Abstract
BackgroundOpioids are the cornerstone of treatment for moderate to severe pain, but chronic use leads to maladaptations that include: tolerance, dependence and opioid-induced hyperalgesia (OIH). These responses limit the utility of opioids, as well as our ability to control chronic pain. Despite decades of research, we have no therapies or proven strategies to overcome this problem. However, murine haplotype based computational genetic mapping and a SNP data base generated from analysis of whole-genome sequence data (whole-genome HBCGM), provides a hypothesis-free method for discovering novel genes affecting opioid maladaptive responses.ResultsWhole genome-HBCGM was used to analyze phenotypic data on morphine-induced tolerance, dependence and hyperalgesia obtained from 23 inbred strains. The robustness of the genetic mapping results was analyzed using strain subsets. In addition, the results of analyzing all of the opioid-related traits together were examined. To characterize the functional role of the leading candidate gene, we analyzed transgenic animals, mRNA and protein expression in behaviorally divergent mouse strains, and immunohistochemistry in spinal cord tissue. Our mapping procedure identified the allelic pattern within the netrin-1 receptor gene (Dcc) as most robustly associated with OIH, and it was also strongly associated with the combination of the other maladaptive opioid traits analyzed. Adult mice heterozygous for the Dcc gene had significantly less tendency to develop OIH, become tolerant or show evidence of dependence after chronic exposure to morphine. The difference in opiate responses was shown not to be due to basal or morphine-stimulated differences in the level of Dcc expression in spinal cord tissue, and was not associated with nociceptive neurochemical or anatomical alterations in the spinal cord or dorsal root ganglia in adult animals.ConclusionsWhole-genome HBCGM is a powerful tool for identifying genes affecting biomedical traits such as opioid maladaptations. We demonstrate that Dcc affects tolerance, dependence and OIH after chronic opioid exposure, though not through simple differences in expression in the adult spinal cord.Electronic supplementary materialThe online version of this article (doi: 10.1186/1471-2164-15-345) contains supplementary material, which is available to authorized users.
Highlights
Opioids are the cornerstone of treatment for moderate to severe pain, but chronic use leads to maladaptations that include: tolerance, dependence and opioid-induced hyperalgesia (OIH)
Haplotype-based computational genetic mapping (HBCGM), which used SNP databases generated from limited sequence information, has identified several involved genes including those coding for β2-adrenergic and 5-HT3 receptors that affect opiate responses, and these findings were translated to humans [8,15]
We focus on identifying genetic factors affecting OIH, we previously noted that the strain dependencies of OIH, tolerance and physical dependence were correlated [13]
Summary
Opioids are the cornerstone of treatment for moderate to severe pain, but chronic use leads to maladaptations that include: tolerance, dependence and opioid-induced hyperalgesia (OIH). These responses limit the utility of opioids, as well as our ability to control chronic pain. Genetic analysis offers a hypothesis-free approach that could be used to discover the mechanisms mediating maladaptive responses to opiates, and this information could lead to novel therapeutic strategies. We expect genetic approaches in mice to be informative, since there are very large differences in the response of inbred strains to opiates, including the development of opioid analgesia, tolerance, dependence and hyperalgesia [8,9,10,11,12,13,14]. HBCGM, which used SNP databases generated from limited sequence information, has identified several involved genes including those coding for β2-adrenergic and 5-HT3 receptors that affect opiate responses, and these findings were translated to humans [8,15]
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