Stroke, as a cerebrovascular disease represented by ischemia or hemorrhage, has a high incidence rate. Most patients will have different degrees of disability, and even die. [1, 2]. According to statistics, ischemic stroke caused by cerebral artery occlusion accounts for about 80% of all strokes[3]. Previous evidence has shown that ischemia injury leads to pathological changes in cells, and a large number of inflammatory cytokines will be released as a result.[4]. Then, with the increase of inflammatory cytokines, inflammatory cascades are generated, such as the further activation of microglia, the increase of nucleotide binding oligomerization domain receptor protein 3 (NLPR3) inflammasome, the destruction of the blood-brain barrier, and the aggravation of brain edema and cell death. And eventually aggravate brain injury[5]. The recombinant tissue plasminogen activator (Rt-PA) is a commonly used clinical treatment, but the narrow time window limits its large-scale application, and many people have not received timely and effective treatment[6]. Therefore, it is urgent to develop new prevention or treatment methods for brain injury caused by ischemic stroke.
The massive research indicated that giving multisensory stimuli (such as motor, visual, auditory, etc.) after cerebral ischemia injury can be effective in ameliorating the neurological dysfunction of patients[7]. The enriched environment (EE) is a noninvasive intervention modality that includes multifaceted stimuli such as motor stimulation, cognitive stimulation, social stimulation, and sensory stimulation[8] (Fig. 1.A,B). Compared with the standard environment (SE), the EE has small toys with different functions and shapes, and a larger activity space for exploration. Secondly, more participating members in the space provide social stimulation, and items will be changed regularly to maintain the novelty of the environment. In the EE, we can have more opportunities to receive multiple stimuli and provide a favorable environment for the repair of the body. In related studies on neurological impairment, the recovery of brain injury after EE intervention has confirmed this view[9, 10]. Accumulating evidence suggests that exposure to EE after brain damage can ameliorate neurological outcome by reducing cell death, promoting motor function recovery, promoting neurogenesis, reducing microglial activation, and altering gene expression. However, there are still few relevant studies on whether EE pretreatment intervention can play a protective role and what its specific mechanism is.
After ischemic stroke, the brain tissue lacks necessary oxygen and energy, resulting in the disorder of neuronal metabolism, the destruction of ion balance and cell homeostasis, and eventually neuronal apoptosis occurs and the formation of infarct core and ischemic penumbra[11]. A series of cascades such as inflammation and apoptosis are triggered. It further damages neurons, endothelial cells, and glial cells, exacerbating ischemic penumbra injury[12, 13]. Inflammation is essential throughout the pathological process of ischemic stroke. After brain injury, the inflammatory response profoundly influences disease progression and recovery. The activation of glial cells, mainly microglia, is the first manifestation of the inflammatory response. Neurotoxic substances such as inflammatory cytokines, glutamate and nitric oxide can be induced by activated microglia[14, 15]. In addition to this, microglia can induce the release of proinflammatory cytokines[16]. Numerous studies have shown that cerebral ischemia induces a severe neuroinflammatory response, manifested by an increased number of inflammasomes and proinflammatory cytokines[17]. Importantly, inhibiting inflammation can significantly attenuates brain damage following ischemia-reperfusion[18, 19]. However, the specific pathway through which different intervention methods inhibit inflammation has not yet been determined. There are still a lot of studies devoted to this, and new mechanisms are constantly added.
The mitogen activated protein kinases (MAPKs), as a group of serine-threonine protein kinases that can be activated by various stimuli, are mainly divided into three subfamilies: p38, JNK and ERK, which mediate fundamental biological processes and cellular responses to external stimuli [20, 21]. P38 and JNK are both related to inflammation and apoptosis. p38 is mainly used in the research of inflammation, while JNK is mainly focused on apoptosis; ERK is related to cell growth and differentiation, and mainly affects the proliferation, migration, and invasion of cancer cells. MAPK has been widely studied in inflammation[22]. Among them, p38MAPK signaling is closely related to ischemic injury mediated by inflammation, and inhibition of p38MAPK signaling can inhibit neuronal apoptosis and inflammation response, protect damaged brain tissue, and ameliorate neurological injury[23]. The main inducement of p38MAPK is stress response such as hypoxia and ultraviolet radiation, and it is participate in governing the synthesis of inflamentary transmitter at the transcriptional as well as translational levels[24]. It is a potential target for anti-inflammatory treatment. So far, the activation mechanism of p38MAPK pathway has been well studied. The p38MAPK cascade, as a central signal pathway, is mainly responsible for transmitting stimuli and other signals to various targets in the cytoplasm and nucleus. Some secondary signal molecules, such as STAT1, STAT3 and STAT4, are located downstream of p38MAPK[25]. Especially, p38mapk can induce activation of STAT1. An increasing body of evidence suggests that that p38MAPK/STAT1 pathway plays a significant role in the inflammatory response[26]. p38MAPK signaling pathway is also participated in the pathological process of stroke, which has been validated in numerous cellular and animal experiments[27–30]. However, it is not clear whether the p38MAPK/STAT1 pathway is associated with the protective effect of EE pretreatment. In addition, the effect of EE pretreatment on p38MAPK/STAT1 pathway still unclear.
In our study, we proposed that EE could also attenuate brain injury after ischemic stroke via p38MAPK/STAT1 pathway. More importantly, we placed the intervention time of EE before modeling to investigate whether advance EE exposure had a protective effect on the brain. We ascertained the influences of EE pretreatment on p38MAPK/STAT1 signaling pathway and inflammatory related cytokines in the contralateral penumbra cortex of adult rats subjected to middle cerebral artery occlusion /reperfusion (MCAO/R) by Western blotting. Our results show that EE pretreatment can inhibit inflammatory response through p38MAPK/STAT1 pathway, thereby reducing brain damage caused by ischemic stroke. More importantly, this research result has certain clinical significance, that is, pleasant mood and relaxed atmosphere are conducive to the prevention and cure of diseases.