Animal Studies

Small-diameter vesicular glutamate transporter 3-lineage (Vglut3) dorsal root ganglion (DRG) neurons play an important role in mechanosensation and thermal hypersensitivity; however, little is known about their intrinsic electrical properties. We therefore set out to investigate mechanisms of excitability within this population. Calcium microfluorimetry analysis of male and female mouse DRG neurons demonstrated that the cooling compound menthol selectively activates a subset of Vglut3 neurons. Whole-cell recordings showed that small-diameter Vglut3 DRG neurons fire menthol-evoked action potentials and exhibited robust, transient receptor potential melastatin 8 (TRPM8)-dependent discharges at room temperature. This heightened excitability was confirmed by current-clamp and action potential phase-plot analyses, which showed menthol-sensitive Vglut3 neurons to have more depolarized membrane potentials, lower firing thresholds, and higher evoked firing frequencies compared with menthol-insensitive Vglut3 neurons. A biophysical analysis revealed voltage-gated sodium channel (Na) currents in menthol-sensitive Vglut3 neurons were resistant to entry into slow inactivation compared with menthol-insensitive neurons. Multiplex hybridization showed similar distributions of tetrodotoxin (TTX)-sensitive Nas transcripts between TRPM8-positive and -negative Vglut3 neurons; however, Na1.8 transcripts, which encode TTX-resistant channels, were more prevalent in TRPM8-negative neurons. Conversely, pharmacological analyses identified distinct functional contributions of Na subunits, with Na1.1 driving firing in menthol-sensitive neurons, whereas other small-diameter Vglut3 neurons rely primarily on TTX-resistant Na channels. Additionally, when Na1.1 channels were blocked, the remaining Na currents readily entered into slow inactivation in menthol-sensitive Vglut3 neurons. Thus, these data demonstrate that TTX-sensitive Nas drive action potential firing in menthol-sensitive sensory neurons and contribute to their heightened excitability.Somatosensensory neurons encode various sensory modalities including thermoreception, mechanoreception, nociception and itch. This report identifies a previously unknown requirement for tetrodotoxin-sensitive sodium channels in action potential firing in a discrete subpopulation of small-diameter sensory neurons that are activated by the cooling agent menthol. Together, our results provide a mechanistic understanding of factors that control intrinsic excitability in functionally distinct subsets of peripheral neurons. Furthermore, as menthol has been used for centuries as an analgesic and anti-pruritic, these findings support the viability of Na1.1 as a therapeutic target for sensory disorders.

Opioid-induced hyperalgesia (OIH) is a serious adverse event produced by opioid analgesics. Lack of an model has hindered study of its underlying mechanisms. Recent evidence has implicated a role of nociceptors in OIH. To investigate the cellular and molecular mechanisms of OIH in nociceptors, , subcutaneous administration of an analgesic dose of fentanyl (30 μg/kg, s.c.) was performed in male rats. Two days later, when fentanyl was administered intradermally (1 μg, i.d.), in the vicinity of peripheral nociceptor terminals, it produced mechanical hyperalgesia (OIH). Additionally, two days after systemic fentanyl, rats had also developed hyperalgesic priming (opioid-primed rats), long-lasting nociceptor neuroplasticity manifested as prolongation of prostaglandin E (PGE) hyperalgesia. OIH was reversed, , by intrathecal administration of cordycepin, a protein translation inhibitor that reverses priming. When fentanyl (0.5nM) was applied to dorsal root ganglion (DRG) neurons, cultured from opioid-primed rats, it induced a mu-opioid receptor (MOR)-dependent increase in [Ca] in 26% of small-diameter neurons and significantly sensitized (decreased action potential rheobase) weakly IB4-positive and IB4-negative neurons. This sensitizing effect of fentanyl was reversed in weakly IB4-positive DRG neurons cultured from opioid-primed rats after treatment with cordycepin, to reverse of OIH. Thus, administration of fentanyl induces nociceptor neuroplasticity, which persists in culture, providing evidence for the role of nociceptor MOR-mediated calcium signaling and peripheral protein translation, in the weakly IB4-binding population of nociceptors, in OIH.Clinically used mu-opioid receptor agonists such as fentanyl can produce hyperalgesia and hyperalgesic priming. We report on an model of nociceptor neuroplasticity mediating this opioid-induced hyperalgesia (OIH) and priming, induced by fentanyl. Using this model, we have found qualitative and quantitative differences between cultured nociceptors from opioid naïve and opioid primed animals, and provide evidence for the important role of nociceptor MOR-mediated calcium signaling and peripheral protein translation, in the weakly IB4-binding population of nociceptors, in OIH. These findings provide information useful for the design of therapeutic strategies to alleviate OIH, a serious adverse event of opioid analgesics.

Central sensitization plays a pivotal role in the maintenance of chronic pain induced by chronic pancreatitis (CP), but cortical modulation of painful CP remains elusive. This study was designed to examine the role of anterior insular cortex (aIC) in the pathogenesis of hyperalgesia in a rat model of CP. CP was induced by intraductal administration of trinitrobenzene sulfonic acid (TNBS). Abdomen hyperalgesia and anxiety were assessed by von Frey filament and open field tests, respectively. Two weeks after surgery, the activation of aIC was indicated by FOS immunohistochemical staining and electrophysiological recordings. Expressions of VGluT1, NMDAR subunit NR2B and AMPAR subunit GluR1 were analyzed by immunoblottings. The regulatory roles of aIC in hyperalgesia and pain-related anxiety were detected via pharmacological approach and chemogenetics in CP rats. Our results showed that TNBS treatment resulted in long-term hyperalgesia and anxiety-like behavior in rats. CP rats exhibited increased FOS expression and potentiated excitatory synaptic transmission within aIC. CP rats also showed up-regulated expression of VGluT1, and increased membrane trafficking and phosphorylation of NR2B and GluR1 within aIC. Blocking excitatory synaptic transmission significantly attenuated abdomen mechanical hyperalgesia. Specifically inhibiting the excitability of insular pyramidal cells reduced both abdomen hyperalgesia and pain-related anxiety. In conclusion, our findings emphasize a key role for aIC in hyperalgesia and anxiety of painful CP, providing a novel insight into cortical modulation of painful CP and shedding light on aIC as a potential target for neuromodulation interventions in the treatment of CP.

Sirtuin1 (SIRT1), which is regulated by microRNA-34a (miR-34a), can modulate pathophysiology processes, including nonalcoholic fatty liver disease and intestinal ischemia/reperfusion injury. We previously reported that SIRT1, an NAD-dependent deacetylase, plays a vital role in the development of neuropathic pain. However, the role of miR-34a/SIRT1 in complete Freund's adjuvant (CFA)-induced inflammatory pain remains unclear. In the present study, we examined miR-34a and SIRT1 in CFA mice. MiR-34a levels increased, while SIRT1 decreased in the spinal cord. Inhibiting miR-34a by intrathecal injection of miR-34a antagomir attenuated CFA-induced pain behavior. Moreover, miR-34a antagomir inhibited the CFA-induced SIRT1 decrease in the spinal cord. Furthermore, the analgesic effect of miR-34a antagomir was abrogated by the SIRT1 inhibitor EX-527. Our data provide support that the underlying mechanisms of miR-34a in promoting inflammatory pain may involve negative regulation of SIRT1.

It has been reported that skin/muscle incision and retraction (SMIR) in the thigh, produces mechanical allodynia in the hind paw, far from the site of incision/retraction. The mechanical allodynia lasts about 22 days, indicating chronic post-operative pain develops. The precise mechanisms, however, are largely unclear. In the current study, we further found that SMIR surgery induced LTP of c-fiber evoked field potentials that lasted at least 4 h. The mRNA and protein level of tumor necrosis factor-alpha (TNFα) and acetylated nuclear factor-kappaB p65 (ac-NF-κB p65) in the lumbar spinal dorsal horn was gradually increased during LTP development, while pretreatment with either TNFα neutralization antibody or NF-κB inhibitor PDTC completely prevented the induction of LTP. Moreover, the expression of Silent information regulator 1 (SIRT1) in the lumbar spinal dorsal horn was decreased and activation of SIRT1 by SRT1720 also prevented the induction of LTP. Importantly, the spinal expression of Liver X receptors (LXRs) was increased, both at mRNA and protein level following SMIR. Application of LXRs agonist T0901317 to the spinal dorsal horn prevented LTP induction following SMIR. Mechanistically, T0901317 enhanced the expression of SIRT1 and decreased the expression of ac-NF-κB p65 and TNFα. Spinal application of SIRT1 antagonist EX-527, 30 min before T0901317 administration, completely blocked the inhibiting effect of T0901317 on LTP, and on expression of ac-NF-κB p65 and TNFα. These results indicated that activation of LXRs prevented SMIR-induced LTP by inhibiting NF-κB/TNFα pathway via increasing SIRT1 expression.