This is a drug-mechanism module, not a disease. Do not create a generic "Opioid Effect" or "Opioid Toxicity" Disease entry from it. The disease entries are the ones that conform: Acute_Opioid_Poisoning on the respiratory branch, Opioid_Use_Disorder on the reward/tolerance branch, Acute_Post-Surgical_Pain and other pain entries on the analgesia branch, and any entry whose treatments include an opioid on the trigger node.
SCOPE. The module covers AGONISM only. Antagonism at the same receptor (naloxone reversal of overdose, naltrexone for relapse prevention, and the peripherally acting mu-opioid receptor antagonists used for opioid-induced constipation) is a separate module, because the consequence chain differs rather than simply inverting; it is planned as mu-opioid receptor antagonism in projects/GPCR.md. Kappa- and delta-receptor pharmacology, the mixed agonist-antagonists, and the noradrenergic and GABAergic physiology of opioid withdrawal outside the reward circuit are out of scope and stay on the disorder entries.
BUPRENORPHINE belongs here rather than in a third module: it is a partial agonist at the same receptor and engages the same trigger node, and its clinically decisive property is a ceiling on the respiratory branch with no ceiling on analgesia over the same dose range (PMID:16547090). That ceiling is recorded as evidence on the respiratory-depression consequence node and in the buprenorphine treatment entry, not as a separate mechanism.
BOUNDARY WITH addiction_cycle. The reward branch here stops at the consequence node "Opioid Reward and Reinforcement", which is the same event as the addiction_cycle trigger node "Acute Mesolimbic Dopamine Reinforcement", reached through the opioid-specific disinhibition pharmacology this module supplies. That node is the declared hand-off: an opioid use disorder entry should conform to both modules, to this one for how the drug reaches dopamine release and to addiction_cycle for what repeated reward then does to the circuit. Do not re-model incentive salience, the ventral-to-dorsal striatal shift, or the extended-amygdala stress systems here. Tolerance and dependence are a separate consequence node in this module, fed by the receptor-desensitization branch rather than by the dopamine branch; addiction_cycle's own opponent-process node ("Reward Deficit and Stress-System Recruitment") is the circuit-level adaptation and is not the same claim.
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. GO has no term for a pre-Botzinger-complex-specific respiratory rhythm process, so the respiratory branch uses the generic nervous-system regulation of respiratory gaseous exchange and carries the anatomical specificity in locations.
BIASED AGONISM IS NOT SETTLED and is recorded as a hypothesis group (biased_agonism_separates_analgesia_from_side_effects, status EMERGING) plus an open knowledge gap, not as a module node. The original beta-arrestin2 account of respiratory depression did not replicate in a three-laboratory consortium (PMID:32052419), and the therapeutic indices of the G-protein-biased compounds track their intrinsic efficacy rather than their bias factors (PMID:32854452). A conforming entry must not assert that oliceridine or any other biased agonist spares a branch of this module.
Mu-Opioid Receptor Agonist Binding and Activation
trigger
The trigger and the drug-target node. An opioid agonist - endogenous (beta-endorphin, enkephalins) or exogenous (morphine, fentanyl, oxycodone, methadone, buprenorphine) - occupies the mu-opioid receptor encoded by OPRM1, a seven-transmembrane receptor that couples to inhibitory G proteins. Agonists differ on this node in efficacy rather than in mechanism: methadone is a full agonist and buprenorphine a partial agonist at the same receptor, which is what sets the ceiling recorded on the respiratory branch. Receptor expression across nociceptive, brainstem respiratory, enteric and mesolimbic circuits is what makes one receptor event produce four distinct consequences.
Downstream
-
Gi/o Coupling and Adenylyl Cyclase Inhibition
Agonist-bound receptor activates pertussis-toxin-sensitive Gi/o heterotrimers, whose alpha subunit inhibits adenylyl cyclase.
-
Mu-Opioid Receptor Desensitization and Internalization
The same agonist occupancy exposes the receptor to kinase-mediated regulation, which over continued exposure uncouples it from these transducers.
Show evidence (2 references)
"All four receptors are G-protein coupled and activate inhibitory G proteins."
Establishes the trigger of this module: agonist occupancy of an opioid receptor activates inhibitory G proteins, the step every downstream node depends on. Evidence source is OTHER because this is a review article.
"Opioids evoke analgesia through activation of opioid receptors (predominantly the μ opioid receptor) in the central nervous system."
Names the mu-opioid receptor as the receptor through which opioid analgesia is produced, which is why this module is receptor-specific rather than generic to the opioid receptor family.
Gi/o Coupling and Adenylyl Cyclase Inhibition
amplifier
Activated mu-opioid receptor engages inhibitory Gi/o heterotrimeric G proteins. The dissociated G alpha-i subunit inhibits adenylyl cyclase and lowers cyclic AMP, reducing protein kinase A activity and cAMP-dependent calcium influx. This is the classical biochemical signature of opioid receptor activation, and its compensatory reversal - upregulation of adenylyl cyclase and cAMP signalling during chronic exposure - is the adaptation that the tolerance and dependence consequence node records.
Downstream
-
G Beta-Gamma Gating of GIRK Current and Calcium Channel Inhibition
The G beta-gamma subunits released in the same activation step act directly on potassium and calcium channels, in parallel with the cyclase arm.
Show evidence (1 reference)
"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 Gi coupling and the adenylyl cyclase inhibition that define this node, including the pertussis-toxin sensitivity that established it.
G Beta-Gamma Gating of GIRK Current and Calcium Channel Inhibition
amplifier
The ion-channel arm, and the one that carries most of the acute neuronal effect. G beta-gamma released from the activated Gi/o heterotrimer opens G-protein-gated inwardly rectifying potassium channels (Kir3/GIRK), increasing potassium conductance, and binds directly to voltage-gated calcium channels of the P/Q-, N- and L-types, reducing calcium current by making pore opening harder to reach from a given voltage step. The two effects are complementary rather than redundant: the potassium current acts postsynaptically and somatically to silence tonic firing, while the calcium inhibition acts presynaptically to cut transmitter release.
Downstream
-
Neuronal Hyperpolarization and Reduced Neurotransmitter Release
Increased potassium conductance hyperpolarizes the neuron while reduced presynaptic calcium entry lowers evoked transmitter release.
Show evidence (2 references)
"In several reports, the inhibitory effects of opioids on neural excitability were shown to be mediated by interactions of opioid receptors with G protein-regulated inwardly rectifying potassium channel (Kir3)"
Supports the GIRK/Kir3 half of this node as the route by which opioid receptors reduce neuronal excitability.
"Opioid receptor induced inhibition of calcium conductance is mediated by binding of the dissociated Gβγ subunit directly to the channel."
Supports the calcium-channel half of this node and attributes it to direct G beta-gamma binding, which is why both channel effects are placed on one node downstream of Gi/o activation.
Neuronal Hyperpolarization and Reduced Neurotransmitter Release
central effector
The central effector and the key conformance target: the single rate-limiting cellular event of mu-opioid receptor agonism. The GIRK current hyperpolarizes the neuron and silences tonic activity, while reduced presynaptic calcium entry lowers the probability of transmitter release. Everything below this node is the same event read out in a different neuronal population, which is why the module has one trunk and four branches rather than four mechanisms. Conforming entries substitute the population their disease concerns (dorsal horn projection neurons and primary afferent terminals, pre-Botzinger complex rhythm-generating neurons, enteric neurons, mesolimbic GABAergic interneurons) and keep this node unchanged.
Downstream
-
Spinal and Supraspinal Antinociception
In nociceptive circuits the same event suppresses transmission of nociceptive input and engages descending modulation.
-
Brainstem Respiratory Rhythm Depression
In the brainstem inspiratory rhythm generator the same event slows respiratory rate.
-
Enteric Neuron Inhibition and Slowed Propulsive Motility
In the enteric nervous system the same event suppresses the neural drive to propulsive motility and fluid secretion.
-
Mesolimbic Dopamine Disinhibition
In the ventral tegmental area the neuron hyperpolarized is a GABAergic interneuron, so the net effect on the dopamine neuron is excitatory.
Show evidence (2 references)
"This process causes cellular hyperpolarization and inhibits tonic neural activity."
States the hyperpolarization half of the central effector as the consequence of G-protein-gated potassium channel activation.
"Under basal conditions the μ-opioid agonist DAMGO (3 μM) reduced the rate of miniature EPSCs in 68% of neurons"
Measures the reduced-transmitter-release half of the central effector directly: a mu-opioid agonist lowers the frequency, not the amplitude, of miniature excitatory events, the signature of a presynaptic release effect. Evidence source IN_VITRO (patch-clamp recordings in rat spinal cord slices).
Spinal and Supraspinal Antinociception
effector
The analgesia branch. In the superficial dorsal horn of the spinal cord, mu-opioid receptor agonism inhibits glutamate release from primary afferent terminals and hyperpolarizes second-order neurons, suppressing transmission of nociceptive input. Supraspinally, receptor agonism in the periaqueductal gray and the rostral ventromedial medulla disinhibits descending pain-modulating pathways that in turn inhibit spinal nociceptive transmission - the same hyperpolarization producing a net excitatory effect because the cell silenced is inhibitory. The two sites are synergistic, which is why systemic, intrathecal and epidural routes all produce analgesia. GO has no term for transmission of nociceptive input through the dorsal horn, so the node is bound to sensory perception of pain, which is the process this suppressed transmission is a step of: the binding names the pathway the step belongs to and the modifier records the direction, while the anatomical specificity is carried in locations. The second-order cell is bound to the broad neuron of the dorsal spinal cord rather than to dorsal horn interneuron, because the population includes lamina I projection neurons and CL has no class for those.
Downstream
-
Analgesia
Suppressed nociceptive transmission at both levels raises the threshold for, and reduces the magnitude of, the behavioural pain response.
Show evidence (2 references)
"The superficial dorsal horn of the spinal cord is a major site of μ-opioid receptor mediated analgesia"
Places the spinal site of this branch. The quote is the paper's introduction restating established pharmacology rather than its own electrophysiological result, hence quote_role BACKGROUND; the sentence names no study design, so evidence_source is OTHER rather than a grade the sentence does not state.
"The activation of opioid receptors at these locations directly inhibit neurons, which in turn inhibit spinal cord pain transmission"
Supports the supraspinal half of this branch: receptor agonism in descending pain-modulating regions inhibits neurons that themselves inhibit spinal pain transmission, the disinhibition logic of supraspinal analgesia.
Analgesia
consequence
The therapeutic consequence of the module: a dose-dependent reduction in pain report and in nociceptive behaviour. Unlike the respiratory branch this one has no ceiling with a partial agonist over the clinical dose range, which is the pharmacological basis for buprenorphine's therapeutic index. Conforming pain entries substitute their own pain context (acute post-surgical, cancer, neuropathic) and the outcome measure used in it.
Show evidence (1 reference)
"In contrast, buprenorphine 0.4 mg caused a large and long-lived analgesic effect with a maximum increase in pain tolerance current of 23.8 (sd 7.4) mA occurring at 130 min after drug administration."
Quantifies the analgesic consequence in humans and shows it continuing to grow with dose, the contrast that makes the respiratory ceiling on the other branch meaningful. Evidence source HUMAN_CLINICAL (healthy volunteers, experimental electrical pain tolerance).
Brainstem Respiratory Rhythm Depression
effector
The toxicity branch, and the one that kills. Mu-opioid receptors are expressed on rhythm-generating neurons of the pre-Botzinger complex in the ventrolateral medulla, the inspiratory rhythm generator, where the same hyperpolarization slows and can abolish inspiratory rhythm. The opioid-sensitive subpopulation corresponds to the neurokinin-1 receptor-expressing neurons of the complex, and the depression is receptor-mediated: animals lacking mu-opioid receptors do not show respiratory rate depression at all. Other brainstem sites, notably the Kolliker-Fuse and parabrachial nuclei, modulate the response, so a conforming entry should not treat the pre-Botzinger complex as the only site.
Downstream
-
Opioid-Induced Respiratory Depression
Slowed inspiratory rhythm, with reduced respiratory drive and consciousness, produces ventilatory insufficiency.
Show evidence (4 references)
"By locally manipulating neurotransmission in the adult rat, we identify the critical site of the medulla, the preBötzinger complex, that mediates opioid-induced respiratory depression in vivo."
Localizes this branch to the pre-Botzinger complex by local manipulation in vivo. Evidence source MODEL_ORGANISM (adult rat).
"neurokinin-1 receptor-expressing preBötzinger complex neurons constitute the critical site mediating opioid-induced respiratory rate depression"
Identifies the neurochemical identity of the opioid-sensitive rhythm-generating population, which is the cell type bound on this node.
"We then compared respiratory responses to fentanyl between wild-type and Oprm1 full knock-out mice and found that the lack of MORs prevented respiratory rate depression from occurring."
Establishes that this branch requires the mu-opioid receptor itself rather than another opioid receptor or a non-receptor action, by abolishing it in Oprm1 knockout mice. The same study excludes somatostatin-expressing cells as the responsible population, which is why the cell type here is bound only to the generic glutamatergic rhythm-generating neuron.
+ 1 more reference
Opioid-Induced Respiratory Depression
consequence
The lethal consequence of the module and the mechanism of fatal opioid overdose: depression of respiratory drive which, with reduced consciousness and upper-airway obstruction, causes ventilatory insufficiency and ultimately apnea. This is the node an acute opioid poisoning entry conforms to. Unlike analgesia, this branch shows a ceiling with a partial agonist: over a two-fold dose range buprenorphine's respiratory depression did not increase in magnitude or timing while its analgesia grew, which is why buprenorphine is modelled on the same trigger node as the full agonists rather than in a separate module.
Show evidence (2 references)
"Opioid-induced respiratory depression (OIRD), the primary cause of opioid-induced death, is the neural depression of respiratory drive which, together with a decreased level of consciousness and obstructive sleep apnea, cause ventilatory insufficiency."
Defines this consequence node and its clinical significance as the primary cause of opioid-induced death.
"We conclude that over the dose range tested buprenorphine displays ceiling in respiratory effect but none in analgesic effect."
The buprenorphine ceiling, recorded on this branch as the module's notes require. In healthy volunteers peak respiratory depression was indistinguishable between 0.2 and 0.4 mg per 70 kg while analgesia increased, dissociating the two branches by agonist efficacy rather than by transducer.
Enteric Neuron Inhibition and Slowed Propulsive Motility
effector
The gastrointestinal branch. Mu-opioid receptors on enteric neurons of the myenteric and submucosal plexuses are inhibited by the same mechanism, suppressing the neural drive to coordinated propulsive contraction and to intestinal fluid secretion while sphincter tone rises. Because the receptors are peripheral, this branch is the one that a peripherally restricted antagonist can reverse without reversing analgesia - the rationale for the PAMORAs, which belong to the planned antagonism module rather than here.
Downstream
-
Opioid-Induced Constipation
Reduced propulsive motility with reduced fluid secretion and raised sphincter tone produces constipation.
Show evidence (2 references)
"In the gut, opioids exert their action on the enteric nervous system, where they bind to the myenteric and submucosal plexuses, causing dysmotility, decreased fluid secretion and sphincter dysfunction, which all leads to opioid-induced bowel dysfunction (OIBD)."
States the cellular substrate and the three physiological effects of this branch, and the anatomical plexuses bound on the node. Evidence source is OTHER because this is a review article.
"Opioid receptors are abundant in multiple regions of the central nervous system and the peripheral nervous system including enteric neurons."
Supports the peripheral receptor expression that makes this branch separable from the central ones by a peripherally restricted drug.
Opioid-Induced Constipation
consequence
The dose-limiting non-lethal consequence of the module. Constipation is the most severe component of opioid-induced bowel dysfunction, does not show the tolerance that develops to analgesia and respiratory depression, and is frequently what limits opioid dose in chronic pain. Conforming entries record their own management arm; the peripherally restricted antagonists that reverse it act on the enteric branch upstream of this node.
Show evidence (2 references)
"One of the most severe symptoms is constipation, which can be assessed using different scales for subjective assessment."
Identifies constipation as the most severe symptom of the bowel dysfunction this branch produces, the consequence conforming entries point at.
"Dose-limiting adverse symptoms of OIBD can lead to insufficient pain treatment."
Supports the dose-limiting character of this consequence, which is what makes it a mechanism worth modelling rather than a side note.
Mesolimbic Dopamine Disinhibition
effector
The reward branch, and the one place in the module where the central effector produces excitation. In the ventral tegmental area the neuron carrying the mu-opioid receptor is a local GABAergic interneuron, not the dopamine neuron; hyperpolarizing it removes spontaneous GABA-mediated inhibitory input to the dopamine cells, which are excited by disinhibition and increase dopamine release in the nucleus accumbens. This is the opioid-specific pharmacology that sits immediately upstream of the addiction_cycle module's trigger node; the circuit consequences of repeated reward are modelled there, not here.
Downstream
-
Opioid Reward and Reinforcement
Increased mesolimbic dopamine release is the proximate reinforcing signal that produces euphoria and makes the drug reinforcing.
Show evidence (2 references)
"Intracellular recordings from these cells in slices of rat brain in vitro showed that opioids do not affect the principal (dopamine-containing) neurons but hyperpolarize secondary (GABA-containing) interneurons."
The result that establishes the disinhibition architecture of this branch: the hyperpolarized cell is the GABAergic interneuron, not the dopamine neuron. Evidence source IN_VITRO (intracellular recordings in rat brain slices).
"In vivo, this would lead to excitation of the dopamine cells by disinhibition, which would be expected to contribute to the positive reinforcement seen with mu-receptor agonists such as morphine and heroin."
The authors' inference from the slice result to reinforcement in the intact animal. Marked INDIRECT because the reinforcement claim follows from the recorded disinhibition rather than being measured in this study.
Mu-Opioid Receptor Desensitization and Internalization
modifier
The modulatory branch that converts acute signalling into tolerance. Phosphorylation of the agonist-occupied receptor by G protein-coupled receptor kinases recruits beta-arrestin, uncoupling the receptor from its transducers (homologous desensitization) and promoting endocytosis. Agonists differ in how strongly they engage this route - DAMGO engages the GRK-arrestin mechanism efficiently while morphine desensitizes the receptor largely without it - so the mechanism is agonist-specific rather than a single pathway, and how much of clinical tolerance it accounts for is contested. Chronic exposure adds a compensatory arm in the opposite direction, upregulation of adenylyl cyclase and cAMP signalling, which reverses the trunk of this module.
Downstream
-
Opioid Tolerance and Dependence
Receptor uncoupling and endocytosis, with the compensatory cyclase upregulation, reduce the effect of a given dose.
Show evidence (2 references)
"For agonists with high relative efficacy for endocytosis, G protein dissociation and conformational changes favorable to GRK phosphorylation drive desensitization and endocytosis."
States the GRK-phosphorylation-to-desensitization-and-endocytosis sequence that defines this node, and that an agonist's relative efficacy for endocytosis governs it. Evidence source is OTHER because this is a review article.
"An emerging concept is that some agonists such as DAMGO regulate MOR by selectively engaging GRK-arrestin mechanisms, while others such as morphine selectively engage non-GRK-arrestin mechanisms."
Supports the agonist-specificity caveat in this node's description, and is the reason the node is a modifier rather than an obligatory step in the trunk.
Opioid Reward and Reinforcement
consequence
The acute consequence of the reward branch: increased dopamine release in the nucleus accumbens produces euphoria and makes the drug reinforcing. This is the module's declared hand-off to addiction_cycle - it is the same event as that module's trigger node "Acute Mesolimbic Dopamine Reinforcement", reached here through the opioid-specific disinhibition pharmacology. What repeated reinforcement then does to the neurocircuitry, and the three-stage progression to compulsive use, is modelled there and not here; an opioid use disorder entry should conform to both modules. It is a separate node from tolerance and dependence because the two have different proximate causes in this module - this one is driven by the dopamine branch, the other by receptor-level adaptation - and because reinforcement is present from the first dose while tolerance requires repeated exposure. GO has no term for reward or reinforcement (`runoak -i ols:go search "reward"` returns nothing), so the node is bound to dopamine secretion in the accumbens, which is the measurable event the reinforcement claim rests on; this is the same binding addiction_cycle uses on the node this hands off to.
Show evidence (2 references)
"Through this indirect mechanism, opioids increase dopamine release in the nucleus accumbens, producing their other characteristic effect, euphoria."
States this consequence and its anatomical site, and that it is reached indirectly - through the disinhibition modelled on the upstream node rather than by a direct action on the dopamine neuron. Evidence source is OTHER because this is a review article synthesizing the mechanism rather than reporting a study.
"By binding to mu opioid receptors expressed on GABAergic interneurons or afferents within the VTA, opioids are assumed to disinhibit VTA DA neurons and increase DA release to the NAc, an effect that underlies the subjective experience of reward and euphoria induced by all drugs of abuse"
Traces the whole reward branch in one sentence, from receptors on VTA GABAergic interneurons through disinhibition to accumbens dopamine release and the subjective reward, which is the chain this node terminates. Note the source's own hedge, "are assumed to"; the direct electrophysiological support for the disinhibition step sits on the upstream node.
Opioid Tolerance and Dependence
consequence
The chronic-exposure consequence. Receptor-level desensitization and internalization, together with compensatory upregulation of adenylyl cyclase and cAMP signalling that reverses the trunk of this module, produce loss of effect at a given dose, physical dependence, and withdrawal on cessation. Tolerance is uneven across the module's branches - it develops to analgesia and to respiratory depression but not to constipation - which is clinically decisive and is the reason an entry cannot treat the branches as scaling together over time. Which signalling route carries clinical tolerance is not established, so this node makes no claim about which arm of the trunk the adaptation reverses.
Show evidence (3 references)
"Chronic opioid use leads to cellular adaptations, including receptor phosphorylation, desensitization, and recruitment of β-arrestin, which uncouple MOR from downstream signaling pathways."
Connects the modifier branch to this consequence: the receptor-level adaptations uncouple the receptor from the transducers that carry the trunk of this module. Evidence source is OTHER because this is a review article.
"These changes, along with compensatory upregulation of adenylyl cyclase (AC) and cAMP signaling, underlie the development of tolerance, dependence, and withdrawal, however the exact signaling pathways responsible remain unknown."
States the compensatory cyclase arm and the resulting tolerance, dependence and withdrawal, and records that the responsible pathways are not established - the reason this node makes no claim about which branch of the trunk the adaptation reverses.
"A major problem in treating painful conditions is that tolerance limits the long-term utility of opioid agonists."
States the clinical weight of this consequence, which is what makes tolerance a modelled endpoint of the module rather than a footnote on the analgesia branch.
Full Mu-Opioid Receptor Agonist Analgesics
Platform:
Small molecule
The opioid analgesics in ordinary clinical use - morphine, fentanyl, oxycodone and their congeners - acting as full agonists at the mu-opioid receptor. They engage the trigger node of this module and therefore produce every branch of it together: analgesia, respiratory depression, constipation and reward are one pharmacology, not a therapeutic effect plus separable side effects. This is the drug-target design pattern for a conforming pain or poisoning entry: point the treatment at the receptor activation node, and read the adverse effects off the module's branches.
Evidence: 1
Methadone (Long-Acting Full Agonist Maintenance)
Platform:
Small molecule
A long-acting full agonist at the mu-opioid receptor, used as agonist maintenance treatment for opioid use disorder and, less commonly, as an analgesic. Mechanistically it engages the same trigger node as the other full agonists; what makes it a maintenance drug is pharmacokinetic rather than pharmacodynamic - a long half-life and oral route give sustained receptor occupancy that suppresses withdrawal and produces cross-tolerance to illicit opioids without the repeated peaks that drive reinforcement. Methadone's non-opioid actions, including NMDA receptor antagonism and hERG-related QT prolongation, are outside this module and belong on the entries that prescribe it.
Evidence: 2
Buprenorphine (Partial Agonist)
Platform:
Small molecule
A partial agonist at the mu-opioid receptor, used for opioid use disorder maintenance and for analgesia. It belongs on the same trigger node as the full agonists because the receptor action is the same; the difference is intrinsic efficacy, and its clinical consequence is a ceiling on the respiratory branch - over a two-fold dose range in healthy volunteers peak respiratory depression did not grow while analgesia grew substantially. The ceiling is a property of this agonist on that branch and is recorded there; it should not be read as evidence that the branches are carried by different transducers, which is the separate and unsettled biased-agonism question.
Evidence: 2