(A) Images of microglia that had migrated through a porous membrane and were then stained with crystal violet. neurons and a delayed increase of MMP-2 in satellite cells and spinal astrocytes after spinal nerve ligation [29]. These studies underscore the value of investigating functional glial properties versus simply observing morphological reactivity. Opioids C The current gold standard Opioids are among the most potent analgesic agents available for clinical use and are the gold standard for treating acute, post-surgical and cancer pain; however, their use in neuropathic pain is often limited by the development of analgesic tolerance and unwanted side effects that are unmasked during the resulting dose escalation. Much of the early research on combating opioid tolerance focused on determining potential neuronal mechanisms of tolerance formation and designing opioid analogs with improved tolerance profiles. Beitner-Johnston demonstrated that naltrexone, a -opioid receptor antagonist, limited the development of tolerance to opioids [30]. This finding led to the synthesis of new -opioid receptor agonists, mixed -receptor agonist and -receptor antagonists, and partial -receptor agonists, which have all failed to improve upon morphine and thus have only limited value in the treatment of pain. Opioid receptor desensitization involves the NMDA receptor cascade. Preclinical [31,32] and preliminary clinical studies [33,34] suggest that the blockade of Rabbit Polyclonal to Integrin beta1 NMDA reduces opioid-induced tolerance; however, large-scale clinical trials using NMDA antagonists in conjunction with opioids to limit tolerance formation have produced disappointing results [35]. Song and Zhao have identified a casual link between glial activation and morphine tolerance [36?]. In accordance with these findings, it has been demonstrated that spinal CR3/CD11b, GFAP (an astrocyte marker) and expression of the cytokines IL-1, IL-6 and TNF increase following chronic morphine administration [37?]. Also, chronic subcutaneous and intrathecal morphine administration induces analgesic tolerance within 6 and 3 days, respectively [38]. The burgeoning body of research implicating glia in opioid tolerance led to the investigation of whether selectively targeting glial cells can provide a potential method for attenuating opioid tolerance. Both inhibiting chronic opioid-induced glial reactivity using propentofylline [39] and inhibiting proinflammatory cytokines [40] attenuate morphine-induced tolerance. Minocycline, which inhibits microglial migration [41], attenuates the development of anti-nociceptive tolerance to chronic morphine through inhibition of p38 MAPK in activated microglia [42]. These studies indicate that microglial migration may have a critical role in morphine tolerance, as has been demonstrated in neuropathic pain states [43,44?]. It was discovered that morphine enhances microglial Iba-1 expression [RJ Horvath, unpublished data] and migration [45] toward ADP in a -opioid receptor-dependent manner (Figure 2). It was proposed that chronic opioid administration induces microglial reactivity and migration toward the dorsal horn, which leads to increased proinflammatory/algesic factor production and neuronal sensitization. Microglial migration might thus prove to be an attractive pharmacological target to inhibit the induction of opioid tolerance. Open in a separate window Figure 2 Migration of morphine-treated primary neonatal rat cortical microglia toward ATPPrimary neonatal rat cortical microglia were harvested, incubated with morphine (0, 1 or 100 nM) for 2 h, then allowed to migrate toward ADP (10 M) for 2 h. (A) Images of microglia that had migrated through a porous membrane and were then stained with SNS-032 (BMS-387032) crystal violet. (B) Microglial migration was quantified by counting ten random fields at 40x magnification for each membrane (n = 3 for all treatments). Error bars represent the standard error of the mean. *p 0.05. Cannabinoids and neuroimmune interactions SNS-032 (BMS-387032) The cannabinoid system regulates and modulates both neuronal and immune functions using at least two protein-coupled cannabinoid receptors (CBRs), CBR1 and CBR2. CBR1s are expressed in the brain, spinal cord and peripheral nerves, and are responsible for the psychotropic effects of cannabinoids [46C50]. Neuronal CBR1s are synthesized in cells of the dorsal root ganglia and are inserted by axonal transport onto terminals in the.CBR2s are expressed peripherally in immune cells [58] and keratinocytes [59]. a delayed increase of MMP-2 in satellite cells and spinal astrocytes after spinal nerve ligation [29]. These studies underscore the value of investigating functional glial properties versus simply observing morphological reactivity. Opioids C The current gold standard Opioids are among the most potent analgesic agents available for clinical use and are the gold standard for treating acute, post-surgical and cancer pain; however, their use in neuropathic pain is often limited by the development of analgesic tolerance and unwanted side effects that are unmasked during the resulting dose escalation. Much of the early research on combating opioid tolerance focused on determining potential neuronal mechanisms of tolerance formation and designing opioid analogs with improved tolerance profiles. Beitner-Johnston demonstrated that naltrexone, a -opioid receptor antagonist, limited the development of tolerance to opioids [30]. This finding led to the synthesis of new -opioid receptor agonists, mixed -receptor agonist and -receptor antagonists, and partial -receptor agonists, which have all failed to improve upon morphine and thus have only limited value in the treatment of pain. Opioid receptor desensitization entails the NMDA receptor cascade. Preclinical [31,32] and initial medical studies [33,34] suggest that the blockade of NMDA reduces opioid-induced tolerance; however, large-scale medical tests using NMDA antagonists in conjunction with opioids to limit tolerance formation have produced disappointing results [35]. Music and Zhao have identified a casual link between glial activation and morphine tolerance [36?]. In accordance with these findings, it has been shown that spinal CR3/CD11b, GFAP (an astrocyte marker) and manifestation of the cytokines IL-1, IL-6 and TNF increase following chronic morphine administration [37?]. Also, chronic subcutaneous and intrathecal morphine administration induces analgesic tolerance within 6 and 3 days, respectively [38]. The burgeoning body of study implicating glia in opioid tolerance led to the investigation of whether selectively focusing on glial cells can provide a potential method for attenuating opioid tolerance. Both inhibiting chronic opioid-induced glial reactivity using propentofylline [39] and inhibiting proinflammatory cytokines [40] attenuate morphine-induced tolerance. Minocycline, which inhibits microglial migration [41], attenuates the development of anti-nociceptive tolerance to chronic morphine through inhibition of p38 MAPK in SNS-032 (BMS-387032) triggered microglia [42]. These studies show that microglial migration may have a critical part in morphine tolerance, as has been shown in neuropathic pain claims [43,44?]. It was discovered that morphine enhances microglial Iba-1 manifestation [RJ Horvath, unpublished data] and migration [45] toward ADP inside a -opioid receptor-dependent manner (Number 2). It was proposed that chronic opioid administration induces microglial reactivity and migration toward the SNS-032 (BMS-387032) dorsal horn, which leads to improved proinflammatory/algesic factor production and neuronal sensitization. Microglial migration might therefore prove to be a good pharmacological target to inhibit the induction of opioid tolerance. Open in a separate window Number 2 Migration of morphine-treated main neonatal rat cortical microglia toward ATPPrimary neonatal rat cortical microglia were harvested, incubated with morphine (0, 1 or 100 nM) for 2 h, then allowed to migrate toward ADP (10 M) for 2 h. (A) Images of microglia that had migrated through a porous membrane and were then stained with crystal violet. (B) Microglial migration was quantified by counting ten random fields at 40x magnification for each membrane (n = 3 for those treatments). Error bars represent the standard error of the mean. *p 0.05. Cannabinoids and neuroimmune relationships The cannabinoid system regulates and modulates both neuronal and immune functions using at least two protein-coupled cannabinoid receptors (CBRs), CBR1 and CBR2. CBR1s are indicated in the brain, spinal cord and peripheral nerves, and are responsible for the psychotropic effects of cannabinoids [46C50]. Neuronal CBR1s are synthesized in cells of the dorsal root ganglia and are put by axonal transport onto terminals in the periphery [51]. Additionally, CBR1s will also be indicated in microglial cells and may act as immune modulators in the CNS [52C54]. CBR2 activation generates a host of peripheral immune effects, including rules of cell migration, inhibition of cell proliferation, reduction of cytokine production, downregulation of surface marker manifestation and impairment of cell functions [55C57]. CBR2s are indicated peripherally in immune cells [58] and keratinocytes [59]. CBR2s also exist in the CNS, primarily in microglia and perivascular microglia cells, in healthy human being and rat brains [60]. CBR2s regulate microglial migration [61] and proliferation [62]. CBR1 manifestation is enhanced in the spinal dorsal.