H1 (Histamine) Receptors

Summary

H1 histamine receptors are G-protein coupled receptors that play an important role in allergic and anaphylactic reactions. Binding of histamine to H1 receptors leads to the activation of a Gq protein subunit, which leads to several downstream signalling effects. In the airways, H1 receptor signaling leads to increased mucus production and bronchospasm which can contribute to asthma. In blood vessels, H1 signalling leads to vasodilation and increased vascular permeability, causing hypotension and edema.  Lastly, histamine signalling at H1 receptors in skin leads to pruritus and urticaria. Antihistamine drugs work by blocking H1 receptors and are used to treat allergic and anaphylactic reactions, as well as to supplement asthma treatment.

Key Points

• H1 (Histamine) Receptors
  • GPCR that responds to histamine
    • Activates a Gq subunit
  • Locations
    • Airway epithelium
    • Vascular smooth muscle
    • Endothelium
    • Sensory nerves, immune cells
  • Actions
    • Increases airway mucus production
      • Antihistamines may be used to treat seasonal allergies (rhinitis) or nasal congestion
    • Bronchoconstriction
      • Histamine signaling plays a role in allergic asthma
      • Cromolyn and Nedocromil treat asthma by stabilizing mast cells to prevent histamine release
    • Vasodilation
      • Histamine can cause vasodilation by mediating release of NO
      • Severe anaphylaxis may lead to systemic vasodilation, reducing vascular resistance and inducing hypotension (shock)
      • Antihistamines and epinephrine used to treat anaphylactic shock
      • Red man syndrome (with vancomycin) is caused by cutaneous vasodilation due to histamine release
    • Urticaria (hives)
      • Increased vascular permeability causes local edema and inflammation under skin, resulting in itchy (pruritic) rash
    • Pain
      • Nerve endings detect histamine as a signal for tissue injury