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and intracisternal injections have been conducted on animals with fur (9).
Injections into the central nervous system showed dose-dependent reproducible scratching (10–16). In the study of peripherally (from the skin) induced
itch, scratching was elicited by substances injected into the skin of mice (17–
23), guinea pigs (24), and rats (1,19,25–27). Among these species, various
reactions to injected chemical substances were observed. Intradermal injections of prostaglandin E2 induced a strong itch–scratch response in guinea
pigs (24), but did not elicit scratching in mice (17). In humans, histamine is
the best-characterized pruritogen, but it did not induce scratching in guinea
pigs (24). In mice, Kuraishi et al. (21) found no involvement of histamine in
scratching behavior, while Inagaki et al. (20), reported the involvement of
both histamine H1 and histamine H2 receptors in passive, cutaneous,
anaphylaxis-induced scratching behavior in mice. Reaction to the substances varies greatly from animals to humans and also among animal species.
This clearly makes it difficult to suggest a valid model for screening different
mediators in rodents, because the experimental results may not be relevant
to humans. In animal models of pruritus, it is therefore necessary to use
chemical substances known to induce itch in humans. Scratching in different
species is then recorded.
When trying to differentiate between scratching due to itch or pain,
both pruritogenic and algesiogenic agents have been injected in mice. Only
the pruritogenic agents induced scratching behavior (21). Scratching behavior in rats can be attributed to pain (1). Nevertheless, serotonin is able to
elicit dose-dependent scratching behavior in rats after intradermal injection—but when a very high concentration was injected (leading to skin necroses), only little scratching was observed (28).
II.
THE SEROTONIN MODEL FOR SCRATCHING IN RATS
A model for peripherally induced scratching in rats in response to intradermal injections of serotonin was recently developed (28). Because scratching decreases after daily repeated injections of serotonin in the same rat,
each rat only received one injection. Injections were given in the neck, as rats
can reach this skin area only with their hind paws. After intradermal
injections, the rats were transferred to cages and video recorded for 2.5 hr
(29) (Fig. 1).
Itch profile curves were obtained from viewing the videotapes—the
number of scratch sequences was registered in 5-min intervals. In this way, a
profile of itch intensity for each rat is recorded. An example of profiles for
two selected concentrations and saline are shown in Figure 2. Typically, itch
profile curves show an increasing number of scratch sequences until
Itch Models in Animals
133
Figure 1 Set-up for studying rats. The rats were recorded with two black and white
videocameras (Topica model TP 606 A/3). These were placed above the cages, so that
they could record the rats from above. Recordings were made in authentic real-time
speed and videotaped on a time-lapse videotape recorder, Sony model SVT-L230P.
They were evaluated visually at triple speed. In night recordings infrared lamps
(Imax=940 nm, spectral bandwidth **=50 nm) were used. The lamps were placed
behind the cages and induced no significant lighting of the cages or the animals.
maximum at about half an hour, followed by a decline. When the concentration of injected serotonin is increased from 0.01 mg/mL, the area under
curve (AUC) of the itch profile curves also increases, primarily as a result of
longer duration of scratching (Fig. 2).
Usually, there was a lag time of 5–10 min before scratching began.
This could represent the time for local distribution and absorption of
serotonin into the bloodstream, or the lag time could simply be a transient
neuronal disturbance as a result of the injection trauma. The lag time could
also be a result of metabolism of serotonin (i.e., serotonin in the skin being
metabolized into a more active pruritogenic substance).
The AUC for each itch profile curve can be obtained, and can be used
for investigations of the dose–response relationship. As mentioned, different
concentrations of serotonin also produce different itch profile curves. It is
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Figure 2 Itch profile curves of scratching in response to different concentrations of
serotonin injected into the neck of rats. Scratching was registered in 5-min intervals.
(Modified curve from Ref. 25.)
seen from Figure 3 that when the concentration of serotonin rises, the AUC
also increases.
In contrast to injections in the neck, injections of serotonin above the
tail root elicited no scratching at all, neither on the site of injection, nor
elsewhere. Thus scratching of the neck in this model is ascribed to a local
stimulus from the injected skin area, and not as a response to systemic
absorption of serotonin into the bloodstream. No indication of a systemic
effect of serotonin was found in this model.
Serotonin is a known histamine liberator, but neither histamine itself
nor the histamine releasing compound 48/80 induced scratching in Sprague–
Dawley rats used in this model. Thus scratching in the rat due to injections
of serotonin is probably elicited in a histamine-independent way. In
humans, serotonin is a local pruritogen and has its own pruritogenic potency, not only acting over histamine containing mast cells (30).
It is very difficult to know if scratching is a result of itch, pain, or some
other sensation. The question is very central to itch research in animals. In a
study by De Castro-Costa et al. (1), both morphine and acetylsalicylate, but
not the antihistamine drug astemizole, depressed scratching behavior in
Itch Models in Animals
135
Figure 3 The dose–response relationship of scratching as a function of the concentration of serotonin injected in the rostral back. The number of scratch sequences
(mean scratch) is the area under the curve (AUC) of all the itch curves of serotonin
concentrations tested. The data fitted a sigmoid curve as a function of log10 to the
concentration injected. Error bars represents F2 S.E.M., n=10. (Modified curve from
Ref. 25.)
arthritic rats, and it was concluded that scratching was due to pain. On the
other hand, Kuraishi et al. (21) induced scratching behavior in mice by
pruritogenic (compound 48/80 and substance P) but not algesiogenic agents
(capsaicin and dilute formalin). In the present study, we found no indications of histamine being crucial to scratching behavior in rats, not even in a
concentration of 10 mg/mL, so H1-receptor antagonists would hardly be
able to reduce scratching behavior in rats (28). Furthermore, scratching
activity was greatly reduced in the present study when necroses developed
on the injection site (Fig. 3). If serotonin-induced scratching in rats was due
to pain, then one would expect that necrosis of the skin would lead to very
intense scratching and not very little scratching. Therefore, we believe that
scratching in the present study was due to a pruritic sensation.
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In conclusion, when using the serotonin model for pruritus in rats,
serotonin is a reproducible local pruritic substance eliciting scratching. The
model may be especially useful in research and development of topical
antipruritics of the nonhistaminic type. Furthermore, the model appears
relevant since serotonin is already recognized as a weak local pruritogen in
humans, as well as for various other purposes in pruritus research.
III.
EXAMPLE OF DRUG TESTING USING THE SEROTONIN
MODEL
The above-mentioned model was used to test the antipruritic potential of
four salicylic compounds, all with different skin penetration characteristics
(31). There is a strong need for antipruritic substances for treating itch in
clinical dermatology, and in one recent human study, topically applied
acetylsalicylic acid has been described to rapidly decrease histamine-induced
itch (32).
Eighteen rats were studied for 6 weeks. Prior to serotonin injections (2
mg/mL, 50 AL), 10 AL of test substances were applied to a circular area 18
mm in diameter.
The four substances were all solubilized to a concentration of 5% w/w.
Skin penetration of the salicylic compounds had previously been characterized by using the microdialysis technique. After serotonin injections,
scratching was monitored by video recordings. Compared to the vehicle, a
lower number of scratch sequences were seen after application of slow
penetrating salicylic compounds. After application of fast penetrating drugs,
no difference was observed. Furthermore, the number of scratch sequences
was lower than with vehicle throughout the 1.5-hr study period.
From the above-mentioned study, it can be concluded that topical
application of diethylamine salicylate and salicylamide could suppress
serotonin-induced scratching in rats. Furthermore, the antipruritic effect
seems to be related to slow drug release of the two substances. The results
may be clinically relevant because serotonin induces itch in humans.
REFERENCES
1.
2.
De Castro-Costa M, et al. Scratching behaviour in arthritic rats: a sign of
chronic pain or itch? Pain 1987; 29:123–131.
Woodward DF, Conway JL, Wheeler LA. Cutaneous itching models. In:
Itch Models in Animals
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
137
Maibach HI, Lowe NJ, eds. Models in Dermatology. Basel: Karger, 1985:187–
197.
Thomsen JS, Benfeldt E, Serup J. Suppression of spontaneous scratching in
hairless rats by sedatives but not by antipruritics. Skin Pharmacol Appl Skin
Physiol 2002; 15:218–224.
Chavaz P, Faucher F, Saurat JH. Dermatosis of hairless rats fed a hypomagnesic diet–pathology and immunology. Dermatologica 1984; 169: 105–111.
Neckermann G, Bavandi A, Meingassner JG. Atopic dermatitis-like symptoms
in hypomagnesaemic hairless rats are prevented and inhibited by systemic or
topical SDZ ASM 981. Br J Dermatol 2000; 142:669–679.
Claverie-Benureau A, Lebel B, Gaudin-Harding F. Magnesium deficiency allergy-like crisis in hairless rats. A suggested model for inflammation studies. J
Physiol 1980; 76:173–175.
Bavandi A, Meingassner JG, Becker S. Diet-induced dermatitis response of
hairless rats to systemic treatment with cyclosporin A (Sandimmun), cyclosporin H and FK506. Exp Dermatol 1992; 1:199–205.
Thomsen JS, Nielsen PL, Serup J. The hypomagnesic rat model: dermatitis
prone hairless rats with mild Magnesium depletion fed a diet low in lipids did
not develop pruritic dermatitis. Skin Pharmacol Appl Skin Physiol 2002.
Submitted.
Kuraishi Y, Yamaguchi T, Miyamoto T. Itch–scratch responses induced by
opioids through central mu opioid receptors in mice. J Biomed Sci 2000; 7: 248–
252.
Bergasa NV, et al. Plasma from patients with the pruritus of cholestasis induces
opioid receptor-mediated scratching in monkeys. Life Sci 1993; 53:1253–1257.
Gmerek DE, Cowan A. An animal model for preclinical screening of systemic
antipruritic agents. J Pharmacol Methods 1983; 10:107–112.
Sakurada T, et al. Nociceptin-induced scratching, biting and licking in mice:
involvement of spinal NK1 receptors. Br J Pharmacol 1999; 127:1712–1718.
Takahashi H, et al. Mechanism of pruritus and peracute death in mice induced
by pseudorabies virus (PRV) infection. J Vet Med Sci 1993; 55:913–920.
Thomas DA, Hammond DL. Microinjection of morphine into the rat medullary dorsal horn produces a dose-dependent increase in facial scratching.
Brain Res 1995; 695:267–270.
Tohda C, Yamaguchi T, Kuraishi Y. Intracisternal injection of opioids induces
itch-associated response through mu-opioid receptors in mice. Jpn J Pharmacol
1997; 74:77–82.
Yamaguchi T, Kitagawa K, Kuraishi Y. Itch-associated response and antinociception induced by intracisternal endomorphins in mice. Jpn J Pharmacol
1998; 78:337–343.
Andoh T, Kuraishi Y. Intradermal leukotriene B4, but not prostaglandin E2,
induces itch-associated responses in mice. Eur J Pharmacol 1998; 353:93–96.
Andoh T, et al. Substance P induction of itch-associated response mediated by
cutaneous NK1 tachykinin receptors in mice. J Pharmacol Exp Ther 1998;
286:1140–1145.
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19. Hayashi I, Majima M. Reduction of sodium deoxycholic acid-induced scratching behaviour by bradykinin B2 receptor antagonists. Br J Pharmacol 1999;
126:197–204.
20. Inagaki N, et al. Participation of histamine H1 and H2 receptors in passive
cutaneous anaphylaxis-induced scratching behavior in ICR mice. Eur J Pharmacol 1999; 367:361–371.
21. Kuraishi Y, et al. Scratching behavior induced by pruritogenic but not algesiogenic agents in mice. Eur J Pharmacol 1995; 275:229–233.
22. Rojavin MA, et al. Antipruritic effect of millimeter waves in mice: evidence for
opioid involvement. Life Sci 1998; 63:L251–L257.
23. Sugimoto Y, et al. Effects of histamine H1 receptor antagonists on compound
48/80-induced scratching behavior in mice. Eur J Pharmacol 1998; 351:1–5.
24. Woodward DF, et al. Characterization of a behavioral model for peripherally
evoked itch suggests platelet-activating factor as a potent pruritogen. J Pharmacol Exp Ther 1995; 272:758–765.
25. Berendsen HH, Broekkamp CL. A peripheral 5-HT1D-like receptor involved in
serotonergic induced hindlimb scratching in rats. Eur J Pharmacol 1991; 194:
201–208.
26. Khasabov SG, et al. Modulation of afferent-evoked neurotransmission by 5HT3 receptors in young rat dorsal horn neurones in vitro: a putative mechanism of 5-HT3 induced anti-nociception. Br J Pharmacol 1999; 127:843–852.
27. Kubota K, et al. Pharmacological characterization of capsaicin-induced body
movement of neonatal rat. Jpn J Pharmacol 1999; 80:137–142.
28. Thomsen JS, et al. Scratch induction in the rat by intradermal serotonin: a model
for pruritus. Acta Derm-Venereol 2001; 81:250–254.
29. Thomsen JS. Itch models and effect of topical antipruritic substances. PhD
thesis, University of Copenhagen, 2001.
30. Weisshaar E, Ziethen B, Gollnick H. Can a serotonin type 3 (5-HT3) receptor
antagonist reduce experimentally-induced itch? Inflamm Res 1997; 46:412–416.
31. Thomsen JS, et al. The effect of topically applied salicylic compounds on serotonin-induced scratching behaviour in hairless rats. Exp Dermatol 2002;
11:370–375.
32. Yosipovitch G, et al. Topically applied aspirin rapidly decreases histamineinduced itch. Acta Derm-Venereol 1997; 77:46–48.
14
Human Itch Models, with Special
Emphasis on Itch in SLS-Inflamed
and Normal Skin
Jens Schiersing Thomsen
Gentofte University Hospital, Copenhagen, Denmark
I.
OBSERVATIONS IN HUMANS
Experimental itch studies have, until now, focused on eliciting itch by one or
two mediators. In clinical dermatology, itch is typically seen in inflammatory
dermatoses, containing a whole orchestra of inflammatory mediators. Consequently, several research groups have provided information about inflammatory mediators, such as prostaglandins and opiates, as being potentiators
of conventional itch mediators (1,2). In this way, different mediators may
contribute to itch sensation (3), and in particular, injection of two different
mediators in normal skin has been studied (4), with synergy demonstrated
(1,5,6).
The alternative to eliciting itch with concomitant experimental mediators has been to induce itch in patients with different kinds of pruritic or
inflammatory dermatoses [e.g., urticaria (7), itching psoriasis (7), and unclassified pruritus (7)]. Itch in patients has typically been compared with
healthy volunteers (8,9).
In recent years, patients suffering from atopic dermatitis (AE), in
particular, have been studied in experimental itch studies (10,11). Histamine,
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acetylcholine, and other itch inducers have been used to induce itch in AE
patients and healthy volunteers in the same studies (7,10,12). One study
included both AE patients suffering from acute eczema and AE patients
during a symptom-free period (12).
However, comparison between patients and healthy volunteers is problematic when assessing the influence of inflammation on experimentally
induced itch, because the two groups do not match each other. Differences
in itch sensation between the groups could be attributed to factors other than
the inflamed skin (e.g., cerebral mechanisms, or differences in Baseline State).
Typically, itch is scored by using a 100-mm visual analog scale (VAS)
(12–14). Both the time interval until itch is perceived (itch latency) (8,15,16)
and itch threshold (i.e., the lowest substance concentration eliciting itch)
(17,18) are measured. Itch duration (7,8,16), itch magnitude (16), and a
combination of these, namely the Total itch index, Tii (or area under the
curve, AUC), are often quantified.
Scratch intensity has also been used as a more objective measure of itch
sensation (19–23). However, itch is subjective in nature and scratching can
only be a surrogate of self-grading (24).
Associated skin symptoms such as wheal (cutaneous edema) and flare
areas are often quantified in itch studies (25–27). Ultrafiltration from the
postcapillary venules creates the wheal (28), while flare is a vasodilatation
resulting from a local axonal skin reflex (28,29). The C-fiber-mediated
responses (flare and itch) often correlate (28,30), while the exclusively
vascular wheal typically does not correlate with the two others (28,31). The
area of itchy skin can be measured in itch models (32). Alloknesis (or itchy
skin) means another sensation, and itch is induced by touching the surrounding area of, e.g., an insect bite (33–35). The area of alloknesis can be
measured. Changes in skin blood flow have also been quantified in itch
studies by laser Doppler flowmetry (10,12).
II.
THE SLS-INFLAMED SKIN MODEL FOR PRURITUS
IN HUMANS
Instead of mimicking inflamed skin seeking for new itch potentiators in a twomediator system, we aimed to establish an itch model in humans comprising
both normal and experimentally inflamed skin, using volunteers as their own
controls (36).
The skin of five selected test sites on one volar forearm was pretreated
for 24 hr with large Finn Chambers containing 1% sodium lauryl sulfate
(SLS) used as a standard contact irritant to induce inflammation.