Which statement regarding the mechanism of neuraxial opioids is true?

Which statement regarding the mechanism of neuraxial opioids is true?

• A. They act by blocking NMDA receptors in the brain.
• B. They target mu receptors in the spinal cord to reduce substance P release.
• C. They inhibit acetylcholine release at the neuromuscular junction.
• D. They increase dopamine release in the nucleus accumbens.

Answer: (B)

Neuraxial opioids produce analgesia mainly by activating mu-opioid receptors located in the spinal dorsal horn. When these receptors are engaged on presynaptic terminals of nociceptive fibers, they inhibit voltage-gated calcium channels, which reduces the release of substance P and other excitatory neurotransmitters. On postsynaptic neurons, mu receptor activation increases potassium conductance, causing hyperpolarization and making neurons less excitable. This combination dampens the transmission of pain signals up the spinal cord, providing effective analgesia. The statement that they target mu receptors in the spinal cord to reduce substance P release best captures this mechanism. Other options describe actions not characteristic of neuraxial opioids, such as NMDA receptor blockade, acetylcholine release at the neuromuscular junction, or dopamine release in reward pathways.

Neuraxial opioids produce analgesia mainly by activating mu-opioid receptors located in the spinal dorsal horn. When these receptors are engaged on presynaptic terminals of nociceptive fibers, they inhibit voltage-gated calcium channels, which reduces the release of substance P and other excitatory neurotransmitters. On postsynaptic neurons, mu receptor activation increases potassium conductance, causing hyperpolarization and making neurons less excitable. This combination dampens the transmission of pain signals up the spinal cord, providing effective analgesia. The statement that they target mu receptors in the spinal cord to reduce substance P release best captures this mechanism. Other options describe actions not characteristic of neuraxial opioids, such as NMDA receptor blockade, acetylcholine release at the neuromuscular junction, or dopamine release in reward pathways.