Pharmacotherapy for Depression and Bipolar Disorder

Exam Tip: If asked why antidepressants take weeks to work, focus on the transition from acute neurotransmitter increases to chronic neuronal adaptations, such as the downregulation of inhibitory autoreceptors and the upregulation of the cAMP/BDNF pathway.

Major Antidepressant Classes

Monoamine Oxidase Inhibitors (MAOIs)

MAOIs are the oldest class of antidepressants. They work by inhibiting the enzyme Monoamine Oxidase (MAO), which is responsible for metabolizing monoamines like NE and 5-HT in the presynaptic terminal. By blocking this enzyme, more neurotransmitters remain available for release into the synapse. However, MAOIs carry significant risks. Because they prevent the breakdown of tyramine (found in certain foods), patients may experience dangerous spikes in blood pressure. They can also interact dangerously with drugs that enhance NE function, such as cocaine or certain cold medications.

Tricyclic Antidepressants (TCAs)

TCAs act by binding to presynaptic transporter proteins to block the reuptake of both NE and 5-HT, thereby prolonging the duration of their action in the synapse. Despite their efficacy, they have a low therapeutic index, meaning the gap between a therapeutic dose and a toxic dose is small. Overdose can be fatal, leading to cardiovascular issues (arrhythmias, tachycardia), delirium, and convulsions. Common side effects include sedation due to histamine blockade and anticholinergic effects like dry mouth and constipation.

Second-Generation Antidepressants (SSRIs and SNRIs)

Second-generation drugs are generally preferred due to improved safety profiles:

• Selective Serotonin Reuptake Inhibitors (SSRIs): These primarily block the reuptake of 5-HT. They are widely used for depression, anxiety, and OCD. A critical risk is serotonin syndrome, a potentially life-threatening condition that occurs when SSRIs are combined with other serotonergic agents.
• Serotonin-Norepinephrine Reuptake Inhibitors (SNRIs): These are dual modulators that enhance both NE and 5-HT function. An example is Mirtazapine, which blocks $\alpha_{2}$-autoreceptors to increase the release of both neurotransmitters.

Emerging and Third-Generation Therapies

Recent developments have moved toward mechanisms that bypass the monoamine system entirely. Ketamine is a prominent example; it acts as an NMDA receptor antagonist. Unlike traditional antidepressants, ketamine provides rapid antidepressant effects, likely by activating the mTOR signaling cascade, which promotes rapid dendritic spine growth and neurogenesis.

Other experimental therapies include CRF receptor antagonists, agomelatine (which targets circadian regulation), and galanin agonists.

Bipolar Disorder Pharmacotherapy

Treatment for bipolar disorder differs from unipolar depression because the goal is to stabilize mood swings—preventing both manic "highs" and depressive "lows".

Lithium

Lithium is considered the most effective treatment for bipolar disorder. It is uniquely effective at reducing the risk of suicide and can eliminate manic episodes without causing significant sedation. Its mechanism involves enhancing 5-HT actions and reducing catecholamine activity, potentially through the inhibition of GSK-3, which helps regulate circadian rhythms. However, lithium has a very low therapeutic index, necessitating regular blood monitoring to prevent severe toxicity.

Anticonvulsants

Anticonvulsant medications are also used as mood stabilizers and are considered as effective as lithium for some patients:

• Valproate (Depakote): Works by increasing GABA levels in the brain. It is important to note that valproate is teratogenic (can cause birth defects).
• Carbamazepine (Tegretol): Acts by inhibiting NE reuptake and affecting adenosine receptors.