Recent findings suggest that itch produced by intradermal insertion of cowhage spicules in human is histamine independent. Neuronal mechanisms underlying nonhistaminergic itch are poorly understood. To investigate which nerve fibers mediate cowhage induced itch in man, action potentials were recorded from cutaneous C-fibers of the peroneal nerve in healthy volunteers using microneurography. Mechano-responsive and -insensitive C-nociceptors were tested for their responsiveness to cowhage spicules, histamine, and capsaicin. Cowhage spicules induced itching and activated all tested mechano-responsive C-units (24/24, but no mechano-insensitive C-fibers (0/17). Histamine also induced itch, but in contrast to cowhage, it caused lasting activation only in mechano-insensitive units (8/12). In mechano-responsive C-units, histamine caused no or only short and weak responses unrelated to the time course of itching. Capsaicin injections activated four of six mechano-responsive fibers and three of four mechano-insensitive C-fibers. Cowhage and histamine activate distinctly different nonoverlapping populations of C-fibers while inducing similar sensations of itch. We hypothesize that cowhage activates a pathway for itch that originates peripherally from superficial mechano-responsive (polymodal) C-fibers and perhaps other afferent units. It is distinct from the pathway for histamine-mediated pruritus and does not involve the histamine-sensitive mechano-insensitive fibers.

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Infusion of the chemotherapeutic agent oxaliplatin leads to an acute and a chronic form of peripheral neuropathy. Acute oxaliplatin neuropathy is characterized by sensory paresthesias and muscle cramps that are notably exacerbated by cooling. Painful dysesthesias are rarely reported for acute oxaliplatin neuropathy, whereas a common symptom of chronic oxaliplatin neuropathy is pain. Here we examine the role of the sodium channel isoform Na V 1.6 in mediating the symptoms of acute oxaliplatin neuropathy. Compound and single-action potential recordings from human and mouse peripheral axons showed that cooling in the presence of oxaliplatin (30-100 μM; 90 min) induced bursts of action potentials in myelinated A, but not unmyelinated C-fibers. Whole-cell patch-clamp recordings from dissociated dorsal root ganglion (DRG) neurons revealed enhanced tetrodotoxin-sensitive resurgent and persistent current amplitudes in large, but not small, diameter DRG neurons when cooled (22°C) in the presence of oxaliplatin. In DRG neurons and peripheral myelinated axons from Scn8a med/med mice, which lack functional Na V 1.6, no effect of oxaliplatin and cooling was observed. Oxaliplatin significantly slows the rate of fast inactivation at negative potentials in heterologously expressed mNa V 1.6r in ND7 cells, an effect consistent with prolonged Na V open times and increased resurgent and persistent current in native DRG neurons. This finding suggests that Na V 1.6 plays a central role in mediating acute cooling-exacerbated symptoms following oxaliplatin, and that enhanced resurgent and persistent sodium currents may provide a general mechanistic basis for cold-aggravated symptoms of neuropathy.chemotherapy | peripheral nerve | abnormal axonal excitability | repetitive action potential discharge C linical use of the highly effective chemotherapeutic oxaliplatin is compromised by an acute and a chronic form of peripheral neuropathy. Acutely, 85-90% of patients exhibit muscle fasciculations (1, 2), sensory paresthesias, and occasional dysesthesias (3), all triggered by mild cooling. Although chronic oxaliplatin-induced neuropathy has been recently linked to changes in the expression and sensitivity of transient receptor potential (TRP) channels TRPM8 and TRPA1 (4, 5), two-pore domain potassium channels (TREK1, TRAAK) and the hyperpolarization-activated channel HCN1 (6), the mechanism underlying acute oxaliplatin neuropathy remains unresolved. Several candidate mechanisms have been proposed including potassium channel blockade (7), calcium chelation (8), and alterations in voltage-gated sodium channel (Na V ) kinetics (9, 10), but none adequately account for motor and sensory symptoms nor their exacerbation by cooling.