Transient receptor potential cation channel subfamily M member 8 (TRPM8), also known as the cold and menthol receptor 1 (CMR1), is a protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 that in humans is encoded by the TRPM8 gene

Gene

A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

.
TRPM8 is an ion channel

Ion channel

Ion channels are pore-forming proteins that help establish and control the small voltage gradient across the plasma membrane of cells by allowing the flow of ions down their electrochemical gradient. They are present in the membranes that surround all biological cells...

, upon activation it allows the entry of Na⁺ (sodium

Sodium

Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

) and Ca²⁺ (calcium

Calcium

Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

) ions to the cell that leads to depolarization and the generation of an action potential. This eventually leads to the feeling of cold.

The TRPM8 protein is expressed in sensory neurons, and it is activated by cold temperatures and cooling agents, such as menthol

Menthol

Menthol is an organic compound made synthetically or obtained from peppermint or other mint oils. It is a waxy, crystalline substance, clear or white in color, which is solid at room temperature and melts slightly above. The main form of menthol occurring in nature is -menthol, which is assigned...

 and icilin

Icilin

Icilin is a synthetic super-agonist of the transient receptor potential M8 ion channel. Although structurally not related with menthol it produces an extreme sensation of cold both in humans and animals. It is almost 200 times more potent than menthol and 2.5 times more efficacious. Despite their...

 where as WS-12 and CPS-369 are the most selective agonist of TRPM8.

TRPM8 is also expressed in the prostate

Prostate

The prostate is a compound tubuloalveolar exocrine gland of the male reproductive system in most mammals....

, lungs, and bladder where its function is not very well understood.
Cold-patches have traditionally been used to induce analgesia or relief in pain which is caused as result of traumatic injuries.