Two different ionotropic receptors are activated by ATP in rat microglia

Our aim was to assess whether ATP-induced inward currents in microglia are due to a single or more than one purinergic receptor. The ATP dose-response curve showed two components, whose presence might be due to the activation of high and low affinity receptors. The P2Z/P2X7 specific receptor agonist benzoylbenzoyl-ATP (Bz-ATP) and some P2 receptor agonists were tested. The rank order of potency was Bz-ATP >> ATP = 2-methylthio-ATP (2-MeSATP) > α,β-methylene ATP (α,β-meATP) ≥ ADP. β,γ-MethyleneATP (β,γ-meATP), UTP and adenosine were ineffective. The non-specific P2 receptor antagonist suramin antagonized by 92 ± 2 % the inward current induced by 100 μ m ATP, and by 51 ± 8 and 68 ± 6 % those induced by 3 m m ATP and 100 μ m Bz-ATP, respectively. The P2Z/P2X7 antagonist oxidized ATP (oATP) almost abolished the inward current induced by 3 m m ATP or Bz-ATP, but was ineffective against 100 μ m ATP. Inward currents induced by low ATP concentrations (≤ 100 μ m ) were generally followed by an almost complete and irreversible desensitization, while those elicited by ATP ≥ 1 m m showed only a partial decline. Interestingly, the inward current induced by 100 μ m 2-MeSATP showed a large desensitization, while that induced by Bz-ATP did not. In voltage-ramp experiments, the 100 μ m ATP-induced current exhibited a slight inward rectification more visible at negative potentials, while the 3 m m ATP-induced current did not. ATP induced a fast and large increase in [Ca 2+ ] that promptly recovered in the continuous presence of low ATP doses, but did not recover in high ATP doses. As with desensitization, the response to Bz-ATP mimicked that of high doses of ATP. When Ca 2+ mobilization due to P2Y receptors was blocked by thapsigargin-induced Ca 2+ depletion or by pertussis toxin treatment, 10 μ m ATP was still able to induce a Ca 2+ transient, which represented the contribution of the Ca 2+ influx induced by P2X receptors In conclusion, the inward currents and a fraction of the Ca 2+ transients induced by ATP in microglia are due to at least two ATP-sensitive receptor channel types, whose different properties and sensitivity to ATP may be associated with different functional roles.