Final answer:
The activation of mu-opioid receptors by fentanyl inhibits the release of substance P which is involved in pain transmission.
Step-by-step explanation:
The activation of mu-opioid receptors by fentanyl inhibits the release of substance P which is involved in pain transmission. Fentanyl does not directly inhibit the release of gamma-aminobutyric acid (GABA), which reduces neuronal activity, or glutamate, which is also involved in pain transmission. Therefore, the correct answer is b. Substance P.
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