Project 176440

Chronic muscle pain conditions are very prevalent in the Canadian population, particularly in women of child-bearing age. We are studying the aetiology and possible treatments for these conditions, using both human and animal models of chronic jaw muscle pain. We have shown that small injections of acidic salt solution into the jaw muscles of rats causes them to turn their head away more frequently when light pressure is applied to the face. This hypersensitivity lasts for more than four weeks, suggesting that the muscles have become painful. We have also shown that some neurons (muscle spindle afferents) that normally signal the length of the muscle and degree of stretch, show abnormal changes in their electrical properties. We believe that these transformed neurons play a key role in this acid-induced muscle pain, and have shown that they could be helping to activate specific pain receptors through an interaction within muscle stretch-sensitive organs. We will now test several hypotheses in new experiments. In human volunteers, we will see if stimulating muscle spindles causes long-lasting muscle pain to increase. In rats, we will see if muscle stretch receptors release chemicals that can cause pain within the muscle. Also, we will see if head withdrawal is prevented by pre-treatment with substances that interfere with communication between the stretch receptors and the pain receptors within the muscle. When these animals are killed, we will take slices of brain so that we can study the properties of muscle spindle sensory neurons kept in an aqueous medium for many hours. We will see if the abnormal changes in electrical properties still occur after treatments that reduce pain. We will also look into the mechanisms underlying these properties, including sensitivity to acid, and try to find ways to prevent the pathological changes.