Antidepressants increase synaptic monoamine availability through various mechanisms. SSRIs and SNRIs are first-line agents; other classes are used for treatment-resistant cases or specific comorbidities.
You already understand from the DSM-5 framework that major depressive disorder is diagnosed based on a constellation of symptoms—depressed mood, anhedonia, sleep disturbances, cognitive impairment—meeting specified duration and severity thresholds. Antidepressants don't target a single brain deficit but push multiple neurobiological systems in ways that, over weeks, shift the clinical picture. Understanding the mechanisms helps explain both why these drugs work and why they take so long to work.
The leading framework is that depression involves insufficient monoamine neurotransmission—particularly serotonin, norepinephrine, and to a lesser extent dopamine. Selective serotonin reuptake inhibitors (SSRIs)—fluoxetine, sertraline, escitalopram—block the serotonin transporter (SERT), preventing serotonin from being pumped back into the presynaptic neuron after release. More serotonin stays in the synapse longer, increasing activation of postsynaptic serotonin receptors. Serotonin-norepinephrine reuptake inhibitors (SNRIs) like venlafaxine and duloxetine block both SERT and the norepinephrine transporter (NET), making them useful when noradrenergic symptoms predominate—fatigue, concentration problems, and pain comorbidities respond particularly well to norepinephrine enhancement.
The clinical puzzle is that SERT blockade happens within hours, but patients typically don't feel better for 2–4 weeks. This delay has driven revision of the simple "monoamine deficiency" account. The more complete picture involves downstream changes: SSRIs increase BDNF (brain-derived neurotrophic factor) expression over weeks, promote hippocampal neurogenesis, and normalize dysregulated stress-axis (HPA) function. These neuroplastic changes take time to accumulate. This means the acute synaptic effect is likely the trigger, while the therapeutic effect depends on weeks of adaptive changes in receptor sensitivity and neural circuit remodeling. Patients should be counseled to expect a lag before benefit emerges.
When SSRIs and SNRIs fail—typically defined as two adequate trials at therapeutic doses—clinicians turn to alternative classes. Bupropion (an NDRI) targets norepinephrine and dopamine reuptake rather than serotonin, making it useful when sexual dysfunction from SSRIs is intolerable or when motivation and energy are the primary concerns. Mirtazapine works through receptor antagonism rather than reuptake blockade, increasing both serotonergic and noradrenergic transmission indirectly; its sedating and appetite-stimulating properties make it valuable for depression with severe insomnia and weight loss. Tricyclic antidepressants and MAOIs remain effective options with decades of evidence but carry more significant side effect and drug interaction burdens. Selecting among these options means matching the drug's side-effect profile to the patient's clinical picture—the DSM-5 diagnostic categories you learned are the starting point, but comorbidities, prior medication history, and symptom subtypes determine which antidepressant is the right tool for the specific presentation.