Scope decisions recorded for the next curator. The central effector is graded INDIRECT on its cited evidence throughout, and deliberately: the sources describe what histamine does at the H1 receptor, so the claim that blockade removes it is an inference from the unblocked state rather than a measurement of the blocked one. Saying so is more useful than asserting the step directly from a source that does not test it. No transducer is named anywhere in the module. The H1 receptor's coupling is not stated by any abstract cited here, and module snippets are matched against the abstract, so the receptor's signalling is left unasserted rather than filled in from general knowledge -- that is the one claim an ontology binding would have made look sourced. The anticholinergic node is unparented for the same reason and it is a stronger case: those effects are conventionally muscarinic, no cited abstract says so, and an edge from H1 occupancy would have asserted that blocking H1 causes thirst and tachycardia. Itch is bound, after a search: GO:0160025 sensory perception of itch exists and fits, so the node binds all three components of the triad rather than carrying a note about the one that got away. Four entries treat with H1 and H2 antihistamines in a single treatment (Mast_Cell_Activation_Syndrome, Systemic_Mastocytosis, Maculopapular_Cutaneous_Mastocytosis and Monoclonal_Mast_Cell_Activation_Syndrome), and all four conform here. A bundled treatment is not an obstacle to conforming when it binds its components as separate agents: the treatment then carries one target_mechanisms link per receptor, and the link to this module's trigger asserts nothing about the H2 agent beside it. Two of the four already bound both classes as separate agents; the other two had agents missing, which were bound from each treatment's own citation. Splitting such a treatment is positively the wrong move in two of these cases -- Systemic_Mastocytosis' evidence is strategy-level rather than class-specific, so split halves would carry no evidence of their own, and Monoclonal_Mast_Cell_Activation_Syndrome's cited sentence is a diagnostic criterion about the bundle as a whole, which a split would misrepresent. What holds the H2 side of these four back is not bundling but the H2 module's scope: that module is a gastric acid-suppression chain, running parietal-cell cAMP loss to reduced acid output to raised intragastric pH, while these entries use H2 blockade for histamine-driven gastrointestinal symptoms without establishing the acid pathway. An H2-mediated-symptom module distinct from acid suppression is what the H2 side needs, and it does not exist yet.
H1 Receptor Antagonist Occupancy
trigger
An antihistamine binds the histamine H1 receptor and prevents histamine acting there. The agents are not only neutral antagonists: at least some of the class are inverse agonists, which suppress the receptor's constitutive activity rather than merely competing with histamine for it. No location is bound here on purpose: this node is the drug's occupancy of the receptor wherever the receptor is, and both the peripheral and the central arm hang off it, so pinning it to one tissue would contradict its own children.
Used by disorders
Urticaria
as Antihistamine Occupancy of Cutaneous H1 Receptors
Downstream
-
Blockade of H1-Mediated Vasodilatation, Plasma Extravasation and Itch
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Central H1 Receptor Occupancy
Show evidence (2 references)
"H1 receptor antagonists, known as H1-antihistamines (AHs), inactivate the histamine H1-receptor thereby preventing histamine causing the primary symptoms of allergic diseases, such as atopic dermatitis, pollinosis, food allergies, and urticaria."
States the receptor, the action on it and the indications, which is this node.
"Desloratadine is a rapid-acting, once-daily, nonsedating selective H1-receptor antagonist/inverse receptor agonist with proven clinical efficacy in patients with CIU."
Records that an agent of this class is an inverse agonist as well as an antagonist, which is the distinction this node's description draws.
Relief of Weals and Itch
consequence
The clinical endpoint the class is prescribed for. H1 antihistamines are first-line in allergic disease and in chronic urticaria, where they reduce the number and size of weals and the severity of itch, and recover the sleep and daily activity those cost.
Show evidence (2 references)
"Results from randomized, double-blind, placebo-controlled studies of 6 weeks' duration in adults and adolescents with moderate-to-severe CIU indicate that desloratadine significantly minimizes the severity of pruritus, reduces the number and size of hives, and improves disease-impaired sleep and..."
Names the endpoints and the trial design behind them, pooled for one agent of the class.
"AHs, which are classified into first-generation (fgAHs) and second-generation (sgAHs) antihistamines, are the first line of treatment for allergic diseases."
Records the class's first-line standing, which is the clinical weight of this node.
Central H1 Receptor Occupancy
effector
The same receptor sits in the brain, so an agent that crosses the blood-brain barrier occupies it there as well. How much it does is measurable rather than inferred: positron emission tomography quantifies H1 receptor occupancy, and that measurement is what brain penetrability now means for this class. It is also what re-sorted the class -- agents shown not to penetrate were reclassified on the strength of those studies.
Downstream
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Sedation and Cognitive Impairment from Central H1 Occupancy
Show evidence (2 references)
"The use of histamine H1-receptor occupancy (H1RO) based on positron emission tomography (PET) has been developed for the evaluation of brain penetrability."
Establishes that central occupancy of this receptor is measured directly, which is what makes this a node rather than an inference from sedation.
"Based on the results of the H1RO-PET studies, non-brain-penetrating AHs (nbpAHs) have recently been reclassified among sgAHs."
Records that the measurement reorganised the class, which is why central occupancy is modelled as its own step rather than folded into the adverse node.
Sedation and Cognitive Impairment from Central H1 Occupancy
adverse effect
Drowsiness, lassitude and cognitive impairment follow occupancy of brain H1 receptors. This is the central arm's consequence and nothing else: the anticholinergic effects the same agents cause are a separate node, because they are a different receptor. The cost is large enough that the cited review does not recommend first-generation agents for allergic disease at all. The later agents are minimally sedating, and that is what lets them be given at the higher doses where they work better.
Show evidence (3 references)
"Although fgAHs are effective, they cause adverse reactions such as potent sedating effects, including drowsiness, lassitude, and cognitive impairment; anticholinergic effects, including thirst and tachycardia."
Quoted whole because the sentence is one clause list; this node rests on its sedating half -- drowsiness, lassitude and cognitive impairment -- and the efficacy clause that makes it a trade-off rather than a failure. The anticholinergic half is a separate node.
"Consequently, the use of fgAHs is not recommended for allergic diseases."
The strength of the recommendation that follows from the adverse profile.
"Today, sgAHs, which are minimally sedating and, therefore, may be used at more effective doses, are the first-line treatment for alleviating the symptoms of allergic diseases."
Records that reduced sedation buys dose headroom, which is the other half of why the compartment distinction matters clinically.
Anticholinergic Effects of First-Generation Antihistamines
adverse effect
Thirst, tachycardia and the rest of the antimuscarinic picture accompany the first-generation agents. This node has no upstream edge inside the module, and that is the claim: these effects do not follow from H1 occupancy at all, central or peripheral, so giving them one would assert that blocking H1 causes them. They are conventionally attributed to the same molecules binding muscarinic acetylcholine receptors, but no abstract cited here states that, so the upstream step is left unasserted rather than filled in -- the same decision taken for the H1 receptor's own transducer. What the module does assert is that the agents cause them, and that this is part of why the first generation is no longer recommended.
Show evidence (1 reference)
"Although fgAHs are effective, they cause adverse reactions such as potent sedating effects, including drowsiness, lassitude, and cognitive impairment; anticholinergic effects, including thirst and tachycardia."
Quoted whole for the same reason as the sedation node; this node rests on its anticholinergic half -- thirst and tachycardia -- and on the sentence attributing them to the agents rather than to a mechanism, which is why no upstream edge is drawn.
Second-Generation H1 Antihistamine Therapy
Platform:
Small molecule
Cetirizine, loratadine, desloratadine and fexofenadine, given for allergic disease and for chronic urticaria. Minimally sedating, and first-line for both.
Evidence: 2
First-Generation H1 Antihistamine Therapy for Sedation-Dependent Indications
Platform:
Small molecule
Diphenhydramine and its relatives, now confined to the indications where crossing into the brain is the point rather than the cost: antiemesis, motion sickness and vertigo. Kept as its own treatment because the platform is the same but the reason for prescribing is the opposite.
Evidence: 1