MicroPET imaging of 5-HT1A receptors in rat brain: a test–retest [18F]MPPF study

  • Nicolas Aznavour
  • Chawki Benkelfat
  • Paul Gravel
  • Antonio Aliaga
  • Pedro Rosa-Neto
  • Barry Bedell
  • Luc Zimmer
  • Laurent Descarries
Original Article

Abstract

Purpose

Earlier studies have shown that positron emission tomography (PET) imaging with the radioligand [18F]MPPF allows for measuring the binding potential of serotonin 5-hydroxytryptamine1A (5-HT1A) receptors in different regions of animal and human brain, including that of 5-HT1A autoreceptors in the raphe nuclei. In the present study, we sought to determine if such data could be obtained in rat, with a microPET (R4, Concorde Microsystems).

Methods

Scans from isoflurane-anaesthetised rats (n = 18, including six test–retest) were co-registered with magnetic resonance imaging data, and binding potential, blood to plasma ratio and radiotracer efflux were estimated according to a simplified reference tissue model.

Results

Values of binding potential for hippocampus (1.2), entorhinal cortex (1.1), septum (1.1), medial prefrontal cortex (1.0), amygdala (0.8), raphe nuclei (0.6), paraventricular hypothalamic nucleus (0.5) and raphe obscurus (0.5) were comparable to those previously measured with PET in cats, non-human primates or humans. Test–retest variability was in the order of 10% in the larger brain regions (hippocampus, medial prefrontal and entorhinal cortex) and less than 20% in small nuclei such as the septum and the paraventricular hypothalamic, basolateral amygdaloid and raphe nuclei.

Conclusions

MicroPET brain imaging of 5-HT1A receptors with [18F]MPPF thus represents a promising avenue for investigating 5-HT1A receptor function in rat.

Keywords

Binding potential Serotonin receptors Brain imaging Autoreceptors Positron emission tomography 

Introduction

The brain serotonin [5-hydroxytryptamine (5-HT)] system regulates a variety of functions and behaviours (reviewed in [1, 2]) via a neuronal network pervading most regions of the neuraxis [3, 4, 5, 6, 7, 8] and operating through at least 14 5-HT receptor subtypes [9]. Particular attention has been paid to the characterisation of the structural, pharmacological and functional properties of the serotonin1A receptors (5-HT1AR; e.g. [10]) owing to their implication in different neurological and psychiatric diseases [11] and their established role in the action of antidepressants/anxiolytics, such as the selective serotonin reuptake inhibitors (reviewed in [12]). 5-HT1AR act as autoreceptors on the soma and dendrites of 5-HT neurons themselves, in the raphe dorsalis nucleus (RN), for example, and as somatodendritic (so-called post-synaptic) heteroreceptors in territories of 5-HT projection, such as hippocampus and cerebral cortex. Because 5-HT!A autoreceptors negatively control the firing and release of 5-HT neurons (reviewed in [13]), their desensitisation, together with increased sensitivity and signalling of 5-HT1A heteroreceptors [14, 15, 16, 17, 18], is generally viewed as accounting for the increases in 5-HT neurotransmission underlying the efficacy of antidepressant treatments.

Several ligands have been developed to image 5-HT1AR in vivo with positron emission tomography (PET), among which the selective 5-HT1A receptor antagonist, 4-2′-(methoxyphenyl)-1-[2″-(N-2″-pyridinyl)-p-fluorobenzamido]ethyl-piperazine (MPPF), labelled with [18F]fluorine: [18F]MPPF [19, 20]. Preliminary radiopharmacological experiments have indicated that the regional distribution of [18F]MPPF in rat brain matches the density of 5-HT1AR binding sites [21, 22]. [18F]MPPF has been successfully used for PET studies in cat, monkey and human brain (reviewed in [23]).

An initial microPET study with [18F]MPPF has already shown high binding of this radioligand in the hippocampus of anaesthetised rat [21]. [18F]MPPF microPET imaging is of particular interest in this species, in view of the detailed knowledge of its 5-HT system and the increasing use of rodent models for investigating a variety of pathological conditions and their treatment, including mood disorders. Moreover, combined immuno-electron microscopic studies of 5-HT1AR and β-microprobe measurement of the in vivo binding of [18F]MPPF in rat have shown that after acute treatment with the prototypical SSRI, fluoxetine (Prozac), selective decreases in [18F]MPPF binding are associated with 5-HT1A autoreceptor internalisation in the dorsal raphe nucleus [24, 25]. Moreover, under similar conditions, decreases in binding potential (BP) have been measured with PET in the dorsal raphe nucleus of cat and human [26, 27]. From a brain imaging perspective, PET scanners designed to image small animals offer the advantage of having better spatial resolution (<1.85 mm) than full-size PET scanners (4 mm) but lower sensitivity. In this context, it was deemed useful to assess the applicability of the [18F]MPPF microPET strategy for quantifying 5-HT1AR binding in the brain of living rats and its reliability over time, notably in an anatomical region as small as the DRN.

Materials and methods

Animals

All procedures involving animals and their care were conducted in strict accordance with the Guidelines for Care of Laboratory Animals of the French Ministère de l’Agriculture et de la Forêt (87–848) and the European Economic Community (86–60, EEC) and the Guide to the Care and Use of Experimental Animals (Ed2) of the Canadian Council on Animal Care. The magnetic resonance imaging (MRI) protocol was approved by the Ethics Committee of the Centre Léon Bérard (Lyon, France) and the microPET imaging protocol by the McGill University Animal Care Committee (Montreal, QC, Canada).

MRI data were obtained from a single adult male, Sprague–Dawley rat (Élevage Dépré, Saint Doulchard, France) weighing 250 g. The microPET experiments were carried out in 13 adult male, Sprague–Dawley rats (Charles River, St-Constant, QC, Canada) weighing 250 ± 25 g. These rats were housed individually at a constant temperature (20–22°C) and under a 12-h light/dark cycle, with free access to food and water.

Magnetic resonance imaging

The MRI study was carried out on a 7-T Bruker BioSpin system (Bruker Biospin, Ettlingen, Germany) using a surface coil of 23 mm in diameter. After anaesthesia by inhalation of isoflurane (5% for 2 min followed by 2%) in air (2 L/min), the rat was placed in a heated mat-cushioned body holder (37°C), with its head immobilised by a hard palate piece. The MRI acquisition (BIO 7T, Bruker BioSpin MRI GmbH, resolution) consisted of a 3D anatomical scan of 38-min duration. A 22.5 × 20 × 22.5-mm field of view (FOV) was used to obtain a 256 × 128 matrix divided in 45 slices, with a resolution of 88 × 156 × 500 μm.

MicroPET scans

[18F]MPPF was obtained by nucleophilic fluoration of a nitro precursor as previously described in detail [20]. The radiochemical yield was in the order of 20–25% at the end of synthesis, with a specific activity of 37–111 GBq/μmol.

MicroPET imaging of [18F]MPPF was performed with a MicroPET R4 (Siemens Preclinical Solutions, Knoxville, TN, USA) scanner, which has a volumetric resolution better than 15.6 μl within 20 mm of the centre of the FOV [28]. The spatial resolution of this scanner at the centre of the FOV is 1.85 mm full-width at half-maximum (FWHM) in the axial direction and 1.66 mm FWHM in the transaxial and tangential directions.

After isoflurane anaesthesia as above, the rat was placed in the body holder with its head affixed to the incisor bar. The laser guidance system at the front of the gantry was used to centre the brain in the FOV. A 10-min transmission scan was acquired using a rotating 57Co point source, followed by a bolus injection of 7.4–11.1 MBq of [18F]MPPF in the tail vein. Radioactivity was then measured in 27 sequential time frames of increasing duration (8 × 30, 6 × 60, 5 × 120 and 8 × 300 s) for a total duration of 60 min.

Sinograms were normalised, corrected for randoms, scatter, attenuation, dead time and radio element decay and finally reconstructed with a filtered backprojection (Hanning filter of cut-off 0.5 cycles/pixels). This allowed the dynamic study of 18 volumes, each with a 128 × 128 matrix divided in 63 slices and 0.6 mm3 in voxel size.

Analysis of microPET data

Dynamic PET volumes from 0 to 60 min were integrated and manually co-registered with MRI data (Register-1.3.6 and Minc-2.0.9, MNI-BIC Software, Montreal, Canada) using a rigid body transformation with 6 degrees of freedom. The obtained transformation matrix was then applied to the dynamic PET volumes. A 20-mm3 ellipse was drawn in the centre of the caudal area of cerebellum on the co-registered MRI (Display-1.3, MNI-BIC Software).

This allowed us to estimate the binding parameters of the tracer according to the simplified reference tissue model (SRTM), as previously applied to [11C]WAY100635 [29] and to [18F]MPPF in humans [30, 31] and very recently to [18F]MPPF in rats [32]. The SRTM is based on the analytic solution of the compartment model for estimating three parameters without the use of an arterial sampling input function. In this model, BP is the ratio of available receptor density to receptor affinity, with \({\text{BP}} \approx B\prime_{\max } /K\prime_{\text{d}} \) (\(B\prime_{\max } \) being the apparent Bmax and \(K\prime_{\text{d}} \) the apparent dissociation constant), k2 is the tracer’s efflux between the vascular system and R1 is the ratio of plasma to brain transport constant in the volume of interest (VOI) and reference region (\(K_1 /K\prime_1 \)). The cerebellum was used as the region of reference since it contains very few 5-HT1AR in adult rat [33, 34].

Parametric images of BP, k2 and R1 were calculated from individual voxel time–activity curves using Receptor Parametric Mapping software [35]. BP volumes were automatically co-registered using PET-to-PET cross-correlation with 7 degrees of freedom (mni_autoreg-0.99.3, MNI-BIC Software). The same transformation matrices were applied to parametric images of k2 and R1. All 18 PET volumes of BP were averaged into a single volume, which was then fused with the MRI to draw the regions containing BP and regrouped into anatomical VOIs (Display-1.3, MNI-BIC Software). The Paxinos and Watson’s stereotaxic atlas of the rat brain [36] was used as an anatomical reference to identify eight regions on this MRI and draw the corresponding VOIs for medial prefrontal cortex (27 mm3), septum (8 mm3), hippocampus (100 mm3), raphe nuclei (3 mm3), entorhinal cortex (15 mm3), amygdala (7 mm3), paraventricular hypothalamic nucleus (4 mm3) and raphe obscurus (2 mm3). Regional radioactivity concentration (kilobecquerel per cubic centimetre) was also measured in the dynamic PET volumes for each VOI and plotted versus time.

Test–retest variability (Eq. 1), intraclass correlation coefficient (Eq. 2) and the coefficient of variation (Eq. 3) were computed with the following equations:
$${\text{Test - retest}}\,{\text{variability}}\left( {{\text{TRV}}} \right) = 100 \cdot \frac{{\left| {{\text{scan}}_{\text{1}} {\text{ - scan}}_{\text{2}} } \right|}}{{{{\left( {{\text{scan}}_{\text{1}} {\text{ - scan}}_{\text{2}} } \right)} \mathord{\left/ {\vphantom {{\left( {{\text{scan}}_{\text{1}} {\text{ - scan}}_{\text{2}} } \right)} {\text{2}}}} \right. \kern-\nulldelimiterspace} {\text{2}}}}}$$
(1)
$${\text{Intraclass}}\,{\text{correlation}}\,{\text{coefficient}}\left( {{\text{ICC}}} \right) = \frac{{{\text{MSBS - MSWS}}}}{{{\text{MSBS + }}\left( {n{\text{ - 1}}} \right){\text{MSWS}}}}$$
(2)
$${\text{Coefficient}}\,{\text{of}}\,{\text{variation}}{\left( {{\text{CV}}} \right)} = 100 \cdot \frac{\sigma }{\mu }$$
(3)
MSBS and MSWS are the mean sum of squares between and within subjects, respectively; n is the number of within-subject measurements, μ is the mean value and σ is the standard deviation. ICC represents the ratio of between-subject variance to total variance and is the appropriate metric for assessing within-subject reliability. Therefore, ICC values will be particularly high when within-subject (i.e. within-subject between-session) variance is low and between-subject variance is high. CV is a measure of dispersion and often referred to as the relative standard deviation. Regional BP values were checked for normality using d’Agostino and Pearson’s omnibus normality test and then compared by one-way ANOVA with Bonferroni’s post test. Student’s two-tailed paired t test was also used to assess regional differences in BP between test and retest scans (GraphPad Prism version 5, GraphPad Software, San Diego, CA, USA).

Statistical parametric mapping (SPM) analysis

Co-registered parametric images of BP were smoothed with a 1-mm Gaussian blur, and a voxelwise paired t test was performed in SPM5 (Wellcome Trust Centre for Neuroimaging at UCL, UK) to examine differences between test and retest scans (voxel size 0.1 × 0.2 × 0.5 mm; 1,474,560 voxels; height threshold T = 7.17, p < 0.001 uncorrected; extent threshold k = 10 voxels, p = 0.031).

Pretreatment with 5-HT1A receptor agonist

A single rat was injected with a saline solution containing the 5-HT1A agonist 8-OH-DPAT (5 mg/kg, i.p.; Sigma-Aldrich, Oakville, Ontario, Canada) 60 min before undergoing a [18F]MPPF microPET scan.

Results

As previously observed in PET studies in cat [26, 37] and human (reviewed in [23]), the regional radioactivity time curves after i.v. injection in the rat indicated an initial surge of [18F]MPPF throughout brain, followed within minutes by a relatively rapid washout over the next hour, but at varying rates according to the region examined (Fig. 1). At all time points, the highest radioactivity was that measured in the hippocampus, followed by septum, medial prefrontal cortex and raphe nuclei. The radioactivity in cerebellum dropped sharply in the first 5 min after the bolus injection of [18F]MPPF and remained low at all time points thereafter (Fig. 1).
Fig. 1

Regional radioactivity in the hippocampus, septum, medial prefrontal cortex, raphe nuclei and cerebellum after the bolus injection of [18F]MPPF. Mean from 18 scans in 12 rats, including the test–retest in six of these rats

Receptor parametric mapping revealed the presence of specific 5-HT1A BP in the medial prefrontal cortex, septum, amygdala, hippocampus, entorhinal cortex, raphe nuclei, paraventricular hypothalamic nucleus and raphe obscurus (Figs. 2 and 3). The co-registration of BP and MRI images confirmed the raphe nuclei as the origin of [18F]MPPF binding in mesencephalon, which was clearly discernible and easily delineated from its immediate surround in the coronal (Fig. 2) as well as sagittal and horizontal planes (Fig. 3).
Fig. 2

Pseudo-colour visualisation of [18F]MPPF BP (a, d, g, j, m), MRI views (b, e, h, k, n) and fusion of both type of images (c, f, i, l, o) at different coronal levels across rat brain. Data from 18 microPET scans in 12 rats and one MRI, as described in the “Materials and methods”. In b, e, h and n, diagrams from Paxinos and Watson’s atlas of the rat brain [36] are superposed on the MRI images to identify regions of interest. MPFC Medial prefrontal cortex, Spt septum, PVN paraventricular hypothalamic nucleus, Am amygdala, Hip hippocampus, RN raphe nuclei, Ent entorhinal cortex, Ce cerebellum, ROb, raphe obscurus. Pseudo-colour scale: BP 0.3–2

Fig. 3

Pseudo-colour visualisation of [18F]MPPF BP (a, d), MRI views (b, e) and fusion of both types of images (c, f) as in Fig. 2, but in the sagittal (a, b, c) and the horizontal (d, e, f) planes. MPFC Medial prefrontal cortex, Spt, septum, PVN paraventricular hypothalamic nucleus, Hip hippocampus, RN raphe nuclei, Ce cerebellum, ROb raphe obscurus. Pseudo-colour scale: BP 0.3–2

As measured with the SRTM (Table 1), the highest BP value was that in hippocampus (1.10 ± 0.20), followed by entorhinal (1.09 ± 0.28), septum (1.08 ± 0.19), medial prefrontal cortex (1.00 ± 0.16), amygdala (0.84 ± 0.11), DRN (0.58 ± 0.11), paraventricular hypothalamic nucleus (0.53 ± 0.14) and raphe obscurus (0.52 ± 0.16). The comparison of regional BP values indicated that most of the regions examined were significantly different from one another as shown in Fig. 4.
Fig. 4

Regional values of BP, R1 and k2. Data from 18 microPET scans, as described in the “Materials and methods”. Regions showing statistically significant differences are linked by hooks with asterisks in front of the differing region(s). *p < 0.05, **p < 0.01 and ***p < 0.001 by one-way ANOVA with Bonferroni’s post test. MPFC Medial prefrontal cortex, Spt septum, Hip hippocampus, RN raphe nuclei, Ent entorhinal cortex, Am amygdala, PVN paraventricular hypothalamic nucleus, ROb raphe obscurus

Table 1

[18F]MPPF BP (binding potential), k2 (tracer’s efflux in the vascular system) and R1 (ratio of plasma to brain transport constant) were measured with the simplified reference tissue model (SRTM), as described in the “Materials and methods

Region

BP (mean ± SD)

k2 (mean ± SD)

R1 (mean ± SD)

Medial prefrontal cortex

1.00 ± 0.16

0.015 ± 0.002

0.82 ± 0.16

Septum

1.08 ± 0.20

0.013 ± 0.003

0.75 ± 0.15

Paraventricular hypothalamic nucleus

0.53 ± 0.14

0.012 ± 0.003

0.85 ± 0.16

Amygdala

0.84 ± 0.11

0.011 ± 0.003

0.70 ± 0.13

Hippocampus

1.20 ± 0.19

0.012 ± 0.002

0.77 ± 0.12

Entorhinal cortex

1.09 ± 0.28

0.011 ± 0.003

0.81 ± 0.12

Raphe nuclei

0.58 ± 0.11

0.012 ± 0.003

0.96 ± 0.22

Nucleus raphe obscurus

0.52 ± 0.16

0.010 ± 0.005

0.92 ± 0.19

Mean ± SD from 18 scans in 12 rats, including the test–retest in six of these rats

The estimated regional values of k2 ranged from 0.010 ± 0.005 in the raphe obscurus to 0.015 ± 0.002 in the medial prefrontal cortex (Table 1). Regional differences in k2 were only found in the medial prefrontal cortex, which differed significantly from the entorhinal cortex (p < 0.05) and raphe obscurus (p < 0.01; Fig. 4).

Regional R1 values ranged from 0.70 ± 0.13 in the amygdala to 0.96 ± 0.22 in the DRN (Table 1). They differed significantly in DRN not only from those measured in the septum (p < 0.05) and amygdala (p < 0.001) but also between the latter region and raphe obscurus (p < 0.01; Fig. 2).

As shown in Table 2, the average percent changes between test and retest BP values ranged from 0.3% in the basolateral amygdaloid nucleus to 10.2% in the DRN, while BP test–retest variability ranged from 7% in hippocampus to 34% in the raphe obscurus. ICC values ranged from to −0.06 in the amygdala to 0.92 in the entorhinal cortex. The test–retest variability in DRN (18%) was comparable to that in the paraventricular hypothalamic nucleus (17%). There were no statistically significant differences between test and retest BP values when compared by Student’s paired t test in the VOIs (Table 2) or by voxelwise analysis with SPM (data not shown).
Table 2

Coefficient of variation (CV), test–retest variability (TRV) and intraclass correlation coefficient (ICC) were computed, as described in the “Materials and methods

Region

BP test (mean ± SD)

CV (%)

BP retest (mean ± SD)

CV (%)

Mean change (|%|)

TRV (% ± SD)

t test (p)

ICC

Medial prefrontal cortex

0.95 ± 0.14

14.6

1.03 ± 0.06

6.2

8.5

9.7 ± 10.3

0.16

0.54

Septum

1.11 ± 0.11

10.2

1.05 ± 0.16

15.5

5.5

13.3 ± 9.9

0.51

0.29

Paraventricular hypothalamic nucleus

0.50 ± 0.09

17.8

0.51 ± 0.03

6.0

1.7

17.3 ± 7.8

0.86

0.07

Amygdala

0.81 ± 0.10

13.0

0.81 ± 0.08

9.5

0.3

15.0 ± 8.9

0.97

−0.06

Hippocampus

1.10 ± 0.17

15.6

1.08 ± 0.13

11.9

2.4

11.7 ± 6.0

0.71

0.67

Entorhinal cortex

1.08 ± 0.17

16.1

1.10 ± 0.17

15.4

2.6

6.9 ± 4.2

0.48

0.92

Raphe nuclei

0.52 ± 0.08

14.6

0.57 ± 0.09

15.3

10.2

18.4 ± 5.5

0.29

0.41

Nucleus raphe obscurus

0.48 ± 0.12

24.7

0.46 ± 0.14

30.6

2.6

34.5 ± 23.7

0.89

0.22

Means ± SD from six rats (two scans per rat)

Pretreatment with the 5-HT1A agonist 8-OH-DPAT completely blocked the binding of [18F]MPPF in all cerebral regions (data not shown).

Discussion

The radioactivity curves here obtained from five regions of rat brain, in the hour following the bolus injection of [18F]MPPF, were comparable to those previously measured with PET in rats [21], cats [20, 26, 37, 38], non-human primates [21, 39] and humans [30, 40, 41, 42]. The hippocampus displayed the highest radioactivity, followed by septum, medial prefrontal cortex and raphe nuclei. A similar distribution pattern has also been reported using in vitro [19, 33, 43, 44] or ex vivo [45, 46] autoradiography to measure the density of radioligand binding to 5-HT1AR in rat brain. Furthermore, the specificity of [18F]MPPF binding was confirmed by the pretreatment with 8-OH-DPAT, which completely blocked [18F]MPPF binding in every brain region containing 5-HT1A receptors.

The present report provides regional estimates of [18F]MPPF BP, k2 and R1 values for rat brain. As shown in Table 3, the BP values were in the same range as those previously reported in various species using PET. Very recently, Millet et al. [32] have reported strikingly similar [18F]MPPF BP values in the hippocampus and raphe nuclei of rats using the YAP-(S)PET scanner, even though their data were not corrected for attenuation or scatter.
Table 3

5-HT1A BP values in selected PET studies

Study

Present

[32]

[59]

[26]

[60]

[31]

[27]

Radioligand

[18F]MPPF

[18F]MPPF

[11C]RWAY

[18F]MPPF

[18F]PWAY

[18F]MPPF

[18F]MPPF

Species

Rat

Rat

Rat

Cat

Monkey

Human

Human

Region

Prefrontal cortex

1 ± 0.2

1.2 ± 0.1

1.1 ± 0.1

1.9 ± 0.1

0.8 ± 0.1

0.7 ± 0.2

Septum

1.1 ± 0.2

1.4 ± 0.1

Hippocampus

1.2 ± 0.2

0.9 ± 0.5

1.5 ± 0.2

1.5 ± 0.2

1.4 ± 0.2

1.7 ± 0.5

Entorhinal cortex

1.1 ± 0.2

1.3 ± 0.2

Raphe nuclei

0.5 ± 0.1

0.5 ± 0.4

0.6 ± 0.1

0.5 ± 0.1

0.3 ± 0.1

0.6 ± 0.2

The realignment procedure used in the determination of PET volumes, as described in the “Materials and methods”, was crucial in obtaining reliable measurements from the various regions examined (see [47]). In this way, a single template could be used to extract BP, k2 and R1 values from each volume, thus reducing the risk of a sampling bias associated with the drawing of ROIs. This method allowed us to estimate BP in regions that are not commonly sampled in microPET studies, such as the septum, PVN, amygdala, DRN and even smaller regions such as the raphe obscurus (Robs).

Despite our best efforts, however, a number of methodological constraints could not be avoided. Foremost, in view of the limited resolution of the microPET camera and the size of the smaller regions examined (raphe nuclei, PVN, RObs), it was likely that the BP values in these regions were underestimated due to partial volume effects, as previously noted with [11C]WAY-100635 in human NRD [29, 48]. This underestimation might have been less with [18F]MPPF than [11C]WAY-100635 because of the lower energy of the 18F radionuclide, yet significant. On the other hand, a potential source of overestimation was the accumulation of radioactivity in the Harderian glands, located behind the eyes (visible in Figs. 2 and 3), which could have increased readings from adjacent regions such as the medial prefrontal cortex. It should also be noted that anaesthetics may affect the binding of radioligands to 5-HT1AR by modulating the affinity state of G-protein-coupled receptors [49], as convincingly shown to be the case for dopamine receptors [50, 51].

Nevertheless, the reproducibility and reliability of our sampling method was evidenced by a relatively low variability in repeated measurements of BP and high ICC, at least in the larger brain regions examined (see Table 2). The regional values of test–retest variability were generally lower than those reported with [11C]WAY-100635 in humans (e.g. hippocampus 26 ± 21%, [48]), but on par with those found with [18F]MPPF in cats (e.g. hippocampus 7 ± 6%, [26]) and humans (e.g. hippocampus 7 ± 3%, [31]; e.g. hippocampus 11 ± 11%, [27]). In the hippocampus, [18F]MPPF BP test–retest variability was in the same range as those reported with [11C]raclopride (14.0%) and [18F]FECT (7.7%) in rat striatum [47]. These values were also comparable to the test–retest variability of DVR measured with [11C]raclopride (8.3%) in rat striatum [52]. Test–retest variability was typically higher (and ICC lower) in smaller brain regions. This was particularly noticeable in the RObs. In fact, it was not expected to even be able to identify such a small anatomical region as a site of [18F]MPPF binding, in view of the resolution and sensitivity of the microPET scanner. The low ICC values found in certain regions could also reflect the fact that intersubject variability is relatively low in rats as opposed to human subjects [53]. This could imply that, in [18F]MPPF microPET studies involving repeated scans, it might not be indispensable to use the same animal as its own control.

MicroPET imaging of 5-HT1A receptors with [18F]MPPF represents a valuable asset to study the activation of 5-HT1A brain receptors in vivo. Pharmacological compounds, which bind to 5-HT1A with high affinity, occupy 5-HT1A receptor binding sites and/or displace [18F]MPPF binding, as we have observed with WAY-100635 in cats (unpublished data) and 8-OH-DPAT in rats. The use of this technique might help to determine whether atypical antipsychotics with low affinity for 5-HT1A receptors in vitro actually interact with 5-HT1A heteroreceptors in vivo [54, 55] and also to better characterise the activation and internalisation of 5-HT1A autoreceptors by agonists [25, 56], SSRIs [24, 26, 27, 57] and atypical antipsychotics [58].

Notes

Acknowledgments

The authors are grateful to Shadreck Mzengeza for his help with the radiochemistry. The work was funded by the Canadian Institutes for Health Research (operating grants to C.B. and L.D.). N.A. held a postdoctoral fellowship, and L.D. benefitted from an infrastructure grant from the Fonds de la Recherche en Santé du Québec.

Conflict of interest

The authors declare that they have no competing financial interests.

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Copyright information

© Springer-Verlag 2008

Authors and Affiliations

  • Nicolas Aznavour
    • 1
    • 10
  • Chawki Benkelfat
    • 1
    • 2
  • Paul Gravel
    • 1
    • 2
  • Antonio Aliaga
    • 3
  • Pedro Rosa-Neto
    • 4
  • Barry Bedell
    • 2
    • 3
  • Luc Zimmer
    • 5
    • 6
  • Laurent Descarries
    • 7
    • 8
    • 9
  1. 1.Department of PsychiatryMcGill UniversityMontrealCanada
  2. 2.Department of Neurology and NeurosurgeryMcGill UniversityMontrealCanada
  3. 3.Department of Small Animal Imaging LaboratoryMcGill UniversityMontrealCanada
  4. 4.Molecular NeuroImaging LaboratoryDouglas HospitalMontrealCanada
  5. 5.ANIMAGE DepartmentCERMEPLyonFrance
  6. 6.Université Lyon 1 and CNRSLyonFrance
  7. 7.Department of Pathology and Cell BiologyUniversité de MontréalMontrealCanada
  8. 8.Department of PhysiologyUniversité de MontréalMontrealCanada
  9. 9.GRSNCUniversité de MontréalMontrealCanada
  10. 10.EPFL, SV, BMILaboratory of Neuroenergetics and Cellular DynamicsLausanneSwitzerland

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