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 antipsy-chotic 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 ziprasi-done’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-HT1Aagonist activity of
ziprasi-done 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-HT1Areceptor 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, cloza-pine, and olanzapine increased dopamine release to ap-proximately 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 re-lease. The 5-HT1Aagonist 8-OHDPAT produced a robust
increase in cortical dopamine (DA) release without affect-ing 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 adminis-tration of WAY-100635 completely blocked the cortical DA increase produced by 8-OHDPAT and significantly attenuated the ziprasidone- and clozapine-induced corti-cal DA increase. WAY-100635 pretreatment had no effect on the olanzapine-induced DA increase.
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 Psychia-try 2000;48:229 –237 © 2000 Society of Biological Psychiatry
Key Words:Antipsychotics, dopamine, prefrontal cortex, 5-HT2A/D2antagonist, 5-HT1Aagonist, microdialysis
Introduction
A
dministration of classical dopamine (DA) D2receptorantagonists 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-HT2Areceptors compared
with D2receptors (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, chemi-cally unrelated to any available antipsychotic drug (Howard et al 1996), with a unique combination of pharmacologic activities at serotonergic, dopaminergic,
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.
and adrenergic receptors and transporters (Seeger et al 1995; Zorn et al 1999). Thus, in addition to having selectivity for 5-HT2Aand D2receptors, 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 po-tency as a 5-HT1Areceptor agonist, this profile
differen-tiates 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; Volonte´ et al 1997). This enhancement of cortical DA release may underlie efficacy against negative symptoms because the negative symp-tomatology 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 dopami-nergic 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
/5-HT2A antagonists clozapine and olanzapine, as well as
with the effects of the D2receptor 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-HT1Aagonist, 8-OHDPAT, was included in these
stud-ies. 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 (AP13.2 mm, ML60.7 mm, DV26.0 mm) or striatum (AP10.7 mm, ML63.0 mm, DV27.5 mm; Paxinos and Watson 1997), of male Sprague–Dawley rats (290 –330 g, Charles River, Wilmington, MA) under ketamine/xylazine anes-thesia. 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 mi-croperfusion 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 (30mL) were continuously collected online and automatically injected every 25 min on a 15033 mm, C18 3mHypersil 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 pre-viously 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 hydro-chloride hydrate), olanzapine (2-methyl-4-(4-methyl-1-piper-azinyl)-10H-thieno[2,3-B][1,5]benzodiazepine), MDL 100,907 MDL 100,907, and 8-OHDPAT
Ki(nmol/L)
D2 5-HT2A 5-HT1A
Ziprasidone 4.8 0.42 3.4
Clozapine 83 16 120
Olanzapine 11 6.2 2,800
Haloperidol 0.71 45 1,100
MDL 100,907a .4,000 0.4 .1,000
8-OHDPAT 2,700 .5,000 1.4
Data from Seeger et al (1995).
(R-(1)-a -(2,3-dimethoxy-phenyl)-1-[2-(4-fluorophenyl)ethyl]-4-pi-peridinemethanol), and WAY-100635 (N -(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl-N-(2-pyridinyl)cyclohexane-carboxamide trihy-drochloride) were synthesized at Pfizer Inc., Central Research Division (Groton, CT). Clozapine (8-chloro-11-(4-methyl-1-piperazinyl)-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). Emul-phor 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 Chemika-BioChemika (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 percentage6 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 ex-pressed as areas under the curve, calculated from the percentages increase above baseline over a 5-hour period after drug admin-istration (area under the curve [AUC0 –5 hours] 6SEM). Dose–
response curves were analyzed by multiple comparisons with Tukey’s honestly significant difference generalized method us-ing orthogonal contrast to compare overall effects of like drugs and doses (cortex vs. striatum and vehicle 1 drug vs. WAY-1006351drug).
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 concen-trations 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 cor-tex 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 Figure 1. Time courses of the effects of 10 mg/kg orally administered (p.o.) zi-prasidone, 3.2 mg/kg p.o. clozapine, and 10 mg/kg p.o. olanzapine on extra-cellular levels of dopamine (DA) in the rat prefrontal cortex and striatum. Ar-rows indicate oral drug administration at t5 0. Data are expressed as per-centage of basal levels6 SEM (n5
3– 4). Statistical significance of DAcortexvs. DAstriatum: *p,.05 and
moderate doses of olanzapine (0.3–10 mg/kg p.o.) pro-duced comparable increases in cortical as in striatal DA release, but after 32 mg/kg p.o., maximal DA increases in
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 ef-fects 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)529.38,p,.0001; clozapine:F(1,14)525.40, p,.0002; olanzapine: F(1,25)530.5,p,.0001].
To investigate the role of 5-HT1Areceptor 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 antag-onist 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.), cloza-pine (1–10 mg/kg SC), and olanzacloza-pine (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.
Cortex and Striatum and in the Prefrontal Cortex after WAY-100635 (WAY) Pretreatment
Drug mg/kg
Maximal effect on DA release (% of basal6SEM)
Striatum Cortex
Cortex1WAY pretreatment
Vehicle 9969 108610 103612
Ziprasidone 0.3 89612 12468
1.0 12169 16466 14066
3.2 141619 176610 134618 10.0 14765 277617a 142612c 32.0 189610 372631a
Clozapine 1.0 12668 154612 13468
3.2 13363 350626 196610c
p.o.3.2 11069 203624b
10.0 175617 507621b 321640c
Olanzapine 0.3 14667 11764
1.0 129610 183613 3.2 150614 182616
10.0 225619 316620 298625 32.0 178623 416642a
Haloperidol 0.3 23868 172612a
MDL 100,907 1.0 10567 11266
8-OHDPAT 0.5 111612 293633b 11868c
Data are expressed as percentages of basal levels6SEM (n53–7). Ziprasi-done 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 DAcortexvs. DAstriatum:
ap,.05. bp,.01.
DAveh1drugvs. DAWAY1drug:
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)56.15,p5 .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 (p5.587). Furthermore, WAY-100635 did not significantly change the overall increases in striatal DA
release produced by 10 mg/kg ziprasidone, clozapine, or olanzapine (p5.68).
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 D2receptor antagonist
halo-peridol, 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, 8-OHDPAT 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
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 in-creases in cortical and striatal DA release, higher doses progressively enhance cortical DA release, resulting in Figure 3. Time courses of the effects of 10 mg/kg orally administered (p.o.) zi-prasidone, 3.2 mg/kg subcutaneous clo-zapine, 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, ex-pressed as percentage of basal DA lev-els6 SEM (n5 3–5). Statistical sig-nificance of DAvehicle 1 drug vs.
DA
WAY-1drug: *p,.05 and **p,.01.
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 WAY-100635. Data are expressed as area under the curve above basal DA levels over 5 hours after drug administration6SEM (n5
3– 6). Statistical significance of DAveh1drugvs. DAWAY1drug:
highly significant overall differences between the cortical and striatal effects of the three compounds. In contrast, the typical D2antagonist 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 halo-peridol 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; Volonte´ 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 accum-bens, whereas higher doses (10 mg/kg SC) increase cortical DA release more than striatal DA release (Kuroki et al 1999; Li et al 1998; Volonte´ 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 signifi-cant 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 5-HT1AReceptor 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-HT1Aagonists 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-HT1Areceptor activation in
the antipsychotic-induced enhancement of cortical DA release. We confirmed that the selective 5-HT1Aagonist 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 com-pletely 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,
WAY-100635 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 anti-psychotic-induced striatal DA increase is consistent with the finding that 5-HT1Areceptor 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 cloza-pine are unique among the clinically used antipsychotics in that both are 5-HT1A receptor agonists
(Newman-Tancredi et al 1998). In vitro, ziprasidone has high affinity for rat (Ki5 3.4 nmol/L) and human (Ki52.5 nmol/L)
Figure 5. Time courses of the effects of 0.3 mg/kg of the D2antagonist
haloper-idol, administered subcutaneously (SC); 1.0 mg/kg of the 5-HT2A antagonist
MDL 100,907 SC; and 0.5 mg/kg of the 5-HT1Aagonist 8-OHDPAT SC on
dopamine (DA) release in the rat pre-frontal cortex and striatum. Arrows in-dicate drug administration at t 5 0. Data are expressed as percentages of basal levels6SEM (n53– 4). Statis-tical significance of DAcortex vs.
5-HT1Areceptors 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-HT1Areceptors (Mason and Reynolds
1992; Newman-Tancredi et al 1996). Additional evidence for in vivo 5-HT1Areceptor agonist properties of
ziprasi-done was provided by a recent study that showed that the decrease in dorsal raphe cell firing produced by ziprasi-done, 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 recep-tors underlies the 5-HT1A–independent cortical DA
in-creases observed. Antagonism of D2 receptors probably does not play a major role in the cortical DA increases produced by ziprasidone, clozapine, and olanzapine, be-cause D2receptor 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,
be-cause all three antipsychotics are potent 5-HT2A/2C
recep-tor antagonists and blockade of these receprecep-tors 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-HT2Areceptor contributes to enhanced DA release in the prefrontal cortex by mixed 5-HT2A/2Cantagonists.
Alter-natively, it is conceivable that specific combinations of receptor interactions produce a preferential increase in cortical DA release, such as the simultaneous blockade of 5-HT2Aand D2receptors (Andersson et al 1995; Kuroki et
al 1999) or the combination of 5-HT2A antagonist and
partial 5-HT1Aagonist activity (Ishii et al 1999).
Clinical Significance
Increasing dopaminergic neurotransmission in the prefron-tal 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 antipsychot-ics. 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; Volonte´ 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 symp-toms 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-HT1Areceptor agonist
properties of clozapine (Mason and Reynolds 1992; New-man-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-HT1Areceptors not only increases prefrontal cortex DA
release but is also known to reduce D2receptor antagonist-induced 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; Christof-fersen 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).
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 treat-ment 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, Con-necticut, 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 perform-ing the statistical analyses of the data.
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