Deletion of the preprotachykinin A gene in mice does not reduce scratching behavior elicited by intradermal serotonin

Automated acoustic detection of mouse scratching

Itch is an aversive somatic sense that elicits the desire to scratch. In animal models of itch, scratching behavior is frequently used as a proxy for itch, and this behavior is typically assessed through visual quantification. However, manual scoring of videos has numerous limitations, underscoring the need for an automated approach. Here, we propose a novel automated method for acoustic detection of mouse scratching. Using this approach, we show that chloroquine-induced scratching behavior in C57BL/6 mice can be quantified with reasonable accuracy (85% sensitivity, 75% positive predictive value). This report is the first method to apply supervised learning techniques to automate acoustic scratch detection.

Transmission of pruriceptive signals

In this chapter we discuss the many recent discoveries of the mechanisms by which itch is transmitted: the neurotransmitters and the responses they trigger, the mechanisms by which specific neuronal targets are activated, and the specificity of the pathways. Current data reveal that DRG neurons and spinal cord cells use a remarkably selective set of transmitters to convey pruritic information from the periphery to the brain: glutamate and Nppb are released from primary itch-sensory cells; these molecules activate secondary spinal cord pruriceptive-specific neurons, which in turn utilize Grp to activate tertiary pruriceptive-selective neurons. Intersecting this basic linear excitatory pathway, inhibitory input from dynorphin and neurons that express the somatostatin receptor modify itch sensation. Cumulatively, these studies paint an elegantly simple picture of how itch signals are transformed and integrated in the spinal cord and open new avenues for research efforts aimed at understanding and better treating itch.

Neural processing of itch

While considerable effort has been made to investigate the neural mechanisms of pain, much less effort has been devoted to itch, at least until recently. However, itch is now gaining increasing recognition as a widespread and costly medical and socioeconomic issue. This is accompanied by increasing interest in the underlying neural mechanisms of itch, which has become a vibrant and rapidly-advancing field of research. The goal of the present forefront review is to describe the recent progress that has been made in our understanding of itch mechanisms.

Investigational drugs for pruritus

INTRODUCTION

Chronic pruritus (CP), defined as itch lasting for > 6 weeks, is a burdensome symptom of several different diseases, dermatological and systemic, with a high negative impact on the quality of life of patients. Given the manifold aetiologies of CP, therapy is often difficult. In recent years, however, novel substances have been developed for treatment of certain CP entities and identified targets.

AREAS COVERED

In this review, the authors present a survey of targets currently believed to be promising (H4R, IL-31, MOR, KOR, GRPR, NGF, NK-1R, TRP channels) and related investigational drugs that are in the preclinical or clinical stage of development. Some substances have already undergone clinical testing, but only one of them (nalfurafine) has been licensed so far. Many of them are most likely to exert their effects on the skin and interfere there with the cutaneous neurobiology of CP.

EXPERT OPINION

Currently, the most promising candidates for new therapeutic agents in CP are neurokinin-1 receptor antagonists and substances targeting the kappa- or mu-opioid receptor, or both. They have the potential to target the neuronal pathway of CP and are thus of interest for several CP entities. The goal for the coming years is to validate these concepts and move forward in developing new drugs for the therapy of CP.

Roles for substance P and gastrin-releasing peptide as neurotransmitters released by primary afferent pruriceptors

Recent studies support roles for neurokinin-1 (NK-1) and gastrin-releasing peptide (GRP) receptor-expressing spinal neurons in itch. We presently investigated expression of substance P (SP) and GRP in pruritogen-responsive primary sensory neurons and roles for these neuropeptides in itch signaling. Responses of dorsal root ganglion (DRG) cells to various pruritogens were observed by calcium imaging. DRG cells were then processed for SP, GRP, and isolectin B-4 (IB4; a marker for nonpeptidergic neurons) immunofluorescence. Of pruritogen-responsive DRG cells, 11.8-26.8%, 21.8-40.0%, and 21.4-26.8% were immunopositive for SP, GRP, and IB4, respectively. In behavioral studies, both systemic and intrathecal administration of a NK-1 receptor antagonist significantly attenuated scratching evoked by chloroquine and a protease-activated receptor 2 agonist, SLIGRL, but not histamine, bovine adrenal medulla peptide 8-22 (BAM8-22), or serotonin. Systemic or intrathecal administration of a GRP receptor antagonist attenuated scratching evoked by chloroquine and SLIGRL but not BAM8-22 or histamine. The GRP receptor antagonist enhanced scratching evoked by serotonin. These results indicate that SP and GRP expressed in primary sensory neurons are partially involved as neurotransmitters in histamine-independent itch signaling from the skin to the spinal cord.

Itch signaling in the nervous system

Itch is a major somatic sensation, along with pain, temperature, and touch, detected and relayed by the somatosensory system. Itch can be an acute sensation, associated with mosquito bite, or a chronic condition, like atopic dermatitis (29, 59). The origins of the stimulus can be localized in the periphery or systemic, and associated with organ failure or cancer. Itch is also a perception originating in the brain. Itch is broadly characterized as either histamine-dependent (histaminergic) or histamine-independent (nonhistaminergic), both of which are relayed by subsets of C fibers and by the second-order neurons expressing gastrin-releasing peptide receptor (GRPR) and spinothalamic track (STT) neurons in the spinal cord of rodents. Historically, itch research has been primarily limited to clinical and psychophysical studies and to histamine-mediated mechanisms. In contrast, little is known about the signaling mechanisms underlying nonhistaminergic itch, despite the fact that the majority of chronic itch are mediated by nonhistaminergic mechanisms. During the past few years, important progress has been made in understanding the molecular signaling of itch, largely due to the introduction of mouse genetics. In this review, we examine some of the molecular mechanisms underlying itch sensation with an emphasis on recent studies in rodents.

Enhanced scratching evoked by PAR-2 agonist and 5-HT but not histamine in a mouse model of chronic dry skin itch

Chronic itch is a symptom of many skin conditions and systemic disease, and it has been hypothesized that the chronic itch may result from sensitization of itch-signaling pathways. We induced experimental chronic dry skin on the rostral back of mice, and observed a significant increase in spontaneous hindlimb scratches directed to the dry skin. Spontaneous scratching was significantly attenuated by a PAR-2 antibody and 5-HT2A receptor antagonist, indicating activation of these receptors by endogenous mediators released under dry skin conditions. We also observed a significant increase in the number of scratch bouts evoked by acute intradermal injections of a protease-activated receptor (PAR)-2 agonist and serotonin (5-HT), but not histamine. We additionally investigated if pruritogen-evoked activity of dorsal root ganglion (DRG) neurons is enhanced in this model. DRG cells from dry skin mice exhibited significantly larger responses to the PAR-2 agonist and 5-HT, but not histamine. Spontaneous scratching may reflect ongoing itch, and enhanced pruritogen-evoked scratching may represent hyperknesis (enhanced itch), both potentially due to sensitization of itch-signaling neurons. The correspondence between enhanced behavioral scratching and DRG cell responses suggest that peripheral pruriceptors that respond to proteases and 5-HT, but not histamine, may be sensitized in dry skin itch.

Absence of histamine-induced itch in the African naked mole-rat and "rescue" by Substance P

Recent research has proposed a pathway in which sensory neurons expressing the capsaicin activated ion channel TRPV1 are required for histamine-induced itch and subsequent scratching behavior. We examined histamine-induced itch in the African naked mole-rat (Heterocephalus glaber) and found that although naked mole-rats display innate scratching behavior, histamine was unable to evoke increased scratching as is observed in most mouse strains. Using calcium imaging, we examined the histamine sensitivity of naked mole-rat dorsal root ganglia (DRG) neurons and identified a population of small diameter neurons activated by histamine, the majority of which are also capsaicin-sensitive. This suggested that naked mole-rat sensory neurons are activated by histamine, but that spinal dorsal horn processing of sensory information is not the same as in other rodents. We have previously shown that naked mole-rats naturally lack substance P (SP) in cutaneous C-fibers, but that the neurokinin-1 receptor is expressed in the superficial spinal cord. This led us to investigate if SP deficiency plays a role in the lack of histamine-induced scratching in this species. After intrathecal administration of SP into the spinal cord we observed robust scratching behavior in response to histamine injection. Our data therefore support a model in which TRPV1-expressing sensory neurons are important for histamine-induced itch. In addition, we demonstrate a requirement for active, SP-induced post-synaptic drive to enable histamine sensitive afferents to drive itch-related behavior in the naked mole-rat. These results illustrate that it is altered dorsal horn connectivity of nociceptors that underlies the lack of itch and pain-related behavior in the naked mole-rat.

Dorsal horn neurons expressing NK-1 receptors mediate scratching in rats

Itch is thought to be signaled by pruritogen-responsive neurons in the superficial spinal dorsal horn. Many neurons here express the substance P NK-1 receptor. We investigated whether neurotoxic destruction of spinal NK-1-expressing neurons affected itch-related scratching behavior. Rats received intracisternal substance P conjugated to saporin (SP-SAP), or saporin (SAP) only (controls), and were subsequently tested for scratching behavior elicited by intradermal 5-hydroxytryptamine. SAP controls exhibited dose-related hindlimb scratching, which was significantly attenuated in SP-SAP-treated rats. There was a virtual absence of NK-1 immunoreactive neurons in superficial laminae of the upper cervical and medullary dorsal horn in SP-SAP-treated rats. These results indicate that superficial dorsal horn neurons expressing NK-1 receptors play a key role in spinal itch transmission.

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 dermatologic 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 (i.d.) 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 i.d. 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.

Scratching behavior and Fos expression in superficial dorsal horn elicited by protease-activated receptor agonists and other itch mediators in mice

Protease-activated receptor (PAR)-2 and PAR-4 are implicated in nonhistaminergic itch. We investigated dose dependence, tachyphylaxis, and cross-tachyphylaxis of itch-associated scratching elicited by intradermal injections of PAR-2 and PAR-4 agonists, serotonin (5-hydroxytryptamine, 5-HT), and histamine in ICR mice, as well as mu-opioid modulation of PAR-2 agonist-evoked scratching. Each agent elicited dose-related increases in scratch bouts. Scratching elicited by the PAR-4 agonist and histamine both exhibited significant tachyphylaxis but no cross-tachyphylaxis with each other. Scratching evoked by 5-HT did not exhibit significant tachyphylaxis but did exhibit significant cross-tachyphylaxis to scratching evoked by the PAR-2 and PAR-4 agonists and histamine. Naltrexone and high-dose morphine (10 mg/kg) attenuated PAR-2 agonist-evoked scratching, whereas lower dose morphine (1 mg/kg) had no effect. High-dose morphine also significantly increased circling behavior, which may have interfered with scratching. The PAR-2 agonist and 5-HT produced overlapping distributions of Fos-like immunoreactivity in the superficial dorsal horn. These results indicate that PAR-2 and PAR-4 agonists, histamine, and 5-HT elicit itch-related scratching and activate superficial dorsal horn neurons that may participate in scratch reflex and ascending itch signaling pathways.

Influence of genotype, dose and sex on pruritogen-induced scratching behavior in the mouse

Itch features considerable interindividual variability in humans, and initial studies using animal models have demonstrated a likely role of genetic factors in mediating such variability. In an attempt to systematically study genetic mediation of itch in the mouse such that gene identification by linkage mapping might be achieved, we examined scratching behavior induced by histamine and chloroquine in mice of 11 inbred mouse strains. Multiple chloroquine drug doses were used, revealing the existence of inverted-U dose-response relationships in every strain, allowing us to determine strain-dependent peak scratching behavior over the entire dose range. Peak chloroquine-induced scratching varied by 2.5-fold in this set of strains; scratching behavior shows moderate heritability in the mouse. The present data also reveal, for the first time, significant sex differences in pruritogen-induced scratching behavior, with female mice scratching an average of 23% more than males. Finally, a comparison of the strain means obtained here with previously collected data using nociceptive assays revealed a suggestive negative genetic correlation between chloroquine-induced itch and thermal pain, such that strains sensitive to pain are resistant to itch and vice versa. This finding may have implications both for our understanding of itch pathophysiology and for the identification of itch-related genes.

Quantitative assessment of directed hind limb scratching behavior as a rodent itch model

Hind limb scratching is used increasingly as an itch model in rodents. Scratching is usually quantified as the number of scratching bouts over a 60 min period. Since the antipruritic effect of scratching should depend on the total time of skin contact, then the duration of scratching bouts and within-bout scratching frequency may also be important factors. Therefore, we measured these parameters during episodes of scratching directed toward the site (nape of neck) of intradermal injection of serotonin in Sprague-Dawley rats. Serotonin elicited significantly more scratching bouts than saline. There was a biphasic pattern of scratching over time, with peaks at 10-20 and 40-50 min. Although cumulative bout duration (2-min intervals) had a similar biphasic distribution, the mean individual bout duration (2.1 s) did not change significantly over time. Within-bout scratching frequency remained constant over time at 8 Hz. The number of scratching bouts was suppressed in a dose dependent manner by naltrexone (3 and 5 mg/kg), while the individual bout duration and the within-bout frequency were not significantly different compared with serotonin-evoked scratching without naltrexone. These results validate the total number of scratching bouts as an indicator of the magnitude of itch-related scratching.