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Skeletal metastases are frequently accompanied by chronic pain that is mechanoceptive in nature. Mechanistically, cancer-induced bone pain (CIBP) is mediated by peripheral sensory neurons innervating the cancerous site, the cell bodies of which are housed in the dorsal root ganglia (DRG). How these somatosensory neurons encode sensory information in CIBP remains only partly explained. Using a validated rat model, we first confirmed cortical bone destruction in CIBP but not sham-operated rats (day 14 after surgery, designated "late"-stage bone cancer). This occurred with behavioural mechanical hypersensitivity (Kruskal-Wallis H for independent samples; CIBP vs sham-operated, day 14; P < 0.0001). Next, hypothesising that the proportion and phenotype of primary afferents would be altered in the disease state, dorsal root ganglia in vivo imaging of genetically encoded calcium indicators and Markov Cluster Analysis were used to analyse 1748 late-stage CIBP (n = 10) and 757 sham-operated (n = 9), neurons. Distinct clusters of responses to peripheral stimuli were revealed. In CIBP rats, upon knee compression of the leg ipsilateral to the tumour, (1) 3 times as many sensory afferents responded (repeated-measures analysis of variance: P < 0.0001 [vs sham]); (2) there were significantly more small neurons responding (Kruskal-Wallis for independent samples (vs sham): P < 0.0001); and (3) approximately 13% of traced tibial cavity afferents responded (no difference observed between CIBP and sham-operated animals). We conclude that an increased sensory afferent response is present in CIBP rats, and this is likely to reflect afferent recruitment from outside of the bone rather than increased intraosseous afferent activity.
Learn More >We investigated the contribution of nucleus locus ceruleus (LC) to the development of pain-associated affective behavior. Mice of both sexes were subjected to sciatic nerve cuffing, a model of peripheral nerve injury, and monitored for 45 days. Although the thermal and mechanical thresholds were equally decreased in both males and females, only the male mice developed anxiodepressive-like behavior, which was complemented by suppressed hippocampal neurogenesis. Furthermore, the LC activity was lower in males when compared with females subjected to sciatic cuffing. Next, we used a chemogenetic approach to modulate the activity of LC projections to the dentate gyrus of the hippocampus in females without cuffs and in males with sciatic cuffs. Sustained inhibition of the LC projections to the dentate gyrus for 15 days induced anxiodepressive-like behavior and reduced the hippocampal neurogenesis in females. Activation of the LC projections to the dentate gyrus for 15 days prevented the development of anxiodepressive-like behavior and increased the hippocampal neurogenesis in males with cuffs. In sum, we demonstrated that the LC projections to the hippocampus link the sensory to the affective component of neuropathic injury and that the female mice are able to dissociate the nociception from affect by maintaining robust LC activity. The work provides evidence that sex differences in LC response to pain determine the sex differences in the development of pain phenotype.
Learn More >Cognitive impairment is one of the most common complications associated with chronic pain. Almost 20% of chronic pain patients suffer from cognitive impairment, which may substantially influence their quality of life. Levels of major excitatory neurotransmitters in the central nervous system and alterations in the glutamatergic system may influence cognitive function and the pain sensory pathway. In this study, we adopted the spared nerve injury model to establish the progress of chronic pain and investigated the mechanism underlying the cognitive aspect related to it. At behavioral level, using the novel-object recognition test, mechanical hypersensitivity was observed in peripheral nerve-injured rats because they exhibited recognition deficits. We showed a dramatic decrease in hippocampal glutamate concentration using nuclear magnetic resonance and reduced glutamatergic synaptic transmission using whole-cell recordings. These were associated with deficient hippocampal long-term potentiation induced by high-frequency stimulation of the Schaffer collateral afferent. Ultra-high-performance liquid chromatography revealed lower levels of D-serine in the hippocampus of the spared nerve injury rats and that D-serine treatment could restore synaptic plasticity and cognitive dysfunction. The reduction of excitatory synapses was also increased by administering D-serine. These findings suggest that chronic pain has a critical effect on synaptic plasticity linked to cognitive function and may built up a new target for the development of cognitive impairment under chronic pain conditions.
Learn More >Physical disuse could lead to a state of chronic pain typified by complex regional pain syndrome type I due to fear of pain through movement (kinesiophobia) or inappropriate resting procedures. However, the mechanisms by which physical disuse is associated with acute/chronic pain and other pathological signs remain unresolved. We have previously reported that inflammatory signs, contractures, disuse muscle atrophy, spontaneous pain-like behaviors, and chronic widespread mechanical hyperalgesia based on central plasticity occurred after 2 weeks of cast immobilization in chronic post-cast pain (CPCP) rat model. In this study, we also demonstrated dystrophy-like changes, both peripheral nociceptive signals and activation of the central pain pathway in CPCP rats. This was done by the following methods: (1) vascular permeability (Evans blue dye) and inflammatory- and oxidative stress-related messenger RNA changes (real-time quantitative polymerase chain reaction); (2) immunofluorescence of pERK and/or c-Fos expression in the spino-parabrachio-amygdaloid pathway; and (3) blockade of nociceptive-related signals using sciatic nerve block. Furthermore, we demonstrated tactile allodynia using an optogenetic method in a transgenic rat line (W-TChR2V4), cold allodynia using the acetone test, and activation of dorsal horn neurons in the chronic phase associated with chronic mechanical hyperalgesia using c-Fos immunofluorescence. In addition, we showed that nociceptive signals in the acute phase are involved in chronic pathological pain-like behaviors by studying the effects of sciatic nerve block. Thus, we conclude that physical disuse contributes to dystrophy-like changes, spontaneous pain-like behavior, and chronic widespread pathological pain-like behaviors in CPCP rats after 2 weeks of cast immobilization.
Learn More >Heat shock protein 90 (Hsp90) is a ubiquitous signal transduction regulator, and Hsp90 inhibitors are in clinical development as cancer therapeutics. However, there have been very few studies on the impact of Hsp90 inhibitors on pain or analgesia, a serious concern for cancer patients. We previously found that Hsp90 inhibitors injected into the brain block opioid-induced antinociception in tail flick, paw incision, and HIV neuropathy pain. This study extended from that initial work to test the cancer-related clinical impact of Hsp90 inhibitors on opioid antinociception in cancer-induced bone pain in female BALB/c mice and chemotherapy-induced peripheral neuropathy in male and female CD-1 mice. Mice were treated with Hsp90 inhibitors (17-AAG, KU-32) by the intracerebroventricular, intrathecal, or intraperitoneal routes, and after 24 hours, pain behaviors were evaluated after analgesic drug treatment. Heat shock protein 90 inhibition in the brain or systemically completely blocked morphine and oxymorphone antinociception in chemotherapy-induced peripheral neuropathy; this effect was partly mediated by decreased ERK and JNK MAPK activation and by increased protein translation, was not altered by chronic treatment, and Hsp90 inhibition had no effect on gabapentin antinociception. We also found that the Hsp90 isoform Hsp90α and the cochaperone Cdc37 were responsible for the observed changes in opioid antinociception. By contrast, Hsp90 inhibition in the spinal cord or systemically partially reduced opioid antinociception in cancer-induced bone pain. These results demonstrate that Hsp90 inhibitors block opioid antinociception in cancer-related pain, suggesting that Hsp90 inhibitors for cancer therapy could decrease opioid treatment efficacy.
Learn More >Transcribed ultraconserved regions are a novel class of long noncoding RNAs and are completely conserved in humans, rats, and mice. Transcribed ultraconserved regions have been implicated in diverse biological processes; however, very little is currently known about their role in pain modulation. Here, we found that the level of the spinal transcribed ultraconserved region uc.153 was significantly increased in a mouse model of sciatic nerve chronic constriction injury (CCI)-induced chronic neuropathic pain. The knockdown of spinal uc.153 prevented and reversed chronic constriction injury-induced pain behaviours and spinal neuronal sensitization. By contrast, the overexpression of spinal uc.153 produced pain behaviours and neuronal sensitization in naive mice. Moreover, we found that uc.153 participates in the regulation of neuropathic pain by negatively modulating the processing of pre-miR-182-5p. Collectively, our findings reveal an important role for uc.153 in pain modulation and provide a novel drug target for neuropathic pain therapy.
Learn More >The prefrontal ventrolateral orbital cortex (VLO) is involved in antinociception. It has been found that dopamine receptors, adrenoceptors, serotonin receptors and μ-opioid receptors are involved in this effect through direct/indirect activation of the VLO output neurons. However, the effect of CB1 receptors on the VLO modulation of pain has not been studied. In this study, we investigated whether activation of CB1 receptors in the VLO modulates nociception. A common peroneal nerve (CPN) ligation model was used to induce neuropathic pain in male mice. On day 13 after CPN ligation, spontaneous firing of the VLO pyramidal neurons was recorded and CB1 receptor level in the VLO was detected. Mechanical allodynia was measured after HU210 was microinjected into the VLO. Relative contribution of CB1 receptors on GABAergic neurons and glutamatergic neurons was determined by CB1 receptor knockdown using a viral strategy. Our data indicated that on day 13 after nerve injury, spontaneous firing of the VLO pyramidal neurons reduced significantly but was enhanced by intraperitoneal injection of HU210 (20 μg/kg), a potent CB1 receptor agonist. Expression of CB1 receptor in the VLO was up-regulated. Microinjection of HU210 into the VLO attenuated allodynia, and this effect was blocked by pre-microinjection of specific CB1 receptor antagonist AM281. Deletion of CB1 receptors on GABAergic neurons in the VLO can completely block HU210-induced analgesia. Thus, it can be concluded that activation of CB1 receptors on GABAergic interneurons in the VLO may be involved in analgesia effect of cannabinoids.
Learn More >Pain produced by bone cancer is often severe and difficult to treat. Here we examined effects of Resolvin D1 (RvD1) or E1 (RvE1), antinociceptive products of ω-3 polyunsaturated fatty acids, on cancer-induced mechanical allodynia and heat hyperalgesia. Experiments were performed using a mouse model of bone cancer produced by implantation of osteolytic ficrosarcoma into and around the calcaneus bone. Mechanical allodynia and heat hyperalgesia in the tumor-bearing paw were assessed by measuring withdrawal responses to a von Frey monofilament and to radiant heat applied on the plantar hind paw. RvD1, RvE1, and cannabinoid receptor antagonists were injected intrathecally. Spinal content of endocannabinoids was evaluated using UPLC-MS/MS analysis. RvD1 and RvE1 had similar antinociceptive potencies. ED for RvD1 and RvE1 in reducing mechanical allodynia were 0.2 pg (0.53 fmol) and 0.6 pg (1.71 fmol), respectively, and were 0.3 pg (0.8 fmol) and 0.2 pg (0.57 fmol) for reducing heat hyperalgesia. Comparisons of dose-response relationships showed equal efficacy for reducing mechanical allodynia, however, efficacy for reducing heat hyperalgesia was greater for of RvD1. Using UPLC-MS/MS we determined that RvD1, but not RvE1, increased levels of the endocannabinoids Anandamide and 2-Arachidonoylglycerol in the spinal cord. Importantly, Resolvins did not alter acute nociception or motor function in naïve mice. Our data indicate, that RvD1 and RvE1 produce potent antiallodynia and antihyperalgesia in a model of bone cancer pain. RvD1 also triggers spinal upregulation of endocannabinoids that produce additional antinociception predominantly through CB2 receptors.
Learn More >This study aims to explore whether orexin 1 receptors (Orx1R) in the ventrolateral periaqueductal gray matter (vlPAG) play a role in the modulation of migraine headaches in adult male Wistar rats. To model chronic migraine-associated pain, nitroglycerin (NTG) (5 mg/kg/IP) was administered to test subjects every second day for 9 days. After the last NTG injection, rats were randomly separated into the following groups (n = 6): orexin-A (OrxA) groups that received intra-vlPAG OrxA (25, 50, and 100 pM), an Orx1R antagonist group, a SB-334867 (20 μM) group; and a SB-334867 (20 μM) + OrxA (100 pM) group. After 10 min, migraine-associated behavioral symptoms were recorded in all animals for up to 90 min. Light-dark chamber and hot plate tests were used for assessing light aversion and thermal hyperalgesia, respectively. Calcitonin gene-related peptide (CGRP)-positive cells were detected in the trigeminal nucleus caudalis (Vc) by immunofluorescence microscopy. NTG caused significant freezing behavior, which was prevented by all OrxA doses. Moreover, OrxA (100 pM) could obstruct NTG-induced increases in facial rubbing and decreases in climbing and body grooming. Furthermore, NTG-induced light aversion and thermal hyperalgesia were attenuated by OrxA at doses of 50 and 100 pM. The effects of OrxA were significantly blocked by SB-334867 (20 μM). Besides, OrxA (100 pM) decreased NTG-induced CGRP upregulation. The data revealed that the activation of Orx1Rs in the vlPAG is effective in relieving NTG-induced migraine symptoms mainly by the downregulation of CGRP in the Vc of rats.
Learn More >Chronic pain is one of the most challenging and debilitating symptoms to manage after traumatic brain injury (TBI), yet the underlying mechanisms remain elusive. The disruption of normal endogenous pain control mechanisms has been linked to several forms of chronic pain and may play a role in pain after TBI. We hypothesized therefore that dysfunctional descending noradrenergic and serotonergic pain control circuits may contribute to the loss of diffuse noxious inhibitory control (DNIC), a critical endogenous pain control mechanism, weeks to months after TBI. For these studies, the rat lateral fluid percussion model of mild TBI was used along with a DNIC paradigm involving a capsaicin-conditioning stimulus. We observed sustained failure of the DNIC response up to 180-days post injury. We confirmed, that descending α adrenoceptor-mediated noradrenergic signaling was critical for endogenous pain inhibition in uninjured rats. However, augmenting descending noradrenergic signaling using reboxetine, a selective noradrenaline reuptake inhibitor, failed to restore DNIC after TBI. Furthermore, blocking serotonin-mediated descending signaling using selective spinal serotonergic fiber depletion with 5, 7-dihydroxytryptamine was also unsuccessful at restoring endogenous pain modulation after TBI. Unexpectedly, increasing descending serotonergic signaling using the selective serotonin reuptake inhibitor escitalopram and the serotonin-norepinephrine reuptake inhibitor duloxetine restored the DNIC response in TBI rats at both 49- and 180- days post injury. Consistent with these observations, spinal serotonergic fiber depletion with 5, 7-dihydroxytryptamine eliminated the effects of escitalopram. Intact α adrenoceptor signaling, however, was not required for the serotonin-mediated restoration of DNIC after TBI. These results suggest that TBI causes maladaptation of descending nociceptive signaling mechanisms and changes in the function of both adrenergic and serotonergic circuits. Such changes could predispose those with TBI to chronic pain.
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