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The Journal of Neuroscience, August 1, 2002, 22(15):6347-6352
Multiple Muscarinic Acetylcholine Receptor Subtypes Modulate
Striatal Dopamine Release, as Studied with
M1-M5 Muscarinic Receptor Knock-Out Mice
Weilie
Zhang,
Masahisa
Yamada,
Jesus
Gomeza,
Anthony
S.
Basile, and
Jürgen
Wess
Laboratory of Bioorganic Chemistry, National Institutes of Diabetes
and Digestive and Kidney Diseases, Bethesda, Maryland 20892
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ABSTRACT |
A proper balance between striatal muscarinic
cholinergic and dopaminergic neurotransmission is required for
coordinated locomotor control. Activation of striatal muscarinic
acetylcholine receptors (mAChRs) is known to modulate striatal dopamine
release. To identify the mAChR subtype(s) involved in this activity, we
used genetically altered mice that lacked functional
M1-M5 mAChRs [knock-out (KO) mice]. In
superfused striatal slices from wild-type mice, the non-subtype-selective muscarinic agonist oxotremorine led to
concentration-dependent increases in potassium-stimulated
[3H]dopamine release (by up to 60%). The lack of
M1 or M2 receptors had no significant effect on
the magnitude of these responses. Strikingly, oxotremorine-mediated
potentiation of stimulated striatal [3H]dopamine
release was abolished in M4 receptor KO mice, significantly increased in M3 receptor-deficient mice, and significantly
reduced (but not abolished) in M5 receptor KO mice.
Additional release studies performed in the presence of tetrodotoxin
suggested that the dopamine release-stimulating M4
receptors are probably located on neuronal cell bodies, but that the
release-facilitating M5 and the release-inhibiting
M3 receptors are likely to be located on nerve terminals.
Studies with the GABAA receptor blocker bicuculline methochloride suggested that M3 and M4
receptors mediate their dopamine release-modulatory effects via
facilitation or inhibition, respectively, of striatal GABA release.
These results provide unambiguous evidence that multiple mAChR subtypes
are involved in the regulation of striatal dopamine release. These
findings should contribute to a better understanding of the important
functional roles that the muscarinic cholinergic system plays in
striatal function.
Key words:
acetylcholine; dopamine release; knock-out mice; muscarinic receptors; oxotremorine; striatum
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INTRODUCTION |
It is well documented that a proper
balance between striatal muscarinic cholinergic and dopaminergic
neurotransmission is required for coordinated locomotor control
(Hornykiewicz, 1981 ; Graybiel, 1990 ; Di Chiara et al., 1994 ; Calabresi
et al., 2000 ; Kaneko et al., 2000 ). Consistent with this concept,
muscarinic antagonists are clinically useful in the treatment of
Parkinson's disease (Fahn et al., 1990 ), a disorder caused by the
relative lack of striatal dopamine resulting from the loss of
dopaminergic neurons in the substantia nigra pars compacta
(Hornykiewicz, 1981 ; Graybiel, 1990 ).
The release of striatal ACh from intrinsic cholinergic interneurons is
modulated by dopamine via activation of different dopamine receptor
subtypes (Di Chiara et al., 1994 ). Reciprocally, ACh-mediated activation of striatal muscarinic acetylcholine receptors (mAChRs) is
known to facilitate striatal dopamine release, as has been shown in
both in vitro (Lehmann and Langer, 1982 ; Raiteri et al., 1984 ; Schoffelmeer et al., 1986 ; Kemel et al., 1989 ) and in
vivo (Xu et al., 1989 ; De Klippel et al., 1993 ; Smolders et al.,
1997 ) studies. Stimulation of striatal mAChRs can also result in
reduced striatal dopamine release, at least under certain experimental conditions (De Belleroche and Bradford, 1978 ; Kemel et al., 1989 ; Xu et
al., 1989 ; De Klippel et al., 1993 ).
The mAChR family consists of five molecularly distinct subtypes
(M1-M5), all of which are
expressed in the striatum in a complex, overlapping manner (Weiner et
al., 1990 ; Levey et al., 1991 ; Bernard et al., 1992 ; Yasuda et al.,
1993 ; Hersch et al., 1994 ; Yan et al., 2001 ). From a therapeutic point
of view, identification of the mAChR subtype(s) modulating striatal
dopamine release should be of considerable interest. Classical
pharmacological studies have led to contradictory results regarding the
nature of the mAChR subtypes involved in this activity (Raiteri et al.,
1984 ; Schoffelmeer et al., 1986 ; Xu et al., 1989 ; De Klippel et al., 1993 ), reflecting the limited receptor subtype selectivity of the
muscarinic agonists and antagonists used in these studies (Caulfield,
1993 ; Wess, 1996 ).
To study the physiological and pathophysiological roles of individual
mAChRs in a more direct manner, we (Gomeza et al., 1999a ,b ; Miyakawa et
al., 2001 ; Yamada et al., 2001a ,b ; Fisahn et al., 2002 ) and others
(Hamilton et al., 1997 ; Matsui et al., 2000 ; Gerber et al., 2001 ) used
gene targeting techniques to generate M1-M5 mAChR-deficient
mice. In the present study, we performed systematic dopamine release
experiments using superfused striatal slices prepared from
M1-M5 mAChR knock-out (KO)
mice. Specifically, we compared the effects of oxotremorine, a
non-subtype-selective muscarinic agonist, on potassium-stimulated
[3H]dopamine release in wild-type (WT)
and M1-M5 mAChR KO mice.
We provide evidence that multiple mAChRs are involved in modulating
striatal dopamine release. Our findings are consistent with the concept
that stimulation of M4 and
M5 receptors facilitates stimulated striatal
dopamine release, whereas activation of M3 receptors inhibits this release.
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MATERIALS AND METHODS |
Animals. The generation of homozygous
M1-M5 receptor KO mice
[genetic background: 129/SvEv × CF1 (M1,
M3, M4, and M5) or 129J1 × CF1 (M2)] has been
described previously (Gomeza et al., 1999a ,b ; Yamada et al., 2001a ,b ;
Fisahn et al., 2002 ). For each KO strain, the corresponding WT mice
were used in parallel as controls. All experiments were performed with
adult male mice that were at least 8 weeks of age. Mouse genotyping was
performed by PCR analysis of mouse-tail DNA.
Dopamine release studies. Striatal slices (250 × 250 µm) prepared from one mouse were pooled and dispersed in 25 ml of
oxygenated (95% O2 and 5%
CO2) Krebs-Ringer buffer (in
mM: 11.5 glucose, 25 NaHCO3, 1.2 MgCl2, 1.2 NaH2PO4, 118 NaCl, 4.8 KCl,
2.5 CaCl2, and 0.004 Na2EDTA, pH 7.4,) at 33°C for 20 min. Slices
were incubated with [3H]dopamine (48.2 Ci/mmol; PerkinElmer Life Sciences, Boston, MA) for 30 min at a
final concentration of 0.2 µM in the presence of the anti-oxidant ascorbate (5 mM) and the
monoamine oxidase inhibitor pargyline (10 µM) to reduce the metabolism of
[3H]dopamine. To prevent the uptake of
[3H]dopamine into serotonergic and
noradrenergic terminals, citalopram (1 µM) and
desipramine (5 µM) were added. After rinsing,
slices were transferred to a superfusion system (SF-12; Brandel,
Gaithersburg, MD) and superfused at 33°C at a constant rate of 0.4 ml/min. Striatal slices prepared from one mouse were aliquoted into six
superfusion chambers (~25-35 slices per chamber), allowing the
construction of complete oxotremorine concentration-response curves
(Fig. 1). Fractions were collected every
4 min beginning after a 60 min superfusion. Two 2 min periods of 20 mM KCl were applied after 72 (S1) and 104 (S2) min of
superfusion. Tetrodotoxin (TTX) (600 nM)
was added at the beginning of superfusion when indicated. TTX is a
highly selective sodium-channel blocker that blocks the conduction of
action potentials along axons. This pharmacological action can be used
in release assays to prevent indirect postsynaptic regulation of
neurotransmitter release involving neuronal activity and can help to
determine whether release-modulatory receptors are located at nerve
terminals or at other cellular locations (cell bodies and/or
dendrites).

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Figure 1.
Effect of oxotremorine on potassium-stimulated
[3H]dopamine release in striatal slices from
M1-M4 mAChR KO mice and corresponding WT
controls. Striatal slices that had been preincubated with
[3H]dopamine were depolarized with 20 mM KCl, and the resulting [3H]dopamine
outflow was quantitated in the absence and in the presence of the
indicated concentrations of oxotremorine. Data are expressed as the
percentage increase in [3H]dopamine release above
control levels (no oxotremorine). The WT curves shown in
C and D are identical. Because the
M3 and M4 mAChR KO mice were both 129/SvEv × CF1 hybrids (50 of 50), only one WT strain of the same
genetic background was tested in parallel with these mutant mice. Each
data point represents the mean ± SEM from 6-11 independent experiments (mice). Asterisks indicate
significant differences between responses in KO versus WT preparations
(*p < 0.05; **p < 0.01;
Student's t test followed by the Holm correction
for multitesting adjustment).
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Drugs were added to the superfusion buffer 20 min before
S2. The efflux of tritium collected was
calculated as a percentage of the total tritium present in the slices
at the start of the fraction considered. The net efflux of tritium
after S1 (fractions 5 and 6) and
S2 (fractions 13 and 14) was calculated by
subtracting the average of three fractions (expected basal value)
before KCl stimulation (S1, fractions 2-4;
S2, fractions 10-12). At the end of the
[3H]dopamine release experiments,
tissues from each chamber were solubilized with 200 µl of 1N NaOH,
and tritium was determined in superfusate samples and tissues via
liquid scintillation counting. The results were expressed as the
S2/S1 ratio of release or
as the percentage increase in
[3H]dopamine release above control using
the following equation: {[S2/S1 (drug)] [S2/S1 (no
drug)]}/[S2/S1 (no
drug)] × 100.
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RESULTS |
To shed light on the potential roles of striatal
M1-M5 mAChRs in modulating
striatal dopamine release, we performed in vitro dopamine
release studies using superfused striatal slices prepared from WT and
M1-M5 mAChR-deficient
mice. Initially, cellular dopamine pools were radioactively labeled by
incubating striatal slices with
[3H]dopamine. Subsequently, potassium
(20 mM)-stimulated
[3H]dopamine release was measured either
in the absence (S1 phase) or in the presence
(S2 phase) of drugs, as described in Materials and Methods. A large body of studies indicates that such measurements yield results that are physiologically relevant (for review, see Starke
et al., 1989 ).
Basal and potassium-stimulated
[3H]dopamine outflow (approximately
threefold to fourfold above basal) did not differ significantly between
the individual mAChR KO mice and the corresponding WT controls (data
not shown).
Multiple mAChRs are involved in modulating stimulated dopamine
release in the striatum
Incubation of striatal slices from WT mice with oxotremorine, a
non-subtype-selective muscarinic agonist, led to
concentration-dependent increases in stimulated
[3H]dopamine release (maximum
stimulation, ~40-60%) (Fig. 1), which is in agreement with work
published previously (Lehmann and Langer, 1982 ; Raiteri et al., 1984 ;
Schoffelmeer et al., 1986 ). This increase in dopamine release was
completely abolished in the presence of atropine (10 µM),
confirming the involvement of mAChRs (data not shown).
As shown in Figure 1A,B, the lack of
M1 or M2 receptors had no
significant effect on oxotremorine-mediated increases in
[3H]dopamine output. However, this
response was significantly enhanced in striatal preparations from
M3 receptor KO mice (Fig. 1C)
(p < 0.05 at 10 µM
oxotremorine), suggesting that the activation of striatal
M3 receptors has an inhibitory effect on
stimulated dopamine release in WT mice. Strikingly, the ability of
oxotremorine to facilitate stimulated striatal
[3H]dopamine release was totally
abolished in M4 receptor KO mice (Fig.
1D). Similar results were obtained with striatal
slices prepared from M2/M4
receptor double KO mice (data not shown). These findings indicate that
M4 receptors play a key role in promoting mAChR-dependent increases in striatal dopamine output.
We have reported previously that oxotremorine-mediated
enhancement of striatal dopamine release was impaired (but not
abolished) in striatal slices derived from M5
receptor KO mice (Yamada et al., 2001a ). Together, these data clearly
indicate that multiple mAChRs are involved in modulating the magnitude
of stimulated dopamine release in the mouse striatum.
TTX has differential effects on oxotremorine-mediated
modulation of simulated striatal dopamine release in WT and
M3 and M5 receptor KO mice
We then wanted to investigate whether mAChR-mediated
modulation of stimulated striatal dopamine release was dependent on the activation of mAChRs on (dopaminergic) nerve terminals or whether it
required more complex neuronal circuits. To address this issue, we
examined to what extent oxotremorine-mediated modulation of striatal
dopamine release was altered in the presence of TTX, a selective
sodium-channel blocker that prevents the propagation of action
potentials along axons.
In the presence of TTX (600 nM), oxotremorine-mediated
increases in potassium-stimulated
[3H]dopamine release were abolished in
striatal preparations from WT mice (Fig.
2A). Interestingly, TTX
treatment unmasked opposing modulatory effects of oxotremorine on
stimulated [3H]dopamine release in
striatal preparations from M3 and
M5 receptor KO mice. In tissues from
M3 receptor KO mice, oxotremorine (in the
presence of TTX) retained the ability to induce small but significant
increases in potassium-stimulated
[3H]dopamine release (Fig.
2B), most likely because of the presence of
release-facilitating M5 receptors and the absence
of release-inhibiting M3 receptors. However, in
preparations from M5 receptor KO mice, oxotremorine treatment (in the presence of TTX) significantly decreased
potassium-stimulated [3H]dopamine
release (Fig. 3A), probably
because of the presence of release-inhibiting M3
and the absence of release-facilitating M5
receptors.

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Figure 2.
Effect of TTX on oxotremorine-mediated modulation
of potassium-stimulated [3H]dopamine release in
striatal slices from WT and M3 receptor KO mice. In the
presence of TTX (600 nM), oxotremorine had no significant
effect on stimulated [3H]dopamine release in WT
preparations (A) but induced a significant
enhancement in [3H]dopamine output in striatal
slices from M3 receptor KO mice (B).
Each bar represents the mean ± SEM of
S2/S1 values from six or seven
independent experiments (mice). Micromolar concentrations are shown.
Asterisks indicate significant differences from the
control group (no oxotremorine) (*p < 0.05;
**p < 0.01; one-way ANOVA followed by
Dunnett's test).
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Figure 3.
Effect of TTX on oxotremorine-mediated modulation
of potassium-stimulated [3H]dopamine release in
striatal slices from M5 receptor KO mice. In the presence
of TTX (600 nM), oxotremorine induced a decrease in
potassium-stimulated [3H]dopamine release in
striatal slices from M5 receptor KO mice
(A). This effect was blocked by the
GABAA receptor antagonist bicuculline methochloride
(B). Each bar represents the
mean ± SEM of S2/S1 values from
eight independent experiments (mice). Micromolar concentrations are
shown. Asterisks indicate significant differences from
the control group (no drug) (*p < 0.05;
**p < 0.01; one-way ANOVA followed by Dunnett's
test).
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In the striatum, M3 receptors are
expressed by a subgroup of GABAergic projection neurons (Hersch et al.,
1994 ; Yan et al., 2001 ), raising the possibility that the
release-inhibiting effects of M3 receptors that
persist in the presence of TTX are attributable to an increase in GABA
release. Consistent with this hypothesis, the incubation of TTX-treated
striatal slices from M5 receptor KO mice with the
GABAA receptor antagonist bicuculline
methochloride (10 and 30 µM) completely prevented
oxotremorine (30 µM)-mediated inhibition of dopamine
release (Fig. 3B).
As outlined above, oxotremorine lacked the ability to
facilitate stimulated dopamine release in striatal slices from
M4 receptor KO mice (Fig. 1D).
In the striatum, M4 receptors are abundantly expressed by GABAergic projection neurons (Weiner et al., 1990 ; Bernard
et al., 1992 ; Hersch et al., 1994 ; Santiago and Potter, 2001 ).
Therefore, we speculated that the dopamine release-facilitating activity of M4 receptors might be attributable to
M4 receptor-mediated reductions in GABA release.
In agreement with this hypothesis, incubation of striatal slices from
WT mice with bicuculline methochloride (100 µM)
mimicked the stimulatory effect of oxotremorine (30 µM) on potassium-induced
[3H]dopamine release (Fig.
4). Combining bicuculline methochloride (30 or 100 µM) and oxotremorine (30 µM) did not increase the stimulated [3H]dopamine outflow from WT
preparations any more than observed with either drug alone (Fig.
4).

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Figure 4.
Effect of the GABAA
receptor antagonist bicuculline methochloride on oxotremorine-mediated
enhancement of potassium-stimulated [3H]dopamine
release in striatal slices from WT mice. When administered alone,
bicuculline methochloride (100 µM) mimicked the
release-facilitating effect of oxotremorine; however, when
coadministered with oxotremorine, bicuculline methochloride did not
further enhance [3H]dopamine output. Each
bar represents the mean ± SEM of
S2/S1 values from eight independent
experiments (mice). Micromolar concentrations are shown.
Asterisks indicate significant differences from the
control group (no drug) (** p < 0.01;
one-way ANOVA followed by Dunnett's test).
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DISCUSSION |
Properly regulated dopamine release in the striatum is of
fundamental importance for extrapyramidal locomotor control (Graybiel, 1990 ; Di Chiara et al.; 1994 ; Calabresi et al., 2000 ). The present study was designed to identify specific mAChR subtypes involved in
modulating this process. Previous studies using classical
pharmacological tools have led to conflicting results regarding the
molecular identity of the mAChR subtypes involved in the regulation of
striatal dopamine release, probably because of the limited degree of
receptor subtype selectivity of the muscarinic agonists and antagonists used in these studies. For example, although it has been proposed that
muscarinic agonist-induced enhancement of striatal dopamine release is
mediated by M1 receptors (Raiteri et al., 1984 ;
Xu et al., 1989 ; Smolders et al., 1997 ), Schoffelmeer et al. (1986) suggested that this activity is dependent on the stimulation of non-M1 mAChRs. However, it has been proposed that
muscarinic agonist-induced inhibition of striatal dopamine release is
mediated by M2 receptors (Xu et al., 1989 ; De
Klippel et al., 1993 ).
Receptor localization studies have shown that all five
mAChRs (M1-M5) are
expressed in the striatum (Weiner et al., 1990 ; Bernard et al., 1992 ;
Yasuda et al., 1993 ; Hersch et al., 1994 ; Yan et al., 2001 ), raising
the possibility that multiple mAChRs play a role in the modulation of
striatal dopamine release. To address this issue in a more direct
manner, we performed systematic dopamine release studies using
superfused striatal slices from M1-M5 mAChR-deficient
mice. Previous studies have shown that disruption of specific mAChR
genes has no significant effect on the expression levels of the
remaining four mAChR subtypes in different regions of the brain,
including the striatum (Hamilton et al., 1997 ; Gomeza et al., 1999a ,b ;
Miyakawa et al., 2001 ; Yamada et al., 2001b ; Fisahn et al., 2002 ; Zhang
et al., 2002 ).
Consistent with previous findings (Lehmann and Langer,
1982 ; Raiteri et al., 1984 ; Schoffelmeer et al., 1986 ), incubation of
WT striatal slices with the non-subtype-selective muscarinic agonist
oxotremorine led to concentration-dependent increases in
potassium-stimulated [3H]dopamine
release (Fig. 1). The magnitude of this response was not affected by
the lack of M1 or M2
receptors (as studied with M1 and
M2 receptor KO mice) (Fig.
1A,B), both of which are expressed at relatively high
levels in the striatum (Weiner et al., 1990 ; Levey et al., 1991 ;
Bernard et al., 1992 ; Hersch et al., 1994 ). These results clearly
indicate that striatal M1 and
M2 receptors do not play a significant role in
the regulation of striatal dopamine output, in contrast to the
conclusions drawn based on classical pharmacological studies using
muscarinic ligands of limited receptor subtype selectivity.
Interestingly, in vivo microdialysis studies showed recently
that M1 receptor-deficient mice have
significantly elevated levels of extracellular dopamine in the
striatum, probably because of increased dopamine release (Gerber et
al., 2001 ). As discussed by Gerber et al. (2001) , it is possible that
the M1 receptors modulating striatal dopamine
release are located on either striatal or extrastriatal (e.g.,
cortical) neurons projecting to the striatum. Because the lack of
M1 receptors had no significant effect on basal
or stimulated dopamine release in striatal slice preparations (this
study), the increase in extracellular dopamine levels observed in
vivo is most likely attributable to the absence of extrastriatal (e.g., cortical) M1 receptors mediating
inhibition of striatal dopamine release through an indirect neuronal pathway.
Strikingly, muscarinic agonist-mediated increases in stimulated
[3H]dopamine output were totally
abolished in striatal slices from M4 receptor KO
mice (Fig. 1D), suggesting that
M4 receptors play a key role in mediating this
activity. Similarly, TTX treatment (600 nM) of WT
striatal slices also completely abolished the dopamine release-facilitating effects of oxotremorine (Fig.
2A), suggesting that this activity requires the
propagation of action potentials. In the striatum,
M4 receptors are abundantly expressed by medium spiny GABAergic projection neurons (Weiner et al., 1990 ; Bernard et
al., 1992 ; Hersch et al., 1994 ; Santiago and Potter, 2001 ), where they
are preferentially located on cell bodies and dendritic shafts and
spines (Hersch et al., 1994 ; Bernard et al., 1999 ). M4 receptors, like M2
receptors, are coupled to G-proteins of the
Gi/Go family, which, among
other cellular effects, can reduce neuronal activity by inhibiting
different classes of calcium channels (Caulfield, 1993 ; Hille, 1994 ;
Howe and Surmeier, 1995 ). Interestingly, in WT striatal slices, the
GABAA receptor antagonist bicuculline methochloride mimicked the release-facilitating effect of oxotremorine when administered alone; however, when coadministered with
oxotremorine, bicuculline methochloride did not further enhance
[3H]dopamine output (Fig. 4). Together,
these observations are consistent with a model in which the activation
of M4 receptors present on GABAergic projection
neurons inhibits GABA release, resulting in reduced
GABAA receptor-mediated inhibition of dopamine
release from dopaminergic nerve endings. This model is supported by
previous studies that indicate that muscarinic agonists inhibit GABA
release in the striatum (Marchi et al., 1990 ; Sugita et al., 1991 ),
that dopaminergic neurons in the substantia nigra receive synaptic input from striatal GABAergic neurons (Bolam and Smith, 1990 ), that
dopamine release-inhibiting GABAA receptors exist
on dopaminergic striatal nerve endings (Ronken et al., 1993 ), and that
ACh-mediated enhancement of striatal
[3H]dopamine release may be mediated by
the collaterals of GABAergic inhibitory neurons (Kemel et al.,
1989 ).
We have reported previously that oxotremorine-mediated potentiation of
stimulated dopamine release was significantly reduced (by ~50%) at
an intermediate oxotremorine concentration (10 µM) in
striatal slices derived from M5 receptor KO mice
(Yamada et al., 2001a ), suggesting that M5
receptors also contribute to ACh-mediated enhancement of striatal
dopamine release. However, maximum oxotremorine responses were not
significantly affected by the lack of M5
receptors (Yamada et al., 2001a ), in contrast to the total lack of
oxotremorine activity observed in the absence of
M4 receptors (Fig. 1D).
In contrast, oxotremorine-mediated enhancement of
stimulated [3H]dopamine release was
significantly increased in striatal slices from
M3 receptor KO mice (Fig. 1C),
indicating that M3 receptor stimulation activates
a pathway that inhibits striatal dopamine release. The opposing effects
of M3 versus M5 receptor
stimulation on striatal dopamine release may explain the observation
that oxotremorine had no net effect on stimulated dopamine release in
striatal slices from M4 receptor KO mice (Fig.
1D).
When release studies were performed in the presence of TTX (which is
predicted to block the M4 receptor-mediated
enhancement of dopamine release), oxotremorine administration led to
small but significant increases in stimulated dopamine release in
striatal slices from M3 receptor KO mice (Fig.
2B) but to a significant decrease in dopamine output
in preparations from M5 receptor KO mice (Fig.
3A). These observations provide additional support for the
concept that M3 and M5
receptors mediate opposing effects on stimulated dopamine release in
the striatum. Because M3 and M5 receptor-mediated modulation of striatal
dopamine release persists in the presence of TTX, it is likely that the
M3 and M5 receptors involved in this activity are located on nerve terminals.
M5 receptor mRNA is the only mAChR subtype mRNA
detectable in the dopamine-containing cells of the substantial nigra
pars compacta (Vilaro et al., 1990 ; Weiner et al., 1990 ), strongly suggesting that the dopamine release-facilitating
M5 receptors are located on dopaminergic nerve
terminals. Based on these findings (Vilaro et al., 1990 ; Weiner et al.,
1990 ), together with the fact that M3 and
M5 receptors (as well as M1
receptors) couple to a similar set of G-proteins
(Gq family) (Caulfield, 1993 ; Wess, 1996 ), it is
unlikely that the dopamine release-inhibiting M3 receptors are colocalized with M5 receptors on
dopaminergic terminals. M3 receptors are
expressed, although apparently at relatively low levels, in a subset of
GABAergic projection neurons (Hersch et al., 1994 ; Yan et al., 2001 ),
raising the possibility that M3 receptors located
on GABAergic nerve terminals inhibit dopamine release by stimulating
GABA release. Consistent with this concept, the
GABAA receptor antagonist bicuculline
methochloride completely blocked the oxotremorine-mediated reduction in
dopamine release observed with TTX-treated striatal slices from
M5 receptor KO mice (Fig. 3B). The
existence of presynaptic striatal M3 receptors has also been demonstrated via immunoelectron microscopy (Hersch et
al., 1994 ).
In summary, our data provide strong evidence that multiple mAChR
subtypes are involved in the regulation of striatal dopamine release.
Whereas activation of M4 and
M5 receptors facilitates striatal dopamine
release, stimulation of M3 receptors inhibits this process. Striatal M1 and
M2 receptors do not seem to play significant
roles in modulating striatal dopamine output. Our results underscore
the usefulness of muscarinic receptor mutant mice to delineate the
roles of individual mAChR subtypes in the regulation of striatal
function. A better understanding of the role of the striatal muscarinic
cholinergic system may open new perspectives for the treatment of
Parkinson's disease and other extrapyramidal movement disorders.
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FOOTNOTES |
Received Feb. 28, 2002; revised April 30, 2002; accepted May 7, 2002.
This work was supported by a Cooperative Research and Development
Agreement between the National Institute of Diabetes and Digestive and Kidney Diseases (J.W.) and the Eli Lilly Research Laboratories.
Correspondence should be addressed to Dr. Jürgen Wess, Molecular
Signaling Section, Laboratory of Bioorganic Chemistry, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Building 8A, Room B1A-05, 8 Center Drive MSC
0810, Bethesda, MD 20892-0810. E-mail: jwess{at}helix.nih.gov.
M. Yamada's present address: Laboratory for Cell Culture Development,
Brain Science Institute, RIKEN, Saitama 351-0198, Japan.
J. Gomeza's present address: Max-Planck-Institut für
Hirnforschung, Neurochemie, Deutschordenstrasse 46, D-60528
Frankfurt/Main, Germany.
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