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BACKGROUND Gut microbiota, a consortium of diverse microorganisms residing in the gastrointestinal tract, has emerged as a key player in neuroinflammatory responses, supporting the functional relevance of the "gut-brain axis." Chronic-constriction injury of the sciatic nerve (CCI) is a commonly used animal model of neuropathic pain with a major input from T cell-mediated immune responses. In this article, we sought to examine whether gut microbiota influences CCI neuropathic pain, and, if so, whether T-cell immune responses are implicated. METHODS We used a mixture of wide-spectrum oral antibiotics to perturbate gut microbiota in mice and then performed CCI in these animals. Nociceptive behaviors, including mechanical allodynia and thermal hyperalgesia, were examined before and after CCI. Additionally, we characterized the spinal cord infiltrating T cells by examining interferon (IFN)-γ, interleukin (IL)-17, and Foxp3. Using a Foxp3-GFP-DTR "knock-in" mouse model that allows punctual depletion of regulatory T cells, we interrogated the role of these cells in mediating the effects of gut microbiota in the context of CCI neuropathic pain. RESULTS We found that oral antibiotics induced gut microbiota changes and attenuated the development of CCI neuropathic pain, as demonstrated by dampened mechanical allodynia and thermal hyperalgesia. Percentages of IFN-γ-producing Th1 cells and Foxp3+ regulatory T cells were significantly different between animals that received oral antibiotics (Th1 mean = 1.0, 95% confidence interval [CI], 0.9-1.2; Foxp3 mean = 8.1, 95% CI, 6.8-9.3) and those that received regular water (Th1 mean = 8.4, 95% CI, 7.8-9.0, P < .01 oral antibiotics versus water, Cohen's d = 18.8; Foxp 3 mean = 2.8, 95% CI, 2.2-3.3, P < .01 oral antibiotics versus water, Cohen's d = 6.2). These T cells characterized a skewing from a proinflammatory to an anti-inflammatory immune profile induced by gut microbiota changes. Moreover, we depleted Foxp3+ regulatory T cells and found that their depletion reversed the protection of neuropathic pain mediated by gut microbiota changes, along with a dramatic increase of IFN-γ-producing Th1 cell infiltration in the spinal cord (before depletion mean = 2.8%, 95% CI, 2.2-3.5; after depletion mean = 9.1%, 95% CI, 7.2-11.0, p < .01 before versus after, Cohen's d = 5.0). CONCLUSIONS Gut microbiota plays a critical role in CCI neuropathic pain. This role is mediated, in part, through modulating proinflammatory and anti-inflammatory T cells.

BACKGROUND: The incremental dose of opioids used in chronic pain management often leads to a reduced opioid analgesic effect, opioid misuse and addiction. Central dopamine dysfunction contributes to the chronicity of pain and a decreased opioid analgesic effect. Methylphenidate (MPH/Ritalin) enhances central dopamine function by inhibiting dopamine re-uptake. In this study, we used a rat model of chronic pain to examine whether combination of MPH with morphine would improve the morphine analgesic effect under a chronic pain condition. METHODS: Tibiotarsal joint Complete Freund’s Adjuvant (CFA) injection in rats was utilized to induce chronic nociception. The analgesic effect of low-dose MPH (0.25 mg/kg), low-dose morphine (2.5 mg/kg), and their combination was examined in CFA rats. Nociceptive behavior was assessed by von Frey test. Conditioned place preference and open field tests were used to examine the rewarding behavior and locomotor activity in rats respectively. RESULTS: Our findings are as follows: (1) in CFA rats with chronic pain, 2.5 mg/kg of morphine had less analgesic effect than 10 mg/kg of morphine at 28 days after injury (95% confidence intervals for difference of means of von Frey threshold in gram: −11.9 [−6.5 to −17.3]); (2) in the one hour time window of 30 to 90 minutes after injection, the combination of MPH (0.25 mg/kg) with morphine (2.5 mg/kg) increased synergistically and prolonged the analgesic effect in CFA rats as compared with MPH or morphine alone (P=0.01 for MPH by morphine interaction, and 95% confidence intervals for difference of means of von Frey threshold in gram: 3.3 [1.37 to 6.12] for the combination versus MPH and 3.2 [1.35 to 5.74] for the combination versus morphine); (3) at the low dose (0.25 mg/kg), MPH did not increase locomotor activity (MOR+MPH versus MOR, P=0.13), nor significantly enhanced morphine reward behavior (MOR+MPH versus MOR, P=0.63) in CFA rats. CONCLUSIONS: Our data suggest that a combination therapy using low-dose MPH and morphine may produce a morphine-sparing effect in chronic pain management.

As opioid abuse and addiction have developed into a major national health crisis, prescription of opioids for pain management has become more controversial. However, opioids do help some patients by providing pain relief and improving the quality of life. To better understand the addictive properties of opioids under chronic pain conditions, we used a conditioned place preference (CPP) paradigm to examine the rewarding properties of morphine in rats with persistent nociception.Spared nerve injury (SNI) model was used to induce persistent nociception in rats. Nociceptive behavior was assessed by von Frey test. CPP test was used to examine the rewarding properties of morphine.Our findings are as follows: (1) SNI rats did not show a difference compared with sham rats in magnitude of morphine-induced CPP 1 day after last morphine injection (2-way analysis of variance; for SNI versus sham, F[1,42] = 0.014, P = .91; and 95% confidence intervals for difference of means, -5.9 [-58 to 46], 0.76 [-51 to 53], and 0.90 [-51 to 53] for 2.5, 5, and 10 mg/kg, respectively); (2) increasing morphine dosage (2.5, 5, and 10 mg/kg) did not further increase the magnitude of CPP in both sham and SNI rats (for dosage: F[2,42] = 0.94, P = .40); and (3) morphine-induced CPP persisted in sham rats but extinguished in SNI rats when tested at 8 days after last morphine injection (for sham versus SNI: Bonferroni correction, P.006 for both 5 and 10 mg/kg doses; and 95% confidence intervals for difference of means, 80.3 [19.7-141] and 87.0 [26.3-148] for 5 and 10 mg/kg, respectively).Our data provide new evidence supporting the notion that the brain's reward circuitry changes in the context of persistent pain. This observational study suggests that future investigation into the neurobiology of opioid reward requires consideration of the circumstances in which opioid analgesics are administered.

Orofacial neuropathic pain caused by trigeminal nerve injury is a debilitating condition with limited therapeutic options. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels regulate neuronal excitability and are involved in the development and maintenance of chronic pain. However, the effect of HCN channel activity in the Gasserian ganglion on trigeminal neuropathic pain has not been examined. We evaluated nociceptive behaviors after microinjection of the HCN channel blockers ZD7288 or ivabradine into the Gasserian ganglion in rats with trigeminal nerve injury. Both blockers dose-dependently ameliorated evoked and spontaneous nociceptive behavior in rats with trigeminal neuropathic pain. Moreover, the clinically available HCN channel blocker ivabradine showed a prolonged antinociceptive effect. In the Gasserian ganglion, HCN1 and HCN2 are major HCN isoforms. After trigeminal nerve injury, the counts of HCN1 as well as HCN2 immuno-positive punctae were increased in the ipsilateral Gasserian ganglions. These results indicate that the increased HCN channel activity in the Gasserian ganglion directly contributes to neuropathic pain resulting from trigeminal nerve injury. Perspective Trigeminal nerve damage-induced orofacial pain is severe and more resistant to standard pharmacological treatment than other types of neuropathic pain. Our study suggests that targeting HCN channel activities in the Gasserian ganglion may provide an alternative treatment of trigeminal neuropathy including trigeminal neuralgia.

Clinical evidence indicates that cognitive impairment is a common comorbid condition of chronic pain. However, the cellular basis for chronic pain-mediated cognitive impairment remains unclear. We report here that rats exhibited memory deficits after spared never injury (SNI). We found that levels of stable microtubule (MT) were increased in the hippocampus of the rats with memory deficits. This increase in stable MT is marked by α-tubulin hyperacetylation. Paclitaxel, a pharmacological MT stabilizer, increased the level of stable MT in the hippocampus and induced learning and memory deficits in normal rats. Furthermore, paclitaxel reduced long-term potentiation (LTP) in hippocampal slices and increased stable MT (evidenced by α-tubulin hyperacetylation) levels in hippocampal neuronal cells. Intracerebroventricular infusion of nocodazole, a MT destabilizer, ameliorated memory deficits in rats with SNI induced nociceptive behavior. Expression of HDAC6, an α-tubulin deacetylase, was reduced in the hippocampus in rats with cognitive impairment. These findings indicate that peripheral nerve injury (e.g. SNI) affects the MT dynamic equilibrium, which is critical to neuronal structure and synaptic plasticity.

Chemotherapy-induced pain is a dose-limiting condition that affects 30% of patients undergoing chemotherapy. We found that the gut microbiota promotes the development of chemotherapy-induced mechanical hyperalgesia. Oxaliplatin-induced mechnical hyperalgesia was reduced in germ-free mice and in those mice pretreated with antibiotics. Restoration of the microbiota of germ-free mice abrogated this protection. These effects appear to be mediated, in part, by TLR4 expressed on hematopoietic cells, including macrophages.

Introduction: The aim of this study was to examine the effect of electroacupuncture (EA) on trigeminal neuropathic pain in rats and explore the potential mechanism underlying the putative therapeutic effect of EA. Methods: Trigeminal neuropathic pain behavior was induced in rats by unilateral chronic constriction injury of the distal infraorbital nerve (dIoN-CCI). EA was administered at ST2 ( Sibai) and Jiachengjiang. A total of 60 Sprague Dawley rats were divided into the following four groups ( n = 15 per group) to examine the behavioral outcomes after surgery and/or EA treatment: sham (no ligation); dIoN-CCI (received isoflurane only, without EA treatment); dIoN-CCI+EA-7d (received EA treatment for 7 days); and dIoN-CCI+EA-14d (received EA treatment for 14 days). Both evoked and spontaneous nociceptive behaviors were measured. Of these, 12 rats ( n = 4 from sham, dIoN-CCI, and dIoN-CCI+EA-14d groups, respectively) were used to analyze protein expression of hyperpolarization-activated cyclic nucleotide-gated (HCN) channel in the Gasserian ganglion (GG) by immunohistochemistry. Results: dIoN-CCI rats exhibited mechanical allodynia and increased face-grooming activity that lasted at least 35 days. EA treatment reduced mechanical allodynia and face-grooming in dIoN-CCI rats. Overall, 14 days of EA treatment produced a prolonged anti-nociceptive effect as compared to 7-day EA treatment. The counts of HCN1 and HCN2 immunopositive puncta were increased in the ipsilateral GG in dIoN-CCI rats and were reduced by 14 days of EA treatment. Discussion: EA treatment relieved trigeminal neuropathic pain in dIoN-CCI rats, and this effect was dependent on the duration of EA treatment. The downregulation of HCN expression may contribute to the anti-nociceptive effect of EA in this rat model of trigeminal neuropathic pain.

BACKGROUND: Tissue injuries such as surgery and trauma are usually accompanied by simultaneous development of acute pain, which typically resolves along with tissue healing. However, in many cases, acute pain does not resolve despite proper tissue repair, rather, it transitions to chronic pain. In this study, we examined whether proliferator-activated receptor-gamma coactivator-1α (PGC-1α), a master regulator of mitochondria biogenesis, is implicated in pain chronification after burn injury in mice. METHODS: We used PGC-1α(+/+) and littermates PGC-1α(+/−) mice of both sex. Burn injury was induced on these mice. Hindpaw mechanical withdrawal thresholds and thermal withdrawal latency were examined. RESULTS: Hindpaw mechanical withdrawal thresholds and thermal withdrawal latencies were comparable at baseline between PGC-1α(+/−) and PGC-1α(+/+) mice. After burn injury, both PGC-1α(+/+) and PGC-1α(+/−) mice exhibited an initial dramatic decrease of withdrawal parameters at days 3 and 5 post-injury. While PGC-1α(+/+) mice fully recovered their withdrawal parameters to pre-injury levels by day 11 to 14, PGC-1α(+/−) mice failed to recover those parameters during the same time frame, regardless of sex. Moreover, we found that PGC-1α(+/−) mice resolved tissue inflammation in a similar fashion to PGC-1α(+/+) mice using a chemiluminescence-based reactive oxygen species imaging technique. CONCLUSIONS: Taken together, our data suggest that PGC-1α haploinsufficiency promotes pain chronification after burn injury.

The number of studies on trigeminal nerve injury using animal models remains limited. A rodent model of trigeminal neuropathic pain was first developed in 1994, in which chronic constriction injury (CCI) is induced by ligation of the infraorbital nerve (IoN). This animal model has served as a major tool to study trigeminal neuropathic pain. Unfortunately, the surgical procedure in this model is complicated and far more difficult than ligation of peripheral nerves (eg, sciatic nerve). The aim of this study was to improve on the current surgical procedure of IoN ligation to induce trigeminal neuropathic pain in rats. We show that the IoN can be readily accessed through a small facial incision. CCI can be induced by ligation of a segment at the distal IoN (dIoN). This dIoN-CCI procedure is simple, minimally invasive, and time-saving. Our data show that the dIoN-CCI procedure consistently induced acute as well as chronic nociceptive behaviors in rats. Daily gabapentin treatment attenuated mechanical allodynia and reduced face-grooming episodes in dIoN-CCI rats. Perspective The orofacial pain caused by trigeminal nerve damage is severe and perhaps more debilitating than other types of neuropathic pain. However, studies on trigeminal neuropathic pain remain limited. This is largely because of the lack of proper animal models because of the complexity of the existing surgical procedures required to induce trigeminal nerve injury. Our improved dIoN-CCI model is likely to make it more accessible to study the cellular and molecular mechanisms of neuropathic pain caused by trigeminal nerve damage.