Grewo Lim

Brain indoleamine 2,3-dioxygenase contributes to the comorbidity of pain and depression

Pain and depression are frequently comorbid disorders, but the mechanism underlying this association is unknown. Here, we report that brain indoleamine 2,3-dioxygenase 1 (IDO1), a rate-limiting enzyme in tryptophan metabolism, plays a key role in this comorbidity. We found that chronic pain in rats induced depressive behavior and IDO1 upregulation in the bilateral hippocampus. Upregulation of IDO1 resulted in the increased kynurenine/tryptophan ratio and decreased serotonin/tryptophan ratio in the bilateral hippocampus. We observed elevated plasma IDO activity in patients with both pain and depression, as well as in rats with anhedonia induced by chronic social stress. Intra-hippocampal administration of IL-6 in rats, in addition to in vitro experiments, demonstrated that IL-6 induces IDO1 expression through the JAK/STAT pathway. Further, either Ido1 gene knockout or pharmacological inhibition of hippocampal IDO1 activity attenuated both nociceptive and depressive behavior. These results reveal an IDO1-mediated regulatory mechanism underlying the comorbidity of pain and depression and suggest a new strategy for the concurrent treatment of both conditions via modulation of brain IDO1 activity.

Upregulation of spinal cannabinoid-1-receptors following nerve injury enhances the effects of Win 55,212-2 on neuropathic pain behaviors in rats

Exogenous cannabinoids are effective in attenuating neuropathic pain behaviors induced by peripheral nerve injury, but the mechanisms of their effectiveness remain unclear. Here we examined the expression of spinal cannabinoid‐1‐receptors (CB1Rs) following chronic constriction sciatic nerve injury (CCI) and its relation to the effects of a CBR agonist (Win 55,212‐2) on neuropathic pain in rats. CCI induced a time‐dependent upregulation of spinal CB1Rs primarily within the ipsilateral superficial spinal cord dorsal horn as revealed by both Western blot and immunohistochemistry. This CCI‐induced CB1R upregulation was at least in part mediated through tyrosine kinase receptors (Trk), because intrathecal treatment with the Trk inhibitor K252a (1 &mgr;g) for postoperative days 1–6 significantly reduced the CB1R upregulation in CCI rats. At the intracellular level, the mitogen‐activated protein kinase (ERK‐MAPK) inhibitor PD98059 (1 &mgr;g) prevented, while the protein kinase C inhibitor chelerythrine (10 &mgr;g) partially reduced, the CCI‐induced CB1R upregulation when each agent was administered intrathecally for postoperative days 1–6. Importantly, the CCI‐induced upregulation of spinal CB1Rs enhanced the effects of Win 55,212‐2 on both thermal hyperalgesia and mechanical allodynia, since inhibition of the CB1R upregulation by PD98059 resulted in a significant reduction of the effects of Win 55,212‐2 in CCI rats. These results indicate that upregulation of spinal CB1Rs following peripheral nerve injury may contribute to the therapeutic effects of exogenous cannabinoids on neuropathic pain.

Central glucocorticoid receptors modulate the expression and function of spinal NMDA receptors after peripheral nerve injury.

Central glucocorticoid receptors (GRs) and NMDA receptors (NMDARs) have been shown to play a significant role in the mechanisms of neuropathic pain after peripheral nerve injury; however, how central GRs and NMDARs interact in this process remains unknown. Here we show that the expression and function of spinal NMDARs after peripheral nerve injury were modulated by central GRs. Chronic constriction nerve injury (CCI) in rats induced a time-dependent upregulation of NR1 and NR2 subunits of the NMDAR within the spinal cord dorsal horn ipsilateral to CCI. The upregulation of NMDARs was significantly diminished by intrathecal administration (twice daily for postoperative days 1-6) of either the GR antagonist RU38486 or an antisense oligonucleotide against GRs. Moreover, this CCI-induced expression of NMDARs was significantly attenuated in rats receiving intrathecal treatment with an interleukin-6 (IL-6) antiserum and in mice with protein kinase Cγ (PKCγ) knock-out. Because IL-6 and PKCγ mediated the upregulation of central GRs after CCI as demonstrated previously, the results suggest that IL-6 and PKCγ served as cellular mediators contributing to the GR-mediated expression of NMDARs after CCI. Functionally, nociceptive behaviors induced by NMDAR activation and CCI were reversed by a single intrathecal administration of the GR antagonist RU38486. Conversely, a single intrathecal injection with the noncompetitive NMDAR antagonist MK-801 reversed neuropathic pain behaviors exacerbated by the GR agonist dexamethasone in CCI rats. These data suggest that interactions between central GRs and NMDARs through genomic and nongenomic regulation may be an important mechanism critical to neuropathic pain behaviors in rats.

Expression of Central Glucocorticoid Receptors after Peripheral Nerve Injury Contributes to Neuropathic Pain Behaviors in Rats

Peripheral glucocorticoid receptors (GRs) play a significant role in the anti-inflammatory effects of glucocorticoids; however, the role of central GRs in nociceptive behaviors after peripheral nerve injury (neuropathic pain behaviors) remains unknown. Here we show that the development of neuropathic pain behaviors (thermal hyperalgesia and mechanical allodynia) induced by chronic constriction nerve injury (CCI) in rats was attenuated by either the GR antagonist RU38486 (4 = 2 > 1 = 0.5 μg) or a GR antisense oligonucleotide administered intrathecally twice daily for postoperative days 1-6. The development of thermal hyperalgesia and mechanical allodynia after CCI also was prevented in adrenalectomized rats, whereas the GR agonist dexamethasone (100 μg/kg) given subcutaneously twice daily for postoperative day 1-6 restored CCI-induced neuropathic pain behaviors in the adrenalectomized rats. Mechanistically, CCI induced a time-dependent and region-specific expression of neuronal GRs primarily within the spinal cord dorsal horn ipsilateral to nerve injury, which showed a time course parallel to that of the development of neuropathic pain behaviors. Moreover, the expression of neuronal GR after CCI was mediated in part through an elevated spinal level of interleukin-6 (IL-6) and protein kinase Cγ (PKCγ), because intrathecal treatment with an IL-6 antiserum, a PKC inhibitor (cheryrithrine), or PKCγ knock-out substantially reduced the expression of neuronal GRs as well as neuropathic pain behaviors after CCI. These findings indicate a central role of neuronal GRs in the mechanisms of neuropathic pain behaviors in rats and suggest a potential role for GR antagonists in clinical management of neuropathic pain.

Expression of Spinal NMDA Receptor and PKCγ after Chronic Morphine Is Regulated by Spinal Glucocorticoid Receptor

Spinal NMDA receptor (NMDAR), protein kinase C (PKC), and glucocorticoid receptor (GR) have all been implicated in the mechanisms of morphine tolerance; however, how these cellular elements interact after chronic morphine exposure remains unclear. Here we show that the expression of spinal NMDAR and PKCγ after chronic morphine is regulated by spinal GR through a cAMP response element-binding protein (CREB)-dependent pathway. Chronic morphine (10 μg, i.t.; twice daily for 6 d) induced a time-dependent upregulation of GR, the NR1 subunit of NMDAR, and PKCγ within the rats spinal cord dorsal horn. This NR1 and PKCγ upregulation was significantly diminished by intrathecal coadministration of morphine with the GR antagonist RU38486 or a GR antisense oligodeoxynucleotide. Intrathecal coadministration of morphine with an adenylyl cyclase inhibitor (2′,5′-dideoxyadenosine) or a protein kinase A inhibitor (H89) also significantly attenuated morphine-induced NR1 and PKCγ expression, whereas intrathecal treatment with an adenylyl cyclase activator (forskolin) alone mimicked morphine-induced expression of GR, NR1, and PKCγ. Moreover, the expression of phosphorylated CREB was upregulated within the spinal cord dorsal horn after chronic morphine, and a CREB antisense oligodeoxynucleotide coadministered intrathecally with morphine prevented the upregulation of GR, NR1, and PKCγ. These results indicate that spinal GR through the cAMP-CREB pathway played a significant role in NMDAR and PKCγ expression after chronic morphine exposure. The data suggest that genomic interaction among spinal GR, NMDAR, and PKCγ may be an important mechanism that contributes to the development of morphine tolerance.

Exacerbated mechanical allodynia in rats with depression-like behavior.

Although a clinical connection between pain and depression has long been recognized, how these two conditions interact remains unclear. Here we report that both mechanical allodynia and depression-like behavior were significantly exacerbated after peripheral nerve injury in Wistar-Kyoto (WKY) rats, a genetic variation of Wistar rats with demonstrable depression-like behavior. Administration of melatonin into the anterior cingular cortex contralateral to peripheral nerve injury prevented the exacerbation of mechanical allodynia with a concurrent improvement of depression-like behavior in WKY rats. Moreover, there was a lower plasma melatonin concentration and a lower melatonin receptor expression in the anterior cingular cortex in WKY rats than in Wistar rats. These results suggest that there exists a reciprocal relationship between mechanical allodynia and depression-like behavior and the melatoninergic system in the anterior cingular cortex might play an important role in the interaction between pain and depression.

Spinal leptin contributes to the pathogenesis of neuropathic pain in rodents

Pain after nerve injury, a phenomenon referred to as neuropathic pain, is a debilitating clinical condition, but the underlying mechanisms remain unclear. As leptin, an adipocytokine produced mainly by nonneuronal tissue, has been implicated in the regulation of neuronal functions, we examined the role of leptin in neuropathic pain using a rat model of the condition chronic constriction sciatic nerve injury (CCI). We report that leptin critically contributed to pain behaviors following CCI. Specifically, spinal administration of a leptin antagonist prevented and reversed neuropathic pain behaviors in rats. Further examination revealed that levels of both leptin and the long form of the leptin receptor (Ob-Rb) were substantially increased within the ipsilateral spinal cord dorsal horn after peripheral nerve injury. Mechanistic studies showed that leptin upregulated the expression of both the spinal NMDA receptor and IL-1beta through the JAK/STAT pathway. Furthermore, these CCI-induced behavioral and cellular responses were diminished in leptin-deficient mice and mimicked by spinal administration of exogenous leptin in naive rats. Our findings reveal a critical role for spinal leptin in the pathogenesis of neuropathic pain and suggest what we believe to be a novel form of nonneuronal and neuronal interactions in the mechanisms of pathological pain.

Evidence for a long-term influence on morphie tolerance after previous morphine exposure: role of neuronal glucocorticoid receptors

&NA; Opioid analgesic tolerance is a pharmacological phenomenon that overtime diminishes the opioid analgesic effect. However, it remains unknown as to whether a previous opioid exposure would have a long‐term influence on opioid tolerance upon subsequent opioid administration. Here, we show that the onset and degree of antinociceptive tolerance to a subsequent cycle of morphine exposure were substantially exacerbated in rats made tolerant to and then recovered from previous morphine administration, indicating a long‐term influence from a previous morphine exposure on the development of morphine tolerance. Mechanistically, morphine exposure induced a cyclic AMP and protein kinase A‐dependent upregulation of neuronal glucocorticoid receptors (GR) within the spinal cord dorsal horn, which was maintained after discontinuation of morphine administration and significantly enhanced upon a second cycle of morphine exposure. Prevention of the GR upregulation with GR antisense oligonucleotides as well as inhibition of GR activation with the GR antagonist RU38486 effectively prevented the exacerbated morphine tolerance after subsequent cycles of morphine exposure. The results indicate that a previous morphine exposure could induce lasting cellular changes mediated through neuronal GR and influence morphine analgesia upon a subsequent exposure. These findings may have significant implications in clinical opioid therapy and substance abuse.

A rat model of unilateral hindpaw burn injury: slowly developing rightwards shift of the morphine dose-response curve.

&NA; Management of pain after burn injury is an unresolved clinical issue. In a rat model of hindpaw burn injury, we examined the effects of systemic morphine on nociceptive behaviors following injury. Injury was induced by immersing the dorsal part of one hindpaw into a hot water bath (85 °C) for 4, 7, or 12 s under pentobarbital anesthesia. Mechanical allodynia to von Frey filament stimulation and thermal hyperalgesia to radiant heat were assessed. Burn injury induced by the 12‐s (but not 4‐, or 7‐s) hot water immersion resulted in reliable and lasting mechanical allodynia and thermal hyperalgesia evident by day 1. In addition, there was an upregulation of protein kinase Cγ and a progressive downregulation of μ‐opioid receptors within the spinal cord dorsal horn ipsilateral to injury as revealed by immunohistochemistry and Western blot. In both injured and sham rats, the anti‐nociceptive effects of subcutaneous morphine were examined on post‐injury days 7 and 14. While the morphine AD50 dose was comparable on day 7 between burn (1.61 mg/kg) and control (1.7 mg/kg) rats, the morphine dose–response curve was shifted to the right in burn‐injured rats (4.6 mg/kg) on post‐injury day 14 as compared with both the injured rats on post‐injury day 7 and sham rats on day 14 (1.72 mg/kg). These data indicate that hindpaw burn injury reliably produces persistent mechanical allodynia and thermal hyperalgesia and that the reduced efficacy of morphine anti‐nociception in chronic burn injury may be in part due to a downregulation of spinal μ‐opioid receptors.

Leptin enhances NMDA-induced spinal excitation in rats: A functional link between adipocytokine and neuropathic pain

&NA; Recent studies have shown that leptin (an adipocytokine) played an important role in nociceptive behavior induced by nerve injury, but the cellular mechanism of this action remains unclear. Using the whole‐cell patch‐clamp recording from rat’s spinal cord slices, we showed that superfusion of leptin onto spinal cord slices dose‐dependently enhanced N‐methyl‐D‐aspartate (NMDA) receptor‐mediated currents in spinal cord lamina II neurons. At the cellular level, the effect of leptin on spinal NMDA‐induced currents was mediated through the leptin receptor and the JAK2/STAT3 (but not PI3K or MAPK) pathway, as the leptin effect was abolished in leptin receptor‐deficient (db/db) mice and inhibited by a JAK/STAT inhibitor. Moreover, we demonstrated in naïve rats that a single intrathecal administration of leptin enhanced spontaneous biting, scratching, and licking behavior induced by intrathecal NMDA and that repeated intrathecal administration of leptin elicited thermal hyperalgesia and mechanical allodynia, which was attenuated by the noncompetitive NMDA receptor antagonist MK‐801. Intrathecal leptin also upregulated the expression of NMDA receptors and pSTAT3 within the rat’s spinal cord dorsal horn, and intrathecal MK‐801 attenuated this leptin effect as well. Our data demonstrate a relationship between leptin and NMDA receptor‐mediated spinal neuronal excitation and its functional role in nociceptive behavior. Since leptin contributes to nociceptive behavior induced by nerve injury, the present findings suggest an important cellular link between the leptin’s spinal effect and the NMDA receptor‐mediated cellular mechanism of neuropathic pain. A functional link is demonstrated between leptin, an adipocytokine, and the cellular mechanisms of neuropathic pain via enhancement of function and expression of spinal N‐methyl‐D‐aspartate receptors.