Analgesic Mechanisms In Patients With Chronic Pain

Dental &Craniofacial Research

Investigators

Linked publications & trials

Abstract

Our current focus is to use candidate gene studies to identify molecules that affect the outcome in chronic nerve pain conditions and would therefore might be high priority targets for pain-relieving drug treatments. Our primary clinical condition of interest is lumbar root pain from herniated intervertebral disc. This is one of the most costly chronic pain conditions in the US population. It is ideal for candidate gene studies because the anatomy and clinical presentation of disc herniation is so uniform, and it is common enough to allow the collection of the large numbers of patients essential to candidate gene studies. Because the physiology of the nerve root is similar in the lumbar spine and in the facial region (trigeminal nerve root) we will use results from the common spinal condition to understand less common orofacial nerve root pain conditions, including idiopathic trigeminal neuralgia (?tic douloureaux?) and traumatic and cancer related trigeminal nerve disorders. We are also carrying out randomized controlled trials of treatments for chronic lumbar nerve root pain, and providing our genetics expertise to other research groups who study other kinds of pain disorders and mood disorders. In the first of our lumbar spinal root pain clinical trials to be published, Khoromi et al. (J Pain, in press) showed that the anticonvulsant drug, topiramate, reduced chronic nerve root pain by about 18% compared to placebo in the 29 patients who completed a two period crossover study. This is the first report of any drug being helpful in this type of pain over a period of several months. However, the usefulness of the drug is lessened by frequent side effects such as sleepiness and change in taste. We completed other randomized trials of opioids, of the antidepressant drug nortriptyline, of the combination of those two drugs, and of magnetic belt treatment of chronic nerve root pain, as well as a study of the effects of chronic pain and opioid treatment on sex and adrenal hormone secretion. Our group?s other primary publications this year focused on our initial steps in developing genetic tests to find out if variants in pain-processing genes affect the outcome of chronic lumbar nerve root pain. The first step is to identify common variants in these genes in approximately 100 individuals each in several different ethnic groups and then prepare an efficient test of these genes that will maximize genetic information and minimize the expense. Our publications describe the structure of gene variation with a level of detail much greater than the information being developed by NIH?s public HapMap project, which uses only 36 patients per ethnic group and studies fewer variants in each gene. The studies describe which variants are usually found with which other variants. An important result of each study is to identify large blocks of DNA which have stayed together for tens of thousands of years. After one understands this, future studies only need to test a few variants on each block to know what all of the coding is on the block, which lowers genetic testing expense and allows the detection of unsuspected disease gene variants that lie between the coding sites that are tested. Our Genes and Immunity paper (Belfer et al., 2004) describes the patterns of variation in inflammatory cytokines IL1-beta, IL-6, and TNF-alpha, which are important mediators of inflammation in nerve injury, surgical, and arthritis pain, and may affect the outcome of many infectious and autoimmune diseases including periodontal disease. Our first J Human Genetics paper (Belfer et al., 2004) describes the variation in the norepinephrine transporter gene, a molecule that reduces the amount of a ?fight or flight? transmitter that produces arousal, alertness, anxiety, and increased pulse rate and blood pressure. A second J Human Genetics paper (Belfer et al., 2005) describes the variation in the alpha adrenergic 2A, 2B, and 2C receptors, which are important for regulating mood, blood pressure, heart rate, and the response to some pain-relieving drugs and the body?s own pain relievers. Our Molecular Genetics and Genomics paper (Buzas et al, 2004) analyzes the variation in the alpha adrenergic 1 class of genes, important for maintaining or elevating the blood pressure, and for the generation of nerve injury pain. Our European Journal of Human Genetics paper (Belfer et al., 2005) analyzes the variation in the beta adrenergic genes, which are important for the treatment of coronary disease, asthma, and anxiety. While we were applying these assays to our own lumbar spine pain study samples, we had the opportunity to use these assays in collaboration with other groups who collected patients who were studied for traits other than pain. For example, in a collaborative study with NIMH and Howard University (Neumeister et al., 2005), we helped Neumeister to show that a common deletion in the alpha-2C adrenergic receptor is associated with excessive release of the catecholamines norarenaline and adrenaline, which may account for some of the higher risk of hypertension, congestive heart failure, and depression in African-Americans. Twenty-nine healthy African-Americans genotyped for alpha-2 adrenergic receptor subtype polymorphisms underwent tritiated-noradrenaline and adrenaline intravenous infusion and arterial blood sampling for measurements of rates of entry of endogenous noradrenaline and adrenaline into arterial plasma (total body spillovers) by the tracer dilution technique. Eleven subjects were homozygotes for the alpha 2C deletion polymorphism, 9 intravenous infusion of the alpha-2-adrenergic receptorr antagonnist, yohimbine. Administration of yohimbine produced larger, more sustained increments in noradrenaline spillover, heart rate, and anxiety in homozygotes for the receptor molecule deletion than in the other groups. Because temporomandibular joint area pain (TMD) is an important orofacial condition, we offered our genetics expertise to William Maixner?s group at the University of North Carolina Dental School, who had done a longitudinal study of normal young women to determine the risk factors for developing TMD. Our Human Molecular Genetics paper (Diatchenko et al, 2005) reports a study in 200 normal young woment were tested for sensitivity to 16 laboratory pain tests, and genotyped for polymorphisms of the COMT gene. We identified three genetic variants (haplotypes) of COMT that we designated as low pain sensitive (LPS), average pain sensitive (APS), and high pain sensitive (HPS) that account for 11% of variability in experimental pain perception. This result is confirmed by the data gathered by following these women for three years for new pain complaints. The presence of even a single LPS haplotype diminishes, by as much as 2.3 times, the risk of developing myogenous temporomandibular joint disorder (TMD), a common musculoskeletal pain condition. The LPS haplotype produces much higher levels of COMT enzymatic activity compared to the APS or HPS haplotypes. Thus, three major haplotypes determine the levels of COMT activity that inversely correlate with pain sensitivity and the risk of developing TMD, and possibly other related chronic pain conditions. This is the first study that has identified a genetic polymorphism associated with the risk of developing a chronic pain condition, and helped these investigators obtain funding for a definitive study of 3200 subjects. We also coauthored a paper on the methods of analyzing and presenting clinical trial results (Farrar et al., J Pain Symptom Management, in press) and five review articles or chapters on methods of pain research and treatment.