IL-33 signaling in sensory neurons promotes dry skin itch.

BACKGROUND: Chronic pruritus, or itch, is common and debilitating, but the neuroimmune mechanisms that drive chronic itch are only starting to be elucidated. Recent studies demonstrate that the IL-33 receptor (IL-33R) is expressed by sensory neurons. However, whether sensory neuron-restricted activity of IL-33 is necessary for chronic itch remains poorly understood. OBJECTIVES: We sought to determine if IL-33 signaling in sensory neurons is critical for the development of chronic itch in 2 divergent pruritic disease models. METHODS: Plasma levels of IL-33 were assessed in patients with atopic dermatitis (AD) and chronic pruritus of unknown origin (CPUO). Mice were generated to conditionally delete IL-33R from sensory neurons. The contribution of neuronal IL-33R signaling to chronic itch development was tested in mouse models that recapitulate key pathologic features of AD and CPUO, respectively. RESULTS: IL-33 was elevated in both AD and CPUO as well as their respective mouse models. While neuron-restricted IL-33R signaling was dispensable for itch in AD-like disease, it was required for the development of dry skin itch in a mouse model that mirrors key aspects of CPUO pathology. CONCLUSIONS: These data highlight how IL-33 may be a predominant mediator of itch in certain contexts, depending on the tissue microenvironment. Further, this study provides insight into future therapeutic strategies targeting the IL-33 pathway for chronic itch.

Early Life Nociception is Influenced by Peripheral Growth Hormone Signaling.

A number of cellular systems work in concert to modulate nociceptive processing in the periphery, but the mechanisms that regulate neonatal nociception may be distinct compared with adults. Our previous work indicated a relationship between neonatal hypersensitivity and growth hormone (GH) signaling. Here, we explored the peripheral mechanisms by which GH modulated neonatal nociception under normal and injury conditions (incision) in male and female mice. We found that GH receptor (GHr) signaling in primary afferents maintains a tonic inhibition of peripheral hypersensitivity. After injury, a macrophage dependent displacement of injury-site GH was found to modulate neuronal transcription at least in part via serum response factor (SRF) regulation. A single GH injection into the injured hindpaw muscle effectively restored available GH signaling to neurons and prevented acute pain-like behaviors, primary afferent sensitization, and neuronal gene expression changes. GH treatment also inhibited long-term somatosensory changes observed after repeated peripheral insult. Results may indicate a novel mechanism of neonatal nociception.SIGNIFICANCE STATEMENT Although it is noted that mechanisms of pain development in early life are unique compared with adults, little research focuses on neonatal-specific peripheral mechanisms of nociception. This gap is evident in the lack of specialized care for infants following an injury including surgeries. This report evaluates how distinct cellular systems in the periphery including the endocrine, immune and nervous systems work together to modulate neonatal-specific nociception. We uncovered a novel mechanism by which muscle injury induces a macrophage-dependent sequestration of peripheral growth hormone (GH) that effectively removes its normal tonic inhibition of neonatal nociceptors to promote acute pain-like behaviors. Results indicate a possible new strategy for treatment of neonatal postsurgical pain.

Cannabinoid Receptor 1 Expression in Human Dorsal Root Ganglia and CB13-Induced Bidirectional Modulation of Sensory Neuron Activity.

Cannabinoid receptors have been identified as potential targets for analgesia from studies on animal physiology and behavior, and from human clinical trials. Here, we sought to improve translational understanding of the mechanisms of cannabinoid-mediated peripheral analgesia. Human lumbar dorsal root ganglia were rapidly recovered from organ donors to perform physiological and anatomical investigations into the potential for cannabinoids to mediate analgesia at the level of the peripheral nervous system. Anatomical characterization of in situ gene expression and immunoreactivity showed that 61 and 53% of human sensory neurons express the CB1 gene and receptor, respectively. Calcium influx evoked by the algogen capsaicin was measured by Fura-2AM in dissociated human sensory neurons pre-exposed to the inflammatory mediator prostaglandin E2 (PGE2) alone or together with CB13 (1 µM), a cannabinoid agonist with limited blood-brain barrier permeability. Both a higher proportion of neurons and a greater magnitude of response to capsaicin were observed after exposure to CB13, indicating cannabinoid-mediated sensitization. In contrast, membrane properties measured by patch-clamp electrophysiology demonstrated that CB13 suppressed excitability and reduced action potential discharge in PGE2-pre-incubated sensory neurons, suggesting the suppression of sensitization. This bidirectional modulation of sensory neuron activity suggests that cannabinoids may suppress overall membrane excitability while simultaneously enhancing responsivity to TRPV1-mediated stimuli. We conclude that peripherally restricted cannabinoids may have both pro- and anti-nociceptive effects in human sensory neurons.

Systemic growth hormone deficiency causes mechanical and thermal hypersensitivity during early postnatal development.

Injury during early postnatal life causes acute alterations in afferent function and DRG gene expression, which in addition to producing short-term sensitivity has the potential to influence nociceptive responses in adulthood. We recently discovered that growth hormone (GH) is a key regulator of afferent sensitization and pain-related behaviors during developmental inflammation of the skin. Peripheral injury caused a significant reduction in cutaneous GH levels, which corresponded with the observed hypersensitivity. However, it has yet to be determined whether GH deficiency (GHD) is sufficient to drive peripheral sensitization in uninjured animals. Here, we found that systemic GHD, induced by knockout of the GH release hormone receptor (GHRHr), was able to induce behavioral and afferent hypersensitivity to peripheral stimuli specifically during early developmental stages. GHD also produced an upregulation of many receptors and channels linked to nociceptive processing in the DRGs at these early postnatal ages (P7 and P14). Surprisingly, P21 GHRHr knockouts also displayed significant alterations in DRG gene expression even though behavioral and afferent hypersensitivity resolved. These data support previous findings that GH is a key modulator of neonatal hypersensitivity. Results may provide insight into whether GH treatment may be a therapeutic strategy for pediatric pain.

Interleukin 1ß inhibition contributes to the antinociceptive effects of voluntary exercise on ischemia/reperfusion-induced hypersensitivity.

Issues of peripheral circulation have been increasingly suggested as an underlying cause of musculoskeletal pain in many conditions, including sickle cell anemia and peripheral vascular disease. We have previously shown in our model of transient ischemia and reperfusion (I/R) injury of the forelimb that individual group III and IV muscle afferents display altered chemosensitivity and mechanical thresholds 1 day after injury. Functional alterations corresponded to increased evoked and spontaneous pain-related behaviors and decreased muscle strength and voluntary activity-all actions that echo clinical symptoms of ischemic myalgia. These behavioral and physiological changes appeared to originate in part from the action of increased interleukin 1ß (IL1ß) in the injured muscles at its upregulated IL1 receptor 1 within the dorsal root ganglion. Here, we describe that two days of voluntary wheel running prior to I/R blocks both injury-induced IL1ß enhancement and the subsequent development of ischemic myalgia-like behaviors. Furthermore, the protective effects of 2 days prior exercise on the I/R-evoked increases in pain-related behaviors were also paralleled with systemic injection of the IL1 receptor antagonist during I/R. Interleukin 1 receptor antagonist treatment additionally prevented the I/R-induced changes in mechanical and chemical sensitivity of individual primary muscle afferents. Altogether, these data strengthen the evidence that transient I/R injury sensitizes group III and IV muscle afferents via increased IL1ß in the muscles to stimulate ischemic myalgia development. Targeting IL1ß may, therefore, be an effective treatment strategy for this insidious type of muscle pain.

Upregulation of P2Y1 in neonatal nociceptors regulates heat and mechanical sensitization during cutaneous inflammation.

Abstract: The upregulation of various channels and receptors classically linked to sensory transduction from the periphery tightly correspond with changes in the responsiveness of specific subpopulations of primary afferents to mechanical and heat stimulation of the skin at different ages. Previous reports in adults have suggested that the purinergic adenosine diphosphate receptor, P2Y1 can specifically regulate sensory neuron responsiveness to heat stimuli in addition to neurochemical alterations in primary afferents during cutaneous inflammation. To determine if the upregulation of P2Y1 found in the dorsal root ganglia of neonatal mice with cutaneous inflammation initiated at postnatal day 7 (P7) was responsible for the specific alteration in heat sensitivity found in faster conducting ("A"-fiber) nociceptors, we assessed the response properties of cutaneous afferents using an ex vivo hairy hindpaw skin-saphenous nerve-dorsal root ganglion-spinal cord preparation in conjunction with nerve-targeted knockdown of P2Y1. We found that P2Y1 knockdown during neonatal cutaneous inflammation was sufficient to reduce the sensitization of "A"-fiber nociceptors to heat stimuli. Surprisingly, we also found that nerve-specific downregulation of P2Y1 could reduce the observed sensitization of these afferent subtypes to mechanical deformation of the skin. Immunocytochemical analysis of dorsal root ganglia showed that P2Y1 may mediate its effects through modulation of the injury-induced increase of transient receptor potential vanilloid type 1 receptor. This suggests that the upregulation of P2Y1 in cutaneous nociceptors during early life peripheral inflammation can regulate the sensitization of myelinated nociceptors to both mechanical and heat stimuli possibly through modulation of transient receptor potential vanilloid type 1 expression.

Growth hormone regulates the sensitization of developing peripheral nociceptors during cutaneous inflammation.

Cutaneous inflammation alters the function of primary afferents and gene expression in the affected dorsal root ganglia (DRG). However, specific mechanisms of injury-induced peripheral afferent sensitization and behavioral hypersensitivity during development are not fully understood. Recent studies in children suggest a potential role for growth hormone (GH) in pain modulation. Growth hormone modulates homeostasis and tissue repair after injury, but how GH affects nociception in neonates is not known. To determine whether GH played a role in modulating sensory neuron function and hyperresponsiveness during skin inflammation in young mice, we examined behavioral hypersensitivity and the response properties of cutaneous afferents using an ex vivo hairy skin-saphenous nerve-DRG-spinal cord preparation. Results show that inflammation of the hairy hind paw skin initiated at either postnatal day 7 (P7) or P14 reduced GH levels specifically in the affected skin. Furthermore, pretreatment of inflamed mice with exogenous GH reversed mechanical and thermal hypersensitivity in addition to altering nociceptor function. These effects may be mediated through an upregulation of insulin-like growth factor 1 receptor (IGFr1) as GH modulated the transcriptional output of IGFr1 in DRG neurons in vitro and in vivo. Afferent-selective knockdown of IGFr1 during inflammation also prevented the observed injury-induced alterations in cutaneous afferents and behavioral hypersensitivity similar to that after GH pretreatment. These results suggest that GH can block inflammation-induced nociceptor sensitization during postnatal development leading to reduced pain-like behaviors, possibly by suppressing the upregulation of IGFr1 within DRG.