Two different molecular mechanisms underlying progesterone neuroprotection against ischemic brain damage

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 LTP induction. The present study focused on the molecular mechanisms underlying the neuroprotection exerted by P4 administration at 1 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 σ1 receptor since the σ1 receptor antagonist NE100 perfectly mimicked the acute P4-neuroprotection and also attenuated the NMDA-induced [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 σ1 receptor to inhibit NMDAr-Ca2+ influx and a delayed one by an activation of P4R-mediated Src-ERK signaling pathway.