Differential Cortico-Thalamic reorganization in Opioid-Induced hyperalgesia and neuropathic pain male rats.

HDAC6 is a class IIB histone deacetylase expressed at many levels of the nociceptive pathway. This study tested the ability of novel and selective HDAC6 inhibitors to alleviate sensory hypersensitivity behaviors in mouse models of peripheral nerve injury and peripheral inflammation. We utilized the murine spared nerve injury (SNI) model for peripheral nerve injury and the Complete Freund's Adjuvant (CFA) model of peripheral inflammation. We applied the Von Frey assay to monitor mechanical allodynia. Using the SNI model, we demonstrate that daily administration of the brain-penetrant HDAC6 inhibitor, ACY-738, abolishes mechanical allodynia in male and in female mice. Importantly, there is no tolerance to the antiallodynic actions of these compounds as they produce a consistent increase in Von Frey thresholds for several weeks. We observed a similar antiallodynic effect when utilizing the HDAC6 inhibitor, ACY-257, which shows limited brain expression when administered systemically. We also demonstrate that ACY-738 and ACY-257 attenuate mechanical allodynia in the CFA model of peripheral inflammation. Overall, our findings suggest that inhibition of HDAC6 provides a promising therapeutic avenue for the alleviation of mechanical allodynia associated with peripheral nerve injury and peripheral inflammation.

Pin-evoked tachycardia: A new measure of neuropathic pain

Spinal Cord Stimulation Modulates Gene Expression in the Spinal Cord of an Animal Model of Peripheral Nerve Injury.

Novel analgesics that do not suppress the respiratory drive are urgently needed. Glutamate signaling through α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors plays important roles in central pain circuits. AMPAkines augment AMPA receptor function and have been shown to stimulate the respiratory drive to oppose opioid-induced hypoventilation. However, their role in chronic pain states remains unknown. The authors studied AMPAkines (CX546 and CX516) in rat spared nerve injury (SNI) model of neuropathic pain and Complete Freund's Adjuvant (CFA) model of inflammatory pain. They measured the effect of AMPAkines on mechanical and cold allodynia. They also evaluated their effect on depressive symptoms of pain using the forced swim test, as time of immobility on this test has been used as a measure for behavioral despair, a feature of depression. The authors found that CX546, compared with dimethyl sulfoxide (DMSO) control, reduced both mechanical and sensory allodynia in SNI (DMSO group, n = 9; CX546 group, n = 11) and CFA models (both DMSO and CX546 groups, n = 9). They found that CX546, compared with control, also reduced depressive symptoms of pain by decreasing immobility on the forced swim test in both SNI (both DMSO and CX546 groups, n = 8) and CFA models (both DMSO and CX546 groups, n = 10). Finally, they found that CX516, compared with control, also reduced mechanical and cold allodynia in the SNI model (both DMSO and CX516 groups, n = 10). AMPAkines alleviate pain hypersensitivity as well as depression-like behavior associated with long-lasting nerve injury and inflammatory insult.

Following surgical repair after peripheral nerve injury, neuropathic pain diminishes in most patients but can persist in a small proportion of cases, the mechanism of which remains poorly understood. Based on the spared nerve injury (SNI), we developed a rat nerve repair (NR) model, where a delayed reconstruction of the SNI-injured nerves resulted in alleviating chronic pain-like behavior only in a subpopulation of rats. Multiple behavioral measures were assayed over 11-week presurgery and postsurgery periods (tactile allodynia, pain prick responses, sucrose preference, motor coordination, and cold allodynia) in SNI (n = 10), sham (n = 8), and NR (n = 12) rats. All rats also underwent resting-state functional magnetic resonance imaging under anesthesia at multiple time points postsurgery, and at 10 weeks, histology and retrograde labeling were used to calculate peripheral reinnervation. Behavioral measures indicated that at approximately 5 weeks postsurgery, the NR group separated to pain persisting (NR persisting, n = 5) and recovering (NR recovering, n = 7) groups. Counts of afferent nerves and dorsal root ganglion cells were not different between NR groups. Therefore, NR group differences could not be explained by peripheral reorganization. By contrast, large brain functional connectivity differences were observed between NR groups, where corticolimbic reorganization paralleled with pain recovery (repeated-measures analysis of variance, false discovery rate, P < 0.05), and functional connectivity between accumbens and medial frontal cortex was related both to tactile allodynia (nociception) and to sucrose preference (anhedonia) in the NR group. Our study highlights the importance of brain circuitry in the reversal of neuropathic pain as a natural pain-relieving mechanism. Further studies regarding the therapeutic potentials of these processes are warranted.

<b>Abstract ID 20519</b> <b>Poster Board 80</b> Histone deacetylase 6 (HDAC6) is a Class IIb histone deacetylase, which is primarily located in the cytoplasm and plays an important role in cell cycle progression and transcriptional regulation. Earlier studies from our group and others have shown that HDAC6 inhibitors alleviate sensory signs of chemotherapy-induced and nerve injury-induced peripheral neuropathy. Furthermore, our recent data show that downregulation of HDAC6 in the dorsal root ganglia (DRG), achieved by injection of the sciatic nerve of adult male HDAC6^fl\flmice with AAV8-Cre-EGFP vectors, results in recovery from mechanical allodynia in paclitaxel chemotherapy induced peripheral neuropathy (CIPN) model. We demonstrate that DRG-knockdown of HDAC6 prevents the development of mechanical allodynia after the paclitaxel CIPN model. We also use the murine spared nerve injury (SNI) model of neuropathic pain to determine the impact of the peripherally acting HDAC6 inhibitor ACY1215 (Regenacy Pharmaceutics) on the alleviation of sensory hypersensitivity behaviors. Using the von Frey assay, we demonstrate that treatment with ACY1215 (30mg/kg) leads to recovery from mechanical allodynia developed after SNI injury in both male and female mice without affecting locomotor activity. We performed bulk tissue RNA Sequencing analysis to understand transcriptomic events and pathways in the DRG associated with the antiallodynic actions of ACY1215. We found over 800 differentially expressed genes (DEG) in the SNI condition between vehicle and ACY1215-treated animals. Most of the pathways affected by these DEGs are associated with inflammatory disorders and B-cell mechanisms, suggesting a potential impairment of circulating immune cell infiltration in to the DRG after injury, which has been shown to be necessary for the induction of sensory hypersensitivity. Furthermore, a separate bulk RNA-seq analysis of L3-6 DRGs 7 weeks after SNI highlighted a significant upregulation of several alpha- and beta-tubulin transcripts. Prior work by others suggests that this may be a compensatory response to nerve injury-induced tubulin destabilization. Therefore, we hypothesize that HDAC6 inhibition may address this molecular maladaptation by increasing acetylated tubulin levels and promoting stability and we are using Western Blot analysis to assess alpha- and beta- tubulin acetylating in the DRG following peripheral nerve injury with and without treatment with HDAC6 inhibitors. An alternative hypothesis is that HDAC6 acts as a transcription factor, and it targets several genes that are associated with hereditary neurodegenerative diseases. We hypothesize that several of these genes are regulated under SNI conditions that are rescued under the presence of ACY1215. We are testing this hypothesis by use of qPCR and Western Blot analysis. Our findings highlight a promising therapeutic role of HDAC6 inhibitors for the alleviation of sensory hypersensitivity behaviors associated with peripheral nerve injury. Future work will define the mechanisms underlying HDAC6 action in the DRG and will test if interventions in HDAC6 activity in subjects suffering from prolonged peripheral nerve injury may reverse sensory and affective manifestations of neuropathic pain. <b>Support/Funding Information:</b> RO1 NS086444 NINDS.

The spared nerve injury (SNI) model of neuropathic pain was first developed by Decosterd and Woolf in 2000 in Sprague Dawley rats to enhance reproducibility of injury and behavioral responses resulting from a partial nerve injury. Given the differences in methodology and inconsistent behavioral data published in the SNI model of neuropathic pain in mice, and given that interspecies behavioral comparisons using the same peripheral nerve injury are presently lacking, in this study we assessed the development of mechanical and cold allodynia for five weeks in C57BL/6 mice and Sprague Dawley rats that underwent SNI. In rats and mice, the tibial and peroneal branches were ligated then severed, leaving the sural branch intact. By controlling several factors in the surgical procedure and behavioral tests, we found that rats developed and maintained strong mechanical and robust cold allodynia immediately following the injury that was maintained for the duration of the experiment (five weeks). In comparison, mice developed mechanical allodynia to a lesser magnitude which peaked at 2 weeks, but did not develop cold allodynia. We found both temporal and qualitative differences in the development of allodynic behaviors between SNI-mice and SNI-rats. Parallel analysis of interspecies differences can be exploited to reveal novel molecular players leading to divergent pain behaviors.

BackgroundDysregulation of voltage-gated sodium channels (Navs) is believed to play a major role in nerve fiber hyperexcitability associated with neuropathic pain. A complete transcriptional characterization of the different isoforms of Navs under normal and pathological conditions had never been performed on mice, despite their widespread use in pain research. Navs mRNA levels in mouse dorsal root ganglia (DRG) were studied in the spared nerve injury (SNI) and spinal nerve ligation (SNL) models of neuropathic pain. In the SNI model, injured and non-injured neurons were intermingled in lumbar DRG, which were pooled to increase the tissue available for experiments.ResultsA strong downregulation was observed for every Navs isoform expressed except for Nav1.2; even Nav1.3, known to be upregulated in rat neuropathic pain models, was lower in the SNI mouse model. This suggests differences between these two species. In the SNL model, where the cell bodies of injured and non-injured fibers are anatomically separated between different DRG, most Navs were observed to be downregulated in the L5 DRG receiving axotomized fibers. Transcription was then investigated independently in the L3, L4 and L5 DRG in the SNI model, and an important downregulation of many Navs isoforms was observed in the L3 DRG, suggesting the presence of numerous injured neurons there after SNI. Consequently, the proportion of axotomized neurons in the L3, L4 and L5 DRG after SNI was characterized by studying the expression of activating transcription factor 3 (ATF3). Using this marker of nerve injury confirmed that most injured fibers find their cell bodies in the L3 and L4 DRG after SNI in C57BL/6 J mice; this contrasts with their L4 and L5 DRG localization in rats. The spared sural nerve, through which pain hypersensitivity is measured in behavioral studies, mostly projects into the L4 and L5 DRG.ConclusionsThe complex regulation of Navs, together with the anatomical rostral shift of the DRG harboring injured fibers in C57BL/6 J mice, emphasize that caution is necessary and preliminary anatomical experiments should be carried out for gene and protein expression studies after SNI in mouse strains.

Whether electroacupuncture (EA) stimulation at different frequencies has a similar effect on spared nerve injury (SNI) as other neuropathic pain models, and how EA at different frequencies causes distinct analgesic effects on neuropathic pain is still not clear. Adult male Sprague-Dawley rats were randomly divided into sham SNI, SNI, 2 Hz, 100 Hz and sham EA groups. Paw withdrawal threshold (PWT) and paw withdrawal latency (PWL) were measured. EA was performed once a day on days 1 to 14 after SNI. The expressions of transient receptor potential cation subfamily V member 1 (TRPV1) and peripheral purinergic P2X receptor 3 (P2X3) were determined by western blotting and immunofluorescence. TRPV1 siRNA and P2X3 siRNA were administered by intrathecal injection. TRPV1 or P2X3 agonists were combined with EA. There were significant decreases in PWT, but no changes in PWL in the 14 days after SNI. EA using 2- or 100-Hz stimulation similarly increased PWT at every time point. The cytosol protein expression of P2X3 in the L4-L6 dorsal root ganglia (DRG) increased, but the expression of TRPV1 decreased in the SNI model. Both these effects were ameliorated by EA, with 2-Hz stimulation having a stronger effect than 100-Hz stimulation. Blocking either TRPV1 or P2X3 specific siRNAs attenuated the decreased PWT induced by SNI. Administration of either a TRPV1 or P2X3 agonist inhibited EA analgesia. 2- and 100-Hz EA similarly induced analgesic effects in SNI. This effect was related to up-regulation and down-regulation, respectively, of cytosol protein expression of P2X3 and TRPV1 in L4-L6 DRG, with 2 Hz having a better effect than 100 Hz.

NGF and BDNF expression in mouse DRG after spared nerve injury

Assessment and analysis of mechanical allodynia-like behavior induced by spared nerve injury (SNI) in the mouse

Chimeric peptide MCRT (YPFPFRTic-NH2 ) was a multifunctional ligand of opioid and neuropeptide FF (NPFF) receptors and reported to be potentially antalgic in acute tail-flick test. Here, we developed spared nerve injury (SNI) model to explore its efficacy in chronic neuropathic pain. Analgesic tolerance, opioid-induced hyperalgesia and gastrointestinal transit were measured for safety evaluation. Intracerebroventricular (i.c.v.) and intraplantar (i.pl.) injections were conducted as central and peripheral routes, respectively. Results demonstrated that MCRT alleviated neuropathic pain effectively and efficiently, with the ED50 values of 4.93nmol/kg at the central level and 3.11nmol/kg at the peripheral level. The antagonist blocking study verified the involvement of mu-, delta-opioid and NPFF receptors in MCRT produced anti-allodynia. Moreover, the separation of analgesia from unwanted effects was preliminarily achieved and that MCRT caused neither analgesic tolerance nor hyperalgesia after chronic i.c.v. administration, nor constipation after i.pl. administration. Notably, the local efficacy of MCRT in SNI mice was about one thousandfold higher than morphine and ten thousandfold higher than pregabalin, indicating a great promise in the future treatment of neuropathic pain.

BB14, a Nerve Growth Factor (NGF)-like peptide shown to be effective in reducing reactive astrogliosis and restoring synaptic homeostasis in a rat model of peripheral nerve injury

Cyclooxygenase 2 expression in the spared nerve injury model of neuropathic pain

Nav1.7 is closely associated with neuropathic pain. Hydrogen sulfide (H2S) has recently been reported to be involved in numerous biological functions, and it has been shown that H2S can enhance the sodium current density, and inhibiting the endogenous production of H2S mediated by cystathionine β-synthetase (CBS) using O-(carboxymethyl) hydroxylamine hemihydrochloride (AOAA) can significantly reduce the expression of Nav1.7 and thus the sodium current density in rat dorsal root ganglion (DRG) neurons. In the present study, it was shown that the fluorescence intensity of H2S was increased in a spared nerve injury (SNI) model and AOAA inhibited this increase. Nav1.7 is expressed in DRG neurons, and the expression of CBS and Nav1.7 were increased in DRG neurons 7, 14 and 21 days post-operation. AOAA inhibited the increase in the expression of CBS, phosphorylated (p)-MEK1/2, p-ERK1/2 and Nav1.7 induced by SNI, and U0126 (a MEK blocker) was able to inhibit the increase in p-MEK1/2, p-ERK1/2 and Nav1.7 expression. However, PF-04856264 did not inhibit the increase in CBS, p-MEK1/2, p-ERK1/2 or Nav1.7 expression induced by SNI surgery. The current density of Nav1.7 was significantly increased in the SNI model and administration of AOAA and U0126 both significantly decreased the density. In addition, AOAA, U0126 and PF-04856264 inhibited the decrease in rheobase, and the increase in action potential induced by SNI in DRG neurons. There was no significant difference in thermal withdrawal latency among each group. However, the time the animals spent with their paw lifted increased significantly following SNI, and the time the animals spent with their paw lifted decreased significantly following the administration of AOAA, U0126 and PF-04856264. In conclusion, these data show that Nav1.7 expression in DRG neurons is upregulated by CBS-derived endogenous H2S in an SNI model, contributing to the maintenance of neuropathic pain.