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Volume 16, Number 13,
Issue of July 1, 1996
pp. 4231-4239
Copyright ©1996 Society for Neuroscience
Amphetamine and Dopamine-Induced Immediate Early Gene Expression
in Striatal Neurons Depends on Postsynaptic NMDA Receptors and
Calcium
Christine Konradi,
Jean-Christophe Leveque, and
Steven E. Hyman
Molecular and Developmental Neuroscience Laboratory and Department
of Psychiatry, Massachusetts General Hospital and Harvard Medical
School, Boston, Massachusetts 02114
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
FOOTNOTES
REFERENCES
ABSTRACT
Amphetamine and cocaine induce the expression of both immediate
early genes (IEGs) and neuropeptide genes in rat striatum. Despite the
demonstrated dependence of these effects on D1
dopamine receptors, which activate the cyclic AMP pathway, there are
several reports that amphetamine and cocaine-induced IEG expression can
be inhibited in striatum in vivo by NMDA receptor
antagonists. We find that in vivo, the NMDA receptor
antagonist MK-801 inhibits amphetamine induction of c-fos
acutely and also prevents downregulation of IEG expression with chronic
amphetamine administration. Such observations raise the question of
whether dopamine/glutamate interactions occur at the level of
corticostriatal and mesostriatal circuitry or within striatal neurons.
Therefore, we studied dissociated striatal cultures in which midbrain
and cortical presynaptic inputs are removed. In these cultures, we find
that dopamine- or forskolin-mediated IEG induction requires
Ca2+ entry via NMDA receptors but not via L-type
Ca2+ channels. Moreover, blockade of NMDA
receptors diminishes the ability of dopamine to induce phosphorylation
of the cyclic AMP responsive element binding protein CREB. Although
these results do not rule out a role for circuit-level
dopamine/glutamate interactions, they demonstrate a requirement at the
cellular level for interactions between the cyclic AMP and NMDA
receptor pathways in dopamine-regulated gene expression in striatal
neurons.
Key words:
dopamine;
amphetamine;
NMDA;
striatum;
immediate
early genes;
Fos;
CREB
INTRODUCTION
Amphetamine is a psychostimulant drug of abuse
that produces long-term changes in behavior including sensitization,
tolerance, and dependence (Klawans et al., 1978 ; Segal et al., 1980 ;
Robinson and Becker, 1986 ). Although it is well established that
amphetamine is an indirect dopamine agonist (Moore, 1977 ; McMillan,
1983 ; Butcher et al., 1988 ), the biological mechanisms by which
dopamine receptor stimulation produces prolonged behavioral changes
remain unknown. Based on the extended time course of drug-induced
alterations in behavior, amphetamine-induced regulation of striatal
gene expression, including immediate early genes (IEGs), has been
sought and documented (Graybiel et al., 1990 ; Moratalla et al., 1992 ;
Nguyen et al., 1992 ; Konradi et al., 1994 ; Cole et al., 1995 ). Several
IEGs regulated by amphetamine encode nuclear proteins that can, in
turn, activate or repress the expression of other genes. Thus, IEGs are
potential candidates for involvement in long-term alterations in neural
function that may affect behavior.
D1 dopamine receptors have been shown to be
required for the dopamine-dependent activation of IEG expression both
in intact rat striatum in vivo and in dissociated primary
striatal cultures (Graybiel et al., 1990 ; Moratalla et al., 1992 ;
Nguyen et al., 1992 ; Konradi et al., 1994 ; Cole et al., 1995 ).
D1 dopamine receptors are positively coupled to
adenylyl cyclase (Kebabian and Calne, 1979 ; Monsma et al., 1990 ), thus
activating the cyclic AMP pathway. One major nuclear target of the cAMP
pathway is a transcription factor, the cyclic AMP responsive element
binding (CREB) protein, which binds cyclic AMP response elements (CREs)
and activates transcription when phosphorylated on its
Ser133 (Montminy and Bilezikjian, 1987 ; Gonzalez
and Montminy, 1989 ). The promoter of the IEG c-fos contains
several CREs (Sassone-Corsi et al., 1988b ). We have shown previously
that the c-fos gene is induced by amphetamine in striatum in
a CREB-dependent manner (Konradi et al., 1994 ).
In addition to the cyclic AMP-dependent protein kinase,
calcium/calmodulin-dependent protein kinases can phosphorylate CREB on
Ser133 as well as on other serines (Dash et al.,
1991 ; Sheng et al., 1991 ; Sun et al., 1994 ). Nonetheless, activation of
the cyclic AMP pathway has been shown to be necessary and sufficient
for induction of CREB-regulated genes in several transformed cell lines
(Nguyen et al., 1990 ). Thus, it is somewhat puzzling that
amphetamine-mediated induction of c-fos and zif
268 gene expression has been reported to be inhibited by NMDA
receptor antagonists in vivo (Snyder-Keller, 1991 ; Ohno et
al., 1994 ; Wang et al., 1994 ). Hypotheses that have been adduced to
explain these observations generally have focused on neural circuitry:
NMDA-receptor blockade has been hypothesized to inhibit the release of
dopamine and, perhaps, serotonin within the striatum or to inhibit
dopamine D1 receptor function (Kashihara et al.,
1990 ; Johnson and Jeng, 1991 ; Krebs et al., 1991 ; Wang, 1991 ).
Moreover, marked differences might exist between the regulation of
intracellular signaling pathways in the transformed cell lines studied
previously versus striatal neurons; for example, the mode of
Ca2+ entry, including its cellular localization,
might influence its interactions with the cAMP pathway in regulating
gene expression. Because a number of potential substrates exist for
dopamine/glutamate interactions within the circuitry controlling
striatal inputs, we studied both whole animal preparations and also
dissociated embryonic day 18 (E18) neuron-enriched primary striatal
cultures in which the confounding variables introduced by circuitry
would be eliminated. To ensure that our results were not biased by
peculiarities of the regulation of particular IEGs, (e.g.,
c-fos autoregulation) (Sassone-Corsi et al., 1988a ), the
regulation of multiple IEGs was studied.
MATERIALS AND METHODS
Drugs and drug paradigms. Dopamine, the dopamine
D1 receptor agonists SKF-38393 and SKF-82958, the
NMDA receptor antagonists D( )-2-amino-5-phosphonopentanoic acid (APV)
and (+)-MK-801 hydrogen maleate (MK-801), the L-type
Ca2+ channel blockers nifedipine and verapamil,
and amphetamine were obtained from RBI (Natick, MA). EGTA was obtained
from Sigma (St. Louis, MO). Fos antibody for immunocytochemistry was
obtained from Oncogene Science (Ab-2) (Uniondale, NY), and
immunocytochemistry was performed as described (Konradi et al.,
1994 ).
S(+)-amphetamine sulfate equivalent to 4 mg/kg free base was
administered in the rat experiments intraperitoneally. MK-801 hydrogen
maleate equivalent to 0.5 to 1 mg/kg free base also was administered
intraperitoneally. For acute rat experiments, amphetamine was
administered 30 min before killing. When used, MK-801 pretreatment was
given 15 min before amphetamine. Chronic experiments were carried out
for 12 d, with single daily injections of drugs. For details, see
Table 1.
Table 1.
Treatment paradigms for chronic intraperitoneal injections
of rats (see Fig. 2)
Day of treatment
|
Days
1-11
|
Day 12
|
| Group |
Initial |
15 min
later |
Initial |
15 min
later |
|
|
| 1 |
Saline |
Saline |
Saline |
Saline |
Control |
| 2 |
Saline |
Saline |
Saline |
Amph* |
Acute
amph |
| 3 |
MK-801 |
Saline |
MK-801 |
Saline |
Chronic
MK-801 |
| 4 |
Saline |
Amph |
Saline |
Amph |
Chronic
amph |
| 5 |
MK-801 |
Amph |
MK-801 |
Amph |
Chronic amph blocked
by MK-801 |
| 6 |
MK-801 |
Amph |
Saline |
Amph |
Chronic amph
blocked by MK-801, unblocked at last injection |
|
|
*Amph, S(+)-amphetamine sulfate. Drug concentrations: amph, 4 mg/kg; MK-801, 1 mg/kg.
|
|
For experiments in E18 primary striatal cultures, drugs were added 30 min before harvest. When used, MK-801 pretreatment was given 30 min to
1 hr before other drugs. For experiments in
Ca2+-free media, cells were washed and then fed
with these media 6 or 18 hr before drug stimulation.
MK-801 binding. Analysis of
[3H]MK-801 binding was performed as described
previously (Kornhuber et al., 1989 ). Equilibrium was reached in the
presence of glutamate and glycine within 4 hr. Ligand-receptor
association was carried out for 4 or 18 hr at room temperature in the
presence of 5 µM
L-glutamic acid, 5 µM
glycine, and 10 µM MgCl2,
in Tris/HCl buffer, pH 7.4. Nonspecific binding was determined in the
presence of 100 µM unlabeled MK-801. Total
protein concentration per sample was 0.08 mg to 0.1 mg (method of
Bradford, 1976 ), and total sample volume was 250 µl. Concentrations
of [3H]MK-801 ranged from 0.3 to 40 nM for ``hot saturation'' or 1 nM [3H]MK-801 and 0.6 to
78 nM unlabeled MK-801 for ``cold saturation''
studies. The ligand program (Munson and Rodbard, 1980 ) was used for
data analysis. Linear Scatchard plots were obtained for ``hot
saturation'' experiments with extracts from rat striatum, rat cortex,
and rat hippocampus, thus fitting a single site model.
Animals. Male Sprague-Dawley rats (200-250 gm) were used
for all experiments. They were housed four to a cage on a 12/12 hr
light/dark cycle. All injections were intraperitoneal. Animals were
killed by rapid decapitation for the preparation of RNA. All
experiments were performed in at least six rats.
Primary striatal culture. Striata were dissected out under a
stereomicroscope from 18-d-old Sprague-Dawley rat fetuses. Tissue was
suspended in 2 ml of medium (DMEM/F12, Gibco, Gaithersburg, MD) with
the following supplements per liter of medium: 4.5 gm of glucose, 10 ml
of penicillin/streptomycin liquid (Gibco), 10% Nu-serum 1 (Collaborative Biomedical Products, Bedford, MA),
Na+ bicarbonate to pH 7.6, with 0.2% DNase. The
tissue was dissociated mechanically with a fire-narrowed Pasteur
pipette, and the cells resuspended in medium without DNase to
106 cells/ml and plated in six-well plates
(Costar, Cambridge, MA) at 2 × 106 cells/well.
Plates were pretreated with 2 ml of a 1:500 diluted solution of
polyethylenimine (Sigma) in 50 mM sodium borate
buffer, pH 7.4, for 24 hr, washed twice with PBS, left with medium for
about 4 hr, and aspirated just before plating. After 18 hr, cells were
placed on defined medium to prevent glial growth [DMEM/F12 with 1×
B27 supplement, 0.5 gm of glucose/100 ml, 1 ml of
penicillin-streptomycin liquid/100 ml (Gibco), and
Na+ bicarbonate to pH 7.6]. All experiments were
performed in duplicate with cells 6-8 d in culture and repeated at
least once in an independent dissection. As determined by HPLC
analysis, glutamate levels in the medium on the day of the experiments
ranged from 1 to 5 µM. Neuron-to-astroglia
ratio was below 25:1, as established by immunocytochemical staining
with glial fibrillary acid protein (Dako, Carpenteria, CA), and
counterstained with 1% cresyl violet. For a subset of experiments,
which are indicated, DMEM was substituted for DMEM/F12. DMEM medium
without CaCl2 was purchased from Gibco. Drug
treatments were compared in all media used to ensure that none of the
media unexpectedly affected gene expression or drug activity.
Northern blot analysis of rat brain striata. Total striatal
RNA (7-10 µg) was prepared as described (Berger and Chirgwin, 1989 )
and size separated on 1.2% denaturing agarose gel (see below). All
samples were electroblotted onto GeneScreen (DuPont), UV
autocross-linked (Strata- linker, Stratagene, La Jolla, CA), baked
in a vacuum oven at 80°C for 2 hr, and hybridized with
32P-labeled DNA probes (cyclophilin,
c-jun, and zif 268; random oligonucleotide method
using nonamers) (Feinberg and Vogelstein, 1983 ) or
32P-labeled RNA probes (c-fos and
jun-B; Gemini system, Promega, Madison, WI). Cyclophilin
mRNA was used as an unregulated internal reference probe to control for
loading differences (Danielson et al., 1988 ). A phosphorimager
(Molecular Dynamics, Sunnyvale, CA) was used for data analysis.
Northern blot analysis of primary striatal cultures.
Striatal cultures were lysed in 500 µl of lysis buffer (50 mM Tris, pH 8.0, 100 mM
NaCl, 5 mM MgCl2, 0.5%
NP40). After 5 min incubation on ice, cells were scraped into
microcentrifuge tubes and centrifuged for 2 min at 14,000 rpm and
4°C, and the supernatant transferred and SDS added to a 0.2% final
concentration. Cells were extracted twice with phenol/chloroform,
followed by a chloroform extraction and ethanol precipitation. Equal
amounts of RNA (2 µg) were resuspended in sample buffer (1 M paraformaldehyde and 50% formamide in MOPS
buffer) and size separated on 1.2% denaturing agarose gel (1 M paraformaldehyde) in MOPS buffer (20 mM MOPS, pH 7.0, 5 mM
sodium acetate, 1 mM EDTA).
Electrophoretic mobility shift assay. Medium was aspirated
and plates frozen on liquid nitrogen. On the day of the experiment,
cells were scraped in 200 µl of sonication buffer [20
mM Hepes, 25% glycerol, 0.5 M KCl, 1.5 mM
MgCl2, 0.4 mM EDTA, 5 mM DTT, 1 mM EGTA, 0.5 mM PMSF, 1 mM NaF, and 5 µM microcystin (Calbiochem, San Diego, CA)]
and sonicated for 5 sec. After centrifuging for 10 min at 14,000 × g and 4°C, the pellet was discarded and the supernatant
used for electrophoretic mobility-shift assays.
For binding, 4 µl of lysate was incubated on ice for 10 min in
binding buffer (15 µl total volume) containing 10 mM Hepes, pH 7.9, 10% glycerol, 0.1 mM EDTA, 8 mM
MgCl2, 2 mM DTT, 10 mg/ml
poly (dI-dC) at 4°C with or without CREB or phosphoCREB antiserum
(UBI, Lake Placid, NY). The phosphoCREB antiserum recognizes CREB
phosphorylated on Ser133 (Ginty et al., 1993 ) and
was used at a concentration of 0.25 µl/lane. In the presence of
antiserum, preincubation was extended to 15 min. After preincubation, 1 ng of 32P-labeled double-stranded oligonucleotide
was added. Samples were incubated for 10 min at 23°C and then
electrophoresed through a 4% nondenaturing polyacrylamide gel (30:1
acrylamide-bisacrylamide) in 0.25 × TBE (1× TBE contains 89 mM Tris/89 mM boric
acid/2.5 mM EDTA, pH 8.0) (Sambrook et al., 1989 )
and 3% glycerol. The gel subsequently was dried and
autoradiographed.
The ATF and calcium and cyclic AMP response element (CaRE)
oligonucleotides were synthesized with partial BamHI sites
and annealed in the presence of 20 mM
NaPO4, 1 mM EDTA, and 100 mM KCl. The partial BamHI sites of the
double-stranded oligonucleotides then were filled in with
32P-labeled dCTP and unlabeled dATP, dGTP, and
dTTP. Sequence of oligonucleotides used in electrophoretic mobility
shift assays were: ATF: 5 GATCGCTGACGTCAGGG 3
(Hoeffler et al., 1988 ) and c-fos CaRE site:
5 GATCCCCGTGACGTTTACA 3 (Sheng et al., 1990 ).
The partial BamHI site is shown in italic, core consensus
sequences are in bold.
Cyclic AMP levels were determined with the cyclic AMP
[3H]assay system from Amersham (Arlington
Heights, IL). Levels of cyclic AMP were determined per well of six-well
tissue culture plates (~2 × 106 cells).
Experiments were performed in duplicate and repeated at least three
times per treatment condition.
RESULTS
MK-801 inhibits acute amphetamine-induced IEG expression and
chronic amphetamine-induced downregulation of IEG expression in
rat striatum in vivo
The effects of increasing concentrations of MK-801 on
amphetamine-induced (4 mg/kg) IEG expression in rat striatum were
examined by Northern analysis. When MK-801 (0.5 mg/kg) was given 15 min
before amphetamine (4 mg/kg), there was a minimal block of
amphetamine-induced c-fos mRNA (Fig. 1) and
of jun-B, c-jun, and zif 268 mRNAs
(data not shown). Inhibition was apparent with MK-801 pretreatment
(0.75 mg/kg). With MK-801 (1.0 mg/kg), amphetamine-induced IEG
expression was reduced by more than 50% for each of the IEGs studied:
induction of c-fos mRNA was reduced from eightfold to
twofold; jun-B mRNA induction was reduced from sixfold to
threefold; c-jun mRNA induction was reduced from fourfold to
1.5-fold; and zif 268 induction was reduced from 2.5-fold to
no induction (Fig. 1, bottom five panels; all data
normalized to cyclophilin, bottom panel). These data thus
support previous findings demonstrating an inhibition of dopamine
receptor-mediated IEG expression by MK-801 (Snyder-Keller, 1991 ; Torres
and Rivier, 1993 ; Ohno et al., 1994 ; Wang et al., 1994 ). MK-801 (1 mg/kg) independently induced Fos immunocytochemical staining in
cortical areas, but not in the striatum (data not shown) (see also
Dragunow and Faull, 1990 ). MK-801-mediated induction of cortical Fos
(Dragunow and Faull, 1990 ) may be mediated by inhibition of GABAergic
interneurons, thus disinhibiting neurons regulated by GABA (Olney et
al., 1989 ).
Fig. 1.
The NMDA antagonist MK-801 blocks
amphetamine-induced IEG induction in vivo. Northern blots of
rat striatal RNA probed with c-fos, jun-B,
c-jun, and zif 268. Rats received two
intraperitoneal injections 15 min apart and were killed 30 min after
the second injection. Three separate experiments are shown for each
condition. The conditions were as follows: Saline, two
injections of saline; MK-801, MK-801 at the doses shown on
the left followed by saline; Amphetamine, saline
followed by amphetamine (4 mg/kg); MK-801/Amphetamine,
MK-801 at the doses shown on the left followed by
amphetamine (4 mg/kg). A significant inhibition of IEG induction can be
seen with MK-801 (1 mg/kg). Cyclophilin was used as internal loading
control. Each lane is derived from a different rat; n = 9; results were highly replicable across animals; three individual
experiments shown.
[View Larger Version of this Image (46K GIF file)]
Repeated treatment with amphetamine or cocaine has been shown to
downregulate the inducibility of multiple IEGs (Hope et al., 1992 ),
potentially representing a component of amphetamine tolerance. With
chronic amphetamine administration (4 mg/kg, i.p., for 12 d) (see
Table 1 for treatment paradigms), we observed downregulation of IEG
induction as expected (Fig. 2, compare lanes
2 and 4; similar results for c-jun and
zif 268 not shown). Because MK-801 inhibited acute induction
of IEGs (Fig. 1), we hypothesized that it also might block
downregulation of IEG induction. Rats, therefore, were pretreated with
MK-801 (1 mg/kg) before each injection of amphetamine (4 mg/kg) for
11 d. On day 12, the MK-801 pretreatment was omitted (and replaced
with saline). Rats treated in this manner demonstrated 50% greater
amphetamine-induced IEG mRNA than rats treated with a single acute dose
of amphetamine (Fig. 2, compare lanes 2 and 6)
and did not exhibit chronic amphetamine-induced downregulation of IEG
induction (Fig. 2, compare lanes 4 and 6).
Fig. 2.
Chronic amphetamine-induced downregulation of IEG
mRNA in vivo is blocked by MK-801 pretreatment. Northern
blots of rat striatal mRNA probed with c-fos and
jun-B. Rats were treated for 12 d as summarized in
Table 1. Lane 1 represents the control condition; lane
2, the acute amphetamine condition; lane 3, chronic
MK-801 condition; lane 4, chronic amphetamine administration
(4 mg/kg); lane 5, chronic amphetamine administration
preceded by MK-801 pretreatment with each dose; and lane 6,
chronic amphetamine condition preceded by MK-801 pretreatment for
11 d with MK-801 pretreatment replaced by saline on day 12. Acute
treatment with amphetamine (4 mg/kg) leads to a marked induction of
IEGs (lane 2); chronic amphetamine administration
(lane 4) leads to a reduced response. This downregulation of
IEG inducibility is blocked by pretreatment with MK-801 before
amphetamine injections, i.e., when MK-801 is omitted at the last
injection, the response to amphetamine (lane 6) is similar
to the acute, indeed greater (lane 2), rather than the
chronic (lane 4), amphetamine condition. Rats were killed 30 min after the last injection. Cyclophilin was used as internal loading
control; n = 6.
[View Larger Version of this Image (22K GIF file)]
MK-801 acts at a single binding site in the rat strain used
MK-801, which is a noncompetitive NMDA antagonist (Wong et al.,
1986 ), has been reported to interact with sites other than the NMDA
receptor channel. In particular, MK-801 has two binding sites in human
brain frontal cortex (Kornhuber et al., 1989 ), one of which may be the
nicotinic acetylcholine receptor (Kavanaugh et al., 1989 ; Kornhuber et
al., 1989 ; Amador and Dani, 1991 ). Because there has been conflicting
evidence about the number of binding sites in rodent brain (Wong et
al., 1988 ; Javitt and Zukin, 1989 ), we performed
[3H]MK-801 binding analyses of multiple rat
brain areas (Table 2) to investigate
the specificity of MK-801 binding in our rat strain. With increasing
concentrations of [3H]MK-801 in the binding
assay (``hot saturation''), all areas examined yielded linear
Scatchard analyses (Fig. 3; Table 2), which are
consistent with a single MK-801 binding site. With a different binding
method (``cold saturation'') using increasing concentrations of
unlabeled MK-801 to displace a single concentration of
[3H]MK-801, the binding data failed to fit a
linear plot (data not shown), but also were not consistent with
multiple binding sites. Taken together, the lack of linearity is a
mathematical distortion attributable to a difference in the affinity of
unlabeled and labeled MK-801 for its binding site, rather than two
different binding sites. Interestingly, the conflicting reports about
number of binding sites in rodents coincide with the use of these two
different methods (Wong et al., 1988 ; Javitt and Zukin, 1989 ). Even in
the absence of glutamate and glycine,
[3H]MK-801 binding (using ``hot saturation'')
still was linear, with decreased binding affinity
(KD values approximately doubled), but no
change in the maximum number of binding sites
(BMAX). The results are consistent with
previously published data in mouse brain (Foster and Wong, 1987 ; Wong
et al., 1988 ) and suggest that MK-801 interacts solely with the NMDA
receptor in the rat strain used in these experiments. Thus, the
attenuation of amphetamine-induced gene expression by MK-801 likely is
attributable to a blockade of NMDA receptors.
Table 2.
Binding affinities and maximum binding sites for
[3H]MK-801 in rat striatum, cortex, and
hippocampus
|
Mean |
±SEM |
N (of
triplicates) |
|
| Striatum |
| KD |
3.3
nM |
0.42 |
2 |
| BMAX |
1.57
pmol/mg
protein |
0.61 |
| Cortex |
| KD |
4.05
nM |
0.58 |
5 |
| BMAX |
2.35
pmol/mg
protein |
0.66 |
| Hippocampus |
| KD |
4.7
nM |
2.55 |
2 |
| BMAX |
4.13
pmol/mg protein |
0.18 |
|
|
Each experiment was done in triplicate.
|
|
Fig. 3.
Representative Scatchard plot of
[3H]MK-801 binding in rat striatum. A ``hot
saturation'' experiment is shown. See Table 2 for all
KD and BMAX
values.
[View Larger Version of this Image (12K GIF file)]
In dissociated striatal neurons, D1 dopamine
receptor-mediated IEG induction depends on Ca2+ entry via
NMDA receptors, but not via L-type Ca2+ channels
To examine whether the observed interaction between NMDA receptors
and dopamine receptors occurs only at the level of striatal circuitry
or whether significant interactions occur intracellularly within
striatal neurons, we turned to dissociated neuron-enriched primary
striatal cultures in which the natural presynaptic dopamine and
glutamate inputs have been removed. Levels of both c-fos and
jun-B mRNA were examined in nearly all experiments in
primary striatal cultures, but because no significant differences were
observed in the responsiveness of these two genes, data are shown for
c-fos mRNA only. As determined by HPLC analysis on the day
of the experiment, glutamate levels in the culture medium range between
1 and 5 µM, whereas aspartate levels are below
detection limit (detection limit was 1 µM).
Dopamine and the D1 agonists SKF 38393 and SKF
82958 all induced c-fos in the dissociated cultures (Fig.
4A, B) consistent with our
previous findings (Konradi et al., 1994 ; Cole et al., 1995 ).
Surprisingly, however, MK-801 (50 µM) or the
competitive NMDA receptor antagonist APV (100 µM) blocked the induction of c-fos
(Fig. 4B) and jun-B mRNA (data not shown) by
dopamine or the D1 agonists. MK-801 (1 µM) also blocked dopamine-mediated
c-fos induction after a 4 hr preincubation (Fig.
4A) (earlier time points not examined). A prolonged
incubation was used with the lower concentration of MK-801, because
[3H]MK-801 binding required 4 hr to reach
equilibrium in the presence of glutamate and glycine (data not shown).
To avoid long exposure to MK-801, we chose a higher, saturating
concentration for a short period of time for most of the experiments
shown.
Fig. 4.
A, B, NMDA receptor
antagonists inhibit dopamine-induced IEG expression in rat primary
striatal cultures. Northern blots from rat primary striatal culture
probed with c-fos. A, Cultures were treated for
30 min with dopamine (50 µM). Pretreatment for
4 hr with MK-801 (1 µM) blocked
c-fos induction; n = 4. B,
Cultures were treated for 30 min with either dopamine (50 µM), SKF 38393 (50 µM),
or SKF 82958 (50 µM). Pretreatment for 30 min
with MK-801 (50 µM) or APV (100 µM) prevented c-fos induction;
n = 8.
[View Larger Version of this Image (21K GIF file)]
NMDA receptor channels are permeable to Ca2+
(MacDermott et al., 1986 ; Hollmann and Heinemann, 1994 ). Therefore, we
tested the role of Ca2+ in dopamine-mediated
c-fos (Fig. 5A, B)
expression in the striatal cultures. When cells were grown for 6 hr
(data not shown) or 18 hr (Fig. 5A) in
CaCl2-free medium (DMEM/F12 medium normally
contains 1 mM CaCl2; DMEM
normally contains 1.8 mM
CaCl2), the ability of dopamine to induce
c-fos was markedly reduced. Alternatively, addition of EGTA
(10 mM) to standard medium 6 hr (data not shown)
or 18 hr (Fig. 5B) before the addition of dopamine markedly
inhibited the induction of c-fos (Fig. 5B) after
dopamine stimulation.
Fig. 5.
A, Ca2+-poor
medium reduces dopamine-induced IEG expression significantly. Northern
blots from rat primary striatal culture probed with c-fos.
Cultures were treated for 30 min with dopamine (50 µM) in regular medium
(+Ca2+ DMEM) or CaCl2-free
medium ( Ca2+ DMEM). Experiments were performed
6 (data not shown) and 18 hr after media change; n = 4. B, EGTA reduces dopamine-mediated IEG induction
significantly. Northern blots from rat primary striatal culture probed
with c-fos. Cultures were treated for 30 min with dopamine
(50 µM) in control medium or in the presence of
10 mM EGTA. Experiments were performed 6 (data
not shown) and 18 hr after addition of EGTA; n = 4.
[View Larger Version of this Image (37K GIF file)]
Because D1 receptor-mediated activation of
protein kinase A (PKA) enhances the activity of L-type
voltage-sensitive Ca2+ channels in striatum
(Surmeier et al., 1995 ), we tested the effects of the L-type
Ca2+ channel blockers nifedipine (20 nM to 100 µM) and
verapamil (20 nM to 100 µM) on dopamine-mediated IEG induction (Fig.
6; the effects of L-type channel blockers at 100 nM and 500 nM are shown).
Neither drug inhibited dopamine-mediated c-fos induction;
indeed at higher concentrations (µM range)
(data not shown) verapamil increased the induction of c-fos.
Treatment with depolarizing concentrations of KCl induced
c-fos expression (Fig. 7); this induction
also was blocked by MK-801 (Fig. 7).
Fig. 6.
L-type Ca2+ channel blockers
do not inhibit dopamine-induced c-fos induction in primary striatal
cultures. Northern blots from rat primary striatal culture probed
with c-fos. The L-type Ca2+ channel
blockers nifedipine and verapamil (concentrations shown for both
inhibitors, 100 and 500 nM) do not block
dopamine- or SKF 82958-induced c-fos expression;
n = 4.
[View Larger Version of this Image (22K GIF file)]
Fig. 7.
KCl-mediated c-fos induction is
inhibited by MK-801. Northern blots from rat primary striatal culture
probed for c-fos mRNA. MK-801 (50 µM) inhibited dopamine-mediated (50 µM) c-fos induction in the presence
and absence of KCl (25 mM) as well as
c-fos induced by KCl alone (compare lanes control and
MK-801 in the presence of KCl to control lane
without KCl). All treatments in duplicate; n = 6.
[View Larger Version of this Image (19K GIF file)]
Acting via D1 receptors, dopamine causes robust
increases in cyclic AMP levels in this primary dissociated striatal
culture system (Cole et al., 1995 ). Therefore, we examined the
possibility that the NMDA receptor antagonists were exerting their
inhibitory effects on IEG induction by altering cyclic AMP levels.
Dopamine and dopamine agonists markedly induced cyclic AMP generation
10 min after stimulation by the cultures. MK-801 did not affect
dopamine agonist-mediated induction of cyclic AMP (Fig.
8).
Fig. 8.
The NMDA receptor antagonist MK-801 does not
affect dopamine-induced cyclic AMP levels in primary striatal cultures.
MK-801 (50 µM) does not affect dopamine- (50 µM) or SKF 82958- (50 µM) induced cyclic AMP levels 10 min after
stimulation; n = 3.
[View Larger Version of this Image (33K GIF file)]
We have shown previously that dopamine receptor-mediated
c-fos induction in striatal neurons is dependent on the
constitutively expressed transcription factor CREB (Konradi et al.,
1994 ), which interacts with the c-fos gene at its CaRE site
(Sheng et al., 1990 ) and is activated by phosphorylation of
Ser133 (Gonzalez and Montminy, 1989 ). To
investigate further the interaction between dopamine and NMDA receptor
pathways, we therefore examined the effects of MK-801 and APV on
dopamine-mediated CREB phosphorylation in striatal neurons (Fig.
9). In gel-mobility shift assays using the
c-fos CaRE site as a probe (Fig. 9) and a palindromic CREB
binding site (ATF) for comparison (which gave identical results to the
CaRE probe) (data not shown), we observed no quantitative difference in
total CREB binding following dopamine or NMDA receptor antagonists. An
antiserum raised against total CREB protein produced a complete
supershift in all lanes (data not shown) (see also Konradi et al.,
1995 ). When we examined phosphorylation of CREB, using a specific
antiserum directed against Ser133 phosphoCREB
(Ginty et al., 1993 ) for supershifts, we found significant effects of
dopamine, which induced a fourfold increase (n = 4) in
the specific phosphoCREB band. Treatment of cultures with MK-801 (50 µM) or APV (100 µM) in
the absence of dopamine decreased Ser133
phosphoCREB below basal levels (Fig. 9, right). Both of the
NMDA receptor antagonists blocked dopamine-induced CREB
phosphorylation, i.e., CREB phosphorylation remained at basal levels
(Fig. 9; right). KCl treatment also induced CREB
phosphorylation (twofold increase as demonstrated by a supershift with
the phosphoCREB antiserum), which was blocked by MK-801 (Fig.
10).
Fig. 9.
Dopamine-induced CREB phosphorylation is blocked
by NMDA receptor antagonists. In the absence of phosphoCREB antiserum,
no quantitative difference in CREB binding to the CaRE or palindromic
CREB/ATF (data not shown) site is observed. The phosphoCREB antiserum
interacts with part of the bound complex (supershifted band) in
experiments performed 15 min after stimulation with dopamine (50 µM). Pretreatment of cultures for 30 min with
MK-801 (50 µM) or APV (100 µM) prevents phosphorylation of CREB, as
demonstrated by the lack of a supershifted band in the respective
lanes; n = 4.
[View Larger Version of this Image (55K GIF file)]
Fig. 10.
KCl-induced CREB phosphorylation is blocked by
MK-801. In the absence of phosphoCREB antiserum, no quantitative
difference in CREB binding to the CaRE site is observed
(Control). The phosphoCREB antiserum interacts with part of
the bound complex (creating the supershifted band, see
arrow) in experiments performed 15 min after stimulation
with dopamine (50 µM) or KCl (25 mM). Pretreatment of cultures for 30 min with
MK-801 (50 µM) prevents phosphorylation of CREB
in both paradigms, as demonstrated by the lack of a supershifted band
in the respective lanes; n = 6.
[View Larger Version of this Image (54K GIF file)]
To examine whether Ca2+ and NMDA receptor
dependence is a general mechanism for cyclic AMP pathways of which the
D1 pathway is an example, we examined the effect of NMDA receptor
antagonists on forskolin-mediated IEG expression. Forskolin, which
stimulates adenylyl cyclase directly, bypasses the
D1 dopamine receptor and is a strong activator of
c-fos in primary striatal cultures (Konradi et al., 1994 ).
Surprisingly, we found that forskolin-induced IEG expression also is
inhibited by MK-801 and APV (Fig. 11) in the
cultures.
Fig. 11.
Forskolin-mediated c-fos
expression is inhibited by NMDA receptor antagonists. Northern blots
from rat primary striatal cultures probed with c-fos.
Cultures were treated for 30 min with forskolin (1-5
µM). Pretreatment for 30 min with MK-801 (50 µM) or APV (100 µM)
inhibited c-fos induction. Increasing forskolin
concentrations were able to partially overcome the block of
c-fos induction exerted by the competitive NMDA antagonist
APV; n = 4.
[View Larger Version of this Image (19K GIF file)]
DISCUSSION
The transient induction of IEG transcription factors in striatal
neurons is a well-studied cellular response to amphetamine and cocaine
administration (Graybiel et al., 1990 ; Young et al., 1991 ; Hope et al.,
1992 ; Nguyen et al., 1992 ; Konradi et al., 1994 ; Kosofsky et al.,
1995 ). As a cellular marker, the induction of IEGs has been used to map
neurons and circuits activated by amphetamine and cocaine. Moreover, it
has been proposed that IEGs might couple some of the acute actions of
these drugs to long-term alterations in neural function. Dopamine
receptor-mediated IEG induction has been shown to exhibit some
properties of tolerance with chronic administration (Hope et al.,
1992 ), thus paralleling results of behavioral studies (De Montis et
al., 1992 ).
Based on the ability of the D1 dopamine receptor
antagonist SCH23390 to block amphetamine-induced striatal IEG
expression in vivo (Graybiel et al., 1990 ; Nguyen et al.,
1992 ) and dopamine-induced IEG expression in striatal cultures (Konradi
et al., 1994 ), it has been argued that D1
receptor-mediated activation of the cyclic AMP pathway is a critical
mechanism of IEG induction. Evidence that the NMDA receptor antagonist
MK-801 (Ohno et al., 1994 ; Wang et al., 1994 ) also blocks
amphetamine-induced IEG induction in vivo does not provide
information as to whether the substrate of the dopamine/glutamate
interaction is at the level of striatal circuits or at the level of
intracellular signaling within individual striatal neurons.
In this report, we characterized further the effects of MK-801 on
amphetamine-induced IEG expression in vivo. Most notably, we
find that MK-801 pretreatment before daily doses of amphetamine blocks
the downregulation of IEG induction that has been proposed as a model
of biochemical tolerance. Indeed, with daily MK-801 pretreatment, the
striatum is, if anything, hyper-responsive to amphetamine. MK-801
previously has been shown to block behavioral responses to cocaine or
amphetamine including sensitization (Itzhak and Stein, 1992 ; Wolf and
Jeziorski, 1993 ; Hoffman, 1994 ; Ohno et al., 1994 ) or reverse tolerance
(Karler et al., 1989 ) and tolerance (De Montis et al., 1992 ). The
present data provide a molecular correlate of the ability of MK-801 to
inhibit an effect of chronic amphetamine administration. This finding
may provide clues to the cellular mechanisms by which NMDA receptor
antagonists may block long-term behavioral effects of
psychostimulants.
Because most of the in vivo data, including our own, have
been obtained with MK-801, we investigated the possibility that MK-801
exerts its action via receptors other than the NMDA receptor. In
multiple rat brain areas and under different binding conditions, only a
single, high-affinity MK-801 binding site was observed in the strain of
rats used in our experiments. Moreover, in primary striatal cultures,
the effects of MK-801 were not significantly different than the effects
of APV, an unrelated competitive NMDA receptor antagonist.
Studies in intact animals cannot address the question of whether
NMDA/D1 dopamine receptor interactions depend on
circuitry or intraneuronal interactions. Indeed, it has been
demonstrated that glutamate receptors including NMDA receptors on
presynaptic dopamine terminals can modulate dopamine release (Kashihara
et al., 1990 ; Johnson and Jeng, 1991 ; Krebs et al., 1991 ; Wang, 1991 ).
To distinguish a potential postsynaptic-intracellular interaction from
such a presynaptic-intercellular mechanism, we used dissociated
primary striatal cultures, a preparation that removes presynaptic
terminals. In these cultures, we supplied glutamate and dopamine in the
media, glutamate continuously, and dopamine at discrete times to model
the phasic stimulation produced by amphetamine. We found that even in
dissociated cultures, blockade of NMDA receptors by MK-801 or APV
substantially inhibited the ability of dopamine to induce IEGs. Thus,
there is an intracellular substrate that may account for a significant
component of the glutamate/dopamine interaction observed in
vivo.
To elucidate further the requirement for NMDA receptors in
dopamine-mediated IEG expression, we investigated the role of
Ca2+ entry and found that extracellular
Ca2+ was required. Complex interactions between
the cyclic AMP pathway and L-, N-, and P-type
Ca2+ channels have been documented within acutely
dissociated adult striatal neurons (Surmeier et al., 1995 ). In
particular, D1 stimulation has been shown to
activate L-type Ca2+ channels (Surmeier et al.,
1995 ), yielding a potential mechanism of interaction between
D1 receptors and Ca2+ in
regulating gene expression. However, L-type Ca2+
channel blockers did not inhibit dopamine-induced IEG expression.
Previous studies have demonstrated that in hippocampal neurons,
different modes of Ca2+ entry can activate
c-fos expression via different intracellular pathways (Lerea
et al., 1992 ; Bading et al., 1993 ; Lerea and McNamara, 1993 ). We find
that in striatal cultures, both cyclic AMP-mediated IEG expression and
KCl depolarization-mediated IEG expression require active NMDA
receptors.
There was a strong correlation between the effects of NMDA receptor
blockers on c-fos mRNA and on phosphorylation of CREB,
consistent with our previous findings that amphetamine-induced
c-fos expression in the striatum is dependent on
transcription factor CREB (Konradi et al., 1994 ). However, these
experiments do not rule out roles for other signaling pathways or
transcription factors in cyclic AMP or
Ca2+-regulated gene expression (Runkel et al.,
1991 ; Robertson et al., 1995 ), especially for genes other than
c-fos. It is noteworthy that induction of c-jun,
which lacks a consensus serum response element (SRE) or CRE (Hattori et
al., 1988 ), was inhibited by NMDA receptor antagonists in
vivo.
Bading et al. (1993) had found, using transfected c-fos
promoter constructs, that in hippocampal neurons the SRE was primarily
responsible for induction of c-fos gene expression by the
NMDA receptor pathway, whereas the CaRE element mediated
c-fos gene expression via L-type Ca2+
channels. In those studies, NMDA receptor activation by itself was
inadequate to stimulate transcription from a single CRE (the
c-fos CaRE element), although a possible role for CREs in
NMDA receptor-mediated c-fos gene expression was not ruled
out. The experiments described here do not rule out some role for the
c-fos SRE in D1-mediated regulation.
Other evidence, which supports a role for CREs in dopamine-mediated
gene expression, is that in the same striatal culture system used for
the present studies, dopamine-regulated prodynorphin gene expression
has been mapped previously to three CREs that interact with CREB (Cole
et al., 1995 ). The dopamine-inducible prodynorphin constructs did not
contain an SRE-like element or a functional AP-1 element (Douglass et
al., 1994 ; Cole et al., 1995 ). In comparing our findings with those of
Bading et al. (1993) , it must be noted that both the cell type and the
activated signaling pathways differ, i.e., in our striatal culture
paradigm, the cAMP pathway was activated in association with the NMDA
receptor pathway; in the previous studies of hippocampal neurons, the
cAMP pathway was not activated.
We conclude that D1 dopamine receptor-mediated
IEG expression in striatal neurons requires functional NMDA receptors
and Ca2+. These findings do not rule out
significant circuit-based interactions between dopamine and NMDA
receptors in vivo. Additional studies are needed to address
the level at which the dopamine/cyclic AMP and NMDA receptor pathways
interact within cells. Mechanisms deserving additional investigation
include cyclic AMP/PKA-stimulated phosphorylation of NMDA receptors,
which might alter Ca2+ influx (see also Chen and
Huang, 1992 ; Cepeda et al., 1993 ) as well as interactions at the level
of protein kinases and transcription factors. It has become clear that
D1 dopamine receptor-mediated regulation of
striatal gene expression is far more complex than it first appeared,
even for well-characterized genes such as c-fos.
FOOTNOTES
Received Jan. 22, 1996; revised April 9, 1996; accepted April 11, 1996.
This work was supported by Public Health Service Grants DA07134 and
DA00257.
Correspondence should be addressed to Dr. Konradi, Laboratory of
Molecular and Developmental Neuroscience, Massachusetts General
Hospital, CNY 2, 149, 13th Street, Charlestown, MA 02129.
Dr. Hyman's current address: Office of the Director, National
Institute of Mental Health, 5600 Fishers Lane, Rockville, MD
20857.
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