Opioid receptor signaling

Category: receptor_pharmacology

Overview

Gi/o-coupled GPCRs activated by endogenous opioid peptides (β-endorphin, enkephalins, dynorphins) and exogenous opioid drugs. Downstream: Gi/o → adenylyl cyclase inhibition → reduced cAMP + PKA; Gβγ → opens K+ channels (hyperpolarization) + closes Ca2+ channels (reduces neurotransmitter release). MOR = analgesia + euphoria + respiratory depression + constipation + miosis (the canonical opioid effects). KOR = spinal analgesia + dysphoria + sedation + diuresis. DOR = mood + minor analgesia. β-arrestin recruitment vs G-protein activation underlies "biased agonism" — OLICERIDINE is a MOR G-protein-biased agonist with less β-arrestin recruitment, marketed claim of less respiratory depression per analgesic equivalent (clinical evidence modest). Mixed agonists/antagonists (NALBUPHINE κ + MOR-partial; BUPRENORPHINE MOR-partial) have a ceiling effect on respiratory depression. NALOXONE / naltrexone are antagonists at all three.

Organ Systems

• nervous

Pathway Steps

1. opioid-receptor-agonist → gi-go-activation — via MOR / KOR / DOR; conformational change → Gi/o exchange. Opioid receptors (μ, δ, κ) are Gi/Go-coupled GPCRs; agonists (endorphins, enkephalins, morphine) activate the inhibitory G protein. The μ-receptor mediates most analgesia plus the euphoria, respiratory depression, and dependence of clinical opioids — the basis of both their efficacy and their danger.
2. gi-go-activation → adenylyl-cyclase-inhibition — via reduced cAMP → reduced PKA → multiple downstream. Through Gαi, opioid receptors inhibit adenylyl cyclase, lowering cAMP. Chronic opioid exposure causes compensatory upregulation of this cyclase system, so on withdrawal cAMP rebounds — a key molecular driver of opioid dependence and the withdrawal syndrome.
3. gi-go-activation → k-channel-opening-ca-closing — via Gβγ → hyperpolarization + reduced presynaptic Ca2+ → neurotransmitter release ↓. Via Gβγ, opioid receptors open potassium channels (hyperpolarizing neurons) and close voltage-gated calcium channels (reducing neurotransmitter release). These two ion-channel actions are the proximate mechanism of opioid analgesia — dampening neuronal excitability and transmission in pain pathways.

Known Modulators

• morphine (activator) — MOR full agonist
• oxycodone (activator) — MOR full agonist
• buprenorphine (activator) — MOR partial agonist + κ antagonist; ceiling on respiratory depression
• nalbuphine (activator) — KOR agonist + MOR partial agonist/antagonist
• naloxone (inhibitor) — MOR + KOR + DOR antagonist (reversal)
• hydrocodone (activator) — μ-opioid receptor (semisynthetic). most-prescribed US opioid; CYP2D6 → hydromorphone active metabolite (poor metabolizers reduced efficacy)
• codeine (activator) — μ-opioid receptor (prodrug → morphine via CYP2D6). CYP2D6 PMs have minimal effect; ultra-rapid metabolizers have severe respiratory depression risk (pediatric deaths)
• hydromorphone (activator) — μ-opioid receptor. 5-10× more potent than morphine; preferred in renal impairment (no active metabolites)
• oxymorphone (activator) — μ-opioid receptor. oxycodone's O-demethylated active metabolite; PO + IV; CV instability + drug abuse signal
• fentanyl (activator) — μ-opioid receptor. ~100× morphine potency; TD + IV + buccal; primary illicit overdose driver post-2014
• methadone (activator) — μ-opioid receptor + NMDA antagonist. opioid use disorder + chronic pain; long + variable t½ → accumulation risk; QT prolongation
• tramadol (activator) — μ-opioid (weak) + SNRI. dual mechanism; CYP2D6 → O-desmethyltramadol active metabolite; serotonin-syndrome risk with SSRIs
• tapentadol (activator) — μ-opioid + NRI. dual mechanism; less constipation than pure μ-agonists; class II controlled
• pentazocine (activator) — κ-agonist + μ partial agonist/antagonist. mixed agonist-antagonist; ceiling effect on respiratory depression; can precipitate withdrawal in opioid-dependent users
• butorphanol (activator) — κ-agonist + μ partial agonist/antagonist. mixed agonist-antagonist; intranasal for migraine; psychomimetic effects via κ
• remifentanil (activator) — μ-opioid receptor. ultra-short-acting μ-agonist; esterase metabolism → context-insensitive half-life ~3 min; OR anesthesia adjunct
• alfentanil (activator) — μ-opioid receptor. short-acting μ-agonist; fast onset (small Vd) + offset; OR/ICU
• sufentanil (activator) — μ-opioid receptor. ~5-10× fentanyl potency; OR + chronic intrathecal; sublingual SDT for breakthrough cancer pain
• naltrexone (inhibitor) — μ-opioid (competitive antagonist) + κ. opioid + alcohol use disorder; PO daily or monthly depot IM; precipitated withdrawal in opioid-dependent
• ldn (inhibitor) — μ-opioid (low-dose, paradoxical microglial modulation). low-dose naltrexone; off-label for chronic pain + autoimmune; paradoxical anti-inflammatory mechanism debated

References

• PMID 11152760
• PMID 30792513