Probes for INS

Primary afferents are responsible for transmitting signals produced by noxious stimuli from the periphery to the spinal cord. Mu and delta opioid receptors (MOP and DOP) have analgesic properties and are highly expressed in dorsal root ganglia (DRG) neurons. In humans, spinal DOP is almost exclusively located on central terminals of DRG neurons, whereas in rodents, it is expressed both on presynaptic terminals and spinal neurons. In this study, we aimed to assess the distribution of MOP and DOP in the DRGs of mice and rats. Using in situ hybridization and immunofluorescence, we visualized MOP and DOP mRNA together with various neuronal markers. In rats and mice, we show that both receptors are expressed, albeit to different extents, in all types of neurons, namely, large and medium myelinated neurons (NF200-positive), small nonpeptidergic (IB4- or P2X3R-positive) and peptidergic C fibres (Tac1-positive). Overall, DOP mRNA was found to be mainly expressed in large and medium myelinated neurons, whereas MOP mRNA was mainly found in C fibres. The distribution of MOP and DOP, however, slightly differs between rats and mice, with a higher proportion of small nonpeptidergic C fibres expressing DOP mRNA in mice than in rats. We further found that neither morphine nor inflammation affected the distribution of the receptor mRNA. Because of their location, our results confirm that MOP and DOP have the potential to alleviate similar types of pain and that this effect could slightly differ between species.

Neuropathic pain affects up to 10% of the total population and no specific target is ideal for therapeutic need. The sodium leak channel (NALCN), a non-selective cation channel, mediates the background Na+ leak conductance and controls neuronal excitability and rhythmic behaviors. Here, we show that increases of NALCN expression and function in dorsal root ganglion (DRG) and dorsal spinal cord contribute to chronic constriction injury (CCI)-induced neuropathic pain in rodents. NALCN current and neuronal excitability in acutely isolated DRG neurons and spinal cord slices of rats were increased after CCI which were decreased to normal levels by NALCN-siRNA. Accordingly, pain-related symptoms were significantly alleviated by NALCN-siRNA-mediated NALCN knockdown and completely prevented by NALCN-shRNA-mediated NALCN knockdown in rats or by conditional NALCN knockout in mice. Our results indicate that increases in NALCN expression and function contribute to CCI-induced neuronal sensitization; therefore, NALCN may be a novel molecular target for control of neuropathic pain.

Peripheral neuropathic pain induced by the chemotherapeutic cisplatin can persist for months to years after treatment. Histone deacetylase 6 (HDAC6) inhibitors have therapeutic potential for cisplatin-induced neuropathic pain since they persistently reverse mechanical hypersensitivity and spontaneous pain in rodent models. Here, we investigated the mechanisms underlying reversal of mechanical hypersensitivity in male and female mice by a two-week treatment with an HDAC6 inhibitor, administered 3 days after the last dose of cisplatin. Mechanical hypersensitivity in animals of both sexes treated with the HDAC6 inhibitor was temporarily reinstated by a single injection of the neutral opioid receptor antagonist 6β-naltrexol or the peripherally restricted opioid receptor antagonist naloxone methiodide. These results suggest that tonic peripheral opioid ligand-receptor signaling mediates reversal of cisplatin-induced mechanical hypersensitivity after treatment with an HDAC6 inhibitor. Pointing to a specific role for delta opioid receptors (DORs), Oprd1 expression was decreased in dorsal root ganglion neurons following cisplatin administration, but normalized after treatment with an HDAC6 inhibitor. Mechanical hypersensitivity was temporarily reinstated in both sexes by a single injection of the DOR antagonist naltrindole. Consistently, HDAC6 inhibition failed to reverse cisplatin-induced hypersensitivity when DORs were genetically deleted from advillin+ neurons. Mechanical hypersensitivity was also temporarily reinstated in both sexes by a single injection of a neutralizing antibody against the DOR ligand met-enkephalin. In conclusion, we reveal that treatment with an HDAC6 inhibitor induces tonic enkephalin-DOR signaling in peripheral sensory neurons to suppress mechanical hypersensitivity.SIGNIFICANCE STATEMENT:Over a quarter of cancer survivors suffer from intractable painful chemotherapy-induced peripheral neuropathy (CIPN), which can last for months to years after treatment ends. HDAC6 inhibition is a novel strategy to reverse CIPN without negatively interfering with tumor growth, but the mechanisms responsible for persistent reversal are not well understood. We built on evidence that the endogenous opioid system contributes to the spontaneous, apparent resolution of pain caused by nerve damage or inflammation, referred to as latent sensitization. We show that blocking the delta opioid receptor or its ligand enkephalin unmasks CIPN in mice treated with an HDAC6 inhibitor (latent sensitization). Our work provides insight into the mechanisms by which treatment with an HDAC6 inhibitor apparently reverses CIPN.

Sensory neurons of the dorsal root ganglion (DRG) are critical for maintaining tissue homeostasis by sensing and initiating responses to stimuli. While most preclinical studies of DRGs are conducted in rodents, much less is known about the mechanisms of sensory perception in primates. We generated a transcriptome atlas of mouse, guinea pig, cynomolgus monkey, and human DRGs by implementing a common laboratory workflow and multiple data-integration approaches to generate high-resolution cross-species mappings of sensory neuron subtypes. Using our atlas, we identified conserved core modules highlighting subtype-specific biological processes related to inflammatory response. We also identified divergent expression of key genes involved in DRG function, suggesting species-specific adaptations specifically in nociceptors that likely point to divergent function of nociceptors. Among these, we validated that TAFA4, a member of the druggable genome, was expressed in distinct populations of DRG neurons across species, highlighting species-specific programs that are critical for therapeutic development.