Mood Disorders and the Neurotransmitter Networks Norepinephrine and γ-Aminobutyric Acid (GABA)
9 min read In this article
A concise overview of mood disorders and their neurobiology: the mood spectrum, how to tell unipolar from bipolar depression, mixed features and disease progression, the roles of norepinephrine (NE) and γ-aminobutyric acid (GABA), the main biological hypotheses of depression, and how linking symptoms to brain circuits can guide treatment.
Description of mood disorders
Mood disorders are conditions in which a disturbance of mood is the central feature. The DSM-5-TR groups them into two families (American Psychiatric Association [APA], 2022):
- Depressive disorders: major depressive disorder (MDD), persistent depressive disorder (dysthymia), premenstrual dysphoric disorder and disruptive mood dysregulation disorder. MDD requires at least five of nine symptoms for at least 2 weeks, including depressed mood or loss of interest or pleasure (anhedonia).
- Bipolar and related disorders: bipolar I disorder (at least one manic episode lasting 1 week or more, or requiring hospitalization), bipolar II disorder (hypomania lasting at least 4 days plus major depression), and cyclothymic disorder.
Mood disorders affect not only emotion but also energy, sleep, appetite, thinking, motivation and physical health. They are leading causes of disability worldwide.
Mood spectrum
Rather than separate boxes, many clinicians view mood disorders as a spectrum running from pure depression to pure mania, with many mixed and “soft” bipolar presentations between them (Stahl, 2021).
MDD, dysthymia
both poles at once
depression + hypomania
full mania
The spectrum view matters clinically. Patients in the middle often look “depressed” but respond poorly, or even get worse, with standard antidepressant monotherapy.
Distinguishing unipolar depression from bipolar depression
A depressive episode looks similar in both disorders, and many people with bipolar disorder first present with depression, so misdiagnosis is common. Clues that point toward bipolar depression include:
| Feature | More suggestive of bipolar depression |
|---|---|
| Family history | A first-degree relative with bipolar disorder |
| Age at onset | Early onset (adolescence or early 20s) |
| Course | Many brief, recurrent episodes; abrupt onset and offset |
| Symptom profile | Atypical features (hypersomnia, increased appetite, leaden heaviness), psychotic features, psychomotor slowing |
| Response to antidepressants | Switch into hypomania or mania, rapid “wear-off,” or agitation and irritability |
| Timing | Postpartum onset |
| Mixed symptoms | Racing thoughts, irritability or increased energy during depression |
Clinicians should always ask about past episodes of elevated mood. Screening tools such as the Mood Disorder Questionnaire can help (Hirschfeld et al., 2000).
Mixed features: are mood disorders progressive?
The DSM-5 “with mixed features” specifier recognizes episodes in which at least three symptoms of the opposite pole occur during a depressive or manic episode, such as depression with racing thoughts and increased energy (APA, 2022). Mixed states are associated with greater severity, higher suicide risk and poorer response to antidepressants.
Evidence suggests that, for many patients, mood disorders can be progressive:
- Kindling: early episodes are often triggered by stress, while later episodes can occur spontaneously, with shorter well periods between them (Post, 1992).
- Residual symptoms after incomplete recovery predict relapse.
- Repeated or untreated episodes are linked to more cognitive impairment and lower treatment response.
This supports early diagnosis, treatment to full remission (not just “response”), and relapse prevention.
Neurobiology of mood disorders
Mood disorders are not caused by one chemical “imbalance.” They involve interacting neurotransmitter systems, brain circuits, stress hormones, inflammation, neurotrophic factors and genetic vulnerability. Two networks are especially important: norepinephrine and GABA.
Neurotransmitters
| Norepinephrine (NE) | GABA | |
|---|---|---|
| Role | Arousal, attention, energy, alertness, stress response | The brain’s main inhibitory neurotransmitter: calming, reducing anxiety, regulating sleep |
| Where it comes from | Locus coeruleus in the brainstem, projecting to the prefrontal cortex, limbic system, hypothalamus, cerebellum and spinal cord | Interneurons throughout the brain; made from glutamate by glutamic acid decarboxylase |
| Key receptors | α1 and β1 (postsynaptic); α2 (presynaptic autoreceptors that switch off NE release) | GABA-A (chloride channel; site of benzodiazepines and neurosteroids); GABA-B (G-protein coupled) |
| Inactivation | Reuptake by the NE transporter (NET); breakdown by MAO and COMT | Reuptake by GABA transporters (GAT) |
| Link to mood symptoms | Low NE activity: fatigue, poor concentration, low energy. Excess NE: anxiety, agitation, insomnia, mania | Low GABA tone: anxiety, insomnia, irritability; altered GABA function has been reported in depression |
| Drugs acting on it | SNRIs, TCAs, bupropion (NDRI), mirtazapine (α2 antagonist), MAOIs | Benzodiazepines (short-term); neurosteroid GABA-A modulators (brexanolone, zuranolone) for postpartum depression |
Serotonin, dopamine, glutamate and histamine interact with these systems. For example, NE and serotonin regulate each other, and GABA interneurons control glutamate output.
The monoamine hypothesis of depression
The classic monoamine hypothesis proposed that depression results from a deficiency of monoamines (serotonin, norepinephrine and dopamine). It came from early observations: reserpine, which depletes monoamines, could cause depression, while MAOIs and tricyclic antidepressants, which increase them, relieved it.
The hypothesis has major limitations. Antidepressants raise monoamine levels within hours, but mood improves only after weeks. Depleting monoamines does not cause depression in most healthy people. An umbrella review found no consistent evidence that depression is caused by low serotonin (Moncrieff et al., 2023), although that conclusion has been debated. Monoamines remain important treatment targets, but they are not the full explanation of the cause.
The monoamine receptor hypothesis and neurotrophic factors
The monoamine receptor hypothesis shifted the focus from neurotransmitter levels to receptors and downstream signaling. It proposes that low monoamine levels lead to compensatory up-regulation of receptors. Antidepressants then gradually down-regulate them, which fits the delay of several weeks before benefit.
These receptor changes alter gene expression, including genes for brain-derived neurotrophic factor (BDNF), which supports the survival and growth of neurons and synapses. Chronic stress reduces BDNF, especially in the hippocampus and prefrontal cortex, and antidepressant treatment increases it (Duman & Monteggia, 2006).
Beyond monoamines: the neuroplasticity and neuroprogression hypothesis of depression
Current models view depression as a disorder of impaired neuroplasticity, meaning a reduced ability of neural circuits to adapt:
- Chronic stress and HPA-axis overactivity: high cortisol may contribute to dendritic atrophy and reduced hippocampal volume.
- Inflammation and oxidative stress: raised inflammatory cytokines in some patients may impair neurotransmission and neurogenesis.
- Neuroprogression: repeated episodes may cause cumulative changes in brain structure and function, which could explain worsening course and treatment resistance (Berk et al., 2011).
- Glutamate and rapid-acting treatments: ketamine and esketamine (NMDA receptor antagonists) can relieve depression within hours. They appear to trigger glutamate release, BDNF signaling and rapid synapse formation, which strongly supports the neuroplasticity model.
Symptoms and circuits in mood disorders
Each symptom of depression or mania can be linked, as a working model, to a brain circuit regulated by particular neurotransmitters (Stahl, 2021):
| Symptom | Key brain regions | Main neurotransmitters |
|---|---|---|
| Depressed mood | Amygdala, ventromedial prefrontal cortex | 5-HT, NE, DA |
| Anhedonia / low motivation | Nucleus accumbens (reward circuit), prefrontal cortex | DA, NE |
| Fatigue / low energy | Prefrontal cortex, striatum, spinal cord | NE, DA |
| Poor concentration | Dorsolateral prefrontal cortex | NE, DA |
| Sleep disturbance | Hypothalamus, thalamus, brainstem | 5-HT, NE, GABA, histamine |
| Appetite / weight change | Hypothalamus | 5-HT, NE |
| Psychomotor agitation or slowing | Striatum, cerebellum | DA, NE |
| Guilt / worthlessness / suicidality | Ventromedial and orbitofrontal cortex, amygdala | 5-HT, NE |
| Anxiety (common comorbidity) | Amygdala, cortico-striato-thalamo-cortical loops | GABA, 5-HT, NE |
Symptom-based treatment selections
Because no single drug fixes every symptom, a symptom-based approach breaks the diagnosis into its symptoms, links each to a likely circuit and neurotransmitter, and chooses or combines treatments that target them. This is especially useful for residual symptoms that persist after initial treatment:
- Fatigue, low energy, poor concentration (often “NE/DA deficient”): consider agents that boost NE and DA, such as bupropion, SNRIs or certain augmenting agents.
- Insomnia and anxiety (often linked to excess NE arousal or low GABA tone): consider more sedating agents such as mirtazapine or trazodone, or psychotherapy such as CBT-I. Use benzodiazepines only short-term and with caution.
- Bipolar depression: avoid antidepressant monotherapy because of the risk of switching or mixed states. Evidence-based options include lithium, quetiapine, lurasidone, cariprazine, lumateperone and olanzapine-fluoxetine.
- Mixed features: mood stabilizers or atypical antipsychotics are preferred over antidepressant monotherapy.
- Treatment-resistant depression: consider augmentation, esketamine, neuromodulation (ECT, rTMS) and evidence-based psychotherapy.
Symptom-based selection is a clinical heuristic that complements, not replaces, evidence-based guidelines, careful diagnosis, safety monitoring and shared decision-making.
Note: This article is for education only and is not medical advice. Medication decisions must be made by a qualified prescriber who knows the patient.
Summary and call to action
Mood disorders lie on a spectrum from unipolar depression to bipolar I disorder, and mixed features and repeated episodes can make the illness progressively harder to treat. Recognizing bipolar clues early prevents harmful misdiagnosis. Neurobiologically, depression has moved beyond the simple monoamine hypothesis. It now involves receptor changes, neurotrophic factors such as BDNF, impaired neuroplasticity and neuroprogression. Norepinephrine networks drive arousal, energy and attention, and GABA networks provide the inhibition that calms anxiety and supports sleep. Linking each symptom to its circuit and neurotransmitter helps clinicians tailor treatment and aim for full remission.
Call to action: If you are a nursing, psychology or psychiatric nurse practitioner student working on an assignment about mood disorders, psychopharmacology or neurotransmitter networks, use this outline to structure your paper, and always support your discussion with current, peer-reviewed sources. Need help? Our qualified tutors can help you produce an original, well-referenced paper. Place your order today.
References
American Psychiatric Association. (2022). Diagnostic and statistical manual of mental disorders (5th ed., text rev.). https://doi.org/10.1176/appi.books.9780890425787
Berk, M., Kapczinski, F., Andreazza, A. C., Dean, O. M., Giorlando, F., Maes, M., Yücel, M., Gama, C. S., Dodd, S., Dean, B., Magalhães, P. V. S., Amminger, P., McGorry, P., & Malhi, G. S. (2011). Pathways underlying neuroprogression in bipolar disorder: Focus on inflammation, oxidative stress and neurotrophic factors. Neuroscience & Biobehavioral Reviews, 35(3), 804–817.
Duman, R. S., & Monteggia, L. M. (2006). A neurotrophic model for stress-related mood disorders. Biological Psychiatry, 59(12), 1116–1127.
Hirschfeld, R. M. A., Williams, J. B. W., Spitzer, R. L., Calabrese, J. R., Flynn, L., Keck, P. E., Jr., Lewis, L., McElroy, S. L., Post, R. M., Rapport, D. J., Russell, J. M., Sachs, G. S., & Zajecka, J. (2000). Development and validation of a screening instrument for bipolar spectrum disorder: The Mood Disorder Questionnaire. American Journal of Psychiatry, 157(11), 1873–1875.
Moncrieff, J., Cooper, R. E., Stockmann, T., Amendola, S., Hengartner, M. P., & Horowitz, M. A. (2023). The serotonin theory of depression: A systematic umbrella review of the evidence. Molecular Psychiatry, 28(8), 3243–3256. https://doi.org/10.1038/s41380-022-01661-0
Post, R. M. (1992). Transduction of psychosocial stress into the neurobiology of recurrent affective disorder. American Journal of Psychiatry, 149(8), 999–1010.
Stahl, S. M. (2021). Stahl’s essential psychopharmacology: Neuroscientific basis and practical applications (5th ed.). Cambridge University Press.
