Bradykinin-induced microglial migration mediated by B₁- bradykinin receptors depends on Ca²⁺ influx via reverse-mode activity of the Na⁺/Ca²⁺ exchanger

Bradykinin (BK) is produced and acts at the site of injury and inflammation. In the CNS, migration of microglia toward the lesion site plays an important role pathologically. In the present study, we investigated the effect of BK on microglial migration. Increased motility of cultured microglia was mimicked by B₁ receptor agonists and markedly inhibited by a B ₁ antagonist, but not by a B₂ receptor antagonist. BK induced chemotaxis in microglia isolated from wild-type and B ₂-knock-out mice but not from B₁-knock-out mice. BK-induced motility was not blocked by pertussis toxin but was blocked by chelating intracellular Ca²⁺ or by low extracellular Ca²⁺, implying that Ca²⁺ influx is prerequisite. Blocking the reverse mode of Na⁺/Ca²⁺ exchanger (NCX) completely inhibited BK-induced migration. The involvement of NCX was further confirmed by using NCX⁺/^- mice; B₁-agonist-induced motility and chemotaxis was decreased compared with that in NCX^+/+ mice. Activation of NCX seemed to be dependent on protein kinase C and phosphoinositide 3-kinase, and resultant activation of intermediate-conductance (IK-type) Ca²⁺-dependent K⁺ currents (I_K(Ca)) was activated. Despite these effects, BK did not activate microglia, as judged from OX6 staining. Using in vivo lesion models and pharmacological injection to the brain, it was shown that microglial accumulation around the lesion was also dependent on B₁ receptors and I_K(Ca)). These observations support the view that BK functions as a chemoattractant by using the distinct signal pathways in the brain and, thus, attracts microglia to the lesion site in vivo.