Herein, we show that a single injection of P4(4 mg/kg) at 1 h or 48 h, but not 96 h, before middle cerebral artery occlusion (MCAO) produces significant protective effects against the ischemia-induced neuronal death and the deficits in spatial cognition and UP induction. The present study focused on the molecular mechanisms underlying the neuroprotection exerted by P4 administration at I h and 48 h pre-MCAO, termed acute and delayed P4-neuroprotection, respectively. Pharmacology suggested that P4-receptor(P4R) cascading to a Src-ERK1/2 signaling mediated the delayed P4-neuroprotection. To support this, it was observed by anti-phosph-ERK1/2 immunoblots that a single injection of P4 triggered a P4R-mediated persistent increase in ERK1/2 phosphorylation and their nuclear translocation for 48 h. In contrast, the acute P4-neuroprotection did not depend on the P4R-mediated Src-ERK1/2 signaling. Instead, the acute P4-administration attenuated the NMDA-induced rise in the intracellular calcium concentration([Ca2+] (i) ) that may be a primary cause for MCAO-induced neuronal injury. This effect seemed to be exerted by an antagonism of sigma(1) receptor since the sigma(1) receptor antagonist NE100 perfectly mimicked the acute P4-neuroprotection and also attenuated the NM- DA-incluced [Ca2+] (i), increase. These findings suggest that the P4 neuroprotection involves two independent processes depending on the timing of P4 administration before MCAO: an acute protection by antagonizing sigma (1) receptor to inhibit NMDAr-Ca2+ influx and a delayed one by an activation of P4R-mediated SrcERK signaling pathway. (C) 2008 Elsevier Ltd. All rights reserved. Herein, we show that a single injection of P4 (4 mg / kg) at 1 h or 48 h, but not 96 h before middle cerebral artery occlusion (MCAO) produces protective effects against the ischemia-induced neuronal death and the deficits in spatial cognition and UP induction. The present study focused on the molecular mechanisms underlying the neuroprotection exerted by P4 administration at I h and 48 h pre-MCAO, termed acute and delayed P4-neuroprotection, respectively. Pharmacology suggested that P4-receptor (P4R ) was casted to a Src-ERK1 / 2 signaling mediated the delayed P4-neuroprotection. It was observed by anti-phosph-ERK1 / 2 immunoblots that a single injection of P4 triggered a P4R-mediated persistent increase in ERK1 / 2 phosphorylation and their nuclear translocation for 48 h. In contrast, the acute P4-neuroprotection did not depend on the P4R-mediated Src-ERK1 / 2 signaling. Instead, the acute P4-administration attenuated the NMDA-induced rise in the intracellular calcium This effect seemed to be exerted by an antagonistic of sigma (1) receptor since the sigma (1) receptor antagonist NE100 perfectly mimicked the acute P4-neuroprotection and also attenuated the NM-DA-incluced [Ca2 +] (i), increase. These findings suggest that the P4 neuroprotection involves two independent processes depending on the timing of P4 administration before MCAO: an acute protection by antagonizing sigma ) receptor to inhibit NMDAr-Ca2 + influx and a delayed one by an activation of P4R-mediated SrcERK signaling pathway. (C) 2008 Elsevier Ltd. All rights reserved.