Modulation of [18F]fluorodopa (FDOPA) kinetics in the brain of healthy volunteers after acute haloperidol challenge

www.elsevier.com/locate/ynimg NeuroImage 30 (2006) 1332 – 1339

Modulation of [18F]fluorodopa (FDOPA) kinetics in the brain of healthy volunteers after acute haloperidol challenge Ingo Vernaleken,a,e,* Yoshitaka Kumakura,b Paul Cumming,b Hans-Georg Buchholz,c Thomas Siessmeier,c Peter Stoeter,d Matthias J. Mu¨ller,e Peter Bartenstein,c and Gerhard Gru¨nder a,e a

Department of Psychiatry and Psychotherapy, RWTH Aachen University, Aachen, Germany PET Centre, Aarhus University Hospitals and Centre for Functionally Integrated Neuroscience, Aarhus, Denmark c Department of Nuclear Medicine, University of Mainz, Mainz, Germany d Department of Neuroradiology, University of Mainz, Mainz, Germany e Department of Psychiatry, University of Mainz, Mainz, Germany b

Received 9 February 2005; revised 3 November 2005; accepted 9 November 2005 Available online 24 January 2006 In animal studies, acute antipsychotic treatment was shown to enhance striatal DOPA-decarboxylase (DDC) activity. However, this phenomenon has not been demonstrated in humans by positron emission tomography (PET). Therefore, we investigated acute haloperidol effects on DDC activity in humans using [18F]fluorodopa (FDOPA) PET. Nine healthy volunteers were scanned with FDOPA in drug-free baseline conditions and after 3 days of haloperidol treatment (5 mg/day). A continuous performance test (CPT) was administered in both conapp ditions. The net blood – brain clearance of FDOPA (K in ) in striatum, mesencephalon, and medial prefrontal cortex was calculated by app volume-of-interest analysis. The macroparameter K in is a composite of several kinetic terms defining the distribution volume of FDOPA in brain (VeD) and the relative activity of DOPA decarboxylase (k D 3 ). Therefore, compartmental kinetic analysis was used to identify the app physiological basis of the observed changes in K in period. The app magnitude of K in was significantly increased in the putamen (18%) and mesencephalon (36%). Furthermore, VeD in the brain was increased by 15%. Increments of k D 3 in the basal ganglia did not attain statistical significance. The significant worsening of CPT results did not correlate with changes in FDOPA utilization. The present PET results indicate potentiation of FDOPA utilization in human basal ganglia by acute haloperidol treatment, apparently due to increased availability throughout the brain. The stimulation of DDC cannot be excluded due to insufficient statistical power in the estimation of k D 3 changes. D 2005 Elsevier Inc. All rights reserved.

* Corresponding author. Department of Psychiatry and Psychotherapy, RWTH Aachen University, Pauwelsstrasse 30, 52074 Aachen, Germany. Fax: +49 241 80 3389654. E-mail address: [email protected] (I. Vernaleken). Available online on ScienceDirect (www.sciencedirect.com). 1053-8119/$ - see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.neuroimage.2005.11.014

Introduction The acute pharmacological blockade of dopamine receptors leads to increased synthesis and metabolism of dopamine in brain tissue, as shown by an increase in the concentrations of dopamine metabolites in brain, cerebrospinal fluid (CSF), or urine (Carlsson and Lindqvist, 1963; Karoum et al., 1994; O’Keeffe et al., 1970). This phenomenon is attributed to the pharmacological blockade of presynaptic autoreceptors regulating dopamine synthesis and release from dopamine fibers. Activation of the enzyme tyrosine hydroxylase, the penultimate enzyme in the biosynthesis of dopamine, is also provoked in the brain by acute treatment with antipsychotics (el Mestikawy et al., 1986; Lerner et al., 1977; Onali et al., 1988). In recent years, it has become apparent that the activity of the ultimate enzyme in the pathway, DOPA decarboxylase, can also influence the overall rate of dopamine synthesis in brain (Gjedde et al., 1993; Opacka-Juffry and Brooks, 1995). Indeed, acute treatment with antipsychotics activates DOPA decarboxylase measured in homogenates from rodent striatum, suggesting modulation by presynaptic autoreceptors (Cho et al., 1997; Cumming et al., 1997; Hadjiconstantinou et al., 1993; Zhu et al., 1992). In studies of the decarboxylation of [3H]DOPA to [3H]dopamine in the living rat brain, acute treatment with an antipsychotic likewise increased the activity of DOPA decarboxylase in striatum (Cumming et al., 1997), whereas treatment with MAO inhibitor decreased the activity (Cumming et al., 1995), possibly due to increased occupancy of autoreceptors by dopamine. Furthermore, in an FDOPA-PET study, acute treatment of pigs with haloperidol stimulated the striatal activity of DOPA decarboxylase (Danielsen et al., 2001). Thus, the observed modulations of DOPA decarboxylase activity in the animal brain are consistent with regulation of the enzyme by presynaptic dopamine autoreceptors. Acute stimulation of dopamine synthesis capacity followed by the later decrease in spontaneously firing dopamine cells (depo-

I. Vernaleken et al. / NeuroImage 30 (2006) 1332 – 1339

larization block) may be an important mechanism of antipsychotic action in the treatment of schizophrenia (Grace and Bunney, 1986; Pickar, 1988). Whereas the initial activation of DOPA decarboxylase by antipsychotics seems well-documented in experimental animals, little is known about the corresponding effects in the human brain. In a recent 6-[18F]-l-m-tyrosine/PET study, the effect of acute risperidone on DOPA decarboxylase activity was tested in human volunteers (Mamo et al., 2004); the treatment did not discernibly alter the tracer utilization in human striatum, calculated using a reference tissue method, although the authors reported increased retention of this tracer in the striatum of rats treated with higher doses of antipsychotic drugs. However, the choice of the radioligand and the atypical features of risperidone might be responsible for the negative findings in the human PET study. We hypothesized that acute challenge with haloperidol enhances the utilization of FDOPA in the brain of healthy volunteers. To test this hypothesis, the net blood – brain clearance of FDOPA (K inapp) (Martin et al., 1989) was mapped in nine volunteers in a baseline condition and after a 3-day treatment with haloperidol at a therapeutic dose for the treatment of schizophrenia. The macroparameter K inapp is a composite of several kinetic terms defining the distribution volume of FDOPA in the brain (V eD) and the magnitude of k 3D. Consequently, the physiological basis of pharmacologically-evoked changes in the magnitude of K inapp can be ambiguous (Kumakura et al., 2004). Therefore, a constrained compartmental kinetic analysis (Cumming and Gjedde, 1998; Gjedde et al., 1991; Hoshi et al., 1993; Huang et al., 1991) was used to identify the effects of haloperidol treatment on specific steps in the distribution and metabolism of FDOPA in the brain of the subjects. Additionally, the influence of haloperidol on cognitive functions (sustained attention) and extrapyramidal side effects as well as possible correlations between changes in these clinical parameters and the FDOPA uptake were investigated.

1333

biperidene. Side effect ratings and neuropsychological tests had been carried out under baseline conditions and directly before the second PET scan.

Neuropsychological testing and side effect ratings All subjects were assessed for extrapyramidal symptoms (EPS) in the baseline and haloperidol conditions using the Simpson – Angus scale (Simpson and Angus, 1970) with 10 items each ranging from 1 (no complaints) to 5 (severe side effects). In order to test for effects of treatment on cognitive function (sustained attention), we performed the continuous performance test before and during haloperidol treatment (Rosvold et al., 1956). We used the computerized Munich version (CPT-M) of the continuous performance test (Kathmann et al., 1996). During this test, 480 blurred digits are presented successively on a 15-inch monitor for 42 ms. Between every presentation, an interstimulus interval of one second was interposed. The task was to press a button as quickly as possible every time the digit ‘‘0’’ (25% of all stimuli, randomly distributed) was presented. A training phase of 160 presentations preceded the main task. Outcome measures were the mean reaction time, the percentage of correctly identified ‘‘0’’ digits (hits), the non¯ )), the sensitivity (dV), and the parametrical sensitivity ( P(A ¯ decision bias (ln(b)). P(A), dV, and ln(b) are parameters according to signal detection theory (SDT), calculated as described previously (Aaronson and Watts, 1987). The sensitivity indices describe the subject’s ability to differentiate between target and non-target stimuli, while the response criterion expresses the amount of evidence the subject requires in deciding whether a ¯ ) and dV are highly given stimulus is a target. In general, P(A correlated (r = 0.90 – 0.95).

PET data acquisition Materials and methods The study was approved by the Ethics Committee of the University of Mainz and the German radiation safety authorities in accordance with national and international standards.

Subjects Nine male subjects gave written, informed consent to participate in the study. All subjects underwent physical and mental-state examinations and were found to be free of any mental disorder. They reported no intake of drugs, in particular centrally-acting agents, for at least 6 weeks. None of the subjects suffered from clinically significant somatic or neurological complaints. The subjects’ age ranged from 23 to 64 years (mean T SD: 34 T 14 years). A T1-weighted 3D gradient echo magnetic resonance scan was performed to check for possible anatomical abnormalities and for anatomical co-registration (vide infra). Two PET scans with FDOPA were performed, one at baseline and one after treatment with haloperidol. Every subject received haloperidol 5 mg/day as a single oral dose at the evening over a period of 3 days prior to the second scan. All subjects were admitted to the psychiatric ward for observation during the period of haloperidol treatment. Directly after the second PET scan, one subject suffered an episode of torticollis which resolved immediately after treatment with

Decarboxylation of FDOPA in peripheral tissues was blocked by oral administration of carbidopa (Merck Sharp and Dome, 2 mg/kg body weight) 1 h prior to the injection of FDOPA. All PET recordings were obtained with the Siemens ECAT EXACT whole-body PET, which has a field-of-view of 16.2 cm in 47 planes, an interplane spacing of 3.375 mm, and an axial resolution of 5.4 mm FWHM. After a brief attenuation scan, a sequence of 30 emission frames lasting a total of 124 min was recorded. Frame length increased progressively according to the following schedule: 3  20 s; 3  1 min, 3  2 min, 3  3 min, 15  5 min, and 3  10 min. A mean of 200 MBq FDOPA (SD: 40.7 MBq, range: 137 – 263 MBq) was injected intravenously as a bolus into a cubital vein. During the first 10 min after FDOPA injection, radioactivity concentration in blood from a radial artery was recorded at intervals of 1 s using an on-line gamma counter cross-calibrated to the tomograph; thereafter, a series of 15 arterial blood samples were drawn manually according to the following schedule: 2  2 min, 2  3 min, 2  5 min, and 9  10 min, and blood radioactivity concentration was measured using well-counter. The fractions of untransformed FDOPA and its major plasma metabolite 3-O-methyl-[18F]fluorodopa (OMFD) were measured by reverse-phase high performance liquid chromatography (Cumming et al., 1993) in plasma samples prepared from arterial blood collected at 5, 10, 15, 20, 30, 45, 60, 90, and 120 min. The

1334

I. Vernaleken et al. / NeuroImage 30 (2006) 1332 – 1339

continuous input functions of plasma FDOPA and OMFD were calculated by interpolation of the bi-exponential function to the measured fractions (Gillings et al., 2001), and were then used to calculate, using a non-linear regression, the magnitudes of the apparent whole body activity of catechol-O-methyltransferase with respect to FDOPA (k 0, min 1) and the rate constant for the elimination of OMFD from plasma (k 1, min 1), an index of renal function (Cumming et al., 1993). The entire dynamic PET sequences, consisting of 30 frames, were realigned and corrected for interframe head-motion using a rigid-body transformation with six degrees of freedom, and employing an MRI derived 4D-template specific for FDOPA (Kumakura et al., 2004; Reilhac et al., 2003). After motion correction, individual summed emission images were calculated. The summed images of the baseline condition were registered to the MNI stereotaxic brain, using the affine transformation of the AIR algorithm (Woods et al., 1992) in order to fit to a grey matter MRI template of MNI average brain (Collins et al., 1998) with its striatum intensity doubled in order to emulate the radioactivity distribution of the summed FDOPA emission images. Likewise, the summed images of treatment condition were also registered to the MNI stereotaxic brain after interscan head realignment using rigidbody transformation in order to avoid interscan misregistration of the images. After resampling of the dynamic emission recordings, time-radioactivity curves were extracted by binary masking ROIs of bilateral putamen (9.7 cm3), bilateral caudate nucleus (8.5 cm3), bilateral medial frontal cortex (35.3 cm3), midbrain (5.9 cm3), and cerebellum (48.3 cm3) placed on the probabilistic anatomical map. Parametric maps of the net blood – brain clearance of FDOPA (K inapp) were obtained by linear graphical analysis of data recorded in the interval 16 – 54 min, with subtraction of the time-radioactivity curve measured in cerebellum (Martin et al., 1989). The compartmental model used for the kinetic analysis of FDOPA cerebral uptake and metabolism is illustrated in Fig. 1. In brief, the magnitude of the apparent distribution volume of FDOPA in cerebellum (VeD, ml g 1) was estimated by non-linear regression of a onecompartment model, in which the plasma volume was fixed at 0.05 ml g 1. OMFD and FDOPA in plasma were assumed to have a permeability ratio ( q) equal to 1.5, and furthermore, were assumed to have a common distribution volume in the brain. The rate constant for DOPA decarboxylase (k 3D, min 1) was assumed to be nil in cerebellum. A two-compartment model with VeD fixed to the observation in cerebellum (Gjedde et al., 1991; Huang et al., 1991) and the plasma volume fixed at 0.05 ml g 1 was fitted to the first 60 min of the time-radioactivity curves recorded in caudate, putamen, and mesencephalon in order to estimate the magnitude of k 3D. The assumptions underlying this constrained reduction of the FDOPA model are discussed in detail elsewhere (Cumming and Gjedde, 1998).

Statistical analyses Means and standard deviations were calculated for FDOPA kinetic estimates by region, and for EPS and cognitive test scores. Differences between baseline and endpoint values were analyzed by means of Wilcoxon matched-pairs signed-ranks tests. Proportional change scores ([endpoint baseline] / baseline * 100) relative to baseline estimates were computed for k 3D, K inappestimates, neuropsychological, and EPS parameters. Spearman rank correlations were calculated for treatment-related changes in

Fig. 1. A scheme describing FDOPA and OMFD kinetics in plasma and the brain. The total radioactivity concentration in a brain region is distributed into three compartments: intravascular space, extravascular tissue (precursor pool), and metabolite compartment (trapped tracer). FDOPA in plasma is metabolized by catechol-O-methyltransferase at apparent rate constant k D 0 (min 1), and the product OMFD is eliminated from circulation at rate 1 constant k M 1 (min ). The model defines the unidirectional blood – brain M clearances (ml g 1 min 1) of FDOPA (K D 1 ) and OMFD (K 1 ), and the M corresponding rate constants for clearance back to circulation (k D 2 , k2 ; min 1). The coefficient of brain tissue methylation of FDOPA, k D , 5 is assumed to be negligible throughout the brain. The magnitude of the rate 1 constant for decarboxylation of [18F]fluorodopa (k D 3 , min ) is assumed to 18 be zero in cerebellum. The [ F]fluorodopamine formed in striatum is and its subsequent metabolites are assumed to diffuse from the brain as a single compartment (k loss, min 1) (modified after Gjedde et al., 1991).

kinetic outcome parameters and cognitive tests and EPS scores. In all analyses, the two-tailed level of statistical significance was set at a = 0.05. Due to the exploratory character of the study, no adjustment for multiple testing was performed.

Results The mean rate constant for the O-methylation of FDOPA in plasma (k 0D) increased from 0.0125 T 0.0027 min 1 in the baseline condition to 0.0158 T 0.0038 min 1 after haloperidol ( P = 0.011), while the mean rate constant for elimination of OMFD from plasma (k 1M) did not differ in the baseline (0.0208 T 0.0045 min 1) and haloperidol (0.0213 T 0.0059 min 1) conditions. The mean estimate of the distribution volume of FDOPA in cerebellum was 0.79 T 0.16 ml g 1 in the baseline and 0.91 T 0.20 ml g 1 after haloperidol ( P = 0.038; two-tailed Wilcoxon test). Mean parametric maps of K inapp suggest that the treatment enhanced the net clearance of FDOPA to the basal ganglia (Fig. 2). The mean magnitudes of K inapp and k 3D in caudate, putamen, midbrain, and medial frontal cortex (where k 3D could not be calculated) are listed in Table 1. Whereas the mean magnitudes of k 3D were not significantly changed by treatment, the mean magnitude of K inapp increased by 18% in putamen ( P = 0.038) and by 36% in the midbrain ( P = 0.038) under the haloperidol condition. Neuropsychological findings are summarized in Table 2. Neuropsychological test results showed significant treatment effects on the sensitivity index (dV) and the non-parametric sensitivity ¯ )) of the continuous performance test. Reaction time and index ( P(A number of hits were unaffected by the haloperidol challenge. Baseline test scores and difference in test scores between the two

I. Vernaleken et al. / NeuroImage 30 (2006) 1332 – 1339

1335

Fig. 2. The net influx of FDOPA to the brain (K inapp) in a paramedian sagittal (left), horizontal (middle), and coronal (right) planes of healthy control subjects in a baseline condition (top) and after haloperidol treatment (bottom). Each map is the mean of nine determinations, transformed into the common stereotaxic space. Images were blurred by Gaussian filter with a FWHM of 6 mm.

conditions did not correlate significantly with individual changes in either K inapp or k 3D in any brain region. Extrapyramidal symptoms documented by the SAS were on average 10.6 T 1.1 before and 21.4 T 6.6 after treatment (increase by 102% T 62%; P = 0.012). There were no statistically significant correlations (Spearman) between changes in K inapp or k 3D and the severity of extrapyramidal side effects evoked by the treatment (caudate nucleus: r = 0.50, P = 0.17; putamen: r = 0.33, P = 0.39; medial frontal cortex: r = 0.37, P = 0.33; midbrain: r = 0.37, P = 0.33).

Discussion Antagonism of pre- and postsynaptic dopamine D2-receptors is a property of all clinically effective antipsychotic compounds.

However, the underlying mechanism resulting in antipsychotic efficacy is not entirely understood and the simple interpretation that alleviation of positive symptoms of schizophrenia is due to blockade of dopamine receptors is called into question. In particular, a majority of authors emphasize a substantial delay in clinical antipsychotic response, although this was recently questioned in a metaanalysis of antipsychotic treatment investigations (Agid et al., 2003). Furthermore, approximately one-third of psychotic patients fail to respond adequately to dopamine antagonistic therapies, even with daily doses of antipsychotics resulting in 80 – 90% occupancy of dopamine D2/3 receptors (Essock et al., 1996; Juarez-Reyes et al., 1995). Therefore, there is no simple relationship between attenuation of psychotic symptoms and blockade of dopamine neurotransmission. Electrophysiological studies of rats treated chronically with antipsychotic drugs have revealed a substantial decrease in

Table 1 K inapp and VeD pre- and posthaloperidol Baseline K inapp Caudate nucleus Putamen Medial prefrontal cortex Midbrain

(ml g

Haloperidol 1

1

min )

kD 3

1

(min )

K inapp (ml g

1

min 1)

1 kD 3 (min )

0.0075 T 0.0019 (0.0056 – 0.0119) 0.0118 T 0.0022 (0.0085 – 0.0146) 0.0003 T 0.0009 ( 0.0011 – 0.0015)

0.0331 T 0.0053 (0.0219 – 0.0395) 0.0439 T 0.0042 (0.0382 – 0.0509) n.d.a

0.0080 T 0.0014 (+7%; n.s.) (0.0063 – 0.0110) 0.0139 T 0.0034 (+18%; P = 0.038) (0.0104 – 0.0202) 0.0003 T 0.0011 (+10%; n.s.) ( 0.0016 – 0.0022)

0.0366 T 0.0164 (+10%; n.s.) (0.0194 – 0.0699) 0.0484 T 0.0125 (+10%; n.s.) (0.0357 – 0.0651) n.d.a

0.0031 T 0.0016 (0.0011 – 0.0052)

0.0083 T 0.0037 (0.0043 – 0.0139)

0.0042 T 0.0015 (+36%; P = 0.038) (0.0023 – 0.0074)

0.0105 T 0.0045 (+27%; n.s.) (0.00577 – 0.167)

app The effect of haloperidol on the net blood – brain clearance of FDOPA (K in , ml g 1 min 1) and the apparent decarboxylation rate constant for FDOPA (k D 3, 1 min ) in caudate, putamen, medial prefrontal cortex, and midbrain. The mean (mean T SD) of nine determinations in healthy volunteers is presented, along with the range of individual estimates. The effect of treatment on the magnitude of K inapp was significant in the putamen and midbrain (2-tailed Wilcoxon test). n.d. = not detectable. a Not detected.

1336

I. Vernaleken et al. / NeuroImage 30 (2006) 1332 – 1339

Table 2 Haloperidol-induced changes in CPT—results CPT

Pretreatment Posttreatment Difference Significance

Reaction (ms)

Hits (%)

¯ )a P(A

dVb

log(b)c

571 T 178 596 T 178 25.7 (4%) n.s.

36 T 22.1 32 T 18.1 3.1 ( 9%) n.s.

0.8 T 0.1 0.7 T 0.1 0.03 ( 3%) P = 0.038

1.3 T 0.7 1.2 T 0.7 0.16 ( 12%) P = 0.028

1.3 T 0.7 1.4 T 1.3 0.03 (3%) n.s.

Baseline and posthaloperidol (for 3 days 5 mg/day) outcome parameters of the continuous performance test (CPT) in nine healthy volunteers (2-tailed Wilcoxon test). Depicted are the average pre- and posttreatment values, their corresponding standard deviations, and the resulting differences. Statistical significance was tested by 2-tailed Wilcoxon rank order test. a Non-parametric sensitivity index. b Sensitivity index. c Decision bias.

spontaneously firing dopamine cells, a phenomenon known as depolarization block (Grace and Bunney, 1986). In accordance with this observation in rats, the delayed clinical response to antipsychotic treatment (Rabinowitz et al., 2001) has been attributed to eventual decline of dopaminergic firing rates in the ventral tegmental area, following an acute period of increased activity (Grace et al., 1997; Pickar, 1988). In support of this claim, concentrations of the dopamine metabolite homovanillic acid (HVA) in the CSF of human patients initially increase after initiation of antipsychotic treatment, but decline after prolonged treatment in those who respond clinically to the treatment (Sharma et al., 1993). Plasma HVA concentrations during haloperidol treatment likewise suggest a time-dependent modulation of dopamine synthesis, eventually proceeding to a state of depolarization block, which may underlie the delayed clinical benefits (Pickar et al., 1986; Yoshimura et al., 2003). We have previously reported that subchronic and clinically-effective haloperidol treatment over a period of 4 weeks reduced the cerebral utilization of FDOPA in the brain of patients with schizophrenia (Gru¨nder et al., 2003), which we interpreted to reveal the presence of depolarization block. The present study was intended to test for activation of FDOPA utilization and dopamine synthesis in the brain of healthy subjects, during the acute phase of treatment with clinically-relevant doses of haloperidol, and to correlate these PET measures with changes in cognitive performance and EPS. FDOPA is reversibly transferred across the brain by facilitated diffusion, and is a substrate of DOPA decarboxylase in living brain; retention of the decarboxylated metabolite [18F]fluorodopamine in vesicles accounts for the specific signal in FDOPA/PET studies (Fig. 1). However, results of PET studies in vivo and biochemical studies ex vivo indicate that FDOPA in brain is not committed to dopamine synthesis, even in regions with high activity of DOPA decarboxylase (see Cumming and Gjedde, 1998). It follows that modulation of DOPA decarboxylase activity can potentially influence the rate of dopamine synthesis by altering the branching ratio for the several metabolic fates of DOPA (Cumming and Gjedde, 1998; Gjedde et al., 1993). Increased trapping of exogenous DOPA decarboxylase substrates has been observed ex vivo in striatum of rats treated with high acute doses of the antipsychotic medications cis-flupenthixol (Cumming et al., 1997), haloperidol, or risperidone (Mamo et al., 2004) and in living pigs treated with a high dose of haloperidol (Danielsen et al., 2001), consistent with regulation of the enzyme by autoreceptors. Thus, acute modulation of DOPA metabolism by dopamine receptors seems well-described in experimental animals, but the corresponding regulation in the human brain is

poorly documented. FDOPA-PET can, in principle, be used for testing the modulation of dopamine synthesis capacity in living brain. Indeed, results of a multi-tracer PET study suggest that compensatory increases in FDOPA utilization can occur in patients with early Parkinson’s disease (Lee et al., 2000). In general, the available results support the claim that therapeutic benefits of antipsychotics are obtained when firing of dopamine neurons is arrested following prolonged treatment, i.e. depolarization block. However, currently, there is no in vivo evidence that acute antidopaminergic treatment initially increases dopamine synthesis capacity in humans. In an earlier PET study, acute treatment with risperidone (3 mg 2 h prior to the scan) failed to alter the apparent utilization of the DOPA decarboxylase substrate 6-[18F]-l-m-tyramine in striatum of six healthy volunteers, although an approximately 20-fold higher dose enhanced its trapping in striatum of rats (Mamo et al., 2004). Although previous PET-studies have suggested that a 3 mg/day risperidone treatment in patients (under stable dose regimen) results in 60 – 70% occupancy of dopamine D2/3 receptors (Yasuno et al., 2001), the acute risperidone dose may account for the lack of effect in the earlier PET study. In addition, the brief interval between oral risperidone ingestion and the PET examinations may be a confounding factor. Finally, the use of an atypical antipsychotic with significant 5 HT2a antagonistic properties may complicate the interpretation of the study by Mamo et al. (2004). The choice of radioligand is another key distinction between the present FDOPA study and the earlier report by Mamo et al. (2004), in which the rate of trapping of 6-[18F]-l-m-tyrosine in striatum was estimated by graphical analysis using a reference input. This tracer yields higher contrast between striatum and surrounding cerebral cortex than does FDOPA (Brown et al., 1999; Doudet et al., 1999). However, the physiological interpretation of 6-[18F]-lm-tyrosine PET studies may be complicated by the failure of the decarboxylated metabolite 6-[18F]-l-m-tyramine to enter into vesicles (Endres et al., 1997), and its rapid catabolism by monoamine oxidase, yielding [18F]hydroxyphenylacetic acid (Jordan et al., 1998; Wahl et al., 1999). Since the acidic metabolites of [18F]fluorodopamine diffuse from rat striatum with a half-life of 15 min (Cumming et al., 1994), it remains unclear what accounts for the prolonged retention in striatum of radioactivity derived from 6[18F]-l-m-tyrosine. Thus, for diverse methodological reasons, results of the present study cannot be compared directly with the earlier study by Mamo et al. (2004). In the present study, we found that haloperidol treatment evoked an enhancement in FDOPA net influx (K inapp) to the striatum

I. Vernaleken et al. / NeuroImage 30 (2006) 1332 – 1339

and mesencephalon, which was evident by inspection of the parametric maps. The position of peak influx was shifted ventrally by about 5 mm in the haloperidol condition, a result which seems comparable to our earlier report of enhanced FDOPA utilization specifically in the ventral striatum of normal volunteers treated with the NMDA-antagonist amantadine (Deep et al., 1999). Thus, FDOPA utilization may be particularly sensitive to pharmacological modulation in the ventral striatum. However, in the present volumeof-interest analysis, the effect of acute haloperidol on K inapp was statistically significant in putamen and midbrain. Earlier studies have shown that levodopa synthesis is elevated in striatum and, to a lesser extent, also in the substantia nigra of rats treated with haloperidol (Argiolas et al., 1982; Magnusson et al., 1987). Thus, the present parametric mapping study suggests that dopamine synthesis capacity in the human mesencephalon may be particularly sensitive to modulation by acute haloperidol challenge. In many previous PET studies, FDOPA utilization has been assessed by calculating the net influx to the brain relative to the arterial plasma input (K inapp). However, net influx of FDOPA is a macroparameter representing the composite of the distribution across the blood – brain barrier, the relative activity of DOPA decarboxylase (k 3D), and other factors (Cumming and Gjedde, 1998). In the present compartmental analysis, numerical increases in the magnitude k 3D were not significant. However, the standard deviation of the mean as a percentage of the mean estimates tended to be higher for k 3D than for K inapp or VD e , and was especially high for k 3D in caudate and putamen in the posthaloperidol condition, suggesting that variable neurochemical response to the pharmacological challenge reduced the statistical power of the method for detecting changes in k 3D in the present group of nine subjects. The results of the present compartmental analysis suggested that the enhanced net FDOPA influx could be attributed to an increase in the magnitude of VD e , i.e. increased bioavailability of the tracer in the brain. We have noted an identical phenomenon in normal volunteers challenged with levodopa, which we attributed to facilitation of amino acid transport at the blood – brain barrier via indirect agonism of adrenergic receptors (Kumakura et al., 2004). Likewise, the magnitude of VD e tended to be increased in an earlier study of subchronic haloperidol for the treatment of schizophrenia (Gru¨nder et al., 2003). In the present study, the formation of OMFD was slightly enhanced in the haloperidol condition, which might conceivably have biased the kinetic analysis, but since this was not observed in the earlier subchronic study, it cannot be invoked as an explanation for the present observation. We are unaware of a physiological mechanism which might account for potentiation of amino acid transport by acute haloperidol. The present protocol for haloperidol treatment (3 days  5 mg/ day) resulted in 70 – 80% occupancy of dopamine receptors in an earlier PET study (Xiberas et al., 2001). Furthermore, the 3 day treatment phase was chosen on the basis of earlier studies reporting increases in plasma HVA during the first week of oral antipsychotic treatment but not much earlier than 24 h after intramuscularly haloperidol injection (Davidson et al., 1987; Pickar et al., 1986). Thus, the cerebral synthesis of endogenous dopamine is likely to have been elevated during the second FDOPA scan of the present study, although we were unable to link the enhanced FDOPA influx firmly to an activation of the enzyme DOPA decarboxylase on the basis of compartmental kinetic analysis. Cognitive impairment is a major aspect of the psychopathology of schizophrenia (Liu et al., 2002; McGrath et al., 1997; Wiedl et al., 2001). Pharmacotherapy with antipsychotic medication, espe-

1337

cially atypical antipsychotics, can improve cognitive functioning in patients with schizophrenia (Bilder et al., 2002; Potkin et al., 2001). However, little is known about the effects of antipsychotics on cognitive performance in healthy volunteers. In one earlier study, haloperidol or the atypical antipsychotic amisulpride interfered with performance of tests for sustained attention, psychomotor functioning, and affective function in healthy volunteers (Ramaekers et al., 1999). The present study appears to be the first use of the CPT in healthy volunteers challenged with an antipsychotic agent. Haloperidol significantly interfered with two out of three subscores in ¯ ) and dV). These the SDT of the continuous performance test ( P(A parameters represent that portion of the signal – noise distribution attributable to veridical perception of the visual stimulus used in the test. SDT parameters of CPT are highly sensitive for measuring sustained attention in schizophrenia, but are calculated from the scores in classical parameters, i.e. omission and commission score and reaction time. Thus, haloperidol disturbs sustained attention in healthy subjects, in contrast to its ameliorative effects reported in patients with schizophrenia. In the present analysis, there were no trends towards significant correlation between indices of FDOPA utilization and CPTparameters or the severity of extrapyramidal side effects in the haloperidol condition. We speculate that the individual changes in FDOPA utilization may be related to differences in the basal tonus of dopamine at autoreceptors such that subjects with low occupancy already had relatively high activity of DOPA decarboxylase, which could not be discernibly stimulated by haloperidol treatment in the present group of nine subjects. We are currently conducting a FDOPA/PET study in a larger cohort of normal subjects in order to assess the relationship between basal FDOPA utilization and cognitive performance after a haloperidol challenge. In summary, we have tested the effects of acute haloperidol on the cerebral utilization of FDOPA in nine healthy volunteers. The treatment enhanced the net influx of FDOPA (K inapp) to putamen and mesencephalon, suggesting regulation of dopamine synthesis capacity by autoreceptors. However, the results of compartmental analysis showed that this effect could be attributed to an increase in the distribution volume of FDOPA in the brain; numerical increases in the magnitude of the estimate of the DOPA decarboxylase activity were not statistically significant. Thus, the present study reveals enhanced utilization of FDOPA in the brain of healthy subjects after acute haloperidol treatment, but does not firmly link this phenomenon to activation of the biochemical pathway for dopamine synthesis. The haloperidol treatment interfered with sustained attention in healthy subjects, but the cognitive effects did not correlate with individual changes in FDOPA utilization. Acknowledgments We thank Prof. Frank Ro¨sch and colleagues for the detection of FDOPA metabolites. This work was supported by the National Science Foundation (Denmark), the Research Fund of the University of Mainz, and the state Rheinland-Pfalz, Germany. References Aaronson, D., Watts, B., 1987. Extensions of Grier’s computational formulas for AV and BW to below-chance performance. Psychol. Bull. 102, 439 – 442.

1338

I. Vernaleken et al. / NeuroImage 30 (2006) 1332 – 1339

Agid, O., Kapur, S., Arenovich, T., Zipursky, R.B., 2003. Delayed-onset hypothesis of antipsychotic action: a hypothesis tested and rejected. Arch. Gen. Psychiatry 60, 1228 – 1235. Argiolas, A., Melis, M.R., Fadda, F., Gessa, G.L., 1982. Evidence for dopamine autoreceptors controlling dopamine synthesis in the substantia nigra. Brain Res. 234, 177 – 181. Bilder, R.M., Goldman, R.S., Volavka, J., Czobor, P., Hoptman, M., Sheitman, B., Lindenmayer, J.P., Citrome, L., McEvoy, J., Kunz, M., Chakos, M., Cooper, T.B., Horowitz, T.L., Lieberman, J.A., 2002. Neurocognitive effects of clozapine, olanzapine, risperidone, and haloperidol in patients with chronic schizophrenia or schizoaffective disorder. Am. J. Psychiatry 159, 1018 – 1028. Brown, W.D., DeJesus, O.T., Pyzalski, R.W., Malischke, L., Roberts, A.D., Shelton, S.E., Uno, H., Houser, W.D., Nickles, R.J., Holden, J.E., 1999. Localization of trapping of 6-[(18)F]fluoro-l-m-tyrosine, an aromatic l-amino acid decarboxylase tracer for PET. Synapse 34, 111 – 123. Carlsson, A., Lindqvist, M., 1963. Effect of chlorpromazine or haloperidol on formation of 3methoxytyramine and normetanephrine in mouse brain. Acta Pharmacol. Toxicol. (Copenh) 20, 140 – 144. Cho, S., Neff, N.H., Hadjiconstantinou, M., 1997. Regulation of tyrosine hydroxylase and aromatic l-amino acid decarboxylase by dopaminergic drugs. Eur. J. Pharmacol. 323, 149 – 157. Collins, D.L., Zijdenbos, A.P., Evans, A.C., 1998. Improved automatic gross cerebral structure segmentation. Forth International Conference on Functional Mapping of the Human Brain. Cumming, P., Gjedde, A., 1998. Compartmental analysis of dopa decarboxylation in living brain from dynamic positron emission tomograms. Synapse 29, 37 – 61. Cumming, P., Leger, G.C., Kuwabara, H., Gjedde, A., 1993. Pharmacokinetics of plasma 6-[18F]fluoro-l-3,4-dihydroxyphenylalanine ([18F]Fdopa) in humans. J. Cereb. Blood Flow Metab. 13, 668 – 675. Cumming, P., Kuwabara, H., Gjedde, A., 1994. A kinetic analysis of 6[18F]fluoro-l-dihydroxyphenylalanine metabolism in the rat. J. Neurochem. 63, 1675 – 1682. Cumming, P., Kuwabara, H., Ase, A., Gjedde, A., 1995. Regulation of DOPA decarboxylase activity in brain of living rat. J. Neurochem. 65, 1381 – 1390. Cumming, P., Ase, A., Laliberte, C., Kuwabara, H., Gjedde, A., 1997. In vivo regulation of DOPA decarboxylase by dopamine receptors in rat brain. J. Cereb. Blood Flow Metab. 17, 1254 – 1260. Danielsen, E.H., Smith, D., Hermansen, F., Gjedde, A., Cumming, P., 2001. Acute neuroleptic stimulates DOPA decarboxylase in porcine brain in vivo. Synapse 41, 172 – 175. Davidson, M., Giordani, A.B., Mohs, R.C., Horvath, T.B., Davis, B.M., Powchik, P., Davis, K.L., 1987. Short-term haloperidol administration acutely elevates human plasma homovanillic acid concentration. Arch. Gen. Psychiatry 44, 189 – 190. Deep, P., Dagher, A., Sadikot, A., Gjedde, A., Cumming, P., 1999. Stimulation of dopa decarboxylase activity in striatum of healthy human brain secondary to NMDA receptor antagonism with a low dose of amantadine. Synapse 34, 313 – 318. Doudet, D.J., Chan, G.L., Jivan, S., DeJesus, O.T., McGeer, E.G., English, C., Ruth, T.J., Holden, J.E., 1999. Evaluation of dopaminergic presynaptic integrity: 6-[18F]fluoro-l-dopa versus 6-[18F]fluoro-l-mtyrosine. J. Cereb. Blood Flow Metab. 19, 278 – 287. el Mestikawy, S., Glowinski, J., Hamon, M., 1986. Presynaptic dopamine autoreceptors control tyrosine hydroxylase activation in depolarized striatal dopaminergic terminals. J. Neurochem. 46, 12 – 22. Endres, C.J., Swaminathan, S., DeJesus, O.T., Sievert, M., Ruoho, A.E., Murali, D., Rommelfanger, S.G., Holden, J.E., 1997. Affinities of dopamine analogs for monoamine granular and plasma membrane transporters: implications for PET dopamine studies. Life Sci. 60, 2399 – 2406. Essock, S.M., Hargreaves, W.A., Dohm, F.A., Goethe, J., Carver, L., Hipshman, L., 1996. Clozapine eligibility among state hospital patients. Schizophr. Bull. 22, 15 – 25. Gillings, N.M., Bender, D., Falborg, L., Marthi, K., Munk, O.L., Cumming,

P., 2001. Kinetics of the metabolism of four PET radioligands in living minipigs. Nucl. Med. Biol. 28, 97 – 104. Gjedde, A., Reith, J., Dyve, S., Leger, G., Guttman, M., Diksic, M., Evans, A., Kuwabara, H., 1991. Dopa decarboxylase activity of the living human brain. Proc. Natl. Acad. Sci. U. S. A. 88, 2721 – 2725. Gjedde, A., Leger, G.C., Cumming, P., Yasuhara, Y., Evans, A.C., Guttman, M., Kuwabara, H., 1993. Striatal l-dopa decarboxylase activity in Parkinson’s disease in vivo: implications for the regulation of dopamine synthesis. J. Neurochem. 61, 1538 – 1541. Grace, A.A., Bunney, B.S., 1986. Induction of depolarization block in midbrain dopamine neurons by repeated administration of haloperidol: analysis using in vivo intracellular recording. J. Pharmacol. Exp. Ther. 238, 1092 – 1100. Grace, A.A., Bunney, B.S., Moore, H., Todd, C.L., 1997. Dopamine-cell depolarization block as a model for the therapeutic actions of antipsychotic drugs. Trends Neurosci. 20, 31 – 37. Gru¨nder, G., Vernaleken, I., Mu¨ller, M.J., Davids, E., Heydari, N., Buchholz, H.G., Bartenstein, P., Munk, O.L., Stoeter, P., Wong, D.F., Gjedde, A., Cumming, P., 2003. Subchronic haloperidol downregulates dopamine synthesis capacity in the brain of schizophrenic patients in vivo. Neuropsychopharmacology 28, 787 – 794. Hadjiconstantinou, M., Wemlinger, T.A., Sylvia, C.P., Hubble, J.P., Neff, N.H., 1993. Aromatic l-amino acid decarboxylase activity of mouse striatum is modulated via dopamine receptors. J. Neurochem. 60, 2175 – 2180. Hoshi, H., Kuwabara, H., Leger, G., Cumming, P., Guttman, M., Gjedde, A., 1993. 6-[18F]fluoro-l-dopa metabolism in living human brain: a comparison of six analytical methods. J. Cereb. Blood Flow Metab. 13, 57 – 69. Huang, S.C., Yu, D.C., Barrio, J.R., Grafton, S., Melega, W.P., Hoffman, J.M., Satyamurthy, N., Mazziotta, J.C., Phelps, M.E., 1991. Kinetics and modeling of l-6-[18F]fluoro-dopa in human positron emission tomographic studies. J. Cereb. Blood Flow Metab. 11, 898 – 913. Jordan, S., Bankiewicz, K.S., Eberling, J.L., VanBrocklin, H.F., O’Neil, J.P., Jagust, W.J., 1998. An in vivo microdialysis study of striatal 6[18F]fluoro-l-m-tyrosine metabolism. Neurochem. Res. 23, 513 – 517. Juarez-Reyes, M.G., Shumway, M., Battle, C., Bacchetti, P., Hansen, M.S., Hargreaves, W.A., 1995. Effects of stringent criteria on eligibility for clozapine among public mental health clients. Psychiatr. Serv. 46, 801 – 806. Karoum, F., Chrapusta, S.J., Egan, M.F., 1994. 3-Methoxytyramine is the major metabolite of released dopamine in the rat frontal cortex: reassessment of the effects of antipsychotics on the dynamics of dopamine release and metabolism in the frontal cortex, nucleus accumbens, and striatum by a simple two pool model. J. Neurochem. 63, 972 – 979. Kathmann, N., Wagner, M., Satzger, W., Engel, R.R., 1996. Vigilanzmessung auf Verhaltensebene: Der Continuous Performance Test-Mu¨nchen (CPT-M). In: Moller, H.-J., Engel, R.R., Hoff, P. (Eds.), Befunderhebung in der Psychiatrie: Lebensqualita¨t, Negativsymptomatik und andere aktuelle Entwicklungen. Springer, Wien, Austria, pp. 331 – 338. Kumakura, Y., Danielsen, E.H., Reilhac, A., Gjedde, A., Cumming, P., 2004. Levodopa effect on [F]fluorodopa influx to brain: normal volunteers and patients with Parkinson’s disease. Acta Neurol. Scand. 110, 188 – 195. Lee, C.S., Samii, A., Sossi, V., Ruth, T.J., Schulzer, M., Holden, J.E., Wudel, J., Pal, P.K., de la Fuente-Fernandez, R., Calne, D.B., Stoessl, A.J., 2000. In vivo positron emission tomographic evidence for compensatory changes in presynaptic dopaminergic nerve terminals in Parkinson’s disease. Ann. Neurol. 47, 493 – 503. Lerner, P., Nose, P., Gordon, E.K., Lovenberg, W., 1977. Haloperidol: effect of long-term treatment on rat striatal dopamine synthesis and turnover. Science 197, 181 – 183. Liu, S.K., Chiu, C.H., Chang, C.J., Hwang, T.J., Hwu, H.G., Chen, W.J., 2002. Deficits in sustained attention in schizophrenia and affective disorders: stable versus state-dependent markers. Am. J. Psychiatry 159, 975 – 982.

I. Vernaleken et al. / NeuroImage 30 (2006) 1332 – 1339 Magnusson, O., Mohringe, B., Fowler, C.J., 1987. Comparison of the effects of dopamine D1 and D2 receptor antagonists on rat striatal, limbic and nigral dopamine synthesis and utilisation. J. Neural Transm. 69, 163 – 177. Mamo, D., Remington, G., Nobrega, J., Hussey, D., Chirakal, R., Wilson, A.A., Baker, G., Houle, S., Kapur, S., 2004. Effect of acute antipsychotic administration on dopamine synthesis in rodents and human subjects using 6-[18F]-l-m-tyrosine. Synapse 52, 153 – 162. Martin, W.R., Palmer, M.R., Patlak, C.S., Calne, D.B., 1989. Nigrostriatal function in humans studied with positron emission tomography. Ann. Neurol. 26, 535 – 542. McGrath, J., Scheldt, S., Welham, J., Clair, A., 1997. Performance on tests sensitive to impaired executive ability in schizophrenia, mania and well controls: acute and subacute phases. Schizophr. Res. 26, 127 – 137. O’Keeffe, R., Sharman, D.F., Vogt, M., 1970. Effect of drugs used in psychoses on cerebral dopamine metabolism. Br. J. Pharmacol. 38, 287 – 304. Onali, P., Olianas, M.C., Bunse, B., 1988. Evidence that adenosine A2 and dopamine autoreceptors antagonistically regulate tyrosine hydroxylase activity in rat striatal synaptosomes. Brain Res. 456, 302 – 309. Opacka-Juffry, J., Brooks, D.J., 1995. l-Dihydroxyphenylalanine and its decarboxylase: new ideas on their neuroregulatory roles. Mov. Disord. 10, 241 – 249. Pickar, D., 1988. Perspectives on a time-dependent model of neuroleptic action. Schizophr. Bull. 14, 255 – 268. Pickar, D., Labarca, R., Doran, A.R., Wolkowitz, O.M., Roy, A., Breier, A., Linnoila, M., Paul, S.M., 1986. Longitudinal measurement of plasma homovanillic acid levels in schizophrenic patients. Correlation with psychosis and response to neuroleptic treatment. Arch. Gen. Psychiatry 43, 669 – 676. Potkin, S.G., Fleming, K., Jin, Y., Gulasekaram, B., 2001. Clozapine enhances neurocognition and clinical symptomatology more than standard neuroleptics. J. Clin. Psychopharmacol. 21, 479 – 483. Rabinowitz, J., Hornik, T., Davidson, M., 2001. Rapid onset of therapeutic effect of risperidone versus haloperidol in a double-blind randomized trial. J. Clin. Psychiatry 62, 343 – 346. Ramaekers, J.G., Louwerens, J.W., Muntjewerff, N.D., Milius, H., de Bie, A., Rosenzweig, P., Patat, A., O’Hanlon, J.F., 1999. Psychomotor,

1339

cognitive, extrapyramidal, and affective functions of healthy volunteers during treatment with an atypical (amisulpride) and a classic (haloperidol) antipsychotic. J. Clin. Psychopharmacol. 19, 209 – 221. Reilhac, A., Sechet, S., Boileau, I., Gunn, R., Evans, A., Dagher, A., 2003. Motion correction for PET ligand imaging. Human Brain Mapping. , New York City, NY. Rosvold, H.E., Mirsky, A.F., Sarason, I., Bransome, E.D., Beck, L.H., 1956. A continuous performance test of brain damage. J. Consult. Psychol. 20, 343 – 350. Sharma, R.P., Javaid, J.I., Janicak, P.G., Davis, J.M., Faull, K., 1993. Homovanillic acid in the cerebrospinal fluid: patterns of response after four weeks of neuroleptic treatment. Biol. Psychiatry 34, 128 – 134. Simpson, G.M., Angus, J.W., 1970. A rating scale for extrapyramidal side effects. Acta Psychiatr. Scand., Suppl. 212, 11 – 19. Wahl, L.M., Chen, J.J., Thompson, M., Chirakal, R., Nahmias, C., 1999. The time course of metabolites in human plasma after 6-[(18)F]fluorol-m-tyrosine administration. Eur. J. Nucl. Med. 26, 1407 – 1412. Wiedl, K.H., Wienobst, J., Schottke, H.H., Green, M.F., Nuechterlein, K.H., 2001. Attentional characteristics of schizophrenia patients differing in learning proficiency on the Wisconsin Card Sorting Test. Schizophr. Bull. 27, 687 – 695. Woods, R.P., Cherry, S.R., Mazziotta, J.C., 1992. Rapid automated algorithm for aligning and reslicing PET images. J. Comput. Assist. Tomogr. 16, 620 – 633. Xiberas, X., Martinot, J.L., Mallet, L., Artiges, E., Loc, H.C., Maziere, B., Paillere-Martinot, M.L., 2001. Extrastriatal and striatal D(2) dopamine receptor blockade with haloperidol or new antipsychotic drugs in patients with schizophrenia. Br. J. Psychiatry 179, 503 – 508. Yasuno, F., Suhara, T., Okubo, Y., Sudo, Y., Inoue, M., Ichimiya, T., Tanada, S., 2001. Dose relationship of limbic-cortical D2-dopamine receptor occupancy with risperidone. Psychopharmacology (Berl) 154, 112 – 114. Yoshimura, R., Ueda, N., Shinkai, K., Nakamura, J., 2003. Plasma levels of homovanillic acid and the response to risperidone in first episode untreated acute schizophrenia. Int. Clin. Psychopharmacol. 18, 107 – 111. Zhu, M.Y., Juorio, A.V., Paterson, I.A., Boulton, A.A., 1992. Regulation of aromatic l-amino acid decarboxylase by dopamine receptors in the rat brain. J. Neurochem. 58, 636 – 641.