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. 2012 Dec 1;35(12):1615–1623. doi: 10.5665/sleep.2230

Sleep Deprivation Increases Cerebral Serotonin 2A Receptor Binding in Humans

David Elmenhorst 1,✉, Tina Kroll 1, Andreas Matusch 1, Andreas Bauer 1,2
PMCID: PMC3490354  PMID: 23204604

Abstract

Study Objectives:

Serotonin and its cerebral receptors play an important role in sleep-wake regulation. The aim of the current study is to investigate the effect of 24-h total sleep deprivation on the apparent serotonin 2A receptor (5-HT2AR) binding capacity in the human brain to test the hypothesis that sleep deprivation induces global molecular alterations in the cortical serotonergic receptor system.

Design:

Volunteers were tested twice with the subtype-selective radiotracer [18F]altanserin and positron emission tomography (PET) for imaging of 5-HT2ARs at baseline and after 24 h of sleep deprivation. [18F]Altanserin binding potentials were analyzed in 13 neocortical regions of interest. The efficacy of sleep deprivation was assessed by questionnaires, waking electroencephalography, and cognitive performance measurements.

Setting:

Sleep laboratory and neuroimaging center.

Patients or Participants:

Eighteen healthy volunteers.

Interventions:

Sleep deprivation.

Measurements and Results:

A total of 24 hours of sleep deprivation led to a 9.6% increase of [18F]altanserin binding on neocortical 5-HT2A receptors. Significant region-specific increases were found in the medial inferior frontal gyrus, insula, and anterior cingulate, parietal, sensomotoric, and ventrolateral prefrontal cortices.

Conclusions:

This study demonstrates that a single night of total sleep deprivation causes significant increases of 5-HT2AR binding potentials in a variety of cortical regions although the increase declines as sleep deprivation continued. It provides in vivo evidence that total sleep deprivation induces adaptive processes in the serotonergic system of the human brain.

Citation:

Elmenhorst D; Kroll T; Matusch A; Bauer A. Sleep Deprivation Increases Cerebral Serotonin 2A Receptor Binding in Humans. SLEEP 2012;35(12):1615-1623.

Keywords: [18F]altanserin, human, positron emission tomography, serotonin 2A receptor, sleep deprivation

INTRODUCTION

Several neurotransmitter systems are involved in the regulation of the sleep-wake cycle. Prolonged wakefulness causes alterations of neurotransmitter levels that are most likely accompanied by changes in respective receptor densities. In humans only a few receptor types have been investigated in this regard,1 including adenosine A1 receptors2 and dopamine D2/D3 receptors.3 So far, there are no data available for receptors of the serotonergic system although they are decisive contributors to circadian rhythmicity and important targets for drug treatment of depression. Moreover, depressive symptoms can be improved by sleep deprivation.

A widely accepted theory of sleep-wake regulation is the two-process model that postulates an interplay of a specific phase of circadian rhythm and a homeostatic sleep debt that sum up the sleep propensity. Sleep deprivation of 24 h primarily affects the homeostatic regulator, which allows investigation of the driving molecular basis of this process.

The serotonergic raphe nuclei with their widespread cortical projections are part of the monoaminergic wake promoting system. Accordingly, cortical serotonin levels are high during wakefulness, reduced during slow wave sleep (SWS), and virtually quiescent during rapid eye movement sleep.4,5 During sleep deprivation the serotonin release is even higher than during the previous wake period, as animal findings suggest. Elevated serotonin levels have been measured in the hippocampus of sleep-deprived rats6 and even during the subsequent recovery period.7 Higher serotonin levels have also been found in dissected dorsal raphe and suprachiasmatic nuclei of sleep-deprived rats.8

Serotonin acts via 14 different receptors9 including the 5-HT1 and 5-HT2 subtypes, which are important for the sleep-regulating actions of serotonin. 5-HT2AR agonists (such as mescaline and psylocybin) enhance wakefulness and reduce SWS, whereas 5-HT2AR antagonists (such as eplivanserin and volianserin) increase SWS and were evaluated as hypnotic agents in phase III clinical trials.5 Blockade of the 5-HT2AR by eplivanserin mimicked certain aspects of the effect of sleep deprivation on recovery sleep.10 Additionally, there is no increase in electroencephalography (EEG) delta power density (an indicator of previous sleep debt) after sleep deprivation in 5-HT2AR knockout mice compared with their wild-type littermates.11 We therefore used the radiofluorinated form of altanserin, a high affinity and selective 5-HT2AR antagonist for imaging of human cerebral 5-HT2ARs in vivo with position emission tomography (PET).12,13 Its low test-retest variability and high reliability14,15 are important prerequisites for the comparative study design used in the current study.

The aim of the current study was to investigate the effect of 24-h total sleep deprivation on 5-HT2AR availability in the human brain to test the hypothesis that sleep deprivation induces global molecular alterations in the cortical serotonergic receptor system. For this purpose, volunteers were examined with [18F]altanserin PET, wake EEG, and the psychomotor vigilance task (PVT) on two subsequent days – after normal sleep and after 24 h of sleep deprivation

MATERIALS AND METHODS

Subjects and Study Design

The study was approved by the Ethics Committee of the Medical Faculty of the University of Düusseldorf, Germany, the German Federal Institute for Drugs and Medical Devices, and the German Federal Office for Radiation Protection.

Eighteen healthy volunteers were recruited via advertisements and were reimbursed for participation. In one volunteer the metabolite analysis failed on the first examination day. In another volunteer highly differing free fractions of radioligand between the two scans were detected during image analysis. Two volunteers did not refrain from caffeine intake before the study, a finding that was revealed retrospectively by plasma caffeine levels from 2-3 mg/L. Although there is no evidence that caffeine influences [18F]altanserin binding or 5-HT2ARs densities, both volunteers were excluded from analysis to rule out possible indirect influences. Consequently, a total of 14 volunteers were included (eight males and six females, mean age 47.4 ± 5.2 y, range 40 – 55 yr).

After giving written informed consent, all volunteers were screened for the following exclusion criteria: history of neurologic or psychiatric diseases, sleep disorders, shift and/or night work, head injury, and alcohol and/or substance abuse.

Some volunteers were taking medication: levothyroxine (n = 3), iodide (n = 1), valsartan + allopurinol (n = 1), and metformin (n = 2). Medication was kept constant during the experiment. All volunteers were nonsmokers and reported to be moderate caffeine consumers with intake of 3.1 ± 1.9 cups (0.15 L) of coffee (or caffeine equivalent) per day. Caffeine intake was not allowed for at least 36 h before the first PET scan. Compliance was confirmed by determination of caffeine plasma levels assessed by liquid chromatography/mass spectrometry. The average caffeine plasma concentration was 0.39 ± 0.4 mg/L.

All volunteers underwent two [18F]altanserin PET scans at the same time of the day on two consecutive days under identical conditions; the first scan after a night with normal sleep and the second one after a night of sustained wakefulness. Volunteers were investigated in groups of two per day with a time lag of 1.5 h between measurements. Time of injection of the radioligand was between 09:33 and 13:33 on both days. Staff members monitored sleep-deprived volunteers during the entire night between the two PET scans to prevent them from falling asleep. During the 60-min period of image acquisition, volunteers were requested to keep their eyes open. In addition, a video system was installed to observe the volunteers in the scanner. As soon as they closed their eyes longer than usual, volunteers were addressed and supported to stay awake. Volunteers reported sleep duration in a sleep diary for one week before the first PET scan. Volunteers were asked to sleep for at least 8 h in a regular schedule during these days. In addition, duration of sleep in the last two nights before the study was controlled by actigraphy (SomnoWatch, SOMNOmedics GmbH, Randersacker, Germany). The wristwatch-sized actigraph was worn on the nondominant wrist. Ambient light level and movements were recorded during 48 h before the first PET scan.

Waking EEG

At least one hour before PET scanning, volunteers were placed in a quiet environment and supplied with electrodes in the C3, C4, A1, and A2 positions for EEG and above and below the lateral angulus oculi for electrooculography. Volunteers rested on a bed in a slightly elevated position for approximately 20 min before a waking EEG was performed, which consisted of 3 min with eyes closed and 5 min with eyes open. Patients were instructed to relax and to fixate on a spot on the ceiling in 2.5-m distance during the eyes-open period. An observer monitored the session continuously and addressed the patients when signs of drowsiness were detected (e.g., rolling eye movements or a reduction in alpha activity). Signals were recorded with a polygraphic amplifier (Somnoscreen PSG EEG-10-20, Somnomedics, Germany), conditioned by a high-pass filter (0.3 Hz) and a low-pass filter (30 Hz) and digitized (128 Hz, 16 bit). The Domino software (version 2.1, Somnomedics) was used to export raw signals in the EDF+ data format. EDF+ data were then imported into MATLAB (The Math Works Inc., Natick, MA, USA) with routines from the BIOSIG Toolbox (A. Schlögl, BIOSIG:a free and open source software library for biomedical signal processing, 2003 – 2010, available online: http://BIOSIG.SF.NET) and further analyzed by using tools from the signal processing toolbox of MATLAB. For each 2-sec epoch, power spectra were calculated by fast Fourier transform routine. Data were previously conditioned by applying a Hanning window and linear detrending. For the 5-min periods with eyes open, activity was averaged from each artifact-free 2-sec epoch as follows: alpha (power 8-12 Hz), lower alpha (power 7-9 Hz16), theta (power 5-8 Hz), and delta (power 0.5-5 Hz). Artifacts were identified by an automated blink detection algorithm and subsequent visual inspection. Automatic artifact detection was performed by squaring the electrooculography signal and excluding every epoch containing a data point that was more than four times higher than the mean of the squared signal. Relative difference in theta and delta activity of the average of the C3/A2 and C4/A1 derivations were expressed as (sleep deprivation – baseline) / baseline activity. One participant with an extraordinarily high eye blink frequency was excluded from EEG analyses because more than 90% of the epochs of both days had to be discarded due to resulting artifacts.

Cognitive Performance Measurements

Participants conducted a 10-min reaction-time task (PVT)17 immediately before PET scanning. In the morning of the first test day participants were trained once in conducting the task. They were asked to perform tests as fast and correct as possible.

Participants had to respond to a LED signal lighting up in irregular intervals (1.5-10 s) on a handheld pocket personal computer by pressing a key as quickly as possible. After each trial the reaction time (in ms) was displayed. The number of presented signals depended on the reaction times of each participant. In this sample the number of presented signals averaged 69.7 ± 10.8 per 10-min trial. Reaction times equal to or longer than 500 ms were regarded as lapses and excluded from analysis. Furthermore, reaction times that were shorter than 130 ms were most probably reactions without stimulus (false starts) and therefore also excluded from analysis. Reaction speed (equals: 1/reaction time) for each reaction was assessed and 10% fastest responses were averaged.

Before and after each PET scan as well as during the nights, participants' sleepiness was screened with the Stanford Sleepiness Scale (SSS) and a visual analog scale (VAS). The SSS is a 7-point Likert scale ranging from very awake to nearly asleep. The VAS ranged from 0 to 1 (very sleepy to very awake). Median SSS and VAS values obtained within the last 15 min before the start of each scan were used for further analysis.

PET Acquisition

[18F]Altanserin PET was performed as previously described12,18 using a bolus/infusion schedule. Scanning took place with participants in the supine position and a quiet ambience. Participants' heads were immobilized in the canthomeatal orientation by a vacuum cushion. PET data were acquired in three-dimensional mode on a Siemens ECAT Exact HR+ scanner (Siemens-CTI, Knoxville, TN, USA) equipped with a circular lead shield to reduce scatter radiation from outside the field of view. [18F]Altanserin was administered as a combination of bolus (2 min) and constant infusion (178 min, bolus/infusion ratio of Kbol = 2.1 h) to rapidly approach an equilibrium of the radioligand in blood and brain. Emission was recorded between 120–180 min after injection in 10-min time frames. Head movements of the participants were recorded with a Polaris optical tracking system (Northern Digital Inc, Waterloo, Ontario, Canada). Head positions were permanently monitored and, if necessary, manually corrected under guidance of a video system and reference skin marks. Data sets were fully corrected for random coincidences, scatter radiation and attenuation (10 min 68Ge/68Ga-transmission scan directly before emission scan), Fourier rebinned into two-dimensional sinograms, and reconstructed by filtered back projection (Shepp filter, 2.5 mm width) with a voxel size of 2 × 2 × 2.43 mm3 (63 slices).

Heparinized venous plasma samples (antecubital vein catheter) were taken at 2, 5, 10, 20, 30, 45, 60, and 120 min after injection and every 10 min during PET emission acquisition. Additional venous blood samples were taken before injection to assess the fraction of free [18F]altanserin in plasma (unbound to proteins, denoted by fP), the recovery of pure [18F]altanserin, and plasma caffeine levels. Radioactivity in whole blood and plasma samples (separated from whole blood samples by 3 min centrifugation at 1,000 g) was measured in an automated gamma counter (1480 WIZARD, Wallac-ADL GmbH, Freiburg, Germany) that was cross-calibrated weekly with the PET scanner. The fraction of nonmetabolized [18F]altanserin was determined in all samples by selective liquid-liquid extraction with quantification of the recovery of total radioactivity followed by thin-layer chromatography.19

Image Processing

To exclude structural brain abnormalities and to define anatomic regions, individual high-resolution magnetic resonance imaging (MRI) data sets were acquired (Magnetom Trio 3T scanner, Siemens, Erlangen, Germany) using a three-dimensional T1-weighted MPRAGE sequence (voxel size 1 mm3). MRI was oriented according to the anterior-posterior commissure (AC-PC line) using the MPI-Tool software (version 6.42, ATV GmbH, Germany). The MRI was segmented (SPM5, Wellcome Trust Centre for Neuroimaging, London, UK) and the gray matter compartment was converted into a binary mask by assigning all voxels with a probability higher than 0.1 to gray matter.

A realignment of later PET frames to the first frame was performed with SPM2 (Wellcome Trust Centre for Neuroimaging, London, UK). Summed PET images of the first scan were manually coregistered to individual MRI data using the MPI-Tool software. Summed PET data of the second scan were aligned to the coregistered PET images of the first scan using the mutual information algorithm of the MPI-Tool software. All transformations were checked visually and applied to each 10-min frame.

Binding Potential

The proposed outcome parameter of [18F]altanserin PET is the binding potential (BPP, mL/cm3)20 related to the plasma compartment activity of [18F]altanserin (CP). The BPP was calculated according to BPP = (Cvoxel – Creference) / CP (with Creference being the average concentration in the reference region cerebellum) for each voxel (Cvoxel) and frame.13 The average of all frames resulted in the final parametric image of [18F]altanserin BPP.

As a substantial fraction of radiometabolites crosses the blood-brain barrier during [18F]altanserin bolus plus constant infusion experiments, the binding potential related to the nondisplaceable compartment (BPND) or distribution volume (VT) cannot be determined reliably as the fraction of radiometabolites is included in the model. BPP is therefore the only accurate outcome parameter of 5-HT2AR densities in [18F]altanserin bolus plus constant infusion experiments.

Region of Interest Analysis

The volume-weighted average BPP values for the neocortex served as primary outcome. According to the atlas template implemented in PMOD software (PMOD v.2.95, PMOD Group, Zurich, Switzerland, AAL atlas [Anatomical Automatic Labeling, merged version]21), neocortex was anatomically defined as being composed of the following side-averaged regions of interest (ROI): precentral and postcentral gyrus (gy), rolandic operculum, supplementary motor area, superior, medial and inferior frontal gy, rectal gy, insula, calcarine sulcus, cuneus, lingual gy, occipital lobe, fusiform gy, supramarginal gy, angular gy, precuneus, paracentral lobule, Heschl gy, parietal lobe, and temporal lobe. BPP images were normalized to the template space and BPP values were individually read out from the parametric image voxels classified as gray matter on each participants' MRI. In detail, individual 1 mm3 voxel size MRI data sets were spatially normalized to the Montreal Neurological Institute (MNI)/International Consortium for Brain Mapping (ICBM) 152 T1 template as supplied with SPM2. The MRI-based nonlinear transformation matrix was transferred to the parametric BPP images.

For detailed regional analysis, ROI were defined on individual, not normalized, MRI data using the PVElab software (PVElab pipeline program, v. 2.0, PVEOut project, Neurobiology Research Unit, Rigshospitalet, Copenhagen University Hospital, Denmark).22

As the field of view of PET was smaller than that covered by MRI, ROI were adjusted manually with the PMOD software if ROI size protruded from one of the two corresponding PET field of views. Within the cerebellar ROI only PET voxels classified as gray matter were used to generate time-activity curves (Creference). A subset of cortical ROI defined by PVElab was merged, side averaged, and used for readout of regional BPP values from the parametric image voxels previously classified as gray matter.

Statistical Analysis

All results are reported as average values (mean ± standard deviation) unless otherwise noted. Statistical significance of BPP was assessed with a mixed model (proc MIXED, SAS Institute Inc. Cary, North Carolina, USA) repeated measure analysis of variance (rmANOVA) with condition (baseline versus sleep deprivation) and ROI as within-subject factors and sex as a between-subject factor. Paired t-tests were used for pairwise comparisons (baseline versus sleep deprivation) of BPP, PET imaging characteristics, EEG recording, PVT, and self-ratings (except for SSS ratings where the nonparametric Wilcoxon test was used). Pearson product moment correlations were used to assess relationships between relative changes ((sleep deprivation – baseline) / baseline) in BPP and EEG power spectral analysis, PVT, and self-ratings on VAS (baseline – sleep deprivation). Analysis of association between self-ratings on SSS and BPP was done with Spearman rank correlation. To correct for multiple comparisons the false discovery rate (FDR) procedure23 (proc MULTTEST, SAS) was applied to the P values obtained by the regional BPP analyses.

RESULTS

Experimental Framework

The average sleep duration the night before the first scan was 6.6 ± 1 h according to the self-rating of the participants and 6.6 ± 1.2 h according to actigraphy. At the time of scan-start, participants were awake on average for 6.9 ± 1.1 h at baseline and for 30.6 ± 0.8 h after sleep deprivation. Self-reported wake-up times and actigraphy times differed only by a few minutes.

PET scans at baseline and sleep deprivation condition were not significantly different with regard to time of day of scanning, injected activity or mass of injected altanserin, fP value, or the fraction of parent compound in plasma. An overview of these data is given in Table 1. There was no significant difference between the participants who underwent scanning first or second during the day (BPP neocortical region: 1.45 vs. 1.37, P = 0.45).

Table 1.

Statistical data for baseline and sleep deprivation with regard to PET imaging characteristics, self ratings, performance measurements, and wake EEG recordings, respectively

graphic file with name aasm.35.12.1615.t01.jpg

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The free fraction of ligand in plasma was determined by ultrafiltration. It is well known that this method is error-prone and highly variable for ligands with a high fraction of protein binding such as [18F]altanserin. As reported previously,13,15 the accuracy of the data did not improve by correcting with the fP value: BPF (which is the binding potential in relation to the free fraction corrected plasma activity) correlated significantly with fP (r = -0.62, P < 0.00001), whereas BPP did not (r = 0.18, P = 0.2).

Effects of Sleep Deprivation

Mean parametric images of BPP before and after sleep deprivation are depicted in Figure 1. Sleep deprivation significantly increased the binding potential of [18F]altanserin in the neocortical ROI (AAL) by 9.6% from 1.36 ± 0.26 to 1.49 ± 0.27 mL/cm3 (paired t-test, P = 0.024). The further explorative analysis of the automatically delineated ROI (PVElab) with a rmANOVA showed a significant main effect of condition (P < 0.0001) and ROI (P < 0.0001) but not of sex (P = 0.08) or of the condition*ROI interaction (P = 0.99). Significant increases (FDR corrected paired t-test) were found in the following ROI: medial inferior frontal gyrus, insula, anterior cingulate, parietal, sensomotoric, and ventrolateral prefrontal cortices. The respective BPP of the investigated ROI are reported in Table 2. Average BPP increased in all ROI and in most participants. For example, in the ventrolateral prefrontal cortex 11 out of 14 participants showed an increase of individual BPP. Individual BPP values for this region are plotted in Figure 2. An overview of the relative changes of BPP ranging from 5% (orbitofrontal cortex) to 15% (insula) is given in Figure 3.

Figure 1.

Figure 1.

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MRI (A) and parametric images representing binding potential (BPP) of [18F]altanserin (5-HT2A receptor binding) at baseline (B) and after sleep deprivation (C). Note the overall cerebral increase of BPP values after sleep deprivation. Spatially normalized average images (n = 14).

Table 2.

Regional [18F]altanserin binding potentials (BPP) before and after 1 night of sleep deprivation (n = 14)

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Figure 2.

Figure 2.

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Individual changes of binding potential (BPP) of [18F]altanserin (5-HT2A receptor binding) in the atlas neocortical region (mean of left and right). Straight lines indicate participans with increased BPP values, dotted lines indicate participants with decreased BPP values.

Figure 3.

Figure 3.

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Average relative changes (sleep deprivation – baseline) / baseline of the binding potential (BPP) of [18F]altanserin (5-HT2A receptor binding) in different brain regions indicating the effect of sleep deprivation (n = 14). Error bars denote SEM. Ctx, cortex; DLPFC, dorsolateral prefrontal cortex; VLPFC, ventrolateral prefrontal cortex. *Significantly different from zero, paired t-test, P < 0.05 controlled for false discovery rate.

There were clear trends with regard to reaction times of the PVT (prolonged after sleep deprivation by 5.8 ± 19 ms) and the 10% fastest reaction speeds (reduced by 3.4 ± 6.1%; P = 0.067) (Table 1).

During the waking EEG recordings the power density of the delta-frequency range was significantly increased ((sleep deprivation – baseline) / baseline) after sleep deprivation by 33.7 ± 32% (P = 0.003, n = 13). Power density in the theta range showed a significant increase of 25.6 ± 40% (P = 0.038, n = 13). No significant differences were found in the alpha or lower alpha band.

Questionnaires (VAS and SSS) revealed a significant increase of sleepiness after sleep deprivation (Table 1). The correlation analysis for changes in self-ratings (sleep deprivation – baseline) versus EEG power-density changes (see previous text) gave a positive significant correlation (theta: r = 0.68, P = 0.014; delta: r = 0.6, P = 0.039, n = 13). No significant correlations were found between the self-reports and the EEG changes toward the cognitive performance measures.

Correlative Analysis of Receptor Densities and Sleep-Related Measures

The change in 5-HT2AR availability correlated significantly with the time spent awake at time of scanning in some regions, and in several regions with the relative change in PVT performance. The respective significance levels and correlation coefficients are reported in Table 3. A representative plot for the superior frontal gyrus atlas region is depicted in Figure 4. No significant correlations were found between the subjective sleepiness rating (VAS) or the delta and theta EEG power density versus the relative changes in BPP.

Table 3.

Correlation between the relative changes of the [18F]altanserin binding potential BPP and the time spent awake (n = 14) or relative changes in psychomotor vigilance task (PVT, n = 14) between baseline and sleep deprivation

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Figure 4.

Figure 4.

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Significant correlations between the relative changes of [18F]altanserin binding potentials (BPP) and the time awake after sleep deprivation (left column, n = 14) as well as the relative changes in psychomotor vigilance task (PVT) performance (10% fastest reaction speeds, right column, n = 14), respectively. The graphs depict the superior frontal gyrus atlas region. Correlation coefficients and significance levels for the other atlas regions are given in Table 3.

DISCUSSION

The current study demonstrates that a single night of total sleep deprivation leads to a global increase of specific [18F]altanserin binding in the human neocortex. This finding points to an upregulation of 5-HT2AR density because the PET outcome parameter BPP is directly proportional to the concentration of available 5-HT2AR as has been shown by comparison with in vitro methods.14 To our knowledge this is the first investigation of the effect of sleep deprivation on the cerebral density of the 5-HT2AR. So far, no direct in vivo or in vitro data are available with regard to prolonged wakefulness and its consequences for cerebral 5-HT2AR in humans or animals. Findings in rodents suggest that during sleep deprivation the serotonin release is higher than during the previous wake period (as discussed in the introduction). Sleep deprivation might therefore alter serotonin levels, which in turn changes both the actual occupancy and the medium- to long-term expression of 5-HT2ARs. In molecular terms, 5-HT2ARs are upregulated throughout the period of sleep deprivation as shown in the current study.

The observed effect size of approximately 10% change in receptor availability is in accordance with earlier findings of studies investigating behavioral or pathologic influences on neuroreceptor regulation. For example, a significant 6.4% decrease of dopamine D2 receptor availability in humans in caudate was reported after 24 h of sleep deprivation using [11C]raclopride PET.3 These data were recently confirmed by the same group reporting a 5.1% decrease in ventral striatum.24 Otherwise it was shown with [11C]DASB PET that depending on the season 5-HTtransporter BPND in putamen varies between winter and summer by approximately 11%.25 In a study of patients with epilepsy, an approximately 8% increase of opioid receptor availability in the epileptic focus 8.5 h after a spontaneous seizure was measured with [11C]diprenorphine PET, with a gradual return to normal levels.26 Participants suffering from a major depressive disorder showed a decrease of receptor availability from 8% to 30% using [18F]altanserin PET.27 Given these pathologic data, we did not expect that alterations caused by sleep deprivation were in the same range as those changes caused by severe neurologic disorders. Instead, an overall difference of 10% is more likely to reflect a physiologic effect.

An intriguing finding of this study is that correlations of sleep deprivation duration versus BPP suggest that, at least in the investigated time span, 5-HT2AR availability seemed to decrease with more time awake. This finding is counterintuitive and opposite of the observed increase in 5-HT2AR availability in the sleep deprivation group as a whole.

Basal serotonin concentrations in the rat brain (microdialysis) have been reported to be in the range of 0.5 nM28 and to rise several-fold with sleep deprivation.7 In turn, higher serotonin levels will result in downregulation of the 5-HT2AR, which is a constant observation for G-protein coupled receptors. However, stimulation of 5-HT2ARs with serotonin or equivalent agonists showed a more complex reaction pattern.29 Internalization of the membrane-bound, green-flourescent-protein-fused 5-HT2AR upon serotonin stimulation (greater than 100 nM) is completed within approximately 10 min in transfected HEK239 cells30 and no internalization was observed at lower serotonin concentrations, even at longer time scales. In contrast, an in vitro study of the effect of serotonin stimulation (10 μM) on rat cerebellar granule cells showed that the binding of [3H]ketanserin and 5-HT2A mRNA increased within hours, indicating an upregulation of 5-HT2AR.31 Moreover, atypical receptor regulations were also reported for 5-HT2AR antagonists: a chronic blockade of 5-HT2AR was often followed by a “paradoxical” downregulation. The increase of 5-HT2AR density in this study is in line with some of the previously mentioned results. However, as 5-HT2AR was investigated at only one time point, it cannot be excluded that there is an early decrease followed by a compensatory increase of 5-HT2ARs just at the time of the PET scan. It is therefore important to establish the specific conditions in humans and in vivo.

Regarding the study design, we decided to investigate all participants in the same order: ‘baseline’ PET scan followed by sleep deprivation and repeated PET scan. Previous studies evaluating the reliability of [18F]altanserin PET in a test-retest design did not find any evidence for a carryover or order effect. Furthermore, the PET tracer methodology makes it highly unlikely to produce any pharmacologic or background (radioactivity) effects with a given time shift of 24 h.

It is also important to ensure that the experimental setting does not induce systematic behavioral changes. To deal with this issue, the behavior of the participants was monitored during the week before the investigation using daily questionnaires about their sleep-wake behavior, sleep quality, fatigue, and overall performance. During the last two days before the experiment participants were controlled by actigraphy. None of the measured items was significantly different the day before the experiment compared with the preceding period. Data from previous studies in which we recorded sleep EEG are in accordance with reports from other groups showing that the first night spent at a sleep laboratory is usually subjectively and objectively disturbed (“first night effect”). Therefore, our participants were allowed to spend the night before the first scan at home, which guaranteed a high degree of “normality” the next morning. Importantly, most of the participants were employees of the Juelich Research Center (staff of approximately 5,000). For that reason, most of the study participants were familiar with the location, which in turn has also contributed to a low stress level. After completing the PET experiment the participants returned to their normal daily working routine. Except for the scanning time and the sleep deprivation period during the night, the daily routine of the participants was therefore mostly identical to the day of the first scan. The current setting assured that sleep deprivation was the main and intended difference between the two scans.

One could also question whether the observed effect size is big enough to be clearly distinguished from normal test-retest variability. It is therefore important to note that on a group level high stability has repeatedly been demonstrated for [18F]altanserin PET. In a reproducibility study the relative difference between test and retest scans was on average only -0.2% in five cortical regions14 (also calculated based on the data of Tables 3 and 4, of reference14, n = 8). In a more recent test-retest evaluation of [18F]altanserin PET15 the average relative difference for 10 cortical regions was comparably low (-0.8%; n = 6). In addition, in a study on the longitudinal stability of [18F]altanserin PET the average relative difference of six cortical regions was -3.2% after a 2-yr period in 12 healthy control participants.32 These reports clearly demonstrate that the observed effect size of 9.6%, although small in absolute terms, is rather big in relation to the reported test-retest variability of [18F]altanserin PET.

Another theoretical constraint derives from our approach to quantify 5-HT2AR with [18F]altanserin BPP. We cannot rule out the possibility that the observed changes might (at least partly) be caused by a change of receptor affinity or by a reduced competition with endogenous serotonin. However, there is no experimental evidence so far that sleep deprivation affects 5-HT2AR affinities and, as previously mentioned, an increase in serotonin concentration after sleep deprivation can be hypothesized. Furthermore, three reports have been published suggesting that [18F]altanserin BPP is insensitive to displacement by endogenous serotonin.12,33,34

There was a clear trend of deterioration in PVT performance after sleep deprivation but levels of significance were not reached. As a potential explanation, training effects might have overlaid to some extent the worsening in reaction speed after sleep deprivation. Our study participants trained the task only once, but the first trials are known to improve reaction speed and reaction time most significantly.35

As previously mentioned, the longer a participant was awake, the increase in BPP was less. The decreasing BPP with increasing waking time might follow a common pattern of sleepiness ratings or cognitive measures after sleep deprivation: after progressive deterioration during the night period of deprivation alertness improves gradually during the following daytime period. In line with this pattern we found that the greater the observed increase in [18F]altanserin BPP, the greater the deterioration in cognitive performance (although we did not find a relation between the self-rating of sleepiness and the BPP changes). Changes in PVT are generally considered to reflect the attentional and arousal state of the participant and have been proven as a measure of sleep loss.36 In a functional MRI study, best performance on the PVT (fast reactions) was found to be associated with increases in the BOLD (blood oxygen level dependent) signal in the middle and inferior frontal and inferior parietal lobe.37 A study on arterial spin labeling perfusion revealed activations or increases of cerebral blood flow during the PVT in middle and inferior frontal cortex, insula, anterior cingulate cortex, and inferior parietal lobe.38 In a PET study of the cerebral metabolic rate of glucose after 24 h of sleep deprivation, a global decrease (8%) was observed with most pronounced cortical changes in prefrontal and parietal cortices.39 These data are in line with the regional pattern of BPP increases and their correlations to PVT, as observed in this study. Most of these areas are considered to be part of the widespread attentional network.40

The current findings of increased 5-HT2AR density might also be related to a recent concept of sleep regulation, the synaptic homeostasis theory.41 It proposes a process of synaptic potentiation, which implies a general increase in receptor density during prolonged wakefulness. The observed increase in 5-HT2AR availability could therefore reflect an increase in synaptic strength.

With regard to the functional consequences of these regulatory processes we found that sleep deprivation led to an increase in the EEG power in the theta and delta bin of the waking EEG, which confirms previous reports indicating an increased sleep propensity in line with the homeostatic model of slow wave sleep enhancement after sleep deprivation. Interestingly, there was no correlation to the BPP of [18F]altanserin. Because increases in the theta power of the waking EEG have been identified as physiologic measures of enhanced sleep propensity,42 the 5-HT2AR may not be primarily involved in this process. It has previously been proposed that serotonergic modulation of the wake-promoting system in the basal forebrain does not induce sleep per se. Instead, serotonin might be a trigger of the behavioral state of drowsiness or quietwaking,43 which prepares for the subsequent onset of sleep. In turn, the role of serotonin receptors is to allow modulating the level of serotonin efficacy in a relatively robust manner because varying concentrations of serotonin are highly dynamic and prone to fluctuations.

The flip-flop circuit model of the transition of sleep and wake44 proposed that a rapid switching to one state is ensured by disinhibition of one state and inhibition of the other state, respectively. The gamma aminobutyric acid-containing neurons in the ventrolateral preoptic area (VLPO) of the hypothalamus seem to inhibit the arousal systems during sleep, which in turn inhibit (when active) the hypothalamic area. Serotonergic inputs from the dorsal raphe nuclei, which are part of this arousal system, inhibit the activity of VLPO neurons. It was speculated that the previously mentioned 5-HT2AR antagonists enhance slow wave sleep by antagonizing the inhibitory input to the sleep promoting cells of the VLPO. The spatial resolution of PET does not allow to report alterations of [18F]altanserin binding in the VLPO. If the generally observed increase in 5-HT2AR availability also applies for the VLPO, the proposed switch may be influenced toward an increased inhibition of VLPO.

Because sleep deprivation as well as serotonergic antidepressants are efficient strategies in the therapy of depression it can be hypothesized that both treatments might share a common molecular basis. With regard to pathologic conditions such as depression it will be important to investigate whether serotonin receptor densities are responding in the pathologic condition to sleep deprivation.

In conclusion, this study provides in vivo evidence of increased levels of 5-HT2AR availability in the human brain after one night of sleep deprivation.

DISCLOSURE STATEMENT

This was not an industry supported study. The authors have indicated no financial conflicts of interest.

ACKNOWLEDGMENTS

Magdalene Vögeling, Angela Weisshaupt, Lutz Tellmann, Elisabeth Theelen, Suzanne Schaden, Hans Herzog, Johannes Ermert, and Heinz H. Coenen are gratefully acknowledged for excellent technical assistance as well as radioligand supply. The authors thank Hans-Peter Landolt for valuable discussions.

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