The CHRM3 receptor, or Cholinergic Receptor Muscarinic 3, is a protein receptor found on the surface of various cells throughout the body. This receptor mediates the effects of acetylcholine, a chemical messenger, which helps control numerous involuntary bodily functions.
Location and General Function
CHRM3 receptors are found in many tissues and organs, including smooth muscles, various glands, and specific regions of the nervous system. As a G protein-coupled receptor (GPCR), CHRM3 functions like a cellular “lock.” Acetylcholine acts as the “key” that fits into this lock, binding to the receptor.
When acetylcholine binds to CHRM3, it activates G proteins, triggering a cascade of intracellular signals. This pathway often increases intracellular calcium levels and modulates other cellular processes, leading to a specific cellular response.
Physiological Roles
CHRM3 plays diverse roles in regulating many involuntary bodily processes. It is involved in the contraction of smooth muscles in various organs. For example, it contributes to the contraction of muscles in the bladder, which is necessary for urination. In the airways, CHRM3 activation leads to bronchoconstriction, narrowing the air passages. The receptor also regulates motility and digestion within the gastrointestinal tract by influencing muscle contractions there.
Beyond muscle contraction, CHRM3 also stimulates glandular secretions throughout the body. It promotes the production of saliva, sweat, and tears. In the eye, CHRM3 mediates pupillary constriction and helps with accommodation, the eye’s ability to adjust focus for near vision. The receptor also has a minor role in regulating heart rate, contributing to the slowing of the heart, although M2 receptors are primarily responsible for this effect.
CHRM3 in Health and Disease
Disruptions in CHRM3 function can contribute to various health conditions. When the CHRM3 receptor in the bladder becomes overactive, it can lead to symptoms of overactive bladder (OAB), such as urinary urgency and incontinence. This heightened activity results in more frequent and sometimes uncontrollable bladder contractions.
In respiratory conditions like asthma and chronic obstructive pulmonary disease (COPD), CHRM3-mediated bronchoconstriction narrows the airways. This contributes to the characteristic symptoms of these diseases, including wheezing and shortness of breath.
Conversely, reduced CHRM3 activity can lead to decreased saliva production, resulting in dry mouth, a condition known as xerostomia. In the eye, dysfunction of CHRM3 can also contribute to glaucoma, a group of eye conditions that can damage the optic nerve and impair vision, partly by affecting intraocular pressure.
Therapeutic Targeting
Understanding the function of CHRM3 allows for the development of targeted medical treatments. Molecules that activate the receptor, mimicking the action of acetylcholine, are known as agonists. Conversely, molecules that block the receptor, preventing acetylcholine from binding, are called antagonists.
Antimuscarinic drugs, which are CHRM3 antagonists, are commonly used in medicine. For instance, medications like oxybutynin and solifenacin are employed to treat overactive bladder by relaxing the bladder muscles, reducing urgency and incontinence. In respiratory conditions such as asthma and COPD, antimuscarinics like ipratropium and tiotropium help relax airway muscles, thereby improving breathing. An example of a CHRM3 agonist is pilocarpine, which is used to treat dry mouth by stimulating saliva production. Pilocarpine also finds use in certain types of glaucoma by constricting pupils and helping to reduce pressure within the eye.