An opioid-naive patient given pentazocine for pain reports unpleasant feelings of dread, hallucinations and dysphoria rather than euphoria. This psychotomimetic profile is best explained by pentazocine's action at:

• A Sigma receptors mediating dysphoria and hallucinations ✓
• B Mu receptors producing excitation of limbic neurons
• C Delta receptors in the cortex
• D NMDA receptors causing dissociative anaesthesia

Explanation

Pentazocine is a mixed agonist-antagonist: it weakly agonizes mu receptors while acting at sigma receptors, which are associated with dysphoria, hallucinations and psychotomimetic effects. This limits its abuse liability but makes it poorly tolerated. Ketamine produces dissociation via NMDA receptors, not pentazocine. Delta receptor activation is not linked to dysphoria, and the effect is not explained by mu agonism since pure mu agonists produce euphoria.

Reference: KD Tripathi Essentials of Medical Pharmacology, 8th ed.

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