M3 (Muscarinic) Receptors

Summary

M3 muscarinic receptors are G-protein coupled receptors that act on many tissues in the body. These receptors respond to acetylcholine binding to activate a Gq-protein subunit, which induces a downstream cascade of second messengers. Physiologically, M3 receptors play an important role in mediating parasympathetic activity, also known as the rest and digest response. For example, M3 receptor activity stimulates body fluid secretions like tears and sweat, bronchoconstriction, bladder and GI motility, miosis or pupil constriction, lens accommodation, and insulin release. Finally, M3 receptor activity plays an important role in vasodilation, by way of increasing nitric oxide and cGMP levels.

Key Points

• M3 Muscarinic Receptors
  • Signaling Pathway
    • Activates a Gq protein subunit
  • Locations
    • Blood vessels
    • GI tract
    • Urinary tract
    • Eye
    • Secretory glands (e.g. pancreas, parietal cells of stomach, salivary gland, etc.)
  • Functions
    • Potentiates most parasympathetic (“rest and digest”) functions
      • Secretions (lacrima, sweat, salivary, gastric acid)
        • M3 antagonists like Glycopyrrolate may be used to reduce airway secretions in preparation for anesthesia
        • M3 stimulation at parietal cells increases stomach acid release
      • Gut peristalsis
        • M3 antagonists like Hyoscyamine / Dicyclomine are used for inhibition of gut activity in irritable bowel syndrome
      • Bronchoconstriction
        • M3 agonists like Methacholine are used to induce bronchoconstriction for measuring airway reactivity in asthma
        • M3 antagonists like Ipratropium and Tiotropium are used to block bronchoconstriction in asthma and COPD
      • Bladder (detrusor) contraction
        • M3 agonists like Bethanechol are used to encourage urination in post-operative cases of urinary ileus
        • M3 antagonists like Terbutaline, Solifenacin, and Oxybutynin are used for inhibition of urinary urgency
      • Pupil constriction (miosis)
        • M3 signaling activates the pupillary sphincter muscle
        • M3 antagonists like Atropine, Homatropine, and Tropicamide are used to induce pupil dilation for ophthalmic exams
      • Lens accommodation (eye focus)
        • M3 stimulation activates the ciliary muscle, which contracts to allow accommodation
      • Insulin release
        • Via parasympathetic stimulation of M3 receptors on pancreas
    • Vasodilation
      • Via nitric oxide (NO) release at blood vessel