Animal Studies

Clinically, pain has an uneven incidence throughout lifespan and impacts more on the elderly. In contrast, preclinical models of pathological pain have typically used juvenile or young adult animals to highlight the involvement of glial populations, proinflammatory cytokines, and chemokines in the onset and maintenance of pathological signalling in the spinal dorsal horn. The potential impact of this mismatch is also complicated by the growing appreciation that the aged central nervous system exists in a state of chronic inflammation because of enhanced proinflammatory cytokine/chemokine signalling and glial activation. To address this issue, we investigated the impact of aging on the expression of genes that have been associated with neuropathic pain, glial signalling, neurotransmission and neuroinflammation. We used qRT-PCR to quantify gene expression and focussed on the dorsal horn of the spinal cord as this is an important perturbation site in neuropathic pain. To control for global vs region-specific age-related changes in gene expression, the ventral half of the spinal cord was examined. Our results show that expression of proinflammatory chemokines, pattern recognition receptors, and neurotransmitter system components was significantly altered in aged (24-32 months) versus young mice (2-4 months). Notably, the magnitude and direction of these changes were spinal-cord region dependent. For example, expression of the chemokine, Cxcl13, increased 119-fold in dorsal spinal cord, but only 2-fold in the ventral spinal cord of old versus young mice. Therefore, we propose the dorsal spinal cord of old animals is subject to region-specific alterations that prime circuits for the development of pathological pain, potentially in the absence of the peripheral triggers normally associated with these conditions.

An electrophysiological technique that can record nerve impulses from a single nerve fiber is indispensable for studying modality-specific sensory receptors such as low threshold mechanoreceptors, thermal receptors, and nociceptors. The teased-fiber single-unit recording technique has long been used to resolve impulses that are likely to be from a single nerve fiber. The teased-fiber single-unit recording technique involves tedious nerve separation procedures, causes nerve fiber impairment, and is not a true single-fiber recording method. In the present study, we describe a new and true single-fiber recording technique, the pressure-clamped single-fiber recording method. We have applied this recording technique to mouse whisker hair follicle preparations with attached whisker afferents as well as to skin-nerve preparations made from mouse hindpaw skin and saphenous nerves. This new approach can record impulses from rapidly adapting mechanoreceptors (RA), slowly adapting type 1 mechanoreceptors (SA1), and slowly adapting type 2 mechanoreceptors (SA2) in these tissue preparations. We have also applied the pressure-clamped single-fiber recordings to record impulses on Aβ-fibers, Aδ-fibers, and C-fibers. The pressure-clamped single-fiber recording technique provides a new tool for sensory physiology and pain research.

Oxytocin reduces primary sensory afferent excitability and produces analgesia in part through a peripheral mechanism, yet its actions on physiologically characterized, mechanically sensitive afferents in normal and neuropathic conditions are unknown. We recorded intracellularly from L4 dorsal root ganglion neurons characterized as low-threshold mechanoreceptors (LTMRs) or high-threshold mechanoreceptors (HTMRs) in female rats 1 week after L5 partial spinal nerve injury or sham control (n = 24 rats/group) before, during, and after ganglionic perfusion with oxytocin, 1 nM. Nerve injury desensitized and hyperpolarized LTMRs (membrane potential [Em] was -63 ± 1.8 mV in sham vs -76 ± 1.4 mV in nerve injury; P < 0.001), and sensitized HTMRs without affecting Em. In nerve-injured rats, oxytocin depolarized LTMRs towards normal (Em = -69 ± 1.9 mV) and, in 6 of 21 neurons, resulted in spontaneous action potentials. By contrast, oxytocin hyperpolarized HTMRs (Em = -68 ± 2.7 mV before vs -80 ± 3.2 mV during oxytocin exposure; P < 0.01). These effects were reversed after removal of oxytocin, and oxytocin had minimal effects in neurons from sham surgery animals. Sensory afferent neurons immunopositive for the vasopressin 1a receptor were larger (34 ± 6.3 μm, range 16-57 μm) than immunonegative neurons (26 ± 3.4 μm, range 15-43 μm; P < 0.005). These data replicate findings that neuropathic injury desensitizes LTMRs while sensitizing HTMRs and show rapid and divergent oxytocin effects on these afferent subtypes towards normal, potentially rebalancing input to the central nervous system. Vasopressin 1a receptors are present on medium to large diameter afferent neurons and could represent oxytocin's target.

Fast inactivation of voltage-gated sodium (Nav) channels is essential for electrical signaling but its mechanism remains poorly understood. Here, we determined the structures of a eukaryotic Nav channel alone and in complex with a lethal α-scorpion toxin, AaH2, by electron microscopy, both at 3.5-A resolution. AaH2 wedges into voltage-sensor domain IV (VSD4) to impede fast activation by trapping a deactivated state in which gating charge interactions bridge to the acidic intracellular C-terminal domain. In the absence of AaH2, the S4 helix of VSD4 undergoes a ~13-Å translation to unlatch the intracellular fast inactivation gating machinery. Highlighting the polypharmacology of α-scorpion toxins, AaH2 also targets an unanticipated receptor site on VSD1 and a pore-glycan adjacent to VSD4. Overall, this work provides key insights into fast inactivation, electromechanical coupling, and pathogenic mutations in Nav channels.

Elevated excitability of primary afferent neurons underlies chronic pain in patients with functional or inflammatory bowel diseases. Recent studies have established an essential role for an enhanced transient receptor potential vanilloid subtype 1 (TRPV1) signaling in mediating peripheral hyperalgesia in inflammatory conditions. Since co-localization of Toll-like receptor 4 (TLR4) and TRPV1 has been observed in primary afferents including the trigeminal sensory neurons and the dorsal root ganglion (DRG) neurons, we test the hypothesis that TLR4 might regulate the expression and function of TRPV1 in primary afferent neurons in TNBS-induced colitis using the TLR4-deficient and the wild type (WT) C57 mice. Despite having a higher disease activity index following administration of TNBS, the TLR4 deficient mice showed less inflammatory infiltration in the colon than the WT mice. Increased expression of TLR4 and TRPV1 as well as increased density of capsaicin-induced TRPV1 current was observed in L4-S2 DRG neurons of WT colitis mice till two weeks post TNBS treatment. In comparison, TLR4 deficient colitis mice had lower TRPV1 expression and TRPV1 current density in DRG neurons with lower abdominal withdrawal response scores during noxious colonic distensions. In WT but not in TLR4-deficient DRG neurons, acute administration of the TLR4 agonist lipopolysacharide (LPS) increased the capsaicin-evoked TRPV1 current. In addition, we found that the canonical signaling downstream of TLR4 was activated in TNBS induced colitis in the WT but not in TLR4-deficient mice. These results indicate that TLR4 may play a major role in regulation of TRPV1 signaling and peripheral hyperalgesia in inflammatory conditions.

Accumulating evidence shows that inhibition of glycogen synthase kinase-3beta (GSK-3β) ameliorates cognitive impairments caused by a diverse array of diseases. Our previous work show that spared nerve injury (SNI) that induces neuropathic pain causes short-term memory (STM) deficits.Here,we reported that GSK-3β activity was enhanced in hippocampus and reduced in spinal dorsal horn following SNI, and the changes persisted for at least 45 d. Repetitive applications of selective GSK-3β inhibitors (SB216763, 5 mg/kg, i.p., 3 times or AR-A014418, 400 ng/kg, i.t., 7 times) prevented STM deficits but did not affect neuropathic pain in SNI rats. Surprisingly, we found that the repetitive SB216763 or AR-A014418 induced a persistent pain hypersensitivity in sham animals. Mechanistically,both β-catenin and brain-derived neurotrophic factor (BDNF) were upregulated in spinal dorsal horn but downregulated in hippocampus following SNI. Injections of SB216763 prevented the BDNF downregulation in hippocampus but enhanced BDNF upregulation in spinal dorsal horn in SNI rats. In sham rats SB216763 upregulated both β-catenin and BDNF in spinal dorsal horn but not affect neither of them in hippocampus. Finally, intravenous injection of interleukin-1beta that induces pain hypersensitivity and memory deficits mimicked the SNI-induced the differential regulation of GSK-3β/β-catenin/BDNF in spinal dorsal horn and in hippocampus. Accordingly, the prolonged opposite changes of GSK-3β activity in hippocampus and in spinal dorsal horn induced by SNI may contribute to Molecular Pain memory deficits and neuropathic pain by differential regulation of BDNF in the two regions. GSK-3β inhibitors that treat cognitive disorders may result in a long-lasting pain hypersensitivity.

Galanin and galanin receptors (GalRs) play important roles in the transmission and modulation of nociceptive information. Our previous research has shown that the expression of GalR1 is upregulated and that GalR1 activation in the nucleus accumbens (NAc) of rats with neuropathic pain has an antinociceptive effect. However, the antinociceptive role of NAc galanin in neuralgia remains unclear. The present study aimed to explore the antinociceptive effect induced by galanin in rats with neuropathic pain and the underlying mechanism. The results showed that the intra-NAc injection of galanin induced a dose-dependent increase in hindpaw withdrawal latency (HWL) to noxious thermal and mechanical stimulation in mononeuropathic rats and that this effect was stronger than that in intact rats. The intra-NAc injection of the non-selective GalR antagonist galantide reduced HWL in the rats with neuropathic pain, but there was no influence of galantide on HWL in intact rats. Moreover, galanin expression in the NAc was upregulated after sciatic nerve ligation. All of these results demonstrate that galanin plays a role in antinociception via binding to GalRs in the NAc of rats and that endogenous galanin is involved in the antinociception after peripheral nerve injury.