Background
Through its mediators, stress can lead to acute or chronic pathological, physical and mental conditions in individuals with a vulnerable genetic, constitutional and/or epigenetic background [
1]. Acute stress may trigger panic attacks and psychotic episodes wile chronic stress may cause physical, behavioral and/or neuropsychiatric manifestations including anxiety, depression, bipolar disorders, executive and/or cognitive dysfunction. Neuropsychiatric manifestations are the cause of high mortality rate across the world. About 450 million persons suffer from mental disturbances or from disturbances of behavior [
2] and about 6.8 million people die every year as a result of neurological disorders [
3]. About 4% of the populations in developed countries are affected by general anxiety disorder [
4]. Precisely, in Africa, about 7.9 to 10% of the population suffer from depression and anxiety disorders, respectively [
5]. According to a report of the Cameroonian Ministry of Health, in Cameroon, a prevalence of 7% of mortality rate is due to psychiatric disorders [
6].
Neuropsychiatric troubles are multifactorial disorders with a multitude of causes, such as environmental causes, stress, genetic, molecular and biochemical alterations. Recent epidemiological data show that chronic stress condition is highly associated to psychiatric mental health impairment: burnout, mood disorders (anxiety, depression), sleep disorders, behavioral disorders (drug addiction, alcohol) [
7].
Modelling of human neuropsychiatric disorders in animals is extremely challenging given the subjective nature of many symptoms, the lack of biomarkers and objective diagnostic tests, and the early state of the relevant neurobiology and genetics. However, our group demonstrated in the last 10 years that oxidative stress is implicated in neurological and psychiatric disorders including Parkinson disease, Alzheimer disease, depression, anxiety disorders, schizophrenia and bipolar disorder [
8‐
10].
Many drugs are useful in the treatment of neuropsychiatric diseases but secondary effects, inaccessibility for some patients, and the high cost of these products for the poor population makes their use problematic [
5]. The therapy of neuropsychiatric diseases is traditionally done with the use of different parts of curative plants. Nevertheless, these treatments are made using decoctions and infusions of these plants or mixtures of several plants [
11,
12].
One of the most commonly used plant in the therapy of mental disorders in sub-Saharan Africa is the sycamore fig or
Ficus sycomorus.
F. sycomorus belongs to the family Moraceae. It is a tree of the northern savannahs, which can reach up to 30 m [
12]. Its curative proprieties are due to abundant active secondary metabolites such as flavonoids [
13] and polyphenols [
3]. Previous studies on
F. sycomorus extracts shows antibacterial effects [
14]; a relaxant activity on muscles and anaesthetic properties [
15]; antioxidant and antidiabetic properties [
16‐
19]; anticonvulsants and antiepileptic properties [
16]. The population traditionally use
F. sycomorus for the treatment of behavioral manifestations related to anxiety and depression. However, no scientific study was made to confirm or to infirm this use of
F. sycomorus. In a recent study, Tijani et al. [
20] showed that the methanolic extract of the bark of this plant has a significant anxiolytic activity when tested in three behavioral models: Elevated plus maze, Elevated zero maze and Open field test.
Using the stem bark of
Morus alba, a plant of the same family, Muhammad et al. [
21,
22] indicated an high correlation and regression between phenolic contents and antioxidant potentials of the extracts, which effective free radical inhibition and CNS-depressant activity. It is now widely known that there is a high rate of comorbid anxiety and depressive disorders in the population. More than 70% of individuals with depressive disorders also have anxiety symptoms [
23]. In the other hand, similar to other Moraceae, the stem bark extract of this plant possesses anticonvulsant activity. It appeared reasonable for us to explore the effect of this extract with both anxiolytic and anticonvulsant activities in some animal models of depression. We thus used the unpredictable chronic mild stress model which is considered by many laboratories to be the animal model of depression that has the greatest validity and translational potential [
24,
25].
In this context, in the present study we decided to determine the effect of the hydromethanolic extract of the stem bark of F. sycomorus on depression, anxiety and amnesia caused by chronic mild unpredictable stress on rats. The bioactive compound were assess by the LC-MS, the antidepressant effect and anxiolytic effect were studied in sucrose consumption test and in elevated plus maze test while the effect on memory was investigated in Y-maze test. Since sucrose preference is attenuated by a diversity of chronic stressors, including chronic mild and unpredictable stress, it was performed twice (before and after UCMS) to assess anhedonic effects in rats. The antioxidant activities of the extract using total glutathione (GSH) content and Lipid peroxidation (measured as malondialdehyde; MDA) in the brain homogenate were taken as reliable indicators for the contribution of free radical generation in oxidative stress.
Discussion
The present study investigated the effects of hydromethanolic extract of F. sycomorus stem barks on depression, anxiety and memory impairment induced by unpredictable chronic mild stress (UCMS) and on the oxidative stress status of the brain, known for its implication in neuropsychiatric disorders. Our results provide additional evidence regarding the involvement of the unpredictable chronic mild stress (UCMS) in the genesis of depression, anxiety and memory impairment by in rats and the role of the oxidative stress is the biological process.
The UCMS model of depression consist of the presentation of a series of varied and unpredictable environmental stressors. In the present study, as stressor, we used food and water deprivation, wet cages, isolation, forced swimming at 20 °C and flashing light. Following such exposure, animals have been reported to exhibit a persistent reduction in responsiveness to pleasurable stimuli, measured by a decrease in their consumption of 1% sucrose solution [
36]. Reductions in sucrose consumption produced by UCMS procedure have been shown to be reversed by chronic treatment with either tricyclic antidepressants or Selective Serotonin Reuptake Inhibitors (SSRIs) [
28].
During our study, animals in a normal psychological state showed a preference for the sucrose solution. However, after 30 days of UCMS there was a significant reduction of sucrose intake (
p < 0.001), suggesting the loss of pleasure to all animals exposed to the stressor, in comparison with normal animals. These results are in accordance with those obtained by Willner et al. [
37], and can be explained by an inactivation of the sub cortical regions of the brain as well as the cortex zones involved in reward and motivation. The treatment with
F. sycomorus extracts has corrected this loss of pleasure for treated animals. In fact, the pre-treatment of animals subject to UCMS with
F. sycomorus hydromethanolic extract at a dose of 100 mg/kg has caused a significant increase of sucrose consumption.
Anhedonia being known as a clinical manifestation of depression [
38], these results suggest that the hydromethanolic extract of
F. sycomorus possesses anti-depressive properties in rats. To confirm this hypothesis, the forced swimming test was used. This test is one of the most commonly used animal models for estimating the anti-depression effects of different classes of drugs in rodents [
39].
This test is based on the observation that animals, after initial escape-oriented movements, develop an immobile posture when placed in an inescapable stressful situation. When antidepressant treatments are given prior to the tests, the subjects will actively persist engaging in escape-directed behavior for longer periods of time than after vehicle treatment [
40]. In the present study, a significant antidepressant-like effect of the extract was observed at the dose of 100 mg/kg (
p < 0.05). However this effect was less than that of imipramine (20 mg/kg), a tricyclic antidepressant used as standard drug in this test (
p < 0.01).
Considering the monoaminergic hypothesis of depression we can thus suggest that the results described above may be explained by the fact that the UCMS impairment of monoaminergic neurotransmission, mainly adrenaline and serotonin and the treatment with the hydromethanolic extract of
F. sycomorus improve the brain level of these neurotransmitters. In fact, it is clearly established that these neurotransmitters are involved in the expression of an antidepressant-like effect in behavioral despair models of depression [
41]. The rats submitted to UCMS exhibited a longer latency to feed while the chronic administration of the extract at all the doses used, as well as imipramine significantly reduce the latency in feeding behavior when compared to the UCMS group (
p < 0.01). This result can be explained by the antidepressant activity of the extract, but it is also well know that compounds with anxiolytic properties can induce positive response in this test, reducing the latency of the animals. In order to be more confident about the nature of the results, the same doses of the extract were used in the elevated plus maze test.
The elevated plus maze test is one of the most used tests to study compounds with anxiolytic effects on rodents [
42]. It is based on the capacity of these rodents to avoid opened arms of the maze which, also in elevation is very anxiogenic, and to prefer closed arms which ensure security. After 30 days of UCMS, rats of the saline group (UCMS without any treatment) showed a significant increase on the time spent in the closed arm (
p < 0.001), in comparison to control animals. These results suggest that the UCMS has effectively contributed in the development of anxiety in animals. This result has also been reported by Frih et al. [
26] and may be due to the inhibitory activity of the GABAergic system [
43]. On the other hand, the treatment of anhedonic rats with
F. sycomorus extracts significantly reduced the number of entries in the closed arms (100 and 200 mg/kg,
p < 0.001) and the number of time spent in the closed arms (100 mg/kg,
p < 0.05). Concomitantly, this treatment has increased the number of entries in the opened arms and the time spent in these arms. These results suggest that the hydromethanolic extract of
F. sycomorus may possess anxiolytic properties. These anxiolytic properties may be due to the action of compounds contained in the extract to interfere with the receptor complex of GABA and/or of the antagonistic action of these compounds on the receptors, causing an increase of the GABA activity [
43]. Therefore, the between-group differences noted in the NFST may be also attributed to the anxiolytic effect of the extract.
In the elevated plus maze test, the number of rearing, of grooming and of head dipping have been registered. According to Augustsson [
44], the reduction of these different parameters indicate a reduction in anxiety behavior. Our results on these three parameters confirm the conclusions taken above on the anxiogenic activity of UCMS and the anxiolytic activity of the methanolic extract of
F. sycomorus bark. In fact, we have noticed a significant increase in the number of rearing (
p < 0.01) and head dipping (
p < 0.05) in the UCMS-non treated group in comparison to normal group. The treatment with the plant extract has caused a significant decrease of the number of rearing and head dipping (200 mg/kg,
p < 0.01) while the effect of imipramine was more significantly important in the inhibition of the head dipping behavior (
p < 0.001).
In the present study, the results showed that, only rats that experience chronic stress would show impaired performance in the Y-maze test. However those that were exposed to the UCMS and hydromethanolic extract of F. sycomorus at doses of 100 mg/kg and 200 mg/kg have significantly improved performance in comparison to stress alone (p < 0.01) in the same test. The effect of imipramine (20 mg/kg) in the prevention of this trouble was more significant than that of the plant extract (p < 0.001). These results clearly indicated that UCMS impair the spatial memory of rat and the hydromethanolic extract of F. sycomorus was able to prevent this deficit. However, at this level of our study, it is difficult for us to hypothesize that only rats exposed to UCMS could have their performance improved in Y-Maze or not. The specific effect of this extract on the memory remains to be determined by a more appropriate methodology.
This experimental model of the memory impairment was also experienced by Henningsen et al. [
45]. In fact, a very close comorbidity exists between stress and neurologic damages. Stress causes at the endocrine level a modification of the circulating level of glucocorticoids such as cortisol, but equally an atrophy of the dendrites of the hippocampus and of the cortex, as well as high expression of the CRH [
46]. This process at the end causes a disruption of the working memory as well as the reduction of the neurotransmitters activity involved in the process of memory [
47]. Some receptors as NMDA receptors are involved in several processes in the central nervous system including synaptic plasticity, which reflects learning and memory. Antidepressant drugs could reverse the negative physiological effects of chronic stress and depression by either normalizing or increasing NMDA receptors, which play a role in learning and memory [
48,
49].
It’s known that flavonoids are powerful antioxidants which act via the suppression of ROS (Reactive Oxygen Species) formation, either by enzyme inhibition or by chelation of oligo-elements involved in the generation of free radicals; by trapping of ROS and/ or rising the regulation of anti-oxidants [
50]. This property of flavonoids may explain the beneficial effect of our extract on chronic stress as shown in the present study. Moreover, the LC-MS analysis, in positive mode (ES+) has shown that the major compound in the
F. sycomorus hydromethanolic extract was a glycoside flavonoid, with the characteristics of the racemosic acid (C
22H
28O
14). Racemosic acid has been isolated for the first time in
F. racemosa [
51]. To date, only its anti-oxidant and its anti-inflammatory properties by the inhibition of the Cox-1 and of 5-LOX is well established [
51]. To the best of our knowledge, the results of this work indicate for the first time the protective effects of the methanolic extract of
F. sycomorus on anxiety and mood disorders.
Our previous studies strongly suggest the central role of the oxidative stress in the neuropsychiatric disorders [
8‐
10]. In the present study there was a decreased level of GSH and increased level of MDA in the rats that experienced chronic stress compared to normal rats. After 30 days, those that were exposed to the UCMS and treated with the hydromethanolic extract of
F. sycomorus at doses of 100 mg/kg and 200 mg/kg showed an increase in the GSH level with a concomitant decrease of MDA in the brain homogenate. The involvement of ROS in the pathogenesis of depressive disorders was confirmed during this study by the positive and significant correlation established between the swimming time vs. GSH and between sucrose preferences vs. GSH, but also the negative correlation although not significant between Swimming time vs. MDA and Sucrose preference vs. MDA. These results suggest that the plant extract has an in vivo antioxidant activity and is capable of ameliorating the effect of ROS in the brain of rats. Grases et al. [
52] also state that depression and anxiety are related to lowered plasma concentrations of antioxidants. Despite these results, some limitations need to be brought to the present work. Due to some technical difficulties we did not separate parts of brains and the biochemical estimations were done using the entire brain rather than the hippocampus. The expression of some protein markers involved in mental disorders was not done. These issues will be seriously taken into account in our upcoming investigations for better liability of the results.