Comprehensive Study Notes on Depressive Disorders and Antidepressant Pharmacology

DEFINITION AND CORE FEATURES OF DEPRESSIVE DISORDERS

• Definition: Depression is defined as a mood disorder characterized by a persistent feeling of sadness and a loss of interest in previously enjoyed activities.
• Prevalence: It is recognized as the most common mental illness.
• Common Features of Depressive Disorders:
  • Persistent moods involving sadness, emptiness, or irritability.
  • Somatic changes (physical bodily symptoms).
  • Cognitive changes that significantly impair an individual's capacity to function effectively.
• Demographic Prevalence: Females are affected by major depression at a rate approximately $2 \times$ more frequently than males.

SYMPTOMATOLOGY AND CLINICAL DIAGNOSIS

• Clinical Presentation of Depressive Episodes:
  • Sad mood and pessimistic worry.
  • Diminished interest in normal daily activities.
  • Mental slowing and poor concentration abilities.
  • Insomnia (disturbed sleep) or increased sleep (hypersomnia).
  • Significant weight loss or weight gain resulting from altered eating habits and activity patterns.
  • Psychomotor agitation (restlessness) or psychomotor retardation (slowing of physical movements).
  • Persistent feelings of guilt and worthlessness.
  • Decreased energy levels and reduced libido.
  • Suicidal ideation.
• Diagnostic Duration: For these symptoms to be classified as a depressive episode, they must occur most days for a period of at least $2\,\text{weeks}$.

CLASSIFICATION OF DEPRESSIVE DISORDERS AND SECONDARY CAUSES

• Primary Types of Depression:
  1. Major Depressive Disorder ($\text{MDD}$, also known as unipolar depression).
  2. Persistent Depressive Disorder ($\text{Dysthymia}$).
  3. Bipolar I and II disorders (previously termed Manic Depressive Illness).
• Secondary Depression: Depressive symptoms may also occur secondary to other medical conditions, including:
  • Hypothyroidism.
  • Parkinson's disease.
  • Other chronic illnesses.

THE PATHOPHYSIOLOGY OF MAJOR DEPRESSIVE DISORDER (MDD)

• The understanding of depression's pathophysiology is based on three primary hypotheses:
  1. Neurotrophic Hypothesis.
  2. Neurotransmitter (Monoamine) Hypothesis.
  3. Neuroendocrine Hypothesis.

THE NEUROTROPHIC HYPOTHESIS

• Focus on BDNF: This hypothesis centers on Brain-Derived Neurotrophic Factor ($BDNF$), which is responsible for:
  • Dendritic spine formation.
  • Synaptogenesis.
  • Neurogenesis.
• Pathological State (Depressed State):
  • Stress and elevated glucocorticoids lead to a decrease in $cAMP$ and $CREB$ (cAMP Response Element-Binding protein) activation.
  • This results in a downregulation or decrease in $BDNF$ gene transcription.
  • Neuronal Effects: Observed physical changes include decreased dendritic branching, decreased dendritic spine density, and general synaptic atrophy.
• Treatment State (Recovery):
  • Treatment with antidepressants (e.g., $SSRIs, SNRIs, TCAs, MAOIs, ketamine$, etc.) increases monoamine levels (e.g., $5-HT, NE, DA$).
  • Increased monoamines lead to increased $cAMP\,CREB$ activation.
  • This stimulates increased $BDNF$ gene transcription.
  • Neuronal Effects: Observed changes include increased dendritic sprouting, increased dendritic spine density, and synaptic strengthening and regeneration.

THE MONOAMINE AND NEUROTRANSMITTER HYPOTHESIS

• Monoamine Deficiency: Suggests depression is caused by a deficiency in the amount or functional capacity of cortical and limbic serotonin ($5-HT$), Norepinephrine ($NE$), and Dopamine ($DA$).
• Glutamate Involvement:
  • Depressed patients often show elevated glutamate content in their cerebrospinal fluid ($CSF$).
  • There are decreased glutamine/glutamate ratios in the plasma of depressed individuals.
• Neuroimaging findings: Structural neuroimaging reveals volumetric changes in brain areas where glutamate neurons and their connections are most abundant.

THE NEUROENDOCRINE HYPOTHESIS

• HPA Axis Abnormalities: Major depression is associated with several abnormalities in the Hypothalamic-Pituitary-Adrenal ($HPA$) axis, including:
  • Elevated cortisol levels.
  • Non-suppression of Adrenocorticotropic Hormone ($ACTH$) release during a Dexamethasone Suppression Test.
  • Chronically elevated levels of Corticotropin-Releasing Hormone ($CRH$).
• Thyroid Dysregulation:
  • Elevations in circulating thyroxine are noted during depressed states.
  • Clinical hypothyroidism can manifest as depressive symptoms, which typically resolve with thyroid hormone supplementation.
• Hormonal Deficiencies and Life Stages:
  • Estrogen Deficiency: Occurs in women during postpartum and postmenopausal periods, playing a role in the etiology of depression.
  • Testosterone Deficiency: Severe deficiency in men is linked to depression.
• Connection to BDNF: Abnormalities in the $HPA$ axis and steroid levels may contribute to the suppression of $BDNF$ gene transcription. The protein synthesis of products like $BDNF$ typically takes $2\,\text{weeks}$ or longer, which coincides with the clinical course of antidepressant treatment lag.

CLASSIFICATION OF ANTIDEPRESSANT PHARMACOTHERAPY

• A. Selective Serotonin Reuptake Inhibitors (SSRIs):
  • Fluoxetine, Paroxetine, Sertraline, Fluvoxamine, Citalopram, Escitalopram.
• B. Serotonin-Norepinephrine Reuptake Inhibitors (SNRIs):
  1. Selective SNRIs: Venlafaxine, Desvenlafaxine, Duloxetine, Milnacipran, Levomilnacipran.
  2. Tricyclic Antidepressants (TCAs): Imipramine, Desipramine, Amitriptyline, Nortriptyline, Clomipramine, Doxepin.
• C. 5-HT Receptor Modulators:
  • Trazodone, Nefazodone, Vortioxetine.
• D. Tetracyclic and Unicyclic Antidepressants:
  • Bupropion, Mirtazapine, Mianserin, Amoxapine, Vilazodone, Maprotiline.
• E. Monoamine Oxidase Inhibitors (MAOIs):
  • Moclobemide, Eprobemide, Selegiline, Phenelzine, Isocarboxazid, Tranylcypromine.
• F. NMDA Receptor Antagonists:
  • Ketamine, Esketamine, Dextromethorphan.
• G. Allosteric Modulators of GABAA:
  • Brexanolone, Zuranolone, Ganaxolone.

MECHANISM OF ACTION AT THE SYNAPTIC LEVEL

• Serotonergic Terminal Action:
  • $5HT$ is synthesized and stored in vesicles.
  • Reuptake is facilitated by the Serotonin Transporter ($SERT$).
  • Inhibitors: $SSRIs, TCAs$, and $SNRIs$ block $SERT$.
  • Autoreceptors: Includes $5HT_{1A}, 5HT_{1B}, 5HT_{1D}$, and $5HT_7$ which modulate release.
  • Metabolism: Monoamine Oxidase ($MAO$) degrades $5HT$. $MAOIs$ block this process.
• Noradrenergic Terminal Action:
  • $NE$ is synthesized and stored in vesicles.
  • Reuptake is facilitated by the Norepinephrine Transporter ($NET$).
  • Inhibitors: $SNRIs$ and $TCAs$ block $NET$.
  • Adrenergic Receptors: Includes $\alpha_1AR, \alpha_2AR$, and $\beta AR$.
  • Signal Transduction: Activation leads to modulation of cell signaling pathways (e.g., $PIP_2 \rightarrow IP_3$) and cell function.

PRINCIPLES OF PHARMACOTHERAPY FOR DEPRESSION

• Mechanism Summary: Reuptake inhibitors block the neuronal serotonin transporter ($SERT$), the norepinephrine transporter ($NET$), or both.
• First Generation Antidepressants: Tricyclic Antidepressants ($TCAs$) enhance monoaminergic neurotransmission by inhibiting reuptake but are associated with more side effects.
• Second Generation Antidepressants: $SSRIs$ and $SNRIs$ are the most commonly used today due to reduced toxicity and improved safety profiles.
• MAOI Mechanism: They inhibit monoamine metabolism, thereby increasing the storage and subsequent release of neurotransmitters.
• Long-term Effects: Chronic treatment results in:
  • Altered adrenergic or serotonergic receptor density or sensitivity.
  • Altered G protein-receptor coupling.
  • Altered cyclic nucleotide signaling.
  • Induction of neurotrophic factors and increased neurogenesis in the hippocampus.

CLINICAL CONSIDERATIONS AND MAINTENANCE TREATMENT

• Therapeutic Lag: Antidepressant response typically exhibits a delay of $3 \text{ to } 4\,weeks$.
• Differential Symptom Response: Sleep disturbances usually improve sooner, whereas mood and cognitive deficits take longer to show improvement.
• Maintenance Phase: Following successful initial treatment, a maintenance phase of $6 \text{ to } 12\,months$ is standard.
• Chronic Depression: Patients who have been depressed for more than $2\,years$ may require life-long pharmacotherapy.
• USFDA Black Box Warning: A warning is issued for the use of $SSRIs$ and other antidepressants in children and adolescents due to observations of increased suicidal ideation and behavior.

SELECTIVE SEROTONIN REUPTAKE INHIBITORS (SSRIs)

• Efficacy: Highly effective for Major Depressive Disorder ($\text{MDD}$).
• Selectivity: They are specifically selective for the inhibition of $SERT$ over $NET$.
• Clinical Indications: Beyond depression, they are used for:
  • Generalized Anxiety Disorder ($GAD$).
  • Post-Traumatic Stress Disorder ($PTSD$) (specifically Sertraline and Paroxetine).
  • Obsessive-Compulsive Disorder ($OCD$).
  • Panic disorder.
  • Premenstrual Dysphoric Disorder ($PMDD$).
  • Bulimia nervosa (specifically Fluoxetine).
• Autoreceptor Dynamics:
  • Initially, $SSRIs$ stimulate $5HT_{1A}$ and $5HT_7$ autoreceptors on cell bodies in the raphé nucleus and $5HT_{1D}$ autoreceptors on terminals, which reduces net $5HT$ synthesis and release.
  • Repeated Treatment: These autoreceptors gradually downregulate and desensitize. Combined with reuptake inhibition, this leads to enhanced net $5HT$ transmission.
• Postsynaptic Effects: Downregulation of $5HT_{2A}$ receptors may contribute to efficacy directly or via heteroreceptors on other neurons.
• Final Therapeutic Response: Mediated by $cAMP$ signaling, phosphorylation of $CREB$, increased $BDNF$, and increased neurogenesis in the hippocampus and subventricular zone.

SSRI PHARMACOKINETICS (ADME)

• Administration: Orally active with once-daily dosing.
• Metabolism (Fluoxetine): Metabolized to the active product norfluoxetine, which has the longest half-life ($t_{1/2}$) of all $SSRIs$. Fluoxetine is available in a once-weekly formulation.
• Hepatic Metabolism: Most $SSRIs$ are metabolized via CYP2D6.
• Tamoxifen Interaction: $SSRIs$ (especially potent inhibitors of $CYP2D6$) prevent the activation of tamoxifen, thereby reducing its anticancer effect.
• Elderly Patients: Escitalopram and citalopram should be dosed carefully in the elderly due to age-dependent decreases in CYP2C19 metabolism.

ADVERSE EFFECTS AND DISCONTINUATION SYNDROME OF SSRIs

• 5HT2 Receptor Overstimulation (Brain): Leads to insomnia, increased anxiety, irritability, decreased libido, and potential worsening of depressive symptoms.
• 5HT2 Receptor Overstimulation (Spinal Cord): Causes sexual side effects including erectile dysfunction, anorgasmia, and ejaculatory delay (most prominent with Paroxetine).
• 5HT3 Receptor Stimulation: Causes gastrointestinal effects like nausea, diarrhea, and emesis.
• Cognitive Effects: While treatment might initially increase anxiety, continued treatment can lead to a "dullness" of intellectual abilities and concentration.
• Discontinuation Syndrome: Caused by sudden withdrawal. Symptoms include dizziness, headache, nervousness, nausea, and insomnia.
  • Intensity: Most intense with drugs having a short half-life, such as Paroxetine, Venlafaxine, and Desvenlafaxine.
  • Fluoxetine Exception: Symptoms are rare due to the long half-life of its active metabolite, norfluoxetine ($1 \text{ to } 2\,weeks$).
• Pregnancy and Neonatal Risks:
  • Paroxetine: Classified as $\text{Category D}$; increased risk of congenital cardiac malformations during the first trimester.
  • Newborn Risks: $SSRI$ use in pregnancy is linked to persistent pulmonary hypertension in newborns.

DRUG INTERACTIONS AND SEROTONIN SYNDROME

• CYP2D6 Interactions: Since fluoxetine and paroxetine are potent inhibitors, coadministration with substrates (e.g., hydrocodone, diphenhydramine) or other inhibitors (e.g., imatinib, quinidine) increases plasma concentrations of the primary drug.
• SSRI + TCA: Coadministration increases exposure to Tricyclic Antidepressants ($TCAs$).
• Serotonin Syndrome:
  • Caused by the combination of $SSRIs$ with $MAOIs$ or other drugs like MDMA (Ecstasy) and synthetic cathinones (bath salts) which release $5HT$.
  • Mechanism: Synergistic increase in extracellular brain serotonin.
  • Characteristics: A triad of cognitive, autonomic, and somatic abnormalities.
  • Symptoms: Hyperthermia, muscle rigidity, myoclonus, tremors, autonomic instability, confusion, irritability, and agitation. Can progress to coma and death.
• Dosing Intervals:
  • Wait at least $14\,days$ after stopping an irreversible $MAOI$ before starting an $SSRI$.
  • Wait at least $5\,weeks$ after stopping Fluoxetine before beginning an $MAOI$ due to the long half-life of norfluoxetine.

SEROTONIN-NOREPINEPHRINE REUPTAKE INHIBITORS (SNRIs)

• Mechanism: Inhibit both $SERT$ and $NET$. Initial feedback via $5HT_{1A/1D}$ autoreceptors slows transmission until desensitization occurs.
• Neurotrophic Impact: Noradrenergic action may contribute to changes in $BDNF$ and $Trk-B$ signaling pathways.
• Clinical Uses (Duloxetine): Approved for Major Depression, fibromyalgia, and neuropathic pain associated with peripheral neuropathy.
• Off-label Uses and Indications (Venlafaxine/SNRIs):
  • Stress urinary incontinence (Duloxetine).
  • Autism, Binge-eating disorders, Hot flashes, and various Pain syndromes.
  • $PTSD$ and $PMDD$ (Venlafaxine).
• Pharmacokinetics:
  • Venlafaxine: Major metabolite is Desmethylvenlafaxine, eliminated by hepatic metabolism and renal excretion. Doses must be reduced for renal or hepatic impairment.
  • Duloxetine: Not recommended for patients with end-stage renal disease or hepatic insufficiency.
• Adverse Effects:
  • Similar to $SSRIs$: Nausea, constipation, insomnia, headache, and sexual dysfunction.
  • Hypertension: Immediate-release Venlafaxine is associated with sustained diastolic hypertension (diastolic $BP > 90\,mmHg$); this risk is lower with extended-release formulations.
  • Pregnancy: Venlafaxine is associated with an increased risk of perinatal complications.
• Important Dosing Intervals:
  • $14\,day$ period required between ending $MAOI$ therapy and starting Venlafaxine.
  • $5\,day$ waiting period required before starting $MAOI$ after ending Duloxetine.