Histamine and its Effects on Body Systems
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Questions and Answers

What mechanism does histamine employ to cause vasodilation in blood vessels?

  • It reduces prostaglandin synthesis in endothelial cells.
  • It activates H1 receptors leading to nitric oxide release. (correct)
  • It directly stimulates sympathetic nervous activity.
  • It inhibits calcium influx in vascular smooth muscle.

    • What is a direct effect of histamine on cardiac function?

  • It decreases heart rate by inhibiting H1 receptors.
  • It reduces myocardial oxygen demand.
  • It causes vasoconstriction resulting in increased peripheral resistance.
  • It has positive chronotropic effects via H2 receptors. (correct)

    • Which of the following effects is associated with histamine's action on the gastrointestinal tract?

  • Inhibition of gastric acid secretion.
  • Induction of diarrhea due to smooth muscle contraction. (correct)
  • Decreased propulsive contractions of the intestines.
  • Relaxation of smooth muscle in the esophagus.

    • How does histamine trigger bronchoconstriction in asthmatics?

    <p>By promoting calcium influx in smooth muscle via H1 receptors.</p> Signup and view all the answers

    What is the role of histamine in increasing capillary permeability?

    <p>It promotes efflux of plasma proteins due to H1 receptor action.</p> Signup and view all the answers

    Which of the following is a consequence of histamine stimulating peripheral nerve endings?

    <p>Itching sensation due to stimulation of sensory fibers.</p> Signup and view all the answers

    What effect does histamine have on gastric acid secretion?

    <p>Stimulates secretion through activation of H2 receptors.</p> Signup and view all the answers

    Which of the following statements about histamine's effects on the cardiovascular system is true?

    <p>Histamine's effects include short-lived vasodilation and increased capillary permeability.</p> Signup and view all the answers

    Which statement about the effect of clozapine in treating psychosis is accurate?

    <p>Clozapine modulates dopamine release differently depending on the secretion levels in various brain areas.</p> Signup and view all the answers

    What is the primary action of bromocriptine in the context of the central nervous system?

    <p>It acts as a dopamine receptor agonist to suppress prolactin secretion.</p> Signup and view all the answers

    Which ergot alkaloid is specifically used to treat postpartum hemorrhage?

    <p>Ergometrem</p> Signup and view all the answers

    What role do ergot alkaloids play in vascular smooth muscle?

    <p>Their effects can vary significantly based on the drug and vessel type.</p> Signup and view all the answers

    Which receptor activity is primarily associated with ergotamine in the treatment of migraines?

    <p>Partial agonist activity at α1 receptors.</p> Signup and view all the answers

    What is a primary condition for which drugs targeting 5-HT receptors are clinically applied?

    <p>Depression and anxiety</p> Signup and view all the answers

    What primarily causes dilation in small blood vessels when 5-HT is present?

    <p>Inhibition of noradrenaline</p> Signup and view all the answers

    Which neuropeptide is released during the activation of the trigeminal nerves associated with migraines?

    <p>Calcitonin Gene-Related Peptide (CGRP)</p> Signup and view all the answers

    What initial cardiovascular effect does intravenous injection of 5-HT typically induce?

    <p>Significant increase in blood pressure</p> Signup and view all the answers

    Activation of which receptors on the intracranial vessels leads to vasoconstriction during a migraine attack?

    <p>5-HT1B receptors</p> Signup and view all the answers

    Which receptor subtype binds 5-HT to enhance platelet aggregation?

    <p>5-HT2A</p> Signup and view all the answers

    What physiological response is primarily caused by the vasodilation during a migraine?

    <p>Hypoxia</p> Signup and view all the answers

    What is the role of 5-HT2 receptor antagonists in migraine treatment?

    <p>Blockade of trigeminal nerve-related pain mechanisms</p> Signup and view all the answers

    In CNS actions, which of the following effects does 5-HT NOT mediate?

    <p>Memory enhancement</p> Signup and view all the answers

    What phenomenon can result from the stimulation of nociceptive sensory nerve endings by 5-HT?

    <p>Neurogenic inflammation</p> Signup and view all the answers

    What is the effect of activating 5-HT1D receptors on presynaptic trigeminal neurons?

    <p>Inhibit release of pro-inflammatory neuropeptides</p> Signup and view all the answers

    Which of the following can trigger the release of neuropeptides that contribute to migraine pain?

    <p>Neurogenic inflammation</p> Signup and view all the answers

    What occurs as a secondary effect when platelets come into contact with injured endothelium?

    <p>Release of 5-HT</p> Signup and view all the answers

    How does the blockade of 5-HT2 receptors alleviate migraine symptoms?

    <p>By reducing excitatory neurotransmission</p> Signup and view all the answers

    Which of the following statements about the receptor-dependent effects of 5-HT in the CNS is correct?

    <p>The subtype of receptor determines post-synaptic excitation or inhibition</p> Signup and view all the answers

    During a migraine, what occurs as a result of mechanical stretch and shunting of blood flow?

    <p>Hypoxia and pain exacerbation</p> Signup and view all the answers

    What best describes the long-term cardiovascular effect of 5-HT after initial intravenous injection?

    <p>Transient increase followed by a decrease</p> Signup and view all the answers

    Which of the following is NOT a primary action of 5-HT in the central nervous system?

    <p>Enhancing muscle contraction</p> Signup and view all the answers

    Which factor does NOT influence the effect of 5-HT on blood vessels?

    <p>Presence of other neurotransmitters</p> Signup and view all the answers

    What is the primary use of Pizotifen in migraine management?

    <p>To prevent future migraine attacks</p> Signup and view all the answers

    What is the primary receptor subtype associated with emesis mediated by 5-HT?

    <p>5-HT3 receptors</p> Signup and view all the answers

    Which drug is an example of a 5-HT3 antagonist used as an antiemetic?

    <p>Ondansetron</p> Signup and view all the answers

    Which treatment strategy is primarily targeted by Selective Serotonin Reuptake Inhibitors (SSRIs)?

    <p>To inhibit serotonin transporters</p> Signup and view all the answers

    In the context of the Monoamine Theory of Depression, what neurotransmitter is primarily linked to depressive symptoms?

    <p>Serotonin</p> Signup and view all the answers

    What is the mechanism of 5-HT1A receptor partial agonists in treating anxiety?

    <p>They act on presynaptic inhibitory receptors</p> Signup and view all the answers

    Which condition is not typically treated with 5-HT3 antagonists?

    <p>Chronic depression management</p> Signup and view all the answers

    Which of the following is a characteristic of SSRIs regarding their impact on neurotransmitters?

    <p>They increase levels of serotonin in the brain</p> Signup and view all the answers

    What type of nausea is commonly prevented by 5-HT3 antagonists?

    <p>Nausea from chemotherapy treatments</p> Signup and view all the answers

    What physiological aspect does the Monoamine Theory of Depression primarily concern?

    <p>Deficit of monoamine neurotransmitters in the limbic system</p> Signup and view all the answers

    Study Notes

    Histamine and its effects

    • Cardiovascular System:
      • Histamine causes vasodilation through two mechanisms: indirect via H1 receptors on the vascular endothelium releasing nitric oxide (NO), and direct activation of H2 receptors on vascular smooth muscle.
      • Histamine increases capillary permeability by acting on H1 receptors on the endothelial vascular cells, leading to an efflux of plasma protein and fluid into the extracellular space and increasing lymph flow, ultimately causing edema.
    • Heart: Histamine increases the rate and force of contraction of the heart through its action on cardiac H2 receptors (positive chronotropic and inotropic effects).
    • Extravascular smooth muscle:
      • Bronchial: Histamine directly contracts smooth muscle via H1 receptors, causing bronchoconstriction. This effect is particularly pronounced in asthmatics.
      • Gastrointestinal tract: Histamine causes contraction of GI smooth muscle, leading to abdominal cramps and diarrhea.
    • Secretory tissue: Histamine stimulates gastric acid secretion by acting on gastric parietal cells through H2 receptors.
    • Peripheral nerve endings: Histamine stimulates various nerve endings, causing sensory effects like itch in the epidermis.

    5-HT (Serotonin) and its effects

    • Cardiovascular effects:
      • The effect of 5-HT on blood vessels varies depending on the size, location, and 5-HT receptor subtype.
      • 5-HT generally constricts large vessels (arteries and veins) by directly acting on vascular smooth muscle cells.
      • 5-HT dilates small vessels by releasing nitric oxide (NO) and inhibiting noradrenaline from sympathetic nerve terminals.
      • Intravenous injection of 5-HT initially increases blood pressure due to large vessel constriction, followed by a slight decrease due to arteriolar dilation.
      • Platelets: While not synthesized in platelets, 5-HT is taken up, stored, and released by them.
        • Platelet activation upon encountering injured endothelium triggers the release of substances promoting platelet aggregation and the subsequent release of 5-HT.
        • 5-HT binding to platelet 5HT2A receptors further enhances aggregation.
    • Peripheral nervous system (PNS): 5-HT stimulates and sensitizes nociceptive sensory nerve endings, contributing to neurogenic inflammation. It may also lead to local release of sensory neuropeptides.
    • Central nervous system (CNS): 5-HT is involved in various CNS-mediated events like migraine, stress and anxiety, mood, vomiting, and appetite. It can also induce hallucination.
      • Cellular effects of 5-HT in the CNS vary based on the receptor subtype and can cause:
        • Post-synaptic excitation
        • Post-synaptic inhibition
        • Pre-synaptic inhibition (autoreceptor).

    Clinical Applications of Drugs Targeting 5-HT

    • CNS conditions:
      • Depression and anxiety
      • Migraine
      • Psychosis
    • Mixed CNS and peripheral conditions:
      • Carcinoid syndrome
      • Nausea/Emesis
      • Appetite

    Migraine

    • Trigeminal-vascular system: The sensory neurons innervating cerebral vessels transmit pain, touch, and temperature sensations to the brain.
    • Pathogenesis of migraine: Activation of trigeminal nerves triggers the release of neuropeptides (e.g., CGRP and Substance P), resulting in vasodilation and inflammation of meningeal and cranial vessels. This causes extravasation of plasma and proteins into the perivascular space and leads to localized edema.
    • Mechanical stretch and edema contribute to acute migraine pain while shunting of blood flow to the affected area can cause hypoxia, further contributing to pain.

    Migraine treatment:

    • 5-HT1 receptor agonists:
      • 5HT1B and 5HT1D receptors are found on vascular smooth muscle in the meningeal arteries and peripheral and central trigeminal neurons.
        • Activation of these receptors constricts intracranial blood vessels, closing shunts and counteracting vasodilation during migraine attacks.
        • This helps alleviate headache pain by inhibiting the release of pro-inflammatory neuropeptides from the trigeminal nerve.
    • 5HT2 receptor partial agonists/antagonists:
      • 5-HT, via 5HT2 receptors, is thought to cause trigeminal nerve excitation, inflammation, and vasodilation in extracerebral vessels, contributing to migraine.
      • Blocking these receptors mitigates these features.
      • Pizotifen, a 5-HT2A-C receptor antagonist, also blocks histamine H1 receptors. It is used prophylactically to prevent migraine attacks but ineffective during active attacks.

    Anti-Emetics

    • Nausea and vomiting: 5-HT release, primarily in the gastrointestinal tract and CNS, plays a key role in emesis (vomiting), mainly mediated via 5HT3 receptors.
      • These receptors are found in the vomit center in the CNS and in peripheral neurons of the GI tract
    • Clinical Implication: 5-HT3 antagonists, like Ondansetron and Tropisetron, are used to prevent nausea and vomiting associated with:
      • Post-surgical operations
      • Chemotherapy treatments (acute nausea)
      • Radiation treatments

    Depression & Anxiety

    • Monoamine theory of depression: Depression is linked to a deficit of monoamine neurotransmitters (e.g., serotonin) in the limbic system.
    • Selective Serotonin Reuptake Inhibitors (SSRIs):
      • SSRIs block the serotonin transporter (SERT), inhibiting 5-HT reuptake, thereby increasing serotonin levels and enhancing serotonergic neurotransmission.
      • They are used to treat depression and anxiety disorders.
    • 5-HT1A Receptor Partial Agonists:
      • These agonists act on 5-HT1A presynaptic inhibitory receptors, enhancing serotonergic activity.
      • They are effective for treating both depression and anxiety.

    Psychosis

    • Psychosis is associated with increased dopamine release in some brain areas and decreased dopamine release in other areas, leading to positive and negative symptoms.
    • Clozapine: This antipsychotic blocks 5HT2A/2C receptors in different brain regions, modulating dopamine release:
      • It increases dopamine release in areas where it is undersecreted.
      • It decreases dopamine release in areas where dopamine is oversecreted.

    Ergot Alkaloids

    • Ergot alkaloids are produced by Claviceps purpurea, a fungus found in grasses and grains.
    • They act on various receptor types: α adrenoceptors, dopamine receptors, and 5-HT receptors.
    • Some have postsynaptic and presynaptic effects.

    Clinical Use of Ergot Alkaloids

    • Migraine: Ergotamine
    • Hyperprolactinemia: Bromocriptine
    • Postpartum hemorrhage (PPH): Ergometrem

    Effects of Ergot Alkaloids on Body Systems

    • CNS:
      • Ergots with dopamine activity, such as bromocriptine, have high selectivity for pituitary dopamine receptors, acting as agonists to suppress prolactin secretion.
    • Vascular smooth muscle:
      • Effects vary with the drug, species, and vessel.
      • Vasoconstriction occurs at nanomolar concentrations.
      • Most ergot alkaloids act as partial agonists on α1 receptors and 5-HT2 receptors.
      • Ergotamine, for example, is used to treat migraines.
    • Uterine smooth muscle:
      • Ergonovine has a high selectivity for uterine smooth muscle, acting as an agonist on α1 receptors and 5-HT receptors.
        • This results in contraction and vasoconstriction, effects that are significantly amplified during pregnancy due to increased α1 receptor expression.

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    Description

    This quiz explores the various effects of histamine on the cardiovascular system, heart function, and smooth muscles, including its role in vasodilation, increased capillary permeability, and bronchoconstriction. Perfect for those studying physiology and pharmacology, it highlights the mechanisms through which histamine operates in different bodily systems.

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