Neurobiology of Bipolar Mood Disorder and Major Depressive Disorder

Introduction

I. NEUROBIOLOGY OF MAJOR DEPRESSIVE DISORDER (MDD)

A. Monoamine Hypothesis (Classic)

• Serotonin deficiency is implicated in depressed mood, anxiety, irritability, and sleep disturbance. This forms the basis of SSRIs and SNRIs.
• Norepinephrine deficiency contributes to fatigue, impaired attention, and psychomotor retardation.
• Dopamine deficiency is linked to anhedonia, loss of motivation, and reduced positive affect.

B. Monoamine Receptor Hypothesis

C. HPA Axis Dysregulation (Neuroendocrine Hypothesis)

• Hypercortisolemia: 20-40% of depressed outpatients and 40-60% of inpatients show elevated urinary free cortisol and 24-hour plasma cortisol.
• Mechanism: Increased CRH from the hypothalamus + decreased hippocampal feedback inhibition.
• Dexamethasone Suppression Test (DST): Depressed patients show initial suppression followed by "escape," reflecting impaired negative feedback. Abnormal in 40-60% of hospitalized depressives, but neither sensitive nor specific enough for diagnostic use.
• Pathological consequence: Chronic hypercortisolemia causes hippocampal neuronal damage (glucocorticoid toxicity), contributing to structural changes.

D. Other Endocrine Abnormalities

E. Brain-Derived Neurotrophic Factor (BDNF) and Neurotrophin Hypothesis

• Stress and glucocorticoids decrease BDNF expression, particularly in the hippocampus.
• Reduced BDNF leads to atrophy of hippocampal neurons, reducing hippocampal volume.
• Antidepressants (all classes) and ECT increase BDNF expression - this temporal delay in BDNF upregulation correlates with the 2-4 week lag in clinical response.
• The "neurotrophic hypothesis" proposes: stress → decreased BDNF → hippocampal atrophy → depressive episode; antidepressants → restored BDNF → neuroplasticity → recovery.

F. Neuroplasticity and Neuroprogression Hypothesis

1. Stress activates glutamate (NMDA) excess at synapses.
2. Glutamate excess, combined with reduced BDNF, leads to synaptic loss and dendritic atrophy.
3. Repeated depressive episodes cause cumulative neuronal damage (neuroprogression).
4. This explains: cognitive decline worsening with each episode, treatment resistance, and structural brain changes seen on MRI.
5. Ketamine (an NMDA receptor antagonist) produces rapid antidepressant effects within hours - bypassing the slow monoamine mechanism - by rapidly restoring synaptic connections.

G. GABAergic Dysfunction

• GABA is the principal inhibitory neurotransmitter. Deficient GABAergic tone (particularly via GABA-A receptors with α2/α3 subunits) is implicated in depression and anxiety.
• Postmortem studies show reduced GABA levels in the prefrontal cortex and hippocampus.
• Brexanolone (a neuroactive steroid / positive allosteric modulator of GABA-A receptors) is approved for postpartum depression, validating this pathway.

H. Neuroinflammatory Hypothesis

• Depressed patients have elevated proinflammatory cytokines: IL-1β, IL-6, TNF-α, CRP.
• Proinflammatory cytokines cross the blood-brain barrier, attracting monocytes and macrophages into the brain.
• Within the brain, these cause: disruption of neurotransmission, oxidative stress, mitochondrial dysfunction, reduced BDNF, altered HPA axis, and epigenetic changes → synaptic loss and neuronal death.
• "Sickness behavior" (fatigue, anorexia, social withdrawal, anhedonia) induced experimentally by cytokines mirrors depressive symptoms.
• MDD associated with immune activation: 30-50% of patients treated with interferon-alpha (for hepatitis C) develop clinical depression.

I. Neuroimaging and Structural Brain Changes in MDD

• Global reduction of anterior cerebral metabolism
• Increased glucose metabolism in limbic regions during depression
• Pulvinar nucleus of thalamus activation
• Heightened amygdala response to negative stimuli
• Lesser response in dorsal striatum and dorsolateral PFC

J. Circuit-Based Understanding: Symptom-Circuit Mapping

K. Genetics of MDD

• Heritability: ~37% (twin studies)
• First-degree relatives have 2.84x increased odds
• GWAS have not found major common variants
• Key candidate genes: SLC6A4 (serotonin transporter, 5-HTTLPR), HTR1A (5-HT1A receptor), DRD4 (dopamine receptor), BDNF Val66Met
• 5-HTTLPR polymorphism: Short (s) allele → reduced serotonin transporter expression → increased vulnerability to stress-induced depression (gene-environment interaction, Caspi et al.)
• Likely polygenic architecture with many small-effect genes

II. NEUROBIOLOGY OF BIPOLAR DISORDER (BD)

A. Relationship to MDD

B. Neurotransmitter Dysregulation in BD

• Excess dopaminergic activity in the mesolimbic circuit → mania (elevated mood, grandiosity, risk-taking, hypersexuality, euphoria)
• Reduced dopamine activity → bipolar depression
• Antipsychotics (D2 blockers) are effective antimanic agents, supporting this model

• Excess NE → mania
• Deficient NE → bipolar depression

• Serotonin dysregulation modulates the amplitude and cycling of mood episodes
• Deficient serotonin creates vulnerability to both poles

• Excess glutamatergic activity contributes to manic and mixed states
• Lamotrigine (glutamate release inhibitor) is effective in bipolar depression

• Reduced GABAergic tone is implicated in the switch from depression to mania
• Valproate enhances GABA and is a first-line mood stabilizer

C. Kindling and Episode Sensitization

• Early episodes are often triggered by identifiable psychosocial stressors
• With repeated episodes, the threshold for triggering a new episode decreases progressively
• Later episodes may occur spontaneously without a clear precipitant
• Analogous to electrical kindling in animal models (repeated subthreshold stimuli eventually produce seizures)
• This explains the clinical observation that the course of BD tends to accelerate over time (cycle frequency increases, duration of euthymia shortens)
• Therapeutic implication: Early and continuous mood stabilizer treatment prevents episode sensitization

D. HPA Axis and Circadian Rhythm Disturbances in BD

• Hypercortisolemia occurs during both manic and depressive phases
• Cortisol levels correlate with episode severity
• Dexamethasone non-suppression also seen in mania
• Circadian rhythm dysregulation is central to BD:
  • Manic episodes often preceded by sleep loss/disruption
  • Sleep deprivation can precipitate mania in vulnerable patients
  • Phase advance of circadian rhythms seen in mania (opposite of phase delay in depression)
  • The suprachiasmatic nucleus (SCN) - the brain's pacemaker - shows abnormal activity in BD
  • This forms the basis of Interpersonal and Social Rhythm Therapy (IPSRT)

E. Second Messenger System Dysregulation in BD

• Gs alpha (stimulatory G-protein subunit) is overactive in BD
• cAMP-PKA signaling is dysregulated

• IP3 and diacylglycerol (DAG) accumulate due to excess PI turnover
• Lithium mechanism: Inhibits inositol monophosphatase → depletes free inositol → dampens PI signaling (the "inositol depletion hypothesis" of lithium's action)

• Elevated PKC activity during mania
• Lithium and valproate both inhibit PKC signaling
• Tamoxifen (PKC inhibitor) shows antimanic properties in trials

• A key downstream signaling molecule
• Lithium directly inhibits GSK-3β → neuroprotective effects, stabilization of β-catenin, promotion of neuroplasticity via BDNF/TrkB pathway

F. Neuroplasticity, BDNF, and Neuroprotection in BD

• BDNF levels are reduced during both manic and depressive episodes and normalize during euthymia
• Repeated mood episodes cause progressive gray matter volume loss (frontal, temporal, limbic regions)
• Lithium has robust neuroprotective effects: increases gray matter volumes, promotes BDNF, inhibits apoptosis via Bcl-2 upregulation
• Valproate also promotes neuroprotective gene expression via HDAC inhibition (epigenetic mechanism)

G. Neuroimaging in Bipolar Disorder

• Increased rates of white matter hyperintensities (deep and periventricular) - more prominent in BD than MDD
• Reduced hippocampal and amygdala volumes (though some studies show amygdala enlargement in early BD)
• Reduced subgenual PFC (Brodmann area 25) volumes
• Reduced thickness of the PFC

• Hyperactivation of the amygdala to emotional stimuli
• Reduced activity of the DLPFC (impaired cognitive control)
• Aberrant activity in the orbitofrontal cortex (OFC) - implicated in impulsivity and reward processing in mania
• Decreased metabolism in the subgenual ACC during depression; normalized with treatment

H. Genetics of Bipolar Disorder

• Heritability: 70-80% (the highest of any psychiatric disorder)
• Monozygotic twin concordance: ~50-80%
• First-degree relatives have 5-10x increased risk
• Genetic overlap with MDD and schizophrenia (shared loci: CACNA1C, ANK3, SYNE1)
• Key risk genes:
  • CACNA1C (voltage-gated L-type calcium channel alpha-1C subunit) - most replicated GWAS finding
  • ANK3 (ankyrin G) - involved in axon initial segment assembly
  • CLOCK gene and other circadian rhythm genes
  • DGKH (diacylglycerol kinase eta) - in PI signaling pathway
  • NRG1 (neuregulin-1) - shared with schizophrenia risk
• Polygenic architecture with rare large-effect variants also playing a role

I. Symptom-Circuit Mapping in Mania

III. SHARED NEUROBIOLOGICAL FEATURES

IV. SUMMARY TABLE

Conclusion

• Stahl's Essential Psychopharmacology: Neuroscientific Basis and Practical Applications (Chapter 6 - Mood Disorders)
• Kaplan & Sadock's Synopsis of Psychiatry (Chapter 7 - The Neurobiology of Depression)
• Kandel's Principles of Neural Science, 6th Edition (Chapter 61 - Mood and Anxiety Disorders)
• Katzung's Basic and Clinical Pharmacology, 16th Edition (Chapter on Lithium and Mood Stabilizers)