Part 2

Nature and Symptoms of Depression

  • Definition: Depression is a mood disorder that affects a person's thoughts, feelings, and overall functioning. It is the most common affective disorder and is characterized by the following symptoms:

    • Sadness or feeling down

    • Loss of interest in usual activities (anhedonia)

    • Sleep disturbances (either sleeping too much or too little)

    • Changes in appetite (eating too much or too little)

    • Fatigue or persistent tiredness

    • Poor concentration, and in some cases, suicidal thoughts

Pathogenesis of Depression

  • The exact cause of depression is not fully understood, but several theories exist. One prominent theory is the Monoamine Theory, which suggests that depression may result from low levels of key neurotransmitters such as serotonin, noradrenaline, and dopamine. These neurotransmitters play critical roles in regulating mood.

  • There is no single cause, and it is thought to be a combination of:

    • Genetic factors: Family history of depression may increase risk.

    • Psychosocial factors: Stressful life events and trauma can contribute.

    • Biological factors: Differences in brain function and neurotransmitter levels are implicated in the condition.

Types of Antidepressant Drugs

  • Antidepressants work primarily by increasing the levels of neurotransmitters in the brain to enhance mood. The main classes include:

a) Monoamine Uptake Inhibitors

  • These drugs increase the availability of monoamine neurotransmitters in the synaptic cleft by blocking their reuptake. This prolongs their action and enhances communication between neurons.

    • Tricyclic Antidepressants (TCAs):

      • Action: Block the reuptake of serotonin (5-HT) and noradrenaline (NorA), but affect other receptors leading to side effects.

      • Side effects: Dry mouth, blurred vision, urinary retention, constipation, weight gain, sedation, and potential to prolong QT interval, impacting heart rhythm.

      • Danger: Overdose can be lethal, particularly due to cardiac arrhythmias.

      • Examples: Imipramine, Amitriptyline.

    • Selective Serotonin Reuptake Inhibitors (SSRIs):

      • Action: Specifically block serotonin reuptake, making them generally safer with fewer side effects.

      • Side effects: Nausea, weight changes, insomnia, sexual dysfunction, and agitation.

      • Examples: Fluoxetine, Sertraline, Citalopram.

    • Selective Serotonin and Noradrenaline Reuptake Inhibitors (SNRIs):

      • Action: Block reuptake of both serotonin and noradrenaline, sharing similarities with TCAs but with more targeted action.

      • Side effects: Nausea, dizziness, insomnia, agitation, and sometimes hypertension.

      • Examples: Venlafaxine, Duloxetine.

b) Monoamine Oxidase Inhibitors (MAOIs)

  • These inhibit the enzyme monoamine oxidase (MAO), which is responsible for breaking down neurotransmitters like serotonin, noradrenaline, and dopamine, leading to increased levels.

  • Side effects: Dry mouth, urinary retention, insomnia, weight gain, and risk of hypertensive crisis when combined with tyramine-rich foods like cheese and certain wines.

  • Risk: Dangerous interactions with other serotonergic medications ( SSRIs and SNRIs) and certain foods.

  • Examples: Phenelzine, Tranylcypromine.

How Do Antidepressants Work?

  • Antidepressants primarily work by increasing levels of neurotransmitters in the brain.

    • Short-term: They block neurotransmitter transporters (NET and SERT), preventing the reuptake of serotonin and noradrenaline, which increases their availability in the synapse and aids mood enhancement.

    • Long-term: Chronic drug usage results in neuroadaptive changes, including downregulation of autoreceptors, normalizing neurotransmission over time, which explains why these drugs may take several weeks to show their full effects.

Common Antidepressants and Their Side Effects

1. Tricyclic Antidepressants (TCAs)

  • Action: Block both serotonin and noradrenaline reuptake while also affecting other receptors.

  • Side effects: Dry mouth, blurred vision, urinary retention, constipation (antimuscarinic effects), weight gain, sedation, and cardiac risks such as QT interval prolongation.

  • Examples: Imipramine, Amitriptyline.

2. Selective Serotonin Reuptake Inhibitors (SSRIs)

  • Action: Block only serotonin reuptake, providing a safer and better-tolerated option.

  • Side effects: Nausea, weight changes, insomnia, sexual dysfunction, and restlessness.

  • Examples: Fluoxetine, Sertraline.

3. Selective Serotonin and Noradrenaline Reuptake Inhibitors (SNRIs)

  • Action: Block both serotonin and noradrenaline reuptake, akin to TCAs but with fewer side effects.

  • Side effects: Nausea, dizziness, insomnia, agitation, and potential hypertension.

  • Examples: Venlafaxine, Duloxetine.

4. Monoamine Oxidase Inhibitors (MAOIs)

  • Action: Inhibit MAO-A, increasing levels of serotonin, dopamine, and noradrenaline.

  • Side effects: Dry mouth, urinary retention, insomnia, weight gain, and a risk of hypertensive crisis when ingested with tyramine-rich foods.

  • Risk: Dangerous interactions with certain medications and foods containing tyramine.

  • Examples: Phenelzine, Tranylcypromine.

Serotonin Syndrome: A Dangerous Side Effect

  • Serotonin syndrome can develop from excess accumulation of serotonin, often due to the combination of serotonergic medications (SSRIs, SNRIs, TCAs, MAOIs).

    • Symptoms: Agitation, confusion, fever, excessive sweating, tremors, and in severe cases, it may be fatal.

    • Treatment: Immediate cessation of the drugs involved and administration of cyproheptadine, an antihistamine that can antagonize serotonin.

The Role of Serotonin

  • Serotonin is pivotal in regulating mood, sleep, and appetite. Antidepressants like SSRIs function by enhancing serotonin availability, facilitating improved mood and cognitive function. However, the exact mechanisms may vary across different classes of antidepressants, with full therapeutic effects often taking several weeks to manifest.

Summary

  • Depression is associated with low neurotransmitter levels, particularly serotonin, noradrenaline, and dopamine.

  • Antidepressants aim to augment neurotransmitter availability, yet their effects may take several weeks to become apparent.

  • The various classes of antidepressants differ in their mechanisms of action, side effect profiles, and safety:

    • TCAs: Effective but carry overdose risks.

    • SSRIs: Most commonly prescribed, safer but can lead to sexual side effects.

    • SNRIs: Block reuptake of both serotonin and noradrenaline, with fewer side effects compared to TCAs.

    • MAOIs: Rarely used due to risks of life-threatening food and drug interactions.

  • Serotonin syndrome is a serious potential side effect linked to excessive serotonin levels associated with treatment.