**Question:** Mode of action of Fluoxetine is -

A. Inhibition of reuptake of serotonin
B. Inhibition of reuptake of norepinephrine
C. Direct action on serotonin receptors
D. Direct action on norepinephrine receptors

**Correct Answer:** A. Inhibition of reuptake of serotonin

**Core Concept:**
Fluoxetine is a selective serotonin reuptake inhibitor (SSRI) used as an antidepressant medication. The primary mode of action of SSRIs, such as Fluoxetine, is to increase the availability of serotonin in the synaptic cleft by inhibiting the reuptake of serotonin back into the presynaptic neuron.

**Why the Correct Answer is Right:**
The correct answer, A, highlights the primary mode of action of Fluoxetine, which is to inhibit the reuptake of serotonin. This leads to a higher concentration of serotonin in the synaptic cleft, which plays a crucial role in improving mood and alleviating symptoms of depression.

**Why Each Wrong Option is Incorrect:**

B. Inhibition of reuptake of norepinephrine (Option B): Fluoxetine primarily affects serotonin reuptake, not norepinephrine. Norepinephrine is primarily involved in the autonomic nervous system, cardiovascular regulation, and the fight-or-flight response, while serotonin is linked to mood, anxiety, and sleep regulation.

C. Direct action on serotonin receptors (Option C): Fluoxetine primarily acts as an inhibitor of serotonin reuptake, not directly affecting serotonin receptors. SSRIs do not directly activate or inhibit serotonin receptors but indirectly enhance serotonergic neurotransmission by preventing the reuptake of serotonin.

D. Direct action on norepinephrine receptors (Option D): As explained earlier, Fluoxetine primarily affects serotonin reuptake, not norepinephrine receptors.

**Clinical Pearl:**
SSRIs, including Fluoxetine, are a vital component of antidepressant therapy, particularly for treating major depressive disorder, obsessive-compulsive disorder, panic disorder, and premenstrual dysphoric disorder. By selectively targeting serotonin, SSRIs aim to improve neurotransmitter balance and enhance serotonergic neurotransmission, which helps alleviate depressive symptoms.