What is the initial mechanism of action for selective serotonin reuptake inhibitors in treating major depressive disorder?

• A. Blocking serotonin receptors
• B. Increasing serotonin reuptake
• C. Inhibiting presynaptic reuptake of serotonin
• D. Enhancing dopamine signaling

Answer: (C)

Selective serotonin reuptake inhibitors (SSRIs) primarily function by inhibiting the presynaptic reuptake of serotonin. This means that when these medications are administered, they bind to the serotonin transporter (SERT) on the presynaptic neuron, preventing the reabsorption of serotonin (5-HT) back into that neuron after it has been released into the synaptic cleft.

As a result of this inhibition, more serotonin remains available in the synaptic cleft, leading to increased stimulation of postsynaptic serotonin receptors. This mechanism is critical in the treatment of major depressive disorder, as it helps to maximize the serotonergic signaling that is often found to be deficient in patients with depression.

While other mechanisms of action may play a role in the broader context of mood regulation and treatment, the initial and primary action of SSRIs is specifically related to their impact on serotonin reuptake, making it the correct choice in this scenario.