The Role of Neurotransmitters in Mood Disorders

Millions of individuals worldwide are impacted by mood disorders, which are mental health diseases that include bipolar disorder and depression. The causes of these disorders have been the subject of extensive investigation, with neurotransmitters appearing as a central theme in the understanding of their underlying mechanisms. In the brain, neurotransmitters are chemical messengers that control mood, feelings, thought processes, and general mental health. Monoamine neurotransmitters, which include serotonin, dopamine, and norepinephrine, are the most significant neurotransmitters in the pathophysiology of mood disorders and the processes by which antidepressants function (1).

Serotonin

The neurotransmitter most frequently linked to depression is serotonin (3). Serotonin suppresses pain and supports mood, hunger, and sleep regulation. The theory that certain depressed individuals have lower serotonin transmission is supported by research. Low levels of serotonin byproducts have been associated with an increased risk of suicide (4). The frequency of suicide and the emergence of violent behaviours were linked to a high quantity of 5-hydroxyindoleacetic acid (5-HIAA), a serotonin metabolite, in the cerebrospinal fluid (CSF) (2).

Studies have indicated that serotonin levels in the prefrontal cortex and hippocampus - two areas of the brain linked to major depressive disorder (MDD) - are frequently lower in depressed patients. The unpleasant emotional states, exhaustion, and lack of drive that are common in depressed people may be attributed to this deficiency. Selective serotonin reuptake inhibitors (SSRIs), which increase the availability of serotonin in the synaptic cleft, are widely used to treat depression, because the serotonin hypothesis of depression suggests that an imbalance or lack of serotonin causes mood abnormalities (5).

According to the serotonin hypothesis, depression's pathophysiology is caused by reduced serotonin pathway activation. Studies on "tryptophan depletion" - in which an acute dietary manipulation is used to produce a transient lowering of brain serotonin activity by diminishing the availability of its precursor amino acid, tryptophan - provide the strongest evidence that serotonin plays a role in the pathophysiology of depression. Tryptophan depletion may not cause significant mood changes in healthy individuals without depression risk factors. Patients who have recovered from depression and are not taking medication may exhibit temporary, clinically relevant symptoms of depression (5).

Low serotonin levels during the depressed phase of bipolar disease may worsen feelings of sadness, hopelessness, exhaustion, and difficulties focusing. These symptoms are similar to those of depression, which is frequently associated with serotonin insufficiency (9). Serotonin levels may change during manic or hypomanic episodes. Manic symptoms including increased energy and impulsivity may be associated with serotonin system hypersensitivity or overactivity. High levels of serotonin can occasionally cause a person with bipolar disorder to go from depression to mania, especially if the medication is the cause (antidepressants, for example) (8).

A large part of the pharmacological treatment for depression has been developed with the serotonin hypothesis in consideration, especially with the introduction of SSRIs. Serotonin is made more accessible in the brain by SSRIs because they prevent serotonin from being reabsorbed into neurons. Many individuals' symptoms of anxiety and depression have been demonstrated to be reduced by elevated serotonin levels. Recent research, however, shows the complexity of mood disorders may not be entirely explained by the serotonin hypothesis alone. SSRIs do not work for everyone. The theory that serotonin depletion is the only cause of mood disorders is complicated by the current understanding of how serotonin interacts with other neurotransmitters and brain systems to control mood (5).

Dopamine

Another important neurotransmitter that regulates mood is dopamine, which is very important when it comes to motivation, pleasure, and rewards (6). Dopamine is linked to the brain's reward system, where it affects our perception of and reaction to events that are enjoyable. It also affects motivation, focus, and mental processes including memory and learning. Abnormal dopamine levels have been linked to mood disorders such as bipolar disorder and depression. Depression symptoms such as anhedonia (the inability to feel pleasure), motivation deficit, and overall negativity can all be attributed to reduced dopamine activity. This is because dopamine dysfunction impairs the brain's capacity to control reward pathways, which makes it challenging for people to experience excitement or pleasure from activities they used to enjoy (6).

Manic and depressive episodes fluctuate in people with bipolar illness. Dopamine activity frequently rises during manic episodes, which can cause increased energy, adrenaline, and impulsive actions. The rise is short-lived, and a person may experience a depressive episode when their dopamine levels fall (7). A dopamine metabolite called homovanillic acid (HVA) is seen in lower concentrations in the CSF during depressive episodes but increases during manic episodes (2).

Medications that aim to block dopamine receptors or make more dopamine available are frequently used to treat mood disorders - such as antipsychotics, dopamine reuptake inhibitors, and mood stabilizers. To aid with depressive symptoms, many mood stabilizers such as lithium and antidepressants increase dopamine transmission (7). Antipsychotics, such as olanzapine and risperidone, block dopamine receptors. Antidepressants, such as bupropion, increase the amount of dopamine accessible in the brain by preventing the reuptake of dopamine and norepinephrine. Lithium, a mood stabilizer, affects brain dopamine levels, which helps control mood swings associated with bipolar disease.

Norepinephrine

Norepinephrine is essential for the body's stress response, arousal, and alertness. It aids in the body's reaction to stressful circumstances by taking part in the "fight-or-flight" response. Additionally, norepinephrine has a part in mood, sleep, and attention regulation. Abnormal norepinephrine levels in mood disorders have been seen in anxiety and depression. Depression and low norepinephrine levels are frequently linked, especially when lethargy, exhaustion, and cognitive impairment are present. Norepinephrine regulates energy and focus, and a shortage of it might lead to the fatigue and foggy thinking associated with depression (8). High and low norepinephrine levels are linked to manic episodes and depressive episodes, respectively, in individuals with bipolar disorder.

Norepinephrine levels are frequently raised in anxiety disorders, which can lead to hyperarousal, restlessness, and increased stress reactions (8). Anxiety disorders are characterized by excessive concern, agitation, and difficulties concentrating, which can be brought on by an overactivation of norepinephrine pathways in the brain. Treatments for mood disorders that target norepinephrine have been developed. Antidepressants in the class known as serotonin–norepinephrine reuptake inhibitors (SNRIs) increase the availability of serotonin and norepinephrine in the brain by preventing their reuptake. Since norepinephrine is involved in pain control, SNRIs are especially useful for patients who have physical symptoms such as fatigue and chronic pain in addition to depressive symptoms (8).

References

1. Neurotransmission in mood disorders. (n.d.). https://psychiatrie.lf1.cuni.cz/file/5882/Neurotransmission_in_mood_disorders.pdf
2. Lee, J. G., Woo, Y. S., Park, S. W., Seog, D., Seo, M. K., & Bahk, W. (2022). Neuromolecular etiology of bipolar disorder: possible therapeutic targets of mood stabilizers. Clinical Psychopharmacology and Neuroscience, 20(2), 228–239. https://doi.org/10.9758/cpn.2022.20.2.228
3. Sekhon, S., & Gupta, V. (2023, May 8). Mood disorder. StatPearls - NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK558911/
4. Harvard Health. (2022, January 10). Depression: chemicals and communication. https://www.health.harvard.edu/depression/depression-chemicals-and-communication
5. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8066851/
6. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5401767/
7. Ressler, K. J., & Nemeroff, C. B. (1999). Role of norepinephrine in the pathophysiology and treatment of mood disorders. Biological Psychiatry, 46(9), 1219–1233. https://doi.org/10.1016/s0006-3223(99)00127-4
8. Serotonin and norepinephrine reuptake inhibitors (SNRIs). (2019, October 5). Mayo Clinic. https://www.mayoclinic.org/diseases-conditions/depression/in-depth/antidepressants/art-20044970
9. Mahmood, T., & Silverstone, T. (2001). Serotonin and bipolar disorder. Journal of Affective Disorders, 66(1), 1–11. https://doi.org/10.1016/s0165-0327(00)00226-3