifying property; (c) channel blocking mec hanism 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 du ring the first 2 days of subcultivation. Its occurrence was strongly d ependent on the preparation, varying from 0% to almost 80%, and it dec reased to 13% of the cells after three days in culture. It showed the folowing features: (i) threshold of activation close to -30 mV; (ii) s igmoid current onset; (iii) voltage-dependent kinetics; and (iv) sensi tivity to 4-aminopyridine (4-AP) and tetraethylammonium (TEA). Further more, we detected two ion currents not previously described in microgl ia: (i) a slowly activating outward current which appeared at potentia ls more positive than +20 mV and with a reversal potential close to 0 mV, tentatively identified as a proton current; and (ii) a Cl- conduct ance identified in ion substitution experiments as the current sensiti ve to the Cl- channel blocker SITS. The two agents, LPS (20-2,000 ng/m l) and IFN-gamma (10-100 u/ml), shared the following effects: (a) enha ncement of membrane capacitance, and (b) increase of OR current amplit ude and frequency of occurrence. Moreover, IFN-gamma was also able to increase IR current density, especially in cells with ameboid morpholo gy, while LPS was ineffective. We conclude that the voltage-dependent ion channel pattern of microglia is more complex than previously thoug ht and that activating agents such as LPS and IFN-gamma share some ele ctrophysiological effects, but differ in others. (C) 1995 Wiley-Liss, Inc.