Adenosine is an important factor to participate

Adenosine is an important factor to participate the generation of ATP. What’s more, adenosine has significant endogenous protective effect such as suppressing the release of excitatory amino acid, reducing the permeability of cell membrane to calcium ion, expanding blood vessel of brain, preventing inflammatory cells from adhesion and infiltration, restraining the platelet aggregation, and alleviating the damage on blood vessel endothelium from inflammatory cells and free radical (Boros et al., 2016, Steensrud et al., 2010, Wever et al., 2011). Previous study indicated that the tolerance to ischemia administrated by adenosine was regulated by adenosine A1 receptor (Melani et al., 2014). Hu et al. discovered that LIP could obviously reduce the infarcts area in focal 17-aag ischemia-reperfusion of rats, which could be partly reverse by injection of adenosine A1 receptor antagonist DPCPX (Hu et al., 2012). Moreover, DPCPX can aggravate ischemia-reperfusion injury of isolated cultured brain cortical neurons of rats (Zhong et al., 2015). These above findings indicated that adenosine performed a significant endogenous protection in ischemia-reperfusion injury. Based on these results, the investigation observed the effect of DPCPX on the brain protection and the up-regulation of p38 MAPK and ERK induced by LIP. Thionin staining showed that the rats suffered from brain ischemia exhibited obvious DND in the CA1 subfield. The destruction of pyramidal neurons was effectively prevented by LIP, which indicated that LIP could induce brain ischemic tolerance. However, pretreatment with DPCPX reversed the effect of LIP. Furthermore, the results of immunohistochemistry and Western blot indicated that the expression of p38 MAPK and ERK decreased significantly after DPCPX administration compared with LIP + BI group. These results indicated that adenosine participates in the neuroprotective effect on brain and the up-regulation of p38 MAPK and ERK induced by LIP. In order to further examine that adenosine is involved in the induction of brain ischemic tolerance and up-regulation of p38 MAPK and ERK, the effect of adenosine on brain ischemia and expression of p38 MAPK and ERK was observed. In adenosine + BI group, we found that only scattered death of the neurons in hippocampal CA1 subfield, and the expression of p38 MAPK and ERK in the CA1 subfield increased significantly. The neuroprotection of adenosine in the studies is consistent with the previous reports (Ordonez et al., 2010). Combining these experimental results, it could be concluded that adenosine participated in the mediation of neuroprotection and up-regulation of p38 MAPK and ERK in CA1 subfield of hippocampus in rats during the induction of brain ischemic tolerance by LIP. What are the possible downstream effects of p38 MAPK and ERK in the induction of brain ischemic tolerance by LIP? Recently, Zhang et al demonstrated that erythropoietin exerted a protective effect in renal ischemia/reperfusion injury via the STAT6/MAPK/NF-κB pathway (Zhang et al., 2018). So, p38 MAPK and ERK might play neuroprotection via triggering NF-κB in the acquisition of brain ischemic tolerance by LIP. SOD is another possible downstream signal of p38 MPK and ERK. Zaheer et al found that the glial maturation factor-activated SOD was blocked by the p38 MAPK inhibitor SB 203580, indicating p38 MAPK increased SOD activity (Zaheer et al., 2004). These assumptions need to be confirmed in the future study. In fact, the mechanisms of neuroprotection of LIP are complexity, which relates to humoral pathway and neural pathway, and they can not be separated absolutely. Previous reports pointed out that LIP could protect myocardium by activating HIF1I-EPO route, whereas denervation of bilateral kidneys could reverse the protective effect on myocardium induced by LIP (Oba et al., 2015). In mice myocardium ischemia-reperfusion injury model, Lim et al. also testified that ischemia and reperfusion 3 times on left lower limb could reduce the infarct size of myocardium through humoral pathway and neural pathway (Lim et al., 2010). These studies provided reliable clues to us on investigating neuro-humoral mechanisms of neuroprotection and up-regulation of p38 MAPK and ERK induced by LIP. If it could be deeply confirmed that the neuro-humoral mechanisms, we could mimic the protection of LIP by manual intervention in the future for alleviating ischemia-reperfusion injury. For example, nerve impulse generated by electric stimulation or specific drugs imitate the neural mechanism of LIP to perform protective effect. Exogenous specific agonist or antagonist of humoral factors was administrated in order to realize the up-regulation or reduction of related humoral factors induced by LIP. Combining above methods to imitate neuro-humoral mechanisms of LIP is also viable. Under specific circumstances, in which brain ischemia-reperfusion probably occurs, LIP may play a potential role as a novel therapeutic target for brain ischemic injury in patients.