Cold Hypersensitivity In Rats Research Articles

5-HT3a receptor contributes to neuropathic pain by regulating central sensitization in a rat with brachial plexus avulsion

PurposeAs a frequently occurring complication resulting from brachial plexus avulsion (BPA), neuropathic pain significantly impacts the quality of life of patients and places a substantial burden on their families. Recent reports have suggested that the 5-HT3a receptor may play a role in the development and regulation of neuropathic pain. The current study aimed to explore the involvement of the 5-HT3a receptor in neuropathic pain resulting from BPA in rats. MethodsA rat model of neuropathic pain was induced through brachial plexus avulsion (BPA). The pain thresholds of the rats were measured after BPA. The spinal dorsal horn (SDH) of rats was collected at day 14 after surgery, and the expression and distribution of the 5-HT3a receptor were analyzed using immunohistochemistry and western blotting. The expression levels of various factors related to central sensitization were measured by western blot, including c-Fos, GFAP, IBA-1, IL-1β and TNF-α. The effects of 5-HT3a receptor antagonists on hyperalgesia were assessed through behavioral tests after intrathecal administration of ondansetron. Additionally, at 120 min postinjection, the SDH of rats was acquired, and the change of expression levels of protiens related to central sensitization were measured by western blot. ResultsBPA induced mechanical and cold hypersensitivity in rats. The 5-HT3a receptor was increased and mainly distributed on neurons and microglia in the SDH after BPA, and the level of central sensitization and expression of inflammatory factors, such as c-Fos, GFAP, IBA-1, IL-1β and TNF-α, were also increased markedly. Ondansetron, which is a selective 5-HT3a receptor antagonist, reversed the behavioral changes caused by BPA. The antagonist also decreased the expression of central sensitization markers and inflammatory factors. ConclusionThe results suggested that the 5-HT3a receptor is involved in neuropathic pain by regulating central nervous system sensitization in a rat brachial plexus avulsion model. Targeting the 5-HT3a receptor may be a promising approach for treating neuropathic pain after brachial plexus avulsion.

Kv4.3 Channel Dysfunction Contributes to Trigeminal Neuropathic Pain Manifested with Orofacial Cold Hypersensitivity in Rats

Trigeminal neuropathic pain is the most debilitating pain disorder but current treatments including opiates are not effective. A common symptom of trigeminal neuropathic pain is cold allodynia/hyperalgesia or cold hypersensitivity in orofacial area, a region where exposure to cooling temperatures are inevitable in daily life. Mechanisms underlying trigeminal neuropathic pain manifested with cold hypersensitivity are not fully understood. In this study, we investigated trigeminal neuropathic pain in male rats following infraorbital nerve chronic constrictive injury (ION-CCI). Assessed by the orofacial operant behavioral test, ION-CCI animals displayed orofacial cold hypersensitivity. The cold hypersensitivity was associated with the hyperexcitability of small-sized trigeminal ganglion (TG) neurons that innervated orofacial regions. Furthermore, ION-CCI resulted in a reduction of A-type voltage-gated K+currents (IA currents) in these TG neurons. We further showed that these small-sized TG neurons expressed Kv4.3 voltage-gated K+channels, and Kv4.3 expression in these cells was significantly downregulated following ION-CCI. Pharmacological inhibition of Kv4.3 channels with phrixotoxin-2 inhibited IA-currents in these TG neurons and induced orofacial cold hypersensitivity. On the other hand, pharmacological potentiation of Kv4.3 channels amplified IA currents in these TG neurons and alleviated orofacial cold hypersensitivity in ION-CCI rats. Collectively, Kv4.3 downregulation in nociceptive trigeminal afferent fibers may contribute to peripheral cold hypersensitivity following trigeminal nerve injury, and Kv4.3 activators may be clinically useful to alleviate trigeminal neuropathic pain.SIGNIFICANCE STATEMENTTrigeminal neuropathic pain, the most debilitating pain disorder, is often triggered and exacerbated by cooling temperatures. Here, we created infraorbital nerve chronic constrictive injury (ION-CCI) in rats, an animal model of trigeminal neuropathic pain to show that dysfunction of Kv4.3 voltage-gated K+channels in nociceptive-like trigeminal ganglion (TG) neurons underlies the trigeminal neuropathic pain manifested with cold hypersensitivity in orofacial regions. Furthermore, we demonstrate that pharmacological potentiation of Kv4.3 channels can alleviate orofacial cold hypersensitivity in ION-CCI rats. Our results may have clinical implications in trigeminal neuropathic pain in human patients, and Kv4.3 channels may be an effective therapeutic target for this devastating pain disorder.

Oxaliplatin-induced changes in expression of transient receptor potential channels in the dorsal root ganglion as a neuropathic mechanism for cold hypersensitivity

Transient receptor potential (TRP) receptors are involved in the development of chemotherapy-induced peripheral neuropathic pain, which is a common side effect of selected chemotherapeutic agents such as oxaliplatin. However, the precise contribution of TRPs to this condition remains unknown. Cold hypersensitivity is the hallmark of oxaliplatin-induced neuropathy, so we used a preclinical model of oxaliplatin-induced cold hypersensitivity in rats to determine the effects of oxaliplatin on TRP channels. To this end, immunohistochemistry was used to examine TRP vanilloid 1 (TRPV1), TRP ankyrin 1 (TRPA1), and TRP melastatin 8 (TRPM8) expression in the rat dorsal root ganglion (DRG) after 4days of oxaliplatin treatment. Behavioral assessment using the acetone spray test showed that oxaliplatin significantly increased acute cold hypersensitivity after 4days of treatment. Double-staining immunohistochemistry showed that 4days after oxaliplatin treatment, there was increased co-expression of TRPA1 and TRPV1 in isolectin B4-positive small-sized DRG neurons, as well as a significant increase in the co-localization of TRPM8 and neurofilament 200 in medium-sized DRG neurons. In addition, in situ hybridization revealed that TRPV1 protein was co-expressed with TRPA1 mRNA on day 4 after oxaliplatin administration. Thus, at an early stage following oxaliplatin treatment there is an increased expression of TRPA1 and TRPV1 in small-sized DRG neurons and of TRPM8 in medium-sized DRG neurons. Collectively, these changes may contribute to the development of oxaliplatin-induced peripheral neuropathic pain.

Pharmacological comparison of a nonhuman primate and a rat model of oxaliplatin‐induced neuropathic cold hypersensitivity

Oxaliplatin is a first‐line treatment for colorectal cancer. However, shortly following treatment, cold‐evoked hypersensitivity appears in the extremities and over time, the pain is such that oxaliplatin dosing may need to be markedly reduced or even terminated. There is currently a lack of efficacious treatments for oxaliplatin‐induced peripheral neuropathy, which is due in part to the difficulty in translating findings obtained from preclinical rodent models of chemotherapy‐induced peripheral neuropathy. Nonhuman primates (NHP) are phylogenetically closer to humans than rodents and may show drug responses that parallel those of humans. A significant decrease in tail withdrawal latency to 10°C water (“cold hypersensitivity”) was observed beginning 3 days after intravenous infusion of oxaliplatin (5 mg/kg) in Macaca fascicularis. A single treatment of duloxetine (30 mg/kg, p.o.) ameliorated oxaliplatin‐induced cold hypersensitivity, whereas pregabalin (30 mg/kg, p.o.) and tramadol (30 mg/kg, p.o.) did not. By contrast, in rats, no significant cold hypersensitivity, or increased responsiveness to acetone applied to the hind paws, was observed 3 days after the first injection of oxaliplatin (5 mg/kg, i.p., once per day, two injections). Therefore, rats were tested after six treatments of oxaliplatin, 17 days after the first treatment. All analgesics (30 mg/kg, p.o.) significantly ameliorated cold hypersensitivity in rats. The activity of analgesics in the oxaliplatin‐treated macaques parallel clinical findings. The current results indicate that the NHP could serve as a bridge species to improve translatability of preclinical findings into clinically useful treatments for oxaliplatin‐induced peripheral neuropathy.

Effects of electroacupuncture on oxaliplatin-induced neuropathic cold hypersensitivity in rats

This study investigated whether and how electroacupuncture (EA) attenuates cold hypersensitivity (allodynia) in a rat model of oxaliplatin-induced neuropathic pain. Cold allodynia [evaluated by immersing the tail into cold water (4°C) and measuring the withdrawal latency] was induced 3days after an oxaliplatin administration (6mg/kg, i.p.). EA stimulation (2/100Hz, 0.3-ms pulse duration, 0.2-0.3mA) was delivered to ST36 acupoint or non-acupoint for 20min. Low-frequency (2Hz) EA at ST36 relieved cold allodynia more effectively than high-frequency EA at ST36 or low-frequency EA at non-acupoint. Naloxone (opioid antagonist, 2mg/kg, i.p.) completely blocked such EA-induced anti-allodynia, whereas phentolamine (α-adrenergic antagonist, 2mg/kg, i.p.) did not. Moreover, plasma β-endorphin levels significantly increased right after the end of EA and subsequently decreased. These results indicate that low-frequency EA at ST36 in rats has a marked relieving effect on oxaliplatin-induced cold allodynia that is mediated by the endogenous opioid, but not noradrenergic, system.

Meteorin reverses hypersensitivity in rat models of neuropathic pain

Neuropathic pain is caused by a lesion or disease to the somatosensory nervous system and current treatment merely reduces symptoms. Here, we investigate the potential therapeutic effect of the neurotrophic factor Meteorin on multiple signs of neuropathic pain in two distinct rat models. In a first study, two weeks of intermittent systemic administration of recombinant Meteorin led to a dose-dependent reversal of established mechanical and cold hypersensitivity in rats after photochemically-induced sciatic nerve injury. Moreover, analgesic efficacy lasted for at least one week after treatment cessation. In rats with a chronic constriction injury (CCI) of the sciatic nerve, five systemic injections of Meteorin over 9days dose-dependently reversed established mechanical and thermal hypersensitivity as well as weight bearing deficits taken as a surrogate marker of spontaneous pain. The beneficial effects of systemic Meteorin were sustained for at least three weeks after treatment ended and no adverse side effects were observed. Pharmacokinetic analysis indicated that plasma Meteorin exposure correlated well with dosing and was no longer detectable after 24hours. This pharmacokinetic profile combined with a delayed time of onset and prolonged duration of analgesic efficacy on multiple parameters suggests a disease-modifying mechanism rather than symptomatic pain relief. In sciatic nerve lesioned rats, delivery of recombinant Meteorin by intrathecal injection was also efficacious in reversing mechanical and cold hypersensitivity. Together, these data demonstrate that Meteorin represents a novel treatment strategy for the effective and long lasting relief from the debilitating consequences of neuropathic pain.

Acute activation of the spinal cord metabotropic glutamate subtype-5 receptor leads to cold hypersensitivity in the rat

Activation of spinal cord dorsal horn ionotropic glutamate receptors leads to pain-related behaviors. However, the role of spinal metabotropic glutamate receptors (mGlu), particularly the mGlu5 receptor subtype, in nociception has not been well characterized. A recently described subtype selective and potent mGluR5 antagonist, 2-methyl-6-(phenylethynyl)pyridine (MPEP) was used to evaluate the role of the mGlu5 receptor in cold sensitivity. Intrathecal (i.t.) injection of group I (mGlu1 and mGlu5 receptors) mGlu receptor-selective agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) increased the hind paw frequency and duration of lifting of rats placed on a cold (4 °C) surface, a behavior similarly observed in rats with a chronic constriction injury (CCI) of the sciatic nerve. In contrast, rats i.t. injected with DHPG did not display increased lifting when placed on a room temperature surface. I.t. injection of MPEP before i.t. injection of DHPG blocked DHPG-evoked cold hypersensitivity, suggesting that activation of spinal mGlu5 receptors induces this behavioral response. In contrast, i.t. injection of MPEP after i.t. injection of DHPG had no effect. In addition, i.t. injection of MPEP did not affect cold hypersensitivity in rats with a CCI. These data suggest that acute activation of spinal cord mGlu5 receptors results in increased sensitivity to cold, but ongoing cold hypersensitivity does not involve activation of the mGlu5 receptor.