Neuroscience & Medicine, 2012, 3, 314-320
http://dx.doi.org/10.4236/nm.2012.33036 Published Online September 2012 (http://www.SciRP.org/journal/nm)
Sex Differences in Scratching Behaviors Induced by
Intradermal Injections of Pruritogenic Chemicals in
C57BL/6 Mice
Liang Liang1,2,3#, Yong Han4#, Ming Zhang1#, Chunwei Liu1,5#, Yabin Xie1#, Wenjuan Han1, Sanjue Hu1,
Hua Zhang2*, Hui Xu1*
1Institute of Neuroscience, Fourth Military Medical University, Xi’an, China; 2Department of Neurosurgery, Tangdu Hospital, Fourth
Military Medical University, Xi’an, China; 3Department of Neurosurgery, General Hospital of People’s Liberation Army Chendu
Military Region, Chengdu, China; 4Department of Thoracic Surgery, Tangdu Hospital, Fourth Military Medical University, Xi’an,
China; 5Class, School of Stomatology, Fourth Military Medical University, Xi’an, China.
Received June 14th, 2012; revised July 13th, 2012; accepted July 20th, 2012
ABSTRACT
Pruritus is an individual unpleasant sensation of human sensory nervous system. In the physiological condition it ex-
certs a self-protective mechanism to protect the skin against external harmful agents. Pruritoceptive itch is also a major
symptom of skin diseases and a common reason for consulting a dermatologist in clinic. It has been well known that
both histamine-dependent and histamine-independent pathways mediate acute and chronic itch sensations. Previous
studies have showed common neural pathways partially shared by itch and pain sensations, and significant sex differ-
ences in pain sensation. However, sex difference in itch sensation has not been given too much attention as the majority
of itch studies were done in male mice or rats till now. In the present study, we compared the scratching behaviors in-
duced by pruritogenic agents in male and female C57BL/6 mice. The results showed that both male and female exhib-
ited scratching behaviors in response to the intradermal injection of histamine-dependent and histamine-independent
pruritogenic chemicals. Moreover, the number of scratching behaviors in response to compound 48/80 and chloro-
quine were significantly higher in female. These results suggested that sex differences occurred in histamine-dependent
compound 48/80-induced and histamine-independent chloroquine-induced itch sensations, but not in histamine-inde-
pendent SLIGRL-NH2-induced itch sensation.
Keywords: Scratching; Pruritus; Sex Difference; Compound 4880 ; Chloroquine; SLIGRL-NH2
1. Introduction
Pruritus, or itch, is defined as an unpleasant sensation
that elicits the desire or reflex to scratch [1]. Pruritocep-
tive itch, or acute itch, serves as a physiological self-
protection as other cutaneous sensations such as pain,
touch and etc., to prevent the skin from harmful external
enviroment. Pruritoceptive itch originates peripheral skin
and is transmitted by slow-conducting subsets of unmye-
linated C nerve fibres [2]. Chronic itch is also a bother-
some symptom in some skin and systemic diseases, for
example, atopic dermatitis, cholestasis and etc. [2-4]. In
addition, acute or chronic itch is also one adverse side-
effect of many drugs including antimalarial drug chloro-
quine, opioid and so on [5]. It has been identified that
both histamine-dependent and histamine-independent
pathways mediate acute and chronic itch sensations [6].
Compound 48/80 which is known to elicit itch by de-
granuating mast cells to release histamine has been
proved to be a more reliable pruritogen in the study of
itch [7]. The protease activated receptor 2 (PAR2) ago-
nist SLIGRL-NH2 has been recently proved to be a his-
tamine-independent pruritogen [8,9]. For a long time, it
is considered that itching and pain have neural pathways
partially [6]. Low intensity stimulation of unmyelinated
polymodal C fiber results in sensation of pruritus
whereas high intensity stimulation causes pain [1]. Itch
can be suppressed by pain-evoking mechanical or ther-
mal stimuli. Patients with spinally administered μ-opioid
agonists frequently experience itch [10]. Recently, gas-
trin-releasing peptide receptor (GRPR) and MrgprA3, or
one member of a family of G protein-coupled receptors,
are identified to mediate itch sensation along itch signal-
#These authors contributed equally to this work.
*Corresponding autho
r
.

Sex Differences in Scratching Behaviors Induced by Intradermal Injections of Pruritogenic Chemicals in C57BL/6 Mice 315
ing pathways [11,12]. Thus, pruritus is not merely a sub-
modality of pain, but also an individual sensation of our
sensory nerve system.
Sex-associated differences in the pain perception and
its modulation have been widely confirmed in humans
and animals as well. Female tend to exhibit more pain
sensitivity, lower pain threshold and tolerance than male
[13-16]. However, sex-associated differences in itch
sensation and its modulation remain unclear. Despite it
was reported that female displayed a higher chronic
scratching behavior than male in a model of autoimmune
disease of MRL/lpr mice [17], and it was also observed
that female were more severely affected by pruritus
among chronic urticaria patients [10], the occurrence of
pruritus was not correlated with sex of the patients in the
clinical research of patients undergoing hemodialysis
[11]. Therefore, sex-associated difference in itch sensa-
tion is still unclear. In addition, the majority of itch stud-
ies were done in male mice or rats nowadays. Thus, our
study was designed to ascertain the sex-associated differ-
ence in itch sensation induced by intradermal injections
of pruritogenic chemicals compound 48/80, SLIGRL-
NH2 and chloroquine as well in C57BL/6 mice of both
sexes.
2. Materials and Methods
2.1. Animals
Both male and female C57BL/6 mice (6 - 8-week-old)
were purchased from the Fourth Military Medical
University. All experiments were carried out according
to the guidelines of the Institutional Animal Care and
Use Committee of the Fourth Military Medical Uni-
versity.
2.2. Itch Behaviors
The mice were allowed to acclimate to the observation
room for one week. Three days before experiments, a 1.5
cm diameter circular area was shaved at the back of the
neck where intradermal injections were given. Each
mouse was habituated to Plexiglas observation cylinders
( 9 × 9 × 13 cm) at least 30 minutes per day for three
days prior to experiments. Pruritogenic compounds (i.e.,
compound 48/80, chloroquine and SLIGR L-NH2 as well)
were subcutaneously injected into the nape of the neck at
a volume of 50 μl after acclimatization. Scratch- ing
behavior was observed for 30 minutes at 5-minute
intervals. Video cameras located above the glass floor
and recorded activity for the next 30 minutes. Digital vi-
deo files were later scored for the total number of
scratching strokes to the injected area during the 30
minutes recorded. A bout of scratching was defined as
continuous scratch movements with hind paws directed
at the area around the injection site. Scratching behavior
was quantified by recording the number of scratching
bouts at 5-minute intervals over the 30-minute observa-
tion period. One scratch was considered to be a lifting of
the hind limb towards the shaved area and then a
replacing of the limb back to the floor or licing the hind
limb, regardless of how many scratching strokes took
place between those two movements [9,12,18]. As to
intradermal injections, the following drugs were dissolved
in sterile saline and administered at a volume of 50 μl:
compound 48/80 (100 μg/50 μl), SLIGRL-NH2 (100
μg/50 μl), chloroquine (200 μg/50 μl).
2.3. Data Analysis
Data were presented as means ± sem. Statistical analysis
was performed with repeated measure analysis of vari-
ance or Student’s unpaired t-test. P < 0.05 was con-
sidered statistically significant.
3. Results
3.1. Scratching Behaviors Induced by
Intradermal Injections of Pruritogenic
Chemicals in Male C57BL/6 Mice
Compared with the injection of saline (n = 13), com-
pound 48/80 (100 g/50 l，n = 10) induced scratching
behaviors in male C57BL/6 mice, which often began
seconds after the intradermal injection and was sustained
within 30 minutes (Figures 1(a) and 1(b)). After com-
pound 48/80 was injected, the scratching behavior was
peaked at 15 minutes in male mice. The Analysis of total
scratching numbers in the 30 minutes showed a signify-
cant scratching effect after the injection in male C57BL/6
mice (Student’s unpaired t-test. P < 0.01, Figure 1(b)).
Similarly, two histamine-independent pruritogenic agents,
chloroquine（200 g/50 l, (Figures 1(c) and 1(d)) and an
agonist of the prote-ase-activated receptor 2 (PAR2), or
SLIGRL-NH2 (100 g/50 l, Figures 1(e) and 1(f)), also
induced signifycant scratching behaviors, which often
began in several seconds after the intradermal injection
and was sustained within 30 minutes. Analysis of total
scratching numbers in the 30 minutes showed both
chloroquine (n = 12) and SLIGRL-NH2 (n = 12) induced
significant itchy scratching behaviors after the injection
(Student’s unpaired t-test, P < 0.01).
3.2. Scratching Behaviors Induced by
Intradermal Injections of Pruritogenic
Chemicals in Female C57BL/6 Mice
Compared with the injections of saline (n = 12) in female
C57BL/6 mice, compound 48/80 (n = 12) induced
scratching behavior, which often began seconds after the
intradermal injection and was sustained within 30 min-
utes (Figures 2(a) and 2(b)). Analysis of total scratching
numbers in the 30 minutes after the injection showed a

Sex Differences in Scratching Behaviors Induced by Intradermal Injections of Pruritogenic Chemicals in C57BL/6 Mice
316
(a)
(b)
2
(c)
2
(d)
(e)
(f)
Figure 1. Scratching behaviors induced by the intradermal
injection of pruritogenic compounds in male C57BL/6 mice.
(a) The time course of every 5 minutes of histamine-depen-
dent compound 48/80-evoked significant scratching behave-
iors during 30 minutes in male mice (n = 10) (repeated
measure analysis of variance, P < 0.01, saline group, n = 13);
(b) The total number of scratches of histamine-dependent
compound 48/80 during 30 minutes in male mice (n = 10)
(Student’s unpaired t-test, P < 0.01, saline group, n = 13); (c )
The time course of every 5 minutes of histamine-indepen-
dent chloroquine-evoked significant scratching behaviors
during 30 minutes in male mice (n = 12) (repeated measure
analysis of variance, P < 0.01, saline group, n = 13); (d) The
total number of scratches of histamine-independent chloro-
quine during 30 minutes in male mice (n = 12) (Student’s
unpaired t-test, P < 0.01, saline group, n = 13); (e) The time
cour se of eve ry 5 minute s of histam ine-in depende nt SLIG RL-
NH2-evoked significant scratching behaviors during 30
minutes in male mice (n = 12) (repeated measure analysis of
variance, P < 0.01, saline group, n = 13); (f) The total num-
ber of scratches of histamine-independent SLIGRL-NH2 dur-
ing 30 minutes in male mice (n = 12) (Student’s unpaired
t-test, P < 0.01, saline group, n = 13).
significant scratching effect in female C57BL/6 mice
(Figure 2(b)). Similarly, two histamine-independent pru-
ritogenic agents, chloroquine and an agonist of the pro-
tease-activated receptor 2 (PAR2) also induced signify-
cant scratching behaviors, which often began in several
seconds after the intradermal injection and was sustained
within 30 minutes. After compound 4880 was injected,
the scratching behavior of female mice was peaked at 20
minutes. Analysis of total scratching numbers in the 30
minutes showed chloroquine (n = 13, Student’s unpaired
t-test, P < 0.05, Figures 2(c) and (d)) and SLIGRL-NH2
(n = 12, Student’s unpaired t-test, P < 0.01, Figures 2(e)
and (f)) induced significant scratching behaviors after the
injection.
3.3. Significant Scratching Behaviors Induced by
Intradermal Injections of Compound 48/80
and Chloroquine, but Not Sligrl-NH2, in
Female C57bl/6 Mice
After the injection of compound 48/80, the scratching
behavior was peaked at 20 minutes in female mice. The
scratching behavior induced by intradermal injection of
compound 48/80 (100 mg/50 ml) was significantly

Sex Differences in Scratching Behaviors Induced by Intradermal Injections of Pruritogenic Chemicals in C57BL/6 Mice 317
(a)
(b)
2
(c)
2
(d)
(e)
(f)
Fi g ure 2. Scr atchi ng be havior s induce d by intr adermal i njec-
tion of pruritogenic chemicals in female C57BL/6 mice. (a)
The time course of every 5 minutes of histamine-dependent
compound 48/80-evoked significant scratching behaviors
during 30 minutes in female mice (n = 12) (repeated meas-
ure analysis of variance, P < 0.01, saline group, n = 12); (b)
The total number of scratches of histamine-depe ndent com-
pound 48/80 during 30 minutes in female mice (n = 12).
(St ude nt’ s un pai red t-test, P < 0.01, saline group, n = 12); (c )
The time course of every 5 minutes of histamine-indepen-
dent SLIGRL-NH2-evoked significant scratching behaviors
during 30 minutes in female mice (n = 12) (repeated meas-
ure analysis of variance, P < 0.01, saline group, n = 12); (d)
The total number of scratches in female mice after the in-
jection of pruritogen or saline in a period of 30 min. Com-
pound 48/80 and SLIGRL-NH2 both induced significantly
higher number of scratching behaviors than saline group
(Student’s unpaired t-test, P < 0.01); (e) The time course of
every 5 minutes of histamine-independent chloroquine-
evoked significant scratching behaviors during 30 minutes
in female mice (n = 12) (repeated measure analysis of vari-
ance, P < 0.01, saline group, n = 12); (f) The total number of
scratches of histamine-independent chloroquine during 30
minutes in female mice (n = 13) (Student’s unpaired t-test, P
< 0.01, saline group, n = 12).
higher in female mice (n = 12) compared with male mice
(n = 10, repeated measure analysis of variance, P < 0.01,
Figures 3(a) and 3(b)). The scratching behaviors in-
duced by intradermal injection of histamine-independent
pruritogenic agent chloroquine was significantly higher in
female mice (n = 12) compared with male mice (n = 10,
repeated measure analysis of variance, P < 0.01, Figures

Sex Differences in Scratching Behaviors Induced by Intradermal Injections of Pruritogenic Chemicals in C57BL/6 Mice
318
3(c) and 3(d)). The scratching behavior induced by in-
tradermal injection of PAR2 agonist SLIGRL-NH2 in
male mice (n = 12) showed no significant difference
from female mice (n = 12, repeated measure analysis of
variance, P > 0.05, Figures 3(e) and 3(f)).
quine as well in C57BL/6 mice.
Tons of evidence demonstrate that sex-associated dif-
ferences are universal in the pain perception and its
modulation. Female exhibits not only more pain sensi-
tivety, lower pain threshold but also tolerance in experi-
mental model and clinical patients as well [14-16,19,20].
Moreover, electrophysiological and imaging studies re-
vealed that different central neurophysiological activeties
to noxious stimuli may contribute to sex-associated dif-
ferences in the perception and modulation of pain. Here,
4. Discussion
The present study showed that the sex-associated differ-
ence in itch sensation induced by intradermal injections
of pruritogenic chemicals compound 48/80 and chloro-
2
2
Figure 3. Significantly higher scratching behaviors induced by intradermal injection of compound 48/80 and chloroquine in
female mice. (a) After the injection of compound 48/80, the scratching behavior was peaked at 20 minutes in female mice,
whereas the peak was arised at 15 minutes after injection in male mice (repeated measure analysis of variance, P < 0.01); (b)
The scratching behavior induced by intradermal injection of compound 48/80 (100 mg/50 ml) was significantly higher in fe-
male mice (n = 12) compared with male mice (n = 10, Student’s unpaired t-test, P < 0.01); (c) The scratching behaviors in-
duced by intradermal injection of chloroquine (200 mg/50 ml) in mal e and female mice (n = 12, repeated measure analysis of
variance, P < 0.01); (d) The scratching behavior induced by intradermal injection of chloroquine was significantly higher in
female mice (n = 12) compared with male mice (n = 10, Student’s unpaired t-test, P < 0.01); (e) The scratching behavior in-
duced by intradermal injection of PAR2 agonist SLIGRL-NH2 in male mice (n = 12) showed no significant difference from
female mice (n = 12, repeated measure analysis of variance, P > 0.05); (f) The total number of scratches was not significantly
different after the injection of SLIGRL-NH2 in a period of 30 minutes between female and male mice (Student’s unpaired
t-test, P > 0.05).

Sex Differences in Scratching Behaviors Induced by Intradermal Injections of Pruritogenic Chemicals in C57BL/6 Mice 319
we presented evidence that both male and female exhib-
ited scratching behaviors in response to both hista [21,22].
Compared to pain sensation, it is still poorly known about
sex-associated differences in itch sensation mine-de-
pendent and histamine-independent pruritogenic chemi-
cals, and sex-associated differences in itch sensation in-
duced by intradermal injections of histamine-dependent
pruritogenic compound 48/80 and histamine-indepen-
dent pruritogenic chloroquine. Histamine is one well
characterized itch mediator till now. In clinic, female
displayed larger wheal responses than male after admini-
stration of histamine by iontophoresis to human subjects
[23]. It was reported that female had higher itching
scores following burns, especially at 3 months postburn
[24]. Female patients were also more severely affected
by pruritus among the sufferer from chronic urticaria
[10]. In experimental studies, there were a few studies
showed that female had more significant scratching be-
haviors induced by the injection of chloroquine [25] and
that female MRL/lpr mice displayed higher chronic
scratching behaviors in a pruritic animal model [17]. All
these results indicated that there may exist sex-associated
differences in itch sensation. Our present study further
provided the evidence that female mice exhibited higher
scratching scores in response to intradermal injections of
histamine-dependent pruritogenic compound 48/80 and
histamine-independent pruritogenic chloroquine. Surpris-
ingly, there was no sex-assocaited difference in itch sen-
sation induced by the intradermal injection of hista-
mine-independent pruritogenic PAR2 agonist. Our re-
sults indicate different mechanisms of chloroquine and
PAR2 agonist might contribute to histamine-independent
itch sensation. Previous study showed that histamine
receptor H1 (H1R) densities of the central histaminergic
neurons in female were higher than male [26]. It is likely
that different distribution of histamine receptor H1 in the
male and female may contribute to the sex-associated
differences in itch senstion induced by histamine-de-
pendent pruritogenic compound 48/80 either through
central or peipheral pruritus pathways. Recently, it is
reported that sex hormone may accelerate the decay of
the histamine-induced peak calcium response in cultured
human umbilical vein endothelial cells [27]. Sex hor-
mones also modulated the intracellular Ca2+ response
induced by histamine in cultured airway smooth muscle
from female patients [28]. Thus, it is very likely that sex
hormones contribute to the sex-associated differences
induced by histamine dependent pruritogenic compound
48/80. But this is not able to explain the reason that the
sex-associated differences induced by histamine-inde-
pendent pruritogenic chloroquine.
Furthermore, our results showed that histamine-inde-
pendent pruritogenic chloroquine induced more itch sen-
sation in female mice. This is in line with that of Green’s
group [25]. It indicated that other endogenous and ex-
ogenous factors such as sex hormones, and genetic fac-
tors might be involved in sex related differences in itch
sensations. The estrous cycle is a short-term, 4 - 5-day
period, in mice according to vaginal cytology [29]. In
clinic, estrogen sensitive women is easy to have estrogen
dermatitis with pruritus [30]. It indicated that estrogen
cycles might significantly affect pruritus behaviors. Thus,
the precise analysis of influence of estrous cycle on
scratching behaviors remained to be further investigated.
In conclusion, the present study demonstrates that the
sex differences occurred in itch sensation induced by in-
tradermal injections of histamine-dependent pruritogen
compound 48/80 and histamine-independent prurito- gen
chloroquine. Investigation into the impact of sex dif-
ferences in itch sensation may enhance our understanding
of the itch transmitting system.
5. Acknowledgements
This work was supported by NSFC (Grant No. 30870830
to Dr. H. X.) and Ministry of Education Foundation for
returned overseas students (HG3503 to Dr. H. X.).
REFERENCES
[1] A. Ikoma, M. Steinhoff, S. Stander, G. Yosipovitch and
M. Schmelz, “The Neurobiology of Itch,” Nature Reviews,
Neuroscience, Vol. 7, No. 7, 2006, pp. 535-547.
[2] G. Yosipovitch, M. W. Greaves and M. Schmelz, “Itch,”
The Lancet, Vol. 361, No. 9358, 2003, pp. 690-694.
doi:10.1016/S0140-6736(03)12570-6
[3] E. Weisshaar, M. J. Kucenic and A. B. Fleischer, “Pruri-
tus: A Review,” Acta Derm Venereology, Vol. 213, 2003,
pp. 5-32.
[4] S. Stander, E. Weisshaar, M. Steinhof, T. A. Luger and D.
Metze, “Pruritus—Pathophysiology, Clinical Features and
Therapy—An Overview,” Journal of the German Society
of Dermatology, Vol. 1, No. 2, 2003, pp. 105-118.
doi:10.1046/j.1610-0387.2003.02023.x
[5] A. Reich, S. Stander and J. C. Szepietowski, “Drug-Induced
Pruritus: A Review,” Acta Derm Venereologica, Vol. 89,
No. 3, 2009, pp. 236-244. doi:10.2340/00015555-0650
[6] S. Davidson and G. J. Giesler, “The Multiple Pathways
for Itch and Their Interactions with Pain,” Trends Neuro-
sciences, Vol. 33, No. 12, 2010, pp. 550-558.
doi:10.1016/j.tins.2010.09.002
[7] Y. Kuraishi, T. Nagasawa, K. Hayashi and M. Satoh,
“Scratching Behavior Induced by Pruritogenic but Not
Algesiogenic Agents in Mice,” European Journal of Phar-
macology, Vol. 275, No. 3, 1995, pp. 229-233.
doi:10.1016/0014-2999(94)00780-B
[8] M. Steinhoff, U. Neisius, A. Ikoma, M. Fartasch, G. Heyer,
P. S. Skov, T. A. Luger and M. Schmelz, “Proteinase Acti-
vated Receptor-2 Mediates Itch: A Novel Pathway for Pru-
ritus in Human Skin,” Journal of Neuroscience, Vol. 23,
No. 15, 2003, pp. 6176-6180.

Sex Differences in Scratching Behaviors Induced by Intradermal Injections of Pruritogenic Chemicals in C57BL/6 Mice
320
[9] S. G. Shimada, K. A. Shimada and J. G. Collins, “Scratch-
ing Behavior in Mice Induced by the Proteinase-Activated
Receptor-2 Agonist, SLIGRL-NH2,” European Journal of
Pharmacology, Vol. 530, No. 3, 2006, pp. 281-283.
doi:10.1016/j.ejphar.2005.11.012
[10] A. Mlynek, M. Magerl, M. Hanna, S. Lhachimi, I. Baiardini,
G. W. Canonica, Z. Brzoza, A. Kasperska-Zajac, B. Rogala,
A. Zalewska-Janowska, T. Zuberbier and M. Maurer, “The
German Version of the Chronic Urticaria Quality-of-Life
Questionnaire: Factor Analysis, Validation, and Initial
Clinical Findings,” Allergy, Vol. 64, No. 6, 2009, pp.
927-936. doi:10.1111/j.1398-9995.2008.01920.x
[11] P. Dyachenko, A. Shustak and D. Rozenman, “Hemodi-
alysis-Related Pruritus and Associated Cutaneous Mani-
festations,” International Journal of Dermatology, Vol.
45, No. 6, 2006, pp. 664-667.
doi:10.1111/j.1365-4632.2005.02592.x
[12] Y.-G. Sun and Z.-F. Chen, “A Gastrin-Releasing Peptide
Receptor Mediates the Itch Sensation in the Spinal Cord,”
Nature, Vol. 448, No. 7154, 2007, pp. 700-703.
[13] C. C. A. Palmeira, H. A. Ashmawi and I. P. Posso, “Sex
and Pain Perception and Analgesia,” Brazilian Journal of
Anesthesiology, Vol. 61, No. 6, 2011, pp. 814-828.
[14] K. J. Berkley, S. S. Zalcman and V. R. Simon, “Sex and
Gender Differences in Pain and Inflammation: A Rapidly
Maturing Field,” American Journal of Physiology Re-
gulatory Integrative and Comparative Physiology, Vol.
291, No. 2, 2006, pp. R241-R244.
doi:10.1152/ajpregu.00287.2006
[15] R. B. Fillingim, C. D. King, M. C. Ribeiro-Dasilva, B.
Rahim-Williams and J. L. Riley, “Sex, Gender, and Pain:
A Review of Recent Clinical and Experimental Findings,”
Journal of Pain, Vol. 10, No. 5, 2009, pp. 447-485.
doi:10.1016/j.jpain.2008.12.001
[16] C. S. Weisse, K. K. Foster and E. A. Fisher, “The Influ-
ence of Experimenter Gender and Race on Pain Reporting:
Does Racial or Gender Concordance Matter?” Pain Medi-
cine, Vol. 6, No. 1, 2005, pp. 80-87.
doi:10.1111/j.1526-4637.2005.05004.x
[17] H. Umeuchi, Y. Kawashima, C. A. Aoki, T. Kurokawa, K.
Nakao, M. Itoh, K. Kikuchi, T. Kato, K. Okano, M. E.
Gershwin and H. Miyakawa, “Spontaneous Scratching
Behavior in MRL/Lpr Mice, a Possible Model for Pru-
ritus in Autoimmune Diseases, and Antipruritic Activity
of a Novel K-Opioid Receptor Agonist Nalfurafine Hy-
drochloride,” European Journal of Pharmacology, Vol.
518, No. 2-3, 2005, pp. 133-139.
doi:10.1016/j.ejphar.2005.06.019
[18] Q. Liu, Z. Tang, L. Surdenikova, S. Kim, K. N. Patel, A.
Kim, F. Ru, Y. Guan, H. J. Weng, Y. Geng, B. J. Undem,
M. Kollarik, Z. F. Chen, D. J. Anderson and X. Dong,
“Sensory Neuron-Specific GPCR Mrgprs Are Itch Re-
ceptors Mediating Chloroquine-Induced Pruritus,” Cell,
Vol. 139, No. 7, 2009, pp. 1353-1365.
doi:10.1016/j.cell.2009.11.034
[19] J. M. Tall and T. Crisp, “Effects of Gender and Gonadal
Hormones on Nociceptive Responses to Intraplantar Car-
rageenan in the Rat,” Neuroscience Letters, Vol. 354, No.
3, 2004, pp. 239-241. doi:10.1016/j.neulet.2003.09.081
[20] J. Lei, L. Jin, Y. Zhao, M. Y. Sui, L. Huang, Y. X. Tan, Y.
K. Chen and H. J. You, “Sex-Related Differences in De-
scending Norepinephrine and Serotonin Controls of Spi-
nal Withdrawal Reflex during Intramuscular Saline In-
duced Muscle Nociception in Rats,” Experimental Neu-
rology, Vol. 228, No. 2, 2011, pp. 206-214.
doi:10.1016/j.expneurol.2011.01.004
[21] L. A. Henderson, S. C. Gandevia and V. G. Macefield,
“Gender Differences in Brain Activity Evoked by Muscle
and Cutaneous Pain: A Retrospective Study of Single-Trial
Fmri Data,” Neuroimage, Vol. 39, No. 4, 2008, pp. 1867-
1876. doi:10.1016/j.neuroimage.2007.10.045
[22] P. E. Paulson, S. Minoshima, T. J. Morrow and K. L.
Casey, “Gender Differences in Pain Perception and Pat-
terns of Cerebral Activation during Noxious Heat Stimu-
lation in Humans,” Pain, Vol. 76, No. 1-2, 1998, pp.
223-229. doi:10.1016/S0304-3959(98)00048-7
[23] W. Magerl, R. A. Westerman, B. Mohner and H. O.
Handwerker, “Properties of Transdermal Histamine Ionto-
phoresis: Differential Effects of Season, Gender, and Body
Region,” Journal of Investigative Dermatology, Vol. 94,
No. 3, 1990, pp. 347-352.
doi:10.1111/1523-1747.ep12874474
[24] N. E. Van Loey, M. Bremer, A. W. Faber, E. Middelkoop
and M. K. Nieuwenhuis, “Itching Following Burns: Epi-
demiology and Predictors,” British Journal of Dermatol-
ogy, Vol. 158, No. 1, 2008, pp. 95-100.
[25] A. D. Green, K. K. Young, S. G. Lehto, S. B. Smith and J.
S. Mogil, “Influence of Genotype, Dose and Sex on Pru-
ritogen-Induced Scratching Behavior in the Mouse,” Pain,
Vol. 124, No. 1-2, 2006, pp. 50-58.
doi:10.1016/j.pain.2006.03.023
[26] P. Ghi, M. Orsetti, S. R. Gamalero and C. Ferretti, “Sex
Differences in Memory Performance in the Object Rec-
ognition Test. Possible Role of Histamine Receptors,” Phar-
macology Biochemistry and Behavior, Vol. 64, No. 4, 1999,
pp. 761-766. doi:10.1016/S0091-3057(99)00143-4
[27] V. M. Miller, L. Li and G. C. Sieck, “Endothelium De-
pendent Effects of Estrogen on Vasomotor Tone. Conse-
quences of Nongenomic Actions,” Vascular Pharmacol-
ogy, Vol. 38, No. 2, 2002, pp. 109-113.
doi:10.1016/S0306-3623(02)00134-9
[28] E. A. Townsend, M. A. Thompson, C. M. Pabelick and Y.
S. Prakash, “Rapid Effects of Estrogen on Intracellular
Ca2+ Regulation in Human Airway Smooth Muscle,”
American Journal of Physiology Lung Cellular and Mo-
lecular Physiology, Vol. 298, No. 4, 2011, pp. L521-L530.
doi:10.1152/ajplung.00287.2009
[29] S. L. Byers, M. V. Wiles, S. L. Dunn and R. A. Taft,
“Mouse Estrous Cycle Identification Tool and Images,”
PLoS One, Vol. 7, No. 4, 2012, Article ID: e35538.
doi:10.1371/journal.pone.0035538
[30] O. A. Leylek, S. Unlu, S. Ozturkcan, A. Cetin, M. Sahin
and E. Yildiz, “Estrogen Dermatitis,” European Journal
of Obstetrics & Gynecology and Reproductive Biology,
Vol. 72, No. 1, 1997, pp. 97-103.