The use of opioids for the treatment of chronic pain has increased dramatically over the past decade [32, 33]. However, as indicated above, the responsiveness to opioids in chronic pain patients may vary depending on individual differences but also on the type of pain [34, 35]. For instance, a number of studies have evidenced that neuropathic pain patients are insensitive to opioids. The pharmacodynamic response to a given opioid depends on the nature of the receptor to which the opioid binds and its affinity for the receptor. During recent years, a wide range of candidate genes relevant to pain has been highlighted [8] and these genes are associated with a variety of molecules involved in pain processing, including neurotransmitters and their receptors and transporters, metabolic enzymes, ion channels, intracellular enzymes and second messengers. Minor changes in allelic variability, like SNPs, may affect the functionality of the encoded protein, resulting in an alteration of the protein activity or in the transcription rate leading to lower or higher amounts of the relevant protein [36]. An example of this is the decreased μ-opioid receptor mRNA and protein expression in human brain tissue in the presence of the OPRM1 118 G variant [37].

This study focused on the genes for the μ-opioid peptide (MOP) receptor, which mediates opioid effects, the ABCB1 transporter of opioids, and the calcium channel gene fragment α2δ, which affects the G-protein of the opioid receptor. The effects of these gene variants were in the present study measured in a group of patients with chronic pain and the effects were correlated with the perception of pain, the incidence of opioid side effects, plasma levels of β-endorphin and opioid sensitivity.

An important finding in the present study is that the plasma levels of β-endorphin the endogenous ligand for the MOP receptor were significantly lower in the chronic pain patients compared to control subjects. This observation agrees with previous reported findings showing that plasma levels of β-endorphin correlate inversely with pain levels in patients with various pain syndromes [38–42], i.e. the plasma concentration of this opioid peptide are lower in patients with poorly controlled pain but increase with pain relief [42]. Interestingly, in the present study opioid responders had higher plasma concentration of β-endorphin than non-responders.

Our results also demonstrate that high responders to remifentanil tended to be more likely to have the minor allele of OPRM1 than normal responders, non-responders or controls. However, there were too few patients to confirm any statistical significance in this regard. The study reported by Landau et al. [15] indicating that women in labor who were homozygous for the minor OPMR1 allele required less intrathecal fentanyl than those with the major allele is in accordance with our results. Similarly, Janicki and co-workers [43] demonstrated that patients with chronic pain who were homozygous for the minor allele used opioids less than those carrying the major allele. Furthermore, men with a higher pain threshold to experimental pressure pain were more likely to have the minor allele of A118G [18]. Similarly, according to a report by Huang et al. [44] women with the minor allele of the OPRM1 receptor gene SNP IVS2+31 G>A have a higher pressure pain threshold than women with the major allele. Taken together, all these findings indicate that the minor alleles of the OPRM1 gene could offer some protection from pain and subsequently have decreased requirement for opioids in chronic pain conditions.

In contrast, recent studies have indicated that patients with malignant disease who have the minor allele of OPRM1 require more morphine than those with the major allele, and that patients who are undergoing abdominal hysterectomy [45] or total knee arthroplasty [46] who have the minor allele require larger intravenous doses of morphine postoperatively than those with the major allele. Also, a previous study has shown that fentanyl is less effective in subjects carrying the G allele of the OPRM1 A118G SNP than those with the A allele, and subjects with the G allele were fount to require more fentanyl for adequate postoperative pain control than those with the A allele [47]. Other studies suggest decreased sensitivity to the analgesic effects of opioids in persons with the minor allele and protection from opioid toxicity and side effects [48, 49]. Thus, in patients with acute postoperative and cancer pain, those with the minor allele of OPRM1 require more morphine than those with the major allele. However, the results of our study suggest that the situation could be different in patients with a phenotype of chronic pain from that in patients with postoperative or cancer pain and healthy subjects.

The consequence of the mutation A>G in the OPMR1 gene is an additional net charge with loss of a putative glucosylation site in the area of the ligand-receptor interaction and this feature may have consequences for the sensitivity to opioids of pain patients, although the phenotype of the pain seems to be essential.

Interestingly, the OPRM1 A118G SNP has also been associated with higher heroin doses in addicted individuals [50]. In one of the first studies of the properties of the A118G allele, Bond et al. found a threefold increase in binding of β-endorphin [10], which could indicate that people with the minor allele might be at risk of opioid addiction. However, this finding has been disputed by Beyer and co-workers [51]. The men with chronic pain in our study who had the minor allele had lower β-endorphin levels than the men with the major allele, which could imply increased binding in the group with the minor allele.

It should be noted that in the present study we present data from the gene analysis as expressed in genotype frequency (see Tables 1, 2, 3) in contrast to many other investigators expressing their data by means of allele frequency. This may give the impression that we report a much higher frequency of the OPRM1 A118G allele in our study population compared many other studies on the Caucasian population. However, calculation of the allele frequency in our material yields values that are in good agreement with those reported from other laboratories [46].

Table 3 Background data Full size table

Our study also suggests that there may be differences between male and female patients regarding the effects of genetic variation in the ABCB1 gene. Men with the minor TT allele had higher β-endorphin levels than men with the major CC allele but the reverse was true for women. Sia and co-workers [52] demonstrated a trend towards a higher risk of postoperative pain in women with the T-allele a difference underlying the potential importance of sex difference. Moreover, variants of the ABCB1 gene may explain some portion of the inter-strain differences in opioid-induced hyperalgesia in mice and perhaps other consequences of chronic opioid administration [53].

Furthermore, patients homozygous for the minor TT allele of the ABCB1 gene experienced more opioid-related side effects such as sweating, muscular tension, stress and sedation than patients with the major CC/CT alleles. This is in agreement with other studies indicating an increased risk of opioid-induced side effects [54], such as early respiratory depression with fentanyl treatment [55]. It has been proposed that this effect involves impairment of P-glycoprotein transport, resulting in higher brain concentrations of the substrate (e.g. remifentanil). However, earlier studies have indicated decreased effects of methadone in patients with the minor TT allele [13, 56] and a smaller increase in R-methadone levels with quetiapine [57]. The consequence of this mutation in the ABCB1 gene may be related to the effectiveness of the transporter P-glycoprotein, encoded by this gene. The transporter P-glycoprotein, is thus known to act on a broad range of prescription medicines, including opioids. The ABCB1_3435C>T SNP has been associated with mRNA, protein and serum levels, and with responses to a number of medical drugs [58].

An interesting observation made in this study is that the minor CACNA2D2 G>A SNP was not found in any of the 16 high responders to remifentanil. This gene encodes a member of the alpha-2/delta subunit family, a protein in the voltage-dependent calcium channel complex. All the high responders to the opioid remifentanil had the major allele of CACNA2D2, suggesting an association between this allele and high opioid sensitivity. Interestingly, genetic variations of the Na+, K+, and Ca2+−channel genes have also been associated with migraine and neuropathic pain [59]. Very few, if any, study on opioid sensitivity in relation to the gene encoding the calcium channel fragment, CACNA2D2 allele, has so far been reported. Interestingly, a recent study demonstrated an association between the voltage-gated calcium channels and the A118G OPRM1 polymorphism [60].

Thus, the effect of the mutation in the CACNA2D2 G>A allele will certainly affect the effectiveness of the opioid. It is also essential for the action of β-endorphin in pain processing pathways. The link between β-endorphin and the calcium channel may be reflected by the observation that this opioid affects the inhibitory action of OPMR1 on the calcium channel expressed on pathways involved in nociception [60].

It is interesting to note that the low number of patients examined in this study was sufficient to provide some significant correlations in the group of patients with chronic pain, a complex bio-psychosocial entity. This was probably due to the homogeneity of the patient groups, and the fact that this study focused on correlating the presence of the gene variation affecting the function of the μ-opioid receptor with its ligand β-endorphin and opioid-related symptoms in a clinical population with the same chronic pain syndrome. This study also presents the gene for a calcium channel fragment as a plausible contributor to individual variations in opioid sensitivity. The sex variations were prominent, as has been demonstrated in other studies. These results take the issue of differences in opioid and pain sensitivity a step further; sex and genetic variations could explain differences in the pharmacokinetics and pharmacodynamics of opioids.