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Tachyphylaxis: Rapidly decreasing response to a drug or physiologically active agent after administration of a few doses. In immunology, it is the rapid immunization against the effect of toxic doses of an extract or serum by previous injection of small doses. (Dorland, 28th ed)Eledoisin: A peptide extracted from the posterior salivary glands of certain small octopi (Eledone spp., Mollusca), or obtained by synthesis. Its actions resemble those of SUBSTANCE P; it is a potent vasodilator and increases capillary permeability. (From Martindale, The Extra Pharmacopoeia, 30th ed, p1364)Methysergide: An ergot derivative that is a congener of LYSERGIC ACID DIETHYLAMIDE. It antagonizes the effects of serotonin in blood vessels and gastrointestinal smooth muscle, but has few of the properties of other ergot alkaloids. Methysergide is used prophylactically in migraine and other vascular headaches and to antagonize serotonin in the carcinoid syndrome.Ganglionic Stimulants: Agents that mimic neural transmission by stimulation of the nicotinic receptors on postganglionic autonomic neurons. Drugs that indirectly augment ganglionic transmission by increasing the release or slowing the breakdown of acetylcholine or by non-nicotinic effects on postganglionic neurons are not included here nor are the nonspecific cholinergic agonists.Amaryllidaceae Alkaloids: Alkaloids derived from TYRAMINE combined with 3,4-dihydroxybenzaldehyde via a norbelladine pathway, including GALANTAMINE, lycorine and crinine. They are found in the Amaryllidaceae (LILIACEAE) plant family.Metiamide: A histamine H2 receptor antagonist that is used as an anti-ulcer agent.Pentagastrin: A synthetic pentapeptide that has effects like gastrin when given parenterally. It stimulates the secretion of gastric acid, pepsin, and intrinsic factor, and has been used as a diagnostic aid.Tyramine: An indirect sympathomimetic. Tyramine does not directly activate adrenergic receptors, but it can serve as a substrate for adrenergic uptake systems and monoamine oxidase so it prolongs the actions of adrenergic transmitters. It also provokes transmitter release from adrenergic terminals. Tyramine may be a neurotransmitter in some invertebrate nervous systems.Histamine: An amine derived by enzymatic decarboxylation of HISTIDINE. It is a powerful stimulant of gastric secretion, a constrictor of bronchial smooth muscle, a vasodilator, and also a centrally acting neurotransmitter.Decamethonium Compounds: Compounds that contain the decamethylenebis(trimethyl)ammonium radical. These compounds frequently act as neuromuscular depolarizing agents.Methacholine Compounds: A group of compounds that are derivatives of beta-methylacetylcholine (methacholine).Dimethylphenylpiperazinium Iodide: A selective nicotinic cholinergic agonist used as a research tool. DMPP activates nicotinic receptors in autonomic ganglia but has little effect at the neuromuscular junction.Atropine: An alkaloid, originally from Atropa belladonna, but found in other plants, mainly SOLANACEAE. Hyoscyamine is the 3(S)-endo isomer of atropine.Capsaicin: An alkylamide found in CAPSICUM that acts at TRPV CATION CHANNELS.