Am J Physiol Regul Integr Comp Physiol 295: R954–R960, 2008.
First published July 23, 2008; doi:10.1152/ajpregu.90406.2008.
Sex-related differences in activity of lower urinary tract in response
to intravesical acid irritation in decerebrate unanesthetized mice
Mitsuharu Yoshiyama,1,2 Hideki Kobayashi,1 Isao Araki,1 Shuqi Du,1 Hidenori Zakoji,1
and Masayuki Takeda1
1
Department of Urology, University of Yamanashi Interdisciplinary Graduate School of Medicine and Engineering,
Chuo, Yamanashi, and 2Yamanashi Rehabilitation Hospital, Fuefuki, Yamanashi, Japan
Submitted 6 May 2008; accepted in final form 21 July 2008
urological field, it has been well documented that prevalence
and incidence of interstitial cystitis, which is a syndrome
consisting of severe refractory bladder symptoms such as
suprapubic pain, urinary frequency, and urgency without a
specific identifiable cause, for women are significantly higher
than those for men (10). What pathophysiological factors are
involved in these sex-related differences?
Here, we proposed the possibilities that sex differences in
mechanisms responsible for transmitting inflammatory pain
signals are associated with degree and frequency of presenting
symptoms in interstitial cystitis or painful bladder syndrome
and thus that nociceptive afferent processing of the two sexes
has different characteristics in the transmissions. Thus the
present study was conducted to compare between female and
male mice responses in the lower urinary tract to acid irritation
and to explore clinical relevance in the differences.
A preliminary report of this study has been presented in
abstract form (41).
MATERIALS AND METHODS
NUMEROUS IN VIVO PHYSIOLOGICAL and pharmacological studies
examining bladder and urethral activity have been conducted
exclusively in one sex of animals in each experimental design,
and thus it is uncertain whether functional responses of female
and male lower urinary tracts are similar. There are only a few
articles detailing differences between females and males in the
lower urinary tract functions. Of the available reports, for
example, one previous study comparing female and male rats
in cystometry revealed that volume thresholds for inducing
micturition were changed in accordance with altered intravesical infusion rates in the female rat, whereas these were
constant regardless of the bladder filling rates in the male (24).
The study suggested the possibility that mechanosensory afferent responses to intensity of the mechanical stimulus were
different between the female and male animals.
Epidemiologic studies have demonstrated sex-specific differences in prevalence and incidence of certain diseases. In the
Animal preparation. Six female and six male mice (C57BL/6,
12–13 wk old, Charles River Laboratories, Yokohama, Japan) weighing 19.3 ⫾ 0.3 and 23.9 ⫾ 0.4 g, respectively, were used in this study.
The animals were housed under a 12:12-h light-dark cycle with
controlled humidity and temperature. Standard pellet diet and water
were available ad libitum. All animal procedures were reviewed and
approved by the University of Yamanashi Institutional Animal Care
and Use Committee. All efforts were made to minimize animal
suffering and to reduce the number of animals used. The animals were
anesthetized with sevoflurane (2–3%) in oxygen (flow rate 0.2 l/min)
during surgery before decerebration. The trachea was cannulated with
a polyethylene tube (PE-90, Clay-Adams, Parsippany, NJ) to facilitate
respiration. The bladder was exposed by way of a midline abdominal
incision. The bladder end of a polyethylene catheter (PE-50, ClayAdams) was heated to create a collar and passed through a small
incision at the apex of the bladder dome, and a suture was tightened
around the collar of the catheter. The catheter was exited near the
xiphoid process.
Precollicular decerebration was performed according to a published
method (32) that included ligating both carotid arteries, followed by
midline incision of head skin with a scalpel and removal of the skull
and forebrain with a fine rongeur and a blunt spatula, respectively.
Sevoflurane was then discontinued. After no further intracranial hemorrhage was visually detected, both lateral flaps of the incised head
skin were sutured together. Experiments were started 2 h after the
decerebration and conducted under unanesthetized conditions (40, 41,
45– 47, 52–56).
Address for reprint requests and other correspondence: M. Yoshiyama,
Yamanashi Rehabilitation Hospital, 855 Komatsu, Kasugai-cho, Fuefuki, Yamanashi, 406-0004, Japan (e-mail: [email protected]).
The costs of publication of this article were defrayed in part by the payment
of page charges. The article must therefore be hereby marked “advertisement”
in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
afferent; bladder tonus; detrusor contractility; efferent; urethral resistance
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0363-6119/08 $8.00 Copyright © 2008 the American Physiological Society
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Yoshiyama M, Kobayashi H, Araki I, Du S, Zakoji H, Takeda M.
Sex-related differences in activity of lower urinary tract in response to
intravesical acid irritation in decerebrate unanesthetized mice. Am J
Physiol Regul Integr Comp Physiol 295: R954 –R960, 2008. First published July 23, 2008; doi:10.1152/ajpregu.90406.2008.—Sex-related differences in lower urinary tract (LUT) activity responding to intravesical
infusion of diluted acetic acid (A/A, pH 3.0) were investigated during
cystometrograms in decerebrate unanesthetized mice. A/A produced a
decrease of intercontraction intervals in both female and male animals,
and the extent of the decrease in male mice was much less than in female
mice [19 ⫾ 5% (P ⫽ 0.03) vs. 65 ⫾ 5% (P ⫽ 0.03); n ⫽ 6 for each],
exhibiting a marked difference between the two groups in response to
acid irritation of the LUT (P ⫽ 0.002). A/A reduced maximal voiding
pressure (MVP) (19 ⫾ 4%, P ⫽ 0.03) but had no effect on pressure
threshold for inducing voiding contraction (PT) (P ⫽ 0.56) in females,
whereas A/A did not change MVP (P ⫽ 1.00) but increased PT (16 ⫾
4%, P ⫽ 0.03) in males. A/A decreased bladder compliances of female
and male mice in a similar fashion (44 ⫾ 10% vs. 24 ⫾ 7%, P ⫽ 0.03
for each). In male mice, A/A produced persistent dribbling of fluid after
voiding contraction phase, which was virtually not seen in females. The
present study demonstrates the differences between female and male
mice in response to noxious stimulation in the LUT: the female bladder
is more sensitive to the acid irritation, while the male urethra is more
irritable to the noxious stimulus. Identification of mechanisms underlying
sex-specific characteristics might be helpful for elucidating pathogenesis
of painful bladder syndrome.
SEX-SPECIFIC DIFFERENCES IN CYSTOMETRY OF MICE
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analysis of variance (ANOVA) were used for statistical analyses, if
applicable. The correlation analysis between various CMG parameters, except for ICI versus ACT, BCD versus 1st PD, BCD versus 2nd
PD, and ACT versus 2nd PD, which partly included each other, was
performed with Spearman rank-order correlation calculations. For all
analyses, P ⬍ 0.05 was considered significant.
Drugs. Sevoflurane (Maruishi Pharmaceutical, Osaka, Japan)
and A/A (99.7%; Nacalai Tesque, Kyoto, Japan) were used in the
present study. To make diluted A/A solution for intravesical
infusion, several drops of 99.7% acetic acid were added to 20 ml
of physiological saline and the pH was adjusted to 3.0 under
monitoring (with compact pH meter B-212, Horiba, Kyoto, Japan)
(final concentration ⬃0.23%).
RESULTS
Saline was infused into the bladders for 2 h before baseline values
were measured. Bladder activity was monitored by way of the cystometry catheter connected to a pressure transducer. Cystometric
recordings were performed by continuously infusing physiological
saline (30 ␮l/min) at room temperature into the bladder to elicit
repetitive voids, which allowed data collection for a large number of
voiding cycles (26). As shown in Fig. 1, the cystometric parameters
measured during continuous cystometrograms (CMGs) were maximal
voiding pressure (MVP, mmHg), which is the peak intraluminal
pressure during voiding; closing peak pressure (CPP, mmHg), which
is the peak pressure during the postvoiding phase of a bladder
contraction; pressure threshold (PT, mmHg), which is the intraluminal
pressure to induce a voiding contraction; resting pressure (RP,
mmHg), which is the lowest pressure immediately after a voiding
contraction; intercontraction interval (ICI, s), which is the time lag
between two voiding cycles; and bladder compliance (BCP, ␮l/
mmHg), which was calculated as fluid volume infused between two
time points of RP and the following PT [i.e., infusion rate ⫻ ICI/60
(␮l)/(PT ⫺ RP) (mmHg)] (14, 18, 43, 47). Bladder contraction
duration (BCD, s) is the sum of the first phase duration (1st PD, s) and
the second phase duration (2nd PD, s). Actual collecting time (ACT,
s) consists of ICI and 2nd PD, which indicates the actual storage phase
until induction of the following voiding contraction during continuous
CMGs.
Acetic acid (A/A) infusion into the bladder has been used widely as
a model for lower urinary tract irritation (57). Effects of diluted A/A
(pH 3.0) intravesically infused for 1 h on bladder activity were
evaluated after the baseline infusion of saline. After the A/A infusion,
saline was again infused into the bladder to examine recovery of the
irritated lower urinary tract.
Data analysis and statistics. Three consecutive storage phases and
voiding contractions immediately before and during A/A infusion
were evaluated as baseline control (i.e., saline infusion) and effect of
acidic irritation, respectively. All values are expressed as means ⫾
SE. Mann-Whitney test, Wilcoxon matched pairs test, and two-way
AJP-Regul Integr Comp Physiol • VOL
Fig. 2. Top: typical bladder activity during continuous saline infusion
cystometrogram (CMG; 30 ␮l/min) in a female mouse. Bottom: effects of
intravesical infusion of diluted acetic acid (pH 3.0) on bladder activity. Note
that diluted acetic acid markedly reduces MVPs and ICIs.
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Fig. 1. A: representative voiding cycle during cystometry in a mouse. B: a
bladder contraction extended from that on right in A. Voiding occurred during
the period indicated in B. ICI, intercontraction interval; BCD, bladder contraction duration; 1st PD, 1st phase contraction duration; 2nd PD, 2nd phase
contraction duration; PT, pressure threshold for inducing micturition contraction; MVP, maximal voiding pressure; CPP, closing peak pressure; RP, resting
pressure. Actual collecting time (ACT) and bladder compliance (BCP) were
calculated as 2nd PD ⫹ ICI and infusion rate ⫻ (ICI/60)/(PT ⫺ RP)
respectively.
Figures 2 and 3 show cystometric activity during intravesical
infusion of saline (Figs. 2 and 3, top) and diluted A/A (Figs. 2
and 3, bottom) in female and male mice, respectively.
Before A/A infusion, baseline values of cystometric parameters in female and male mice were measured as presented in
Table 1. In male mice, MVP was correlated with ICI (r ⫽ 1.00)
and ACT (r ⫽ 1.00) (Fig. 4A), while CPP was correlated with
2nd PD (r ⫽ 1.00) (Fig. 4B) (P ⫽ 0.003 in each analysis). No
statistically significant correlations, however, were found between these parameters in female mice.
Bladder activity was gradually changed during A/A infusion, and the peak effects, which were evaluated for analysis,
were achieved within 40 min of acid intravesical infusion in
both sexes.
In female mice (n ⫽ 6) A/A infusion significantly decreased
MVP (mean ⫽ 19 ⫾ 4%), BCP (mean ⫽ 44 ⫾ 10%), ICI
(mean ⫽ 65 ⫾ 5%), and ACT (mean ⫽ 62 ⫾ 5%), whereas it
increased RP (mean ⫽ 65 ⫾ 14%) (Fig. 5). The acid irritation
did not change PT (P ⫽ 0.56), CPP (P ⫽ 0.09), 1st PD (P ⫽
0.56), 2nd PD (P ⫽ 0.16), or BCD (P ⫽ 0.84) (Fig. 5). No
statistically significant correlations were found during A/A as
well as saline infusion CMGs between any parameters in
female mice.
In male mice (n ⫽ 6) the A/A irritation increased RP
(mean ⫽ 44 ⫾ 13%), PT (mean ⫽ 16 ⫾ 4%), 2nd PD (mean ⫽
115 ⫾ 25%), and BCD (mean ⫽ 86 ⫾ 15%), whereas it
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SEX-SPECIFIC DIFFERENCES IN CYSTOMETRY OF MICE
decreased ICI (mean ⫽ 19 ⫾ 5%), ACT (mean ⫽ 28 ⫾ 4%),
and BCP (mean ⫽ 24 ⫾ 7%) (Fig. 5). The A/A infusion had no
effect on MVP (P ⫽ 1.00), CPP (P ⫽ 0.31), and 1st PD (P ⫽
0.76) (Fig. 5). The A/A infusion produced persistent “fluid
dribbling” at the 2nd PD in all male mice (Fig. 3), which was
not seen in female mice (Fig. 2). Because of the repeated fluid
leakage in the 2nd PD, for analysis the ACT values were
evaluated as identical to the ICIs during A/A infusion in the
male mice. The increase of BCD in males was attributed to
prolongation of 2nd PD, one of the components composing
BCD (Figs. 3 and 5). During A/A infusion CMGs, MVP was
correlated with ICI (r ⫽ 0.94) and ACT (r ⫽ 0.94) (Fig. 6A)
and 2nd PD (r ⫽ 0.87) (data not shown), while CPP was
correlated with 2nd PD (r ⫽ 0.87) (Fig. 6B) (P ⬍ 0.05 in each
analysis).
Figure 7 shows the changes of parameter values in the lower
urinary tract activity after as well as during the A/A infusion
expressed in percentage of baseline (i.e., during initial saline
infusion). The subsequent intravesical saline infusion during
the recovery period reversed the female lower urinary tract
activity altered by acid irritation within 2 h, whereas it
rather suppressed the male bladder activity and tonus (Fig.
7, B–E, G, H).
DISCUSSION
To our knowledge, this is the first report demonstrating
sex-specific differences in reflex activity of the lower urinary
tract responding to intravesical noxious stimulation. The
present study revealed that intravesical acid irritation produced
different responses between female and male mice in ICI,
Fig. 4. Data collected during saline infusion CMGs were calculated by the
Spearman rank-order correlation analysis. Coefficients (r) between MVP and
ICI or ACT (A) and between CPP and 2nd PD (B) are shown.
ACT, BCD, 2nd PD, MVP, CPP, and PT, whereas it caused
common changes in both sexes in RP and BCP. Of the
evaluated parameters, only 1st PD was not affected by acid
irritation to the lower urinary tract. The detailed cystometric
analyses were conducted to facilitate better understanding of
the complex autonomic functions.
The most intriguing discovery in the present study was that
intravesical acid irritation more markedly decreased ICI and
ACT in female mice than in male mice, suggesting that female
bladders were more sensitive to the noxious stimulation than
male bladders. ICI has been generally evaluated as a parameter
indicative of afferent excitability (26). Some attention, however, should be paid to the fact that ICI can be affected by
diverse factors including volume threshold for inducing micturition, voided volume, residual volume (i.e., voiding efficiency), detrusor tonus (i.e., related to bladder compliance), or
combinations of these. In the present study, intravesical A/A
increased RP and decreased BCP in both female and male
mice. The altered RP can be indicative of residual volume
change. However, it seems likely that the increase of RP is
Table 1. Comparisons between female and male mice in cystometric parameters
Group
PT, mmHg
MVP, mmHg
CPP, mmHg
RP, mmHg
BCP, ␮l/mmHg
1st PD, s
2nd PD, s
BCD, s
ICI, s
ACT, s
Female
Male
6.1⫾0.8
4.9⫾0.3
22.8⫾1.2
24.6⫾1.5
17.0⫾2.5
18.2⫾3.6
1.8⫾0.3
2.2⫾0.2
32.9⫾7.4
44.9⫾8.2
8.0⫾0.5
9.7⫾1.1
17.8⫾2.5
25.6⫾4.6
25.8⫾2.6
35.3⫾4.6
257⫾47
195⫾28
275⫾48
221⫾31
Values are means ⫾ SE for 6 mice/group. PT, pressure threshold for inducing micturition contraction; MVP, maximal voiding pressure; CPP, closing peak
pressure; RP, resting pressure; BCP, bladder compliance; 1st PD, 1st phase duration of bladder contraction; 2nd PD, 2nd phase duration of bladder contraction;
BCD, bladder contraction duration; ICI, intercontraction interval; ACT, actual collecting time. No differences were found between female and male mice in all
parameters indicated (by Mann-Whitney test).
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Fig. 3. Top: typical bladder activity during continuous saline infusion CMG
(30 ␮l/min) in a male mouse. Bottom: effects of intravesical infusion of diluted
acetic acid (pH 3.0) on bladder activity. Note that diluted acetic acid produces
little change in MVPs and ICIs. *Fluid dribbling.
SEX-SPECIFIC DIFFERENCES IN CYSTOMETRY OF MICE
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attributed to the decreased BCP and that the influence of
residual volume change seems to be minimal because values
obtained by a calculation of BCP ⫻ RP during A/A infusion
were similar to those during baseline saline infusion in both
groups. Thus we concluded that the decreased ICI or decreased
Fig. 6. Data collected during acetic acid infusion CMGs were calculated by
the Spearman rank-order correlation analysis. Coefficients (r) between MVP
and ICI or ACT (A) and between CPP and 2nd PD (B) are shown.
AJP-Regul Integr Comp Physiol • VOL
ACT was due to decreased volume threshold for inducing
micturition (i.e., increase of afferent excitability) produced by
A/A infusion and that the afferent sensitivity to acid irritation
in female mice was certainly greater than that in males.
Alternatively, the result may be dependent on differences
between females and males in acid permeability of the epithelium.
A/A infusion increased BCD in males, whereas it did not
change the parameter in females. This difference was due to
different responses of second phase contraction to the A/A
irritation between the two groups. In male mice A/A induced
persistent dribbling of fluid out of the urethral orifice accompanying intravesical pressure fluctuation (Fig. 3), which was
not seen in female mice (Fig. 2), during the second phase
contraction. The leakage of fluid occurring in the second phase
contraction seems to be induced as a response to acid irritation
to urethra because it appeared immediately after voiding (i.e.,
after the 1st phase contraction) but not at storage phase,
suggesting a possible involvement of the “urethra-to-bladder”
reflex (13, 19) in the mechanism. In conclusion, the functional
response of the urethra to acid irritation is different between the
two sexes, and the male urethra is more irritable to acid
stimulus.
In the present study MVP was decreased by A/A infusion in
females, whereas it was not changed in males. Decrease of
MVP can be attributed to decreased detrusor contractility,
reduced urethral resistance during micturition, or both. In this
respect, there has been no report of in vitro or in vivo physiological studies detailing the sex differences in detrusor and
urethral responses to acid. The present study also did not
determine the mechanism responsible for the MVP change by
A/A in female mice. It is of value to compare detrusor and
urethral contractile responses to low pH between females and
males in an in vitro experimental setting. Previous studies
showed that a lower concentration of A/A (0.1%) did not
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Fig. 5. Effects of intravesical infusion of diluted acetic acid (pH 3.0) on cystometric parameters in female (E) and male (F) mice. sl, Saline infusion; aa, diluted
acetic acid (pH 3.0) infusion. Comparisons between saline and acetic acid were made by Wilcoxon matched pairs test (*P ⬍ 0.05). Two-way repeated-measures
ANOVA was used to examine whether cystometric responses to diluted acetic acid intravesical infusion were different between female and male mice (#P ⬍
0.05, ##P ⬍ 0.01, ###P ⬍ 0.001). n.s., Not significant.
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SEX-SPECIFIC DIFFERENCES IN CYSTOMETRY OF MICE
change MVP in urethane-anesthetized female rats (20, 57) but
increased MVP in conscious female rats (30). The increased
MVP in the conscious animal might be caused by an excessive
behavioral response such as increase of abdominal pressure
due to the acid irritation passing through the urethra.
CPP is indicative of intravesical pressure increase during
bladder neck/proximal urethra closing. CPP of females responded differently to A/A from that of males (according to
2-way ANOVA), albeit that the A/A irritation did not statistically change the CPP in either female or male mice. The
second phase contraction in which the CPP is involved does
not exhibit micturition during the normal voiding cycle (40),
and its functional role or significance has not been determined
(4). A recent study, on the other hand, showed that a ␤-adrenergic agonist, isoproterenol, decreased CPP without changing
MVP and the decrease of CPP in muscarinic M2 receptordeficient mice was more marked than that in wild-type mice
(16). Birder et al. (4) reported that activation of transient
receptor potential vanilloid (TRPV)4 channels by the selective
agonist 4␣-phorbol 12,13-didecanoate increased CPP. These
studies suggest that CPP is at least related to the mechanisms
via the ␤-adrenergic system and TRPV4 channels in the bladder. Further investigations are warranted to elucidate the relevance of CPP change.
PT of males was altered by A/A differently than PT of
females (Fig. 5A). Fluid volume infused into the bladder to
induce a voiding contraction (i.e., ACT ⫻ infusion rate, ␮l)
and bladder tone (i.e., related to BCP, ␮l/mmHg) approximately have positive and negative relationships, respectively,
with PT (mmHg) in each voiding cycle. Thus changes of the
ACT versus BCP relationship by A/A irritation significantly
affected the PT changes in the two groups. Because BCPs of
female and male mice were decreased by A/A in a similar
fashion (Fig. 5E), the difference in PTs was largely dependent
on degrees of ACT reduction by A/A (Fig. 5J).
AJP-Regul Integr Comp Physiol • VOL
Increased RP and decreased BCP were common changes by
A/A intravesical infusion in both female and male mice.
Changes of these parameters are significantly related. Increases
of RP in the present study were largely due to decreased BCP
(i.e., increased bladder tone), although change in RP can be
also affected by change in residual volume. These can be
confirmed by comparing values obtained by a calculation of
RP ⫻ BCP before and during A/A infusions.
A/A decreased BCP equivalently in female and male mice.
The change in BCP suggested the possibility that pH 3.0 A/A
could pass through the mucosa and reach the detrusor, affecting its tonus during urine storage. Of interest is that normal
saline infusion after A/A (i.e., during recovery time) reversed
the values of CMG parameters changed by A/A within 2 h in
females, whereas it rather suppressed bladder contractions and
further decreased BCP in males. These discrepancies in male
mice should be evaluated in future studies.
There were significant correlations in MVP versus ICI or
ACT and CPP versus 2nd PD in males, while no significant
correlations in evaluated parameters were found in females.
ICI and ACT are parameters affected by afferent excitability,
sympathetic implication, or a combination of both (26, 46),
although they can be also altered by intravesical residual fluid
after voiding. MVP is determined by detrusor contractility (i.e.,
at least parasympathetic component) and urethral resistance
(i.e., sympathetic and somatic components) (12, 43, 46, 47).
Taken together, it seems likely that sympathetic control, at
least, has a key role in significant correlations between MVP
and ICI/ACT in male mice. On the other hand, lower CPP can
be indicative of larger sympathetic participation via ␤-adrenoceptors in the detrusor (16), and thus less time (i.e., shorter 2nd
PD) is required until the detrusor starts playing a role in the
storage phase, in which the sympathetic mechanisms are dominant. Such correlations, however, were not found in female
mice, and the reasons are unknown.
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Fig. 7. Time courses of cystometric parameter % changes during acetic acid and subsequent saline intravesical infusion in female (E) and male (F) mice. sl, saline
infusion; aa, diluted acetic acid (pH 3.0) infusion. Values during acetic acid or subsequent saline (at 1 h, 2 h, and 3 h of recovery phase) infusion were
compared with baseline values of saline infusion by Wilcoxon matched pairs test (*P ⬍ 0.05, using actual values).
SEX-SPECIFIC DIFFERENCES IN CYSTOMETRY OF MICE
Perspectives and Significance
The present results suggest that there are sex-related differences in lower urinary tract activity in response to intravesical
acid stimulus, which may be associated with the differences in
prevalence and incidence of interstitial cystitis or painful bladder syndrome that is more frequently found in women. The
sites responsible for the differences can be at the local organs,
the peripheral nervous system, and/or the central nervous
system (i.e., urothelium, detrusor, dorsal root ganglia, spinal
AJP-Regul Integr Comp Physiol • VOL
cord, brain stem, or combinations of these), and the underlying
mechanisms can be various (e.g., via TRPV1, ASICs, estrogen
receptors, progesterone receptors, nerve growth factor, interleukin-1, 5-hydroxytryptamine, bradykinin) (5, 21, 28, 33).
Further investigations are necessary to elucidate the mechanisms involved in the sex differences and to develop treatments
for the bladder pain.
ACKNOWLEDGMENTS
Present address of S. Du: Dept. of Urology, First Affiliated Hospital, China
Medical University, 155 Nanjingbei St., Heping District, Shenyang, Liaoning
110001 China.
GRANTS
This work was supported by Grants-in-Aid for Scientific Research from the
Ministry of Education, Culture, Sports, Science and Technology of Japan
(Grant No. 19591839) (M. Yoshiyama).
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The present study was performed in decerebrate unanesthetized mice. Decerebration, which removes the forebrain receiving signals from the peripheral organs and supplying both
excitatory and inhibitory inputs to the brain stem, has been
established for in vivo urological research (27, 40, 41, 45– 47,
52–56) and allows the study of involuntary “reflex” activity of
the lower urinary tract as the brain stem (i.e., including periaqueductal gray and pons), spinal cord (i.e., including thoracic
sympathetic neurons and lumbosacral parasympathetic neurons), and peripheral nerves in which neural circuits responsible for voiding and storage (12) are intact. On the other hand,
conscious (9, 18, 43, 49, 50) and anesthetized (14, 24, 26, 42,
48, 51, 57, 58) rodents are mainstays for in vivo studies.
Voluntary micturition “behavior” in conscious animals, however, can be affected by stress from circumstances (31, 36) or
drugs that change the mood of the animal (37, 44), and thus
greater attention should be paid to the use of the model.
Anesthesia such as urethane may cause pharmacological interactions (52) and disturb efficient voiding (56), while it remains
a choice because of its convenience and suitability in in vivo
neurophysiological and pharmacological experiments to study
“reflex” micturition (25, 29).
These studies demonstrated that there were significant sexrelated differences in the reflex bladder response to noxious
stimulation by diluted A/A (pH 3.0), revealing that the female
bladder is more sensitive and vulnerable to the irritation. The
result is compatible with previously reported epidemiologic
studies in human populations (6, 8, 38) and laboratory experiments in animals (1, 3, 7, 17, 34, 35) showing that the female
is highly sensitive to noxious stimuli and pain. On the other
hand, our present results suggest the possibility that the male
urethra is affected more evidently. What mechanisms are
involved in these differences in the lower urinary tract responses? A previous study using immunohistochemical technique revealed large quantitative differences between female
and male mice in gene expression of progesterone receptors
and estrogen receptors in the bladder and urethra (33). It has
been suggested that estrogens modulate pain (2, 11) and
inflammatory response (23) and that progesterone receptors
and estrogen receptors are involved in signal transmission of
pain (15, 22). Furthermore, our recent study (21) validated
sex-specific differences in quantities of gene expressions of
TRPV1 (5) and acid-sensing ion channels (ASICs) (39), both
of which were channels sensitive to extracellular protons (i.e.,
pH), in both mucosal and muscle layers of the mouse bladder.
The differential quantities of these channels distributed in
bladders of females and males can be associated with sexrelated sensitivity to noxious acidic stimulus (e.g., inflammation), and thus it seems reasonable to target these channels for
the treatment of pain in the lower urinary tract.
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