Distinct Effects Of Histamine On Neuroprotection Induced By Hypoxic Preconditioning Or Acid Postconditioning Against Cerebral Ischemia

Cerebral ischemia is a pathological condition in which there is insufficient blood flow to the brain. At present, cerebral ischemia remains to be one of the leading-causes of death and disablity worldwide. In the last decades, efforts have been made to find effective neuroprotective agents in rescuing neuronal viability. However, the achievement is quite limited since most neuroprotective agents are single-targeted while complex cascades have been found in cerebral ischemia. Therefore, in recent years, attention has turned to the brain's own endogenous protective strategies, including pre-and post-conditioning. It was generally accepted that sub-lethal dose (a brief noninjurious episode) of hypoxia or ischemia, can activate endogenous protective mechanisms against lethal ischemia. Alterations in the release of neurotransmitters and expression or activation of numerous proteins are involved in this process, yet the detailed mechanisms are not fully understood.Histamine is recognized as an important neurotransmitter or neuromodulator in the central nervous system (CNS). Histamine is synthesized from histidine by the specific enzyme histidine decarboxylase (HDC). Both in vivo and in vitro studies indicate that histaminergic neurotransmission mediates neuroprotective activity. Inhibition of histamine signaling by a-fluoromethylhistidine (a-FMH, a selective inhibitor of histidine decarboxylase) aggravates cerebral ischemic injury. And administration of histamine or histamine precursor histidine can attenuate brain ischemia-induced injury. In addition, an increase of histamine release occurs in synaptosomal preparations from rats with hypoxia, and in rat cortex or striatum following focal cerebral ischemia. Recently, we also reported that histamine protects against NMDA-induced necrosis in cultured cortical neurons through the H2 receptor/cyclic AMP/PKA pathway. These evidences suggest that histamine plays an important role in brain ischemia. However, the effects of histamine on neuroprotection induced by preconditioning or postconditioning have not been determined. In the present study, we investigated the role of histamine in hypoxic preconditioning. We further explored whether acid postconditioning is neuroprotective, and the relationship between histamine and acid postconditioning using histidine decarboxylase knock-out (HDC-KO) mice.1. Effects of histamine on hypoxic preconditioning-induced stroke tolerance and the related mechanisms in miceWild-type (WT) and HDC-KO mice were preconditioned by 3 hours of hypoxia (8% O2).48 hours later, animals were subjected to 30 min of middle cerebral artery (MCA) occlusion (MCAO) followed by 24 hours of reperfusion. Hypoxic preconditioning improved neurological function and decreased infarct volume in WT mice or HDC-KO mice treated with histamine, but not in HDC-KO mice or WT mice treated with?-FMH. Laser-Doppler flowmetry analysis showed that hypoxic preconditioning ameliorated cerebral blood flow (CBF) in the periphery of the MCA territory during ischemia in WT mice but not HDC-KO mice. Histamine decreased in cortex of WT mice after 2,3 and 4 hours of hypoxia and HDC activity increased after 3 hours of hypoxia. VEGF mRNA and protein expression showed a greater increase after hypoxia than those in HDC-KO or a-FMH treated WT mice. In addition, VEGFR2/Flkl antagonist SU1498 prevented protective effect of hypoxic preconditioning in infarct volume and reversed increased peripheral CBF in WT mice. Therefore, endogenous histamine is an essential mediator of hypoxic preconditioning. It may act by enhancing hypoxia-induced VEGF expression.2. Acid postconditioning induces neuroprotection against bran ischemia not involving histamineCerebral ischemic postconditioning has emerged recently as a kind of endogenous strategy for neuroprotection, which was defined as a sub-threshold ischemic insult applied to the brain after ischemic reperfusion. It has been demonstrated in vivo and in vitro. However, its clinical application was limited. Therefore, it is necessary to find other postconditionig methods which are more applicable for clinic. Acidosis is an important factor in ischemic postconditioning. We hypothesize that acid postconditioning may have neuroprotection against brain ischemia. In corticostriatal slices and cultured cortical neurons, acid postconditioning (pH 6.8) improved the neuronal viability after oxygen-glucose deprivation (OGD)/reperfusion in a time-dependent manner. Postconditioning with acidosis by inhaling 20%CO2 for 5 min at 5 min after reperfusion significantly improved neurological function and decreased infarct size induced by MCAO in WT mice. Acid postconditioning also produced neuroprotection in HDC-KO mice or?-FMH treated WT mice. Acid postconditioning significantly reduced the percentage of TUNEL-positive cells and caspase-3 expression induced by OGD in cultured cortical neurons. Similarly, compared with MCAO alone group, acid postconditioning significantly inhibited the protein expression of caspase-3 and bax. Furthermore, we found that atractyloside (Atra), which opens mitochondrial permeability transition pore (MPTP), aborted neuroprotection of acid postconditioning in vivo. In cultured cortical neurons, acid postconditionig suppressed the OGD/reperfusion-induced dissipation of mitochondrial membrane potential (??m, evaluating MPTP opening) and reactive oxygen species (ROS) generation, and enhanced reduced glutathione (GSH, the major endogenous antioxidant) levels. Postconditioning with MPTP closer cyclosporin A also suppressed the dissipation of mitochondrial membrane potential and ROS generation. And Atra partially reversed ROS inhibition by acid postconditioning. In the MCAO model of mice, the ROS generation at 30 min after reperfusion was inhibited by acid postconditioing in the core and periphery of the MCA territory. These results suggest that acid postconditioning inhibits brain ischemia/reperfusion injury. The neuroprotective effect is likely achieved by anti-apoptosis. And inhibiting mitochondria injury and ROS production by suppressing MPTP opening and enhancing GSH levels may be involved in this process. However, histamine is not involved in acid postconditioing-induced neuroprotection.