Which specific opioid receptor subtype mediates supraspinal and spinal analgesia, miosis, respiratory depression, and euphoria — and what second-messenger system does it couple to?

• A Kappa (KOR) receptor; mediates all four effects equally to mu receptor via Gs coupling
• B Delta (DOR) receptor; mediates euphoria and analgesia via Gq/IP3/DAG signaling
• C Mu (MOR) receptor coupled to Gi/o protein; decreases cAMP (inhibits adenylyl cyclase), opens GIRK K+ channels (hyperpolarization), closes N-type VGCC (reduces neurotransmitter release) ✓
• D Sigma receptor coupled to Gs; mediates psychomimetic effects mistaken for opioid effects

Explanation

The mu-opioid receptor (MOR, OPRM1 gene) mediates supraspinal analgesia (PAG, thalamus), spinal analgesia (dorsal horn), euphoria (nucleus accumbens), miosis (Edinger-Westphal), and respiratory depression (pre-Botzinger complex). MOR is coupled to Gi/Go proteins: inhibiting adenylyl cyclase (reducing cAMP), activating GIRK channels (hyperpolarizing neurons), and inhibiting N-type and P/Q-type voltage-gated calcium channels (reducing synaptic neurotransmitter release). Kappa receptors mediate spinal analgesia, dysphoria, and sedation; delta receptors modulate mu activity and mood; sigma receptors are not true opioid receptors.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.