Intrathecal Injection of GRIP-siRNA Reduces Postoperative Synaptic Abundance of Kainate Receptor GluK2 Subunits in Rat Dorsal Horns and Pain Hypersensitivity

Overexpression of TAFA4 in the dorsal root ganglion ameliorates neuropathic pain in male rats through promoting macrophage M2-Skewing

Neuro-immune interactions between macrophages and primary sensory neurons have been implicated in nerve injury and associated pain. This study aims to explore the function of the TAFA4 as a crucial neuroimmune regulator in modulating macrophage states within the context of neuropathic pain. To elucidate the role of TAFA4 in dorsal root ganglia (DRG) following a chronic constriction injury (CCI) model in male rats, immunofluorescent staining, western blot, flow cytometry analysis and enzyme-linked immunosorbent assay were performed. Microinjection of self-complementary adeno-associated virus expressing TAFA4 mRNA into the L4 and L5 DRGs was conducted to overexpress TAFA4 in the DRGs. Following peripheral nerve injury, we observed a downregulation of TAFA4 in ipsilateral DRG neurons. Restoring this downregulation effectively alleviated the mechanical and thermal nociceptive hypersensitivity by inhibiting pro-inflammatory mediators while promoting the secretion of anti-inflammatory cytokines on day 14 post-CCI. Notably, scAAV-TAFA4 microinjection also facilitated the polarization of macrophages in the DRGs towards the M2 phenotype. Mechanistically, TAFA4 modulates the functions of macrophages in a lipoprotein receptor-related protein 1-dependent manner. Our findings revealed the role of TAFA4 in shifting macrophages in favor of an anti-inflammatory phenotype and enhancing interleukin 10 concentrations in the DRG, suggesting it is a potential analgesic target for alleviating neuropathic pain.

How spinal GABAergic circuits modulate cerebral processing of postsurgical pain

Post-surgical pain affects millions each year, hindering recovery and quality of life. Surgical procedures cause tissue damage and inflammation, leading to peripheral and central sensitization, resulting in pain at rest or mechanical and heat hyperalgesia. In a rat model for post-surgical pain, spinal GABAergic transmission via GABAA receptors reduces mechanical hypersensitivity but has no effect on pain at rest. While fMRI studies show consistent brain activity changes during mechanical stimulation in post-surgical pain, central processing of pain at rest and the role of spinal GABAergic circuits on surgical pain processing is currently unclear. The aim of this study was to evaluate the influence of an acute surgical incision injury, a proxy for post-surgical pain, on the cerebral processing of pain at rest and mechanical hypersensitivity, and to assess the influence of spinal GABAA-circuits on this processing. In rats, a unilateral incision affected sensorimotor and thalamo-limbic subnetworks at rest and following mechanical stimulation, indicating changes in neural processing relevant to pain at rest and mechanical hypersensitivity in post-surgical pain. Enhancing spinal GABAergic tone increased functional connectivity (FC) in parts of these subnetworks during mechanical stimulation, but not at rest, highlighting spino-cerebral interactions in pain regulation relevant for mechanical hypersensitivity and potentially the transisition to chronic pain after surgery but likely not for pain at rest. These findings underscore the complex and interconnected nature of brain networks in post-surgical pain processing, and provide insights into potential spinal targets for pharmacological intervention to alleviate post-surgical pain and prevent it's chronification.

Excitotoxic spinal damage induced by kainic acid impairs locomotion, alters nociception, and reduces CREB nuclear translocation

Our previous in vitro studies showed that excitotoxicity evoked by glutamate analogue kainate (KA) significantly decreased the number of rat spinal neurons and triggered high release of glutamate leading to locomotor network block. Our current objective was to assess the role of CREB as a predictive marker of damage following chemically-induced spinal cord injury by using in vivo and in vitro models. Thus, in vivo excitotoxicity in Balb/c adult mice was induced by KA intraspinal injection, while in vitro spinal cord excitotoxicity was produced by bath-applied KA. KA application evoked significant neuronal loss, deterioration in hindlimb motor coordination and thermal allodynia. In addition, immunohistochemical analysis showed that KA application resulted in decreased number of CREB positive nuclei in the ventral horn and in dorsal layers III-IV. Our data suggests that excitotoxic-induced neuronal loss may be potentially predicted by altered CREB nuclear translocation.

Upregulation of Spinal MDGA1 in Rats After Nerve Injury Alters Interactions Between Neuroligin-2 and Postsynaptic Scaffolding Proteins and Increases GluR1 Subunit Surface Delivery in the Spinal Cord Dorsal Horn

Previous studies suggested that postsynaptic neuroligin-2 may shift from inhibitory toward excitatory function under pathological pain conditions. We hypothesize that nerve injury may increase the expression of spinal MAM-domain GPI-anchored molecule 1 (MDGA1), which can bind to neuroligin-2 and thereby, alter its interactions with postsynaptic scaffolding proteins and increase spinal excitatory synaptic transmission, leading to neuropathic pain. Western blot, immunofluorescence staining, and co-immunoprecipitation studies were conducted to examine the critical role of MDGA1 in the lumbar spinal cord dorsal horn in rats after spinal nerve ligation (SNL). Small interfering ribonucleic acids (siRNAs) targeting MDGA1 were used to examine the functional roles of MDGA1 in neuropathic pain. Protein levels of MDGA1 in the ipsilateral dorsal horn were significantly upregulated at day 7 post-SNL, as compared to that in naïve or sham rats. The increased levels of GluR1 in the synaptosomal membrane fraction of the ipsilateral dorsal horn tissues at day 7 post-SNL was normalized to near sham level by pretreatment with intrathecal MDGA1 siRNA2308, but not scrambled siRNA or vehicle. Notably, knocking down MDGA1 with siRNAs reduced the mechanical and thermal pain hypersensitivities, and inhibited the increased excitatory synaptic interaction between neuroligin-2 with PSD-95, and prevented the decreased inhibitory postsynaptic interactions between neuroligin-2 and Gephyrin. Our findings suggest that SNL upregulated MDGA1 expression in the dorsal horn, which contributes to the pain hypersensitivity through increasing the net excitatory interaction mediated by neuroligin-2 and surface delivery of GluR1 subunit in dorsal horn neurons.

NETO2-GluK2 interaction contributes to postoperative pain hypersensitivity through inducing PKCγ activation and synaptic incorporation of AMPA receptor GluR1 subunits in rat dorsal horn

Important roles in the initiation and maintenance of postoperative pain are played by the functional control of kainate (KA) and α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors in the rat dorsal horn (DH). However, the mechanisms underpinning the cross-talk between spinal KA and AMPA receptors in postoperative pain are poorly understood. We hypothesized that after the rat's plantar incision, the synaptic incorporation of AMPA receptor GluR1 subunits in the DH ipsilateral to the incision would increase due to the interaction between GluK2 and neuropilin tolloid-like 2 (NETO2). Our findings showed that incision stimuli caused severe pain responses, as measured by cumulative pain scores. GluK2-NETO2 but not GluK2-NETO1interaction was upregulated in ipsilateral dorsal horn neurons (DHNs) at 6 h post-incision. At 6 h post-incision, NETO2 small interfering ribonucleic acid (siRNA) intrathecal pretreatment increased mechanical withdrawal thresholds to von Freys and decreased ipsilateral paw cumulative pain scores. Further, PKCγactivation and synaptic abundance of GluK2 and GluR1 subunits in the ipsilateral DH were decreased by intrathecal pretreatment with NETO2 siRNA at 6 h post-incision. In conclusion, our findings imply that GluK2-NETO2 interaction could trigger PKCγactivation and the synaptic incorporation of AMPA receptor GluR1 subunits in rat DHs, which in turn led to the enhanced pain hypersensitivity after surgery. It sheds light on the interplay between KA and AMPA receptors in DHNs, which is thought to contribute to postoperative pain.

Upregulation of Hevin contributes to postoperative pain hypersensitivity by inducing neurexin1β/neuroligin1-mediated synaptic targeting of GluA1-containing AMPA receptors in rat dorsal horn

The astrocytes-secreted active molecule, Hevin considerably contributes in the transsynaptic bridge of neurexin1β/neuligin1 in excitatory synapse. Previous studies have demonstrated that activity-dependent synaptic recruitment of spinal neuroligin1 and GluA1-containing AMPA receptors (AMPARs) is involved in incisional, inflammatory and neuropathic pain. Here, we hypothesized that Hevin induced postoperative pain hypersensitivity by enhancing the neurexin1β/neuroligin1-mediated synaptic targeting of GluA1-containing AMPARs in spinal dorsal horns (DH). Our results showed that plantar incision induced significant postoperative pain behavior, which was described by the cumulative pain scores. At 1 d and 3 d post-incision, Hevin expression was considerably elevated in ipsilateral DHs, although it recovered to baseline value at 5 d following the incision. At 1 d post plantar incision, the neurexin1β/neuroligin1 interactions significantly increased in ipsilateral DHs in rats subjected to incision when compared with those in control rats. Intrathecal pretreatments of small interference RNA targeting Hevin substantially suppressed postoperative pain hypersensitivity and reduced the neurexin1β/neurolgin1 interaction as well as the synaptic targeting of GluA1 in ipsilateral spinal DHs. These data suggest that Hevin induced postoperative pain hypersensitivity by enhancing the neurexin1β/neuroligin1 interaction and subsequent synaptic targeting of GluA1-containing AMPARs in ipsilateral spinal DHs. It provides new insights into the role of Hevin-mediated trans-synaptic regulation in postoperative pain hypersensitivity, which would help develop a novel therapeutic strategy.