What is the primary mechanism of action of opioids used in veterinary analgesia?

• A. Mu receptor agonists
• B. NMDA receptor antagonists
• C. Serotonin reuptake inhibitors
• D. Alpha-2 agonists

Answer: (A)

Opioids provide their pain-relieving effect mainly by activating mu opioid receptors in the central nervous system. When these receptors are engaged, they couple to Gi/o proteins, which leads to a chain of changes: reduced cAMP production, closing of voltage-gated calcium channels, and opening of potassium channels. The result is hyperpolarization and a decrease in neurotransmitter release (such as substance P and glutamate) in the spinal cord and brain, damping the transmission and perception of pain. This mu receptor–driven action underpins the strongest analgesia produced by opioids in veterinary patients, and it also accounts for the common side effects seen with these drugs.

Other options describe different drug actions: NMDA receptor antagonism, seen with ketamine, provides analgesia through a separate mechanism; serotonin reuptake inhibition targets mood and certain chronic pain pathways but is not how classic opioids relieve acute nociception; alpha-2 agonists produce analgesia via noradrenergic pathways and sedation but are not opioids and do not act on opioid receptors.