A genetic analysis of opioid-induced hyperalgesia in mice

Abstract

Background: Opioid-induced hyperalgesia (OIH) is a syndrome of increased sensitivity to noxious stimuli, seen after both the acute and chronic administration of opioids, that has been observed in humans and rodent models. This syndrome may reduce the clinical utility of opioids in treating acute and chronic pain.

Methods: In these studies, the authors measured the propensity of 15 strains of inbred mice to develop mechanical manifestations of OIH. These data were subjected to in silico genetic analysis, which resulted in the association of haplotypic blocks within or near several known genes. Both pharmacologic agents and transgenic mice were used to confirm the functional association of the most strongly linked gene with OIH.

Results: Both baseline mechanical nociceptive thresholds and the percentage changes in these thresholds after 4 days of morphine treatment were found to be highly strain dependent. The haplotypic blocks most strongly associated with the mechanical OIH data were located within the beta2 adrenergic receptor gene (beta2-AR). Using the selective beta2-AR antagonist butoxamine, the authors observed a dose-dependent reversal of OIH. Furthermore, deletion of the beta2-AR gene sharply reduced the mechanical allodynia present after morphine treatment in the wild-type mouse strain. Analysis of the associated beta2-AR haplotypic block identified single nucleotide polymorphisms potentially explaining in part the strain specific differences in OIH.

Conclusions: Genetic variants of the beta2-AR gene seem to explain some part of the differences between various strains of mice to develop OIH. The association of this gene with OIH suggests specific pharmacologic strategies for reducing the impact of OIH on patients consuming opioids.

Figures

Figure 1

Mechanical nociceptive thresholds for the 15 strains of mice used in these studies. Mechanical thresholds were determined at baseline and after having administered morphine for 4 consecutive days. Data are presented as the means +/− SEM, *P

Figure 2

Haplotypic mapping results using data from the 15 strains of mice. For these calculations, the percent decrease in mechanical nociceptive threshold was used as the trait to be mapped. Panel A provides a plot of the significance (1/P) of correlation for each haplotypic block in the database versus the genome as displayed as tandem chromosomes. The block with the highest degree of significance (Adrb2) was part of the gene for the β2 adrenergic receptor. Panel B provides the mapping output in tabular form for the top 10 most strongly linked haplotypic blocks. The P value represents the strength of association. The actual haplotype is color coded such that all strains of mice sharing a haplotype for a specific block also share a color code. For example, the most strongly linked block (Adrb2) has 7 strains sharing the red haplotype, 7 sharing the blue haplotype and one strain uniquely possessing the green type. In this figure the colored blocks are arranged such that the leftmost block represents the strain which displayed the lowest degree of OIH, and the rightmost block is the one which displayed the highest degree of OIH. Also provided is the chromosome on which the haplotypic block is located, the starting position of the block (megabases), and the number of SNPs in the associated block.

Figure 3

The pharmacological reversal of the mechanical and thermal manifestations of OIH using the selective β2-AR antagonist butoxamine. Mice of the C57BL/6J strain were used after 4 days of morphine treatment to induce OIH or after 4 days of saline administration as a control. In panel A, data representing the measurement of mechanical withdrawal thresholds after the subcutaneous administration of butoxamine are presented. In panel B, control mice or mice treated with morphine were administered a just-maximal dose of butoxamine to determine the effects on the thermal manifestations of OIH. Data are presented as the means +/− SEM, *P

Figure 4

The assessment of mechanical and thermal manifestations of OIH in β2-AR null mutant (knockout) mice versus FVB wild type littermates. Panel A displays data demonstrating mechanical manifestations of OIH in control FVB wild type mice, but no significant sensitization in the β2-AR knockouts after 4 days of morphine administration. Panel B provides similar data using thermal nociceptive measurements. Data are presented as the means +/− SEM, *P

Figure 5

The effects of locally injected β2-AR ligands. In panel A, local injection of saline or butoxamine (1 μg ) had no effect on baseline mechanical nociceptive thresholds, while the injection of 1 μg butoxamine but not saline significantly reversed the allodynia displayed by mice after 4 days of morphine treatment (N=8/group). In panel B, separate groups of 5 mice received plantar injections of saline or each of the doses of terbutaline indicated. Data are presented as the means +/− SEM, **P

Figure 6

Diagram of the β2-AR haplotypic block associated with OIH. Arrows indicate the relative positions of the 8 SNPs defining the block.