Pharmacological Treatments of Depression

Introduction to Depression and the Limbic Brain

  • Depression: Most common mood disorder in North America.
    • Recurring and debilitating, impairing social and occupational functioning.
    • Lifetime prevalence: 14.4%.
    • Symptoms (5 out of 9 for 2 weeks):
    • Depressed mood
    • Loss of interest in pleasurable activities
    • Changes in weight or appetite
    • Insomnia or hypersomnia
    • Psychomotor agitation or retardation
    • Fatigue
    • Feelings of worthlessness
    • Suicidal ideation
    • Diminished concentration

Limbic Brain

  • The limbic brain surrounds the brainstem and includes:
    • Amygdala
    • Hippocampus
    • Basal ganglia
    • Cingulate gyrus
    • Connected to frontal cortex and hypothalamus.

Emotion and Motivated Behavior

  • Emotions: Subjective feelings (anger, fear, sadness, etc.) linked to the limbic system.
  • Motivation: Purposeful behavior driven by the mesocorticolimbic dopamine system.

Depression and the Limbic Brain

  • MDD associated with increased limbic engagement (amygdala) and decreased motivation-related area engagement (striatum).
  • Changes in brain activity suggest alterations in neurotransmitter release linked to depression symptoms.

Depression Etiology: Monoamine Hypothesis

  • Monoaminergic neurotransmitters: Dopamine, norepinephrine, and serotonin.
    • Overlapping synthesis and catabolic pathways.
    • Modulatory role in mood, arousal, attention.
    • Alterations linked to mood disorders.
  • Inadequate monoamine neurotransmission linked to depression:
    • Causes include lesser neurotransmitter release, fewer receptors, impaired signal transduction.
    • Evidence from drugs like reserpine (depletes monoamines) and ipronazid (MAO inhibitor) suggests monoamines alter mood symptoms.

Glutamatergic Hypothesis of Depression

  • Depression associated with reduced glutamatergic signaling.
    • Impacts both excitatory and inhibitory functions.
    • Affects long-term potentiation, neurotrophic production (BDNF), and synapse formation.

Drugs Used to Treat Depression

1. Monoamine-Targeting Antidepressants
  • Increased synaptic levels of norepinephrine and serotonin (e.g., MAO inhibitors):
    • MAO inhibitors block neurotransmitter breakdown.
    • Requires dietary restrictions (avoid tyramine).
    • Selective Serotonin Reuptake Inhibitors (SSRIs) and Serotonin Norepinephrine Reuptake Inhibitors (SNRIs) increase extracellular concentrations by blocking reuptake.
    • Limitations: 30-50% efficacy, delayed onset of effects, systemic side effects (nausea, weight loss).
2. Glutamate-Targeting Antagonists (e.g., Ketamine)
  • NMDA receptor antagonist with potential antidepressant effects.
    • Induces a transient glutamate burst leading to synaptic remodeling.
    • Promising for treatment-resistant depression but narrow therapeutic index; requires intravenous administration.
3. Psychedelics (e.g., Psilocybin, LSD)
  • Emerging evidence for improvement in depression symptoms; better outcomes than SSRIs in some studies.
    • Issues with trial blinding and potential side effects like anxiety or suicidal ideation.

Summary

  • Depression involves multiple overlapping mechanisms (monoaminergic, glutamatergic).
  • Monoamine antidepressants effective for some patients but delayed onset is challenging.
  • New treatment avenues with NMDA antagonists and drugs targeting synaptic remodeling show potential.