Activation of superficial dorsal horn neurons in the mouse by a PAR-2 agonist and 5-HT: Potential role in itch

Itch, an unpleasant sensation associated with the desire to scratch, is symptomatic of dermatologie and systemic disorders that often resist antihistamine treatment. Histamine-independent itch mediators include serotonin (5-HT) and agonists of the protease-activated receptor-2 (PAR-2). We used behavior, Fos immunohistochemistry, and electrophysiology to investigate if these mediators activate spinal dorsal horn neurons in a manner consistent with itch. Intradermal (id) injection of the PAR-2 agonist SLIGRL-NH ₂ in the rostral back evoked bouts of directed hindlimb scratches over 20-30 min. Hindpaw injection of SLIGRL-NH ₂ produced Fos staining in superficial dorsal horn which was then targeted for single-unit recording. Small id microinjections of SLIGRL-NH ₂ or 5-HT identified responsive single units in the superficial dorsal horn of mice anesthetized with pentobarbital. Thirty-eight units characterized as wide dynamic range, nociceptive specific, or mechanically insensitive exhibited significantly increased firing after id SLIGRL-NH ₂ for 9 min, to partial (25%) tachyphylaxis with repeated injection. A majority additionally responded to 5-HT (70%), mustard oil (79%), and capsaicin (71%). Seven units isolated with the 5-HT search stimulus exhibited significant and prolonged responses to 5-HT with tachyphylaxis to repeated injections. The majority also responded to SLIGRL-NH ₂, mustard oil, and capsaicin. The prolonged responses of superficial dorsal horn neurons to SLIGRL-NH ₂ and 5-HT suggest a role in signaling itch. However, their responsiveness to algogens is inconsistent with itch specificity. Alternatively, such neurons may signal itch, whereas noxious stimulus levels recruit these and a larger population of pruritogen-insensitive cells to signal pain which masks or occludes the itch signal.