- Serotonin 5-HT1A receptor binding and self-transcendence in healthy control subjects—a replication study using Bayesian hypothesis testing
- No association between serotonin 5-HT 1A receptors and spirituality among patients with major depressive disorders or healthy volunteers
- Positron emission tomography procedures
- Automated region of interest analysis
- Quantification of [carbonyl-11C]WAY-100635 binding
- Statistical analyses
Serotonin 5-HT1A receptor binding and self-transcendence in healthy control subjects—a replication study using Bayesian hypothesis testing
The serotonin system is involved in a wide range of fundamental physiological functions regulation of mood, sleep and appetite (Filip & Bader, 2009).
Furthermore, serotonergic neurotransmission is implicated in higher brain functions such as cognitive performance (Jenkins et al.
, 2016) and in several psychiatric disorders, including depression, autism, anxiety disorders and schizophrenia (Filip & Bader, 2009; Fidalgo, Ivanov & Wood, 2013).
With regard to personality, the serotonin system has been linked to the trait self-transcendence (ST) in both Positron Emission Tomography (PET) and genetic studies (Borg et al., 2003; Ham et al., 2004; Lorenzi et al., 2005; Nilsson et al., 2007; Aoki et al., 2010; Saiz et al.
, 2010; Kim et al., 2015). ST refers to the degree to which an individual feels part of nature and the universe at large, and to extraordinary experiences such as extra sensory perception and sense of a transcendent being or presence (Gillespie et al., 2003).
The association has been interpreted as evidence for a role for the serotonin system in spiritual experiences, as well as providing a putative mechanism for the involvement of serotonin in psychosis, since high scores in ST has been linked to the schizophrenia spectrum disorders (Nitzburg, Malhotra & DeRosse, 2014).
Our group previously reported a negative correlation between 5-HT1A receptor binding potential (BPND), as measured with PET and the radioligand [11C]WAY-100635, and ST as measured using Temperament and Character Inventory (TCI).
The association was strongest for the subscale spiritual acceptance (SA) (Borg et al., 2003). However, the results could not be replicated in a subsequent PET study at an independent centre (Karlsson et al., 2011).
These studies contained 15 and 20 healthy participants, respectively, and therefore, a replication study in a larger sample is required.
The aim of the present study was to perform a replication of our original finding of a negative correlation between 5-HT1A receptor BPND and ST/SA in a larger sample.
In addition to traditional frequentist statistics, we made use of Bayesian hypothesis testing, which allows us not only to test a hypothesis, but also to quantify the relative probability of the observed data under competing hypotheses.
Recently replication Bayes factors (BF) have been introduced (Verhagen & Wagenmakers, 2014; Wagenmakers, Verhagen & Ly, 2016), allowing researchers to evaluate replication success by taking the outcome of the previous study fully into account.
In this way, we aimed to evaluate the relationship between 5-HT1A receptor binding and ST/SA from the perspective both of hypothesis testing without consideration of the magnitude of previous results, and of replication success.
The sample consisted of 50 healthy men: 12 were enrolled as control subjects in a series of different pharmacological studies (for details see Matheson et al. (2015); 38 in a twin study (Borg et al., 2016). Age ranged from 21 to 55 (Mean = 30, SD = 5 years).
The studies were approved by the Regional Ethics Committee in Stockholm and the Radiation Safety Committee of the Karolinska Hospital, and all subjects provided written informed consent prior to their participation in the studies (IRB 2008/60-31/3; for serotonin markers 2013/136-32).
Magnetic Resonance Imaging (MRI) images were acquired using a 1.5TGE Signa system (Milwaukee, WI, USA). T1- and T2-weighted MRI images were acquired for all subjects. The PET system used was Siemens ECAT Exact HR 47 (CTI/Siemens, Knoxville, TN, USA).
All subjects were examined using [11C]WAY-100635; The injected radioactivity was 276 ± 35 MBq (mean; SD). BPND values were calculated for the same regions as examined in the original study (Borg et al.
, 2003): frontal cortex, hippocampus (using the simplified reference tissue model – SRTM) and dorsal raphe nucleus (using a wavelet-based method using the non-invasive Logan plot in order to reduce the noise in this small region). For detailed description see Matheson et al. (2015).
Other regions were not included in the analysis as they were not part of the original study. However, since [11C]WAY100635 BPND is highly correlated between regions, the inclusion of more regions would therefore be unly to provide unique information from the three included regions (Bose et al., 2011).
The Swedish translation of the TCI self-report questionnaire was used (Brändström et al., 1998).
It consists of 238 true/false items covering four temperament dimensions (novelty seeking, harm avoidance, reward dependence, and persistence) and three character dimensions (self-directedness, cooperativeness, and self-transcendence). Individual scores were calculated for ST and its subscale SA.
Pearson’s correlation coefficients and their corresponding p-values were calculated for the correlation between ST/SA and 5-HT1A BPND in the frontal cortex, hippocampus and dorsal raphe nucleus. Two BF tests were performed for each comparison.
Firstly, we calculated a default correlation BF for the association between BPND and the ST/SA scores in frontal cortex, hippocampus and dorsal raphe nucleus respectively. Since we specifically wanted to test a negative correlation, we choose a one-sided default Bayes factor test, with a negative Beta prior of width 1 (i.e.
, flat between −1 and 0) using JASP (JASP Team, 2017). This test compares the predictive adequacy of the null hypothesis H0 (i.e., no correlation) with an alternative hypothesis H– (i.e., a negative correlation) (for more details on Bayes factors, see (Ly, Verhagen & Wagenmakers, 2016; Wagenmakers, Morey & Lee, 2016).
Second, we calculated a replication BF for the correlations for each region as a measure of replication success. This test compares the predictive adequacy of the null hypothesis H0 (i.e., no correlation) with an alternative hypothesis Hr.
The alternative hypothesis is defined as the posterior distribution of the correlation coefficient derived from the original study, assuming a uniform prior before seeing the data of the original study (Wagenmakers, Verhagen & Ly, 2016). We slightly modified of the following source code http://www.josineverhagen.
com/wp-content/uploads/2013/07/RepfunctionscorrelationFINAL1.txt (for plotting purposes) to the code which can be found online at the following address: https://osf.io/x9gjj/. This code was executed using RStudio (RStudio Team, 2017) with R 3.3.2 (R Core Team, 2015). We also reanalysed the results of Karlsson et al. (2011) with these methods.
Bayes factors assess the relative lihood of the observed data under competing hypotheses, yielding a ratio of the relative evidence for one hypothesis over the other. For instance, a BF01 below 3 indicates weak or anecdotal evidence, a BF01>3 moderate and a BF01 >10 strong evidence in favour of the null against the alternative (Jeffreys, 1961).
In this paper, all BFs are presented as the lihood of the null hypothesis relative to the alternative hypothesis (i.e., BF0− specifying a negative correlation as alternative; BF0r specifying the posterior probability distribution of the original correlation as alternative).
The differences between the default and the replication BF tests can be expressed as follows: the default test addresses the question of whether an effect was present or absent given relatively little prior knowledge of the effect size, while the replication test asks whether the effect was similar to what was found before, or absent (Wagenmakers, Verhagen & Ly, 2016).
Two potential sources of bias for this analysis were the inclusion of twin pairs, and the use of cerebellar grey matter as the reference region (Hirvonen et al., 2007).
We therefore performed two additional analyses by (1) randomly excluding one twin from each twin pair (using http://www.random.
org), resulting in a sample size of 31, and (2) using the white matter as a reference region for hippocampus and frontal cortex.
In the present sample of 50 subjects, the BPND of [11C]WAY100635 varied about 4-fold between individuals (Table 1). ST scores ranged from 2 to 24 (mean 9.7, SD 5.8); the SA scores ranged from 0 to 12 (mean 3.9, SD 3.1) (Table 1). There were no significant correlations between regional 5-HT1A receptor binding and scores on ST or SA (Fig. 1, Table 2).
Table 1: TCI scores and BPND in the original study (Borg et al., 2003) and the present replication study.
|ST||9.4 (3.8)||3–15||9.7 (5.8)||2–24|
|SA||4.7 (3.0)||0–9||3.9 (3.1)||0–12|
|Dorsal raphe nuclei||2.2 (0.87)||0.81–4.11||1.7 (0.48)||0.64–2.88|
|Hippocampus||4.7 (1.49)||1.91–7.15||5.1 (1.41)|
No association between serotonin 5-HT 1A receptors and spirituality among patients with major depressive disorders or healthy volunteers
An earlier study (Borg et al., Am J Psychiatry 2003) found an inverse correlation between [carbonyl-11C]WAY-100635 ligand binding to 5-HT1A receptors and scores for self-transcendence, but no other of the six dimensions of the Temperament and Character Inventory, in a group of healthy males.
The aim of this study was to investigate if the finding of an inverse correlation between spirituality and 5-HT1A could be seen in patients suffering from major depressive disorder or replicated among healthy volunteers. A total of 23 patients with major depressive disorder and 20 healthy volunteers were examined with PET using [carbonyl-11C]WAY-100635 as the radioligand.
The personality traits were measured using the Finnish version of the Temperament and Character Inventory and correlated with ligand binding (BP).
No significant correlations were found between the different Temperament and Character Inventory subscales and BP in any of the studied brain regions (amygdala, anterior cingulate cortex, dorsal raphe nuclei, dorsolateral prefrontal cortex, angular gyrus, inferior, middle, and superior temporal gyri, medial prefrontal cortex orbitofrontal cortex, hippocampus, insular cortex, subgenual anterior cingulate cortex, supramarginal gyrus, ventrolateral prefrontal cortex, and posterior cingulate cortex). These results do not support the idea that the serotonin system forms the biological basis of spiritual experiences among patients suffering from major depressive disorder or among healthy volunteers.
In an earlier study1 it was found that the binding potential (BPND, ratio of specifically bound ligand to free and non-specifically bound ligand) of [carbonyl-11C]WAY-100635, a ligand for 5-HT1A receptors, correlated inversely with scores for self-transcendence, but no other of the six dimensions of the Temperament and Character Inventory, in a group of healthy males.
There is some evidence showing that patients suffering from depression score higher in self-transcendence compared with healthy subjects2, 3 and that 5-HT1A receptor availability is lower among patients suffering from major depressive disorder.
4, 5 At the same time, there seems to be no correlation between the severity of depressive symptoms and BPND.4, 5 Thus, low 5-HT1A BPND may reflect a trait- vulnerability and the high self-transcendence a stable personality trait.
In this study, we sought to determine whether the previously reported finding of a negative correlation between spirituality and 5-HT1A binding could also be seen in patients suffering from major depressive disorder and replicated among healthy volunteers. Our hypothesis was that the same pattern between spirituality and 5-HT1A binding could be found among depressive patients and healthy volunteers as was found among healthy men by Borg et al.1
We assessed the association between self-transcendence and 5-HT1A in a sample of patients with major depressive disorder that we demonstrated earlier having lower 5-HT1A receptor binding (BPP) than healthy subjects.
5 Of the 25 patients described in our earlier report, Temperament and Character Inventory data were available from 23 patients (10 males, 13 females, age 40±9 years).
In this subsample, Hamilton depression rating scale scores ranged from 13 to 23 (median 18) and total Beck depression inventory scores ranged from 10 to 40 (median 23).
A total of 22 patients were completely antidepressant-naive, and 1 patient had been treated with citalopram 5 years before the imaging and was free of antidepressant drug therapy since. Arterial plasma data were available for 19 of these patients (for estimating BPP). In addition, we included 20 healthy subjects (11 males, 9 females, age 26±6 years). Each study subject also underwent 1.5 T magnetic resonance imaging.
This study was approved by the Joint Ethical Committee of the University of Turku and Turku University Central Hospital, and was conducted according to the Declaration of Helsinki. After complete description of the study to the subjects, written informed consent was obtained.
Positron emission tomography procedures
Positron emission tomography experiments were carried out as described earlier.6 Injected dose and mass was 236±28 MBq and 1.1±0.7 μg for patients with major depressive disorder, and 252±80 MBq and 2.0±0.9 μg for healthy controls, respectively (means±s.
d.). Arterial plasma samples were collected for the measurement of plasma radioactivity and metabolites according to earlier published schedule,6 and an arterial plasma input curve for [carbonyl-11C]WAY 100635 corrected for metabolites was constructed.
Automated region of interest analysis
PET images were analyzed using an automated region of interest analysis as described earlier.5 In brief, summed (integral) PET images were spatially normalized to a ligand-specific template in the standard stereotactic space using SPM2.
7 These transformation parameters were then applied to dynamic PET images, and a set of regions of interest in the stereotactic were applied to the normalized dynamic PET images to calculate regional time-activity curves.
Regions covered the following brain regions: amygdala, anterior cingulate cortex, dorsal raphe nuclei, dorsolateral prefrontal cortex, angular gyrus, inferior, middle, and superior temporal gyri, medial prefrontal cortex, orbitofrontal cortex, hippocampus, insular cortex, subgenual anterior cingulate cortex, supramarginal gyrus, ventrolateral prefrontal cortex, and posterior cingulate cortex.
Quantification of [carbonyl-11C]WAY-100635 binding
Distribution volume (VT) was estimated as described earlier.6 VT represents the ratio at equilibrium of area under the curve in the brain to that in arterial plasma;8 that is, it corrects the observed brain activity with that observed in arterial plasma.
BP values were estimated indirectly from VT values as the ratio of specifically bound ligand either to total ligand in arterial plasma (BPP) or to free plus non-specifically bound ligand in tissue (BPND).8 BPP was calculated using cerebellar white matter as reference region.
5, 6 BPP is preferred over BPND for estimating specific binding of [carbonyl-11C]WAY-100635 because of its relative independence of changes in non-specific binding.
5, 6 BPND was calculated with the simplified reference tissue model9 using cerebellar gray matter as the reference region to enable direct comparison with an earlier study.1
Analyses were performed on the natural logarithm of BPP.5 The association between Temperament and Character Inventory components and BPP or BPND was evaluated with Pearson's correlation coefficients (R), to enable direct comparison to an earlier study.1 P