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THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS
Copyright © 1999 by The American Society for Pharmacology and Experimental Therapeutics
JPET 288:774 –781, 1999
Vol. 288, No. 2
Printed in U.S.A.
Effects of Antipsychotic Drugs on Extracellular Dopamine
Levels in Rat Medial Prefrontal Cortex and Nucleus
Accumbens1
TOSHIHIDE KUROKI, HERBERT Y. MELTZER and JUNJI ICHIKAWA
Department of Psychiatry, Saga Medical School, Nabeshima, Saga, Japan (T.K.); and Department of Psychiatry, Psychopharmacology Division,
Vanderbilt University School of Medicine, Nashville, Tennessee (H.Y.M., J.I.).
Accepted for publication September 14, 1998
This paper is available online at http://www.jpet.org
Antipsychotic drugs such as haloperidol (HAL) and S-(2)sulpiride (SUL) increase dopamine (DA) release in the striatum (STR) and the nucleus accumbens (NAC), most likely
due to the blockade of presynaptic DA-D2-like autoreceptors
(Westerink and DeVries, 1989). The atypical antipsychotics
clozapine (CLOZ) and amperozide (APZ), both of which have
greater affinities for serotonin (5-hydroxytryptamine,
5-HT)2A receptors relative to D2 receptors (Meltzer et al.,
1989; Leysen et al., 1993), have been reported to produce
greater increases in extracellular DA levels in the medial
prefrontal cortex (mPFC) compared with the STR and the
NAC (Moghaddam and Bunney, 1990; Nomikos et al., 1994;
Pehek and Yamamoto, 1994; Volonté et al., 1997). By contrast, HAL has a limited effect on extracellular DA levels in
Received for publication April 10, 1998.
1
This work was supported in part by U.S. Public Health Services Grant MH
41684, Department of Veterans Affairs grant GCRC MO1RR00080, and the
National Alliance for Research on Schizophrenia and Depression (NARSAD)
Young Investigator Award grant, as well as grants from Elisabeth Severance
Prentiss Foundation and Mr. Larry Freedman. Preliminary data from this
study have been reported in abstract forms of the annual meetings of the
Society for Neuroscience, 1995, and the Society of Biological Psychiatry, 1996.
tipsychotic, significantly increased extracellular DA levels in the
NAC but not in the mPFC. The effects of the six antipsychotic
drugs to increase extracellular DA levels in the mPFC relative to
those in the NAC was positively correlated with the difference
between their pKi values for serotonin (5-hydroxytryptamine,
5-HT2A) and DA-D2 receptors and was inversely correlated to
their pKi values for D2 or D3 receptors, but was not for 5-HT2A
receptors alone. These results are consistent with the hypothesis that the ability of antipsychotic drugs to produce a greater
increase in prefrontal compared with NAC extracellular DA
levels may be related, in part, to weak D2 and D3 receptor
affinity relative to 5-HT2A receptor antagonism.
the mPFC. Compounds like CLOZ and APZ that are relatively more potent as 5-HT2A than as D2 receptor antagonists, e.g., melperone, olanzapine (OLAN), risperidone
(RISP), quetiapine, sertindole, and ziprasidone (Meltzer et
al., 1989; Leysen et al., 1993), have been commonly referred
to 5-HT2A/D2 receptor antagonists (Meltzer, 1995). Therefore, it is of interest to test the hypothesis that all antipsychotic drugs of the 5-HT2A/D2 receptor antagonist type produce a greater increase in extracellular DA levels in the
mPFC compared with the NAC.
Dopaminergic hypofunction in the prefrontal cortex has
been suggested to be related to the etiology of negative symptoms (Davis et al., 1991; Weinberger and Lipska, 1995) and
cognitive dysfunction of schizophrenia (Sawaguchi and Goldman-Rakic, 1991). A number of the 5-HT2A/D2 receptor antagonists, e.g., CLOZ, RISP, OLAN, and sertindole, have
been reported to have a greater ability to improve negative
symptoms than do neuroleptics such as HAL or chlorpromazine (Kane et al., 1988; Chouinard et al., 1993; Meltzer, 1995;
Tollefson and Sanger, 1997). CLOZ and RISP also improve
some aspects of cognitive function (Hagger et al., 1993; Green
ABBREVIATIONS: HAL, haloperidol; SUL, S-(2)-sulpiride; DA, dopamine; STR, striatum; NAC, nucleus accumbens; CLOZ, clozapine; APZ,
amperozide; mPFC, medial prefrontal cortex; OLAN, olanzapine; RISP, risperidone; 5-HT, 5-hydroxytryptamine; AUC, area under the curve; VEH,
vehicle control.
774
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ABSTRACT
The present study was designed to compare the effects of
typical and atypical antipsychotic drugs on extracellular dopamine (DA) levels in the medial prefrontal cortex (mPFC) and the
nucleus accumbens (NAC), using in vivo microdialysis with dual
probe implantation in awake, freely moving rats. Amperozide (2
and 10 mg/kg), clozapine (5 and 20 mg/kg), and olanzapine (10
mg/kg), all of which are atypical antipsychotics, produced
greater increases in extracellular DA levels in the mPFC than in
the NAC. Olanzapine (1 mg/kg), risperidone (0.1 and 1 mg/kg),
also an atypical antipsychotic, and S-(2)-sulpiride (25 mg/kg), a
typical antipsychotic, produced comparable increases in extracellular DA levels in the mPFC and the NAC. S-(2)-sulpiride (10
mg/kg) and haloperidol (0.1 and 1 mg/kg), another typical an-
1999
Antipsychotics and Prefrontal Dopamine
Materials and Methods
Animals. Male Sprague-Dawley albino rats (Zivic-Miller, Pittsburgh, PA) weighing 250 to 300 g were used throughout the study.
Rats were housed two or three per cage and were maintained on a
12-h light/dark cycle and under constant temperature at 22°C, with
ad libitum access to food and water.
Surgery and Microdialysis. Rats were anesthetized with a combination of xylazine (Rompun, 6 mg/kg, i.p.; Miles, Kansas City, KS)
and ketamine hydrochloride (Ketaset, 70 mg/kg, i.p.; Fort Dodge
Lab., Fort Dodge, IA) and mounted in a stereotaxic frame (David
Kopf Instruments, Tujunga, CA). Two stainless guide cannula (21
gauge) with a dummy probe were placed and fixed by cranioplastic
cement onto the cortex dorsal both to the right mPFC and the left
NAC (dual probe implantation). Stereotaxic coordinates of each
probe when implanted were A 13.2, L 10.8 (10° inclination), V 25.5
mm for the mPFC and A 12.0, L 11.7, V 27.5 mm for the NAC,
respectively, relative to bregma; incision bar level: 23.0 mm, according to the atlas of Paxinos and Watson (1986).
Microdialysis probes were constructed in our laboratory according
to the method of Robinson and Whishaw (1988). Briefly, the probe
was a concentric design consisting of 25-gauge stainless steel tubing
(460 mm o.d.) connected to PE20 tubing as an inlet. The outlet
consisted of a fused silica capillary (150 mm o.d.; Polymicro Technologies, Phoenix, AZ) passed through the wall of the PE20, threaded
down through the stainless steel tubing and into a 2-mm length of
sealed dialysis tubing (polyacrylonitrile/sodium methalylsulfonate
polymer, 310 mm o.d., 220 mm i.d., 40,000 Da cutoff, AN69HF, Hospal; CGH Medical, Lakewood, CO). PE10 tubing was attached to the
silica capillary where it exits the probe. All junctions were glued with
2-ton Epoxy (Devon Bearings, Brooklyn Heights, OH). Because the
variability in percentage of recovery of DA between probes varied by
less than 10%, basal extracellular DA levels in each region were
expressed as absolute values and were not corrected by percentage of
recovery.
Three to five days after cannulation, the microdialysis probes were
implanted into the mPFC and the NAC under slight anesthesia with
methoxyflurane (Metofane; Pitman-Moore, Chicago, IL). The probe
was perfused with modified Dulbecco’s phosphate-buffered saline
including Ca11 (NaCl, 138 mM; Na2HPO4, 8.1 mM; KCl, 2.7 mM;
KH2PO4, 1.5 mM; MgCl2, 0.5 mM; and CaCl2, 1.2 mM, at pH 5 7.4)
at 0.2 ml/min through the night. After overnight perfusion for approximately 15 h, the perfusion flow rate was increased to 0.4 ml/min,
and, beginning 1 h later, dialysate samples (12 ml) were collected
every 30 min. After obtaining stable baseline values in the dialysate
such that a percentage of S.E.M. of the three consecutive DA values
in the dialysate differed less than 10% of the mean values, each drug
or vehicle was administered s.c. to rats. The effect of the drug was
followed for at least another 180 min. After the completion of each
experiment, the rats were sacrificed with an overdose of chloral
hydrate. The brains were removed and cut coronally along the puncture created by the probe. The location of the tip was then verified
macroscopically. All experiments were carried out between 10:00 and
16:00 h. The procedures applied in this study were in strict accordance with the National Institutes of Health Guide for the Care and
Use of Laboratory Animals and were approved by the Institutional
Animal Care and Use Committee of Case Western Reserve University in Cleveland, OH, where we completed these experiments.
Dual probe implantation into the mPFC and the NAC used in this
study was done to monitor cortico-subcortical interactions and to
compare drug effects on the two regions. It has been suggested that
the shell, but not the core, of the NAC, projects to the mPFC in the
rat brain and may be the target site for the antipsychotic action of
antipsychotic drugs (Deutch et al., 1993). In this study, dialysis
probes were always implanted above or on the medial side of the
anterior limb of the anterior commissure, a location which may
indicates preference for the shell of the NAC. However, because of
the probe size used in this study, the dialyzed area might also
include, in part, the core of the NAC.
Biochemical Assay. DA concentrations in dialysate samples
were determined by high-performance liquid chromatography with
electrochemical detection as described previously (Kuroki et al.,
1996). DA was separated on the reversed phase column (BDS Hypersil C18, 1.0 3 100 mm, 3 mm particle size; Keystone Scientific,
Bellefonte, PA). The composition of the mobile phase was 50 mM
NaH2PO4 (pH 5 6.0), 20% (v/v) methanol, 8% (v/v) acetonitrile, 450
mg/liter SDS, 1 mM Na2EDTA, 10 mM NaCl, and 500 ml/liter triethylamine. Electrochemical detection controllers (LC-4C; BAS, West
Lafayette, IN) with unijet amperometric detector cells (MF-9080;
BAS) set at 1550;580 mV versus Ag/AgCl reference electrodes were
used to detect DA. The column and detector cell were placed within
a column oven (831 temperature regulator; Gilson, Middlestone, MA)
at 35°C except for the experiment of OLAN administration. Because
an OLAN metabolite was coeluted with DA at the same retention
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et al., 1997) which may be related to frontal lobe function.
Thus, the greater ability of 5-HT2A/D2 receptor antagonists
to increase extracellular DA levels in the cortical regions
compared with subcortical regions may contribute to their
ability to improve negative symptoms (Deutch et al., 1991).
This hypothesis is consistent with preferential in vivo and in
vitro binding of 5-HT2A/D2 receptor antagonists to cortical
5-HT2A compared with striatal D2 receptors (Meltzer et al.,
1989; Leysen et al., 1993; Stockmeier et al., 1993).
Recently, evidence for the modulation of the activity of
mesolimbocortical DA neurons by 5-HT2A receptors has been
reported. Systemic administration of the 5-HT2A/2C receptor
antagonist ritanserin has been demonstrated to enhance the
activity of midbrain DA neurons (Ugedo et al., 1989). Ritanserin has also been reported to potentiate the ability of the
selective D2/3 receptor antagonist raclopride to increase DA
release in the mPFC but not in the STR, whereas ritanserin
alone had no effect on DA release in either region (Andersson
et al., 1995). These data suggest that the 5-HT2A receptor
antagonism, together with blockade of D2-like receptors (D2,
D3, and perhaps D4 subtype), may contribute to the preferential increase in extracellular DA levels in the mPFC compared with the NAC and the STR produced by APZ and
CLOZ. However, this hypothesis is not consistent with the
reports that RISP and OLAN, which are more potent as
5-HT2A than as D2 receptor antagonists (Bymaster et al.,
1996a; Schotte et al., 1996), produced comparable increases
in extracellular DA levels in the mPFC and the NAC or the
STR (Hertel et al., 1996; Volonté et al., 1997).
The present study was designed to characterize antipsychotic drugs of the 5-HT2A/D2 receptor antagonist type in
terms of their relative ability to produce a greater increase in
extracellular DA levels in the mPFC compared with the
NAC. Importantly, antipsychotic drugs differ in their affinities for D1, D3, D4, 5-HT2C, 5-HT6, and 5-HT7 receptors
(Meltzer et al., 1989; Roth et al., 1992; Schotte et al., 1996).
These affinities may also influence their effects on extracellular DA levels in the mPFC, the NAC, and the STR. Therefore, this study also investigated the relationships between
relative potency to increase extracellular DA levels and in
vivo affinities for multiple 5-HT and DA receptors as indicated by pKi values obtained from published sources (Table
1). Six antipsychotic drugs were chosen on the basis of the
difference in in vitro and in vivo affinity between rat cortical
5-HT2A and striatal D2 receptors in the following rank order:
APZ . CLOZ . RISP . OLAN . HAL . SUL.
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Kuroki et al.
Vol. 288
logarithms of the % net AUC in the mPFC and the NAC. A probability of less than 0.05 (p , .05) was considered significant in the
present study.
Results
Basal Extracellular DA Levels in the mPFC and the
NAC. Basal extracellular DA levels (mean 6 S.E.M. fmol/10
ml/30 min, not corrected by % recovery) in the dialysates
obtained from all rats used in this study were 1.72 6 0.09
(n 5 71) for the mPFC and 11.04 6 0.46 (n 5 77) for the NAC,
respectively. The basal extracellular DA levels in the NAC
were about 6-fold higher than in the mPFC (F1,146 5 370.90,
p , .001 by one-way ANOVA). There was no significant
difference in basal extracellular DA levels in each brain
region between the various treatment groups. In a preliminary experiment, the injection of either 0.1 M tartaric acid,
deionized water, or physiological saline (0.9% NaCl) had no
significant effect on extracellular DA levels in either region.
Therefore, the combined data of the effects of both 0.1 M
tartaric acid and deionized water on extracellular DA levels
were used as the vehicle control (VEH) for the statistical
analysis and the graphic presentation (n 5 6 for each region).
Time-Dependent Effects on Extracellular DA Levels
in the mPFC and the NAC during the 180-min Period
after Drug Injections Compared With VEH. Repeatedmeasure ANOVA for each treatment group in both regions
with post hoc comparison, as compared with VEH, is indicated as follows: APZ (10 mg/kg) produced time-dependent
effects on extracellular DA levels in the mPFC and the NAC,
whereas APZ (2 mg/kg) had no time-dependent effect in either region. Both doses of CLOZ (5 and 20 mg/kg), OLAN (1
and 10 mg/kg), and RISP (0.1 and 1 mg/kg) had significant
time-dependent effects in both regions. SUL (25 mg/kg) significantly increased extracellular DA levels in both regions,
whereas SUL (10 mg/kg) had a significant effect in the NAC
but not in the mPFC. Both doses of HAL (0.1 and 1 mg/kg)
increased extracellular DA levels in the NAC but not in the
mPFC (Figs. 1 and 2).
The time course of extracellular DA levels in the mPFC or
the NAC was comparable with peak DA levels in the mPFC
at 60 min for all drugs studied except OLAN and SUL, which
peaked at 90 to 120 min. Elevated extracellular DA levels
were still present in the mPFC 180 min after drug administration. The same was true for the increase in extracellular
DA levels in the NAC.
TABLE 1
Binding affinity (pKi and ED50 values) of six antipsychotic drugs for DA and 5-HT receptor subtypes
pKi Values
ED50 Values
Drug
APZ
CLOZ
RISP
OLAN
HAL
SUL
D1
D2
D3
6.2
6.8
6.2b
6.6b
7.0
,5.0d
6.3
7.0
8.9
7.8b
9.0
7.8d
6.3a
6.4b
7.9b
7.3b
7.7b
7.6d
D4
8.0
7.8
7.6b
8.0
6.0d
5-HT1A
5-HT2A
5-HT2C
5-HT2A-D2
D2
5-HT2A
5-HT2A/D2
5.9a
6.7b
6.6b
,6.0c
5.5b
8.0
8.3
10.1
8.7b
7.7
,5.0
5.9
8.1
7.5
8.1b
5.6
,5.0
1.7
1.3
1.2
0.9
21.3
,23.0
.100
5.2
0.3
15
0.2
86
1.1
0.9
0.1
6
15
.340
,0.01
0.2
0.2
0.4
10
.3.9
The pKi values for D1, D2, D4, 5-HT2A, and 5-HT2C receptors were obtained from Meltzer et al. (1989), Stockmeier et al. (1993), Roth et al. (1992, 1995), except where noted.
The ED50 values (mg/kg) for in vivo 5-HT2A and D2 receptor occupancy were obtained from Stockmeier et al. (1993) except for OLAN. Data for OLAN refers to Bymaster et
al. (1996b). The ED50 values of sulpiride are data of a racemic compound.
a
Data refers to Leysen et al. (1993).
b
Data refers to Schotte et al. (1996).
c
Data refers to Bymaster et al. (1996a).
d
Data refers to Seeman and Van Tol (1995).
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period of the chromatogram under the above-mentioned conditions,
the column temperature was lowered to 28°C to obtain clear separation of DA for the experiment of OLAN injection. The data were
analyzed with an integrator (HP 3396 Series II; Hewlett-Packard,
Avondale, PA). The detection limit was 0.4 fmol for DA at a 3:1
signal-to-noise ratio. All reagents used for high-performance liquid
chromatography with electrochemical detection were purchased
from Fisher Scientific (Pittsuburgh, PA) and Sigma Chemical Co.
(St. Louis, MO).
Drugs. Six antipsychotic drugs were chosen on the basis of the
difference in in vitro and in vivo affinity between rat cortical 5-HT2A
and striatal D2 receptors (Table 1). The doses of antipsychotic drugs
except OLAN were chosen on the basis of the ED50 values for displacing in vivo [3H]N-methylspiperone binding to cortical 5-HT2A/
olfactory tubercle D2 receptors (Stockmeier et al., 1993). The choice
of the doses of OLAN was according to the ED50 values for inhibition
of ex vivo [3H]ketanserin binding to cortical 5-HT2A receptors and
inhibition of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline-induced inactivation of striatal D2 receptors (Bymaster et al., 1996b).
APZ, CLOZ, RISP, and OLAN at the doses used in this study may
predominantly occupy 5-HT2A receptors compared with D2 receptors
in rat brain, whereas SUL and HAL predominantly occupy D2 receptors compared with 5-HT2A receptors, although discrepancies between in vivo and in vitro data have been reported for some antipsychotic drugs, e.g., racemic SUL (Meltzer et al., 1989; Stockmeier et
al., 1993).
APZ-HCl (Pharmacia LEO Therapeutics AB, Malmö, Sweden) was
dissolved in deionized water. CLOZ-HCl (Sandoz, East Hanover,
NJ), RISP (Janssen, Titasville, NJ), OLAN (Eli Lily, Indianapolis,
IN), HAL (McNeil, Spring House, PA), and SUL (Research Biochemical Inc., Natick, MA) were each dissolved in 0.1 M tartaric acid
because of their lesser solubility in water. Drugs or vehicle (0.1 M
tartaric acid or deionized water) in a volume of 1.0 ml/kg were
administered s.c. to randomly assigned rats.
Data Analysis. The mean value of three consecutive samples
before drug injection was set at 100% as the predrug basal value. The
percentage of net area under the curve (% net AUC) was obtained
from the net increase for a 180-min period after drug or vehicle over
each predrug basal level, and expressed as percentage of predrug
basal levels. All data were statistically evaluated with StatView 4.50
(Abacus Concepts Inc., Berkeley, CA). Basal extracellular DA levels
in the mPFC and the NAC in the various treatment groups were
subject to one-way analysis of variance (ANOVA). The time-dependent effects of drugs were analyzed by repeated-measure ANOVA
followed by the Fisher’s protected least significant difference post
hoc pairwise comparison procedures. Spearman rank correlation was
analyzed to examine the relationship between the greater effect on
extracellular DA levels in the mPFC compared with the NAC, as
indicated by the difference between the values of log[mPFC] and
log[NAC], and the pKi values of antipsychotic drugs for 5-HT and DA
receptor subtypes. The values of log[mPFC] and log[NAC] are the
1999
Effects on the Percentage of Net AUC of Extracellular DA Levels in the mPFC and the NAC. Both doses of
antipsychotic drugs studied except HAL produced significant
increases in the % net AUC in the mPFC compared with
VEH. APZ (2 mg/kg) showed a significant increase in the %
net AUC in the mPFC when determined by one-way ANOVA
despite nonsignificant time-dependent effects determined by
repeated-measure ANOVA with post hoc comparison. HAL (1
but not 0.1 mg/kg) had a significant effect in the mPFC. APZ
(10 but not 2 mg/kg) produced a significant increase in the %
777
Fig. 2. Time-dependent effects of RISP (0.1 and 1 mg/kg, s.c.), SUL (10
and 25 mg/kg, s.c.), and HAL (0.1 and 1 mg/kg, s.c.) on extracellular DA
levels in the mPFC (left) and the NAC (right). The arrow indicates the
drug injection time. Data are mean 6 S.E.M. of corresponding time points
at 30-min intervals, expressed as percentage of the predrug basal levels.
The numbers of rats in each treatment group are presented in Table 2.
Repeated-measure ANOVA with post hoc comparison in each treatment
group, as compared with the VEH, is indicated as follows: RISP (0.1 and
1 mg/kg) had significant time-dependent effects in the mPFC (F2,11 5
13.43, p , .05 and p , .001) and the NAC (F2,11 5 34.21, p , .001 and p ,
.001). SUL (25 but not 10 mg/kg) significantly increased extracellular DA
levels in the mPFC (F2.13 5 5.28, p , .01), whereas both doses of SUL (10
and 25 mg/kg) had a significant effect in the NAC (F2,15 5 22.71, p , .001
and p , .01). Both doses of HAL (0.1 and 1 mg/kg) increased extracellular
DA levels in the NAC (F2,13 5 11.19, p , .01 and p , .001), but not the
mPFC.
net AUC in the NAC. Both doses of all of the other antipsychotic drugs had significant effects in the NAC (Table 2).
Because of the great difference in absolute values of extracellular DA levels and variance of % converted values after
drug injection between the mPFC and the NAC, statistical
analysis by ANOVA is not applicable to the regional comparison of the effects of antipsychotic drugs on extracellular DA
levels. However, as can be seen in Table 2, APZ (2 and 10
mg/kg) and CLOZ (5 and 20 mg/kg) clearly showed greater
effects on extracellular DA levels in the mPFC than in the
NAC. OLAN (10 mg/kg) had a slightly greater effect on ex-
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Fig. 1. Time-dependent effects of APZ (2 and 10 mg/kg, s.c.), CLOZ (5 and
20 mg/kg, s.c.), and OLAN (1 and 10 mg/kg, s.c.) on extracellular DA
levels in the mPFC (left) and the NAC (right). The arrow indicates the
drug injection time. Data are mean 6 S.E.M. of corresponding time points
at 30-min intervals, expressed as percentage of the predrug basal levels.
The numbers of rats in each treatment group are presented in Table 2.
Repeated-measure ANOVA for each treatment group in both regions with
post hoc comparison, as compared with the VEH, is indicated as follows:
APZ (10 mg/kg) produced time-dependent effects on extracellular DA
levels in the mPFC (F2,14 5 18.02, p , .001) and the NAC (F2,17 5 10.70,
p , .001), whereas APZ (2 mg/kg) had no time-dependent effect in either
region. CLOZ (5 and 20 mg/kg) increased extracellular DA levels in the
mPFC (F2,16 5 7.48, p , .05 and p , .001) and the NAC (F2,16 5 6.89, p ,
.05 and p , .01). OLAN (1 and 10 mg/kg) also produced time-dependent
effects in the mPFC (F2,16 5 17.72, p , .05 and p , .001) and the NAC
(F2,15 5 10.15, p , .01 and p , .001).
Antipsychotics and Prefrontal Dopamine
778
Kuroki et al.
Vol. 288
TABLE 2
Effects on the % net AUC of extracellular DA levels in the mPFC and the NAC
% net AUC (mean 6 S.E.M.)
Drug
VEH
APZ
CLOZ
OLAN
RISP
SUL
HAL
mg/kg
0
2
10
5
20
1
10
0.1
1
10
25
0.1
1
mPFC
NAC
log[mPFC]2log[NAC]
9 6 37 (6)
351 6 82** (6)
1310 6 287*** (5)
321 6 70** (6)
556 6 142** (7)
202 6 66* (7)
560 6 82*** (6)
208 6 26* (4)
358 6 79*** (4)
153 6 36* (6)
266 6 101** (4)
88 6 29 (4)
137 6 45* (6)
213 6 14 (6)
16 6 23 (9)
135 6 21*** (5)
78 6 34* (7)
133 6 38* (6)
181 6 70** (6)
280 6 39** (6)
265 6 76*** (4)
454 6 33*** (4)
384 6 49*** (8)
286 6 60** (4)
300 6 100** (5)
371 6 52*** (7)
1.35
0.99
0.62
0.63
0.05
0.30
20.10
20.11
20.40
20.03
20.54
20.43
The values of log[mPFC]2log[NAC] are the differences in the logarithm of % net AUC between the mPFC and the NAC. Numbers of rats in each treatment group are in
parentheses. * p , .05, ** p , .01, *** p , .001 by one-way ANOVA compared to the vehicle control.
0.81, p , .01). The difference in the pKi values between D2
and 5-HT2A receptors was not significantly correlated with
the pKi values for D2 (r 5 20.60, p . .1) or D3 (r 5 0.64, p .
.1) receptors (Table 1).
Discussion
The major findings of the present study are: 1) at certain
doses, some but not all atypical antipsychotic drugs have
preferential effects to increase extracellular DA levels in the
mPFC compared with the NAC; and 2) the ability of antipsychotic drugs to increase extracellular DA levels in the mPFC
compared with the NAC is positively correlated with the
difference in the affinities of antipsychotic drugs for 5-HT2A
and D2 receptors. It is also inversely correlated with the
affinities for D2 or D3 receptors. There is no correlation
between the affinities for 5-HT2A receptors alone and the
preferential effects in the mPFC.
This study was undertaken to clarify the importance of the
affinities of antipsychotic drugs for 5-HT2A and D2 receptors
for the differential clinical and biochemical effects of typical
and atypical antipsychotic drugs. It was previously postulated that the difference in pKi values for these two receptors
was relevant to their ability to produce minimal extrapyramidal symptoms and might also contribute to their possible
advantages as antipsychotic drugs (Meltzer et al., 1989).
These drugs may also differ with regard to their ability to
treat negative symptoms and to improve cognitive function.
Negative symptoms and cognitive impairment have been
suggested to be related to frontal lobe function and, in particular, to the alteration of cortical dopaminergic activity
(Sawaguchi and Goldman-Rakic, 1991; Murphy et al., 1996).
5-HT2A receptors located on pyramidal cells in the cerebral
cortex may also be the site of action of antipsychotic drugs
(Jakab and Goldman-Rakic, 1998). Therefore, it is of importance to determine whether typical and atypical antipsychotic drugs differ with regard to their ability to increase
extracellular DA levels in the mPFC relative to the NAC. If
so, this difference may be related to their affinities for
5-HT2A and D2 receptors.
The six antipsychotic drugs studied included four atypical
antipsychotic drugs (CLOZ, APZ, RISP, and OLAN) and two
typical antipsychotic drugs (HAL and SUL) (Meltzer, 1995).
Despite the fact that only six drugs were studied, the range of
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tracellular DA levels in the mPFC than in the NAC, whereas
OLAN (1 mg/kg) produced a comparable increase in the
mPFC compared to the NAC. RISP (0.1 and 1 mg/kg) produced comparable effects on extracellular DA levels in the
two regions. SUL (25 mg/kg) also showed a comparable increase in the mPFC compared to the NAC, whereas SUL (10
mg/kg) as well as HAL (0.1 and 1 mg/kg) increased extracellular DA levels in the NAC to a greater extent than in the
mPFC.
Thus, the six antipsychotic drugs may be grouped on the
basis of their effects on extracellular DA levels in the mPFC
and the NAC. Three of the drugs (CLOZ, APZ, and OLAN)
produced greater increases in extracellular DA levels in the
mPFC compared with the NAC at at least one dose studied,
whereas two drugs (SUL and HAL) had greater effects in the
NAC compared with the mPFC at at least one dose studied.
Relationship Between the Affinities (pKi Values) for
Receptor Subtypes and the Ability to Increase Extracellular DA Levels in the mPFC Compared With the
NAC. To determine the relationship between pKi values and
regional effects of antipsychotic drugs on extracellular DA
levels, the difference in the values of log[% net AUC of DA]
between the mPFC and the NAC (log[mPFC] 2 log[NAC])
was determined in this study (Table 2). This measure is more
appropriate for this purpose than the comparison of absolute
extracellular DA levels in each region between the mPFC and
the NAC because basal extracellular DA levels are significantly different (6-fold) between these regions, and the % net
AUC values are less affected by the difference in basal levels.
Analysis of Spearman rank correlation coefficients for the
six antipsychotic drugs revealed that the difference in the
values of log[% net AUC of DA] between the mPFC and the
NAC (log[mPFC] 2 log[NAC]) significantly correlated with
the difference in the pKi values between 5-HT2A and D2
receptors (r 5 0.79, p , .01) and inversely correlated with
their pKi values for D2 (r 5 20.89, p , .01) and D3 receptors
(r 5 20.86, p , .01) (Fig. 3). However, the log[mPFC] log[NAC] values had no significant correlation with the pKi
values for 5-HT2A receptors (r 5 0.23) or other receptor
subtypes (D1, D4, 5-HT1A, and 5-HT2C receptors, r 5 20.24,
0.04, 0.31 and 0.51, respectively). The log[mPFC] or log[NAC]
values alone were not significantly correlated with the pKi
values for DA and 5-HT receptor subtypes, except for the
log[NAC] versus D2 (r 5 0.79, p , .01) or D3 receptors (r 5
1999
the differences in their relative affinities for 5-HT2A and D2
receptors was 1000-fold. Because the D2 and D3 receptor
affinities of the six drugs are highly correlated, it was not
possible to separate out the importance of D2 versus D3
receptor blockade with regard to the effect on extracellular
DA levels in the mPFC and NAC. Further study is needed to
determine the relative importance of D2 and D3 receptors to
the ability to increase extracellular DA levels in the two
regions.
Both doses of APZ, CLOZ, OLAN, and RISP had greater or
comparable effects on extracellular DA levels in the mPFC
compared with the NAC. By contrast, SUL (10 mg/kg) and
both doses of HAL had much greater effects on extracellular
DA levels in the NAC than in the mPFC, leaving open the
779
possibility that an equal or greater effect on extracellular DA
levels in the mPFC compared with the NAC may be characteristic of an atypical antipsychotic drug. These results are
generally consistent with previous reports by others
(Moghaddam and Bunney, 1990; Nomikos et al., 1994; Hertel
et al., 1996). Volonté et al. (1997) reported a greater effects of
OLAN (0.3, 1 and less than 3 mg/kg) in the STR than in the
mPFC, whereas we found a greater effect of OLAN (10 but
not 1 mg/kg) in the mPFC than in the NAC. It should be
noted that SUL (25 mg/kg) produced a comparable increase
in extracellular DA levels in the mPFC relative to the NAC.
This effect may be related to the reported beneficial effects of
SUL to decrease negative symptoms (Mauri et al., 1996).
HAL, which has minimal beneficial effect on negative symptoms, produced the smallest increase in extracellular DA
levels in the mPFC at both doses studied. These data support
the possibility that there could be a relationship between
extracellular DA levels in the mPFC and improvement in
negative symptoms.
Quetiapine, another 5-HT2A/D2 receptor antagonist, has a
low extrapyramidal symptoms profile similar to that of
OLAN, yet there is no evidence that it has beneficial effects
on negative symptoms or cognition in schizophrenia (Arvanitis et al., 1997). Recently, quetiapine was reported not to
increase extracellular DA levels in the mPFC (Volonté et al.,
1997). These findings may be consistent with the hypothesis
that increased prefrontal extracellular DA levels may be
important to the ability of 5-HT2A/D2 receptor antagonists to
improve negative symptoms. The lesser ability of quetiapine
to increase DA levels in the mPFC may be due to much
weaker affinity of quetiapine for 5-HT2A and D2 receptors
than any other atypical antipsychotic drug studied here
(Schotte et al., 1996). More typical and atypical antipsychotic
drugs need to be studied to further clarify this relationship
between prefrontal extracellular DA levels and negative
symptoms. It will also be important to determine the effect of
chronic administration of these agents on extracellular DA
levels because the effect of chronic treatment would be expected to have greater clinical relevancy.
Although there was a strong inverse correlation between
D2 (or D3) receptor affinities and the ability to increase extracellular DA levels in the mPFC compared with the NAC, it
is most likely that the relationship determining the preferential effect on extracellular DA levels in the mPFC is
5-HT2A relative to D2 receptor affinity. The Spearman rank
correlation coefficient of the difference in pKi values for the
two receptors and the logarithm of the % net AUC for the
mPFC and the NAC was 0.79, accounting for 62% of the
variance in the dependent measure. This correlation is unlikely due to the correlation with pKi values for D2 or D3
receptors because there was no significant correlation between these values and the difference in pKi values for
5-HT2A and D2 receptors. 5-HT2A receptor affinities alone did
not correlate with the greater ability to increase extracellular
DA levels in the mPFC relative to the NAC, although the
selective 5-HT2A receptor antagonists such as M-100,907
were reported to increase extracellular DA levels in the
mPFC (Schmidt and Fadayel, 1995). It would appear that
5-HT2A receptor blockade, in the presence of D2 receptor
blockade that is relatively less effective than that of 5-HT2A
receptor blockade, is able to increase extracellular DA levels
in the mPFC more so than in the NAC. This may be due to
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Fig. 3. Spearman rank correlation analysis showed significant relationships between the pKi values of six antipsychotic drugs for D2 receptors
and the difference in their pKi values between 5-HT2A and D2 receptors,
and the difference of log[% net AUC] between the mPFC and the NAC
(log[mPFC] 2 log[NAC]) (six drugs x two doses). The log[mPFC] 2 log[NAC] values had also significant correlation with the pKi values for D3
receptors (r 5 20.86, p , .01). There was no such correlation with the pKi
values for 5-HT2A receptors (r 5 0.23) or other receptor subtypes (D1, D4,
5-HT1A, and 5-HT2C receptors, r 5 20.24, 0.04, 0.31, and 0.51, respectively).
Antipsychotics and Prefrontal Dopamine
780
Kuroki et al.
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Send reprint requests to: Toshihide Kuroki, M.D., Ph.D., Department of
Psychiatry, Saga Medical School, Nabeshima 5-1-1, Saga 849-8501, Japan.
E-mail: [email protected]
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