Investigation of the pruritogenic effects of histamine, serotonin, tryptase, substance P and interleukin-2 in healthy dogs

Calcium imaging of primary canine sensory neurons: Small-diameter neurons responsive to pruritogens and algogens

INTRODUCTION

Rodent primary sensory neurons are commonly used for studying itch and pain neurophysiology, but translation from rodents to larger mammals and humans is not direct and requires further validation to make correlations.

METHODS

This study developed a primary canine sensory neuron culture from dorsal root ganglia (DRG) excised from cadaver dogs. Additionally, the canine DRG cell cultures developed were used for single-cell ratiometric calcium imaging, with the activation of neurons to the following pruritogenic and algogenic substances: histamine, chloroquine, canine protease-activated receptor 2 (PAR2) activating peptide (SLIGKT), compound 48/80, 5-hydroxytryptamine receptor agonist (5-HT), bovine adrenal medulla peptide (BAM8-22), substance P, allyl isothiocyanate (AITC), and capsaicin.

RESULTS

This study demonstrates a simple dissection and rapid processing of DRG collected from canine cadavers used to create viable primary sensory neuron cultures to measure responses to pruritogens and algogens.

CONCLUSION

Ratiometric calcium imaging demonstrated that small-diameter canine sensory neurons can be activated by multiple stimuli, and a single neuron can react to both a pruritogenic stimulation and an algogenic stimulation.

Characterization of a chloroquine-induced canine model of pruritus and skin inflammation

BACKGROUND

Chloroquine (CQ) is a prototypical systemic and intradermal pruritogen for histamine-independent (nonhistaminergic) itch in mice and humans. The predictive validity of this model is poorly documented in dogs.

HYPOTHESIS/OBJECTIVE

To determine pruritogenic and inflammatory effects of systemic and i.d. CQ injections in healthy dogs.

ANIMALS

Ten healthy purpose-bred laboratory beagles.

METHODS AND MATERIALS

All dogs were randomized to receive i.d. (200 and 400 µg/site), intravenous (2 mg/kg) and subcutaneous (3 mg/kg) CQ injections. Dogs were video-recorded for 30 min after i.d. injections and for 300 min after i.v. and s.c. injections. Buffered saline injections served as controls for each route. Global wheal scores were evaluated at 30 min post-i.d. injection by a blinded investigator.

RESULTS

All dogs showed wheal and erythema at the CQ i.d. injection sites; global wheal scores of each CQ concentration were significantly increased compared to placebo (P ≤ 0.05). Blinded evaluation revealed no significant increase in generalized pruritic behaviour (pruritic seconds) after i.v. or s.c. administration of CQ. Intradermal injections induced mild localized acute pruritic behaviours at the site of injections at 200 µg (P = 0.06) and 400 µg (P = 0.27) CQ in dogs.

CONCLUSION AND CLINICAL SIGNIFICANCE

To the best of the authors' knowledge, this is the first report which shows that i.d. CQ injections may induce acute inflammation in healthy dogs. By contrast to the systemic CQ-induced pruritus reported previously in healthy mice and dogs, no significant pruritic behaviours were observed after CQ injection, regardless of the route of administration.

Dose-dependent pruritogenic and inflammatory effects of intradermal injections of histamine, compound 48/80 and anti-canine IgE in healthy dogs

BACKGROUND

Scratching behaviours associated with intradermal (i.d.) injection of pruritogens such as histamine and compound 48/80 into the skin of mice and humans is the commonly used model to advance itch research and drug development. The predictive validity of this model is poorly documented in dogs.

OBJECTIVES

To evaluate the dose-dependent effects of pruritogenic substances, each with a different mechanism of action, in healthy dogs.

ANIMALS

Ten healthy laboratory beagles.

METHODS AND MATERIALS

All dogs were video-recorded for 30 min post-injection (mpi) of i.d. goat anti-canine IgE (4 and 25 μg/site), histamine and compound 48/80 (50, 100, 200, 400 μg/site); two buffered saline injections served as controls. Two blinded investigators reviewed the pruritic behaviours of all video recordings. Global wheal scores were evaluated at 30 min by a blinded investigator.

RESULTS

All dogs showed wheal and erythema at the pruritogen injection site; global wheal scores at 30 min of each substance significantly increased at all concentrations compared to control (P ≤ 0.05). A blinded evaluation revealed that all pruritogens induced mild acute pruritic behaviours at the site of injection. There was no injection site pain seen in any dog. Compared to controls, injections of pruritogens did not significantly affect the pruritic seconds or occurrence of pruritic episodes for any of the substances.

CONCLUSIONS AND CLINICAL SIGNIFICANCE

These preliminary results suggest that i.d. injections of the studied pruritogens can induce cutaneous wheal and flare response in healthy dogs; but inconsistencies occur in the induction of itch, even with the different concentrations of pruritogens.

Early Activation of Th2/Th22 Inflammatory and Pruritogenic Pathways in Acute Canine Atopic Dermatitis Skin Lesions

Determining inflammation and itch pathway activation in patients with atopic dermatitis (AD) is fraught with the inability to precisely assess the age of skin lesions, thus affecting the analysis of time-dependent mediators. To characterize inflammatory events occurring during early experimental acute AD lesions, biopsy samples were collected 6, 24, and 48 hours after epicutaneous application of Dermatophagoides farinae house dust mites to sensitized atopic dogs. The skin transcriptome was assessed using a dog-specific microarray and quantitative PCR. Acute canine AD skin lesions had a significant up-regulation of genes encoding T helper (Th) 2 (e.g., IL4, IL5, IL13, IL31, and IL33), Th9 (IL9), and Th22 (IL22) cytokines as well as Th2-promoting chemokines such as CCL5 and CCL17. Proinflammatory (e.g., IL6, LTB, and IL18) cytokines were also up-regulated. Other known pruritogenic pathways were also activated: there was significant up-regulation of genes encoding proteases cathepsin S (CTSS), mast cell chymase (CMA1), tryptase (TPS1) and mastin, neuromedin-B (NMB), nerve growth factor (NGF), and leukotriene-synthesis enzymes (ALOX5, ALOX5AP, and LTA4H). Experimental acute canine house dust mite-induced AD lesions exhibit an activation of innate and adaptive immune responses and pruritogenic pathways similar to those seen in humans with acute AD, thereby validating this model to test innovative therapeutics modalities for this disease.

Atopic itch in dogs: pharmacology and modeling

Itch is the most common clinical problem seen in dogs with skin diseases. Although an etiological classification of canine pruritus does not yet exist, most causes would likely fall into the IFSI class I (dermatological) itch. One of the most common causes of canine itch is that associated with atopic dermatitis, and there is randomized controlled trial grade evidence of the efficacy of several antipruritic interventions. At this time, the mainstay of treatment of canine atopic itch relies principally on the use of topical and/or oral glucocorticoids and oral cyclosporine. Type 1 receptor antihistamines are notorious in their inconsistency in reducing pruritus in atopic dogs. A new Janus kinase (JAK)-1 inhibitor has recently been approved for treatment of allergic itch in dogs, and its onset of efficacy is remarkably fast. Modeling itch in dogs can be achieved by allergen sensitization (fleas, house dust mites), and challenges that elicit pruritic manifestation can be used for mechanistic studies as well as for testing of novel anti-itch modalities.

PCR detects bands consistent with the expression of receptors associated with pruritus in canine dorsal root ganglia

BACKGROUND

Various mediators are involved in the induction of itch, i.e. pruritus; however, the in vivo pharmacology of pruritus seems to be different in distinct species, and little is known about receptors that are involved in the induction of itch in dogs. The species differences in the mediation of pruritus might be explained by species differences in receptor expression in the sensory nerves, including the dorsal root ganglia (DRG).

HYPOTHESIS/OBJECTIVES

The aim of the study was to analyse the expression of receptors for various mediators of pruritus in canine DRG.

METHODS

Dorsal root ganglia of 14 dogs, which were euthanized for reasons not related to this study, were analysed. Multiple DRG per dog were dissected and, after homogenization of the DRG tissues, total RNA was isolated, reverse transcribed to complementary DNA and amplified with custom-synthesized primers.

RESULTS

The following receptors were found in canine DRG: transient receptor potential cation channel subfamily V member 1, tachykinin receptor 1, Toll-like receptor 7, endothelin receptor type A, opioid receptors μ1 and κ1, histamine H1 -H4 receptors and the interleukin-31 receptor complex.

CONCLUSIONS AND CLINICAL IMPORTANCE

PCR analysis detected bands consistent with the expression of receptors associated with pruritus in canine DRG.

Cowhage can induce itch in the atopic dog

Itch is a cardinal symptom of atopic dermatitis in humans and dogs. Until now, experimental induction of itch in dogs has proven difficult. The objectives of this study were to determine whether protease-rich spicules, protein extracts and the protease mucunain of the tropical legume cowhage provoked itch and inflammation when rubbed onto canine skin. Native spicules variably induced itch manifestations in about half of the dogs, while challenges with protease-deactivated spicules remained negative. The epicutaneous application of cowhage extract and mucunain after microneedle roller usage also induced pruritus and inflammation. Importantly, there was an interindividual inconsistency in pruritus and inflammation induction and also marked differences in pruritus intensity after challenge. In conclusion, cowhage spicules, protein-rich extracts and mucunain can all induce pruritus and inflammation in dogs as in other species, but the inconsistency of provocation is currently a limitation of this challenge type for future studies of pruritus in dogs.

Double-blinded, placebo-controlled study to evaluate an antipruritic shampoo for dogs with allergic pruritus

Shampoo therapy is frequently used on pruritic dogs. However, there are few double-blinded, placebo-controlled studies of this form of therapy. This randomised, double-blinded, placebo-controlled study evaluated the efficacy of a commercial medicated shampoo (DermaTopic; Almapharm) containing chlorhexidine, lactoferrin, piroctone olamine, chitosan and essential fatty acids in 27 dogs with mild to moderate allergic pruritus without secondary skin infections. All dogs received shampoo therapy with either DermaTopic or a shampoo vehicle as placebo twice weekly for four weeks. The extent of pruritus was evaluated before the study and then on a daily basis by the owners using a visual analogue scale. Before beginning the treatment and after four weeks, the skin lesions were evaluated by an experienced clinician with a validated lesion score (Canine Atopic Dermatitis Extent and Severity Index - CADESI). The pruritus was reduced significantly by both DermaTopic and placebo. However, there was no significant difference between both groups. There was no statistically significant difference in the CADESI scores pre- and post-treatment in either group or between the two types of treatment. This study provides further evidence of the benefit of shampoo therapy for pruritic dogs.

The expression of histamine H4 receptor mRNA in the skin and other tissues of normal dogs

The histamine 4 (H(4)) receptor was first cloned and characterized in 2000 using the human H(3) receptor DNA sequence. The H(4) receptor has been shown to participate in various aspects of inflammation, such as chemotaxis, upregulation of adhesion molecule expression and modulation of cytokine secretion. The primary goal of this study was to determine whether H(4) receptor mRNA is expressed in normal canine skin by performing an RT-PCR. An additional goal was to determine the expression of this receptor in the colon, liver, spleen and kidney. Tissues were collected from five healthy, young-adult pit bull dogs. Samples were immediately placed in RNAlater(®) solution and stored at -20°C until processed. The amplified products in all skin samples in addition to the colon, liver, spleen and kidney (variable expression) had the expected size of 400-500 bp. The sequenced amplicons matched the National Center for Biotechnology Information published sequence for the canine H(4) receptor. The study results showed that canine normal skin expresses the H(4) receptor mRNA. Further studies using immunohistochemistry should be conducted to demonstrate the expression of the H(4) receptor at the protein level and to localize the expression of this receptor in the skin.

Correlation between pruritus score and grossly visible erythema in dogs

The severity of pruritus and the extent and severity of erythema were quantified in 107 dogs presenting with various dermatoses. Pruritus was assessed using a previously validated scale, and erythema was quantified by assessing severity at 72 different body sites. Pruritus scores were either 0, or followed a normal type of distribution, with most dogs having a score in the middle of the range and a few dogs having low or high scores. The median pruritus score was 6.3/10. Erythema scores were heavily skewed towards lower values, with only a few dogs having high scores. The median diffuse erythema score was 6.0/216 and the median score for maculo-papular/pustular erythema was 0/1080. Pruritus and erythema scores were significantly correlated with a Spearman rank correlation coefficient of 0.4062 (P<0.001). However, visual assessment of the data representing the two variables revealed that this was not a consistent biological or clinically relevant correlation. Individual dogs could have a high pruritus score with low erythema score or vice versa. This study raises questions about the use of erythema scoring systems as a primary outcome measure in clinical trials, and also about the role of various inflammatory mediators in the pathogenesis of canine pruritus.