Animal Studies

The size and modular structure of versican and its gene suggests the existence of multiple splice variants. We have identified, cloned and sequenced a previously unknown exon located within the non-coding gene sequence downstream of exon 8. This exon, which we have named exon 8 specifies two stop-codons. mRNAs of the versican gene with exon 8 are predicted to be constitutively degraded by nonsense mediated RNA decay. Here we tested the hypothesis that these transcripts become expressed in a model of neuropathic pain.

The study reported here investigated the role of the central vascular endothelial growth factor-A (VEGF-A) pathway in the development of trigeminal neuropathic pain following nerve injury. A Sprague-Dawley rat model of trigeminal neuropathic pain was produced using malpositioned dental implants. The left mandibular second molar was extracted under anesthesia and replaced with a miniature dental implant to induce injury to the inferior alveolar nerve. The inferior alveolar nerve injury produced a significant upregulation of astrocytic VEGF-A expression in the medullary dorsal horn. The nerve injury-induced mechanical allodynia was inhibited by an intracisternal infusion of VEGF-A164 antibody. Although both VEGF-A Receptor 1 (VEGF-A R1; colocalized with the blood-brain barrier) and VEGF-A Receptor 2 (VEGF-A R2; colocalized with astrocytes) participated in the development of trigeminal neuropathic pain following nerve injury, only the intracisternal infusion of a VEGF-A R1 antibody, and not that of a VEGF-A R2 antibody, inhibited the increased blood-brain barrier (BBB) permeability produced by nerve injury. Finally, we confirmed the participation of the central VEGF-A pathway in the development of trigeminal neuropathic pain by reducing VEGF-A expression using VEGF-A164 siRNA. This suppression of VEGF-A produced significant prolonged anti-allodynic effects. These results suggest that the central VEGF-A pathway plays a key role in the development of trigeminal neuropathic pain following nerve injury through two separate pathways: VEGF-A R1 and VEGF-A R2. Hence, a blockade of the central VEGF-A pathway provides a new therapeutic avenue for the treatment of trigeminal neuropathic pain.

As a protective mechanism, the cornea is sensitive to noxious stimuli. Here, we show that in mice, a high proportion of corneal TRPM8 cold-sensing fibers express the heat-sensitive TRPV1 channel. Despite its insensitivity to cold, TRPV1 enhances membrane potential changes and electrical firing of TRPM8 neurons in response to cold stimulation. This elevated neuronal excitability leads to augmented ocular cold nociception in mice. In a model of dry eye disease, the expression of TRPV1 in TRPM8 cold-sensing fibers is increased, and results in severe cold allodynia. Overexpression of TRPV1 in TRPM8 sensory neurons leads to cold allodynia in both corneal and non-corneal tissues without affecting their thermal sensitivity. TRPV1-dependent neuronal sensitization facilitates the release of the neuropeptide substance P from TRPM8 cold-sensing neurons to signal nociception in response to cold. Our study identifies a mechanism underlying corneal cold nociception and suggests a potential target for the treatment of ocular pain.

Neurons are an integral component of the immune system that functions to coordinate responses to bacterial pathogens. Sensory nociceptive neurons that can detect bacterial pathogens are found throughout the body with dense innervation of the intestinal tract. Here we assessed the role of these nerves in the coordination of host defenses to Citrobacter rodentium. Selective ablation of nociceptive neurons significantly increased bacterial burden 10 days post infection and delayed pathogen clearance. Since the sensory neuropeptide CGRP regulates host-responses during infection of the skin, lung, and small intestine, we assessed the role of CGRP receptor signaling during C. rodentium infection. Although CGRP receptor blockade reduced certain pro-inflammatory gene expression, bacterial burden and Il-22 expression was unaffected. Our data highlight that sensory nociceptive neurons exert a significant host protective role during C. rodentium infection, independent of CGRP receptor signaling.

Pain sensation and aversive behaviors entail the activation of nociceptor neurons, whose function is largely conserved across animals. The functional heterogeneity of nociceptors and ethical concerns are challenges for their study in mammalian models. Here, we investigate the function of a single type of genetically identified C. elegans thermonociceptor named FLP. Using calcium imaging in vivo, we demonstrate that FLP encodes thermal information in a tonic and graded manner over a wide thermal range spanning from noxious cold to noxious heat (8°C-36°C). This tonic-signaling mode allows FLP to trigger sustained behavioral changes necessary for escape behavior. Furthermore, we identify specific transient receptor potential, voltage-gated calcium, and sodium "leak" channels controlling sensory gain, thermal sensitivity, and signal kinetics, respectively, and show that the ryanodine receptor is required for long-lasting activation. Our work elucidates the task distribution among specific ion channels to achieve remarkable sensory properties in a tonic thermonociceptor in vivo.

N-methyladenosine (mA) modification in RNA has been implicated in diverse biological processes. However, very little is currently known about its role in nociceptive modulation. Here, we found that the level of spinal mA modification was significantly increased in a mouse model of Complete Freund's Adjuvant (CFA)-induced chronic inflammatory pain, which was accompanied with the augmentation of methyltransferase-like 3 (METTL3) expression in the spinal cord. Knockdown of spinal METTL3 prevented and reversed CFA-induced pain behaviors and spinal neuronal sensitization. In contrast, overexpression of spinal METTL3 produced pain behaviors and neuronal sensitization in naive mice. Moreover, we found that METTL3 positively modulated the pri-miR-65-3p processing in a microprocessor protein DiGeorge critical region 8-dependent manner. Collectively, our findings reveal an important role of METTL3-mediated mA modification in nociceptive sensitization and provide a novel perspective on mA modification in the development of pathological pain.

Understanding how the nervous system bridges sensation and behavior requires the elucidation of complex neural and molecular networks. Forward genetic approaches, such as screens conducted in C. elegans, have successfully identified genes required to process natural sensory stimuli. However, functional redundancy within the underlying neural circuits, which are often organized with multiple parallel neural pathways, limits our ability to identify 'neural pathway-specific genes', i.e. genes that are essential for the function of some, but not all of these redundant neural pathways. To overcome this limitation, we developed a 'forward optogenetics' screening strategy in which natural stimuli are initially replaced by the selective optogenetic activation of a specific neural pathway. We used this strategy to address the function of the polymodal FLP nociceptors mediating avoidance of noxious thermal and mechanical stimuli. According to our expectations, we identified both mutations in 'general' avoidance genes that broadly impact avoidance responses to a variety of natural noxious stimuli (unc-4, unc-83, and eat-4) and mutations that produce a narrower impact, more restricted to the FLP pathway (syd-2, unc-14 and unc-68). Through a detailed follow-up analysis, we further showed that the Ryanodine receptor UNC-68 acts cell-autonomously in FLP to adjust heat-evoked calcium signals and aversive behaviors. As a whole, our work (i) reveals the importance of properly regulated ER calcium release for FLP function, (ii) provides new entry points for new nociception research and (iii) demonstrates the utility of our forward optogenetic strategy, which can easily be transposed to analyze other neural pathways.