Restraint stress was a significant factor in the study's effects on various aspects of brain function, as shown by the development of anxiety- and depressive-like behaviors as well as a deficit in male rats' spatial learning and memory. This study showed how behavior changes as a result of exercise under conditions of ongoing constraint stress and its underlying mechanisms. We were able to show the beneficial effects of treadmill exercise on CRS-induced behavioral changes and brain monoamine and neurotransmitter levels. Our study concentrated on the molecular mechanisms governing these advancements in rat brain function.
In this investigation, CRS dramatically worsened spatial learning memory and increased anxiety- and depression-like behaviors. The TST showed that the stress group's animals were more immobile than the control group animals. Additionally, the animals in the stress group floated for a longer period of time than the animals in the stress + exercise group, who were more active during FST. The stressed group spent much more time in the dark compartment, according to LDT data, while exercise increased the amount of time spent in the light compartment without changing the number of tunnels connecting the two compartments. The OFT indirectly determined that exercise enhanced the stressed rats' locomotor activity slightly but not significantly.
Physiological benefits of exercise, especially its anxiolytic properties, have been discussed in studies (Kim & Han, 2016). In addition, a recent study demonstrated that exercise prior to CRS induction prevented anxiety and depressive symptoms (Lapmanee et al., 2017). It shown that treadmill exercise clearly had an effect on CRS-induced rats similar to an antidepressant.
In the MWM test, CRS rats displayed a decrease in time spent in the target quadrant, a sign of spatial memory or the ability to recall previously stored information, but the spatial memory of trained animals appeared to be strengthened. The MWM test results supported earlier studies by showing that physical activity reversed stress-related declines in spatial learning and memory (Lapmanee et al., 2017). Exercise has been shown to improve a deficiency in spatial learning caused by the CRS (Lapmanee et al., 2017) or a confluence of many stressors (Zou et al., 2010; Lapmanee et al., 2017). The aforementioned results demonstrate that exercise decreases anxiety- and depressive-like symptoms as well as stress-related deficiencies in spatial learning and memory, possibly via modulating the activity of the HPA axis (Pietrelli et al., 2018) or the serotonergic system (Shin et al., 2017).
Results of behavioral tests made it clear that there is a connection between chronic stress and a number of monoaminergic neurotransmissions (norepinephrine, dopamine, and serotonin). Central monoamine levels are known to change as a result of chronic stress (Alghasham & Rasheed, 2014; Akter et al., 2019). For instance, in the PFC and hypothalamus of animals under chronic stress have reduced levels of 5-HT (Oh et al., 2018). It has been shown that stress changes the amounts and metabolites of 5-HT and DA as well asthe transmission of these chemicals in the PFC and hippocampus (Torres et al., 2002; Mahar et al., 2014). The DA metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) has been reported to be present at higher concentrations in the hypothalamus, hippocampus, frontal cortex, and amygdala of rats (Inoue et al., 1994). Serotonin and its metabolite 5-hydroxyindolacetic acid (5-HIAA) levels were also increased in the rat PFC, nucleus accumbens, hypothalamus, and amygdala. These results cast doubt on the effects of exercise, which is known to modify monoaminergic targets to protect against stress-induced anxiety, depression, and memory loss. While the exact mechanisms are uncertain, exercise can alleviate stress in part through altering the signaling of brain monoamines (Chaouloff, 1989). Several stress-sensitive rodent brain regions, have shown enhanced monoamine activity including 5-HT and DA, following acute exercise sessions (Chaouloff, 1989; Goekint et al., 2012). Additionally, research in rat models indicates that regular exercise induces plasticity by the 5-HT and DA systems, which may alter the amount of activity of these neurotransmitters in areas of the brain that are resistant to stress (Greenwood et al., 2003; Greenwood et al., 2005; Foley & Fleshner, 2008). The effects of stress on BDNF levels, as well as the serotonergic and dopaminergic systems’ activity, are known to be mitigated by exercise (Clark et al., 2015; Heijnen et al., 2015). A full understanding of the mechanisms that physical activity uses to protect against stress could lead to the identification of novel targets for the treatment or prevention of disorders connected to stress. Dopamine has several different roles in the central nervous system (CNS), including controlling locomotor activity, neuroendocrine secretion, motivation, emotional reactions, learning, and memory (Björklund & Dunnett, 2007). In animal studies, aerobic exercise improves DA levels in the striatum, hypothalamus, midbrain, and brain stem, providing proof to the favorable effects of exercise on mood and memory (Foley & Fleshner, 2008).
In the study, treadmill exercise successfully prevented the anxiety and depressive-like behaviors in the rats who had undergone CRS, as well as the deterioration of their spatial learning and memory. It is well established that aerobic exercise improves cognitive abilities, especially spatial memory, which depends on the hippocampus (Suwabe et al., 2018). Regarding chemical mechanisms, it has been demonstrated that BDNF is necessary for regulating exercise-induced brain plasticity, which improves memory and learning (Voss et al., 2013). Studies have revealed that chronic stress and depressive-like symptoms are correlated with a decrease in BDNF synthesis and activity in the hippocampus and frontal cortex, and that lower BDNF levels are associated with increased susceptibility to stress and depression (Duman & Monteggia, 2006; Zhang et al., 2014).
In our study, rat in the exercise group showed significantly higher BDNF expression in the hypothalamus and PFC. Although an increase was observed in the corpus striatum and hippocampus, this increase was not significant. Previous research has shown that stress significantly reduced BDNF expression, especially in the hippocampus. The rise of BDNF following exercise was found to be associated with improvements in learning and memory, improved locomotion, a decrease in anxiety- and depressive-like behaviors. However, it might be claimed that exercise normalizes stress-induced changes not only through BDNF level but also by a process unrelated to BDNF.
Anxiety and panic are two unsettling emotional and behavioral outcomes that are closely associated with serotonergic system dysfunction in the brain (Canteras & Graeff, 2014). The 2-A subtype (5-HT2A) of the main excitatory 5-HT receptors in the brain is important in modulating the 5-HT response to stress (Carhart-Harris & Nutt, 2017). Variety of stressors increase the expression and activation of cortical 5-HT2A receptors (Murnane, 2019). One of the receptors involved in anxiety, stress, and fear responses is 5 HT-2C (Martin et al., 2013; Marcinkiewcz et al., 2016). Studies on experimental animals show that the anxiogenic effects is involved with stimulating these receptors (Greenwood et al., 2012; Vicente & Zangrossi Jr, 2012). Anxiety is reduced in mice with a 5HT2C receptor deletion, and it has been shown that inhibiting the 5HT2C receptor has anxiolytic effects (Wood et al., 2001; Martin et al., 2002).
HTR2A and HTR2C serotonin receptors are variably expressed in the brain, and changes in their activity in response to stress have been linked to depressive-like behaviors (Jiang et al., 2009; Martin et al., 2014). On the other hand, it was found that exercise produced resistance to the anxiety- and depression-like behaviors induced by HT2RC activation (Greenwood et al., 2012). Additionally, administration of an HTR2A antagonist ameliorated the downregulation of BDNF mRNA brought on by immobilization stress in the rat hippocampal region (Vaidya et al., 1999). In our work, we discovered considerably less HTR2A expression in the hypothalamus, hippocampus, PFC, and corpus striatum of the stress + exercise group animals compared to the stress group animals, which suggests that 5-HT signaling has been downregulated. However, there were no discernible differences in HTR2C expression between the groups.
Chronic stress is known to alter the synaptic input and excitability of dopamine receptor D1 (DRD1) and dopamine receptor D2 (DRD2) cells (Anderson et al., 2019) as well as cause DRD1-expressing neurons to atrophy (Shinohara et al., 2018). Repeated stress paradigms impair working memory because DRD1-expressing pyramidal cells, especially those in the PFC, show lower activity (Arnsten, 2015). In our research, we discovered that hippocampus DRD1 and DRD2 expression was significantly higher in the exercise group animals than the other groups. With the exception of DRD1 expression, animals in the exercise group displayed a similar pattern in their hypothalamus. Stress animals were exercised did not significantly benefit from exercise and DRD1 and DRD2 expression was downregulated in the hypothalamus of the stress + exercise group compared to the control group. It can be somewhat surprising to note that exercise negatively affects neurotransmitter levels in stress + exercise groups, despite the present result that exercise reduces stress-related behavioral abnormalities in rats. Possible cause here could be forced treadmill use. According to several studies, in addition to the detrimental physiological reactions connected with stress, forced exercise can also result in psychopathological reactions such as despair and anxiety (Bakshi & Kalin, 2000; Moraska et al., 2000). In one study, rats' anxiety levels were found to increase when forced to exercise (Uysal et al., 2015). The co-development of the two parameters may be the reason why these markers were lower in the stress + exercise group than in the stress group. This makes it possible to evaluate the effects of chronic stress in situations where it is first induced and then exercise is initiated after the stress becomes chronic.
Our research shown that treadmill exercise has anti-depressant and anxiolytic effects in addition to exerting preventive effects on memory dysfunctions via regulating neuroprotective activity through numerous monoamine and neurotransmitter pathways. The molecular and cellular processes behind the antidepressant-like and anti-anxiolytic effects of exercise on the CRS-exposed rat may be represented by increased monoamine neurotransmitters.
In conclusion, exercise can counteract stress-induced anxiety and depressive-like behaviors as well as memory impairment by inducing long-term adaptations in various monoaminergic systems and expression of various proteins in the brain. Evidence suggests that voluntary moderate-intensity exercise may be useful in delaying the onset of mood disorders and memory loss in circumstances where future stress exposure is anticipated. Exercise seems to be a perfect, low-cost solution for both preventing and treating mood and cognitive problems in those who are under stress.