Depression is generally thought of as having feelings of sadness and hopelessness from time to time. However, Major Depressive Disorder, is a debilitating mental disorder characterised by feelings of low mood and aversion to activities that they person may have gained pleasure from before. Key signs or symptoms are changes in mood, thinking, behavior and physical changes and usually accompanies changes in either eating too little/ overeating, sleeping too much/insomnia, fatigue, loss of energy, problems concentrating, irritability and pains or digestive problems that are resistant to treatment. Depressed people may become preoccupied with feelings of low self-worth, inappropriate guilt or regret, helplessness, hopelessness and self hatred. More severe forms of MDD may accompany psychosis/ loss of contact with reality which can include hallucinations or delusions (The Ministry of Health, 2012).

Depression can be caused by many things, including normal reactions to life events such as the death of a loved one. However, over time, the cause of depression has evolved to include psychological, psychosocial, hereditary, evolutionary and biological factors. Psychological treatments focus on the personality, interpersonal communication and learning (eg counselling, psychotherapy), whereas biological theories focus on explaining that the depletion of particular chemicals (neurotransmitters); serotonin, norepinephrine and dopamine, are the main cause of depression and focus on treatment with antidepressant medications (The Association for Behavioral and Cognitive Therapies [ABCT], n.d).

Biological explanations postulate that depression must be linked to changes in the brain. One of the major theories of the biological explanation is the Monoamine Theory of Depression. This theory postulates that depression is caused by having deficient levels of neurotransmitters; dopamine, serotonin and norephedrine. These neurotransmitters are essential to the transfer of information from neuron to neuron, as they act as chemical messengers across neuron synapses. With a depletion of chemical messengers, the information is unable to be passed to the next neuron and therefore this is where the problem is.

Each neurotransmitter has a specific objective, for example serotonin is concerned with regulating other neurotransmitter systems and therefore if there is a depletion in serotonin, then the other neurotransmitters will be out of sync and act in unusual and erratic ways. According to this theory, if there are low levels of serotonin then serotonin will not be passed on at neuronal synapses, and therefore if the job of serotonin is to control other neurotransmitter systems, then consequently, other essential neurotransmitters, dopamine and norephedrine will be unable to carry out their essential tasks (Hoyer, Hanner & Martin, 2001). Serotonin receptors are found widely in the peripheral and central nervous system and produce inhibitory and excitatory responses. It is thought that these receptors influence biological and neurological processes such as anxiety aggression, appetite, cognition, learning, memory, mood, nausea, sleep and thermoregulation (Hoyer et al. 2001) A study conducted in New Zealand, examined a cohort of normal people over a period of time, and hypothesised why stressful experiences lead to depression in some people and not others. It was found that a functional polymorphism in the in the promoter region of the serotonin (5-HT) transporter gene was discovered to moderate and influence the reaction to stressful life events of depression. It was also found that individuals who exhibited one or two copies of the short allele, 5-HTT, showed more depressive symptoms, were more inclined to be suicidal and be diagnosed with clinical depression (Caspi et al., 2003).

If there is a depletion in serotonin, then as already stated above, the other neurotransmitters will be unable to complete their essential tasks within the body