gastrin-releasing-peptide and Neuralgia

Neuropathic itch is clinically important but has received much less attention as compared to neuropathic pain. In the past decade, itch-specific pathways have been characterized on a cellular and molecular level, but their exact role in the pathophysiology of neuropathic itch is still unclear. Traditionally, mutually exclusive theories for itch such as labeled line, temporal/spatial pattern, or intensity theory have been proposed, and experimental studies in mice mainly favor the specificity theory of itch. By contrast, results in humans also suggest a role for spatial and temporal patterns in neuropathic itch. Rarefication of skin innervation in neuropathy could provide a "spatial contrast" discharge pattern, and axotomy could induce de novo expression of the itch-specific spinal neuropeptide, gastrin-releasing peptide, in primary afferent nociceptors, thereby modulating itch processing in the dorsal horn. Thus, clinical neuropathy may generate itch by changes in the spatial and temporal discharge patterns of nociceptors, hijacking the labeled line processing of itch and abandoning the canonical scheme of mutual exclusive itch theories. Moreover, the overlap between itch and pain symptoms in neuropathy patients complicates direct translation from animal experiments and, on a clinical level, necessitates collaboration between medical specialities, such as dermatologists, anesthesiologists, and neurologists.

Topics: Animals; Gastrin-Releasing Peptide; Humans; Neuralgia; Nociceptors; Pruritus

Itch and pain are intimately related and may share similar peripheral and central mechanisms and pathways. However, it has been believed that synaptic glutamate release from a group of peripheral nociceptors is required to sense pain and suppress itch. Although we previously demonstrated that phosphorylation of GluN2B subunits of the NMDA receptor at Tyr1472 is important for central sensitization in a neuropathic pain model of mice with a knock-in mutation of the Tyr1472 site to phenylalanine of GluN2B (Y1472F-KI), the role of NMDA receptors in itch transmission remains unknown. Here, we demonstrated that the scratching behaviors elicited by various pruritogens applied to the cheek and c-fos expression in the region innervated by the trigeminal nerve were markedly attenuated in the Y1472F-KI mice. The c-fos immunoreactivity was co-localized with the receptor of gastrin-releasing peptide (GRP). Scratching behaviors evoked by chloroquine were inhibited by the NMDA receptor antagonists D-AP5 and CP101,606 and by the Src kinase inhibitor PP2. Direct activation of the trigeminal region by intracisternal administration of NMDA and GRP induced robust scratching behaviors, both of which were reduced by the GRP receptor antagonist RC-3095. Taken together, the data obtained in this present study are the first to demonstrate that phosphorylation of GluN2B subunit at Tyr1472 is important for trigeminal transmission of itch and suggest that the NMDA receptor activation occurs upstream of the GRP-GRP receptor pathway.

Topics: Animals; Behavior, Animal; Gastrin-Releasing Peptide; Glutamic Acid; Mice; Neuralgia; Phosphorylation; Pruritus; Receptors, Bombesin; Receptors, N-Methyl-D-Aspartate; Signal Transduction; Spinal Cord

Activated mammalian target of rapamycin (P-mTOR) has been shown to maintain the sensitivity of subsets of small-diameter primary afferent A-nociceptors. Local or systemic inhibition of the mTOR complex 1 (mTORC1) pathway reduced punctate mechanical and cold sensitivity in neuropathic pain and therefore offered a new approach to chronic pain control. In this study, we have investigated the effects of the rapamycin analog temsirolimus (CCI-779) on itch. Bouts of scratching induced by the histamine-dependent pruritogenic compound 48/80 and histamine-independent pruritogens, chloroquine and SLIGRL-NH2, injected intradermally were significantly reduced by local (intradermal) or systemic (intraperitoneal, i.p.) pretreatment with CCI-779. We also investigated the action of metformin, a drug taken to control type 2 diabetes and recently shown to inhibit mTORC1 in vivo. Although the response to nonhistaminergic stimuli was reduced at all of the time points tested, scratching to compound 48/80 was modified by metformin only when the drug was injected 24 hours before this pruritogen. We also examined the colocalization of P-mTOR with gastrin-releasing peptide, a putative marker for some itch-sensitive primary afferents, and found that P-mTOR was coexpressed in less than 5% of gastrin-releasing peptide-positive fibers in the mouse skin. Taken together, the data highlight the role that P-mTOR-positive A-fibers play in itch signaling and underline the importance of the mTORC1 pathway in the regulation of homeostatic primary afferent functions such as pain and itch. The actions of the antidiabetic drug metformin in ameliorating nonhistamine-mediated itch also suggest a new therapeutic route for the control of this category of pruritus.

Topics: Animals; Disease Models, Animal; Gastrin-Releasing Peptide; Histamine; Hypoglycemic Agents; Male; Mechanistic Target of Rapamycin Complex 1; Metformin; Mice; Mice, Inbred C57BL; Multiprotein Complexes; Neuralgia; Phosphoproteins; Protein Kinase Inhibitors; Pruritus; Signal Transduction; Sirolimus; Skin; TOR Serine-Threonine Kinases; Treatment Outcome