The present study evaluated the role of thrombin and its receptors in neurodegeneration and microglial activation. Immunocytochemical evidence indicated that intracortical injection of thrombin resulted in a significant loss of neurons and the activation of microglia in the rat cortex in vivo. Reverse transcription PCR and double-label immunocytochemistry further demonstrated the early and transient expression of pro-inflammatory cytokines and neurotoxic factors as well as their colocalization within activated microglia. The thrombin-induced loss of cortical neurons was partially blocked by N-G-nitro-L-arginine methyl ester hydrochloride, a nitric oxide synthase inhibitor, and by NS-398, a cyclooxygenase-2 inhibitor, indicating that the activation of microglia is involved in the neurotoxicity of thrombin in the cortex in vivo. In addition, thrombin activated cortical microglia, in culture, as indicated by the expression of several pro-inflammatory cytokines and produced cell death in microglia-free, neuron-enriched cortical cultures. However, agonist peptides for thrombin receptors, including protease-activated receptor-1 (SFLLRN), -3 (TFRGAP), and -4 (GYPGKF), failed to activate microglia and were not neurotoxic in culture. Intriguingly, morphological and biochemical evidence indicated that thrombin-induced neurotoxicity but not microglial activation was prevented by hirudin, a specific inhibitor of thrombin. Collectively, the present data suggest that a non-proteolytic activity of thrombin activates microglia and that the proteolytic activity mediates its neurotoxicity. (c) 2006 Elsevier Inc. All rights reserved.