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Vol. 281, Issue 2, 972-982, 1997
Department of Anatomy and Cell Biology, East Carolina University School of Medicine, Greenville, North Carolina
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Abstract |
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 Top
Abstract
Introduction
Materials & Methods
Results
Discussion
Conclusions
References
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Neuropeptide and immediate early gene expression in striatonigral neurons of the normosensitive striatum is induced by mixed D1/D2 receptor agonists and indirect dopamine agonists, such as cocaine and amphetamine. Both D1 and D2 receptor antagonists block these events. In contrast, the partial D1 agonist, SKF-38393, does not evoke striatonigral gene expression in intact rats. These findings have contributed to the idea that both D1 and D2 receptors must be stimulated to evoke gene expression in striatonigral neurons. How these "D1/D2 interactions" are accomplished is unclear in light of the controversy over whether striatonigral neurons express both D1 and D2 receptors. This study addresses these issues by demonstrating that in intact rats 1) a full D1 receptor agonist, SKF-82958, induced behavioral activity and preprodynorphin (PPD) and substance P (SP) gene expression in medium spiny neurons in the dorsal, and especially, in the ventral striatum, 2) either a D1 antagonist, SCH-23390, or a D2 antagonist, eticlopride, blocked these effects, 3) the muscarinic antagonist, scopolamine, augmented PPD and SP mRNA expression induced by SKF-82958 and prevented the ability of eticlopride to block SKF-82958-induced PPD and SP mRNAs and 4) the SKF-82958-induced increase in preproenkephalin mRNA in striatopallidal neurons was blocked by SCH-23390 or scopolamine but not by eticlopride. These data indicate that endogenous acetylcholine attenuates D1 receptor-stimulated PPD/SP gene expression in medium spiny neurons, mediates D1 receptor-stimulated preproenkephalin gene expression in striatopallidal neurons and contributes to D2 receptor involvement in D1-stimulated PPD/SP gene expression.
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Introduction |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
Conclusions
References
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The psychostimulants, cocaine and
amphetamine, stimulate the expression of immediate early genes and the
neuropeptides, PPD and SP, in striatonigral neurons (for review, see
Wang and McGinty, 1996d
) that primarily express D1
receptors (Gerfen et al., 1990; Le Moine and Bloch, 1995).
Psychostimulants also induce PPE mRNA in striatopallidal neurons (Hurd
and Herkenham, 1992; Steiner and Gerfen, 1993; Wang and McGinty, 1996a)
that primarily express D2 receptors (Gerfen et
al., 1990; Le Moine et al., 1990). However, D1 receptor antagonists block the induction of gene
expression in both populations of striatal neurons (Wang and McGinty,
1996a). Even though D1 receptors are implicated in these
responses, the D1 receptor agonist, SKF-38393, does not
mimic the effects of psychostimulants on gene expression unless animals
are depleted of striatal dopamine (Gerfen et al., 1990;
Jiang et al., 1990; Robertson et al., 1990). The
inability of SKF-38393 to induce striatal gene expression in intact
animals may reflect its partial (45-70%) efficacy in stimulating
adenylate cyclase as compared to dopamine (Anderson and Jansen, 1990;
Arnt et al., 1988; O'Boyle et al., 1989).
Indeed, when full D1 agonists are used, such as A-77636
with 134% of the intrinsic activity of dopamine (Kebabian et
al., 1992) or SKF-82958 with 149% of the intrinsic activity of
dopamine (O'Boyle et al., 1989), robust Fos-like
immunoreactivity and c-fos and zif/268 mRNA are
induced in the normosensitive rat striatum (Wirtshafter and Asin, 1994;
Wang and McGinty, 1996b).
D2 receptor antagonists also block the increase in
striatonigral, but not striatopallidal, gene expression induced by
indirect dopamine agonists (Hanson et al., 1987; Smiley
et al., 1990; Sivam, 1989; Daunais and McGinty, 1996; Wang
and McGinty, 1996a). Therefore, co-activation of D1 and
D2 receptors is necessary for induction of gene expression
in striatonigral neurons. This phenomenon is one example of
D1/D2 interactions. The above findings raise a question about whether D1 and D2 receptors are
co-localized on both striatonigral and striatopallidal neurons
(Surmeier, et al., 1993) or, alternatively, whether
D2 agonist effects on striatonigral neurons and
D1 agonist effects on striatopallidal neurons are mediated
indirectly (Keefe and Gerfen, 1995).
Recent studies have provided evidence that cholinergic interneurons are
in a strategic position to mediate D1/D2
interaction transynaptically. First, D1 agonists increase,
whereas D2 agonists decrease, acetylcholine release (Ajima
et al., 1990; Bertorelli and Consolo, 1990; Consolo et
al., 1993; Damsma et al., 1990, 1991). Second,
muscarinic receptor stimulation suppresses PPD and SP mRNA in
striatonigral neurons (Lucas and Harlan, 1995; Wang and McGinty,
1996c). Blockade of muscarinic receptors increases basal, and
potentiates D1-stimulated, PPD and SP mRNA levels in the
intact striatum (Wang and McGinty, 1996c). Therefore, the increase in
SP/PPD mRNA induced by selective D1 agonists may be attenuated by acetylcholine release. Furthermore, by decreasing acetylcholine release, D2 receptor activation would remove
a brake on SP/PPD gene expression and thus enable striatonigral neurons to fully respond to D1 receptor stimulation. In contrast,
muscarinic agonists stimulate PPE mRNA expression in striatopallidal
neurons whereas muscarinic antagonists block psychostimulant-induced
increases in PPE mRNA (Lucas and Harlan, 1995; Wang and McGinty,
1996c). Therefore, by increasing acetylcholine release,
psychostimulants may be able to increase PPE mRNA induction
transynaptically.
In this study, quantitative in situ hybridization was used to examine the contribution of muscarinic receptors to the transynaptic regulation of striatal gene expression induced by D1 receptor activation. Experiment I investigated whether acute injection of the full D1 agonist, SKF-82958, would induce PPD, SP and PPE mRNA expression in the intact rat striatum. Experiment II investigated whether D1 and D2 receptor antagonists would block SKF-82958-stimulated gene expression. Once these questions were answered positively, experiment III investigated whether 1) the muscarinic receptor antagonist, scopolamine, would augment SKF-82958-stimulated PPD and SP, but block PPE gene induction in the striatum and 2) whether scopolamine would prevent the ability of the selective D2 antagonist, eticlopride, to block SKF-82958-induced gene expression.
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Materials and Methods |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
Conclusions
References
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Animals. Adult male Wistar rats (240-270 g, Charles River, Raleigh, NC) were individually housed and maintained on a 12-hr light/dark schedule with food and water provided ad libitum. All animals were handled on a daily basis for at least 2 days before the experiment to minimize stress. On the day of the experiment, the animals received injections and were observed in the quiet home room. All animal use procedures were in strict accordance with the NIH Guide for the Care and Use of Laboratory Animals and were approved by the Institutional Animal Care and Use Committee.
Experimental protocols. Three experiments were carried out in this study. Experiment I examined dose-dependent effects of SKF-82958 (RBI, Natick, MA) on striatal neuropeptide expression. Rats were randomly divided into five groups (n = four per group). Each rat received one injection of saline or one dose of SKF-82958 (0.02, 0.1, 0.5 and 2 mg/kg). Experiment II evaluated the contribution of D1 and D2 dopamine receptors to SKF-82958-stimulated gene expression. The effects of pharmacological blockade of D1 receptors by SCH-23390 (0.1 mg/kg, RBI) or blockade of D2 receptors by eticlopride (0.5 mg/kg, RBI) on SKF-82958- (2 mg/kg) stimulated neuropeptide mRNA expression were investigated in 6 groups (n = 4 per group): saline + saline, SCH-23390 + saline, eticlopride + saline, saline + SKF-82958, SCH-23390 + SKF-82958, eticlopride + SKF-82958. Experiment III was designed to explore whether muscarinic receptors mediated either or both findings from the second experiment: 1) that eticlopride blocked SKF-82958-stimulated PPD and SP gene expression and 2) that SKF-82958 stimulated PPE expression. In this experiment, following pretreatment with saline (four groups, n = four per group) or the nonselective muscarinic antagonist, scopolamine (5 mg/kg) (four groups, n = four per group), rats received injections either of saline + saline, eticlopride (0.5 mg/kg) + saline, saline + SKF-82958 (0.5 mg/kg) or eticlopride (0.5 mg/kg) + SKF-82958 (0.5 mg/kg).
All drugs were freshly prepared in physiological saline and injected s.c. in a volume of 1.2 ml/kg. The interval between injections in experiments II and III was 15 min. The behavior of the rats was rated by two trained raters, who were unaware of the treatment, 5 min before the first injection, every 5 min during the first hour and every 15 min for the next 2 hr after the final injection. Ratings were determined using a nine-point scale modified from Ellinwood and Balster (1974): 1)
asleep, inactive; 2) normal activity, grooming; 3) increased activity;
4) hyperactive running with jerky movement; 5) slow patterned movement
(repetitive exploration); 6) fast patterned movement (repetitive
exploration with hyperactivity); 7) stereotypy (repetitive
sniffing/rearing in one location to the exclusion of other activities);
8) continuous gnawing, sniffing or licking and 9) dyskinesia, seizures.
In situ hybridization histochemistry.
Three hours after a
single injection (experiment I) or the final injection (experiments II
and III), the rats were anesthetized with equithesin (5 ml/kg, i.p.)
and decapitated. A 3-hr time point was chosen because peak induction of
striatal PPD (Wang et al., 1995), PPE (Bannon et
al., 1989) and SP (Bannon et al., 1991; Haverstick
et al., 1989) mRNA expression occurs 3 hr after dopamine stimulation by amphetamine. The brains were removed and frozen in
isopentane at 
40°C and stored at 70°C. Quantitative in
situ hybridization histochemistry to test PPD, SP and PPE mRNA
expression in striatal neurons was performed according to standard
procedures in this laboratory (Wang et al., 1995; Wang and
McGinty, 1995a, b).
). Briefly,
the 14C standards were measured, plotted against known
dpm/mg, and converted to 35S equivalents to generate a
calibration curve. Film background was measured and saved as a "blank
field" to correct uneven illumination. Hybridization signals were
measured under the density slice option. The upper limit was set to
eliminate any background and this value was used to measure all images.
The lower limit was set at the bottom of the LUT scale. The use of
density slicing allows specific analysis of heterogeneous induction
patterns in brain regions (such as patches in the caudate in which the
hybridization signal is above that in the matrix). The hybridization
signal in the caudoputamen was measured using a circle (diameter = 200 pixels). The signals in the NAc were measured using a manual
drawing that outlined the shell or core areas. Quantitative changes
were expressed as 1) the number of labeled pixels per area (area), 2)
mean density of tissue in dpm/mg and 3) integrated density which is the
product of area times mean density. As previously described (Wang and McGinty, 1995b), the integrated density measurement takes into account
not just the average intensity of the signal, but the entire area over
which the signal is expressed above threshold. We have found this value
to be more representative of changes in hybridization signal than mean
density alone. The mean ± S.E.M. of each of these measures was
calculated for each rat by averaging the values in four adjacent
sections at AP 1.2 mm rostral to bregma (Paxinos and Watson, 1986).
Statistics.
A one-way analysis of variance followed by a
Bonferroni (Dunn) comparison of groups using least squares-adjusted
means was performed on the AUC values calculated from plotting
behavioral ratings against time (Smith and McGinty, 1994). Significance
in area, mean density and integrated density between groups was
determined by a nested two-way analysis of variance followed by a
Bonferroni (Dunn) comparison of groups using least squares-adjusted
means.
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Results |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
Conclusions
References
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Behavior.
Behavioral activity ratings from all three
experiments are summarized in figure 1. In experiment I,
SKF-82958 increased behavioral activity in a dose-dependent manner
(fig. 1A). At 0.1 and 0.5 mg/kg, but not 0.02 mg/kg, SKF-82958
significantly increased locomotor activity (sniffing, grooming, rearing
and locomotion). At the highest dose of 2 mg/kg, SKF-82958 initially
induced hyperlocomotion, which included hanging on the bars of the cage
top and jumping. These behaviors were usually replaced by multiple
stereotypical behaviors (continuous sniffing, exploration or rearing in
one place) within 25 to 30 min.
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, b). Coadministration of scopolamine
and SKF-82958 (0.5 mg/kg) induced greater stereotypical activity than
either drug alone. Finally, in rats treated with scopolamine,
eticlopride, and SKF-82958, behavioral activity was not significantly
different than that displayed by rats treated with scopolamine plus
saline or scopolamine plus eticlopride.
Experiment I. Effects of SKF-82958 on striatal neuropeptide mRNA
expression.
Dose-dependent induction of PPD, SP and PPE mRNAs was
seen in the striatum 3 hr after acute injection of SKF-82958 in doses above 0.02 mg/kg (fig. 2). The alterations in integrated
density reflect a change in the mean density of the hybridization
signal and, to a much larger extent, the number of labeled particles per area (data not shown). At the lowest dose (0.02 mg/kg), SKF-82958 had no effect on any of the mRNA levels in any region of the striatum. Reliable increases in all three mRNA hybridization signals were detectable after 0.1 mg/kg and were robust after 0.5 or 2 mg/kg. The
PPD induction in the dorsal striatum (fig. 2A) was characteristically patchy (not shown) whereas the PPD induction in the ventral striatum, including shell and core regions of the NAc and the olfactory tubercle,
was more homogeneous and much greater, especially in the shell area,
than that in the dorsal striatum. SP induction (fig. 2B), unlike the
pattern of PPD mRNA, occurred in a more homogeneous pattern throughout
the dorsal striatum with greatest intensity in the lateral
caudoputamen. In the ventral striatum, extremely strong induction of SP
mRNA occurred in the shell area of the NAc and throughout the olfactory
tubercle and islands of Calleja. However, SP mRNA expression in the
core area of the NAc was not altered by SKF-82958 administration at any
dose (fig. 2B). Induction of the PPE hybridization signal in the dorsal
striatum was also homogeneous but no change was detectable in the
ventral striatum (fig. 2C).
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Experiment II. Effects of D1 and D2
receptor blockade on SKF-82958-stimulated striatal neuropeptide gene
expression.
Pretreatment with the D1 receptor
antagonist, SCH-23390 (0.1 mg/kg), significantly reduced basal levels
of PPD (fig. 3B and 4A and B) and SP
(fig. 3G and 4C) mRNA in the dorsal and ventral striatum whereas the
D2 receptor antagonist, eticlopride (0.5 mg/kg), did not
affect constitutive PPD or SP expression in these regions (fig. 4).
SCH-23390 blocked SKF-82958 (2 mg/kg)-stimulated PPD (figs. 3C
vs. D and 4A and B) and SP (figs. 3H vs. I and
fig. 4C) hybridization in dorsal and ventral striatum. Similarly,
eticlopride severely attenuated SKF-82958-stimulated PPD (figs. 3C
vs. E and 4A and B) and SP (figs. 3H vs. J and
4C) mRNA expression in the dorsal and ventral striatum.
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Experiment III. Effects of muscarinic receptor blockade on
SKF-82958-stimulated neuropeptide gene expression in the presence or
absence of eticlopride.
In this experiment, the increase in PPD
and SP expression induced by 0.5 mg/kg SKF-82958 was blocked by 0.5 mg/kg eticlopride (figs. 5B vs. C and G
vs. H and 6). In the rats pretreated with 5 mg/kg scopolamine, PPD and SP induction in response to saline + SKF-82958 was substantially augmented in the caudoputamen and the shell
areas of the NAc as compared to that in rats pretreated with saline
(figs. 5B vs. D and G vs. I and 6). However,
scopolamine had no significant effect on SKF-82958-stimulated PPD mRNA
in the core area of the NAc (fig. 6B). In the presence of scopolamine, eticlopride did not alter SKF-82958-stimulated PPD (figs. 5D
vs. E and 6A and B) or SP (figs. 5I vs. J and 6C
and D) expression. In fact, the SKF-82958-stimulated PPD and SP mRNAs
were augmented to the same extent by scopolamine in the presence or
absence of eticlopride. In addition, in rats treated with
scopolamine + saline + saline, a moderate increase in the
basal levels of PPD and SP in both dorsal and ventral striatum was
exhibited as compared to that in the saline + saline + saline
group (fig. 6).
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Discussion |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
Conclusions
References
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A series of experiments was conducted in this study to
characterize alterations in neuropeptide gene expression in specific populations of striatal neurons after direct D1 dopamine
receptor stimulation in intact rats. The major findings include the
following. First, the full D1 receptor agonist, SKF-82958,
unlike the partial D1 agonist, SKF-38393, was a potent
stimulator of behavior as well as PPD/SP mRNA in striatonigral neurons
and PPE mRNA in striatopallidal neurons in the normosensitive striatum.
SKF-82958-stimulated PPD/SP expression was displayed in distinct
patterns within the striatum; stronger induction was concentrated in
the NAc than in the caudoputamen. This pattern closely parallels the
stronger induction of c-fos and zif/268
expression in nucleus accumbens than in caudoputamen elicited by this
drug (Wang and McGinty, 1996b). However, the pattern is unique because
all other direct or indirect dopamine agonists induce greater gene
expression in the caudoputamen than in nucleus accumbens. Second,
D1 and D2 receptors cooperatively mediated the
stimulating effects of SKF-82958 on behavior and gene expression as
demonstrated by the equal sensitivity of SKF-82958-stimulated changes
to D1 and D2 blockade. Third, scopolamine
augmented SKF-82958-stimulated behavior and PPD/SP induction whereas it
blocked SKF-82958-stimulated PPE induction, supporting the idea that
striatal acetylcholine inhibits gene expression in striatonigral
neurons and facilitates gene expression in striatopallidal neurons (Di
Chiara et al., 1994; Wang and McGinty, 1996b,c,d). Finally,
reduction of cholinergic transmission by systemic scopolamine prevented
eticlopride from blocking SKF-82958-stimulated PPD/SP induction in rats
that still had elevated motor activity. These data suggest that
muscarinic cholinergic receptors contribute to the ability of
D2 antagonists to block D1-stimulated gene
expression.
The full D1 receptor agonist, SKF-82958, unlike the
partial D1 receptor agonist, SKF-38393, stimulates behavior
and gene expression in intact animals.
In intact rats, SKF-82958
strongly stimulates behaviors (Meyer and Shults, 1993; Murray and
Waddington, 1989; this study), immediate early gene (Wang and McGinty,
1996b) and neuropeptide (this study) gene expression whereas SKF-38393,
commonly regarded as the prototypical D1 agonist, does not
(Gerfen et al., 1990; Jiang et al., 1990; La
Hoste et al., 1993; Paul et al., 1992; Robertson
et al., 1991). Differences in the actions of these two structurally related benzazepine derivatives reflect, in part, the fact
that SKF-82958 more powerfully stimulates D1-coupled adenylate cyclase. SKF-38393, the first compound shown to have a
selective action at the D1 receptor (Setler et
al., 1978), has only 45 to 70% of the maximum efficacy of
dopamine in stimulating adenylate cyclase that is much less than that
of SKF-82958 (149% intrinsic activity of dopamine) as demonstrated in
homogenates of rat striatum (Anderson and Jansen, 1990; O'Boyle
et al., 1989). Furthermore, SKF-38393 has limited ability to
penetrate the blood brain barrier in contrast to SKF-82958 that is more
lipophilic (Pfeiffer et al., 1982). These properties may
contribute to the reason why SKF-82958, but not SKF-38393, possesses
the power to stimulate behavioral and gene expression in intact rats.
Further evidence supporting the importance of the efficacy of
D1 agonists in stimulating adenylate cyclase is provided by
the observation that the full D1 agonist, A-77636 with
134% intrinsic activity (Kebabian et al., 1992), induces
Fos immunoreactivity in the dorsal striatum of intact rats (Wirtshafter
and Asin, 1994).
Is the D2 receptor contribution to SKF-82958-stimulated
PPD and SP gene expression in striatonigral neurons mediated by
acetylcholine?
The PPD/SP induction by SKF-82958 is a
D1-mediated event because blockade of D1
receptors by SCH-23390 prevented the induction. However, the PPD/SP
induction is also mediated by D2 receptors because
D2 receptor blockade by eticlopride significantly
attenuated the increases. How D2 receptors regulate
striatonigral gene expression is puzzling given the controversy over
colocalization of D1/D2 receptors in these
neurons (Gerfen et al., 1990; Le Moine et al., 1990; Le Moine and Bloch, 1995; Surmeier et al., 1993) and
the inhibitory effect of D2 receptor stimulation on
adenylate cyclase activity. Two major alternatives, which involve
transynaptic mechanisms, exist: 1) direct synaptic connections between
D2-expressing striatopallidal neurons and
D1-expressing striatonigral neurons (Yung et
al., 1996) and/or 2) indirect synaptic interactions that are
mediated by interneurons. With regard to the former, because
eticlopride causes an increase in striatal PPE (and GAD) (Soghomonian,
1994) mRNA and enkephalin immunoreactivity, it is assumed that
striatopallidal neurons are activated by D2 receptor
blockade. Such activation may lead to an increase in inhibition of
striatonigral neurons via direct contacts that would result in a
decreased ability of D1 receptor stimulation to increase
PPD/SP expression. Although this pathway may contribute to some types
of D1/D2 interactions, its contribution to the
ability of scopolamine to completely block eticlopride's effects on
SKF-stimulated SP/PPD gene expression is not obvious. Instead, this
study and others (reviewed in Di Chiara et al., 1994 and
Wang and McGinty, 1996d) suggest that cholinergic interneurons are
involved in these D1/D2 interactions. We
hypothesize that D2 dopamine receptor stimulation reduces
cholinergic inhibition of striatonigral gene expression by decreasing
acetylcholine release (Ajima et al., 1990; Bertorelli and
Consolo, 1990; Damsma et al., 1990). The result would be the
same as blocking muscarinic receptors with scopolamine that enables
striatonigral neurons to positively and fully respond to D1
stimulation. In contrast, D2 receptor blockade would
stimulate acetylcholine release and the subsequent muscarinic receptor
stimulation would considerably suppress the responsiveness of
striatonigral neurons to D1 receptor stimulation. A
muscarinic antagonist would prevent the D2 antagonist's effect on striatonigral gene expression by blocking the effects of
acetylcholine release. Moreover, in this study, muscarinic receptor
blockade by scopolamine completely reversed eticlopride's ability to
block SKF-82958-stimulated PPD/SP induction.
Is the D1-mediated increase in PPE mRNA in
striatopallidal neurons mediated by acetylcholine?
In contrast to
the negative regulation of striatonigral PPD/SP gene expression by
striatal acetylcholine, striatopallidal PPE gene expression is
positively regulated by cholinergic neurotransmission. In our previous
study, systemic injection of the muscarinic receptor agonist,
oxotremorine, up-regulated PPE mRNA expression (Wang and McGinty,
1996c). In a parallel way, an increase in endogenous release of
acetylcholine (Consolo et al., 1993; Damsma et
al., 1991; Florin et al., 1992; Lindefors et
al., 1992; Mandel et al., 1994) may contribute to an
increase in PPE mRNA expression in response to amphetamine stimulation
because systemic (Wang and McGinty, 1996c) or intrastriatal (Wang and
McGinty, 1997) administration of scopolamine attenuated amphetamine
induction of PPE mRNA. Furthermore, in this study, scopolamine
abolished SKF-82958-stimulated PPE induction. It is noteworthy that
direct D1 receptor stimulation by SKF-82958 produced higher
levels of PPE induction than did amphetamine administration (Wang and
McGinty, 1996a). The greater effect of SKF-82958 on PPE mRNA expression
is consistent with stronger stimulation of D1
vs. D2 receptors (in contrast to a more equal
stimulation of D1/D2 receptors by amphetamine)
that should result in a larger increase in acetylcholine release (Ajima et al., 1990; Bertorelli and Consolo, 1990; Damsma et
al., 1990).
) or chronic (Caboche et al., 1993; Morris
et al., 1988) SCH-23390 administration decreases PPE mRNA.
However, in contrast to the role of acetylcholine in
D1-stimulated PPE expression as described above, the
D1 contribution to basal expression of PPE is not likely to
be mediated by acetylcholine because scopolamine did not affect the
basal level of PPE mRNA in this study. Direct blockade of
D1 receptors on striatopallidal neurons is also not likely
because of the uncertainty about D1 receptor localization on these neurons. Although the precise mechanism underlying the D1 contribution to maintaining basal PPE mRNA expression is
unclear, it is possible that blockade of D1 receptors on
striatonigral neurons leads to increased striatal dopamine release or
decreased striatal glutamate release via transynaptic mechanisms, which in turn results in reduction of PPE expression.
The increase in PPE expression induced by neuroleptics, such as
eticlopride, is not only mediated by direct blockade of D2 receptors located on striatopallidal neurons but also apparently by
D2 receptors on cholinergic neurons. Because D2
receptor antagonists disinhibit striatal acetylcholine release,
stimulation of muscarinic receptors on striatopallidal neurons, most
likely through M1 receptors that are positively coupled to
phosphoinositide hydrolysis (Hulme et al., 1990) and
expressed by all striatopallidal neurons (Bernard et al.,
1992; Weiner et al., 1990), may contribute to PPE induction by neuroleptics. This notion is supported by substantial evidence from
this study and others that muscarinic receptor blockade attenuates neuroleptic-stimulated Fos (Guo et al., 1992) and enkephalin
immunoreactivity (Hong et al., 1980, 1985) and PPE mRNA in
the striatum (Angulo et al., 1990; Augood et al.,
1992; 1993; Pollack and Wooten, 1992; this study).
Although eticlopride was not able to block SKF-82958-stimulated PPD/SP
expression in the presence of scopolamine, eticlopride still
significantly attenuated SKF-82958-stimulated behaviors after
scopolamine pretreatment. This result may be due to residual enhancement of striatopallidal neuronal activity after eticlopride blockade of D2 receptors that scopolamine only partially
attenuates. Activated striatopallidal neurons would counteract the
influence of the striatonigral pathway on behavioral activation, thus
contributing to the locomotor depressant effects of neuroleptics.
Is the striatum an important site for the dopamine/acetylcholine
interactions in regulation of striatonigral and striatopallidal peptide
gene expression?
Keefe and Gerfen (1995) established that
intrastriatal microinfusion of D1- or
D2-selective antagonists decreases the ability of SKF-38393
and quinpirole coadministration to induce immediate early gene
expression in the striatum of 6-OHDA-lesioned rats. Regarding
cholinergic regulation of striatal gene expression, Nisenbaum et
al. (1994) reported that seven daily injections of scopolamine
prevented the 6-hydroxydopamine lesion-induced elevation of PPE mRNA
and reduction of SP mRNA in the striatum. However, seven daily
intrastriatal injections of scopolamine in a concentration range of
0.5-50 mM were not able to mimic this prevention. In contrast, recent
data from this laboratory (Wang and McGinty, 1997) indicate that
intrastriatal injection of the muscarinic receptor agonist,
oxotremorine, at a concentration of 1.6-8.1 mM, inhibited PPD/SP mRNA
induced by amphetamine and increased PPE mRNA expression. Intrastriatal
injection of the muscarinic receptor antagonist, scopolamine (62 mM),
increased basal levels of PPD/SP expression and augmented
amphetamine-stimulated PPD/SP mRNA expression. Intrastriatal
scopolamine also blocked amphetamine-stimulated PPE mRNA expression.
Furthermore, amphetamine-induced behavioral activity was completely
blocked by intrastriatal oxotremorine and augmented by intrastriatal
scopolamine (Wang and McGinty, 1997). These data are in good accordance
with those observed after acute systemic injection of oxotremorine and
scopolamine (Wang and McGinty, 1996c). Thus, intrastriatal
dopamine/acetylcholine interactions contribute to the regulation of
tonic and phasic striatal neuropeptide gene expression under normal and
dopamine-stimulated conditions. However, the contribution of
extrastriatal cholinergic regulation of dopamine-dependent changes in
striatal peptide gene expression should not be overlooked because it
may preferentially function in specific pathophysiological processes
and under different experimental conditions.
; Wang and
McGinty, 1996d; this study), an hypothesized model of cell-to-cell interactions between dopamine and acetylcholine in the regulation of
striatal peptide gene expression is summarized in figure
8. SKF-82958 increases PPD/SP expression by direct
stimulation of D1 receptors on striatonigral and accumbal
neurons and striatopallidal PPE expression indirectly by facilitating
SP-induced (Anderson et al., 1993) or glutamate-induced
(DeBoer and Abercrombie, 1994) release of acetylcholine or, possibly,
by stimulating D5 receptors that are expressed by a subset
of cholinergic interneurons, at least in primates (Bergson et
al., 1995). Eticlopride increases PPE expression by direct and
indirect mechanisms, i.e., blocking D2 receptors
directly on striatopallidal neurons and increasing the facilitatory
influence of acetylcholine on PPE expression by blocking D2
inhibition of acetylcholine release. The increased acetylcholine
release evoked by eticlopride could also exercise its inhibitory
influence on SKF-82958-stimulated PPD/SP expression. Scopolamine
augments tonic and phasic PPD/SP induction most likely by blocking
M4 receptors on striatonigral neurons and attenuates SKF-82958- or eticlopride-stimulated PPE mRNA induction most likely by
blocking M1 receptors on striatopallidal neurons.
Furthermore, scopolamine prevents the ability of eticlopride to
decrease D1 receptor-stimulated PPD/SP expression by
blocking the interaction of acetylcholine, released by eticlopride,
with its receptors. Further studies will investigate the location and
identification of the specific muscarinic receptor subtypes involved.
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Conclusions |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
Conclusions
References
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This study demonstrates that a full D1 receptor agonist induces neuropeptide mRNA expression in the normosensitive dorsal and ventral striatum. The induction of PPD/SP mRNAs in striatonigral neurons is prevented by D1 and D2 receptor blockade, indicating a participation of D2 receptor tone in the full expression of D1-stimulated gene expression. The induction of PPE mRNA in striatopallidal neurons is blocked by D1 and muscarinic receptor blockade, indicating that the D1-mediated induction of PPE involves transynaptic activation of cholinergic neurotransmission. Finally, because eticlopride failed to prevent SKF-82958-stimulated striatonigral gene expression in the presence of scopolamine, D2 receptor blockade most likely prevents the stimulating effect of SKF-82958 by enhancing inhibitory cholinergic tone on striatonigral neurons.
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Acknowledgments |
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The authors thank Denise C. Mayer and William T. Bohler for their technical assistance.
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Footnotes |
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Accepted for publication January 29, 1997.
Received for publication October 11, 1996.
1 This work was supported by Grant DA03982.
Send reprint requests to: Dr. Jacqueline F. McGinty, Department of Anatomy and Cell Biology, East Carolina University School of Medicine, Greenville, NC 27858-4354.
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Abbreviations |
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PPD, preprodynorphin; SP, substance P; PPE, preproenkephalin; AUC, area under curve; CPu, caudoputamen; NAc, nucleus accumbens.
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References |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
Conclusions
References
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