Main

Selective serotonin (5-HT) reuptake inhibitors (SSRIs) are widely used for the treatment for depression, but about 30–50% of patients do not initially respond to SSRIs (Ferrier, 1999; Nelson, 1999; Barbui and Hotopf, 2001). Clinical studies show that atypical antipsychotics such as risperidone and olanzapine are effective when added to an SSRI in the case of depression in which treatment with an SSRI alone is not effective (called treatment-resistant depression) (O'Connor and Silver, 1998; Ostroff and Nelson, 1999; Shelton et al, 2001). On the other hand, Zhang et al (2000) using in vivo microdialysis techniques demonstrated that combination of olanzapine with fluoxetine produced robust, sustained increases in extracellular dopamine (DA), norepinephrine (NE), and 5-HT levels in rat prefrontal cortex. These effects may contribute to the clinical effect of combination therapy of atypical antipsychotics with SSRI. However, the neurochemical mechanism for the combination effect appears to be complicated, since olanzapine has wide-ranging effects on numerous neurotransmitter systems (Bymaster et al, 1996; Schotte et al, 1996; Zhang and Bymaster, 1999).

SSRIs increase extracellular 5-HT levels and the increased 5-HT may interact with specific subtypes of 5-HT receptors. Previous studies show that 5-HT receptor subtypes modulate DA-D2/3 receptor blockade-induced DA release. Lucas and Spampinato (2000) reported that the DA-D2/3 receptor antagonist haloperidol-induced increase was reduced by the 5-HT2A receptor antagonist SR 46349B and potentiated by the 5-HT2A receptor agonist 1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane in the striatum. In contrast, Liégeois et al (2002) reported that haloperidol-induced DA release was potentiated by the 5-HT2A receptor antagonist M100907 in the prefrontal cortex. These results suggest that 5-HT2A receptors regulate haloperidol-induced DA release in a brain region-specific manner. With respect to 5-HT1A receptors, Ichikawa and Meltzer (1999) reported that the 5-HT1A receptor agonist 8-OH-DPAT potentiated the DA-D2/3 receptor antagonist sulpiride-induced DA release in the prefrontal cortex and the potentiation was abolished by the 5-HT1A receptor antagonist WAY100635. These results suggest that 5-HT1A receptor agonist potentiates prefrontal DA release in combination with DA-D2/3 receptor antagonism. However, it remains to be determined whether combination of typical antipsychotics and SSRIs affects the in vivo release of amine neurotransmitter including DA in the frontal cortex. The present study demonstrates that coadministration of sulpiride and fluvoxamine, an SSRI, selectively activates dopaminergic neurons in the prefrontal cortex, but not in the striatum. We also examined the mechanism for the coadministration-induced increase in prefrontal DA release.

MATERIALS AND METHODS

Animals and Drugs

Male Wistar rats weighing 250–350 g at the beginning of the experiments were used. The animals were maintained under controlled environmental conditions (22±1°C; 12–12 h light–dark cycle, lights on at 0800 h; food and water ad libitum) for at least 1 week before being used in the experiment. The procedures involving the animals and their care were conducted according to Guiding Principles for the Care and Use of Laboratory Animals approved by the Japanese Pharmacological Society. The following drugs were used: fluvoxamine (Solvay Seiyaku KK, Kawagoe, Saitama, Japan); sulpiride (Fujisawa Pharmaceutical Co., Osaka, Japan); N-{2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl}-N-(2-pyridinyl)cyclohexanecarboxamide (WAY100635) (Mitsubishi Pharma Co., Yokohama, Japan); haloperidol (Sigma, St Louis, MO, USA). All other chemicals used were of the highest commercially available purity. Fluvoxamine and WAY100635 were dissolved in saline (0.9% NaCl solution). Sulpiride was dissolved in 0.1 M HCl and was adjusted to pH 6–7 with 0.1 M NaOH. Haloperidol was dissolved in saline containing less than 0.1% v/v acetic acid. The drugs were i.p. injected at 1 ml/kg.

Microdialysis

Rats were anaesthetized with sodium pentobarbital (40 mg/kg, i.p.) and stereotaxically implanted with guide cannula (one site per animal) for the dialysis probe (Eicom, Kyoto, Japan) at the prefrontal cortex (A+3.2 mm, L−0.6 mm, V−5.2 mm, from the bregma and skull) or caudate putamen (A+0.2 mm, L−2.6 mm, V−7.0 mm) (Paxinos and Watson, 1986). The active probe membrane was 3 mm in length. On the day following surgery, the probe was perfused at a constant flow rate of 2 μl/min with Ringer's solution (147.2 mM NaCl, 4.0 mM KCl, and 2.2 mM CaCl2, Fuso Pharmaceutical Industries, Ltd., Osaka, Japan). A stabilization period of 3 h was allowed before the experiment. Two separate experiments were carried out for the determination of 5-HT, DA, and NE release as previously reported (Ago et al, 2002, 2003). In one experiment, 10 min microdialysis samples (20 μl) were taken and then immediately injected onto an HPLC column for a simultaneous assay of 5-HT and DA. In the other experiment, 30 min microdialysis samples (60 μl) were taken and then immediately injected onto an HPLC column for an NE assay.

For the assay of 5-HT and DA, an Eicompak PP-ODS column (4.6 mm i.d. × 30 mm: Eicom) was used, and the potential of the graphite electrode (Eicom) was set to+400 mV against an Ag/AgCl reference electrode. The mobile phase for the 5-HT and DA assay contained 100 mM sodium phosphate buffer (pH 6.0), 500 mg/l decanesulfonic acid, 134 μM EDTA, 1% (vol/vol) methanol. For the NE assay, an Eicompak CA-5ODS column (2.1 mm i.d. × 150 mm: Eicom) was used, and the potential of the graphite electrode (Eicom) was set to +500 mV against an Ag/AgCl reference electrode. The mobile phase for NE assay contained 100 mM sodium phosphate buffer (pH 6.0), 400 mg/l octanesulphonic acid, 134 μM EDTA, 5% (v/v) methanol.

After each experiment, microinjection (0.5 μl) of 1% Evans Blue dye was made through the cannula. The rats were decapitated, and the brains were removed and frozen in cold isopentane. Serial frozen sections were sliced, and the probe position was histologically verified.

Statistics

All microdialysis data were calculated as percent change from dialysate basal concentrations with 100% defined as the average of three fractions before drug administration. The statistical analyses were performed using one-way or two-way analysis of variance (ANOVA) for treatment as between-subjects factor and repeated measures with time as within-subject factor. The statistical analyses were performed using a software package (Stat View 5.0) for an Apple Macintosh computer. The P-values of 5% or less were considered statistically significant. The effects of vehicle treatments were not discussed, since they did not produce significant changes in the in vivo release of 5-HT, DA, and NE in rat prefrontal cortex and striatum.

RESULTS

In the brain microdialysis, basal levels of 5-HT, DA, and NE in dialysate (incorrected for in vitro probe recovery) were expressed as pg/fraction. The basal extracellular 5-HT, DA, and NE levels (means±SEM) in the prefrontal cortex were 0.77±0.06 pg/20 μl, 0.41±0.03 pg/20 μl, and 3.67±0.27 pg/60 μl, respectively (n=80 for 5-HT and DA, n=39 for NE) and striatum were 0.29±0.02 pg/20 μl, 4.62±0.70 pg/20 μl, and 0.91±0.09 pg/60 μl, respectively (n=22 for 5-HT and DA, n=21 for NE).

Figure 1 shows the effects of fluvoxamine and sulpiride alone and in combination on the in vivo release of 5-HT, DA, and NE in rat prefrontal cortex. Fluvoxamine at 10 and 20 mg/kg alone caused a sustained increase in 5-HT release (F(28, 224)=8.690; P<0.0001) (Figure 1a), and at 10 mg/kg caused a slight increase in NE release (F(7, 71)=8.842; P<0.0001). However, it did not affect DA release (F(28, 224)=0.869; NS). Sulpiride (10 mg/kg) alone did not affect the release of 5-HT (F(14, 119)=1.141; NS), DA (F(14, 119)=1.218; NS), and NE (F(7, 63)=1.147; NS). Coadministration of sulpiride at 3 and 10 mg/kg and fluvoxamine (10 mg/kg) significantly increased prefrontal DA release (F(28, 209)=3.196; P<0.0001), while sulpiride did not affect the fluvoxamine-induced increase in 5-HT (F(28, 209)=1.013; NS) or NE release (F(7, 63)=1.693; NS). The combination of sulpiride at 10 mg/kg and fluvoxamine at 20 mg/kg also produced the robust increase in DA release (F(14, 134)=8.511; P<0.0001), while sulpiride did not affect the fluvoxamine-induced increase in 5-HT release (F(14, 134)=0.737; NS). The increasing effect on DA release by sulpiride (10 mg/kg) was significantly greater in combination with 20 mg/kg of fluvoxamine than in that with 10 mg/kg of fluvoxamine (F(14, 134)=2.127; P<0.05).

Figure 1
figure 1

Effects of sulpiride and fluvoxamine alone and in combination on the in vivo release of 5-HT (a), DA (b), and NE (c) in rat prefrontal cortex. Sulpiride at 3 mg/kg (Sul 3) and 10 mg/kg (Sul 10) and fluvoxamine at 10 mg/kg (Flv 10) and 20 mg/kg (Flv 20) were i.p. administered at 0 time (arrow). Vehicle alone did not affect the extracellular levels of monoamines (not shown). Results are means±SEM of four to five rats.

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The augmentation effect on DA release by coadministration of DA-D2/3 antagonist and SSRI was also observed when haloperidol, another typical DA-D2/3 antagonist, was used instead of sulpiride. Figure 2a shows the effects of haloperidol and fluvoxamine alone and in combination on the in vivo release of 5-HT in rat prefrontal cortex. Haloperidol at 0.1 mg/kg alone did not affect the release of 5-HT (F(14, 119)=0.556; NS). The combination of haloperidol at 0.03 or 0.1 mg/kg and fluvoxamine at 10 mg/kg did not affect fluvoxamine-induced increase in 5-HT (F(28, 194)=1.278; NS). Figure 2b shows the effects of haloperidol and fluvoxamine alone and in combination on the in vivo release of DA in rat prefrontal cortex. Haloperidol at 0.1 mg/kg alone did not affect the release of DA (F(14, 119)=1.379; NS). The combination of haloperidol at 0.03 or 0.1 mg/kg and fluvoxamine at 10 mg/kg increased the DA release (F(28, 194)=3.144; P<0.0001). Figure 2c shows the effects of haloperidol and fluvoxamine alone and in combination on the in vivo release of NE in rat prefrontal cortex. Haloperidol at 0.1 mg/kg alone did not affect the release of NE (F(7, 63)=0.894; NS). The combination of haloperidol at 0.1 mg/kg and fluvoxamine at 10 mg/kg did not affect the fluvoxamine-induced increase in NE release (F(7, 71)=1.700; NS).

Figure 2
figure 2

Effects of haloperidol and fluvoxamine alone and in combination on the in vivo release of 5-HT (a), DA (b), and NE (c) in rat prefrontal cortex. Haloperidol at 0.03 mg/kg (Hal 0.03) and 0.1 mg/kg (Hal 0.1) and fluvoxamine at 10 mg/kg (Flv 10) were i.p. administered at 0 time (arrow). Vehicle alone did not affect the extracellular levels of monoamines (not shown). Results are means±SEM of four to five rats.

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The effects of sulpiride and fluvoxamine alone and in combination on the in vivo release of 5-HT, DA, and NE were examined in rat striatum (Figure 3). Fluvoxamine at 10 mg/kg increased 5-HT release (F(14, 149)= 17.230; P<0.0001) in the striatum, while sulpiride at 10 mg/kg decreased 5-HT release (F(14, 119)=4.266; P<0.0001). Sulpiride at 10 mg/kg did not affect the fluvoxamine (10 mg/kg)-induced increase in 5-HT release (F(14, 134)=0.466; NS). Sulpiride at 10 mg/kg increased the DA release (F(14, 119)=20.440; P<0.0001) in the striatum, while fluvoxamine at 10 mg/kg did not affect the DA release (F(14, 149)=1.272; NS). Fluvoxamine at 10 mg/kg did not affect the sulpiride (10 mg/kg)-induced increase in DA release (F(14, 119)=0.603; NS). The NE release in the striatum was not affected by fluvoxamine at 10 mg/kg (F(7, 63)=0.804; NS) and sulpiride at 10 mg/kg (F(7, 71) =0.257; NS) alone, or in combination (F(7, 31)= 1.297; NS).

Figure 3
figure 3

Effects of sulpiride and fluvoxamine alone and in combination on the in vivo release of 5-HT (a), DA (b), and NE (c) in rat striatum. Sulpiride at 10 mg/kg (Sul 10) and fluvoxamine at 10 mg/kg (Flv 10) were i.p. administered at 0 time (arrow). Vehicle alone did not affect the extracellular levels of monoamines (not shown). Results are means±SEM of four to five rats.

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Figure 4 shows the effect of systemic WAY100635 on sulpiride plus fluvoxamine-induced changes in the in vivo release of 5-HT and DA in rat prefrontal cortex. WAY100635 at 0.1 mg/kg did not affect 5-HT and DA release (F(14, 134)=1.465; NS for 5-HT, F(14, 134)=0.423; NS for DA). WAY100635 at 0.1 mg/kg significantly blocked the increase in DA release (F(14, 149)=5.757; P<0.0001), while it potentiated the increase in 5-HT release (F(14, 149)=5.586; P<0.0001). Figure 5 shows the effect of local application of WAY100635 on sulpiride plus fluvoxamine-induced changes in the in vivo release of 5-HT and DA in the prefrontal cortex. Local application of WAY100635 at 10 μM via the microdialysis probe during the experiment did not affect the combination-induced increases in 5-HT and DA release (F(14, 119)=0.863; NS for 5-HT, F(14, 119)=0.653; NS for DA).

Figure 4
figure 4

Effect of systemic WAY100635 on the changes in the in vivo release of 5-HT and DA induced by fluvoxamine plus sulpiride combination in rat prefrontal cortex. Fluvoxamine at 10 mg/kg and sulpiride at 10 mg/kg (Flv 10 + Sul 10) were i.p. administered at 0 time (arrow). WAY100635 at 0.1 mg/kg (WAY 0.1) was injected simultaneously with fluvoxamine and sulpiride. WAY100635 alone did not affect the release of 5-HT and DA. Results are means±SEM of four to five rats.

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Figure 5
figure 5

Effect of local application of WAY100635 on the changes in the in vivo release of 5-HT and DA induced by fluvoxamine plus sulpiride combination in rat prefrontal cortex. Rats were i.p. injected with fluvoxamine at 10 mg/kg plus sulpiride at 10 mg/kg (Flv 10+Sul 10) at 0 time (arrow). Ringer's solution (open circles) or WAY100635 (WAY) at 10 μM (closed circles) was perfused into the cortex via a dialysis probe for the time indicated by the horizontal bar. Results are means±SEM of three to five rats.

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Figure 6 shows the combination effect of local application of fluvoxamine and systemic administration of sulpiride on the in vivo release of 5-HT and DA in rat prefrontal cortex. Local application of fluvoxamine at 50 nM via the microdialysis probe during the experiment increased 5-HT release (F(14, 59)=7.803; P<0.0001), while it did not affect DA release (F(14, 59)=1.321; NS). These effects of local fluvoxamine were not affected by systemic administration of sulpiride at 10 mg/kg (F(14, 119)=0.599; NS for 5-HT, F(14, 119)=0.603; NS for DA). Figure 7 shows the combination effect of local application of sulpiride and systemic administration of fluvoxamine on the in vivo release of 5-HT and DA in rat prefrontal cortex. Local application of sulpiride at 10 μM via the microdialysis probe during the experiment did not affect the 5-HT and DA release (F(14, 59)=1.514; NS for 5-HT, F(14, 59)=0.711; NS for DA). Local sulpiride did not affect the fluvoxamine-induced increase in 5-HT release (F(14, 134)=1.094; NS), while the combination of local, like systemic, sulpiride and fluvoxamine significantly increased the DA release (F(14, 59)=7.282; P<0.0001). The results of the present study are summarized in Table 1.

Figure 6
figure 6

Effect of local application of fluvoxamine plus systemic sulpiride combination on the in vivo release of 5-HT and DA in rat prefrontal cortex. Rats were i.p. injected with vehicle (open circles) or sulpiride at 10 mg/kg (closed circles) (Sul 10) at 0 time (arrow). Fluvoxamine (Flv) at 50 nM was perfused into the cortex via the dialysis probe for the time indicated by the horizontal bar. Results are means±SEM of four rats.

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Figure 7
figure 7

Effect of systemic fluvoxamine plus local application of sulpiride combination on the in vivo release of 5-HT and DA in rat prefrontal cortex. Rats were i.p. injected with fluvoxamine at 10 mg/kg (Flv 10) at 0 time (arrow). Ringer's solution (open circles) or sulpiride (Sul) at 10 μM (closed circles) was perfused into the cortex via a dialysis probe for the time indicated by the horizontal bar. Results are means±SEM of four to five rats.

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Table 1 Summary of the Findings of the Present Study
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DISCUSSION

Zhang et al (2000) reported that the atypical antipsychotics olanzapine in combination with SSRI produced robust, sustained increases in extracellular levels of DA and NE. This effect may explain the clinical effectiveness of the combination therapy for treatment-resistant depression. The neurochemical mechanism for the increases in extracellular levels of monoamines, however, remains unknown. Olanzapine interacts with many receptors other than DA-D2/D3 receptors, while the typical antipsychotic sulpiride is a selective antagonist of DA-D2/D3 receptors (Sokoloff et al, 1990). The present study examined the effect of sulpiride instead of olanzapine on extracellular levels of monoamines to clarify the role of DA-D2/D3 receptor blockade in the synergistic effect of antipsychotic and SSRI. Sulpiride at 10 mg/kg alone did not affect the extracellular levels of these monoamines in the prefrontal cortex. Furthermore, fluvoxamine, an SSRI, at 10 mg/kg alone did not affect extracellular levels of DA in the prefrontal cortex, while it caused a marked increase in extracellular 5-HT levels and a slight increase in the NE levels. Under these conditions, the combination of sulpiride and fluvoxamine caused a marked increase in extracellular DA levels in the prefrontal cortex, while sulpiride did not affect the fluvoxamine-induced increases in the 5-HT and NE levels. The role of DA-D2/D3 receptor blockade in the combination effect is also supported by the observation that haloperidol, like sulpiride, in combination with fluvoxamine increases cortical DA release. In contrast, Zhang et al (2000) reported that the combination of haloperidol with fluoxetine did not increase prefrontal DA levels more than fluoxetine alone. This may be due to a difference in the dose of haloperidol used between the previous (1.0 mg/kg haloperidol) and present (0.03 and 0.1 mg/kg) studies. In this line, Lucas et al (2000) reported that the SSRI citalopram enhanced striatal DA release induced by 0.01 mg/kg, but not 1.0 mg/kg, of haloperidol. These opposite effects of haloperidol may be explained by the effect on presynaptic or postsynaptic DA-D2/D3 receptors. Taken together, the present findings suggest that DA-D2/D3 receptor blockade in combination with 5-HT reuptake inhibition causes a selective increase in DA release in the prefrontal cortex.

Fluvoxamine increases extracellular 5-HT levels not only in the prefrontal cortex but also in other brain regions including the raphe nuclei where serotonergic cell bodies are localized. We first examined whether the increased 5-HT levels in the prefrontal cortex are involved in the effect of the combination of sulpiride and fluvoxamine. The fluvoxamine-induced increase in extracellular levels of 5-HT in the prefrontal cortex was not affected by sulpiride. This observation suggests that the increased levels of prefrontal 5-HT are not involved in the effect of the combination of sulpiride and fluvoxamine. In agreement with this suggestion, the combination of local application of fluvoxamine in the prefrontal cortex, which increased extracellular levels of 5-HT, and systemic sulpiride did not increase the extracellular levels of DA. This result suggests that the increased 5-HT in the prefrontal cortex does not play a role in the combination effect: the sites of action of systemic fluvoxamine which in combination with sulpiride increase DA release are regions other than the prefrontal cortex.

We secondly examined what kind of 5-HT receptor subtypes may be involved in the combination-induced increase in DA release in the prefrontal cortex. We have previously found that 5-HT1A receptor activation causes DA release in the frontal cortex (Sakaue et al, 2000). Furthermore, Ichikawa et al (2001) reported that a combination of the 5-HT2A antagonist M100907 and sulpiride increased DA release in rat prefrontal cortex and this effect was blocked by a low dose of WAY100635. In view of these observations, the present study examined the possible involvement of 5-HT1A receptors in the combination-induced increase in DA release in the prefrontal cortex. We found that the combination effect of sulpiride and fluvoxamine was blocked by a low dose of WAY100635, a 5-HT1A receptor antagonist. In this experiment, WAY100635 markedly enhanced the combination-induced increase in 5-HT release. This is in agreement with the previous observations that WAY100635-induced inhibition of presynaptic 5-HT1A autoreceptors enhances SSRI-induced 5-HT release (Romero et al, 1996; Hjorth et al, 1997; Dawson and Nguyen, 1998). In view of the previous microdialysis study that local application of 5-HT facilitates DA release in the prefrontal cortex (Iyer and Bradberry, 1996), presynaptic 5-HT1A receptor-mediated decrease in 5-HT release may lead to decrease in cortical DA release. It then appears that presynaptic 5-HT1A receptor activation in the raphe is not involved in the combination-induced increase in cortical DA release. However, the present study showed that the combination effect was not affected by local application of WAY100635. This suggests that 5-HT1A receptors responsible for combination-induced increase in cortical DA release are localized in regions other than the prefrontal cortex. This supports the point discussed above that the sites of action of systemic fluvoxamine, which in combination with sulpiride increase DA release, are regions other than the prefrontal cortex. Furthermore, the combination effect was blocked by a low dose of systemic WAY100635 which blocks preferentially presynaptic 5-HT1A receptor-mediated responses (Hajós-Korcsok et al, 1999; Gobert et al, 1999; Ago et al, 2003). These findings suggest that fluvoxamine-induced activation of 5-HT1A receptors, presumably presynaptic 5-HT1A receptors, is involved in the combination effect of fluvoxamine and sulpiride in increasing DA release.

We also examined the site of action of sulpiride in the effect of the combination of sulpiride and fluvoxamine. Previous studies show that DA autoreceptors in the prefrontal cortex are coupled to regulation of the release of DA (Wolf and Roth, 1987; Bean et al, 1990), and prefrontal DA-D2/D3 receptors function as DA autoreceptors (Gobert et al, 1995; Koeltzow et al, 1998). The present study showed that a local application of sulpiride in the cortex, like systemic sulpiride, with systemic fluvoxamine increased extracellular levels of DA. The finding suggests that prefrontal DA-D2/3 receptor blockade plays a role in the combination effect of sulpiride and fluvoxamine in increasing DA release. That is, fluvoxamine increases cortical DA release under the condition of prefrontal DA-D2/3 receptor blockade. The effect of fluvoxamine may be explained by activation of 5-HT1A receptors in regions other than the prefrontal cortex as discussed above. Fluvoxamine-induced increase in 5-HT activates 5-HT1A autoreceptors which reduce serotonergic activity. This may disinhibit dopaminergic activity in the ventral tegmental area. This may lead to an increase in DA release in the prefrontal cortex, to which the ventral tegmental area projects dopaminergic neurons.

Koch et al (2004) have recently demonstrated that the combination effect of olanzapine and fluoxetine on monoamines is observed only in select brain regions such as the prefrontal cortex and hypothalamus. We also observed that the combination of sulpiride and fluvoxamine did not affect extracellular DA levels in the striatum. The region-specific effect of the combination appears to be of significance as discussed by Koch et al (2004). The prefrontal cortex plays an important role in mediating affect and cognition and has been considered to control behaviors with relevance to affect and reward (Miller and Cohen, 2001). The regional specificity for regulation of DA release is also reported in the effect of coadministration of 8-OH-DPAT and sulpiride on DA release (Ichikawa and Meltzer, 1999).

In general, drug metabolism plays a key role in changes in the pharmacological effects induced by the combined administration of two drugs. However, it is unlikely that the effect of the combination of sulpiride and fluvoxamine on DA release is due to pharmacokinetic interactions. In this study, we found that sulpiride did not affect fluvoxamine-induced increase in 5-HT and NE levels. The observation suggests that sulpiride does not affect metabolism of fluvoxamine. Furthermore, we showed that fluvoxamine did not increase the sulpiride-induced increase in the DA levels in the striatum. The result suggests that fluvoxamine does not affect the metabolism of sulpiride. In this line, Imondi et al (1978) reported that most of the sulpiride administered was the unchanged form in the urine and feces in human and monkeys.

Sulpiride has antidepressant activity at doses that are 4–5 times lower than usual neuroleptic values (Benkert and Holsboer, 1984; Maier and Benkert, 1994; Vergoni et al, 1995), and the effect is considered to be mediated by enhanced dopaminergic activity (Serra et al, 1990). The present study shows that coadministration of sulpiride and fluvoxamine causes a selective increase in prefrontal DA release. In view of the previous reports that various drugs with antidepressant potentials increase extracellular DA levels selectively in the rat prefrontal cortex (Tanda et al, 1994, 1996), the present finding suggests that the combination may have an antidepressant effect. In agreement with the suggestion, Renard et al (2001) reported that sulpiride significantly increased the anti-immobility effects of the SSRIs fluvoxamine and paroxetine in a mouse-forced swimming test. In addition, we have recently found in the separate experiment that the combination of sulpiride and fluvoxamine is more effective than either drug alone in reducing the immobility time in a mouse tail suspension test (unpublished). On the other hand, previous studies suggest that atypical antipsychotic drugs may improve negative symptoms and cognitive dysfunction in schizophrenia partly via increasing cortical DA release (Moghaddam and Bunney, 1990; Kuroki et al, 1999; Meltzer and McGurk, 1999). Then, the present neurochemical study implies that the combination of sulpiride and fluvoxamine, like 5-HT1A receptor agonist combined with sulpiride (Ichikawa and Meltzer, 1999), is effective not only for treatment of depression but also for improvement of cognitive dysfunction in schizophrenia.