Article

A Role for Polymodal C-Fiber Afferents in Nonhistaminergic Itch

Department of Neurosurgery, Johns Hopkins University, Baltimore, Maryland 21287, USA.
The Journal of Neuroscience : The Official Journal of the Society for Neuroscience (Impact Factor: 6.34). 07/2008; 28(30):7659-69. DOI: 10.1523/JNEUROSCI.1760-08.2008
Source: PubMed

ABSTRACT

Recent psychophysical and electrophysiological studies in humans suggest the existence of two peripheral pathways for itch, one that is responsive to histamine and a second pathway that can be activated by nonhistaminergic pruritogens (e.g., cowhage spicules). To explore the peripheral neuronal pathway for nonhistaminergic itch, behavioral responses and neuronal activity in unmyelinated afferent fibers were assessed in monkey after topical application of cowhage spicules or intradermal injection of histamine and capsaicin. Cowhage and histamine, but not capsaicin, evoked scratching behavior indicating the presence of itch. In single-fiber recordings, cowhage, histamine and/or capsaicin were applied to the cutaneous receptive field of 43 mechano-heat-sensitive C-fiber (CMH) nociceptors. The majority of CMHs exhibited a prolonged response to cowhage (39 of 43) or histamine (29 of 38), but not to capsaicin (3 of 34). Seven CMHs were activated by cowhage but not histamine. The average response to cowhage was more than twice the response to histamine, and responses were not correlated. The response of the CMHs to a stepped heat stimulus (49 degrees C, 3 s) was either quickly adapting (QC) or slowly adapting (SC). In contrast, the cowhage response was characterized by bursts of two or more action potentials (at approximately 1 Hz). The total cowhage response of the QC fibers (97 action potentials/5 min) was twice that of the SC fibers (49 action potentials/5 min). A subset of QC fibers exhibited high-frequency intraburst discharges ( approximately 30 Hz). These results suggest multiple mechanisms by which CMHs may encode itch to cowhage as well as pain to mechanical and heat stimuli.

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    • "Recent evidence suggests that chemical stimuli that elicit acute itch in humans and animals evoke action potentials in a specific subset of cutaneous nociceptors. These nociceptors, termed 'pruriceptive' because they respond to a pruritic agent, also respond to one or more noxious stimuli (Schmelz et al., 2003; Johanek et al., 2008; Ringkamp et al., 2011; Liu et al., 2012; Han et al., 2013). However, there is little information about whether activity in pruriceptive neurons might be abnormally generated during disorders that cause chronic itch. "
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    ABSTRACT: Itch is a common symptom of diseases of the skin but can also accompany diseases of other tissues including the nervous system. Acute itch from chemicals experimentally applied to the skin is initiated and maintained by action potential activity in a subset of nociceptive neurons. But whether these pruriceptive neurons are active or might become intrinsically more excitable under the pathological conditions that produce persistent itch and nociceptive sensations in humans is largely unexplored. Recently, two distinct types of cutaneous nociceptive dorsal root ganglion neurons were identified as responding to pruritic chemicals and playing a role in itch sensation. One expressed the mas-related G-coupled protein receptor MRGPRA3 and the other MRGPRD (MRGPRA3(+) and MRGPRD(+) neurons, respectively). Here we tested whether these two distinct pruriceptive nociceptors exhibited an enhanced excitability after the development of contact hypersensitivity, an animal model of allergic contact dermatitis, a common pruritic disorder in humans. The characteristics of increased excitability of pruriceptive neurons during this disorder may also pertain to the same types of neurons active in other pruritic diseases or pathologies that affect the nervous system and other tissues or organs. We found that challenging the skin of the calf of the hind paw or the cheek of previously sensitized mice with the hapten, squaric acid dibutyl ester, produced symptoms of contact hypersensitivity including an increase in skin thickness and site-directed spontaneous pain-like (licking or wiping) and itch-like (biting or scratching) behaviours. Ablation of MRGPRA3(+) neurons led to a significant reduction in spontaneous scratching of the hapten-challenged nape of the neck of previously sensitized mice. In vivo, electrophysiological recordings revealed that MRGPRA3(+) and MRGPRD(+) neurons innervating the hapten-challenged skin exhibited a greater incidence of spontaneous activity and/or abnormal after-discharges in response to mechanical and heat stimuli applied to their receptive fields compared with neurons from the vehicle-treated control animals. Whole-cell recordings in vitro showed that both MRGPRA3(+) and MRGPRD(+) neurons from hapten-challenged mice displayed a significantly more depolarized resting membrane potential, decreased rheobase, and greater number of action potentials at twice rheobase compared with neurons from vehicle controls. These signs of neuronal hyperexcitability were associated with a significant increase in the peak amplitude of tetrodotoxin-sensitive and resistant sodium currents. Thus, the hyperexcitability of MRGPRA3(+) and MRGPRD(+) neurons, brought about in part by enhanced sodium currents, may contribute to the spontaneous itch- and pain-related behaviours accompanying contact hypersensitivity and/or other inflammatory diseases in humans.
    Full-text · Article · Feb 2014 · Brain
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    • "Histamine-independent itch was first reported in 1953; papain and cowhage spicules were shown to induce the itch sensation [10]. The papain and cowhage spicules both activate polymodal C-fibers, which are in charge of pain sensation under mechanical and thermal stimuli as well [11]. The receptor target in this case is likely to be proteinase-activated receptor 2 (PAR2) [12], which can be activated by mast cell tryptase released from mast cells in both rat and human skin [13, 14]. "
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    ABSTRACT: This study investigated whether immediate acupuncture effects in the acupoint are histamine dependent. Both histamine injection and manual acupuncture stimulation increased the pain threshold (PT) after treatment compared with the model group (P < 0.01), producing an analgesic effect. After pretreatment with clemastine, an H1 receptor antagonist and an antipruritic, the increase in the animals' pain threshold after acupuncture was suppressed compared with the Acu group (P < 0.01); however, there was no interference with the acupuncture-induced degranulation of mast cells. Pretreatment with disodium cromolyn did not suppress the increase in PT induced by the histamine injection at Zusanli (ST-36). We conclude that in adjuvant-induced arthritic rats, acupuncture analgesic effects are histamine dependent, and this histamine dependence determines the acupoint preference of acupoints away from the target site in acupuncture practice.
    Full-text · Article · Nov 2012 · Evidence-based Complementary and Alternative Medicine
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    • "Additionally, the possibility of pruriceptive or nociceptive information coded within the timing dynamics of the spike train of spinal neurons is unresolved. Interestingly, in response to histamine and cowhage, peripheral C fibers in the monkey were found to fire in a bursting pattern (Johanek et al. 2008). Here we show that pruriceptive STT neurons fired in a bursting pattern to both pruritogens and also to a subsequent application of capsaicin. "
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    ABSTRACT: Itch of peripheral origin requires information transfer from the spinal cord to the brain for perception. Here, primate spinothalamic tract (STT) neurons from lumbar spinal cord were functionally characterized by in vivo electrophysiology to determine the role of these cells in the transmission of pruriceptive information. One hundred eleven STT neurons were identified by antidromic stimulation and then recorded while histamine and cowhage (a nonhistaminergic pruritogen) were sequentially applied to the cutaneous receptive field of each cell. Twenty percent of STT neurons responded to histamine, 13% responded to cowhage, and 2% responded to both. All pruriceptive STT neurons were mechanically sensitive and additionally responded to heat, intradermal capsaicin, or both. STT neurons located in the superficial dorsal horn responded with greater discharge and longer duration to pruritogens than STT neurons located in the deep dorsal horn. Pruriceptive STT neurons discharged in a bursting pattern in response to the activating pruritogen and to capsaicin. Microantidromic mapping was used to determine the zone of termination for pruriceptive STT axons within the thalamus. Axons from histamine-responsive and cowhage-responsive STT neurons terminated in several thalamic nuclei including the ventral posterior lateral, ventral posterior inferior, and posterior nuclei. Axons from cowhage-responsive neurons were additionally found to terminate in the suprageniculate and medial geniculate nuclei. Histamine-responsive STT neurons were sensitized to gentle stroking of the receptive field after the response to histamine, suggesting a spinal mechanism for alloknesis. The results show that pruriceptive information is encoded by polymodal STT neurons in histaminergic or nonhistaminergic pathways and transmitted to the ventrobasal complex and posterior thalamus in primates.
    Preview · Article · Jun 2012 · Journal of Neurophysiology
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