CHARACTERIZATION AND POSSIBLE FUNCTION OF ADENOSINE 5'-TRIPHOSPHATE RECEPTORS IN ACTIVATED RAT MICROGLIA

1 Purinoceptor agonist-induced currents in untreated (proliferating) a nd lipopolysaccharide (LPS; 100 ng ml(-1))-treated (non-proliferating) rat microglial cells in culture were recorded by the whole-cell patch -clamp technique. These cells have two preferred resting membrane pote ntials, one at -35 mV and another one at -70 mV. 2 Most experiments we re carried out in non-proliferating cells. ATP, ATP-gamma-S and <alpha ,beta-MeATP (1-1000 mu M in all cases) evoked an inward current at a h olding potential of -70 mV, followed, in some experiments, by an outwa rd current. At -70 mV 2-methylthio ATP (1-1000 mu M) evoked an inward current, whereas at -35 mV it produced an outward current only. 3 When K+ was replaced in the pipette solution by an equimolar concentration of C-s+ (150 mM), the main outward component of the ATP-gamma-S (10 m u M) induced response disappeared. Instead, an inward current was obta ined. Replacement of K+ by Cs+ did not affect the inward current evoke d by 2-methylthio ATP (300 mu M) 4-Aminopyridine (1-10 mM), however, a lmost abolished this current and unmasked a smaller outward current. 4 The rank order of agonist potency was 2-methylthio ATP > ATP > alpha, beta-MeATP. Adenosine and UTP were inactive. Suramin (300 mu M) and re active blue 2 (50 mu M) antagonized the effect of 2-methylthio ATP (30 0 mu M). 5 1-V relations were determined by delivering fast voltage ra mps before and during the application of 2-methylthio ATP (300 mu M). In the presence of extra- (1 mM) and intracellular (150 mM) Csf, the 2 -methylthio ATP-evoked current crossed the zero current level near 0 m V. When both Cs+ (1 mM) and 4-aminopyridine (1 mM) were present in the bath medium, the intersection of the 2-methylthio ATP current with th e zero current level was near -75 mV. 6 2-Methylthio ATP (1-1000 mu M) induced the same inward current both in proliferating and nonprolifer ating microglia. However, the depolarizing response to 2-methylthio AT P (300 mu M) was larger and longer-lasting in the proliferating cells. When the free Ca2+ concentration in the pipettes was increased from t he standard 0.01 to 1 mu M, the amplitude and duration of this depolar ization was increased in non-proliferating cells. 4-Aminopyridine (1 m M) enhanced the duration, but not the amplitude of responses. 7 ATP an d its structural analogues stimulate microglial purinoceptors of the P -2Y-type. This leads to the opening of non-selective cationic channels and potassium channels. Depending on the resting membrane potential, depolarization or hyperpolarization prevails. Although the inward curr ent produced by 2-methylthio ATP is of similar amplitude in proliferat ing and non-proliferating microglia, the resulting depolarization is s maller in the latter cell type because of the presence of voltage-sens itive, outwardly rectifying potassium channels.