The Role of Genetics and Pain in Drug Tolerance and Addiction
Doctors have concerns about the use of addictive drugs (opioids) such as morphine used for pain. Tolerance and physical dependence can occur when used over a period of time. Animal studies have shown that genetics have an effect on tolerance and dependence. In this study, researchers use mice to look at how pain affects opioid tolerance and dependence.
Two groups of eight mice were included in the study. The test group was given an injection in one paw to cause an inflammatory reaction. The control group was not injected. Then mice in both groups were injected once each day for four days with morphine. The amount of drug given was increased each day.
A drug (Narcan) was used to cause withdrawal from opioids. Narcan works by blocking the cell receptors for opioids. Without the opioids, the body goes into withdrawal. Physical dependence on morphine was measured by counting the number of jumps the mice made for 15 minutes after the Narcan was given.
Different strains of mice had different reactions to the morphine and morphine withdrawal. For example, in the control group some mice jumped less than one time in 15 minutes. Others jumped more than 90 times during the same period. In the test group jumping behavior increased in three-fourths of the mice and decreased in the remaining one-fourth.
What this study shows is that mice that are in pain when given morphine develop an increased tolerance for the drug. This means the effects of opioids on pain were less over time. The mice needed more drug to get the same analgesic effect.
The large differences among strains of mice suggest genetics do play a role in pain response to opioids. The authors conclude both chronic pain and genetics change the "patient's" (mice) response to morphine, tolerance, and physical dependence on the drug.
References: De-Yong L, et al. Chronic Pain and Genetic Background Interact and Influence Opioid Analgesia, Tolerance, and Physical Dependence. In Pain. April 2006. Vol. 121. No. 3. Pp. 232-240.
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