Directory UMM :Data Elmu:jurnal:B:Biological Psichatry:Vol48.Issue3.2000:

5-HT1A Receptor Activation Contributes to
Ziprasidone-Induced Dopamine Release in the Rat
Prefrontal Cortex
Hans Rollema, Yi Lu, Anne W. Schmidt, Jeffrey S. Sprouse, and Stevin H. Zorn
Background: Ziprasidone (Zeldox) is a novel antipsychotic with a unique combination of antagonist activities
at monoaminergic receptors and transporters and potent
agonist activity at serotonin 5-HT1A receptors. 5-HT1A
receptor agonism may be an important feature in ziprasidone’s clinical actions because 5-HT1A agonists increase
cortical dopamine release, which may underlie efficacy
against negative symptoms and reduce dopamine D2
antagonist-induced extrapyramidal side effects. This study
investigated the in vivo 5-HT1A agonist activity of ziprasidone by measuring the contribution of 5-HT1A receptor
activation to the ziprasidone-induced cortical dopamine
release in rats.
Methods: Effects on dopamine release were measured by
microdialysis in prefrontal cortex and striatum. The role
of 5-HT1A receptor activation was estimated by assessing
the sensitivity of the response to pretreatment with the
5-HT1A antagonist, WAY-100635. For comparison, the
D2/5-HT2A antagonists clozapine and olanzapine, the D2
antagonist haloperidol, the 5-HT2A antagonist MDL

100,907 and the 5-HT1A agonist 8-OHDPAT were
included.
Results: Low doses (,3.2 mg/kg) of ziprasidone, clozapine, and olanzapine increased dopamine release to approximately the same extent in prefrontal cortex as in
striatum, but higher doses ($3.2 mg/kg) resulted in an
increasingly preferential effect on cortical dopamine release. The 5-HT1A agonist 8-OHDPAT produced a robust
increase in cortical dopamine (DA) release without affecting striatal DA release. In contrast, the D2 antagonist
haloperidol selectively increased striatal DA release,
whereas the 5-HT2A antagonist MDL 100,907 had no
effect on cortical or striatal DA release. Prior administration of WAY-100635 completely blocked the cortical
DA increase produced by 8-OHDPAT and significantly
attenuated the ziprasidone- and clozapine-induced cortical DA increase. WAY-100635 pretreatment had no effect
on the olanzapine-induced DA increase.

From the Department of Neuroscience, Central Research Division, Pfizer Inc.,
Groton, Connecticut.
Address reprint requests to Hans Rollema, Pfizer Inc., Dept. of Neuroscience,
Central Research Division, Groton CT 06340.
Received December 8, 1999; revised February 8, 2000; accepted February 14, 2000.

© 2000 Society of Biological Psychiatry

Conclusions: The preferential increase in DA release in
rat prefrontal cortex produced by ziprasidone is mediated
by 5-HT1A receptor activation. This result extends and
confirms other in vitro and in vivo data suggesting that
ziprasidone, like clozapine, acts as a 5-HT1A receptor
agonist in vivo, which may contribute to its activity as an
antipsychotic with efficacy against negative symptoms and
a low extrapyramidal side effect liability. Biol Psychiatry 2000;48:229 –237 © 2000 Society of Biological
Psychiatry
Key Words: Antipsychotics, dopamine, prefrontal cortex,
5-HT2A/D2 antagonist, 5-HT1A agonist, microdialysis

Introduction

A

dministration of classical dopamine (DA) D2 receptor
antagonists to patients results in a high incidence of
compliance, limiting acute extrapyramidal side effects

(EPSs) and later occurring tardive dyskinesia. In addition,
for the majority of patients treated with conventional
antipsychotics there is usually greater improvement in
positive than in negative symptoms. The introduction of
clozapine, a highly efficacious antipsychotic with activity
against negative symptoms and minimal EPS liability,
represented a significant improvement, but the use of
clozapine is substantially limited by its potential for severe
side effects (i.e., agranulocytosis; Lieberman et al 1989).
The search for new antipsychotics with a clinical spectrum
like that of clozapine but lacking its serious side effects
has led to a new class of drugs, often referred to as
“atypical” antipsychotics (e.g., risperidone, quetiapine,
olanzapine). These compounds display a higher in vitro
affinity for serotonin (5-HT) 5-HT2A receptors compared
with D2 receptors (Meltzer et al 1989) and are efficacious
as antipsychotics, without resulting in a high incidence of
EPS (Buckley 1997; Lieberman 1996).
Ziprasidone (Zeldox) is a novel antipsychotic, chemically unrelated to any available antipsychotic drug
(Howard et al 1996), with a unique combination of

pharmacologic activities at serotonergic, dopaminergic,
0006-3223/00/$20.00
PII S0006-3223(00)00850-7

230

H. Rollema et al

BIOL PSYCHIATRY
2000;48:229 –237

Table 1. Selected Binding Affinities (Ki in nmol/L) for Rat D2
(Striatum), 5-HT2A (Cortex), and 5-HT1A (Cortex) Receptors
of Ziprasidone, Clozapine, Olanzapine, Haloperidol,
MDL 100,907, and 8-OHDPAT
Ki (nmol/L)

Ziprasidone
Clozapine
Olanzapine

Haloperidol
MDL 100,907a
8-OHDPAT

D2

5-HT2A

5-HT1A

4.8
83
11
0.71
.4,000
2,700

0.42
16
6.2

45
0.4
.5,000

3.4
120
2,800
1,100
.1,000
1.4

Data from Seeger et al (1995).
a
MDL 100,907 data from Schmidt et al (1997).

and adrenergic receptors and transporters (Seeger et al
1995; Zorn et al 1999). Thus, in addition to having
selectivity for 5-HT2A and D2 receptors, it has high affinity
for 5-HT1A, 5-HT1D, and 5-HT2C receptors, as well as
moderate potency as a 5-HT and NE uptake inhibitor and

a1-adrenoceptor antagonist (Table 1). Furthermore, unlike
clozapine and olanzapine, which have high affinities for
the muscarinic m1 receptor, ziprasidone has negligible
affinity for this site (Ki . 4500 nmol/L), which may be
particularly beneficial in the elderly because antagonism at
the muscarinic m1 receptor has been associated with
memory impairment (Zorn et al 1999). Together with
negligible affinity for histaminic receptors, and high potency as a 5-HT1A receptor agonist, this profile differentiates ziprasidone from all other antipsychotic drugs.
Several of the new antipsychotic drugs preferentially
increase prefrontal cortex DA release over that in the
striatum or nucleus accumbens (Hertel et al 1996; Kuroki
et al 1999; Moghaddam and Bunney 1990; Nomikos et al
1994; Pehek et al 1993; Volonté et al 1997). This
enhancement of cortical DA release may underlie efficacy
against negative symptoms because the negative symptomatology in schizophrenia has been hypothesized to be
associated with a functional impairment of mesocortical
dopaminergic transmission (Weinberger and Lipska
1995). To examine the effect of ziprasidone on dopaminergic neurotransmission, we measured extracellular DA
levels in prefrontal cortex and striatum of freely moving
rats following orally administered ziprasidone. The effects

of ziprasidone were compared with those of the D2/5HT2A antagonists clozapine and olanzapine, as well as
with the effects of the D2 receptor antagonist haloperidol
and the selective 5-HT2A receptor antagonist MDL
100,907 (Table 1).
In addition, because ziprasidone and clozapine are
5-HT1A receptor agonists (Mason and Reynolds 1992;
Newman-Tancredi et al 1996, 1998; Schmidt et al 1998;
Seeger et al 1995) we examined the contribution of

5-HT1A receptor activation in enhancing cortical DA
release, by pretreatment with the selective 5-HT1A antagonist WAY-100635. For comparison, the prototypical
5-HT1A agonist, 8-OHDPAT, was included in these studies. A portion of this work was previously presented in
abstract form (Lu et al 1997).

Methods and Materials
Microdialysis
Vertical concentric microdialysis probes (8 mm long, 4 mm
active dialysis AN 69 Hospal membrane) were implanted in the
prefrontal cortex (AP 13.2 mm, ML 60.7 mm, DV 26.0 mm)
or striatum (AP 10.7 mm, ML 63.0 mm, DV 27.5 mm; Paxinos

and Watson 1997), of male Sprague–Dawley rats (290 –330 g,
Charles River, Wilmington, MA) under ketamine/xylazine anesthesia. One day after surgery, the probe inlet was connected via
PEEK tubing (inside diameter 0.005 inches, Upchurch Scientific,
Oak Harbor, WA) and a dual channel fluid swivel system
(Instech Laboratories Inc., Plymouth Meeting, PA) to a microperfusion pump (CMA/100, CMA/Microdialysis, Acton,
MA) and perfused with artificial cerebrospinal fluid (aCSF: 147
mmol/L NaCl, 2.7 mmol/L KCl, 1.3 mmol/L CaCl2, 1.0 mmol/L
MgCl2) at 1.5 mL/min. The probe outlet was connected via
PEEK tubing to a HPLC sample loop and microdialysate samples
(30 mL) were continuously collected online and automatically
injected every 25 min on a 150 3 3 mm, C18 3m Hypersil BDS
column (Keystone Scientific, Bellefonte, PA). The analytes were
separated with a mobile phase containing 75 mmol/L sodium
acetate pH 4.3, 8% MeOH, 7 mg/L EDTA, and 1.1 mmol/L
heptanesulfonic acid, delivered at a flow rate of 0.35 mL/min by
a Shimadzu LC-10AD pump (Shimadzu, Columbia, MD) and
detected amperometrically at a glassy carbon electrode set at 600
mV vs Ag/AgCl (ANTEC DECADE, Leiden, The Netherlands).
The detection limit of the assay was ;0.5 pg DA on column.

Drug Treatment
Test compounds were administered orally (p.o.) by oral gavage
or subcutaneously (SC) in a volume of 2 mL/kg after basal DA
levels had stabilized. Ziprasidone, olanzapine, and clozapine
were administered orally in a mixture of emulphor EL620,
ethanol, and saline (5:5:90; EES). For dose–response studies
clozapine, was also given by SC injection, dissolved in 40%
2-hydroxypropyl-b-cyclodextrin, to allow comparisons with previously published data. Haloperidol, MDL 100,907 and 8-OH
DPAT were dissolved in EES and injected SC. In separate
experiments, rats were pretreated with WAY-100635 (0.1 mg/kg
SC in saline in a volume of 1 mL/kg) 30 min before test
compound administration.

Drugs and Chemicals
Ziprasidone (CP-88,059-01; 5-{2-(4-(1,2-benzisothiazol-3-yl)piperazinyl)ethyl}-6-chloro-1,3-dihydro-2(1H)-indol-2-one hydrochloride hydrate), olanzapine (2-methyl-4-(4-methyl-1-piperazinyl)-10H-thieno[2,3-B][1,5]benzodiazepine), MDL 100,907

5-HT1A Effects of Ziprasidone on DA Release

BIOL PSYCHIATRY
2000;48:229 –237

231

Figure 1. Time courses of the effects of
10 mg/kg orally administered (p.o.) ziprasidone, 3.2 mg/kg p.o. clozapine,
and 10 mg/kg p.o. olanzapine on extracellular levels of dopamine (DA) in the
rat prefrontal cortex and striatum. Arrows indicate oral drug administration
at t 5 0. Data are expressed as percentage of basal levels 6 SEM (n 5
3– 4). Statistical significance of
DAcortex vs. DAstriatum: *p , .05 and
**p , .01.

(R-(1)-a-(2,3-dimethoxy-phenyl)-1-[2-(4-fluorophenyl)ethyl]-4-piperidinemethanol), and WAY-100635 (N-(2-(4-(2-methoxyphenyl)1-piperazinyl)ethyl-N-(2-pyridinyl)cyclohexane-carboxamide trihydrochloride) were synthesized at Pfizer Inc., Central Research
Division (Groton, CT). Clozapine (8-chloro-11-(4-methyl-1piperazinyl)-5H-dibenzo-[b,e][1,4]-diazepine), 8-OHDPAT
((6)8-hydroxy-N,N-dipropylaminotetralin. HBr), haloperidol,
and 2-hydroxyethyl-b-cyclodextrine were purchased from RBI
(Natick, MA), tetrodotoxin from Sigma (St. Louis, MO). Emulphor EL 620 was obtained from Warner Graham (Cockeysville,
MD), methanol from J.T. Baker (Philipsburg, NJ), and all other
analytical grade chemicals were purchased from Fluka ChemikaBioChemika (Ronkonkoma, NY).

Data Analysis
Dialysate concentrations of DA were quantified by comparing
peak heights with those of DA standard solutions. Five p.o. doses
of ziprasidone and olanzapine (0.3, 1.0, 3.2, 10, and 32 mg/kg),
one p.o. dose (3.2 mg/kg) and three SC doses (1.0, 3.2, and 10
mg/kg) of clozapine were investigated. The effects of the drugs
on DA concentrations were monitored for at least 5 hours and
expressed as the mean percentage 6 SEM of basal levels (i.e.,
the average of 5 basal samples before drug injection) to construct
time-response curves (n 5 3–7 for each dose). Statistical
analyses of time-response curves were performed by two-way
analyses of variance for repeated measures and Dunnett’s post
hoc multiple comparisons between baseline and posttreatment
levels. For dose–response curves, the drug effects were expressed as areas under the curve, calculated from the percentages
increase above baseline over a 5-hour period after drug administration (area under the curve [AUC0 –5 hours] 6 SEM). Dose–
response curves were analyzed by multiple comparisons with
Tukey’s honestly significant difference generalized method using orthogonal contrast to compare overall effects of like drugs
and doses (cortex vs. striatum and vehicle 1 drug vs. WAY100635 1 drug).

Results
Effects of Ziprasidone, Clozapine, and Olanzapine
on DA Release
Basal microdialysate DA concentrations (not corrected for
recovery) were 0.30 6 0.02 nmol/L (n 5 115) in the
prefrontal cortex and 4.3 6 0.3 nmol/L (n 5 64) in the
striatum and did not differ statistically between treatment
groups. The neurogenic origin of the DA collected in
cortical and striatal dialysates was routinely verified at the
end of an experiment by a 20-min perfusion with 1
mmol/L tetrodotoxin. This treatment reversibly reduced
DA dialysate concentrations in prefrontal cortex and
striatum to less than 5% of basal levels. Administration of
vehicles did not significantly change extracellular concentrations of DA over the time period that measurements
were performed.
Figure 1 shows time courses of the effects of orally
administered ziprasidone (10 mg/kg), clozapine (3.2 mg/
kg), and olanzapine (10 mg/kg) on extracellular DA levels
(expressed as percentage of basal levels) in prefrontal
cortex and striatum.
The effects of the compounds on DA release follow
comparable time courses, with maximal DA increases
reached within 2 hours and returning to basal levels 5– 6
hours later. Table 2 summarizes the maximal effects of the
three compounds on DA release.
Ziprasidone produced higher DA increases in prefrontal
cortex than in striatum at all doses tested, and this
difference reached statistical significance after 10 and 32
mg/kg.
Clozapine increased extracellular DA in prefrontal cortex significantly more than in striatum after 3.2 and 10
mg/kg. SC, as well as after 3.2 mg/kg p.o. Low and

232

H. Rollema et al

BIOL PSYCHIATRY
2000;48:229 –237

Table 2. Maximal Effects of Ziprasidone, Clozapine,
Olanzapine, Haloperidol, MDL 100,907, and 8-OHDPAT on
Extracellular Dopamine (DA) Levels in the Rat Prefrontal
Cortex and Striatum and in the Prefrontal Cortex after
WAY-100635 (WAY) Pretreatment
Maximal effect on DA release
(% of basal 6 SEM)
Drug
Vehicle
Ziprasidone

Clozapine

Olanzapine

Haloperidol
MDL 100,907
8-OHDPAT

mg/kg

Striatum

Cortex

0.3
1.0
3.2
10.0
32.0
1.0
3.2
p.o. 3.2
10.0
0.3
1.0
3.2
10.0
32.0
0.3
1.0
0.5

99 6 9
89 6 12
121 6 9
141 6 19
147 6 5
189 6 10
126 6 8
133 6 3
110 6 9
175 6 17
146 6 7
129 6 10
150 6 14
225 6 19
178 6 23
238 6 8
105 6 7
111 6 12

108 6 10
124 6 8
164 6 6
176 6 10
277 6 17a
372 6 31a
154 6 12
350 6 26
203 6 24b
507 6 21b
117 6 4
183 6 13
182 6 16
316 6 20
416 6 42a
172 6 12a
112 6 6
293 6 33b

Cortex 1 WAY
pretreatment
103 6 12
140 6 6
134 6 18
142 6 12 c
134 6 8
196 6 10c
321 6 40c

298 6 25

118 6 8c

Data are expressed as percentages of basal levels 6 SEM (n 5 3–7). Ziprasidone and olanzapine were administered orally. Clozapine was administered
subcutaneously and orally (p.o.). Haloperidol, MDL 100,907, and 8-OHDPAT were
administered subcutaneously.
Statistical difference of DAcortex vs. DAstriatum:
a
p , .05.
b
p , .01.
DAveh 1 drug vs. DAWAY 1 drug:
c
p , .05.

moderate doses of olanzapine (0.3–10 mg/kg p.o.) produced comparable increases in cortical as in striatal DA
release, but after 32 mg/kg p.o., maximal DA increases in

cortex were statistically significantly higher than those in
striatum.
The effects of each dose of the drugs on DA release
were also calculated as the area under the curve over a
5-hour period (AUC0 –5 hours) to construct dose–response
curves. For the dose–response studies, clozapine was
administered SC to allow comparisons with published
data. Dose–response curves (Figure 2) depicting the effects of ziprasidone (0.3–32 mg/kg p.o.), clozapine
(0.3–10 mg/kg SC), and olanzapine (0.3–32 mg/kg p.o.)
show that the three compounds elevate cortical and striatal
DA release in a dose-dependent manner. With increasing
doses, the cortical DA increases become increasingly
greater than the corresponding striatal DA increases. This
trend results in highly significant differences between the
slopes of the dose–response curves in prefrontal cortex
versus striatum for all three compounds [ziprasidone:
F(1,20) 5 29.38, p , .0001; clozapine: F(1,14) 5 25.40,
p , .0002; olanzapine: F(1,25) 5 30.5, p , .0001].
To investigate the role of 5-HT1A receptor activation on
the dopaminergic effects of ziprasidone, clozapine, and
olanzapine, rats were pretreated 30 min before drug
administration with a dose of the selective 5-HT1A antagonist WAY-100635, which had no effect by itself (0.1
mg/kg SC).
Figure 3 shows the time courses of the effects of 10
mg/kg p.o. ziprasidone, 3.2 mg/kg SC clozapine, and 10
mg/kg p.o. olanzapine on DA release in rat prefrontal
cortex with and without WAY-100635 pretreatment. The
maximal effects of ziprasidone (1–10 mg/kg p.o.), clozapine (1–10 mg/kg SC), and olanzapine (10 mg/kg p.o.) on
cortical DA release in rat prefrontal cortex in the absence
and presence of WAY-100635 are given in Table 2. Figure
4 shows the dose–response relationship for the cortical

Figure 2. Dose–response curves showing
the effects of orally administered (p.o.)
ziprasidone, subcutaneous clozapine, and
p.o. olanzapine on dopamine (DA) release
in the rat prefrontal cortex and striatum.
Data are expressed as area under the curve
above basal DA levels over 5 hours after
drug administration 6 SEM (n 5 3–7).
Statistical significance of DAcortex vs.
DAstriatum: *p , .05 and **p , .01.

5-HT1A Effects of Ziprasidone on DA Release

BIOL PSYCHIATRY
2000;48:229 –237

233

Figure 3. Time courses of the effects of
10 mg/kg orally administered (p.o.) ziprasidone, 3.2 mg/kg subcutaneous clozapine, and 10 mg/kg p.o. olanzapine
on dopamine (DA) release in the rat
prefrontal cortex in the absence (●) and
presence (E) of WAY-100635, expressed as percentage of basal DA levels 6 SEM (n 5 3–5). Statistical significance of DAvehicle 1 drug vs.
DAWAY- 1 drug: *p , .05 and **p , .01.

effects (as AUC0 –5 hours) of ziprasidone and clozapine
measured with and without WAY-100635 pretreatment.
Pretreatment with WAY-100635 reduced the cortical
DA increases following 1–10 mg/kg p.o. ziprasidone by
50 –70% and the DA increases following 3.2–10 mg/kg SC
clozapine by about 40 – 60%. The overall differences
between the dose–response curves for ziprasidone and
clozapine in rat cortex with and without WAY-100635
pretreatment were highly significant [F(1,16) 5 6.15, p 5
.006, and F(1,17) 5 7.34, p 5 .002, respectively]. In
contrast, WAY-100635 had no effect on the increases in
cortical extracellular DA levels induced by 10 mg/kg p.o.
olanzapine (p 5 .587). Furthermore, WAY-100635 did not
significantly change the overall increases in striatal DA

release produced by 10 mg/kg ziprasidone, clozapine, or
olanzapine (p 5 .68).

Effects of Haloperidol, MDL 100,907, and
8-OHDPAT on DA Release
To compare the effects of the three antipsychotics with
those of singularly selective high-affinity ligands for the
D2, the 5-HT2A, and the 5-HT1A receptors, we also
measured the effects of the D2 receptor antagonist haloperidol, the 5-HT2A receptor antagonist MDL 100,907,
and the 5-HT1A receptor agonist 8-OHDPAT on DA
release in the rat prefrontal cortex and striatum. The
maximal effects of these drugs on cortical and striatal DA
release are listed in Table 2, whereas the time courses are
shown in Figure 5. Haloperidol (0.3 mg/kg SC) produced
a significantly higher increase (p , .05) in striatal DA
release (to 240% of basal levels) than in cortical DA
release (to 170% of basal levels), whereas MDL 100,907
(1 mg/kg SC) did not significantly change cortical or
striatal extracellular DA levels. At 0.5 mg/kg SC, 8OHDPAT increased DA release in rat prefrontal cortex
about threefold (p , .01) without affecting striatal DA
release. The cortical DA increase produced by 8-OHDPAT
was completely blocked by WAY-100635 pretreatment
(Table 2).

Discussion
Cortical versus Striatal Dopaminergic Effects
Figure 4. Dose–response curves showing the effects of orally
administered ziprasidone and subcutaneous clozapine on cortical
dopamine (DA) release in the absence and presence of WAY100635. Data are expressed as area under the curve above basal
DA levels over 5 hours after drug administration 6 SEM (n 5
3– 6). Statistical significance of DAveh 1 drug vs. DAWAY 1 drug:
*p , .05 and **p , .01.

Clozapine, ziprasidone, and olanzapine dose dependently
increase DA release in the prefrontal cortex and, to a lesser
extent, in the striatum of awake rats. Although the lowest
doses tested produce comparable, relatively small increases in cortical and striatal DA release, higher doses
progressively enhance cortical DA release, resulting in

234

BIOL PSYCHIATRY
2000;48:229 –237

H. Rollema et al

Figure 5. Time courses of the effects of
0.3 mg/kg of the D2 antagonist haloperidol, administered subcutaneously (SC);
1.0 mg/kg of the 5-HT2A antagonist
MDL 100,907 SC; and 0.5 mg/kg of
the 5-HT1A agonist 8-OHDPAT SC on
dopamine (DA) release in the rat prefrontal cortex and striatum. Arrows indicate drug administration at t 5 0.
Data are expressed as percentages of
basal levels 6 SEM (n 5 3– 4). Statistical significance of DAcortex vs.
DAstriatum: *p , .05 and **p , .01.

highly significant overall differences between the cortical
and striatal effects of the three compounds. In contrast, the
typical D2 antagonist antipsychotic haloperidol produces a
significantly greater increase in striatal DA release than in
cortical DA release at 0.3 mg/kg SC. These results are in
good agreement with previously reported effects of haloperidol and clozapine on in vivo DA release in rats ( Hertel
et al 1996; Kuroki et al 1999; Li et al 1998; Moghaddam
and Bunney 1990; Volonté et al 1997; Westerink et al
1998). The effects of olanzapine are also consistent with
recent studies showing that DA increases in prefrontal
cortex after low doses (#3 mg/kg SC) of olanzapine are
similar to, or smaller than those in striatum and accumbens, whereas higher doses (10 mg/kg SC) increase
cortical DA release more than striatal DA release (Kuroki
et al 1999; Li et al 1998; Volonté et al 1997). The present
results show that the novel antipsychotic ziprasidone also
preferentially enhances cortical DA release. Ziprasidone
consistently produces higher increases in DA release in the
prefrontal cortex than in the striatum over a wide dose
range (0.3–32 mg/kg p.o.), reaching statistically significant differences after 10 and 32 mg/kg. The cortical effects
of ziprasidone are consistent with a recent observation that
0.3 mg/kg of subcutaneously administered ziprasidone
increases cortical DA release almost twofold (Westerink et
al 1998), comparable to the effects we found after oral
administration of 1–3.2 mg/kg ziprasidone.

Mechanisms of Cortical DA Increase: Role of 5HT1A Receptor Activation
Because the antipsychotics examined in this study have
high affinity for multiple receptor sites, it is reasonable to
assume that specific receptor affinities, or combination of
affinities, are likely responsible for the observed increases
in DA release in rat prefrontal cortex. Thus, the fact that

5-HT1A agonists are known to increase cortical DA release
(Arborelius et al 1993; Wedzony et al 1996) prompted us
to use the selective 5-HT1A antagonist WAY-100635 to
assess the involvement of 5-HT1A receptor activation in
the antipsychotic-induced enhancement of cortical DA
release. We confirmed that the selective 5-HT1A agonist
8-OHDPAT produces a robust increase in cortical DA
release without affecting striatal DA release. This cortical
effect was found to be entirely mediated by 5-HT1A
receptor activation because the DA increase was completely blocked by pretreatment with the selective 5-HT1A
antagonist WAY-100635. Because pretreatment with
WAY-100635 also significantly reduces the ziprasidone
and clozapine responses, it is likely that the increases in
cortical DA release are mediated to a substantial degree
via activation of 5-HT1A receptors. In contrast, WAY100635 has no effect on the olanzapine-induced increase
in cortical DA release, consistent with its negligible
affinity (Ki 5 2800 nmol/L) for 5-HT1A receptors. The
lack of effect of WAY-100635 pretreatment on the antipsychotic-induced striatal DA increase is consistent with
the finding that 5-HT1A receptor activation by 8-OHDPAT
does not increase DA release in striatum, indicating that
5-HT1A receptors are not involved in the striatal DA
effects. This is in contrast to the cortical DA increase,
which is thought to be caused by activation of 5-HT1A
receptors located on DA neurons in the VTA or in regions
that project to the VTA (Arborelius et al 1993).
The results of the WAY-100635 experiments are in
agreement with previous in vitro and in vivo studies that
have provided ample evidence that ziprasidone and clozapine are unique among the clinically used antipsychotics
in that both are 5-HT1A receptor agonists (NewmanTancredi et al 1998). In vitro, ziprasidone has high affinity
for rat (Ki 5 3.4 nmol/L) and human (Ki 5 2.5 nmol/L)

5-HT1A Effects of Ziprasidone on DA Release

5-HT1A receptors and is an agonist at both guinea pig and
human 5-HT1A receptors (Seeger et al 1995; Zorn et al
1999). Clozapine has moderate affinity for rat 5-HT1A
receptors (Ki 5 120 nmol/L) and also exhibits agonist
activity at human 5-HT1A receptors (Mason and Reynolds
1992; Newman-Tancredi et al 1996). Additional evidence
for in vivo 5-HT1A receptor agonist properties of ziprasidone was provided by a recent study that showed that the
decrease in dorsal raphe cell firing produced by ziprasidone, but not by olanzapine, was inhibited by pretreatment
with WAY-100635 (Sprouse et al 1999).

Other Mechanisms of Cortical DA Increase
The cortical DA increases produced by olanzapine and
haloperidol, as well as the residual, 5-HT1A–independent
DA increase produced by ziprasidone and clozapine, are
likely the result of interactions with other neurotransmitter
receptors. Lacking the appropriate pharmacologic tools
required to test all plausible mechanisms, it remains
speculative as to which receptor or combination of receptors underlies the 5-HT1A–independent cortical DA increases observed. Antagonism of D2 receptors probably
does not play a major role in the cortical DA increases
produced by ziprasidone, clozapine, and olanzapine, because D2 receptor blockade has a much greater effect on
striatal than on cortical DA release (Moghaddam and
Bunney 1990; Santiago et al 1993). Furthermore, although
all compounds share moderate to high affinity for the D4
receptor, it is not yet clear to what extent, if any, D4
receptors are involved in the regulation of cortical DA
increase. Therefore, other serotonergic mechanisms may
be involved, in particular 5-HT2 receptor blockade, because all three antipsychotics are potent 5-HT2A/2C receptor antagonists and blockade of these receptors have been
reported to produce increases in cortical DA release
(Nomikos et al 1994; Pehek and Yanming 1997; Schmidt
and Fayadel 1995). Nonetheless, we and more recently
other investigators (Ishii et al 1999) could not reproduce
the observation by Schmidt and Fayadel (1995) that the
selective and potent 5-HT2A antagonist, MDL 100,907,
increases cortical DA release, but found that 5-HT2A
receptor blocking doses (0.1–1 mg/kg SC) of MDL
100,907 had no effect on cortical DA release. It therefore
seems unlikely that blockade of only 5-HT2A receptors
leads to an increased cortical DA release, raising the
possibility that blockade of the 5-HT2C rather than the
5-HT2A receptor contributes to enhanced DA release in the
prefrontal cortex by mixed 5-HT2A/2C antagonists. Alternatively, it is conceivable that specific combinations of
receptor interactions produce a preferential increase in
cortical DA release, such as the simultaneous blockade of
5-HT2A and D2 receptors (Andersson et al 1995; Kuroki et

BIOL PSYCHIATRY
2000;48:229 –237

235

al 1999) or the combination of 5-HT2A antagonist and
partial 5-HT1A agonist activity (Ishii et al 1999).

Clinical Significance
Increasing dopaminergic neurotransmission in the prefrontal cortex is generally thought to be advantageous for the
treatment of schizophrenia, in view of the purported
reduction in mesocortical dopaminergic neurotransmission
in schizophrenic patients (Weinberger and Lipska 1995).
Enhancement of cortical DA release could underlie, at
least in part, improvement in negative and cognitive
symptoms associated with schizophrenia. On the other
hand, because excessive occupation of nigrostriatal D2
receptors is associated with increased motor side effects
(Farde et al 1992), pronounced effects on striatal DA
release are believed to predict EPS liability of antipsychotics. This is in agreement with the clinical profiles of
clozapine and the newer “atypical” D2/5-HT2A receptor
antagonist antipsychotics, which preferentially increase
DA release in prefrontal cortex, (Kuroki et al 1999;
Moghaddam and Bunney 1990; Nomikos et al 1994;
Pehek et al 1993; Volonté et al 1997), with the exception
of risperidone, which increases striatal and cortical DA
release to the same extent (Hertel et al 1996). Using this
logic, the selective effects of ziprasidone on DA release in
prefrontal cortex compared with striatum may also be
beneficial for ameliorating negative and cognitive symptoms while reducing EPS liability.
The fact that ziprasidone acts as a 5-HT1A receptor
agonist in vivo may further contribute to its efficacy as an
antipsychotic with activity against negative symptoms and
low EPS liability. First of all, there is an increasing
awareness, partially based on the 5-HT1A receptor agonist
properties of clozapine (Mason and Reynolds 1992; Newman-Tancredi et al 1996; Rollema et al 1997; this study),
that 5-HT1A receptor agonism may be beneficial for the
treatment of schizophrenic affective symptomatology
(Sharma and Shapiro 1996). In addition, activation of
5-HT1A receptors not only increases prefrontal cortex DA
release but is also known to reduce D2 receptor antagonistinduced catalepsy in rats, an effect thought to predict
clinical EPS ( Andersen and Kilpatrick 1996; Invernizzi et
al 1988; Lucas et al 1997; Wadenberg and Ahlenius 1991)
and dystonia and EPS in monkeys (Casey 1994; Christoffersen and Meltzer 1998). Therefore, 5-HT1A agonism
may reduce motor side effect liability further than that
reportedly achieved by antagonism of 5-HT2A receptors
alone (Bersani et al 1986; Lucas et al 1997; Schmidt and
Seeger 1986).
Although ziprasidone has a unique pharmacologic spectrum of receptor activities that may be linked to the
etiology and treatment of schizophrenia (D2, 5-HT2A,

236

BIOL PSYCHIATRY
2000;48:229 –237

5-HT1D, 5-HT2C, 5-HT, and NE uptake), the demonstration of in vivo 5-HT1A receptor activity reported here
suggests that it is this property that may contribute to all
the features of an ideal antipsychotic drug—low EPS
liability, efficacy against positive and negative symptoms,
and relief from associated depression and anxiety. In
addition, 5-HT1A receptor activation could be one of the
features that is responsible for the negligible increase in
body weight observed with ziprasidone compared with the
considerable weight gain associated with the use of other
atypical antipsychotics (Allison et al 1999). Ziprasidone
could offer advantages over existing agents for the treatment of schizophrenia and has been reported to show a
broad spectrum of clinical benefits as indicated by results
of recent efficacy and toleration studies in patients (Daniel
et al 1999; Goff et al 1998; Keck et al 1998; Tandon et al
1997).

The research was conducted in Pfizer Central Research, Groton, Connecticut, and all animal procedures were approved by the Institutional
Animal Care and Use Committee. The authors are grateful to Dr. David
Raunig (Department of Biometrics, Pfizer Central Research) for performing the statistical analyses of the data.

References
Allison DB, Mentore JL, Heo M, Chandler LP, Cappelleri JC,
Infante MC, Weiden PJ (1999): Antipsychotic-induced
weight gain: A comprehensive research synthesis. Am J
Psychiatry 156:1686 –1696.
Andersen HL, Kilpatrick IC (1996): Prevention by (6)-8hydroxy-2-(di-n-propylamino)tetralin of both catalepsy and
the rises in rat striatal dopamine metabolism caused by
haloperidol. Br J Pharmacol 118:421– 427.
Andersson JL, Nomikos CG, Marcus M, Hertel P, Mathé JM,
Svensson TH (1995): Ritanserin potentiates the stimulatory
effects of raclopride on neuronal activity and dopamine
release selectively in the mesolimbic dopaminergic system.
Naunyn Schmiedebergs Arch Pharmacol 352:374 –385.
Arborelius L-E, Nomikos GG, Hacksell U, Svenson TH (1993):
(R)-8-OHDPAT preferentially increases dopamine release in
rat medial prefrontal cortex. Acta Physiol Scand 148:465–
466.
Bersani G, Grispini A, Marini S, Pasini A, Valducci M, Ciani N
(1986): Neuroleptic-induced extrapyramidal side effects:
Clinical perspectives with ritanserin (R 55667), a new selective 5-HT2 receptor blocking agent. Curr Ther Res 40:492–
499.
Buckley PF (1997): New dimensions in the pharmacologic
treatment of schizophrenia and related psychoses. J Clin
Pharmacol 37:363–378.
Casey DE (1994): Extrapyramidal syndromes: primate models as
predictors of clinical response. Neuropsychopharmacology
10:370S.
Christoffersen CL, Meltzer LT (1998): Reversal of haloperidolinduced extrapyramidal side effects in Cebus monkeys by

H. Rollema et al

8-hydroxy-2-(di-n-propylamino)tetraline and its enantiomers.
Neuropsychopharmacology 18:399 – 402.
Daniel DG, Zimbroff DL, Potkin SG, Reeves KR, Harrigan EP,
Lakshminarayanan M, et al (1999): Ziprasidone 80 mg/day
and 160 mg/day in the acute exacerbation of schizophrenia
and schizoaffective disorder: A 6-week placebo-controlled
trial. Neuropsychopharmacology 20:491–505.
Farde L, Nordström AL, Wiesel FA, Pauli, S, Halldin C, Sedvall
G (1992): Positron emission tomographic analysis of central
D1-dopamine and D2-dopamine receptor occupancy in patients treated with classical neuroleptics and clozapine—
relation to extrapyramidal side effects. Arch Gen Psychiatry
49:528 –544.
Goff DC, Posover T, Herz L, Simmons J, Kletti N, Lapierre K,
et al (1998): An exploratory, haloperidol-controlled, dosefinding study of ziprasidone in hospitalized patients with
schizophrenia or schizoaffective disorder. J Clin Psychopharmacol 18:296 –304.
Hertel P, Nomikos CG, Iurlo M, Svensson TH (1996): Risperidone: Regional effects in vivo on release and metabolism of
dopamine and serotonin in the rat brain. Psychopharmacology
124:74 – 86.
Howard HR, Lowe JA III, Seeger TF, Seymour PA, Zorn SH,
Maloney PR, et al (1996): 3-Benzisothiazolylpiperazine derivatives as potential atypical antipsychotic agents. J Med
Chem 39:143–148.
Invernizzi RW, Cervo L, Samanin R (1988): 8-Hydroxy-2-(din-propylamino)tetralin, a selective serotonin1A receptor agonist, blocks haloperidol-induced catalepsy by an action on
raphe nuclei medianus and dorsalis. Neuropharmacology
27:515–518.
Ishii H, Fowler WL, O’Laughlin IA, Meltzer HY, Ichikawa J
(1999): M100907, a 5HT2A antagonist, potentiates the ability
of R(1)-8-OH-DPAT to increase extracellular dopamine
levels in rat medial prefrontal cortex. Soc Neurosci Abstr
25:1215.
Keck P Jr, Buffenstein A, Ferguson J, Feighner J, Jaffe W,
Harrigan EP, et al (1998): Ziprasidone 40 and 120 mg/day in
the acute exacerbation of schizophrenia and schizoaffective
disorder: A 4-week placebo-controlled trial. Psychopharmacology 140:173–184.
Kuroki T, Meltzer HY, Ichikawa J (1999): Effects of antipsychotic drugs on extracellular dopamine levels in rat medial
prefrontal cortex and nucleus accumbens. J Pharmacol Exp
Ther 288:774 –781.
Li XM, Perry KW, Wong DT, Bymaster FP (1998): Olanzapine
increases in vivo dopamine and norepinephrine release in rat
prefrontal cortex, nucleus accumbens and striatum. Psychopharmacology 136:153–161.
Lieberman JA (1996): Atypical antipsychotic drugs as a first-line
treatment of schizophrenia—a rationale and hypothesis.
J Clin Psychiatry 57(suppl 11):68 –71.
Lieberman JA, Kane JM, Johns CA (1989): Clozapine: Guidelines for clinical management. J Clin Psychiatry 50:329 –338.
Lu Y, Zorn SH, Rollema H (1997): Comparison of the novel
antipsychotic ziprasidone with clozapine: Effects on dopamine release in rat prefrontal cortex and dorsolateral striatum.
Soc Neurosci Abstr 23:1931.
Lucas G, Bonhomme N, DeDeurwaerdere P, LeMoal M, Spampinato U (1997): 8-OH-DPAT, a 5-HT1A agonist and rintan-

5-HT1A Effects of Ziprasidone on DA Release

serin, a 5-HT2A/2C antagonist, reverse haloperidol-induced
catalepsy in rats independently of striatal dopamine release.
Psychopharmacology 131:56 – 63.
Mason SL, Reynolds G.P (1992): Clozapine has sub-micromolar
affinity for 5-HT1A receptors in human brain tissue. Eur
J Pharmacol 221:397–398.
Meltzer HY, Matsubara S, Lee J-C (1989): Classification of
typical and atypical antipsychotic drugs on the basis of
dopamine D1, D2, and serotonin2 pKi values. J Pharmacol
Exp Ther 251:238 –246.
Moghaddam B, Bunney BS (1990): Acute effects of typical and
atypical antipsychotic drugs on the release of dopamine from the
prefrontal cortex, nucleus accumbens, and striatum of the rat: An
in vivo microdialysis study. J Neurochem 54:1755–1760.
Newman-Tancredi A, Chaput C, Verriele L, Millan MJ (1996):
Clozapine is a partial agonist at human serotonin 5-HT1A
receptors. Neuropharmacology 25:119 –121.
Newman-Tancredi A, Gavaudan S, Conte C, Chaput C, Touzard
M, Verriele L, et al (1998): Agonist and antagonist actions of
antipsyhotic agents at 5-HT1A receptos: a [35S]GTPgS binding study. Eur J Pharmacol 355:245–256.
Nomikos CG, Iurlo M, Andersson JL, Kimura K, Svensson TH
(1994): Systemic administration of amperozide, a new atypical antipyschotic drug, preferentially increases dopamine
release in the rat medial prefrontal cortex. Psychopharmacology 115:147–156.
Paxinos G, Watson C (1997): The Rat Brain in Stereotaxic
Coordinates, 3rd ed. San Diego: Academic Press.
Pehek EA, Meltzer HY, Yamamoto BK (1993): The atypical
antipyschotic drug amperozide enhances rat cortical and
striatal dopamine efflux. Eur J Pharmacol 240:107–109.
Pehek EA, Yanming B (1997): Ritanserin administration potentiates amphetamine-stimulated dopamine release in the rat
prefrontal cortex. Prog Neuropsychopharmacol Biol Psychiatry 21:671– 682.
Rollema H, Lu Y, Schmidt AW, Zorn SH (1997): Clozapine
increases dopamine release in prefrontal cortex by 5-HT1A
receptor activation. Eur J Pharmacol 338:R3–R5.
Santiago M, Machado A, Cano J (1993): Regulation of prefrontal
cortical dopamine release by dopamine receptor agonists and
antagonists. Eur J Pharmacol 239:83–91.
Schmidt AW, Lebel LA, Johnson CG, Howard HR Jr, Lowe JA,
Zorn SH (1998): The novel antipsychotic ziprasidone has a
unique human receptor binding profile compared with other
agents. Soc Neurosci Abstr 24:2177.
Schmidt AW, Seeger TF (1986): Serotonin 5-HT2 potency as a
predictor of antipsychotic side effect liability. Soc Neurosci
Abstr 12:479.

BIOL PSYCHIATRY
2000;48:229 –237

237

Schmidt CJ, Fadayel GM (1995): The selective 5-HT2A receptor
antagonist, MDL 100,907, increases dopamine efflux in the
prefrontal cortex of the rat. Eur J Pharmacol 273:273–279.
Schmidt CJ, Kehne JH, Carr AA (1997): MDL 100,907: A
selective 5-HT2A receptor antagonist for the treatment of
schizophrenia. CNS Drug Rev 3:49 – 67.
Seeger TF, Seymour PA, Schmidt AW, Zorn SH, Schulz D,
Lebel LA, et al (1995): Ziprasidone (CP-88,059): A new
antipsychotic with combined dopamine and serotonin receptor antagonist activity. J Pharmacol Exp Ther 275:101–
113.
Sharma RP, Shapiro LE (1996): The 5-HT1A receptor system:
Possible implications for schizophrenic negative symptomatology. Psychiatr Ann 26:88 –92.
Sprouse J, Reynolds LS, Braselton JP, Rollema H, Zorn SH
(1999): Comparison of the novel antipsychotic ziprasidone
with clozapine and olanzapine: Inhibition of dorsal raphe cell
firing and the role of 5-HT1A receptor activation. Neuropsychopharmacology 21:622– 631.
Tandon R, Harrigan E, Zorn SH (1997): Ziprasidone: A novel
antipsychotic with unique pharmacology and therapeutic
potential. J Serotonin Res 4:159 –177.
Volonté M, Monferini E, Cerutti M, Fodritto F, Borsini F (1997):
BIMG 80, a novel potential antipsychotic drug: Evidence for
multireceptor actions and preferential release of dopamine in
prefrontal cortex. J Neurochem 69:182–190.
Wadenberg ML, Ahlenius S (1991): Antipsychotic-like profile of
combined treatment with raclopride and 8-OH-DPAT in the
rat: Enhancement of antipsychotic-like effects without catalepsy. J Neural Transm Gen Sect 83:43–53.
Wedzony K, Mackowiak M, Fijal K, Golembiowska K (1996):
Ipsapirone enhances the dopamine outflow via 5-HT1A
receptors in the rat prefrontal cortex. Eur J Pharmacol
305:73–78.
Weinberger DR, Lipska BK (1995): Cortical maldevelopment,
antipsychotic drugs, and schizophrenia: A search for common
ground. Schizophr Res 16:87–110.
Westerink BHC, De Boer P, De Vries JB, Kruse CG, Long SK
(1998): Antipsychotic drugs induce similar effects on the
release of dopamine and noradrenaline in the medial prefrontal cortex of the rat brain. Eur J Pharmacol 361:27–33.
Zorn SH, Lebel LA, Schmidt AW, Lu Y, Braselton JP,
Reynolds LS, et al (1999): Pharmacological and neurochemical studies with the new antipsychotic ziprasidone.
In: Palomo T, Beninger RJ, Archer T, editors. Interactive
Monoaminergic Disorders, Vol 4. Madrid: Editorial Sintesis, 377–394.