PPA Module 2a Lecture 2.8 on Histamine and its Role in Neurological Systems

Introduction to Biogenic Amines and Histamine

  • Focus on histamine as a central nervous system neurotransmitter.
  • Importance of understanding synthesis, transport, signaling, receptors, contributions to behavior and illness, and medication effects through histamine systems.

Definition and Historical Context

  • Histamine (beta-aminoethylamine):
    • First synthesized in 1907 before biological significance was identified.
    • Autocoid: A substance that is produced by and acts on the same tissue (self-remedy).

Biological Functions and Synthesis of Histamine

  • Functions:
    • Major mediator of anaphylaxis and inflammation.
    • Mediates gastric acid secretion (discussed in autonomic nervous system).
  • Synthesis Mechanism:
    • Formed via decarboxylation of the amino acid histidine.
    • Catalyzed by the enzyme L-histidine decarboxylase.

Storage and Distribution

  • Location in the Body:
    • Present in most mammalian tissues but unevenly distributed.
    • High concentration in sites of potential injury.
    • Stored in granules of mast cells and basophils, and released from there.
    • Predominantly produced in enterochromaffin-like cells of the stomach (stimulates gastric acid increase).
    • Found in the tuberomammillary nucleus of the hypothalamus, stored in synaptic vesicles.

Transport and Metabolism

  • Storage Mechanism:
    • Histamine stored in vesicles via VMAT (Vesicular Monoamine Transporter).
    • Protons traded for histamine concentration in vesicles.
  • Metabolism Pathways:
    • Two major pathways for metabolism in humans:
    1. Histamine N-methyltransferase followed by Monoamine oxidase (MAO) (primary method).
    2. Diamine oxidase followed by phosphoribosyltransferase (less common).
  • Importance of Enzymes:
    • Remember key enzymes: Histamine N-methyltransferase and MAO.

Histamine Receptors

  • Types of Histamine Receptors:
    1. H1 Receptor: GQ coupled.
    2. H2 Receptor: GS coupled.
    3. H3 Receptor: GI and GO.
    4. H4 Receptor: GI.
  • Mechanisms of Action:
    • Excitatory or inhibitory actions depending on G protein signaling.
    • Activation of H1 leads to increased intracellular calcium via PLC activation (IP3 and DAG).
    • Activation of H2 promotes cyclic AMP production through adenylyl cyclase.
    • Activation of H3 and H4 inhibits cyclic AMP, leading to inhibition of neuron activity.

Distribution of Receptors in Body

  • Locations of Receptors:
    • H1 Receptors: Present in smooth muscles, tuberomammillary nucleus (CNS), and on sensory nerves in PNS for nociceptive signaling.
    • H2 Receptors: Located on smooth muscles and parietal cells in the stomach, regulating gastric acid secretion.
    • H3 Receptors: Predominantly presynaptic in CNS (cortex and subcortex).
    • H4 Receptors: Found on basophils and in the thymus.

Physiological and Immunological Functions of Histamine

  • Role in Inflammation:
    • Released from mast cells and basophils during inflammatory responses.
    • Causes swelling, vasodilation, and release of other autocoids.
  • Effects on Pain and Respiratory Function:
    • Causes hypersensitivity to pain, bronchoconstriction, and may mediate anaphylactic airway tightening.
    • Strongly promotes pruritus (itching sensation).

Central Nervous System Pathways

  • Histamine Release in CNS:
    • Tuberomammillary pathway: originates from tuberomammillary nucleus, projects to several brain regions (cortex, amygdala, locus coeruleus, etc.).
    • Increases awareness, arousal, wakefulness, and cognition through acetylcholine release.
  • Impact of Antihistamines:
    • Antihistamines can promote sedation, working through H1 receptors.
    • Can inhibit appetite based on central H1 activity.
  • Regulation of Neurotransmitters:
    • H3 receptors act as auto receptors to inhibit release of neurotransmitters such as acetylcholine, norepinephrine, dopamine, and serotonin.

Examples of Histamine-Related Medications

  • Zyrtec (Cetirizine):
    • H1 receptor antagonist for allergies; induces sleepiness.
  • Famotidine (Pepcid):
    • H2 receptor antagonist; inhibits gastric acid production.
  • Pitolisant:
    • H3 receptor antagonist; used in narcolepsy to promote wakefulness.
  • Thioperamide:
    • H3 receptor antagonist; enhances histamine release by blocking autoreceptors.

Conclusion and Review

  • Understanding histamine's synthesis, transport, signaling, receptor dynamics, and medication implications is crucial for the neurological systems context.
  • Remember the major pathways, receptors, and clinical implications of histamine and related medications for future reference.