**Core Concept**
Tianeptine is an atypical antidepressant that exerts its effects by modulating glutamatergic neurotransmission, specifically through the action on glutamate receptors. This unique mechanism of action differentiates tianeptine from traditional antidepressants that primarily target the serotonin and norepinephrine systems.

**Why the Correct Answer is Right**
Tianeptine acts as a partial agonist at the glutamate receptor subtype mGluR2, which is involved in regulating glutamate release. By activating mGluR2, tianeptine reduces the excitatory neurotransmission of glutamate, thus exerting an antidepressant effect. This mechanism of action is distinct from other antidepressants that may increase the levels of serotonin and norepinephrine in the synaptic cleft.

**Why Each Wrong Option is Incorrect**
**Option A:** Tianeptine does not act primarily as a selective serotonin reuptake inhibitor (SSRI). While SSRIs are a common class of antidepressants, tianeptine's unique mechanism of action is centered around glutamate modulation, not serotonin reuptake inhibition.
**Option B:** Tianeptine does not act as a norepinephrine reuptake inhibitor (NRI). NRIs are another class of antidepressants that increase the levels of norepinephrine in the synaptic cleft, but tianeptine's primary mechanism is through glutamate receptor modulation.
**Option C:** Tianeptine does not act as an atypical antipsychotic. While atypical antipsychotics may have some antidepressant properties, tianeptine's primary indication is as an antidepressant, and its mechanism of action is centered around glutamate modulation, not dopamine receptor antagonism.

**Clinical Pearl / High-Yield Fact**
Tianeptine's unique mechanism of action as a partial agonist at mGluR2 makes it a valuable option for patients who have not responded to traditional antidepressants, highlighting the importance of considering alternative mechanisms of action in treatment-resistant depression.

**Correct Answer: C.**