This is a drug-mechanism module, not a disease. Do not create a generic "Opioid Antagonist Effect" or "Opioid Withdrawal" Disease entry from it. The disease entries are the ones that conform: Acute_Opioid_Poisoning on the respiratory-restoration branch, Opioid_Use_Disorder on the reward-blockade and precipitated-withdrawal branches, Alcohol_Use_Disorder on the endogenous-opioid arm of the reward branch, and any entry whose treatments include a peripherally acting antagonist on the enteric branch.
SCOPE. The module covers competitive antagonism at the mu-opioid receptor only. Agonism at the same receptor, including the partial agonist buprenorphine with its respiratory ceiling, is mu_opioid_receptor_agonism; the decision to model the two directions as separate modules was taken because the consequence chains differ rather than simply inverting - precipitated withdrawal and renarcotization have no counterpart in the agonism module, and the peripherally restricted agents separate a branch that no agonist separates. Kappa- and delta-receptor pharmacology, the mixed agonist-antagonists, naloxone's non-opioid actions, and the GABAergic and glutamatergic physiology of spontaneous opioid withdrawal outside the noradrenergic arm are out of scope and stay on the disorder entries.
INVERSE NODES. Five nodes here are the explicit inverse of a node in mu_opioid_receptor_agonism and say so in their own descriptions, one for one along the trunk and on two of the branches:
- "Loss of Gi/o Coupling and Relief of Adenylyl Cyclase Inhibition" inverts
"Gi/o Coupling and Adenylyl Cyclase Inhibition".
- "GIRK Closure and Recovery of Presynaptic Calcium Current" inverts "G
Beta-Gamma Gating of GIRK Current and Calcium Channel Inhibition", and binds
the same two molecular-function terms with the modifiers reversed.
- "Restored Neuronal Excitability" inverts "Neuronal Hyperpolarization and
Reduced Neurotransmitter Release"; both are the central effector and key
conformance target of their module.
- "Brainstem Respiratory Rhythm Restoration" inverts "Brainstem Respiratory
Rhythm Depression".
- "Enteric Mu-Opioid Receptor Blockade and Restored Propulsive Motility"
inverts "Enteric Neuron Inhibition and Slowed Propulsive Motility".
The agonism module's trunk is three nodes and so is this one, so a conforming entry that models both directions should point at the matching pair at each step rather than reusing one node for both. The two branches with no inverse here are the agonism module's analgesia branch, which antagonism does not separately model because loss of analgesia is simply the agonism branch withdrawn, and its desensitization branch, which is an adaptation to continued agonist exposure rather than anything an antagonist produces.
BOUNDARY WITH addiction_cycle. The reward branch here stops at blockade of the dopamine-reinforcement signal, which is the receptor-level pharmacology upstream of the addiction_cycle trigger node "Acute Mesolimbic Dopamine Reinforcement". That module already carries an "Opioid Receptor Antagonists" treatment entry pointing at its own trigger node for alcohol use disorder and gambling disorder; this module records why the antagonist reaches that node, and does not re-model incentive salience, the ventral-to-dorsal striatal shift, or the extended-amygdala stress systems.
Node term-binding follows module convention (GO, CL and UBERON terms only; the drug identities live on the treatments' therapeutic_agent as CHEBI ids). The receptor gene OPRM1 is bound on the trigger node, reusing the identifier mu_opioid_receptor_agonism binds. Two consequence nodes carry no biological_processes term: GO has no process for an opioid withdrawal syndrome and none for drug reinforcement or relapse. Searches run against the configured GO source immediately before committing this file were `runoak -i ols:go search "withdrawal"` (no results), `runoak -i ols:go search "reward"` (no results) and `runoak -i ols:go search "drug-seeking"` (no results); the nearest hits for the behavioural arm, `GO:0043278` response to morphine and `GO:1990708` conditioned place preference, are deliberately not bound anywhere in this module: neither names the loss of a drug effect that the reward branch asserts, and the branch carries `GO:0014046` dopamine secretion plus prose instead. The locus coeruleus is bound with UBERON's own spelling, `UBERON:0002148` locus ceruleus.
WHAT THIS MODULE DOES NOT CLAIM. The respiratory branch is reversible but the reversal is not durable: naloxone's duration of effect is shorter than that of many opioid agonists, so a single dose can be followed by renarcotization, and illicitly manufactured fentanyl analogs may need higher or repeated dosing. Nalmefene is included as a higher-affinity, longer-acting reversal agent, but whether that is an advantage in overdose is contested: its intranasal approval rests on a bridge to the injectable formulation and a healthy-volunteer pharmacodynamic study rather than a randomized efficacy trial in overdose, and two toxicology societies recommend against replacing naloxone with it because the longer occupancy may lengthen precipitated withdrawal. Both sides are recorded on its treatment entry, including a REFUTE item; the module itself takes no position. Precipitated withdrawal is modelled as a toxicity of the same receptor action, not as a separate drug effect, and the module makes no claim that it can be dissociated from reversal efficacy - that is the open knowledge gap below.
Competitive Mu-Opioid Receptor Antagonist Occupancy
trigger
The trigger and the drug-target node, and the inverse of the agonist-binding trigger of mu_opioid_receptor_agonism. An antagonist - naloxone, nalmefene, naltrexone, or a peripherally restricted agent - occupies the orthosteric site of the mu-opioid receptor encoded by OPRM1 without stabilizing an active receptor conformation, so it displaces an agonist already bound and prevents further agonist binding. The competition is surmountable in both directions, which is the whole clinical character of the node: an agonist of high receptor affinity needs a higher antagonist concentration or a continuous infusion before reversal is achieved, and a further agonist dose can reclaim the receptor. Antagonists differ on this node in pharmacokinetics and distribution rather than in receptor mechanism - how long occupancy lasts, and whether the drug enters the brain at all.
Used by disorders
Opioid Use Disorder
as Competitive Mu-Opioid Receptor Antagonist Occupancy (Naltrexone and Naloxone Target)
Downstream
-
Loss of Gi/o Coupling and Relief of Adenylyl Cyclase Inhibition
An occupied but unactivated receptor no longer engages Gi/o heterotrimers, so the cyclase arm of agonist signalling lapses first and the channel arm with it.
Show evidence (2 references)
"An opioid with high receptor affinity will require greater naloxone concentrations and/or a continuous infusion before reversal sets in compared with an opioid with lower receptor affinity."
States the competitive, affinity-dependent character of the occupancy this node describes: how much antagonist is needed is set by the agonist it has to displace. Evidence source is OTHER because this is a review article.
"there is a risk of inadequate response or re-narcotization after the administration of a single dose of naloxone in patients who have taken large doses or long-acting opioid formulations, as the duration of effect of naloxone is shorter than that of many opioid agonists"
Supports the pharmacokinetic axis on which the treatments differ at this node, and the surmountability that makes a single antagonist dose temporary. Evidence source is OTHER because this is a review article.
Loss of Gi/o Coupling and Relief of Adenylyl Cyclase Inhibition
amplifier
The first amplifier, and the inverse of "Gi/o Coupling and Adenylyl Cyclase Inhibition" in mu_opioid_receptor_agonism. An occupied but unactivated receptor no longer engages inhibitory Gi/o heterotrimers, so the G alpha-i subunit is not dissociated, adenylyl cyclase is released from inhibition and cyclic AMP recovers. The step is quantitatively competitive rather than all-or-none: an antagonist's potency against agonist-driven cAMP inhibition is measured as a concentration producing half-maximal relief, and that concentration rises steeply with the affinity and efficacy of the agonist it has to displace - the cell-level form of the surmountable competition on the trigger node. In a neuron adapted to chronic agonist exposure the cyclase this node releases has itself been compensatorily upregulated, which is why the withdrawal branch downstream is a rebound rather than a return.
Downstream
-
GIRK Closure and Recovery of Presynaptic Calcium Current
With no Gi/o activation there is no free G beta-gamma either, so the channel arm of agonist signalling lapses in the same step as the cyclase arm.
Show evidence (2 references)
"Both antagonists revealed 10 to 100-times higher IC50-values against CAR-mediated cAMP inhibition compared to the other opioids"
Measures this node directly: naloxone and nalmefene are assayed by the concentration at which they relieve agonist-driven cAMP inhibition, and that concentration depends on which agonist is bound. Evidence source IN_VITRO (HEK-293 cells stably expressing the mu-opioid receptor).
"Today it is widely accepted that all four opioid receptor types couple to pertussis toxin sensitive G-proteins including Gαi, to cause inhibition of cAMP formation."
States the coupling and the cyclase inhibition that this node withdraws, in the direction the agonism module records them. Marked INDIRECT because the claim here follows from removing what this sentence asserts rather than being asserted by it. Evidence source is OTHER because this is a review article.
GIRK Closure and Recovery of Presynaptic Calcium Current
amplifier
The second amplifier, and the inverse of "G Beta-Gamma Gating of GIRK Current and Calcium Channel Inhibition" in mu_opioid_receptor_agonism. With no G beta-gamma released from an activated heterotrimer, the G-protein-gated inwardly rectifying potassium channels close and the potassium conductance falls, while the direct G beta-gamma block of P/Q-, N- and L-type voltage-gated calcium channels is lifted and presynaptic calcium current recovers. The agonist-evoked outward potassium current is the measurable handle on this node - it reverses at the potassium equilibrium potential and is barium-sensitive, and an antagonist abolishes the physiological effect it produces within minutes. As in the agonism module the two halves are complementary rather than redundant: the potassium arm governs somatic and postsynaptic firing, the calcium arm governs transmitter release.
Downstream
-
Restored Neuronal Excitability
A fallen potassium conductance depolarizes the neuron back towards threshold while recovered presynaptic calcium entry restores evoked transmitter release.
Show evidence (3 references)
"In voltage-clamp recordings the outward current produced by the opioid agonist ME was concentration dependent, reversed at the potassium equilibrium potential and was blocked by BaCl2"
Identifies the conductance this node closes, by the two criteria that establish it as a GIRK current. Evidence source IN_VITRO (voltage-clamp recordings in rat brainstem slices).
"resulted in robust apneusis (lengthened low amplitude inspiration due to loss of post-inspiratory drive) that was rapidly reversed by the opioid antagonist naloxone"
Shows an antagonist reversing, within minutes, the respiratory effect that the same study attributes to the potassium conductance quoted above, which is this node read out in the intact preparation. Evidence source MODEL_ORGANISM (in situ arterially perfused working heart-brainstem preparation of rat).
"Opioid receptor induced inhibition of calcium conductance is mediated by binding of the dissociated Gβγ subunit directly to the channel."
States the calcium half of this node in the agonist direction, and attributes it to free G beta-gamma - which is exactly what antagonist occupancy leaves unavailable. Marked INDIRECT because the recovery claimed here follows from removing the mechanism this sentence describes. Evidence source is OTHER because this is a review article.
Restored Neuronal Excitability
central effector
The central effector and the key conformance target: the single rate-limiting cellular event of mu-opioid receptor antagonism, and the exact inverse of the mu_opioid_receptor_agonism central effector "Neuronal Hyperpolarization and Reduced Neurotransmitter Release". The hyperpolarization lifts, tonic firing resumes and evoked transmitter release returns. In an opioid-naive neuron this restores baseline excitability; in a neuron adapted to chronic agonist exposure, with adenylyl cyclase and cAMP signalling compensatorily upregulated, removal of the inhibition leaves the cell firing above its prior rate, which is what makes the withdrawal branch a rebound rather than a return. Everything below this node is the same event read out in a different neuronal population, and conforming entries substitute the population their disease concerns (pre-Botzinger complex rhythm-generating neurons, locus coeruleus noradrenergic neurons, mesolimbic GABAergic interneurons, enteric neurons) while keeping this node unchanged.
Downstream
-
Brainstem Respiratory Rhythm Restoration
In the brainstem inspiratory rhythm generator the same event returns respiratory rate towards baseline.
-
Locus Coeruleus Noradrenergic Rebound
In locus coeruleus neurons adapted to chronic agonist exposure the same event releases a compensatorily upregulated cell into hyperactivity.
-
Blockade of Opioid Reward Signalling
In the ventral tegmental area the neuron no longer hyperpolarized is a GABAergic interneuron, so the dopamine neuron loses its disinhibition and the reinforcing dopamine signal is not produced.
-
Enteric Mu-Opioid Receptor Blockade and Restored Propulsive Motility
In enteric neurons the same event returns the neural drive to propulsive contraction, and is reachable by a drug that never enters the brain.
Show evidence (2 references)
"Experiments with agonists and antagonists selective for opioid receptor subtypes indicated that the hyperpolarization of secondary cells involved the mu-receptor."
Receptor-subtype-selective antagonists are what established that the hyperpolarization carried by the agonism module's central effector is mu-receptor-mediated, which is the same thing as saying it is blockable at that receptor. Marked INDIRECT because the recordings measure the agonist effect and its pharmacological identity rather than a therapeutic reversal. Evidence source IN_VITRO (intracellular recordings in rat brain slices).
"Different electrophysiological studies have reported the hyperactivity of LC neurons following naloxone-induced withdrawal in brain slices taken from chronically morphine treated rats"
Supports the second half of this node's description - that in an agonist-adapted neuron removal of the inhibition leaves firing above baseline rather than at it - at the level of recorded cellular activity. The sentence restates established slice electrophysiology rather than this paper's own paragigantocellularis result, hence quote_role BACKGROUND.
Brainstem Respiratory Rhythm Restoration
effector
The therapeutic branch in overdose, and the inverse of "Brainstem Respiratory Rhythm Depression" in mu_opioid_receptor_agonism. Antagonist occupancy of mu-opioid receptors on rhythm-generating neurons of the pre-Botzinger complex and on the modulatory Kolliker-Fuse and parabrachial nuclei lifts the inhibition of the inspiratory rhythm generator, and respiratory rate recovers. The branch is pharmacologically unlike its agonism counterpart in one respect that matters clinically: recovery lasts only as long as antagonist occupancy exceeds what the remaining agonist can reclaim, so the branch has to be sustained rather than merely triggered.
Downstream
-
Reversal of Opioid-Induced Respiratory Depression
Recovered inspiratory rhythm restores ventilation and averts the apnea that is the mechanism of fatal overdose.
Show evidence (1 reference)
"The only treatment currently available to reverse opioid respiratory depression is by naloxone infusion."
Establishes that antagonist occupancy is the route by which depressed respiratory drive is reversed, which is the claim of this branch. Evidence source is OTHER because this is a review article.
Reversal of Opioid-Induced Respiratory Depression
consequence
The consequence an acute opioid poisoning entry conforms to: recovery of respiratory drive and consciousness, and survival of an otherwise fatal overdose. The consequence is real at the population level - naloxone distribution has been followed by reduced overdose mortality - and is time-limited at the individual level, because antagonist occupancy decays faster than the agonist burden in a patient who has taken a large dose or a long-acting formulation. Clinical practice therefore titrates to effect and continues treatment by infusion, and the illicitly manufactured fentanyl analogs have pushed the required dose and the number of repeat administrations upward.
Show evidence (3 references)
"With increased utilization of naloxone there has been a subsequent reduction in mortality following an opioid overdose."
The population-level consequence of this branch. Marked INDIRECT because the mortality trend follows from the reversal rather than measuring the restored respiratory drive itself. Evidence source is OTHER because this is a review article.
"The clinical approach to severe opioid-induced respiratory depression is to titrate naloxone to effect and continue treatment by continuous infusion until chances for renarcotization have diminished."
States the short-duration caveat that distinguishes this consequence from a durable cure, and the titration practice that follows from sharing one cellular effector with the precipitated-withdrawal branch.
"there is growing evidence that higher dose or multiple administrations of naloxone are required to fully reverse IMF related toxicity"
Supports the dose and repeat-administration caveat for high-affinity synthetic agonists, the practical form the surmountable competition on the trigger node takes. Evidence source is OTHER because this is a review article.
Locus Coeruleus Noradrenergic Rebound
effector
The toxicity branch, and the one with no counterpart in the agonism module. Locus coeruleus noradrenergic neurons are inhibited by mu-opioid receptor agonism and adapt to chronic exposure by upregulating adenylyl cyclase and cAMP signalling, so their basal drive is restored against a continuing agonist load. Antagonist occupancy removes the inhibition from an already compensated cell, and firing and norepinephrine release overshoot: an antagonist given to a dependent patient markedly increases norepinephrine release in forebrain targets, and the same agents that reduce that efflux reduce the behavioural signs. This branch is not reached in an opioid-naive patient, which is why it is a toxicity of the drug in one population rather than a property of the drug.
Downstream
-
Precipitated Opioid Withdrawal
Noradrenergic overshoot, with the wider central excitation the same adaptation produces, generates the autonomic and affective withdrawal syndrome.
Show evidence (3 references)
"Low dose naltrexone pre-treatment significantly attenuated withdrawal-induced increases of extracellular norepinephrine in the BNST, with a smaller effect in the FC."
Measures the noradrenergic rebound of this branch directly, as antagonist-precipitated increases in extracellular norepinephrine in a forebrain target of the brainstem noradrenergic nuclei. Evidence source MODEL_ORGANISM (in vivo microdialysis in morphine-dependent rats).
"Studies from various in vivo and in vitro animal models of opioid withdrawal syndrome have led to understanding of its pathobiology which includes complex interrelated pathways leading to adenylyl cyclase superactivation based central excitation."
Supports the cyclase-superactivation half of this node: the adaptation that turns loss of inhibition into central excitation rather than a return to baseline. Evidence source is OTHER because this is a review article.
"Abrupt discontinuation of opioids or administration of opioid antagonists (such as naloxone (NAL)) markedly increases norepinephrine release and triggers systemic withdrawal symptoms"
States the rebound this node asserts, and that an antagonist produces it as directly as abrupt cessation does. The sentence summarises the preclinical literature in this paper's introduction rather than its own imaging result, hence quote_role BACKGROUND and evidence_source MODEL_ORGANISM.
Precipitated Opioid Withdrawal
consequence
The toxic consequence, and the dose-limiting one for every centrally acting antagonist. In an opioid-dependent patient, antagonist occupancy produces within minutes the autonomic, gastrointestinal and affective syndrome that spontaneous cessation would produce over hours - yawning, rhinorrhea, lacrimation, abdominal cramps, restlessness and aversion - together with cortisol release and increased dorsal striatal dopamine release that tracks the severity of the subjective aversion. Clinically this is why overdose reversal is titrated to respiratory effect rather than to full receptor blockade, and why antagonist maintenance for opioid use disorder requires a completed medical withdrawal before induction. It is the reason the peripherally restricted agents exist: an antagonist that does not enter the brain does not reach this branch.
Show evidence (3 references)
"Reversal of opioid toxicity may precipitate an opioid-withdrawal syndrome."
States this consequence as a consequence of the reversal itself, which is what places it on the same trigger node rather than in a separate mechanism. Evidence source is OTHER because this is a review article.
"We found that acute withdrawal precipitated by the opioid antagonist naloxone significantly increased dorsal striatal DA release in OUD participants"
Measures a central correlate of this consequence in dependent humans given an antagonist, which is the population in which the branch is reached. Evidence source HUMAN_CLINICAL (positron emission tomography in men with opioid use disorder).
"Net changes in striatal DA were significantly correlated with a subjective index of withdrawal aversion"
Ties the measured neurochemical change to the aversive experience that makes this consequence dose-limiting, rather than leaving it an imaging finding of unstated clinical weight.
Blockade of Opioid Reward Signalling
effector
The reward branch. With mu-opioid receptors on ventral tegmental area GABAergic interneurons occupied by an antagonist, an exogenous opioid can no longer hyperpolarize them, the dopamine neurons are not disinhibited, and the reinforcing dopamine signal that follows opioid use is not produced - which is read out clinically as loss of drug liking after a heroin challenge. The same blockade applies to the endogenous opioid arm: ethanol consumption releases beta-endorphin and other endogenous opioid peptides into the ventral striatum, and an antagonist competitively impedes their action at the same receptor, which is the mechanism by which naltrexone and nalmefene reduce alcohol-induced reward. Peripherally restricted antagonists do not reach this branch.
Downstream
-
Reduced Reinforcement and Relapse Prevention
Without the reinforcing dopamine signal, opioid or alcohol use is not rewarded, and the probability of return to heavy use falls.
Show evidence (3 references)
"Naltrexone levels above 2 ng/mL blocked nearly all VAS ratings of drug liking after intravenous heroin administration."
Measures the blockade of this branch in humans, and shows it is graded by antagonist exposure rather than all-or-none, consistent with the surmountable competition on the trigger node. Evidence source HUMAN_CLINICAL (laboratory heroin challenge in participants with opioid use disorder).
"Ethanol consumption has been demonstrated to induce the release of dopamine (DA) and endogenous opioids, also known as peptide neurotransmitters, in the ventral striatum"
Establishes the endogenous-opioid arm of this branch, the ligand an antagonist displaces when there is no exogenous opioid to displace. Evidence source is OTHER because this is a review article.
"It has been demonstrated that naltrexone exerts its effects by competitively binding to receptors located within the central and peripheral nervous systems, thereby impeding the actions of endorphins and opioids, including beta-endorphins, which have been implicated in the experience of euphoria..."
Names the competitive displacement of endogenous opioid peptides as the mechanism of the alcohol arm, which is why this branch carries both the exogenous and the endogenous ligand rather than splitting into two nodes.
Reduced Reinforcement and Relapse Prevention
consequence
The therapeutic consequence in substance use disorder: use that produces no reward is not reinforced, so antagonist maintenance reduces relapse to regular opioid use and reduces heavy drinking in alcohol use disorder. Unlike the respiratory branch this consequence requires sustained occupancy over weeks to months, which is what the extended-release formulation supplies, and unlike agonist maintenance it confers no tolerance - so a lapse after antagonist levels fall carries the full overdose risk of the patient's former dose. The adaptations that repeated reward has already produced in the circuit are not reversed by blockade and are modelled in addiction_cycle, which carries its own antagonist treatment entry for the alcohol and gambling indications.
Show evidence (2 references)
"once initiated, both medications were equally safe and effective"
A randomized comparative-effectiveness trial places antagonist maintenance on a par with agonist maintenance for relapse prevention once induction succeeds, which is what makes this consequence a therapeutic endpoint of the module rather than a pharmacological curiosity. Evidence source HUMAN_CLINICAL (multicentre randomised trial in opioid use disorder).
"naltrexone reduced the risk of heavy drinking to 83% of the risk in the placebo group"
Quantifies the same consequence on the endogenous-opioid arm, in a disease where the antagonist displaces no exogenous drug. Evidence source HUMAN_CLINICAL (meta-analysis of randomized trials in alcohol dependence).
Enteric Mu-Opioid Receptor Blockade and Restored Propulsive Motility
effector
The gastrointestinal branch, and the inverse of "Enteric Neuron Inhibition and Slowed Propulsive Motility" in mu_opioid_receptor_agonism. Mu-opioid receptors on enteric neurons of the myenteric and submucosal plexuses are peripheral, so an antagonist designed not to cross the blood-brain barrier - a quaternary amine such as methylnaltrexone, or a pegylated or efflux-substrate molecule such as naloxegol or naldemedine - occupies them while leaving central receptors free. The inhibition of coordinated propulsive contraction is relieved and motility recovers, with central analgesia preserved and the withdrawal branch not reached. This is the one branch of the module that is separable from the others by drug distribution rather than by dose, and it is the clearest demonstration that the module's branches are one receptor action in different populations.
Downstream
-
Relief of Opioid-Induced Constipation
Recovered propulsive motility produces spontaneous bowel movements in patients whose constipation did not respond to laxatives.
Show evidence (3 references)
"As a quaternary amine, methylnaltrexone, a mu-opioid-receptor antagonist, has restricted ability to cross the blood-brain barrier."
States the distributional property that makes this branch separable: the antagonist reaches enteric receptors and not central ones. The sentence is the trial's framing of its own rationale rather than a result, hence quote_role BACKGROUND, and it names no study design, so evidence_source is OTHER.
"Evidence of withdrawal mediated by central nervous system opioid receptors or changes in pain scores was not observed."
The negative result that confirms the separation: blocking the enteric receptors moved neither the analgesia branch of the agonism module nor the precipitated-withdrawal branch of this one. Evidence source HUMAN_CLINICAL (randomized trial in patients with advanced illness).
"Treatment with naloxegol, as compared with placebo, resulted in a significantly higher rate of treatment response, without reducing opioid-mediated analgesia."
Replicates the same separation with an orally administered peripherally acting antagonist of a different chemical class, which is why the branch is modelled as a property of receptor location rather than of one molecule. Evidence source HUMAN_CLINICAL (two phase 3 randomized trials).
Relief of Opioid-Induced Constipation
consequence
The therapeutic consequence of the enteric branch: laxation in patients on stable opioid therapy whose constipation is refractory to laxatives, without loss of analgesia and without withdrawal. Because opioid-induced constipation is the one branch of mu_opioid_receptor_agonism to which tolerance does not develop, this consequence is what often allows an opioid dose to be maintained, and conforming entries record it as a management arm rather than as a reversal of the opioid itself.
Show evidence (2 references)
"Subcutaneous methylnaltrexone rapidly induced laxation in patients with advanced illness and opioid-induced constipation."
States this consequence as the trial's own conclusion, in the population where the enteric branch is clinically dose-limiting. Evidence source HUMAN_CLINICAL (randomized placebo-controlled trial).
"Naldemedine treatment led to a significantly higher responder rate than did placebo and was generally well tolerated."
Replicates the consequence with a third peripherally acting antagonist in chronic non-cancer pain, a different population from the advanced-illness trial. Evidence source HUMAN_CLINICAL (two phase 3 randomized trials).
Naloxone (Short-Acting Overdose Reversal)
Platform:
Small molecule
The competitive mu-opioid receptor antagonist used to reverse opioid-induced respiratory depression, by injection in hospital and by intranasal or intramuscular take-home kits in the community. It engages the antagonist-occupancy trigger node of this module and therefore produces every branch it can reach: respiratory recovery and, in a dependent patient, precipitated withdrawal, from one receptor action. Its defining limitation is pharmacokinetic rather than pharmacodynamic - occupancy decays faster than the agonist burden it was given to displace, so a single dose can be followed by renarcotization and high-affinity synthetic agonists may need higher or repeated dosing.
Evidence: 2
Nalmefene (Longer-Acting Overdose Reversal)
Platform:
Small molecule
An opioid receptor antagonist approved as a nasal spray for emergency treatment of known or suspected opioid overdose, and used for the same purpose by injection. Mechanistically it engages the same trigger node as naloxone; what distinguishes it there is higher receptor affinity and a longer duration of action, and in a healthy-volunteer model of remifentanil-induced respiratory depression a faster onset of reversal. Whether those properties are an advantage in overdose is contested rather than settled, and the dispute belongs on this entry rather than on the module's branches: the intranasal formulation was approved without new clinical data and has not been studied in patients currently overdosing, comparative effectiveness data against naloxone are sparse, and longer occupancy at higher affinity may mean a more prolonged and severe precipitated withdrawal. Two toxicology societies therefore recommend against replacing naloxone with it as the primary antidote. Nalmefene's separate use in alcohol use disorder acts on the endogenous-opioid arm of the reward branch and is recorded with naltrexone below and in addiction_cycle.
Evidence: 5
Extended-Release Naltrexone (Antagonist Maintenance)
Platform:
Small molecule
A long-acting competitive mu-opioid receptor antagonist given as a monthly injection for relapse prevention in opioid use disorder, and orally or by injection for alcohol use disorder. It engages the same trigger node, and what makes it a maintenance drug is sustained occupancy over weeks: plasma levels above a threshold block the subjective effects of a heroin challenge, and as levels fall that blockade is lost. Because the drug produces precipitated withdrawal in anyone still physically dependent, induction requires a completed medical withdrawal first, which is the practical difficulty that distinguishes antagonist from agonist maintenance. In alcohol use disorder the ligand displaced is endogenous rather than exogenous.
Evidence: 2
Peripherally Acting Mu-Opioid Receptor Antagonists (PAMORAs)
Platform:
Small molecule
Methylnaltrexone, naloxegol and naldemedine, antagonists designed not to enter the central nervous system, used for opioid-induced constipation in patients who need to continue opioid therapy. They engage the same receptor occupancy as the reversal agents but only in the periphery, so they reach the enteric branch of this module and not the respiratory, withdrawal or reward branches - laxation without loss of analgesia and without withdrawal. The three differ in how exclusion from the brain is achieved, a quaternary amine against a pegylated molecule and an efflux-transporter substrate, and not in receptor mechanism, which is why they share one treatment entry on one node.
Evidence: 2