 |
INTRODUCTION |
The pelvic parasympathetic ganglion is not only a relay station
that distributes excitatory signals from the CNS to the urogenital organs but also a modulatory site for the neuronal information (Akasu and Nishimura 1995
; de Groat and Booth
1980
; Keast 1995
). Neurons in the pelvic
ganglion receive cholinergic input from the
S2-S4 sacral region of the
spinal cord via the pelvic nerve and adrenergic input from the inferior
mesenteric (sympathetic) ganglia via the hypogastric nerve
(Akasu and Nishimura 1995
; de Groat et al.
1993
; Keast 1995
). Stimulation of the pelvic
nerve evoked an excitatory postsynaptic potential mediated by the
nicotinic actions of acetylcholine (Akasu and Nishimura
1995
; de Groat and Booth 1980
; Gallagher
et al. 1982
).
-Aminobutyric acid (GABA), a typical
inhibitory transmitter in the CNS, may play a role in the lower urinary
tract (Maggi et al. 1983
, 1985a
-c
). GABA
inhibited bladder contractions evoked by stimulation of preganglionic
nerve fibers in the rat major pelvic ganglia (MPG). Neurons in the rat MPG contain GABA, which is released by stimulation of the pelvic nerve
(de Groat 1970
; Kusunoki et al. 1984
).
Binding sites and synthesizing enzymes for GABA exist in the neurons of
the rat MPG (de Groat 1970
; Kusunoki et al.
1984
). In the pelvic ganglia of the cat urinary bladder, GABA
induced a biphasic response, a depolarization followed by an
afterhyperpolarization, which were mediated by changes of
Cl
conductance (Mayer et al.
1983
).
Recently, it has been reported that GABA receptors can be classified as
GABAA and GABAC receptors,
which are ionotropic receptors, or as GABAB
receptors, which are metabotropic receptors coupled to the GTP-binding
protein. GABAA receptors have several subunits (
1-6,
1-3,
1-3,
), which form a pentameric chloride
channel (Macdonald and Olsen 1994
). As a subtype of the
GABAA receptor, GABAC
receptors are probably pentameric Cl
channels
composed of the recently discovered
subunits (
1-3) (Barnard et al. 1998
; Cutting et al.
1991
; Ogurusu and Shingai 1996
). Patch-clamp
studies have shown that the single-channel conductance of the
GABAA receptors was larger than that of the GABAC-receptor channels in retinal cells
(Feigenspan and Bormann 1994
). The ion channel of the
GABAC receptors opened for a longer time and was
less liable to desensitization than most GABAA
receptors. GABAC receptors are not blocked by
bicuculline and are not modulated by barbiturates, benzodiazepines, or
neuroactive steroids (Bormann and Feigenspan 1995
;
Dong et al. 1994
; Feigenspan et al. 1993
; Johnston 1997
; Lukasiewicz 1996
;
Lukasiewicz et al. 1994
; Polenzani et al.
1991
; Qian and Dowling 1993
, 1994
; Wang
et al. 1994
).
The aim of the present study is to examine the contribution of
GABAA and GABAC receptors
to the biphasic response in the neurons of the rat MPG. Application of
GABA induced a complex response that consisted of early and late
depolarizations and a hyperpolarization. It is suggested that
GABAA receptors mediate the early GABA-induced depolarization (early GABAd), whereas
GABAC or GABAAOr receptors (Barnard et al. 1998
) mediate the late GABA-induced
depolarization (late GABAd) and the
hyperpolarization (GABAh). A preliminary account
of some of this work has been published previously (Tsurusaki et
al. 1998
).
 |
METHODS |
Male Wistar rats weighing 200 g were killed by
decapitation. The MPG located on a lateral site of the prostate were
dissociated and then pinned onto silicone elastomer (Sylgard) at the
bottom of a superfusion chamber (0.5 ml total volume). The MPG were
continuously superfused with Krebs solution (3 ml/min) with the
following composition (in mM): 117 NaCl, 4.7 KCl, 2.5 CaCl2, 1.2 MgCl2, 1.2 NaH2PO4, 25 NaHCO3, and 11 glucose (295-305 mOsm). The Krebs
solution was gassed with 95% O2-5%
CO2 and preheated to 32°C at the recording site. In some experiments, MPG neurons were superfused with a modified
Krebs solution containing 85 mM Cl
(low
Cl
solution), where 40.4 mM NaCl was replaced
with Na-isethionate. The pH of the Krebs solution was adjusted to 7.4. Intracellular microelectrodes filled with 3 M K-acetate had tip
resistances of 80-120 M
. Membrane potential and current were
recorded with an Axoclamp 2A (Axon Instruments). The MacLab system
software program (AD Instruments) operating on Apple Computers
(Macintosh 8100/100AV and PowerBook 5300CS) was used to continuously
record the membrane potentials. The membrane potential and membrane
current were continuously monitored with a memory oscilloscope
(Nihon-Kohden, RTA-1100). The voltage and current were also digitized
and stored in the computer (Power Mac 8500: Apple Computer) with a
data-acquisition system (AxoData: Axon Instruments) for later analysis.
The voltage-current (V-I) relationship was obtained by
applying cathodal and anodal current pulses with a duration of 300 ms.
The following drugs were used: GABA, muscimol, imidazole-4-acetic acid
(I4AA) hydrochloride, and baclofen were purchased from SIGMA;
(
)-bicuculline methiodide from Nacalai Tesque; picrotoxin from Wako
Pure Chemical Industries; and cis-4-aminocrotonic acid
(CACA) and trans-4-aminocrotonic acid (TACA) from TOCRIS.
CGP55845A was a gift from CIBA-GEIGY. All drugs were dissolved directly
in the Krebs solution. The data were expressed as mean ± SE.
 |
RESULTS |
Effect of GABA on the membrane potential of MPG neurons
The resting membrane potential and input resistance of rat MPG
neurons were
62 ± 2 mV (n = 159) and 70 ± 5 M
(n = 52), respectively. The action potential had
an afterhyperpolarization with an amplitude of 14 ± 1 mV
(n = 11) and a duration of 333 ± 52 ms
(n = 11). Application of GABA (300 µM) to the
external solution for 1 min induced a biphasic response, an initial
depolarization (GABAd) followed by a
hyperpolarization (GABAh) in 88 (75%) of 117 MPG neurons (Fig. 1A). The
remaining 29 (25%) neurons did not respond to GABA. The input
resistance of the rat MPG neurons decreased during the
GABAd and increased during the
GABAh (Fig. 1A). The falling phase of
the GABA-induced depolarization consisted of early and late components
(Table 1). When the
GABAd reached the maximum amplitude, it initially
declined rapidly toward the resting membrane potential and then
subsequently entered a slow decay phase. Thus the GABA-induced
depolarization sometimes had a plateau on the falling phase (Fig.
1B). The early GABAd measured at the half-maximum amplitude had a duration of 27 ± 5 s
(n = 11). The late GABAd lasted
for 60-140 s; the mean duration of the late GABAd was 129 ± 11 s
(n = 11). The duration of the late
GABAh ranged from 300 to 600 s with a mean
duration of 516 ± 64 s (n = 11) at the
resting membrane potential. The effect of GABA in producing the
depolarization was concentration dependent (Fig. 2). At a concentration of 5 µM, GABA
depolarized the neurons by 4 ± 1 mV (n = 6). Only
a slow depolarizing response was produced by 5-30 µM GABA. GABA (500 µM) produced a maximal response of 22 ± 1 mV (n = 30). The fast depolarization was seen at concentrations of GABA >50
µM (Fig. 2). The GABAh was also concentration
dependent. The minimum effective concentration of GABA was 100 µM
that produced a hyperpolarization with an amplitude of 3 ± 1 mV
(n = 14). GABA (500 µM) produced the maximal
hyperpolarization with an amplitude of 7 ± 2 mV
(n = 30).

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Fig. 1.
GABA-induced responses in a major pelvic ganglion (MPG) neuron.
A, top: continuous pen record of the GABA-induced
depolarization (GABAd) and the following hyperpolarization
(GABAh). Resting membrane potential was 55 mV. Period of
the bath-application of GABA (300 µM) is indicated by a horizontal
bar. Downward deflections on the trace indicate hyperpolarizing
electrotonic potentials evoked by anodal current pulses with an
amplitude of 0.3 nA and a duration of 200 ms. Bottom:
expanded records of the electrotonic potentials. a-d were obtained at
the times marked by the respective letters in the top trace.
B: measurements of the amplitude and duration of the
GABA-induced responses. a and b indicate the amplitude of the early
GABAd and the GABAh. Durations of the late
GABAd and GABAh are shown by c and d.
Inset: expanded record of the GABAd.
Duration of the early GABAd (e) was measured at the
half-maximum amplitude. R.M.P, resting membrane potential.
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Fig. 2.
Concentration-dependent properties of the GABA responses.
A: responses were induced by bath-application of GABA
(10-500 µM) in the Krebs solution. - - -, original membrane
potential. B: concentration-response curves for the
GABA-induced depolarizations ( ) and hyperpolarizations
( ), ± SE of the mean. Number of experiments is
shown in parentheses.
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Effects of agonists and antagonists for GABA receptors
The effects of GABA-receptor agonists on the membrane potential
were examined in rat MPG neurons. Figure 3 shows the effects of
GABA (300 µM), muscimol (100 µM) and baclofen (100 µM) on a single MPG neuron. GABA produced a typical biphasic response in this
neuron (Fig. 3A). Muscimol
(100 µM) produced only a depolarizing response that was similar to
the early GABAd (Fig. 3B and Table 1). The
muscimol-induced depolarization had an amplitude of 22 ± 4 mV
(n = 4) and was associated with a decreased
membrane resistance. The muscimol-induced depolarization was not
followed by an obvious plateau potential on the falling phase or by an
afterhyperpolarization. The duration of the muscimol-induced
depolarization measured at the half-maximum amplitude was 22 ± 3 s (n = 6). Baclofen (10 µM), a selective
GABAB-receptor agonist (Bowery 1989
),
produced no changes in the membrane potential or resistance (Fig.
3C). The effects of the GABAA-receptor
antagonists on the GABA-induced responses were examined in the MPG
neurons (Fig. 4). Bicuculline (60 µM),
a GABAA-receptor antagonist, reduced the amplitude of the
early GABAd by 48 ± 3% (n = 8).
In the neurons treated with bicuculline (60 µM), the amplitude and
duration of the slow depolarization induced by GABA (300 µM) were
9 ± 2 mV (n = 12) and 98 ± 16 s (n = 12), respectively. The GABA-induced
hyperpolarization obtained in the presence of bicuculline (100 µM)
had a duration of 403 ± 41 s (n = 8) and
an amplitude of 4 ± 1 mV (n = 8). The
biphasic GABA responses obtained in the presence of bicuculline (100 µM) were identical to the late GABAd and
GABAh in normal Krebs solution (Table 1). The application
of picrotoxin (100 µM) to a Krebs solution containing bicuculline (60 µM) abolished the residual GABA responses (Fig. 4A).
CGP55845A (3 µM), a selective GABAB-receptor antagonist
(Bowery 1997
), had no significant effect on the
GABA-induced responses in the rat MPG neurons (n = 4).

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Fig. 3.
Effects of GABA (A), muscimol (B) and
baclofen (C) on a rat MPG neuron. Drugs were applied in
the Krebs solution at the time indicated by the horizontal bars.
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Fig. 4.
Effects of bicuculline, picrotoxin and imidazole-4-acetic acid (I4AA)
on the GABA-induced biphasic response. A, top: typical
GABA-induced response obtained from an MPG neuron in Krebs solution.
Middle: GABA-induced response in the presence of
bicuculline (60 µM). Bottom: picrotoxin (100 µM) was
added to the Krebs solution containing bicuculline (100 µM). All
records were from the same cell. Note the bicuculline-resistant,
picrotoxin-sensitive component of the GABA response. B:
effect of I4AA (100 µM) on the GABA-induced responses.
Top: typical response to GABA (1 mM) in an MPG neuron.
Bottom: obtained 5 min after the application of I4AA
(100 µM). These records were obtained from the same cell.
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Properties of a bicuculline-insensitive, picrotoxin-sensitive GABA
response
Recent studies have demonstrated a bicuculline-insensitive,
picrotoxin-sensitive GABA response mediated by
GABAC receptors in retinal bipolar cells
(Bormann and Feigenspan 1995
; Feigenspan and
Bormann 1994
; Feigenspan et al. 1993
;
Johnston 1996
; Qian and Dawling 1995
).
The possibility that the GABA responses are mediated by
GABAC receptors in rat MPG neurons was examined.
It has been reported that I4AA (100 µM) acts as an antagonist at GABAC receptors (Bormann and Feigenspan
1995
; Kusama et al. 1993
; Pan and Lipton
1995
; Qian and Dowling 1994
, 1995
). Figure
4B shows the effect of I4AA (100 µM) on the GABA-induced
responses in MPG neurons. I4AA (100 µM) did not block the early
GABAd but markedly depressed the late
GABAd and the GABAh. Pooled
data from these studies are shown in Table
2. The effects of
GABAC-receptor agonists on the membrane potential
were compared with those of GABA on the same MPG neurons (Fig.
5). In this neuron, GABA (300 µM)
produced a typical biphasic response. CACA (1 mM), a selective agonist for GABAC receptors (Bormann and
Feigenspan 1995
; Johnston 1996
, 1997
), also
produced a biphasic response composed of a depolarization followed by a
hyperpolarization in the same MPG neuron (Fig. 5B). There
was no obvious early component in the CACA-induced biphasic response.
The amplitudes of the depolarization and hyperpolarization induced by
CACA (1 mM) were 12 ± 1 mV (n = 8) and 4 ± 1 mV (n = 12), respectively. The input resistance of
the MPG neurons was decreased during the CACA-induced depolarization
and was increased during the hyperpolarization. The durations of the
CACA-induced responses were similar to those of GABA-induced responses
obtained in the presence of bicuculline (Table 1). The CACA-induced
depolarization and hyperpolarization lasted for 118 ± 4 s
(n = 8) and 463 ± 23 s (n = 8), respectively. TACA (1 mM), another
GABAC-receptor agonist (Bormann and
Feigenspan 1995
; Johnston 1996
), also produced
depolarizing responses with a plateau on the falling phase (Fig.
5C). The amplitude of the TACA (1 mM)-induced depolarization
was 7 ± 1 mV (n = 8). A hyperpolarizing response
with an amplitude of 3.0 ± 0.8 mV (n = 8)
followed the TACA-induced depolarization. The effects of GABA-receptor
antagonists on the CACA-induced biphasic response were examined in MPG
neurons (Fig. 6A). Bicuculline
(100 µM) produced only a small depression of the CACA-induced
responses (Table 2). Picrotoxin (100 µM) blocked the biphasic
response produced by CACA (1 mM). The magnitude of depression of the
CACA-induced responses by picrotoxin is shown in Table 2. Figure
6B shows the effect of I4AA on the CACA-induced
depolarization and hyperpolarization. The CACA (100 µM)-induced
biphasic response was markedly depressed by I4AA (100 µM).
Statistical data for the I4AA-induced depression of the CACA responses
are shown in Table 2.

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Fig. 5.
Biphasic responses induced by GABA and GABAC-receptor
agonists, cis- and trans-4-aminocrotonic
acid (CACA and TACA). A: typical GABA-induced biphasic
response in an MPG neuron, which had a resting membrane potential of
65 mV. B: CACA (100 µM)-induced slow biphasic
response. C: TACA (1 mM)-induced slow biphasic response
in an MPG neuron. All records were obtained from the same cell.
· · · · , original resting membrane
potential.
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Fig. 6.
Effects of bicuculline (A) and I4AA (B)
on CACA-induced responses. A, top: CACA response
obtained from an MPG neuron superfused with Krebs solution. CACA (1 mM)
was applied to the MPG neurons during the period indicated
( ). Bottom: effect of bicuculline on the
CACA-induced response. CACA (1 mM) was applied to the MPG neuron 1 min
after the application of bicuculline (100 µM). , time
of the application of bicuculline (100 µM). B: effect
of I4AA (100 µM) on the response to CACA (1 mM). I4AA (100 µM) was
applied in the superfusing solution 5 min before the application of
CACA. Top and bottom: responses to CACA
taken before and during the application of I4AA (100 µM),
respectively. , time of application of I4AA (100 µM).
, times for the application of CACA (1 mM).
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Reversal potentials of GABA-induced responses
It has been reported that GABAA and
GABAC receptors are ionotropic receptors which
include a Cl
selective channel (Bormann
1988
; Feigenspan and Bormann 1994
; Feigenspan et al. 1993
; Macdonald and Olsen
1994
). In the vesical pelvic ganglia of the cat urinary bladder
wall, the GABA-induced depolarization and afterhyperpolarization were
produced by activation and inhibition of Cl
channels, respectively (Mayer et al. 1983
). Therefore
the contributions of Cl
to the GABA-induced
responses were examined in rat MPG neurons. Direct application of GABA
(1 mM) to MPG neurons by pressure pulses (70 kPa for 600 ms) induced
fast depolarizations with durations of 8-12 s followed by neither a
slow depolarization nor a hyperpolarization (Fig.
7). Bicuculline (20 µM) blocked the
fast depolarization from pressure application of GABA
(n = 5), indicating that it was a
GABAA-receptor-mediated response. The fast
depolarization was also decreased in amplitude, when MPG neurons were
depolarized. Figure 7 shows a linear relationship between the membrane
potential and the amplitude of the early GABAd.
The reversal potential of the early GABAd
obtained by extrapolation of this curve was
32 ± 3 mV
(n = 7). Reversal potentials for the late
GABAd and the GABAh were
also examined in MPG neurons. Electrotonic potentials were produced
by applying cathodal and anodal current pulses with a duration of 300 ms. Figure 8 shows examples of the
voltage-current relationships (V-I curves). The intersection
of the V-I curves obtained before and during the late
GABAd yielded a reversal potential of
43 mV in
this particular neuron. For the same cell, V-I curves taken
before and during the GABAh yielded a reversal of
potential
34 mV. From four other experiments, the reversal potentials
of the late GABAd and GABAh
were
38 ± 2 mV and
34 ± 2 mV, respectively. To examine
the Cl
dependency of the late
GABAd and GABAh, MPG
neurons were superfused with a low-Cl
solution
containing 85 mM Cl
. In this solution, the
reversal potentials of late GABAd and the
GABAh were shifted to
17 ± 3 mV
(n = 5) and
18 ± 3 mV (n = 5),
respectively. Examples of these study are shown in Fig. 9.

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Fig. 7.
Reversal potentials of the early GABAd in MPG neurons.
Amplitudes of the early GABAd produced by pressure (70 kPa
for 600 ms) application of GABA (1 mM) through a micropipette were
plotted against the membrane potential. Vertical lines indicate the SE
of the means obtained from 7 cells. Inset: example of
the early GABAd. Open triangles indicate the times of the
pressure applications of GABA.
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Fig. 8.
Reversal potential of the early GABAd and the
GABAh in an MPG neuron. A, top and
bottom: current injected through a microelectrode and
the membrane potential changes, respectively. Anodal and cathodal
current pulses with a duration of 300 ms were applied to MPG neurons to
evoke electrotonic potentials. B: voltage-current
(V-I) relationships obtained before and during the GABA
responses. , control V-I curve obtained
before the application of GABA. and , obtained
during the late GABAd and the GABAh,
respectively.
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Fig. 9.
Effects of lowering extracellular Cl on the late
GABAd (A and B) and the
GABAh (C and D). In
A and C, neurons were superfused with the
Krebs solution (Cl : 124.4 mM). In B and
D, the concentration of external Cl was
reduced to 85 mM. and and and , taken in the Krebs solution and low
Cl solution, respectively. , reversal potential of
GABA-induced responses.
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 |
DISCUSSION |
The present study demonstrated that GABA induced a depolarization
(GABAd) followed by a hyperpolarization
(GABAh) in neurons of the rat MPG. The initial
GABAd was associated with a decreased membrane
resistance, whereas the GABAh was associated with
an increased membrane resistance. Such a biphasic GABA response
resembles that observed in neurons of the vesical pelvic ganglion of
the cat urinary bladder (Mayer et al. 1983
). The
GABAh was sometimes followed by an additional
slow depolarizing response associated with a decreased membrane
resistance in rat MPG neurons; however, the properties of the late
depolarization remain to be investigated. The
GABAd in rat MPG neurons had two components, the
early GABAd and the late
GABAd, based on their decay time course.
Muscimol, a classic GABAA-receptor agonist,
recently has been shown to be a partial agonist at
GABAC receptors (Johnston 1997
).
Application of muscimol to MPG neurons, however, produced only a fast
depolarization with a duration that was similar to the early
GABAd. Bicuculline blocked the early
GABAd but not the late
GABAd or the GABAh. These results indicate that the early GABAd is mediated
by GABAA receptors in rat MPG neurons.
Recently, the GABA receptors responsible for the
bicuculline-insensitive, picrotoxin-sensitive GABA response were
described as "GABAC receptors" in vertebrate
retina (Bormann and Feigenspan 1995
; Feigenspan
and Bormann 1994
; Johnston 1996
; Qian and
Dowling 1993
, 1995
; Wellis and Werblin 1995
).
GABAC receptors are members of the GABA-gated
chloride ion-channel superfamily of receptors as are
GABAA receptors. Recent classification by
international union of pharmacology (IUPHA) suggest that
GABAC receptors appear to be a subtype of GABA
ionotropic receptors, the GABAA receptor (Barnard et al. 1998
). However, there are some
differences between these two receptor subtypes (Dong et al.
1994
; Feigenspan et al. 1993
; Qian and
Dowling 1993
; Wang et al. 1994
). Molecular
biological studies have indicated that GABAC
receptors are homooligomeric protein complexes formed by
subunits
and are distinct from the heterooligomeric
GABAA-receptor complexes formed by
,
,
(or
) subunits (Cutting et al. 1991
; Macdonald
and Olsen 1994
; Ogurusu and Shingai 1996
).
Patch-clamp studies have shown that the single-channel conductance of
the GABAC receptor is approximately four times larger than that of GABAA-receptor channels in
retinal cells (Feigenspan and Bormann 1994
).
GABAC receptors are more sensitive to GABA than
are GABAA receptors (Feigenspan and
Bormann 1994
; Kusama et al. 1993
). The ion
channel of the GABAC receptor remains open for a
longer time and is less liable to desensitization than most GABAA receptors (Feigenspan et al.
1993
). GABAC receptors are not modulated
by barbiturates, benzodiazepines, or neuroactive steroids (Dong
et al. 1994
; Feigenspan et al. 1993
;
Lukasiewicz et al. 1994
; Polenzani et al.
1991
; Qian and Dowling 1993
, 1994
; Wang
et al. 1994
). GABAC receptors have been
demonstrated to be localized in the CNS (Johnston 1997
).
Other bicuculline-insensitive GABA receptors that fall outside the
GABAABC classification were described in
embryonic brain-stem neurons (Momose-Sato et al. 1995
)
and rat retinal bipolar cells (Pan and Lipton 1995
).
They closely resemble vertebrate GABAC receptors;
but these receptors, called GABAD receptors, were
insensitive to both bicuculline and picrotoxin (Momose-Sato et
al. 1995
; Pan and Lipton 1995
). Another bicuculline-insensitive, picrotoxin-sensitive GABA current (mediated by
Cl
) has been observed in the hippocampus of 0- to 10-day-old rats (Martina et al. 1995
).
The present study also describes bicuculline-insensitive GABA
responses, the late GABAd and the
GABAh, that were completely blocked by picrotoxin
in rat MPG neurons. CACA and TACA, selective and potent
GABAC-receptor agonists (Bormann and
Feigenspan 1995
; Johnston 1996
), produced a slow
depolarizing response associated with a decreased membrane resistance
followed by a slow hyperpolarizing response associated with an
increased membrane resistance in rat MPG neurons. The time courses of
the CACA-induced responses were almost the same as those of the
GABA-induced slow responses. The CACA-induced response was blocked by
picrotoxin but not by bicuculline. I4AA, a
GABAC-receptor antagonist (Bormann and
Feigenspan 1995
; Kusama et al. 1993
; Qian
and Dowling 1994
), selectively depressed the late
GABAd and the GABAh but not
the early GABAd. I4AA also depressed the
CACA-induced responses. These pharmacological properties indicate that
GABA receptors that mediate the late GABAd and
the GABAh in rat MPG neurons belong to the
GABAC (or GABAAOr)
(Barnard et al. 1998
) receptor type.
Previously, it was demonstrated that the GABA-induced depolarization is
produced by the activation of Cl
conductance in
vesical pelvic ganglia (VPG) neurons of the cat urinary bladder
(Mayer et al. 1983
). The reversal potential of the early
GABAd in rat VPG neurons was
32 ± 2 mV, which was
comparable with that for the GABA-induced depolarization in cat pelvic
neurons. Mayer et al. (1983)
have also reported that the
GABA-induced afterhyperpolarization is produced by a depression of the
Cl
conductance in neurons of the cat VPG. In rat MPG, the
GABAh was associated with an increased membrane resistance.
The reversal potentials of the depolarization and the hyperpolarization
induced by GABA in rat MPG neurons are comparable with those observed in cat VPG neurons (Mayer et al. 1983
). When the
concentration of extracellular Cl
was reduced from 125.4 to 85 mM, the reversal potentials of the late GABAd and the
GABAh were shifted to a depolarizing potential according to
the Nernst equation of Cl
. These results suggest that the
Cl
conductance is also responsible for the GABA-induced
biphasic responses in rat MPG.
GABA is known to downregulate the function of the lower urinary tract
and is a putative inhibitory transmitter in the brain, spinal cord
(Desarmenien et al. 1984
; Kontani et al.
1988
; Maggi et al. 1987
; Sillén et
al. 1980
, 1985
), pelvic ganglia (de Groat 1970
;
Maggi et al. 1983
, 1985a
) and neuron-smooth muscle
junction (Chen et al. 1992
; Kusunoki et al.
1984
; Maggi et al. 1985a
,c
). It has been shown
that neurons in the rat MPG contain GABA and its synthesizing enzyme
(de Groat 1970
; Kusunoki et al. 1984
). Binding sites for GABA have also been demonstrated in rat MPG neurons
(Kusunoki et al. 1984
). The results of the present study together with previous studies indicate that the hyperpolarization induces a long-lasting inhibition of urinary bladder activity.
This work was supported by a Grant-in-Aid for Scientific Research
(B) from the Ministry of Education, Science, Sports and Culture of
Japan (08457017).
Address for reprint requests: T. Akasu, Dept. of Physiology, Kurume
University School of Medicine, 67 Asahi-machi, Kurume 830-0011, Japan
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.