Opioid Mechanisms of Action

1. Receptor Classes

• Goodman & Gilman's Pharmacological Basis of Therapeutics, p. 463-464
• Katzung's Basic and Clinical Pharmacology 16e, p. 874-875

2. Core Intracellular Signaling Cascade

α-Subunit Effects

• Inhibits adenylyl cyclase (AC) → reduces cAMP → reduces PKA activity → decreased phosphorylation of many proteins and reduced cAMP-dependent Ca²⁺ influx
• On chronic opioid exposure, AC becomes supersensitized; abrupt withdrawal causes a "cAMP overshoot" - a key molecular basis of withdrawal symptoms

βγ Dimer Effects

• Closes voltage-gated Ca²⁺ channels (VGCCs) on presynaptic terminals → reduced Ca²⁺ influx → inhibited neurotransmitter release
• Opens GIRK channels (G protein-coupled inwardly rectifying K⁺ channels) → K⁺ efflux → membrane hyperpolarization → decreased neuronal firing
• Activates MAPK/ERK and JNK pathways (important for long-term neuroadaptations)
• Activates Ras, Rho, PKC, PLC signaling cascades
• Goodman & Gilman's, p. 465
• Katzung 16e, p. 874

3. Pre- and Postsynaptic Actions

• VGCCs close → reduced release of glutamate (main excitatory amino acid from nociceptive terminals), substance P, acetylcholine, norepinephrine, and serotonin
• This attenuates transmission of pain signals from primary to secondary afferent neurons in the dorsal horn

• K⁺ channel opening → hyperpolarization of secondary neurons → reduced action potential generation

• Functional μ receptors exist on peripheral sensory nerve endings
• Particularly relevant in inflammation; immune cells in injured tissue release β-endorphin, activating peripheral MOR
• This reduces sensory neuron activity and transmitter release locally
• Katzung 16e, p. 874-875

4. Receptor Subtypes and Their Physiological Effects

• Katzung 16e, p. 875

5. Descending Pain Modulation

• In the ventrolateral periaqueductal gray (PAG), opioids inhibit GABA interneurons → releases inhibition on descending pain-modulating neurons (raphe nucleus, noradrenergic nuclei) → these project to the dorsal horn to further suppress pain transmission
• In the ventral tegmental area (VTA), inhibition of GABA interneurons → increased dopamine release in the nucleus accumbens → underlies reward and addictive potential
• Goodman & Gilman's, p. 465

6. Receptor Structure

• Extracellular N-terminus
• 7 transmembrane (TM) alpha-helical domains connected by 3 extracellular and 3 intracellular loops
• Intracellular C-terminus (palmitoylated, forming an extra small alpha-helix)
• Orthosteric binding site within the TM domains - the nitrogen atom and phenolic OH group of morphine-like molecules (or the Tyr moiety of endogenous peptides) form a salt bridge with a conserved aspartate in TM3 and a water-mediated interaction with a histidine in TM6
• Larger endogenous peptides (e.g., dynorphin) extend beyond the orthosteric pocket and interact with extracellular loops for receptor selectivity
• Goodman & Gilman's, p. 465

7. Signal Termination: Desensitization and Internalization

1. GRK (GPCR receptor kinase) phosphorylates the activated, agonist-bound receptor at intracellular loop and C-terminus residues
2. β-arrestin is recruited to the phosphorylated receptor → sterically uncouples the receptor from G proteins (desensitization)
3. β-arrestin scaffolds ERK1/2 and JNK signaling (arrestin-biased signaling)
4. The receptor-arrestin complex is internalized into endosomes
5. Internalized receptor is either recycled to the plasma membrane (resensitization) or targeted for lysosomal degradation (downregulation)

• Goodman & Gilman's, p. 465 (Figure 23-2)

8. Biased Agonism (Functional Selectivity)

• G-protein biased agonists preferentially activate the Gi/o pathway (analgesia, euphoria) while minimizing β-arrestin recruitment (reducing tolerance, constipation, and possibly respiratory depression)
• Oliceridine (approved IV opioid) was designed as a G-protein biased MOR agonist, though clinical evidence shows its safety profile is similar to morphine
• Mice with targeted deletion of β-arrestin develop less morphine tolerance - but still develop dependence - suggesting biased agonism has complex trade-offs

• Kaplan & Sadock's Comprehensive Textbook of Psychiatry, p. 590
• Goodman & Gilman's, p. 465

9. Partial Agonists and Antagonists

• Partial agonists (e.g., buprenorphine) bind the orthosteric site but cannot produce the same maximal response as full agonists regardless of dose; buprenorphine's very high μ-receptor affinity means it displaces full agonists and requires much higher doses of naloxone for reversal
• Antagonists (e.g., naloxone, naltrexone) bind with high affinity but produce no receptor activation; naloxone has ~10x higher affinity for μ than κ and reverses respiratory depression within 2 minutes IV; naltrexone has longer oral duration (up to 24 hours per dose)
• Allosteric modulators act at a distinct non-orthosteric site: positive allosteric modulators can amplify endogenous opioid signaling without directly activating the receptor - a potential future strategy to reduce side effects
• Lippincott Pharmacology, p. 727-728
• Goodman & Gilman's, p. 465

Summary Diagram of Key Mechanisms

Opioid binds MOR/KOR/DOR
         ↓
    Gi/o activation
    ↙            ↘
  α subunit      βγ dimer
     ↓              ↙    ↘
↓ Adenylyl     Close VGCC   Open GIRK (K⁺)
  cyclase       (presynaptic)  (postsynaptic)
↓ cAMP              ↓              ↓
↓ PKA          ↓ Transmitter    Hyperpolarization
               release          ↓ Firing
(Glutamate, SP,
  NE, 5-HT, ACh)

         ↓
   ANALGESIA (+ euphoria, sedation, respiratory depression)

Also via βγ: MAPK/ERK → long-term neuroplasticity, addiction

Opioids: Other Mechanisms of Action

1. Monoamine Reuptake Inhibition (Tramadol & Tapentadol)

• Tramadol's serotonin effect creates risk of serotonin syndrome (especially with SSRIs/MAOIs)
• Tramadol is a prodrug dependent on CYP2D6 for conversion to active M1; poor metabolizers (8% of Whites, up to 7% of African Americans, <0.5% of Asians) have attenuated analgesia
• Tapentadol avoids CYP2D6 dependency (glucuronidation instead) - lower seizure risk and no serotonin syndrome risk compared to tramadol
• Barash Clinical Anesthesia 9e, p. 4678; Katzung 16e, p. 1027

2. NMDA Receptor Interaction and Opioid-Induced Hyperalgesia (OIH)

1. Chronic MOR occupancy activates PKC (protein kinase C)
2. PKC phosphorylates and upregulates/sensitizes NMDA glutamate receptors in spinal dorsal horn neurons
3. Enhanced NMDA receptor activity increases excitability of pain-transmission neurons (central sensitization)
4. The net result is paradoxical increased pain sensitivity - opioid-induced hyperalgesia (OIH)

• A paradoxical phenomenon where pain sensitivity increases during or after escalating opioid treatment
• Particularly well-documented after remifentanil infusions during anesthesia
• Distinct from tolerance (tolerance = reduced effect; OIH = opposite effect - more pain)

• Ketamine (an NMDA antagonist) can prevent or reverse OIH - hence its co-administration with opioids in perioperative settings
• Methadone uniquely among opioids has intrinsic NMDA antagonist activity, which may explain its utility in neuropathic pain and lower propensity for tolerance
• Goodman & Gilman's, p. 474 (system-level counteradaptation); Miller's Anesthesia 10e, p. 2893

3. Adenylyl Cyclase Superactivation - Molecular Basis of Withdrawal

• Normally, opioids inhibit AC → lower cAMP
• Chronically, cells compensate by upregulating AC expression and activity (superactivation)
• When opioid is suddenly withdrawn → the upregulated, now-uninhibited AC produces a massive cAMP overshoot
• This "cAMP storm" drives the autonomic surge of withdrawal: tachycardia, hypertension, diarrhea, hyperthermia, agitation, hyperalgesia, mydriasis, hormonal surges (ACTH, cortisol, pituitary hormones)

• Goodman & Gilman's, p. 474

4. Reward and Mesocorticolimbic Dopamine Disinhibition

1. In the VTA, tonically active GABA interneurons normally suppress dopamine neuron firing
2. MOR activation inhibits these GABA interneurons (by closing VGCCs and opening GIRK channels)
3. With GABA inhibition removed, dopamine neurons fire more freely → increased DA release into the nucleus accumbens (NAc)
4. DA in NAc projects to the ventral pallidum (VP) → generates the positive reinforcing (reward) state
5. MOR on NAc neurons also directly reduces GABA output to VP, further enhancing the reward signal

• Goodman & Gilman's, p. 474-475

5. System-Level Tolerance: Differential Tolerance Development

• Goodman & Gilman's, p. 474

6. Non-Neuronal Mechanisms: Immune System

• Opioid receptors are expressed on macrophages, peripheral microglia, and astrocytes
• Delta-opioid receptors (DOR) on cardiac tissue may afford cardioprotection during ischemia
• β-endorphin released from immune cells in inflamed tissue activates peripheral MOR to produce local analgesia - particularly relevant in inflammatory pain states
• This peripheral immune-mediated opioid effect is why intra-articular opioids (e.g., post-arthroscopy) can provide up to 24 hours of analgesia

• Barash Clinical Anesthesia 9e, p. 1522; Goodman & Gilman's, p. 464

7. Endogenous Opioid Pathway Activation (Non-Drug Mechanisms)

• Stress-induced analgesia - extreme stress triggers endogenous opioid release
• Placebo-induced analgesia - naloxone-reversible, confirming genuine MOR mediation
• Conditioned pain modulation (CPM) - pain at one site is reduced by a second painful stimulus elsewhere ("pain inhibits pain"), mediated partly through endogenous opioid release

• Barash Clinical Anesthesia 9e, p. 1522

Summary: Mechanisms Beyond Classical GPCR Signaling