Morphine is an opioid agonist used for the relief of moderate to severe acute and chronic pain. Morphine can be administered orally as a capsule or tablet, intravenously, intrathecally, or as an epidural. Morphine-6-glucuronide, a metabolite of morphine, is responsible for approximately 85% of the response observed by morphine administration, however morphine may have greater analgesic efficacy. Morphine binds to mu opioid receptors, stimulating the exchange of GTP for GDP on the G-protein complex. As the effector system is adenylate cyclase and cAMP located at the inner surface of the plasma membrane, opioids decrease intracellular cAMP by inhibiting adenylate cyclase. Subsequently, the release of nociceptive neurotransmitters such as GABA is inhibited. Opioids close N-type voltage-operated calcium channels and open calcium-dependent inwardly rectifying potassium channels. This results in hyperpolarization and reduced neuronal excitability. Morphine acts at A delta and C pain fibres in the dorsal horn of the spinal cord. By decreasing neurotransmitter action there is less pain transmittance into the spinal cord. This leads to less pain perception. Morphine also binds to delta and kappa opioid receptors in the nucleus accumbens where the reward pathway is activated.

Morphine Opioid Agonist Pathway References