Shu-Hui Zhao, Li Wang

1. Shanxi medical university

2. Second hospital of Shanxi medical university, Taiyuan, Shanxi 030001, China

Keywords:Depression Stroke Model Animal

ABSTRACT Post-stroke depression (PSD) is one of the complications after stroke, with an incidence of about 31%. There are different opinions on the pathogenesis of PSD, but it is still unclear and needs further study. At present, in addition to clinical research, the establishment and research of PSD animal model is an important part of the basic research of PSD. Therefore, it is very important to establish an effective animal model. In this paper, combined with the current situation at home and abroad, the relevant models are summarized and summarized.

Stroke is the first cause of death in China, which is characterized by high morbidity, high mortality and high disability rate. Poststroke depression (PSD) is a common and treatable complication after stroke, which can show depression and certain somatic symptoms[1]. In the past 5 years, the incidence of PSD is about 31%[1,2]. Both clinical and basic studies have found that the prognosis of stroke is closely related to PSD, that is, PSD will affect the recovery of neurological function, the decrease of activities of daily life and quality, and the most serious consequence is to greatly increase the mortality[3-7]. In May 2016, the guidelines for Stroke Rehabilitation issued by the American Heart Association (American Heart Association, AHA) / American Stroke Association (American Stroke Association, ASA) pointed out[8] that PSD plays a very important role in stroke rehabilitation. Therefore, it is of great significance to study the pathogenesis of PSD and the corresponding research[1,9].

There are different opinions on the pathogenesis of PSD [10-15], At present, it is not completely clear, including biological mechanism (inflammatory mechanism, gene polymorphism mechanism, etc.) and social psychological mechanism and so on. For the study of the pathogenesis of PSD, it is necessary to establish a PSD model. At present, the establishment of PSD model, mainly using SD rats [16-24] or Wistar mice [24-26] as animal research objects, the steps are mainly divided into two parts, first to establish a stroke model, followed by further intervention to form a depression model, and finally to form a PSD model. The evaluation methods of each model and PSD model are described below.

1.Stroke model [27]

1.1 Permanent occlusion of the middle cerebral artery (permanent middle cerebralartery occlusion, pMCAO)

After anesthesia and skin preparation, the animals were cut in the middle of the neck and exposed the operation site to find the common carotid artery, internal carotid artery, external carotid artery and peripheral nerve one by one, ligation of the external carotid artery, temporary ligation of the common carotid artery, temporary ligation of the common carotid artery at the bifurcation of the common carotid artery, insertion of thread and fixation [28]. The disadvantage is that the operation is complex and the mortality rate is high.

1.2 Middle cerebral artery ischemia-reperfusion model

On the basis of MCAO, the embolic material was removed to restore blood flow, but the operation was more complex and the success rate was lower.

1.3 Craniotomy electrocoagulation model of middle cerebral artery

The middle cerebral artery was exposed by craniotomy. The corresponding part of the blood vessel was occluded with an electrode, resulting in infarction in the corresponding subcortical region. The advantage is that the infarct site can be located, and the disadvantage is that the risk of infection is high.

1.4 Middle cerebral artery autologous blood embolism model

Using self-thrombus to observe the de process of middle cerebral artery embolism caused by self-thrombus, the disadvantage is that it is difficult to control the occlusion site.

1.5 Bilateral common carotid artery permanent occlusion model

Bilateral common carotid arteries were ligated, but the disadvantage was high postoperative injury and mortality.

1.6 Bilateral common carotid artery ischemia-reperfusion model

After embolization of bilateral common carotid arteries for a period of time, the embolization was removed to restore blood supply, but the disadvantage was multiple operations and increased risk of infection.

1.7 Local cortical model of light embolization

According to the action of photosensitive substances and light, vascular endothelial injury and platelet aggregation are caused in the corresponding parts, resulting in local cortical ischemia. The advantage is that the embolic site can be controlled artificially. The disadvantage is that the embolization is mostly located in the terminal arterioles, which is not consistent with the mechanism of human middle cerebral artery embolization.

1.8 Other methods [29]

Chemical induction of stroke with drugs, cerebral embolism caused by injection of thrombin at specific sites, cerebral ischemia caused by balloon catheter compression, cerebral ischemia caused by cortical pressure, CCA compression clamping method, intimal injury method, and so on.

2. Depression model

2.1 Electrical stimulation induction (ESI)

This model is the most commonly used to shock the feet of rodents, that is, the animals will be placed in a cage on the live floor, and it will escape immediately or within 5 seconds. If the escape latency of the animal is delayed to 10-19s, or even 20-30s, the depression model can be successfully established [30-32]. The ESI model simulates depressive symptoms in humans, but it lasts for a short time, and most animals recover within a few days. As a result, the model is unable to conduct long-term studies of depression.

2.2 Environment induction (EI)

This model can be divided into two levels of stimulation: mild stimulation, strong stimulation, each can cause rodent behavior despair in a short period of time, called BD test (BDT).

Mild stimulation is also known as chronic unpredictable mild stress (CUMS).The CUMS model [33-36] has been gradually developed to include more than 12 stimulation methods. According to the nature of stimulation, CUMS is usually divided into two categories: physical stimulation (including fasting, space constraint, etc.) and peripheral stimulation (including day and night reversal, wet bedding, etc.). Two kinds of stimuli can be given randomly, once or twice a day. Depression-like behavior can usually be induced by stimulation for 3 to 5 weeks. The disadvantages are that the pretreatment time is too long, the result is unstable and some samples need to be removed after experimental induction.

The intensity of strong stimulation was greater than that of animals in CUMS model [37-38].It includes FST and TST.FST ,means to put the animal in a limited pool and record the immobile time of swimming. TST, in which the animals are hung upside down by their tails and the immobility time is recorded. Both modes made it impossible for animals to escape, resulting in BD. The advantage is that depression-like behavior can be produced in a short period of time, but the results may be affected by other factors, such as multiple measurements can cause adaptation and fatigue, thus reducing the accuracy of the BD model.

2.3 Chemosensory (CI)

CI is called drug induction as well. It has been reported[39] that reserpine is widely used to inhibit signs of depression caused by central nervous system function by consuming dopamine and 5-hydroxytryptamine and reversed by antidepressants, but it is easy to produce false positive antidepressant effects. In addition, withdrawal of serotonin (5-HT), morphine, corticosterone and some psychostimulants may induce depression-like behavior [40-42]. At present, CI model is only used to study antidepressants.

2.4 Social stress-sensitive (SSI)

SSI model can imitate the pattern of human life. There are two kinds of SSI: one is to put weaker and stronger animals together, and the other is to use the method of mother-child separation to simulate the social stress of human babies who lack maternal love. The advantage is that it can simulate the nature of human social pressure, while the disadvantage is that it takes too long to make a model [43]. The mother-child separation model has the most obvious influence in the early stage of life, which is of significant significance for the study of adult depression suffering from early social stress [44].

2.5 Surgical induction of (SI)

Olfactory bulb is related to the limbic system [45]. Resection of bilateral olfactory bulb reduces passive avoidance response and increases plasma steroid levels, leading to depression-like behaviors such as anaesthesia, decreased appetite and decreased libido [46]. The disadvantages are difficult operation, high infection rate and high mortality rate.

2.6 genetically induced (GI)

There are some models that can simulate the symptoms of human depression: Wistar-Kyoto, Fawn-Hooded rat models and so on[47-48]. In addition, transgenic or gene knockout techniques for neurotransmitters, receptors and transporters have been used to produce a mouse model of depression [49-52]. The disadvantage is that the model is too expensive to carry out widely , and the corresponding pathogenesis is uncertain.

3. PSD models

PSD models are the most commonly used compound model [53-55]. Specifically, cerebral ischemia was induced at first, and then the treatment of depression model was superimposed. In order to evaluate the success of the model, three principles have been applied: facial effectiveness (similarity of clinical features), structural effectiveness (repeatability of pathophysiological processes) and predictive effectiveness (sensitivity to intervention). The following will be a corresponding summary and summary of each model.

3.1 MCAO model

That is, cerebral ischemia model, some mice will develop depression, but the duration of depression is very short, in addition, the effectiveness of face and structure is seriously inadequate, and some studies have shown that antidepressants can reverse depressionlike behavior, but the studies about neurotransmitters and receptors are not clear [56-58]. Therefore, a single MCAO model is far from being a PSD model.

3.2 "MCAO+ isolation" model

To verify the effect of social isolation on stroke.Adult mice were kept in captivity for 14 days, and then MCAO, was induced. The results showed that isolation not only tended to depressive behavior in behavioral tests, but also showed significant brain atrophy in isolated mice [59]. Therefore, the "MCAO+ isolation" model has both facial effectiveness and structural effectiveness.

3.3 "MCAO+ isolation + CUMS" model

It is a classical modeling method with depression and cerebral ischemia [16,60]. It is the most widely used PSD model at present. The animals were raised alone after MCAO operation, and CUMS induction was performed 3 or 7 days after operation. Generally speaking, depression-like behavior can be shown in 2 to 3 weeks, but it takes 5 weeks or more for some animals to reach a state of depression [54,61]. The model can be used to study depression caused by environmental stress, and it was found that the infarct size increased in the depression group, indicating that depression aggravated ischemic cerebral infarction [62-63].

3.4 "MCAO+ spatial restraint " model

Firstly, the MCAO model was established, and then the mice were placed in a ventilable closed container, and the mouse tail was left at the opening at the end of the container, so that the mice could not rotate or move in the test tube. Data showed that PSD-like depression phenotype could be detected in MCAO mice after 60 minutes of ischemia, accompanied by decreased levels of BDNF, serotonin and dopamine. Imipramine could reverse all observed markers [64]. The advantage is,that it is clear that 50 minutes of ischemia has a poor effect on depression, 60 minutes of ischemia can induce PSD, ischemia for 70 minutes to aggravate depressive symptoms, but the mortality rate increases.

3.5 " Spatial restraint + middle cerebral artery electrocoagulation (MCA)" model

First, spatial constraints were performed for 2 hours a day and repeated constraints for 7 days. 24 hours after the last fixation, focal cerebral ischemia was induced by permanent closure of the middle cerebral artery. Inflammatory reaction and pressure-related lipid peroxidation could be detected, but no other markers related to depression were found [65]. Therefore, more studies are needed to prove the relationship between the model and PSD.

3.6 "Heat-induced cortex lesions + restraint or CUMS" model

First, heat-induced cortical lesions were induced, and then CUMS was added to induce 3 weeks to produce a typical PSD model [66]. The advantage is that the cortex injury can be induced directionally, but the disadvantage is that the modeling process is complex, high intracranial infection and mortality.

3.7 " Intraventricular injection of drugs " model

Vahid - Ansari reported an innovative model of prefrontal injection of endothelin-1 (ET-1) [67]. Two Male C57BL/6 mice were fed for 2 weeks before operation, and then endothelin-1 was injected into the target brain tissue by stereotactic operation. At present, more and more experimental teams are constantly exploring this model, such as intraventricular injection of inflammatory substances, but the results remain to be seen. The brain of this model can be used to study the relationship between the location of specific lesions and PSD and the corresponding pathophysiological mechanism according to different injection sites.

3.8 "Photochemical cortical lesion + isolation" model

Local ischemia was performed in the cortex by photothrombosis, and the mice were isolated to establish a PSD mouse model. SPT, FST and weight measurements were performed before stroke and two months after social isolation. The results showed that in the experimental group, immobility time increased and sucrose consumption decreased. After the treatment with fluoxetine, the depressive symptoms were significantly improved, and it was found that BDNF in the hippocampus increased[68].The advantage is that the risk of intracranial infection is low and the survival rate is high. The disadvantage is that subsequent isolation takes a long time.

4. The behavioral evaluation of depression

To judge whether the PSD model is successful or not needs to be tested and evaluated by certain means. The following is a summary of the most common evaluation methods.

4.1 Sugar water preference test (SPT) [69]

Mainly observe whether the interest state of animals has changed before and after the experiment, compare the sucrose intake of rats, and indirectly reflect the current interest and state of animals. When the sucrose water consumption of the animals decreased and significantly compared with the initial control, it could reflect that the interest of the PSD animal model was lower than that of the normal group, and then showed a state of depression.

4.2 Field test (Openfieldtest,OFT) [70]

The anxiety state of animals was mainly evaluated. Place the animals in an unfamiliar environment (such as a container that is 1 meter long, 1 meter wide and 0.5 meters high) and record how long the rats will move in or around the central area. If the time of animals in the central area was significantly less than before, the interest and anxiety of PSD animals were lower than those of normal animals, which indirectly indicated the state of depression.

4.3 Forced swimming test (FST) [27]

The animals were placed in a cylindrical container full of water, and the time of struggle and rest was observed. If the time of immobility was prolonged, the animals were considered to have a certain tendency to depression. However, its immobility may also be a fatigue phenomenon, or an adaptation phenomenon caused by a large number of tests, so this method is open to question.

4.4 Tail suspension test (TST) [27]

When the tail of the experimental animal is fixed so that its head is hung down, the animal will show a struggle, and it can be observed that it tries to get out of the predicament, but because it cannot get rid of it, it will appear intermittent immobility, that is, a state of "behavioral despair", which is similar to the state of depression. However, the disadvantages are similar to forced swimming and may be related to fatigue and adaptation, so further research is needed.

5. The prospect and existing problems of PSD model

The establishment of a reasonable model is very important to study the pathological mechanism of PSD. In the above-mentioned PSD model, the "intraventricular drug injection" model has obvious localization characteristics and certain innovation, and has great potential value for the accurate study of the pathophysiological mechanism of PSD, but there is less experimental data and more data is needed to support it. In addition, we can establish a PSD model based on depression-related gene mutations and superimpose it on stroke to study the role of related genotypes in the occurrence of PSD and explore possible treatments. As a disease after stroke, the relationship between PSD and stroke should not be limited to the sequence or causality of events, so more innovative and experimental means are needed to simulate more possible PSD.

At present, there are still many problems about the establishment of PSD model: (1) When to start to add other depressive stimuli after the establishment of stroke model, the most effective need to be further studied. (2) There is no clear unified standard for the stimulation mode and time of CUMS, (3) There is no unified standard for evaluating the success of PSD model, and more indepth research is needed, (4) The evaluation and observation time of PSD model is still short, and the stability of the model cannot be determined. (4) The comparative study of various animals in PSD remains to be supported by data.