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OBJECTIVE Genetic factors are hypothesized to be involved in interindividual differences in opioid sensitivity. Inbred mouse strains that are genetically different and isogenic within each strain are useful for elucidating the genetic mechanisms underlying the interindividual differences in opioid-induced analgesia. METHODS We examined the effects of morphine in 10 inbred mouse strains, including wild-derived strains that have a wide range of genetic diversity, including BLG2, CHD, KJR, MSM, NJL, PGN2, and SWN. We also performed full sequencing of the 5' flanking region and exons of the mouse mu opioid receptor gene Oprm1 and analyzed the association between genotypes and phenotypes in these mice. RESULTS The effects of morphine on locomotor activation and antinociception varied among the inbred strains. The nucleotide differences that cause amino acid substitutions were not found in the Oprm1 gene in the inbred strains analyzed in this study. In the 5' flanking region and 3' untranslated region of the Oprm1 gene, four highly variable regions containing novel short tandem repeat polymorphisms (GA, T, TA, and CA/CT) were identified. The GA, T, and TA repeat numbers were significantly associated with morphine-induced antinociception. CONCLUSION These results suggest that the short tandem repeats in the 5' flanking and 3' untranslated regions of the mu opioid receptor gene are involved in interstrain differences in opioid sensitivity in mice. Wild-derived inbred mouse strains with different numbers of these repeats may be useful models for examining interindividual differences in opioid sensitivity.

Opioids have been suggested to affect feeding behaviour. To clarify the role of mu-opioid receptors in feeding, we measured several parameters relating to food intake in mu-opioid receptor knockout mice. Here, we show that the knockout mice had increased body weight in adulthood, although the intake amount of standard food was similar between the wild-type and knockout littermates. Serum markers for energy homeostasis were not significantly altered in the knockout mice. Hypothalamic neuropeptide Y mRNA, however, was higher in knockouts than in wild-type mice. Our results suggest that the up-regulated expression of neuropeptide Y mRNA might contribute to the increased weights of adult mu-opioid receptor knockout mice.

The mu opioid receptor (MOR) is known to play an essential role in morphine-induced analgesia. MOR-1 mRNA, the major MOR transcript, possesses a long 3′ untranslated region (3′UTR) in both mouse and human species. The sequence of the MOR-1 3′UTR, especially that of its 3′ end region, is conserved between mice and humans. In the MOR-1 3′UTR, AU-rich elements (AREs) are densely localized at the 3′ end region, suggesting low stability of this mRNA. Numerous putative transcription factor-binding motifs are located in the 3′ non-coding regions of the mouse and human MOR genes. The CXBK mouse strain, known as a MOR-deficient strain, possesses a decreased amount of MOR-1 mRNA containing an abnormally long MOR-1 3′UTR with a long nucleotide insertion. This insert might disrupt the stability of the MOR-1 mRNA or might reduce the transcription of the MOR-1 mRNA by separating the transcription factor-binding motifs in the 3′ non-coding region of the MOR gene, thereby decreasing MOR-1 mRNA expression and attenuating morphine-induced analgesia in CXBK mice. These recent findings suggest that the MOR-1 3′UTR is involved in mRNA expression and in the difference in response to morphine. This possible genetic mechanism may provide a good starting point for designing effective pain treatments with opiates.

Individual differences in responses to opioids limit effective pain treatment with these drugs. Identifying the mechanism could help to improve the analgesic effects of them. Since the molecular cloning of the mu opioid receptor (muOR) gene, substantial advances in opioid research have been made, including the discoveries that muOR plays a mandatory role in the analgesic effects of opioids and that the sequence of the muOR gene varies from one individual to another. It is conceivable that the differences in the muOR gene cause individual differences in opioid actions. The present review summarizes the recent advances made in research on human and mouse muOR genes and proposes that the variances in the 3' untranslated region (39-UTR) of the muOR gene might participate in the variability of the opioid response.

Much evidence indicates that calcium channel plays a role in cocaine-induced behavioral responses. We assessed the contributions of Ca(v)2.3 (alpha(1E)) calcium channel to cocaine effects using Ca(v)2.3 knockout mice (Ca(v)2.3-/-). Acute administration of cocaine enhanced the locomotor activity in wild-type mice (Ca(v)2.3+/+), but failed to produce any response in Ca(v)2.3-/- mice. Repeated exposure to cocaine induced the behavioral sensitization and conditioned place preference in both genotypes. Pretreatment with a D1-receptor antagonist, SCH23390, blocked the cocaine-induced place preference in Ca(v)2.3+/+ mice; however, it had no significant effect in Ca(v)2.3-/- mice. Microdialysis and RT-PCR analysis revealed that the levels of extracellular dopamine and dopamine D1 and D2 receptor mRNAs were not altered in Ca(v)2.3-/- mice. These data indicate that Ca(v)2.3 channel contributes to the locomotor-stimulating effect of cocaine, and the deletion of Ca(v)2.3 channel reveals the presence of a novel pathway leading to cocaine rewarding which is insensitive to D1 receptor antagonist.

Changes in expression of voltage-dependent ion channel subunits were examined in the radicular pain state. Furthermore, antinociceptive effects of gabapentin on radicular pain were compared with effects on peripheral neuropathic pain.To clarify molecular substrates involved in the development of radicular pain, and to investigate the responsiveness of radicular pain to gabapentin.Peripheral nerve injuries are known to induce dynamic changes of voltage-dependent Na+ and Ca2+ channel subunits expression in dorsal root ganglion neurons. However, the expression profiles of Na+ and Ca2+ channel subunits in the radicular pain state have not been examined.Two radicular pain models and one peripheral neuropathic pain model were prepared. By using semiquantitative reverse transcriptase-polymerase chain reaction, the expression levels of several Na+ and Ca2+ channel subunits in the dorsal root ganglions of these pain model rats were investigated. The antinociceptive effects of gabapentin were examined in a behavioral study using the aforementioned pain models.All three neuropathic pain operations induced comparable mechanical allodynia and thermal hyperalgesia. The upregulation of the Na(v)1.3 Na+ channel and Ca(v)alpha2delta Ca2+ channel subunits was observed only in the peripheral nerve injury model. A downregulation of the Na(v)1.9 channel was observed in all three pain model rats. A lower dose of gabapentin was significantly more effective in alleviating the mechanical allodynia of rats with radicular pain.The reduction of Na(v)1.9 found in all three models may link to the neuropathic pain state, including radicular pain. The lower sensitivity to gabapentin in rats with peripheral neuropathic pain might be partly explained by the marked upregulation of Ca(v)alpha2delta in the dorsal root ganglions, suggesting that gabapentin may be more effective in radicular pain treatment.

The importance of voltage-dependent Ca2+ channels (VDCCs) in pain transmission has been noticed gradually, as several VDCC blockers have been shown to be effective in inhibiting this process. In particular, the N-type VDCC has attracted attention, because inhibitors of this channel are effective in various aspects of pain-related phenomena. To understand the genuine contribution of the N-type VDCC to the pain transmission system, we generated mice deficient in this channel by gene targeting. We report here that mice lacking N-type VDCCs show suppressed responses to a painful stimulus that induces inflammation and show markedly reduced symptoms of neuropathic pain, which is caused by nerve injury and is known to be difficult to treat by currently available therapeutic methods. This finding clearly demonstrates that the N-type VDCC is essential for development of neuropathic pain and, therefore, controlling the activity of this channel can be of great importance for the management of neuropathic pain.

Opiate analgesics are widely used and abused drugs. Individual differences in opiate sensitivity can hamper effective pain treatments and increase risks of drug abuse. Although genetic factors might affect individual differences in opiate sensitivity, scientific evidence for specific genetic mechanisms that underlie these differences has been sparse. Recent studies using inbred and knockout mice have revealed that the mu opioid peptide (MOP) receptor encoded by the Oprm1 gene has a mandatory role in the analgesic and addictive properties of opiate drugs. Increasing evidence suggests that differences in Oprm1 gene sequences affect the amount of Oprm1 mRNA and sensitivity to opiates, and >100 polymorphisms have been identified in the human OPRM1 gene, some of which are related to vulnerability to drug dependence in some populations. Rapid advances in this research field are leading to improved understanding of the relationships between gene polymorphisms and opiate sensitivities that will enable more-accurate prediction of the opiate sensitivity and opiate requirements in individual patients.

α 1 subunit of the voltage-dependent Ca 2+ channel is essential for channel function and determines the functional specificity of various channel types. α 1E subunit was originally identified as a neuron-specific one, but the physiological function of the Ca 2+ channel containing this subunit (α 1E Ca 2+ channel) was not clear compared with other types of Ca 2+ channels because of the limited availability of specific blockers. To clarify the physiological roles of the α 1E Ca 2+ channel, we have generated α 1E mutant (α 1E −/−) mice by gene targeting. The lacZ gene was inserted in-frame and used as a marker for α 1E subunit expression. α 1E −/− mice showed reduced spontaneous locomotor activities and signs of timidness, but other general behaviors were apparently normal. As involvement of α 1E in pain transmission was suggested by localization analyses with 5-bromo-4-chloro-3-indolyl β- d -galactopyranoside staining, we conducted several pain-related behavioral tests using the mutant mice. Although α 1E +/− and α 1E −/− mice exhibited normal pain behaviors against acute mechanical, thermal, and chemical stimuli, they both showed reduced responses to somatic inflammatory pain. α 1E +/− mice showed reduced response to visceral inflammatory pain, whereas α 1E −/− mice showed apparently normal response compared with that of wild-type mice. Furthermore, α 1E −/− mice that had been presensitized with a visceral noxious conditioning stimulus showed increased responses to a somatic inflammatory pain, in marked contrast with the wild-type mice in which long-lasting effects of descending antinociceptive pathway were predominant. These results suggest that the α 1E Ca 2 + channel controls pain behaviors by both spinal and supraspinal mechanisms.