**3.2 Neuroplasticity**

Neuroplasticity is the brain's ability to modify, change and adapt structure and function throughout life in response to experience. It is highly influenced by physical activity [12]. The increased neuroplastic capacity explains the mental health benefits of various somatic psychiatric treatments, such as selective serotonin reuptake inhibitors and trans-magnetic stimulation [12].

Several systemic factors, modifiable through exercise training, must be present to facilitate neuroplastic changes. Two essential factors include neurotrophins and low neuroinflammation. Neurotrophic growth factors increase with exercise and are linked to neuroplasticity, including modulation of brain-derived neurotrophic factor (BDNF), vascular endothelial growth factor (VEGF), and insulin-like growth factor 1 (IGF-1) [12]. Studies have shown that exercise increases BDNF production

in the population with and without depression [23]. BDNF plays an important role in regulating synapse plasticity, affecting serotonin production in the hippocampus [23], and consequently impacts depressive symptoms. Also, exercise-enhancing VEGF and changing the vascular microenvironment were conducive to delivering oxygen-optimising neurogenesis [24].

#### **3.3 Monoamines neurotransmitters (serotonin, norepinephrine, and dopamine)**

Serotonin, norepinephrine, and dopamine are all monoamine neurotransmitters that modulate our mood. Depression has been consistently associated with impaired monoaminergic neurotransmission [25].

The cytokines can influence the synthesis, release, and reuptake of neurotransmitters [26]. Exercise may also influence depression by elevating the levels of endorphins and monoamines and regulating the hypothalamic-pituitary-adrenal axis.
