Depression is a mood disorder that causes people to feel unhappy and discontent. They may feel unmotivated and have periods of extreme sadness and negativity. It interferes with their daily lives and how they operate, affecting motivation, the ability to sleep, and other daily tasks.

So, what is the cause of depression? Is it because of our biology? Or is it more to do with our psychological state of mind? The biological explanation of depression explores the physical cause of the disorder by analysing the brain and the neurotransmitters involved in vital brain functions.

• We are going to explore the biological explanation of depression.
• First, we will establish the definition of biological depression.
• Then we will highlight the various biological explanations of depression, including the imbalance of neurotransmitters.
• We will also provide an evaluation of the biological explanation of depression.

Fig. 1 - Depression is a mood disorder.

Definition of Biological Depression

Biological depression is defined as a persistent state of low mood, characterised by feelings of sadness, discontent, and hopelessness. Depression lasts for longer than a few days, usually for weeks, months or even years, and affects everyday life. The severity can differ, so some may have mild depression whilst others have severe depression.

Biological Explanation of Depression

The biological explanation of depression suggests that an imbalance in neurotransmitters within the brain causes depression, most notably serotonin. Other neurotransmitters implicated in the biological explanation include noradrenaline and dopamine. Biological explanations of depression also examine the influence of our genes and abnormal brain structures.

One of the main theories surrounding the biological explanation of depression is depletion. This means a reduction of specific neurotransmitters, specifically serotonin and noradrenaline (also known as norepinephrine). As we mentioned above, biological causes for depression also include abnormal brain structures and the influence of our genes (such as the inheritance of depression through your DNA if your family members have depression).

Serotonin and norepinephrine have been closely linked to depression:

• Serotonin is essential in mood modulation and feeling happy and relaxed, among other functions.

• Norepinephrine helps you concentrate and increases your awareness/alertness, and is essential in regulating executive functions (the higher-level functions associated with our ability to problem-solve etc.).

Abnormal brain structures concern how our brains can be affected by depression, in that depression causes structural changes in the brain and how existing abnormalities result in depression (which can also relate to our genes!)

According to Pandya et al. (2012), the amygdala, hippocampus, and the dorsomedial thalamus are all brain regions associated with depression.

However, a lot of research has found multiple areas of abnormal brain structures in patients with depression. Those with anxiety, for instance, also show similar abnormalities, so we cannot definitely say all of these areas are associated fully with depression. We also can't say whether they cause depression or result from depression. This issue is known as not being able to establish cause and effect.

Before we delve into the neurotransmitters a bit more, let's first establish how a nerve impulse works and the role of neurotransmitters in neurone function and communication, which will help us understand their importance.

Neurotransmitters

Typically, a nearby cell will trigger an action potential in your neurone (this is the nerve impulse), and this travels down the axon of your neurone towards the axon terminal and the synapse like a wave.

Once the impulse reaches the axon terminal, the neurotransmitters are released into the synaptic cleft from the presynaptic neurone (the neurone sending the impulse) to the postsynaptic neurone (the neurone receiving the impulse). The neurotransmitter then binds to receptors on the postsynaptic neurone. Depending on the receptor it activates and binds to, the neurotransmitter can affect the postsynaptic neurone.

Fig. 1 - Neurotransmitters affect nerve impulses¹.

After, the neurotransmitters go through reuptake and degradation, so the postsynaptic neurone does not constantly fire (if the neurotransmitters were to remain in the synaptic cleft, they would continue to bind to the receptors and activate your neurone, which usually isn't necessary).

The biological explanation of depression argues that disturbances with your neurotransmitters, most notably serotonin and norepinephrine, cause depression.

Biological Explanation of Depression: Chemical imbalance

As we mentioned above, neurotransmitters are involved in the communication between neurones, and in depression, we are explicitly focused on serotonin and norepinephrine. That's not to say other neurotransmitters are not involved in depression; for instance, dopamine is also associated with the disorder, but for your exam, we will focus on the first two.

Serotonin and Depression

The dominant theory on serotonin and depression is that low serotonin levels produce depression-like symptoms. This theory is quite old, close to 50 years, and is known as the 'serotonin hypothesis'.

Two different mechanisms in the serotonin hypothesis in depression exist.

• First, a reduced level of serotonin in the brain lowers the availability of serotonin overall, and second, there may be some defect or abnormality in the receptors for serotonin on the postsynaptic neurones.
• Both reduce the effects of serotonin, either by limiting the availability of serotonin so it cannot do its job correctly or by restricting the ability of your neurones to respond to the release of serotonin.

As we now know, serotonin is important in mood regulation and feelings of happiness and relaxation. It helps to ward off feelings of fear and anxiety. According to this theory, when serotonin levels are low, it cannot regulate your mood correctly or help you feel happy and relaxed.

Certain biological treatments focus on affecting the levels of neurotransmitters.

Norepinephrine and Depression

Norepinephrine is associated with our ability to concentrate and our alertness/awareness. In those with depression, low levels of norepinephrine cause lower levels of concentration, lower energy levels (which explains why those with depression have motivation issues), and reduced levels of cognitive ability.

Experiments on mice have shown that when the systems involving norepinephrine are enhanced (by increasing norepinephrine transmission), the animals are more resistant and protected from depression-like symptoms during stressful situations (Moret & Briley, 2011).

SNRIs (serotonin-norepinephrine reuptake inhibitors), a biological drug treatment work by preventing the reuptake of norepinephrine.

Biological treatments for depression: antidepressants and SSRIs/SNRIs (selective serotonin reuptake inhibitors/serotonin-norepinephrine reuptake inhibitors)

With the theory of low serotonin levels and issues with norepinephrine causing depression or depression-like symptoms, biological treatments in the form of drugs were introduced to help people with the disorder. These are known as antidepressants, and examples include SSRIs (selective serotonin reuptake inhibitors) and SNRIs (serotonin-norepinephrine reuptake inhibitors).

Research into SSRIs and SNRIs has shown that they effectively treat depression. However, many studies suggest that they are most effective when used alongside psychological treatments, such as cognitive behavioural therapy (CBT). This shows there is some proof and validity to the biological explanation of depression, as treating it biologically has shown an effect! Depression, overall, is biologically treatable.

Viable treatment options are also helpful for the patients, although there are some side effects (such as nausea and weight gain) that we cannot ignore.

Evaluation of the Biological Explanation of Depression

As with all biological explanations, the biological explanation of depression has issues with multiple things. Most notably, these are issues with:

• Reductionism: the biological explanation reduces the disorder to a purely physical perspective and ignores other important psychological factors in explaining and treating the disorder.

• Being too simplistic

• Conflicting evidence/multiple explanations: as we mentioned above, low serotonin levels were the leading theory behind depression for a long time; however, more recent research suggests this is not the case, and this theory is too simplistic.