Novel menthol method points to new pain therapies
point towards new therapies to treat chronic inflammatory and
neuropathic pain indications by targeting the temperature-activated
ion channels.
"Some of these ion channels are considered targets to treat chronic inflammatory and neuropathic pain indications," said Ardem Patapoutian, associate professor at Scripps Research and member of GNF.
"Understanding how small molecules such as menthol affect the function of these proteins could be crucial in designing future drugs that can either activate or block them."
Ion channels are proteins found in the cell membrane that can form a tunnel or channel that allows specific ions to move across the membrane.
When activated, the channel opens, allowing an influx of calcium ions into the axon, an electrical signal that alerts the neuron, which relays the message to the brain.
Using a novel mutagenesis and high-throughput screening approach, the study assayed 14,000 TRPM8 mutants to find mutants that were not enhanced by menthol but were otherwise functioning normally.
The scientists' analysis pinpointed a potential interaction site for menthol, as well as a site that translates binding information to ion channel activity.
It is a well-established method to mutate individual amino acid residues in an ion channel protein and examine the effect that these mutations have on the channel's function. However, the laborious nature of these experiments limits the number of mutant ion channels that can be made and analysed.
"Our new high-throughput screening methodology allowed us to analyze 14,000 mutants out of which we isolated five that specifically affected menthol activity," said research associate Michael Bandell, the lead author of the study.
"Our experiments yielded significant insights into the functional elements of TRPM8 ion channel protein that would have been difficult to obtain using other mutagenesis methods."
Because the methodology can be used to screen for activation or inhibition, it could prove to be useful as a general method to analyse the mechanism by which drugs can activate or inhibit ion channels or other receptors.
Specifically, the new methodology could be used to identify amino acid residues in certain ion channel proteins and G-protein coupled receptors (proteins involved in stimulus-response pathways) that are involved in the interaction with small molecules that affect their function.
Menthol is an ingredient known for its cooling and antibacterial properties. It is used in topical preparations to assist their permeation through the surface of the skin. Menthol replaces the pain message with a cooling sensation in order to provide temporary relief from localised pain.
The reason that menthol creates a cool sensation is that it reacts with thermoreceptors in the skin. It has been estimated that humans have several times more thermoreceptors that are attuned to feel cold than they do those that are sensitised to detect warm.
The study was released in an advanced online version by the journal Nature Neuroscience. It will be published in the journal's April edition (Vol. 9, No. 4).
