THURSDAY, Feb. 20, 2003 (HealthDayNews) -- If you find yourself whining over a paper cut or moaning over a stubbed toe, blame your genes.
The difference between a wimp and a tough guy (or girl) is due in part to a tiny variation of a single gene, a new study says.
In a second report, researchers have identified how neurons form "memories" of past pain. The research could help to explain why some people experience chronic pain even after the injury or inflammation that sparked the pain has subsided.
The discoveries, which appear in the Feb. 21 issue of Science, add to a growing body of research finding that an individual's perception of pain is largely dependent on his genes and brain chemistry.
"We know as clinicians that that there are certain individuals who, given the same exposure to pain, experience it chronically and forever, while there are those who get over the pain quickly," says Dr. Charles Argoff, director of the Cohn Pain Management Center and an assistant professor of neurology at New York University School of Medicine.
"This work is fantastic in that it starts to show that there could be underlying genetic differences that may explain why some people, given the same experience, never get rid of the pain or experience it longer while some barely experience it at all," he adds.
In the study on "pain memories," researchers from Germany and Austria knew from previous research that when a certain group of neurons in the spinal cord are stimulated by pain-related matter called "substance P," abnormally enhanced sensitivity to pain can result.
The researchers determined that the activation of a key set of receptors creates conditions that promote the strengthening of the connections among the neurons, leading to a permanent enhancement of the pain-processing pathways, and, therefore, increasing sensitivity to even the most minor pain.
In the second study, which focused on the genetic differences of pain perception, researchers injected 15 men and 14 women ages 20 to 30 with salt water. The injection simulated the pain someone would feel if he had a chronic condition called temporomandibular joint pain disorder.
During the experiment, the study participants were asked to rate their pain every 15 seconds while researchers monitored their brain activity using positron-emission tomography, or PET scans. The participants were also asked to fill out a detailed questionnaire about their perception of pain and their level of emotional distress after the study.
Researchers found study participants who had a single variation on the COMT (catechol-O-methyl transferase) gene experienced more severe pain and were more troubled by the experience.
Researchers focused on the COMT gene because it contains enzymes that control the metabolism of the neurotransmitters dopamine and noradrenaline. The enzymes act as a sort of brain janitor, breaking down and metabolizing dopamine and noradrenaline, says Dr. Jon-Kar Zubieta, lead author of the study and an associate professor in the departments of radiology and psychiatry at the University of Michigan.
Each person has two copies of the COMT gene, inherited from each parent. The COMT gene carries one of two amino acids: valine ("val"), or methionine ("met"). Therefore, you can have one of three combinations: val-val, met-met or met-val.
People with the val-val combination make powerful COMT that mops up dopamine rapidly, Zubieta says. People with the met-met combination make poor COMT that can't clean up the dopamine in their brains very well.
Those with one copy of each gene variety, the met-val combination, fall somewhere in the middle. The met-val combination is the most common, Zubieta says.
Animal studies have shown that when the dopamine system is highly active, the brain reduces its production of chemicals called enkephalins, Zubieta says. Enkephalins, which are part of the body's pain-control system, regulate and suppress painful or stress-related signals in the brain.
Zubieta and his colleagues found that people with the val-val combination were able to activate the brain's painkilling system better than those with the met-met combination.
Therefore, people with the val-val combination were able to tolerate the most pain, while those with the met-met had the most pronounced response to pain. As researchers suspected, people with the met-val combination fell in between.
"A single gene can impose how your body responds to pain and controls pain," Zubieta says.
The discoveries open the door to identifying other pathways that impact or pain perception and emotions, Argoof says.
More information
For help coping with chronic pain, check out the American Pain Foundation or the American Chronic Pain Association.
