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Volume 17, Number 9,
Issue of May 1, 1997
pp. 3254-3261
Copyright ©1997 Society for Neuroscience
Repeated Cocaine Modifies the Mechanism by which Amphetamine
Releases Dopamine
R. Christopher Pierce and
Peter W. Kalivas
Alcohol and Drug Abuse Program, Washington State University,
Pullman, Washington 99164-6520
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
FOOTNOTES
REFERENCES
ABSTRACT
This study determined whether daily cocaine administration
initiates a calcium requirement for the increase in extracellular dopamine produced by psychostimulants. The increase in extracellular dopamine induced by perfusion of amphetamine through a microdialysis probe in the nucleus accumbens shell was enhanced in cocaine- relative
to saline-pretreated rats. The augmented portion of the amphetamine-induced increase in nucleus accumbens dopamine was abolished by the coperfusion of L- or N-type calcium channel blockers. Inhibition of calcium/calmodulin-dependent protein kinase II (CaM-KII) also prevented the augmented increase in dopamine by amphetamine, whereas inhibition of vesicular exocytosis by botulinum toxin B was
ineffective. When the concentration of extracellular dopamine in the
nucleus accumbens was elevated by blocking the plasmallemal dopamine
transporter with GBR-12909, the augmented increase in extracellular
dopamine in rats sensitized to repeated cocaine was blocked by a
CaM-KII inhibitor. Pretreatment with botulinum toxin B prevented the
increase in extracellular dopamine by GBR-12909 in both
cocaine-pretreated and control rats. Taken together, these results
demonstrate that the psychostimulant-induced enhanced increase in
extracellular dopamine in the nucleus accumbens shell of
cocaine-pretreated rats arises from the induction of calcium- and
CaM-KII-dependent mechanisms.
Key words:
behavioral sensitization;
cocaine;
amphetamine;
GBR-12909;
verapamil;
conotoxin;
diltiazem;
KN-93;
botulinum toxin;
calcium;
calmodulin
INTRODUCTION
The abuse of amphetamine-like psychostimulants is
a medical and social problem throughout the world. The behavioral
stimulant and reinforcing properties of amphetamine-like stimulants are linked to the capacity of these drugs to increase extracellular concentrations of dopamine in the forebrain, notably in the nucleus accumbens (Wise and Bozarth, 1987 ; Koob and Bloom, 1988 ; Nestler, 1992 ;
Di Chiara, 1995 ). Psychostimulant abuse can result in the emergence of
psychopathologies such as paranoid psychosis and panic attacks, and
altered forebrain dopamine transmission is linked to the development of
these behavioral changes (Ellinwood, 1967 ; Segal et al., 1981 ; Post and
Weiss, 1988 ). Similarly, in an animal model of this form of behavioral
sensitization, the repeated administration of amphetamine-like
psychostimulants in rats increases the capacity of amphetamine to
induce motor activity and increase extracellular dopamine in the
nucleus accumbens, particularly after extended withdrawal periods
(Kalivas and Duffy, 1993 ; Wolf et al., 1993 ; Paulson and Robinson,
1995 ).
Recent findings indicate that calcium-dependent protein kinases
are critically involved in activity-dependent synaptic modifications (Calabresi et al., 1994 ; Lisman, 1994 ). It has been reported that calcium also plays an important role in the behavioral plasticity observed after repeated exposure to psychostimulants. Thus, in contrast
to the acute behavioral effect of amphetamine or cocaine, the
sensitized motor response after repeated psychostimulant treatment is
blocked by the systemic administration of L-type calcium channel antagonists (Pani et al., 1990 ; Karler et al., 1991 ; Martin-Iverson and
Reimer, 1994 ). The fact that calcium channel antagonists inhibit the
expression of behavioral sensitization, but not the acute drug effect,
poses the possibility that the repeated administration of
psychostimulants may modify the mechanism of amphetamine-induced dopamine release by invoking a requirement for calcium. Indeed, a
recent microdialysis study indicated that replacing calcium with
magnesium in the dialysis buffer inhibited the augmented increase in
extracellular dopamine in the nucleus accumbens of rats pretreated with
repeated amphetamine (Warburton et al., 1996 ). Furthermore, among
cocaine-pretreated animals, the systemic administration of the L-type
calcium channel antagonist, nimodipine, attenuated the enhanced
increase in dopamine in the striatum resulting from a peripheral
cocaine injection (Pani et al., 1990 ).
To appraise potential mechanisms underlying the calcium requirement for
the augmented increase in extracellular dopamine in cocaine-sensitized
animals, the present study estimated the effects of various compounds
that block calcium channels or inhibit calcium-calmodulin-stimulated kinases on the increase in extracellular dopamine in the nucleus accumbens shell produced by psychostimulants. Two psychostimulants were
used: GBR-12909, which blocks the dopamine transporter in a manner
similar to cocaine (Van der Zee et al., 1980 ; Nomikos et al., 1990 ),
and amphetamine, which induces dopamine release by exchange diffusion
(see Seiden et al., 1993 ). GBR-12909 was substituted for cocaine in all
experiments in which the drug was administered via the microdialysis
probe because of the local anesthetic property of cocaine. The role of
calcium-mediated vesicular exocytosis also was evaluated by pretreating
animals with botulinum toxin B before assessing the effects of
amphetamine or GBR-12909.
MATERIALS AND METHODS
Subjects. All of the experiments described in this
report were performed in accordance with the specifications of the
Washington State University Animal Care and Use Committee. Male Sprague
Dawley rats (Simmonsen Laboratories, Gilroy, CA) were individually
housed with food and water available ad libitum. A 12 hr
light/dark cycle was used with the lights on at 6:30 A.M. All cocaine
injections, behavioral testing, and microdialysis were performed during
the light cycle.
Repeated cocaine or saline treatment. Equal numbers of
subjects were assigned to the cocaine and saline groups. The day before the start of the experiment, all animals were habituated to the photocell boxes (Omnitech Electronics, Columbus, OH) for 3 hr. On the
first treatment day, all animals were habituated to the photocell boxes
for 1 hr. After habituation, animals received either cocaine (15 mg/kg, i.p.; donated by the National Institute of Drug
Abuse) or saline (1.0 ml/kg, i.p.), and behavior
subsequently was monitored for 2 hr. On days 2-6, rats received daily
injections of cocaine (30 mg/kg, i.p.) or saline in the
home cage. On the seventh day, all animals were again habituated to the
photocell boxes for 1 hr. Cocaine (15 mg/kg) or saline
then was administered i.p., and behavior was monitored for 2 hr.
Surgery. Stereotaxic implantation of a dialysis guide
cannula was conducted 2 weeks after the last injection of cocaine or saline. Rats weighing 300 to 350 gm were anesthetized with Equithesin (3.0 ml/kg) and mounted in a stereotaxic apparatus. A
unilateral microdialysis guide cannula (12 mm, 20 gauge stainless
steel) was implanted 2 mm dorsal to the nucleus accumbens shell (1.0 mm
anterior; 0.8 mm lateral relative to bregma; Paxinos and Watson, 1986 )
and cemented in place by affixing dental acrylic to three stainless
steel screws that were tapped into the skull. There was 1 week of
recovery from surgery before all microdialysis experiments.
Microdialysis and measurement of extracellular dopamine. The
dialysis probes were constructed as described by Robinson and Whishaw
(1988) , with approximately 2.0 mm of active dialysis membrane exposed
at the tip. The dialysis experiments were performed 21-22 days after
the last daily cocaine injection. The evening before the experiment,
the probes were inserted through the guide cannulae into the nucleus
accumbens (8.0 mm ventral to the top of the skull). The next day,
dialysis buffer (5 mM KCl, 120 mM NaCl, 1.2 mM CaCl2, 1.2 mM MgCl2,
5.0 mM glucose, plus 0.2 mM PBS to give a pH
value of 7.4 and a final sodium concentration of 120.7 mM)
was advanced through the probe at a rate of 1.9 µl/min via a syringe pump (Harvard Instruments,
Boston, MA). In the first two experiments, after 100 min of baseline,
increasing concentrations of amphetamine (0.1, 1.0, and 10.0 µM) or GBR-12909 (1.0, 10.0, and 100.0 µM) were incorporated into the dialysis buffer for 100 min each. The next
series of experiments were designed to assess the effects of calcium
channel antagonists on the amphetamine- and GBR-12909-induced increase
in dopamine. After baseline, one of the following drugs was included in
the dialysis buffer for the duration of the experiment: the L-type
calcium channel blockers verapamil (100 µM; Research Biochemicals, Natick, MA) and diltiazem (10 µM; Research
Biochemicals), or the N-type channel blocker -conotoxin (1 µM; Research Biochemicals). After baseline and 100 min of
one of the calcium channel antagonists alone, increasing concentrations
of amphetamine (0.1, 1.0, and 10.0 µM) or GBR-12909 (1.0, 10.0, and 100.0 µM) were incorporated into the dialysis
buffer for 100 min each. The effect of the CaM-KII inhibitor KN-93 also
was assessed. After baseline and 100 min of KN-93 (10 µM;
Calbiochem, San Diego, CA) alone, increasing concentrations of
amphetamine (0.1, 1.0, and 10.0 µM) or GBR-12909 (1.0, 10.0, and 100.0 µM) were incorporated into the dialysis buffer for 100 min each. The effect of the negative control compound for KN-93, KN-92, also was assessed. After baseline and 100 min of
KN-92 (10 µM; Calbiochem, San Diego, CA) alone,
increasing concentrations of amphetamine (0.1, 1.0, and 10.0 µM) were incorporated into the dialysis buffer for 100 min each. In the final experiments, the rats were pretreated with a
microinfusion of botulinum toxin B (1.0 ng/µl, 0.3 µl; Sigma, St.
Louis, MO) or saline (0.3 µl) into the nucleus accumbens 15 min
before inserting the microdialysis probe. The next day, after 100 min
of baseline, increasing concentrations of amphetamine (0.1, 1.0, and
10.0 µM) or GBR-12909 (1.0, 10.0, and 100.0 µM) were incorporated into the dialysis buffer for 100 min each. In all cases microdialysis samples were taken every 20 min.
For the measurement of extracellular dopamine, samples were
collected into microfuge tubes containing 20 µl of mobile phase (0.1 M citric acid, 75 mM
Na2HPO4, 1.5 mM heptanosulfonic
acid, 0.1 mM EDTA, 15% methanol, v/v, pH 4.2) plus 2.0 pmol of dihydroxybenzylamine as an internal standard. After collection,
all samples were frozen at 80°C until analyzed. The samples were
subsequently thawed and placed in an autosampler (Gilson Medical
Supplies, Middleton, WI) connected to an HPLC system with
electrochemical detection. The dopamine was separated using a 25 cm
C18 reversed phase column (Bioanalytical Systems, West
LaFayette, IN) and oxidized/reduced using coulometric detection (ESA,
Bedford, MA). Three electrodes were used: a preinjection port guard
cell (+0.4 V) to oxidize the mobile phase, an oxidation analytical
electrode (+0.3 V), and a reduction analytical electrode ( 0.14 V).
Peaks were recorded on a chart recorder and compared with an external
standard curve (10-1000 fmol).
Histology. After the dialysis experiment, the rats
were given an overdose of pentobarbital (>100 mg/kg, i.p.) and
perfused intracardially with PBS, followed by 10% formalin. The brain
was removed and stored in 10% formalin for at least 1 week. The brains were then blocked, and coronal sections (100 µm) were taken at the
level of the nucleus accumbens with a vibratome. The sections were
mounted on gelatin-coated slides and stained with cresyl violet. Probe
and cannula placements were determined according to the atlas of
Paxinos and Watson (1986) by an individual unaware of the behavioral or
neurochemical response of the rats.
Data analysis. The effect of amphetamine or GBR-12909 on
extracellular dopamine was evaluated using a two-way ANOVA with
repeated measures over dose. Pairwise comparisons between cocaine and
saline treatment groups were made with Fisher's LSD. The effect of
calcium channel antagonists on basal extracellular levels of dopamine was examined by averaging the last two samples of baseline collection and comparing this with the average of the last two samples with a
calcium antagonist alone with a paired t test. The effect of botulinum toxin B on basal extracellular dopamine was determined by
comparing the average of the last two baseline samples in rats pretreated with a microinfusion of botulinum toxin B with the average
of the last two baseline samples from rats pretreated with a saline
microinfusion with a t test.
RESULTS
Behavioral sensitization to cocaine
Table 1 shows the total horizontal photocell counts
for the 2 hr after the first and last injection of daily cocaine or
saline. The behavioral response to the last cocaine injection was
enhanced relative to the first cocaine administration in all groups. In contrast, there was no consistent change in the behavioral response to
saline on the seventh compared to the first injection.
Table 1.
Effect of seven daily intraperitoneal administrations of
saline or cocaine on horizontal activity in the
rat
|
n |
Day
1a |
Day
7 |
Statisticsb |
|
| Amphetamine |
| Saline |
13 |
7973 (658)c |
9907 (2032) |
t(12) = 0.929, p
= 0.3711 |
| Cocaine |
12 |
11513 (1464) |
20039 (2565) |
t(11) = 02.39, p
= 0.0358 |
| GBR-12909 |
| Saline |
9 |
4747 (467) |
7588 (1063) |
t(8) = 02.42, p
= 0.0422 |
| Cocaine |
8 |
13119 (2159) |
25159 (3574) |
t(7) = 03.04, p
< 0.0188 |
| Amphetamine + verapamil |
| Saline |
7 |
4002 (627) |
6840 (533) |
t(6) = 03.45, p
= 0.0136 |
| Cocaine |
9 |
14402 (1627) |
25123 (1755) |
t(8) = 09.14, p
= 0.0001 |
| Amphetamine + diltiazem |
| Saline |
10 |
4035 (1036) |
5322 (762) |
t(9) = 01.14, p
= 0.2822 |
| Cocaine |
10 |
13738 (2212) |
19085 (7247) |
t(9) = 02.99, p
= 0.0153 |
Amphetamine + -conotoxin |
| Saline |
10 |
4419 (360) |
5627 (907) |
t(9) = 01.36, p
= 0.2064 |
| Cocaine |
8 |
13460 (2352) |
21193 (3793) |
t(7) = 02.55, p
= 0.0382 |
| Botulinum
toxin-amphetamine |
| Saline |
7 |
5894 (1029) |
6418 (1164) |
t(6) = 0.452, p
= 0.6673 |
| Cocaine |
10 |
15096 (3380) |
21260 (4325) |
t(9) = 01.30, p
= 0.2265 |
| Botulinum
toxin-GBR-12909 |
| Saline |
6 |
3466 (146) |
5962 (1245) |
t(5) = 01.89, p
= 0.1168 |
| Cocaine |
6 |
11826 (4786) |
32261 (4093) |
t(5) = 07.66, p
= 0.0006 |
| Amphetamine + KN-93 |
| Saline |
9 |
3739 (551) |
7578 (1438) |
t(8) = 02.53, p
= 0.0351 |
| Cocaine |
8 |
9266 (2345) |
21481 (2472) |
t(7) = 04.28, p
= 0.0036 |
| Amphetamine + KN-92 |
| Saline |
9 |
7025 (902) |
5583 (911) |
t(8) = 01.13, p
= 0.2935 |
| Cocaine |
8 |
16799 (3311) |
22599 (4717) |
t(7) = 01.14, p
= 0.2900 |
| GBR-12909 + KN-93 |
| Saline |
11 |
5928 (953) |
9270 (1324) |
t(10) = 02.16, p
= 0.0558 |
| Cocaine |
16 |
12376 (1975) |
19257 (2643) |
t(15) = 01.93, p
= 0.0729 |
|
|
a
Cocaine or saline was administered daily
for 1 week. Day 1 corresponds to the first and day 7 the last daily
injection. Horizontal activity was measured on days 1 and 7 after an
intraperitoneal injection of cocaine (15 mg/kg) or saline. On days
2-6, the subjects received an intraperitoneal injection of either
cocaine (30 mg/kg) or saline and were returned to their home cage.
b
Paired t test; degrees of freedom in
parentheses.
c
The data are presented as the total horizontal
photocell counts (SE) recorded over the first 120 min after cocaine or
saline injection.
|
|
Effect of calcium channel blockers, KN-93, KN-92, and botulinum
toxin on basal levels of extracellular dopamine
Table 2 outlines the influence of the calcium
channel blockers, the CaM-KII inhibitor KN-93, the KN-93 control
compound KN-92, or botulinum toxin on basal levels of extracellular
dopamine in the nucleus accumbens shell. Preliminary dose-response
curves were conducted for the channel blockers and KN-93 to determine a
dose that decreased basal extracellular dopamine by 30-50% from baseline. As shown in Table 2, the calcium channel blockers reduced the
extracellular dopamine concentration in the nucleus accumbens by
24-52% relative to baseline in rats pretreated with either saline or
cocaine. Because the preliminary dose-response curve for KN-93 did not
reveal a change in the basal extracellular levels of dopamine, the dose
used was based on IC50 values from in vitro studies allowing for an estimated 10% efficiency of diffusion across
the dialysis membrane (Sumi et al., 1991 ).
Table 2.
Effects of calcium channel antagonists, KN-93, KN-92, or
botulinum toxin B on the baseline concentration of extracellular dopamine in the nucleus accumbens shell
|
n |
Baselinea |
Drugb |
%
Baseline |
|
| Botulinum
toxin-amphetaminec |
| Saline |
13/7 |
39 (7) |
19 (4)* |
49 |
| Cocaine |
12/10 |
41 (8) |
12 (2)* |
29 |
| Botulinum
toxin-GBR-12909 |
| Saline |
9/6 |
31 (13) |
15 (9) |
48 |
| Cocaine |
8/6 |
31 (12) |
15 (15) |
48 |
| Amphetamine + verapamil |
| Saline |
7 |
42 (10) |
29 (8)* |
69 |
| Cocaine |
9 |
29 (6) |
22 (4)* |
76 |
| Amphetamine + diltiazem |
| Saline |
10 |
25 (6) |
16 (3)* |
64 |
| Cocaine |
10 |
54 (15) |
40 (18) |
74 |
| Amphetamine + conotoxin |
| Saline |
10 |
25 (7) |
16 (3)* |
64 |
| Cocaine |
8 |
21 (5) |
12 (4) |
74 |
| Amphetamine + KN-93 |
| Saline |
9 |
32 (8) |
35 (8) |
109 |
| Cocaine |
8 |
33 (5) |
38 (6) |
115 |
| Amphetamine + KN-92 |
| Saline |
9 |
51 (10) |
63 (11) |
124 |
| Cocaine |
8 |
79 (15) |
78 (10) |
99 |
| GBR-12909 + KN-93 |
| Saline |
11 |
25 (3) |
29 (5) |
116 |
| Cocaine |
16 |
35 (6) |
43 (9) |
123 |
|
|
a
Baseline extracellular dopamine in the
nucleus accumbens shell (uncorrected for recovery). These values
represent the mean (SE) fmol/sample of the average of the last two
baseline samples (60-100 min from the onset of the experiment).
b
Alterations in baseline extracellular dopamine
induced by calcium channel antagonists KN-93, KN-92, or botulinum
toxin. These values represent the mean (SE) fmol/sample of the average
of the last two samples taken 60-100 min after one of these compounds was included in the microdialysis buffer (160-200 min from the onset
of the experiment).
c
The baseline and drug data from the botulinum
toxin experiments are from separate groups of rats. The baseline
amphetamine group represents the combination of subjects that either
did (n = 8) or did not (n = 17) receive a saline
microinfusion (0.3 µl/60 sec) into the nucleus accumbens the day
before the microdialysis experiment. The data from these subjects did
not significantly differ and were pooled. All of the animals in the
baseline GBR-12909 group received an intra-accumbens microinfusion of
saline before the implantation of the microdialysis probe. These
baseline samples correspond to the amphetamine and GBR-12909 baseline
data presented in Figure 1. The "drug" column represents animals
that received an intra-accumbal microinfusion of botulinum toxin (1.0 ng/µl, 0.3 µl) the day before the microdialysis experiment (i.e.,
the baseline values of the amphetamine and GBR-12909 experiments
summarized in Fig. 3).
*
Significant difference between the drug and baseline dopamine values
(p < 0.05, t test).
|
|
In contrast to the calcium channel antagonists, KN-93, and KN-92,
which were administered through the microdialysis probe, botulinum
toxin B was microinfused into the nucleus accumbens before inserting
the microdialysis probe. This method of administration was required
because the molecular weight of botulinum toxin B (>100,000) is beyond
the cutoff for diffusion through the dialysis membrane. As shown in
Table 2, this route of administration was effective because botulinum
toxin reduced the baseline levels of dopamine in the nucleus accumbens
51-71% relative to the basal levels obtained in the experiments
summarized in Figure 1.
Fig. 1.
The effect of amphetamine
(A) and GBR-12909 (B) on
extracellular dopamine in the nucleus accumbens 3 weeks after repeated saline or cocaine treatment. Baseline samples were collected for 100 min, and then three increasing concentrations of amphetamine (0.1, 1.0, and 10 µM) or GBR-12909 (1, 10, and 100 µM)
were incorporated into the dialysis buffer for 100 min each (400 min experiment total). The data are depicted as the mean ± SE
fmol dopamine per 20 min sample and were statistically evaluated with a
two-way repeated measures ANOVA (amphetamine dose × pretreatment with repeated measures/dose). For amphetamine there
was a significant main effect of amphetamine
(F(19,437) = 34.42; p < 0.0001) and a significant amphetamine × pretreatment
interaction (F(38,437) = 4.01;
p < 0.0001). For GBR-12909, there was a
significant main effect of amphetamine
(F(19,285) = 6.31; p < 0.0001) and a significant amphetamine × pretreatment
interaction (F(19,285) = 2.104;
p = 0.0051). *Significant difference from saline
(p < 0.05, Fisher's LSD).
[View Larger Version of this Image (18K GIF file)]
Effect of amphetamine or GBR-12909
The data outlined in Figure 1A demonstrate that
amphetamine produced a dose-dependent increase in extracellular
dopamine in the nucleus accumbens shell and that this effect was
significantly enhanced in animals pretreated with cocaine. Likewise,
the capacity of GBR-12909 to increase extracellular dopamine was
enhanced in cocaine- relative to saline-pretreated rats (Fig.
1B).
Effect of calcium channel antagonists
Figure 2 demonstrates that blockade of calcium
channels with verapamil (L-type), diltiazem (L-type), or -conotoxin
(N-type) prevented the augmented increase in extracellular dopamine in cocaine-pretreated animals. None of the calcium channel blockers altered the capacity of amphetamine to increase extracellular dopamine
in rats repeatedly given injections of saline.
Fig. 2.
Effects of L-type (100 µM verapamil
and 10 µM diltiazem) and N-type (1 µM
conotoxin) calcium channel antagonists on amphetamine-induced increases
in extracellular dopamine in the nucleus accumbens of saline- and
cocaine-pretreated rats. After 100 min of baseline, a calcium
antagonist was incorporated into the dialysis buffer for 100 min. For
the next 300 min of the experiment, three increasing doses of
amphetamine (0.1-10 µM, 100 min each) were included in the dialysis buffer in the presence of the same calcium channel blocker. In all three experiments, there was a significant main effect
of amphetamine dose [verapamil (V),
F(24,336) = 15.25, p < 0.0001; conotoxin (C), F(24,384) = 19.21, p < 0.0001; diltiazem (D),
F(24,342) = 60.28, p < 0.0001]. There were no other significant main effects or
interactions.
[View Larger Version of this Image (16K GIF file)]
Effect of botulinum toxin B
Figure 3 shows that, even though the maximal effect
of amphetamine was blunted by pretreatment with botulinum toxin B, the augmented increase in extracellular dopamine among cocaine-sensitized rats was unaffected. In contrast, botulinum toxin B pretreatment eliminated the capacity of GBR-12909 to elevate extracellular dopamine
in both the cocaine and saline groups.
Fig. 3.
The effect of botulinum toxin B on amphetamine
(top)- or GBR-12909 (bottom)-induced
increases in extracellular dopamine in the nucleus accumbens of saline-
and cocaine-pretreated rats. Botulinum toxin B (1.0 ng/µl, 0.3 µl)
was unilaterally microinfused (via a 33 gauge cannula) into the nucleus
accumbens through the microdialysis cannula 15 min before the placement
of the microdialysis probe. For the amphetamine experiment, a two-way
ANOVA revealed a significant main effect of amphetamine dose
(F(19,285) = 24.28, p < 0.0001) and a significant interaction between treatment groups and dose
of amphetamine (F(19,285) = 1.59, p = 0.058). For the GBR-12909 experiment, there
was a significant main effect of GBR-12909 dose
(F(19,190) = 2.12, p = 0.0058); there were no other significant main effects or
interactions. *Significant difference from saline (p < 0.05, Fisher's LSD).
[View Larger Version of this Image (18K GIF file)]
Effect of KN-93 and KN-92
Figure 4 illustrates that, similar to the calcium
channel antagonists (see Fig. 2), inhibition of CaM-KII with KN-93
prevented the augmented increase in extracellular dopamine induced by
amphetamine in cocaine-pretreated animals. KN-93 also inhibited the
augmented increase in extracellular dopamine produced by GBR-12909 in
subjects pretreated with cocaine. KN-93 did not alter the capacity of
amphetamine or GBR-12909 to elevate extracellular dopamine in the
nucleus accumbens of saline-pretreated subjects. In contrast, KN-92,
the control compound for KN-93, did not influence the
amphetamine-induced increase in extracellular dopamine in the nucleus
accumbens in saline-pretreated rats or the augmented release of
dopamine observed in cocaine-sensitized rats.
Fig. 4.
The effect of the CaM-KII blocker, KN-93, on
amphetamine- or GBR-12909-induced increases in extracellular dopamine
in the nucleus accumbens of saline- and cocaine-pretreated rats.
Baseline samples were collected for 100 min, followed by 100 min of
KN-93 (10 µM). For the remaining 300 min of the
experiment, three increasing concentrations of amphetamine (0.1, 1.0, and 10 µM) or GBR-12909 (1, 10, and 100 µM)
were incorporated into the dialysis buffer for 100 min each with 10 µM KN-93. For the amphetamine experiment, there was a
significant main effect of amphetamine dose
(F(19,285) = 26.25; p < 0.0001), with no other significant treatment effects or interactions.
For the GBR-12909 experiment, there was a significant main effect of
GBR-12909 dose (F(19,475) = 9.77;
p < 0.0001), with no other significant treatment
effects or interactions. The effects of the KN-93 control compound,
KN-92, also were assessed on amphetamine-induced increases in
extracellular dopamine in the nucleus accumbens of saline- and
cocaine-pretreated rats. The design of the experiment was identical to
the amphetamine experiment described above, except that KN-92 (10 µM) was substituted for KN-93. The analysis of this
experiment revealed a significant main effect of amphetamine dose
(F(19,285) = 8.77; p < 0.0001) and a significant amphetamine × treatment interaction
(F(19,285) = 1.59; p = 0.05). *Significant difference from saline
(p < 0.05, Fisher's LSD).
[View Larger Version of this Image (23K GIF file)]
Histology
Figure 5A depicts microdialysis probe placements in
the shell of the nucleus accumbens. The dialysis probes were located in the medial compartment of the shell, not in the lateral limb. Many
probes were at the lateral edge of the shell adjacent to the core.
Also, in several cases a portion of the active region of the probes was
located in the ventral neostriatum. Figure 5, B and
C, shows micrographs depicting probe tracts in the nucleus accumbens of rats treated with KN-93 plus GBR-12909 and botulinum toxin
B plus amphetamine through the probe, respectively. Note the lack of
neurotoxicity after either treatment other than the mechanical damage
produced by the microdialysis probe.
Fig. 5.
Histological evaluation of dialysis probe
placements in the medial nucleus accumbens. A,
Composite of the location of all dialysis probe tracts used in data
analysis. B, Micrograph taken at the level of the
nucleus accumbens. In this animal, KN-93 and GBR-12909 were
simultaneously incorporated into the microdialysis buffer.
C, Micrograph of the nucleus accumbens of a rat that was microinfused with botulinum toxin B; amphetamine subsequently was
applied locally to the nucleus accumbens. Scale bar, 100 µM. Note the lack of neurotoxicity in
B and C, other than the mechanical damage
produced by the microdialysis probe. The white arrows
indicate the mechanical damage produced by the microdialysis probe. The black arrows point to intact cells located near the
microdialysis probe.
[View Larger Version of this Image (109K GIF file)]
DISCUSSION
The present results demonstrate that the enhanced increase in
extracellular dopamine in the nucleus accumbens shell of
cocaine-sensitized rats arises from the induction of a requirement for
calcium- and CaM-KII-dependent mechanisms. This finding is consistent
with the calcium-dependent increase in extracellular dopamine induced by dopamine reuptake blockers, such as GBR-12909. However, because the
amphetamine-induced increase in extracellular dopamine occurs in the
absence of calcium-mediated vesicular exocytosis in saline-pretreated rats, the calcium-dependent portion of the amphetamine neurochemical response represents a shift in the mechanism of action of this drug
after the repeated administration of cocaine.
Behavioral sensitization and dopamine in the nucleus accumbens
Consistent with previous findings (Pierce and Kalivas, 1995 ), the
present results indicate that the capacity of locally applied amphetamine to enhance dopamine transmission in the nucleus accumbens shell is enhanced in cocaine-pretreated rats. Our data also indicate that the administration of the selective dopamine reuptake blocker, GBR-12909, through the dialysis probe results in an amplified increase
in extracellular dopamine among cocaine- relative to saline-pretreated
animals. It is important to note that these results were obtained 3 weeks after the last repeated injection of cocaine and that these
changes in dopamine transmission often are not observed during the
first few days after the cessation of a repeated psychostimulant
treatment regimen (Segal and Kuzcenski, 1992a,b; Kalivas and Duffy,
1993 ; Wolf et al., 1993 ; Pierce and Kalivas, 1995 ; Heidbreder et al.,
1996 ).
Role of calcium
The present data clearly indicate a role for L- and N-type calcium
conductances in the augmented increase in extracellular dopamine
induced by amphetamine or GBR-12909 in the nucleus accumbens of
cocaine-sensitized rats. The results of the GBR-12909 experiments are
consistent with evidence that impairing impulse flow or presynaptic calcium influx severely impairs the ability of dopamine reuptake blockers to increase extracellular dopamine (Carboni et al., 1989 ; Westerink et al., 1989 ). In contrast, the elimination of the sensitized portion of the amphetamine-induced increase in dopamine in
cocaine-pretreated rats stands in sharp contrast to the augmentation in
dopamine transmission induced by amphetamine in saline-pretreated rats, which is completely unaffected by the coadministration of calcium channel antagonists (see Fig. 2).
The distinction between the mechanisms of action of GBR-12909 and
amphetamine was verified in the experiments in which animals were
pretreated with an intra-accumbens microinjection of botulinum toxin B,
which cleaves synaptobrevin, thereby preventing vesicular fusion with
the plasma membrane (Burgoyne and Morgan, 1995 ; Schweizer et al.,
1995 ). Botulinum toxin administration abolished the increase in
extracellular dopamine induced by GBR-12909 in both cocaine- and
saline-pretreated rats. The elimination of the GBR-12909-induced increases in dopamine by botulinum toxin B was expected and verifies the effectiveness of the dose used for disrupting vesicular exocytosis. In contrast, impairing calcium-mediated vesicular exocytosis had no
influence on the ability of amphetamine to augment the increase in
extracellular dopamine in cocaine-sensitized rats. However, botulinum
toxin B attenuated the maximal increase in extracellular dopamine
induced by amphetamine in both the cocaine and saline groups,
suggesting that the amphetamine-induced increase in extracellular dopamine may be partially calcium dependent. This finding is
inconsistent with the mechanism of action of amphetamine being mediated
solely by reverse transport (Fisher and Cho, 1979; Liang and Rutledge, 1982 ). Higher concentrations of amphetamine have been proposed not only
to promote reverse transport of dopamine into the extracellular space,
but also to prevent the uptake of dopamine into the cytosol (Kuzcenski,
1983 ). Thus, a component of the increase in extracellular dopamine
induced by amphetamine may depend on vesicular exocytosis. Regardless
of the role exocytosis may play in maintaining the increase in
extracellular dopamine after the administration of high doses of
amphetamine, the present data clearly demonstrate that the augmented
increase in dopamine in cocaine-pretreated subjects was not blocked by
botulinum toxin B and is, therefore, independent of vesicular
exocytosis.
Role of CaM-KII
The CaM-KII inhibitor KN-93 abolished the augmented increase in
extracellular dopamine produced by either amphetamine or GBR-12909 in
rats sensitized to cocaine. The induction of a CaM-KII-dependent mechanism in the expression of behavioral sensitization to
psychostimulants is consistent with biochemical measurements showing
increased calmodulin activity in rats pretreated with daily amphetamine (Gnegy et al., 1991 ). Although the present data do not indicate which
CaM-KII phosphorylated protein(s) is mediating the changes in
amphetamine- and GBR-12909-induced increases in extracellular dopamine,
two potential mechanisms can be considered. A recent in
vitro study reported that the Vmax of the
plasmallemal dopamine transporter is enhanced in the presence of
calcium, and that this calcium-dependent enhancement of dopamine uptake
is regulated by CaM-KII (Uchikawa et al., 1995 ). These results suggest
that the augmented effect of amphetamine among sensitized rats may be
due to a CaM-KII-mediated enhancement of amphetamine uptake by the
plasmallemal dopamine transporter and an associated increase in the
reverse transport of dopamine. However, the fact that CaM-KII activity
was required for the augmented effect of GBR-12909 in the present study
argues against a role for phosphorylation of dopamine transporters.
That is, because GBR-12909 binds to the transporter but is not
transported, a CaM-KII induced increase in transport velocity is not
likely to be involved in the enhanced increase in extracellular
dopamine induced by GBR-12909 in the nucleus accumbens of
cocaine-sensitized rats.
A second possibility resides in the influence of CaM-KII on synaptic
vesicles. CaM-KII promotes the undocking of vesicles from cytoskeletal
proteins in preparation for exocytosis by phosphorylating synapsin I
(Lin et al., 1990 ). Recently, CaM-KII-dependent phosphorylation of
synapsin I was found to be augmented in rats pretreated with repeated
amphetamine (Iwata et al., 1996 ). An amplification of this CaM-KII
dependent transduction pathway in rats behaviorally sensitized to
cocaine may explain the augmented capacity of GBR-12909 to elevate
extracellular dopamine. Because synaptic vesicles must undock before
fusion with the plasmallemal membrane (Burgoyne and Morgan, 1995 ;
Schweizer et al., 1995 ) and vesicular fusion mediates the increase in
extracellular dopamine after dopamine transporter blockade by
GBR-12909, increasing the proportion of undocked vesicles may mediate
the enhanced effect of GBR-12909 on dopamine transmission in
cocaine-pretreated rats. The manner in which the CaM-KII-mediated
undocking of synaptic vesicles augments the effects of amphetamine is
less apparent. One possibility is that the vesicular dopamine
transporter may be more readily accessed by amphetamine in undocked
vesicles, thereby increasing the availability of cytoplasmic dopamine
for reverse transport.
Clinical implications
The data presented herein show that repeated cocaine initiates an
enduring requirement for calcium and the activation of CaM-KII for the
augmented increase in dopamine in the nucleus accumbens induced by
amphetamine or GBR-12909. A pathophysiological change in dopamine
transmission in the nucleus accumbens is a major candidate in the
etiology of psychostimulant-induced psychiatric disorders (Segal et
al., 1981 ; Post and Weiss, 1988 ). Behavioral sensitization and dopamine
transmission in the nucleus accumbens are also implicated in the abuse
liability of psychostimulants (Robinson and Berridge, 1993 ). Whereas a
role for calcium transduction mechanisms in psychostimulant-induced psychopathologies remains to be established, several preclinical studies report that calcium channel antagonists influence
psychostimulant self-administration and conditioned place preference in
a manner similar to dopamine antagonists (Pani et al., 1991 ;
Martellotta et al., 1994 ; Rosenzweig-Lipson and Barrett, 1995).
Collectively, these results suggest that drugs that act on calcium
conductances and associated transduction pathways might serve as
potentially effective therapeutic agents for treating psychostimulant
abuse and possibly psychostimulant-induced psychiatric disorders.
FOOTNOTES
Received Oct. 28, 1996; revised Feb. 3, 1997; accepted Feb. 6, 1997.
This research was supported in part by the Washington State Alcohol and
Drug Abuse Program and U.S. Public Health Service Grants MH-40817,
DA-03906, Research Career Development Award DA-00158 (P.W.K.), and
National Research Service Award DA-05589 (R.C.P.).
Correspondence should be addressed to Dr. Chris Pierce, Department of
VCAPP, Washington State University, Pullman, WA
99164-6520.
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