Skip to main content
NCBI home page
UKPMC Funders Author Manuscripts logoLink to UKPMC Funders Author Manuscripts
. Author manuscript; available in PMC: 2014 Aug 18.
Published in final edited form as: Neuroimage. 2009 Dec 5;50(1):260–266. doi: 10.1016/j.neuroimage.2009.11.077

Acute effect of the anti-addiction drug bupropion on extracellular dopamine concentrations in the human striatum: An [11C]raclopride PET study

Alice Egerton 1,2,3, John P Shotbolt 1,2,4, Paul RA Stokes 1,2, Ella Hirani 5, Rabia Ahmad 5, Julia M Lappin 1,2,3, Suzanne J Reeves 1,2,3, Mitul A Mehta 1,2,3, Oliver D Howes 1,2,3, Paul M Grasby 1,2
PMCID: PMC4135078  EMSID: EMS59978  PMID: 19969097

Abstract

Bupropion is an effective medication in treating addiction and is widely used as an aid to smoking cessation. Bupropion inhibits striatal dopamine reuptake via dopamine transporter blockade, but it is unknown whether this leads to increased extracellular dopamine levels at clinical doses in man. The effects of bupropion on extracellular dopamine levels in the striatum were investigated using [11C]raclopride positron emission tomography (PET) imaging in rats administered saline, 11 or 25mg/kg bupropion i.p. and in healthy human volunteers administered either placebo or 150mg bupropion (Zyban® Sustained-Release). A cognitive task was used to stimulate dopamine release in the human study. In rats, bupropion significantly decreased [11C]raclopride specific binding in the striatum, consistent with increases in extracellular dopamine concentrations. In man, no significant decreases in striatal [11C]raclopride specific binding were observed. Levels of dopamine transporter occupancy in the rat at 11 and 25mg/kg bupropion i.p. were higher than predicted to occur in man at the dose used. Thus, these data indicate that, at the low levels of dopamine transporter occupancy achieved in man at clinical doses, bupropion does not increase extracellular dopamine levels. These findings have important implications for understanding the mechanism of action underlying bupropions’ therapeutic efficacy and for the development of novel treatments for addiction and depression.

Keywords: bupropion; dopamine; imaging positron emission tomography; [11C]raclopride; striatum; addiction, depression, mechanism, smoking, rat, human

Introduction

Bupropion is an effective medication in smoking cessation and has a good safety and side-effect profile (Aubin et al., 2002; Hurt et al., 1997; Jorenby et al., 1999). In addition to its original indication for treatment of depressive disorders, bupropion may also be effective in the treatment of methamphetamine addiction and pathological gambling (Dannon et al., 2005; Elkashef et al., 2008). Elucidation of the pharmacological features of bupropion which most contribute to its clinical efficacy may aid development of novel treatments for smoking cessation and other disorders of addiction.

The precise pharmacological mechanisms that underlie the therapeutic effects of bupropion are unclear (Dwoskin et al., 2006; Paterson, 2009; Warner and Shoaib, 2005). Bupropion weakly inhibits monoamine reuptake to presynaptic terminals through dopamine transporters (DAT), and, to a lesser extent, noradrenaline transporters (Ascher et al., 1995; Damaj et al., 2004; Ferris and Beaman 1983). Via interaction with vesicular monoamine transporter-2, bupropion increases sequestration of cytoplasmic dopamine to vesicles (Rau et al., 2005). At similar concentrations to those which inhibit monoamine transporter function, bupropion also acts as a non-competitive inhibitor of nicotinic acetylcholine receptors (Fryer and Lucas, 1999; Miller et al., 2002; Slemmer et al., 2000).

In rats, microdialysis studies show that acute, systemic, bupropion administration reproducibly and dose-dependently increases striatal extracellular dopamine levels (Bredeloux et al., 2007; Brown et al., 1991; Gazzara and Andersen, 1997; Li et al., 2002; Nomikos et al., 1989; Sidhpura et al., 2007). It has been suggested that increases in striatal dopamine concentrations following bupropion administration may help combat the anhedonia associated with withdrawal from nicotine (or other addictive drugs) and anhedonia in depression (Paterson et al., 2007; 2009; Warner and Shoaib, 2005; Shiffman et al., 2000). However, what is unclear is whether therapeutic doses of bupropion are sufficient to increase extracellular dopamine levels in the human striatum.

In man, this question has been addressed indirectly using molecular imaging with dopamine transporter radioligands, to estimate the degree of DAT occupancy which occurs following repeated bupropion treatment (Árgyelán et al., 2005; Kugaya et al., 2003; Learned-Coughlin et al., 2003; Meyer et al., 2005) or acute administration of the bupropion active metabolite hydroxybupupropion (Volkow et al., 2005). Overall, these studies indicate that, in man, only a small proportion - at most, 20-25% - of striatal DAT sites are occupied at clinical doses of bupropion. This observation has led to proposals that DAT inhibition alone does not explain the therapeutic efficacy of bupropion (Meyer et al., 2005; Kugaya et al., 2003; Paterson et al., 2009; Warner and Shoaib, 2005).

A more direct approach is to investigate the effects of bupropion administration on extracellular dopamine concentrations in the human striatum. Using positron emission tomography (PET) in combination with the D2/3 dopamine receptor radiotracer [11C]raclopride, it is possible to index changes in extracellular dopamine levels in both man and experimental animals, as [11C]raclopride competes with dopamine for D2/3 receptor binding (Laruelle, 2000). As bupropion has negligible affinity at D2/3 dopamine receptors and therefore will not compete with [11C]raclopride directly (Ferris and Beaman, 1983), this non-invasive imaging approach is viable for assessing bupropion-induced changes in extracellular dopamine concentrations in man.

As the relationship between microdialysis and [11C]raclopride PET measures of extracellular DA is complex (Laruelle, 2000), we performed an initial [11C]raclopride PET study in rats to confirm whether bupropion-induced increases in dopamine concentrations are detectable using [11C]raclopride PET. Following positive confirmation, this approach was translated to man in order to determine whether the dose of bupropion used in the UK to aid smoking cessation (150mg Zyban® Sustained-Release) increases extracellular dopamine concentrations in the human striatum.

We investigated the effects of bupropion on striatal dopamine levels while volunteers completed a spatial planning task, previously shown to decrease striatal [11C]raclopride binding potential in healthy volunteers (Lappin et al., 2009), as increases in extracellular dopamine concentrations following dopamine reuptake inhibition are most apparent when dopamine release is stimulated (Volkow et al., 2002). This approach was also selected as stimulation of dopamine release via administration of a behavioral task in combination with dopamine reuptake inhibition by bupropion would additionally provide a relatively safe method of probing striatal dopaminergic function in clinical populations in future studies.

Methods

Initial animal study

Doses of 11 and 25mg/kg bupropion i.p. were selected for the initial study in rats. Microdialysis studies have previously shown increases in extracellular dopamine levels in the rat within this dose range (Bredeloux et al., 2007; Brown et al., 1991; Li et al., 2002; Nomikos et al., 1989; Sidhpura et al., 2007) and the 11mg/kg dose is equivalent to the 150mg human dose as calculated using dose-scaling factors (Mordenti and Chappell, 1989).

All animal experiments were carried out in accordance with the UK Animals (Scientific Procedures) Act, 1986 and associated guidelines. Under isoflurane anesthesia, 14 adult male Sprague–Dawley rats (Harlan Olac, UK) (body weight: mean ± S.D. = 315 ± 46g) were administered either vehicle (0.9% saline, n = 5), 11mg/kg bupropion (Sigma, UK) (n = 3) or 25mg/kg bupropion (n = 6) i.p. 30 minutes prior to [11C]raclopride injection. Rats were positioned in a stereotaxic frame and PET data were acquired using a quad-HIDAC (high-density avalanche chamber) small animal tomograph (Oxford Positron Systems). [11C]Raclopride was administered via a previously catheterized lateral tail vein. The mean ± SD injectate was 0.311 ± 0.032 mCi (11.5 ± 1.2 MBq) with an associated stable content of 0.68 ± 0.23 nmol/kg. Emission data were acquired in list mode for 60 minutes.

To reconstruct scan sinograms, list mode emission data were binned into 0.5mm isotropic voxels using filtered back-projection (Hamming filter, 0.6 cut-off), resulting in a spatial resolution of ~0.5mm full width at half-maximum (FWHM) (Myers and Hume, 2002). Image volumes were then transferred into ANALYZE (www.analyzedirect.com) (Robb and Hanson, 1991). Using a volume of interest (VOI) template (Hume et al., 2001) data were sampled from the dorsal striatum (2 × 140 voxels) and cerebellum (764 voxels). Data analysis was limited to calculation of the specific binding ratio (SBR: the ratio of specifically bound radiotracer (striatum) to free and nonspecifically bound radiotracer (cerebellum), minus 1) during a single 40 minute time-frame, beginning 20 minutes after [11C]raclopride injection; in order to improve count statistics (Hume et al., 2001). Previous studies have shown that [11C]raclopride takes ~20 min to reach dynamic equilibrium in isoflurane-anesthetized rats and that the striatum/cerebellum ratio remains unchanged from 20–60 min after [11C]raclopride injection (Hume et al. 1996) and ratio data acquired in the 20–60-min time frame correlates well with individual binding potential measurements derived from time–activity curves (Houston et al. 2004).

We estimated DAT occupancy under the same experimental conditions as used above: anaesthetized rats were administered vehicle (0.9% saline), 11mg/kg bupropion or 25mg/kg bupropion i.p. Thirty minutes later, ~10μCi [3H]cocaine (Perkin Elmer Life Sciences, UK) was administered i.v., and rats were euthanized 20 minutes following [3H]cocaine administration. The striata and cerebellum were dissected out, solubilized (Soluene®-350, PerkinElmer, UK), and counted for 3H using a LKB scintillation counter with automatic quench factor (Beckman, UK). Counts were normalized against standards and data were calculated as percent injected activity per gram of tissue, normalized for body weight, giving ‘uptake units’. The cerebellum, which contains a very low level of dopamine transporters (Panagopoulos et al., 1991), was used to represent free and non-specifically bound [3H]cocaine. Data are expressed as the striatal:cerebellar SBR. Percentage occupancy of dopamine transporter sites following bupropion administration was calculated as:

Occupancy=([SBRvehicle−SBRbupropion]∕SBRvehicle)×100

Human study

Participants

Ten healthy participants were recruited by public advertisement (80% male; 90% right handed; average age: 47±6.7 years; age range 37 to 58 years). 9 of the 10 subjects were non smokers; the single participant who smoked consumed ~10 cigarettes / day. None of the participants were currently taking any prescribed medication. All participants gave their written, informed consent to be included in the study. Exclusion criteria were pregnancy, any contraindication to PET imaging, current or previous neurological, psychiatric or medical illness including head injury, and alcohol or other recreational drug use or dependency according to DSM-IV criteria. The absence of illicit drugs was confirmed by a urine drugs screen. The study was approved by Hammersmith and Queen Charlotte’s and Chelsea Research Ethics Committee, London, UK and the Administration of Radioactive Substances Advisory Committee.

Study design

Each participant underwent three [11C]raclopride PET scans, performed on separate days and administered in a predetermined randomized order. The scan conditions were as follows: A) Baseline: subjects were administered placebo and the data were acquired at rest; B) Placebo_Task: subjects were administered placebo and data were acquired as subjects performed a spatial planning task; C) Bupropion_Task: subjects were administered bupropion and data were acquired as subjects performed a spatial planning task. Bupropion hydrochloride (150mg Zyban® Sustained Release Tablets, GlaxoSmithKine) and placebo tablets were administered 2.5 hours prior to [11C]raclopride injection, in order that PET data acquisition coincided with peak bupropion plasma levels (Hsyu et al., 1997). All tablets were consumed in the presence of one of the investigators. The participants, but not the study investigators, were blind to the contents of the tablet. Although blood samples were taken mid-way though the scan to assay plasma bupropion levels, these data are not available for technical reasons. The spatial planning task was an adapted one-touch Tower of London task (Owen et al, 1997) presented on a computer touch-screen during the scan, as previously described (Lappin et al., 2009).

PET image acquisition

Data were acquired on an ECAT HR+ 962 scanner (CTI/Seimens) in three-dimensional mode, with an axial field of view of 15.5cm. Head movement was monitored and minimized using a light head-strap. A 10-minute transmission scan was performed prior to radiotracer injection to correct for attenuation and scatter. The spatial planning task commenced 5 minutes before radiotracer injection. [11C]-raclopride was administered as a bolus injection followed by constant rate infusion with a Kbol of 85 minutes (Stokes et al., 2009). The total administered activity was 10.72±0.36 mCi (396.8±13.3 MBq) per scan, with an associated stable content of 2.175±1.355μg.

Image analysis

Head movement was corrected using frame-by-frame (FBF) realignment. Nonattenuation corrected images were used to optimize the FBF realignment process (Dagher et al, 1998). The nonattenuation corrected images were denoised using a level 2, order 64 Battle Lemarie wavelet filter (Turkheimer et al, 1999). A mutual information algorithm (Studholme et al, 1997) was applied for frame realignment to a single frame acquired 40 mins. post-injection, in which there was a high signal-to-noise ratio. Transformation parameters were applied to the corresponding attenuation-corrected dynamic images to generate a movement-corrected dynamic image.

Striatal and cerebellar regions of interest (ROIs) were defined on an atlas (Hammers et al, 2003) in Montreal Neurologic Institute (MNI) space. Striatal ROIs comprised the sensorimotor, associative and limbic functional subdivisions (Martinez et al, 2003). An [11C]-raclopride template (Meyer et al., 1999) was spatially transformed into the individual PET space of each FBF-corrected dynamic image within SPM5 (Wellcome Department of Cognitive Neurology, London; www.fil.ion.ucl.ac.uk/spm), and the resulting transformation parameters were then used to transform the ROI map into individual PET space.

A weighted average add image for the 40-85 min steady-state time period was generated from each FBF-corrected dynamic image using in house software, written in Matlab (version 5; The MathWorks, Inc, Natick, Mass). The transformed ROI map was used to sample counts from the steady-state add image using ANALYZE software. Binding potential (BPND), the ratio at equilibrium of specifically bound radioligand to that of nondisplaceable radioligand in tissue (Innis et al., 2007), was calculated as the ratio of total radioactivity counts in the striatal subdivisions compared to the cerebellum, minus 1, during the 40-85 minute steady-state sampling period.

Statistical analysis

In the rat study, the effects of 11mg/kg and 25mg/kg bupropion on striatal [11C]raclopride SBR and striatal DAT occupancy were determined using 2-tailed independent sample t-tests. For the human study, differences in the amount and specific activity of injected [11C]raclopride across conditions were explored using analysis of variance. [11C]Raclopride BPND values in the associative, sensorimotor and limbic subdivisions were compared across the three scan conditions using repeated measures analysis of variance (ANOVA), with scan condition and side (left or right) as within-subjects factors. Potential effects of scan order on [11C]raclopride BPND were explored using the same approach. Body surface area (BSA) was calculated for each participant using the formula BSA =(W0.425 × H0.725) × 0.007184, where W is weight in kilograms and H is height in centimeters (DuBois and DuBois, 1916). Relationships between BSA and percentage change in [11C]-raclopride BPND in the bupropion_task compared to placebo_task condition were explored using Pearson’s correlation coefficient. All statistical analysis was performed in SPSS 16.0 (Chicago, Illinois), and the threshold for statistical significance was set at an alpha-level of 0.05. All data are reported as mean ± standard deviation.

Results

Initial rat study

Figure 1 illustrates the images that were obtained in control and bupropion-treated rats using the quad-HIDAC tomograph system. In Figure 1, the reduction in [11C]raclopride SBR following the higher dose of 25mg/kg bupropion compared to control values is clearly visible. Individual SBR values obtained in the striatum of control, 11mg/kg bupropion and 25mg/kg bupropion-treated animals are presented in Table 1. In the dorsal striatum, pre-treatment with both 11mg/kg and 25mg/kg bupropion significantly reduced [11C]raclopride SBR (11 mg/kg t(6) = 3.203; p = 0.019; 25 mg/kg bupropion t(6.58) = 9.157; p < 0.001). These decreases in SBR were to the magnitude of 6 ± 3% following 11mg/kg bupropion and 23 ± 7% following 25mg/kg bupropion.

Figure 1. Mean [11C]-raclopride coronal SBR images obtained in rats treated with saline (control), 11mg/kg bupropion or 25mg/kg.

Figure 1

Open in a new tab

The images represent addimages of data collected 20-60 minutes after [11C]-raclopride injection. Distances from bregma (mm) are indicated along the top of the figure.

Table 1. Striatal [11C]raclopride SBR and [3H]cocaine SBR in control rats and following administration of 11 or 25mg/kg bupropion i.p.

All data was acquired in anaesthetized animals. [11C]raclopride SBR was determined from summed PET data acquired 20-60 minutes following [11C]raclopride administration. [3H]cocaine SBR was determined from ex-vivo dissection data collected 20 minutes following [3H]cocaine administration. Bupropion produced significant (p < 0.05) increases in extracellular dopamine concentrations, as indexed by change (Δ) in [11C]raclopride SBR compared to control values, and significant occupancy of dopamine transporter sites as indexed by difference in [3H]cocaine SBR compared to control values.

[11C]raclopride SBR (mean ± S.D.)
Control 11mg/kg Significance ΔSBR 25mg/kg Significance ΔSBR
Bupropion Bupropion
3.25±0.07 3.05±0.11 p = 0.019 6 ± 3% 2.52±0.18 p < 0.001 23 ± 7%
[3H]cocaine SBR (mean ± S.D.)
Control 11mg/kg Significance Occupancy 25mg/kg Significance Occupancy
Bupropion Bupropion
2.00±0.26 1.30±0.37 p = 0.055 35 ± 18% 0.81±0.22 p = 0.004 60±18%
Open in a new tab

Table 1 also presents the [3H]cocaine dopamine transporter occupancy data that were obtained at doses of 11 and 25mg/kg bupropion in the rat. 25mg/kg bupropion produced significant occupancy of the dopamine transporter (t4 = 5.984; p = 0.004) and there was a trend for the same effect at the lower dose of 11mg/kg (t4 = 2.678; p = 0.055). These values corresponded to 35 ± 18% dopamine transporter occupancy with 11mg/kg bupropion and 60 ± 11% dopamine transporter occupancy with 25mg/kg bupropion.

Human study

Spatial planning accuracy offline (mean ± S.D. = 74.4 ± 22.7%; range = 50 to 100%) and in the scanner following placebo administration (mean ± S.D. = 77.3 ± 19.6%; range = 43.8 to 96.3%) were correlated (r = 0.745; p = 0.013). Planning accuracy in the scanner following bupropion administration (mean ± S.D. = 76.3 ± 15.2%; range = 50 to 91.3%) and placebo administration also correlated (r = 0.879; p = 0.001). Bupropion did not significantly affect planning accuracy in the scanner (t(9) = 0.329; p = 0.750). No significant correlations were apparent between planning accuracy and age.

There was no significant difference in either the amount of [11C]raclopride radioactivity injected (p > 0.36) or associated stable content (p > 0.21) across the three scan conditions. Similarly, scan order did not influence BPND in any of the striatal subdivisions (sensorimotor: F2 = 1.167; p = 0.334; associative: F2 = 0.326; p = 0.726; limbic: F2 = 0.801; p = 0.464). As there were no significant associations between age and BPND in the whole striatum or any of the striatal sub-regions, age was not used as a covariate in subsequent analysis. Planning accuracy did not correlate with [11C]raclopride BPND in any of the striatal subdivisions under either the Placebo_Task or Bupropion_Task condition.

The BPND values that were obtained in each of the three scan conditions are presented in Table 2. In the associative striatum, there was a significant overall effect of scan condition (F2 = 4.021; p = 0.036) and side (F1 = 44.895; p < 0.001) on [11C]raclopride BPND but no significant condition by side interaction (F2=1.031; p = 0.297). Post hoc analysis revealed a significant (4.4 ± 5%) increase in [11C]raclopride BPND in the associative striatum in the Bupropion_Task compared to Placebo_Task condition (F2 = 4.021; p = 0.036), but this did not survive correction for multiple comparisons (p = 0.081). Individual BPND values in the associative striatum in the Placebo_Task and Bupropion_Task conditions are presented in Figure 2. Change in [11C]raclopride BPND in the associative striatum in the Bupropion_Task compared to Placebo_Task condition was not significantly correlated with BSA (r=0.462; p = 0.179). There was no significant difference in [11C]raclopride BPND in the associative striatum in the Baseline compared to Placebo_Task conditions (F1 = 1.279; p = 0.287).

Table 2. [11C]raclopride BPND values (mean ± S.D.) in the functional subdivisions of the human striatum under each scan condition.

AST: associative striatum; SMS: sensorimotor striatum; LS: limbic striatum. Effect of condition denotes overall effect of scan condition (Baseline, Placebo_Task or Bupropion_Task) in repeated measures ANOVA, Effect of side denotes overall effect of side (Left or Right) in repeated measures ANOVA.

Baseline Placebo_Task Bupropion_Task Effect of condition Effect of side
Left AST 2.16±0.15 2.15±0.15 2.24±0.12 p = 0.036* p < 0.001 *
Right AST 2.32±0.17 2.26±0.17 2.36±0.12
Left SMS 2.52±0.14 2.52±0.18 2.55±0.18 p = 0.080 p < 0.001 *
Right SMS 2.78±0.17 2.73±0.19 2.88±0.17
Left LS 2.28±0.15 2.29±0.16 2.32±0.22 p = 0.810 p = 0.004 *
Right LS 2.14±0.16 2.11±0.12 2.15±0.10
Open in a new tab
*

p < 0.05.

Figure 2. Individual [11C]raclopride BPND in the associative striatum in the Placebo_Task and Bupropion_Task conditions.

Figure 2

Open in a new tab

Bupropion administration significantly increased BPND (p =0.028).

No significant effects of scan condition on [11C]raclopride BPND were apparent in the sensorimotor (F2 = 2.919; p = 0.080) or limbic (F1 = 0.213; p = 0.810) striatal subdivisions. As in the associative striatum, there were significant effects of side (left or right) on BPND in the sensorimotor (F1 = 48.074; p < 0.001) and limbic subdivisions (F1 = 15.36; p = 0.004) but no significant condition by side interactions were detected.

Discussion

Using [11C]raclopride PET, we sought to determine whether bupropion administration increases extracellular dopamine levels in the rat and human striatum. In rats, bupropion administration decreased striatal [11C]raclopride specific binding, consistent with increases in extracellular dopamine concentrations resulting from inhibition of dopamine reuptake. However, when this approach was translated to man, bupropion administration did not decrease striatal [11C]raclopride BPND, indicating that extracellular dopamine levels were not increased to levels detectable using this approach. These results indicate that, in man, bupropion’s therapeutic efficacy is unlikely to principally derive from marked increases in striatal dopaminergic transmission.

The decreases in [11C]raclopride SBR which we report in anaesthetized rats are accordant with the increases in extracellular dopamine concentrations that are detected using microdialysis following administration of similar doses of bupropion in awake animals (Brown et al., 1991; Li et al., 2002; Nomikos et al., 1989; Sidhpura et al., 2007). Whilst the relationship between dopamine release and change in [11C]raclopride binding potential varies according to the pharmacological nature of the challenge stimulus (Schiffer et al., 2006; Tsukada et al., 1999), previous studies combining microdialysis and [11C]raclopride PET in animals following administration of the dopamine transporter inhibitor methylphenidate, estimate that a 1% change in [11C]raclopride BP relates to a 17% change in dopamine concentration as measured using microdialysis (Schiffer et al., 2006). Applying this relationship to the current data, the 6% and 22% change in SBR observed at 11mg/kg and 25 mg/kg bupropion respectively, would translate to a 102% and 374% change in dopamine concentration as measured using microdialysis. These values are in the range of the percentage increases in striatal dopamine concentrations that have been reported using microdialysis in rats after administration of bupropion at similar doses (Brown et al., 1991; Li et al., 2002; Nomikos et al., 1989; Sidhpura et al., 2007) and therefore validate the use of [11C]raclopride PET imaging to measure changes in striatal dopamine concentrations following bupropion administration.

In the human study, we did not observe any significant decreases in [11C]raclopride BPND in the striatum following bupropion administration, despite the presence of a behavioral task applied to stimulate dopamine release and therefore maximize the influence of dopamine re-uptake inhibition. We therefore conclude that bupropion administration does not markedly increase striatal extracellular dopamine concentrations. Indeed to the contrary, in the associative striatum we detected a small increase in [11C]raclopride BPND, consistent with decreases in extracellular dopamine concentrations, although this did not survive correction for multiple comparisons.

In rats, decreases in [11C]-raclopride BPND occurred at 11 and 25mg/kg bupropion i.p. The 11mg/kg bupropion dose is equivalent to the human dose of 150mg as simply estimated using dose scaling factors (Mordenti and Chapperel, 1989). However, the extensive metabolism of bupropion to the active metabolites hydroxybupropion and threohydrobupropion in man (Schroeder, 1983), occurs to a far lesser extent in rats (Suckow et al., 1986). We did not compare the plasma concentrations of bupropion and its metabolites that were achieved in the rat and human subjects, although, as brain concentrations of bupropion may be some order of magnitude higher than those measured in plasma (Schroeder et al., 1983; Suckow et al., 1986), interpretation would be limited. A better indication of dose equivalence is provided by comparing the degree of striatal DAT occupancy resulting from bupropion administration in rats and man. Here, the lowest (11mg/kg) dose of bupropion investigated in the rat was estimated to occupy at least 35% of DAT sites; in contrast, previous data show the levels of DAT occupancy achieved in man following chronic bupropion dosing are, at most, ~20-25% (Árgyelán et al., 2005; Kugaya et al., 2003; Learned-Coughlin et al., 2003; Meyer et al., 2002). This suggests that higher levels of DAT occupancy were achieved in the rat than the human study, which might explain why significant decreases in [11C]-raclopride BPND following bupropion were observed in rats, but not man.

In the animal literature, the central effects of bupropion are often investigated using doses of 10mg/kg or more. The results of the present study and those previously examining DAT occupancy following bupropion administration in man (Árgyelán et al., 2005; Kugaya et al., 2003; Learned-Coughlin et al., 2003; Meyer et al., 2002), suggest that investigation of the effects of bupropion within a lower dose range would be of increased relevance to human, clinical situation.

The human study was powered (0.8) to reliably detect a 5% change in [11C]-raclopride BPND between scan conditions, based on both previous published data (Mawlawi et al., 2001), and unpublished data acquired in-house on the same scanner. It is unlikely that lack of power precluded observation of decreases in [11C]-raclopride BPND, as in both the associative and sensorimotor striatal divisions [11C]-raclopride BPND was actually increased rather than decreased in 8 of 10 volunteers in the Bupropion_Task compared to Placebo_Task condition. Although we scanned volunteers 2.5 hours after bupropion administration to coincide with peak bupropion plasma concentrations, bupropion metabolite concentrations peak approximately 6 hours following bupropion administration (Hsyu et al., 1997). This raises the possibility that scanning at a later time-point, when dopamine transporter occupancy may have been higher, may have revealed differential effects on [11C]raclopride BPND. It is also possible that repeated administration of bupropion is required to increase striatal dopamine concentrations in man; this hypothesis could be tested using a similar [11C]raclopride PET approach to the present study. However, the low levels of dopamine transporter occupancy observed in man following repeated bupropion administration (Árgyelán et al., 2005; Kugaya et al., 2003; Learned-Coughlin et al., 2003; Meyer et al., 2002) suggest that this is unlikely.

In contrast to our previous study (Lappin et al., 2009), in this sample we did not detect significant decreases in striatal [11C]raclopride BPND during the spatial planning task. Differences between the two studies may explain this. In particular the current but not previous study used placebo tablets, which, as subject expectation may alter dopamine levels, may have masked an effect (Yoder et al., 2008). Furthermore spatial planning accuracy was poorer and more variable in the current sample both offline (74±23% versus 90±10%) and within the scanner (77±20% versus 90±4%), although the age range of subjects in the two studies was similar (mean 47±7 years, range 37-58 years in the present study, mean 53±9 years, range 39 to 68 years in Lappin et al., 2009). This suggests that ability to observe dopamine release during behavioral tasks using [11C]raclopride PET may be particularly sensitive to the precise experimental conditions. We conclude that application of this task, with or without concurrent dopamine re-uptake inhibition, does not provide a robust approach to probing striatal dopamine function in man.

In conclusion, as acute administration of bupropion administration did not result in detectable increases in extracellular dopamine concentrations in the human striatum, this study does not support the involvement of striatal dopamine in the clinical efficacy of bupropion.

Acknowledgements

We would like to thank all the volunteers who participated in this study, and the radiographers and radiochemists at GE Healthcare, Cyclotron Unit, Hammersmith Hospital, who made this study possible. This study was funded by the Medical Research Council and GlaxoSmithKline.

Footnotes

Disclosure / Conflicts of Interest: This study was performed in collaboration with GlaxoSmithKline UK.

References

  1. Argyelán M, Szabó Z, Kanyó B, Tanács A, Kovács Z, Janka Z, Pávics L. Dopamine transporter availability in medication free and in bupropion treated depression: a 99mTc-TRODAT-1 SPECT study. J Affect Disord. 2005;89:115–123. doi: 10.1016/j.jad.2005.08.016. [DOI] [PubMed] [Google Scholar]
  2. Ascher JA, Cole JO, Colin JN, Feighner JP, Ferris RM, Fibiger HC, Golden RN, Martin P, Potter WZ, Richelson E, et al. Bupropion: a review of its mechanism of antidepressant activity. J Clin Psychiatry. 1995;56:395–401. [PubMed] [Google Scholar]
  3. Aubin HJ. Tolerability and safety of sustained-release bupropion in the management of smoking cessation. Drugs. 2002;62(Suppl 2):45–52. doi: 10.2165/00003495-200262002-00005. [DOI] [PubMed] [Google Scholar]
  4. Bredeloux P, Dubuc I, Costentin J. Comparisons between bupropion and dexamphetamine in a range of in vivo tests exploring dopaminergic transmission. Br J Pharmacol. 2007;150:711–719. doi: 10.1038/sj.bjp.0707151. [DOI] [PMC free article] [PubMed] [Google Scholar]
  5. Brown EE, Damsma G, Cumming P, Fibiger HC. Interstitial 3-methoxytyramine reflects striatal dopamine release: an in vivo microdialysis study. J Neurochem. 1991;57:701–707. doi: 10.1111/j.1471-4159.1991.tb03802.x. [DOI] [PubMed] [Google Scholar]
  6. Dagher A, Gunn R, Lockwood G, Cunningham V, Grasby P, Brooks D. Measuring neurotransmitter release with PET: Methodological issues. In: Carson R, Daube-Witherspoon M, Herscovitch P, editors. Quantitative functional brain imaging with positron emission tomography. Academic Press; San Diego: 1998. pp. 449–454. [Google Scholar]
  7. Damaj MI, Carroll FI, Eaton JB, Navarro HA, Blough BE, Mirza S, Lukas RJ, Martin BR. Enantioselective effects of hydroxy metabolites of bupropion on behavior and on function of monoamine transporters and nicotinic receptors. Mol Pharmacol. 2004;66:675–682. doi: 10.1124/mol.104.001313. [DOI] [PubMed] [Google Scholar]
  8. Dannon PN, Lowengrub K, Musin E, Gonopolski Y, Kotler M. Sustained-release bupropion versus naltrexone in the treatment of pathological gambling: a preliminary blind-rater study. J Clin Psychopharmacol. 2005;25:593–596. doi: 10.1097/01.jcp.0000186867.90289.ed. [DOI] [PubMed] [Google Scholar]
  9. DuBois D, DuBois EF. A formula to estimate the approximate surface area if height and weight be known. Arch Intern Medicine. 1916;17:863–871. [Google Scholar]
  10. Dwoskin LP, Rauhut AS, King-Pospisil KA, Bardo MT. Review of the pharmacology and clinical profile of bupropion, an antidepressant and tobacco use cessation agent. CNS Drug Rev. 2006;12:178–207. doi: 10.1111/j.1527-3458.2006.00178.x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  11. Elkashef AM, Rawson RA, Anderson AL, Li SH, Holmes T, Smith EV, Chiang N, Kahn R, Vocci F, Ling W, Pearce VJ, McCann M, Campbell J, Gorodetzky C, Haning W, Carlton B, Mawhinney J, Weis D. Bupropion for the treatment of methamphetamine dependence. Neuropsychopharmacology. 2008;33:1162–1170. doi: 10.1038/sj.npp.1301481. [DOI] [PubMed] [Google Scholar]
  12. Ferris RM, Beaman OJ. Bupropion: a new antidepressant drug, the mechanism of action of which is not associated with down-regulation of postsynaptic beta-adrenergic, serotonergic (5-HT2), alpha 2-adrenergic, imipramine and dopaminergic receptors in brain. Neuropharmacology. 1983;22:1257–1267. doi: 10.1016/0028-3908(83)90198-3. [DOI] [PubMed] [Google Scholar]
  13. Fryer JD, Lukas RJ. Noncompetitive functional inhibition at diverse, human nicotinic acetylcholine receptor subtypes by bupropion, phencyclidine, and ibogaine. J Pharmacol Exp Ther. 1999;288:88–92. [PubMed] [Google Scholar]
  14. Gazzara RA, Andersen SL. The effects of bupropion in vivo in the neostriatum of 5-day-old and adult rats. Brain Res Dev Brain Res. 1997;100:139–142. doi: 10.1016/s0165-3806(97)00006-0. [DOI] [PubMed] [Google Scholar]
  15. Hammers A, Allom R, Koepp MJ, Free SL, Myers R, Lemieux L, Mitchell TN, Brooks DJ, Duncan JS. Three-dimensional maximum probability atlas of the human brain, with particular reference to the temporal lobe. Hum Brain Mapp. 2003;19:224–247. doi: 10.1002/hbm.10123. [DOI] [PMC free article] [PubMed] [Google Scholar]
  16. Houston GC, Hume SP, Hirani E, Goggi JL, Grasby PM. Temporal characterisation of amphetamine-induced dopamine release assessed with [11C]raclopride in anaesthetised rodents. Synapse. 2004;51:206–212. doi: 10.1002/syn.10296. [DOI] [PubMed] [Google Scholar]
  17. Hume SP, Lammertsma AA, Myers R, Rajeswaran S, Bloomfield PM, Ashworth S, Fricker RA, Torres EM, Watson J, Jones T. The potential of high-resolution positron emission tomography to monitor striatal dopaminergic function in rat models of disease. J Neurosci Methods. 1996;67:103–112. [PubMed] [Google Scholar]
  18. Hume S, Hirani E, Opacka-Juffry J, Myers R, Townsend C, Pike V, Grasby P. Effect of 5-HT on binding of [(11)C] WAY 100635 to 5-HT(IA) receptors in rat brain, assessed using in vivo microdialysis and PET after fenfluramine. Synapse. 2001;41:150–159. doi: 10.1002/syn.1069. [DOI] [PubMed] [Google Scholar]
  19. Hurt RD, Sachs DP, Glover ED, Offord KP, Johnston JA, Dale LC, Khayrallah MA, Schroeder DR, Glover PN, Sullivan CR, Croghan IT, Sullivan PM. A comparison of sustained-release bupropion and placebo for smoking cessation. N Engl J Med. 1997;337:1195–1202. doi: 10.1056/NEJM199710233371703. [DOI] [PubMed] [Google Scholar]
  20. Hsyu PH, Singh A, Giargiari TD, Dunn JA, Ascher JA, Johnston JA. Pharmacokinetics of bupropion and its metabolites in cigarette smokers versus nonsmokers. J Clin Pharmacol. 1997;37:737–743. doi: 10.1002/j.1552-4604.1997.tb04361.x. [DOI] [PubMed] [Google Scholar]
  21. Innis RB, Cunningham VJ, Delforge J, Fujita M, Gjedde A, Gunn RN, Holden J, Houle S, Huang SC, Ichise M, Iida H, Ito H, Kimura Y, Koeppe RA, Knudsen GM, Knuuti J, Lammertsma AA, Laruelle M, Logan J, Maguire RP, Mintun MA, Morris ED, Parsey R, Price JC, Slifstein M, Sossi V, Suhara T, Votaw JR, Wong DF, Carson RE. Consensus nomenclature for in vivo imaging of reversibly binding radioligands. J Cereb Blood Flow Metab. 2007;27:1533–1539. doi: 10.1038/sj.jcbfm.9600493. [DOI] [PubMed] [Google Scholar]
  22. Jorenby DE, Leischow SJ, Nides MA, Rennard SI, Johnston JA, Hughes AR, Smith SS, Muramoto ML, Daughton DM, Doan K, Fiore MC, Baker TB. A controlled trial of sustained-release bupropion, a nicotine patch, or both for smoking cessation. N Engl J Med. 1999;340:685–691. doi: 10.1056/NEJM199903043400903. [DOI] [PubMed] [Google Scholar]
  23. Kugaya A, Seneca NM, Snyder PJ, Williams SA, Malison RT, Baldwin RM, Seibyl JP, Innis RB. Changes in human in vivo serotonin and dopamine transporter availabilities during chronic antidepressant administration. Neuropsychopharmacology. 2003;28:413–420. doi: 10.1038/sj.npp.1300036. [DOI] [PubMed] [Google Scholar]
  24. Lappin JM, Reeves SJ, Mehta MA, Egerton A, Coulson M, Grasby PM. Dopamine release in the human striatum: motor and cognitive tasks revisited. J Cereb Blood Flow Metab. 2009;29:554–564. doi: 10.1038/jcbfm.2008.146. [DOI] [PubMed] [Google Scholar]
  25. Laruelle M. Imaging synaptic neurotransmission with in vivo binding competition techniques: a critical review. J Cereb Blood Flow Metab. 2000;20:423–451. doi: 10.1097/00004647-200003000-00001. [DOI] [PubMed] [Google Scholar]
  26. Learned-Coughlin SM, Bergström M, Savitcheva I, Ascher J, Schmith VD, Långstrom B. In vivo activity of bupropion at the human dopamine transporter as measured by positron emission tomography. Biol Psychiatry. 2003;54:800–805. doi: 10.1016/s0006-3223(02)01834-6. [DOI] [PubMed] [Google Scholar]
  27. Li SX, Perry KW, Wong DT. Influence of fluoxetine on the ability of bupropion to modulate extracellular dopamine and norepinephrine concentrations in three mesocorticolimbic areas of rats. Neuropharmacology. 2002;42:181–190. doi: 10.1016/s0028-3908(01)00160-5. [DOI] [PubMed] [Google Scholar]
  28. Martinez D, Slifstein M, Broft A, Mawlawi O, Hwang DR, Huang Y, Cooper T, Kegeles L, Zarahn E, Abi-Dargham A, Haber SN, Laruelle M. Imaging human mesolimbic dopamine transmission with positron emission tomography. Part II. Amphetamine-induced dopamine release in the functional subdivisions of the striatum. J. Cereb. Blood Flow Metab. 2003;23:285–300. doi: 10.1097/01.WCB.0000048520.34839.1A. [DOI] [PubMed] [Google Scholar]
  29. Mawlawi O, Martinez D, Slifstein M, Broft A, Chatterjee R, Hwang DR, Huang Y, Simpson N, Ngo K, Van Heertum R, Laruelle M. Imaging human mesolimbic dopamine transmission with positron emission tomography: I. Accuracy and precision of D(2) receptor parameter measurements in ventral striatum. J Cereb Blood Flow Metab. 2001;21:1034–1057. doi: 10.1097/00004647-200109000-00002. [DOI] [PubMed] [Google Scholar]
  30. Meyer JH, Gunn RN, Myers R, Grasby PM. Assessment of spatial normalization of PET ligand images using ligand-specific templates. Neuroimage. 1999;9:545–553. doi: 10.1006/nimg.1999.0431. [DOI] [PubMed] [Google Scholar]
  31. Meyer JH, Goulding VS, Wilson AA, Hussey D, Christensen BK, Houle S. Bupropion occupancy of the dopamine transporter is low during clinical treatment. Psychopharmacology. 2002;163:102–105. doi: 10.1007/s00213-002-1166-3. [DOI] [PubMed] [Google Scholar]
  32. Miller DK, Sumithran SP, Dwoskin LP. Bupropion inhibits nicotine-evoked [(3)H]overflow from rat striatal slices preloaded with [(3)H]dopamine and from rat hippocampal slices preloaded with [(3)H]norepinephrine. J Pharmacol Exp Ther. 2002;302:1113–1122. doi: 10.1124/jpet.102.033852. [DOI] [PubMed] [Google Scholar]
  33. Mordenti J, Chappell W. The use of interspecies scaling in toxicokinetics. In: Yacobi A, Kelly J, Batra V, editors. Toxicokinetics and New Drug Development. Pergamon Press; New York: 1989. pp. 42–96. [Google Scholar]
  34. Myers R, Hume S. Small animal PET. Eur Neuropsychopharmacol. 2002;12:545–555. doi: 10.1016/s0924-977x(02)00103-7. [DOI] [PubMed] [Google Scholar]
  35. Nomikos GG, Damsma G, Wenkstern D, Fibiger HC. Acute effects of bupropion on extracellular dopamine concentrations in rat striatum and nucleus accumbens studied by in vivo microdialysis. Neuropsychopharmacology. 1989;2:273–279. doi: 10.1016/0893-133x(89)90031-6. [DOI] [PubMed] [Google Scholar]
  36. Owen AM. Cognitive planning in humans: neuropsychological, neuroanatomical and neuropharmacological perspectives. Prog Neurobiol. 1997;53:431–450. doi: 10.1016/s0301-0082(97)00042-7. [DOI] [PubMed] [Google Scholar]
  37. Panagopoulos NT, Papadopoulos GC, Matsokis NA. Dopaminergic innervation and binding in the rat cerebellum. Neurosci Lett. 1991;130:208–212. doi: 10.1016/0304-3940(91)90398-d. [DOI] [PubMed] [Google Scholar]
  38. Paterson NE, Balfour DJ, Markou A. Chronic bupropion attenuated the anhedonic component of nicotine withdrawal in rats via inhibition of dopamine reuptake in the nucleus accumbens shell. Eur J Neurosci. 2007;25:3099–3108. doi: 10.1111/j.1460-9568.2007.05546.x. [DOI] [PubMed] [Google Scholar]
  39. Paterson NE. Behavioural and pharmacological mechanisms of bupropion’s anti-smoking effects: recent preclinical and clinical insights. Eur J Pharmacol. 2009;603:1–11. doi: 10.1016/j.ejphar.2008.12.009. [DOI] [PubMed] [Google Scholar]
  40. Rau KS, Birdsall E, Hanson JE, Johnson-Davis KL, Carroll FI, Wilkins DG, Gibb JW, Hanson GR, Fleckenstein AE. Bupropion increases striatal vesicular monoamine transport. Neuropharmacology. 2005;49:820–830. doi: 10.1016/j.neuropharm.2005.05.004. [DOI] [PubMed] [Google Scholar]
  41. Robb RA, Hanson DP. A software system for interactive and quantitative visualization of multidimensional biomedical images. Australas Phys Eng Sci Med. 1991;14:9–30. [PubMed] [Google Scholar]
  42. Schiffer WK, Volkow ND, Fowler JS, Alexoff DL, Logan J, Dewey SL. Therapeutic doses of amphetamine or methylphenidate differentially increase synaptic and extracellular dopamine. Synapse. 2006;59:243–251. doi: 10.1002/syn.20235. [DOI] [PubMed] [Google Scholar]
  43. Schroeder DH. Metabolism and kinetics of bupropion. J Clin Psychiatry. 1983;144:79–81. [PubMed] [Google Scholar]
  44. Sidhpura N, Redfern P, Rowley H, Heal D, Wonnacott S. Comparison of the effects of bupropion and nicotine on locomotor activation and dopamine release in vivo. Biochem Pharmacol. 2007a;74:1292–1298. doi: 10.1016/j.bcp.2007.06.025. [DOI] [PubMed] [Google Scholar]
  45. Shiffman S, Johnston JA, Khayrallah M, Elash CA, Gwaltney CJ, Paty JA, Gnys M, Evoniuk G, DeVeaugh-Geiss J. The effect of bupropion on nicotine craving and withdrawal. Psychopharmacology. 2000;148:33–40. doi: 10.1007/s002130050022. [DOI] [PubMed] [Google Scholar]
  46. Slemmer JE, Martin BR, Damaj MI. Bupropion is a nicotinic antagonist. J Pharmacol Exp Ther. 2000;295:321–327. [PubMed] [Google Scholar]
  47. Stokes PR, Mehta MA, Curran HV, Breen G, Grasby PM. Can recreational doses of THC produce significant dopamine release in the human striatum? Neuroimage. 2009;48:186–190. doi: 10.1016/j.neuroimage.2009.06.029. [DOI] [PubMed] [Google Scholar]
  48. Studholme C, Hill DL, Hawkes DJ. Automated 3-D registration of MR and CT images of the head. Med. Image Anal. 1996;1:163–175. doi: 10.1016/s1361-8415(96)80011-9. [DOI] [PubMed] [Google Scholar]
  49. Suckow RF, Smith TM, Perumal AS, Cooper TB. Pharmacokinetics of bupropion and metabolites in plasma and brain of rats, mice, and guinea pigs. Drug Metab Dispos. 1986;14:692–697. [PubMed] [Google Scholar]
  50. Tsukada H, Nishiyama S, Kakiuchi T, Ohba H, Sato K, Harada N. Is synaptic dopamine concentration the exclusive factor which alters the in vivo binding of [11C]raclopride?: PET studies combined with microdialysis in conscious monkeys. Brain Res. 1999;841:160–169. doi: 10.1016/s0006-8993(99)01834-x. [DOI] [PubMed] [Google Scholar]
  51. Turkheimer FE, Brett M, Visvikis D, Cunningham VJ. Multiresolution analysis of emission tomography images in the wavelet domain. J. Cereb. Blood Flow Metab. 1999;19:1189–1208. doi: 10.1097/00004647-199911000-00003. [DOI] [PubMed] [Google Scholar]
  52. Volkow ND, Wang GJ, Fowler JS, Logan J, Franceschi D, Maynard L, Ding YS, Gatley SJ, Gifford A, Zhu W, Swanson JM. Relationship between blockade of dopamine transporters by oral methylphenidate and the increases in extracellular dopamine: therapeutic implications. Synapse. 2002;43:181–187. doi: 10.1002/syn.10038. [DOI] [PubMed] [Google Scholar]
  53. Volkow ND, Wang GJ, Fowler JS, Learned-Coughlin S, Yang J, Logan J, Schlyer D, Gatley JS, Wong C, Zhu W, Pappas N, Schueller M, Jayne M, Carter P, Warner D, Ding YS, Shea C, Xu Y. The slow and long-lasting blockade of dopamine transporters in human brain induced by the new antidepressant drug radafaxine predict poor reinforcing effects. Biol Psychiatry. 2005;57:640–646. doi: 10.1016/j.biopsych.2004.12.007. [DOI] [PubMed] [Google Scholar]
  54. Warner C, Shoaib M. How does bupropion work as a smoking cessation aid? Addict Biol. 2005;10:219–231. doi: 10.1080/13556210500222670. [DOI] [PubMed] [Google Scholar]
  55. Yoder KK, Kareken DA, Morris ED. What were they thinking? Cognitive states may influence [11C]raclopride binding potential in the striatum. Neurosci Lett. 2008;430:38–42. doi: 10.1016/j.neulet.2007.10.017. [DOI] [PMC free article] [PubMed] [Google Scholar]

RESOURCES