Karine Madsen

Brain imaging of serotonin 4 receptors in humans with [11C]SB207145-PET

Pharmacological stimulation of the serotonin 4 (5-HT(4)) receptor has shown promise for treatment of Alzheimers disease and major depression. A new selective radioligand, [(11)C]SB207145, for positron emission tomography (PET) was used to quantify brain 5-HT(4) receptors in sixteen healthy subjects (20-45 years, 8 males) using the simplified reference tissue model. We tested within our population the effect of age and other demographic factors on the endpoint. In seven subjects, we tested the vulnerability of radioligand binding to a pharmacolological challenge with citalopram, which is expected to increase competition from endogenous serotonin. Given radiotracer administration at a range of specific activities, we were able to use the individual BP(ND) measurements for population-based estimation of the saturation binding parameters; B(max) ranged from 0.3 to 1.6 nM. B(max) was in accordance with post-mortem brain studies (Spearmans r=0.83, p=0.04), and the regional binding potentials, BP(ND), were on average 2.6 in striatum, 0.42 in prefrontal cortex, and 0.91 in hippocampus. We found no effect of sex but a decreased binding with age (p=0.046). A power analysis showed that, given the low inter-and intrasubject variation, use of the present method will enable detection of a 15% difference in striatum with only 7-13 subjects in a 2-sample test and with only 4-5 subjects in a paired test. The citalopram challenge did not discernibly alter [(11)C]SB207145 binding. In conclusion, the 5-HT(4) receptor binding in human brain can be reliably assessed with [(11)C]SB207145, which is encouraging for future PET studies of drug occupancy or patients with neuropsychiatric disorders.

High familial risk for mood disorder is associated with low dorsolateral prefrontal cortex serotonin transporter binding.

Mood disorders are elicited through a combination of genetic and environmental stress factors, and treatment with selective serotonin reuptake inhibitors ameliorates depressive symptoms. Changes in the serotonin transporter (SERT) binding may therefore occur in depressive patients and in subjects at risk for developing depression. The aim of this study was to explore whether abnormalities in SERT might be present in healthy individuals with familial predisposition to mood disorder. Nine individuals at high familial risk (mean age 32.2+/-4.2 years) and 11 individuals at low risk (mean age 32.4+/-5.0 years) for developing mood disorder were included. The subjects were healthy twins with or without a co-twin history of mood disorder identified by linking information from the Danish Twin Register and the Danish Psychiatric Central Register. Regional in vivo brain serotonin transporter binding was measured with [(11)C]DASB PET. The volumes of interest included the orbitofrontal cortex, the dorsolateral prefrontal cortex, the ventrolateral prefrontal cortex, anterior cingulate, caudate, putamen, thalamus, and midbrain. We found that individuals at high familial risk for mood disorders had a 35% reduction in SERT binding in dorsolateral prefrontal cortex (p=0.014, Bonferroni corrected) and on a trend basis a 15% reduction in anterior cingulate (p=0.018, un-corrected). The depression and symptom scores of the high and the low risk individuals were not significantly different. In conclusion, our data suggest that a low SERT binding in dorsolateral prefrontal cortex represents a trait marker for mood disorders.

Cerebral Serotonin 4 Receptors and Amyloid-β in Early Alzheimer's Disease

The 5-HT4 receptor may play a role in memory and learning and 5-HT4 receptor activation has been suggested to modulate acetylcholine release and to reduce amyloid-β (Aβ) accumulation. The aim of this study was for the first time to investigate the in vivo cerebral 5-HT4 receptor binding in early Alzheimer disease (AD) patients in relation to cortical Aβ burden. Eleven newly diagnosed untreated AD patients (mean MMSE 24, range 19-27) and twelve age- and gender-matched healthy controls underwent a two-hour dynamic [11C]SB207145 PET scan to measure the binding potential of the 5-HT4 receptor. All AD patients and eight healthy controls additionally underwent a [11C]PIB PET scan to measure the cortical Aβ burden. When AD patients were defined on clinical criteria, no difference in cerebral 5-HT4 receptor binding between AD patients and healthy controls was found (p = 0.54). However, when individuals were reassigned to groups according to their amyloid status, the PIB-positive individuals had 13% higher 5-HT4 receptor levels than PIB-negative individuals (p = 0.02) and the importance of classification of groups is emphasized. The 5-HT4 receptor binding was a positively correlated to Aβ burden (p = 0.03) and negatively to MMSE score of the AD patients (p = 0.02). Our data suggests that cerebral 5-HT4 receptor upregulation starts at a preclinical stage of and continues while dementia is still at a mild stage, which contrasts other receptor subtypes. We speculate that this may either be a compensatory effect of decreased levels of interstitial 5-HT, an attempt to improve cognitive function, increase acetylcholine release or to counteract Aβ accumulation.

Loss of serotonin 2A receptors exceeds loss of serotonergic projections in early Alzheimer's disease: a combined [11C]DASB and [18F]altanserin-PET study.

In patients with Alzheimers disease (AD), postmortem and imaging studies have revealed early and prominent reductions in cerebral serotonin 2A (5-HT(2A)) receptors. To establish if this was due to a selective disease process of the serotonin system, we investigated the cerebral 5-HT(2A) receptor and the serotonin transporter binding, the latter as a measure of serotonergic projections and neurons. Twelve patients with AD (average Mini Mental State Examination [MMSE]: 24) and 11 healthy age-matched subjects underwent positron emission tomography (PET) scanning with [(18)F]altanserin and [(11)C]N,N-Dimethyl-2-(2-amino-4-cyanopheylthio)benzylamine ([(11)C]DASB). Overall [(18)F]altanserin binding was markedly reduced in AD by 28%-39% (p = 0.02), whereas the reductions in [(11)C]DASB binding were less prominent and mostly insignificant, except for a marked reduction of 33% in mesial temporal cortex (p = .0005). No change in [(11)C]DASB binding was found in the midbrain. We conclude that the prominent reduction in neocortical 5-HT(2A) receptor binding in early AD is not caused by a primary loss of serotonergic neurons or their projections.

Obesity is associated with high serotonin 4 receptor availability in the brain reward circuitry

The neurobiology underlying obesity is not fully understood. The neurotransmitter serotonin (5-HT) is established as a satiety-generating signal, but its rewarding role in feeding is less well elucidated. From animal experiments there is now evidence that the 5-HT(4) receptor (5-HT(4)R) is involved in food intake, and that pharmacological or genetic manipulation of the receptor in reward-related brain areas alters food intake. Here, we used positron emission tomography in humans to examine the association between cerebral 5-HT(4)Rs and common obesity. We found in humans a strong positive association between body mass index and the 5-HT(4)R density bilaterally in the two reward ‘hot spots’ nucleus accumbens and ventral pallidum, and additionally in the left hippocampal region and orbitofrontal cortex. These findings suggest that the 5-HT(4)R is critically involved in reward circuits that regulate peoples food intake. They also suggest that pharmacological stimulation of the cerebral 5-HT(4)R may reduce reward-related overeating in humans.

Age and sex effects on 5-HT(4) receptors in the human brain: a [(11)C]SB207145 PET study

Experimental studies indicate that the 5-HT4 receptor activation influence cognitive function, affective symptoms, and the development of Alzheimers disease (AD). The prevalence of AD increases with aging, and women have a higher predisposition to both AD and affective disorders than men. This study aimed to investigate sex and age effects on 5-HT4 receptor-binding potentials in striatum, the limbic system, and neocortex. Positron-emission tomographic scans were conducted using the radioligand [11C]SB207145 in a cohort of 30 healthy subjects (mean age 44 years; range 20 to 86 years; 14 men and 16 women). The output parameter, BPND, was modeled using the simplified reference tissue model, and partial volume correction was performed with the Muller-Gartner method. A decline with age of 1% per decade was found only in striatum. Women had a 13% lower 5-HT4 receptor binding in the limbic system. The lower limbic 5-HT4 receptor binding in women supports a role for 5-HT4 receptors in the sex-specific differences in emotional control and might contribute to the higher prevalence of affective diseases and AD in women. The relatively stable 5-HT4 receptor binding with aging contrasts others in subtypes of receptors, which generally decrease with aging.

Lack of association between prior depressive episodes and cerebral [11C]PiB binding

Depressive symptoms are frequent in Alzheimers disease (AD), but it is controversial whether depression is a risk factor for AD. This study measured for the first time cortical amyloid-β (Aβ) levels using [(11)C] Pittsburgh Compound B (PiB) positron emission tomography (PET) in a group of nondemented patients with prior depressive episodes. Twenty-eight elderly patients (mean age 61 years, range 51-75, 18 women) with onset of first depressive episode more than 6 years ago but now remitted from depression and 18 healthy subjects (mean age 61 years, range 50-76, 12 women) were included. All subjects were investigated with cognitive testing, 3T magnetic resonance imaging (MRI) and [(11)C]PiB high resolution research tomography (HRRT) positron emission tomography scan. There was no between-groups difference in [(11)C]PiB binding (p = 0.5) and no associations to number of depressive episodes, cognitive performance, or antidepressant treatment. Patients with late onset of depression had increased severity of white matter lesions (p = 0.04). In this study depressive episodes were not associated with increased levels of [(11)C]PiB. Thus, our results do not support the notion that depressive episodes previously in life are a risk factor for developing AD.

Mass dose effects and in vivo affinity in brain PET receptor studies — a study of cerebral 5-HT4 receptor binding with [11C]SB207145

UNLABELLED Attention to tracer dose principles is crucial in positron emission tomography (PET), and deviations can induce serious errors. In this study, we devise a method for determining receptor occupancy of the mass dose of the radioligand itself and the in vivo affinity. METHODS The approach was used for [(11)C]SB207145, a new PET radioligand for imaging the cerebral 5-HT(4) receptors in humans. Test-retest PET studies with varying specific activities of [(11)C]SB207145 were conducted in seven healthy subjects, and the output parameter regional BP(ND) was modeled. Individual occupancy plots were first computed to estimate the mass dose that saturates 50% of receptors (ID(50)), and subsequently, the maximal mass dose that can be injected (arbitrarily set at an occupancy <5%) was calculated. Scatchard plots were computed to estimate the in vivo K(D). RESULTS Increasing the mass dose resulted in a decrease in BP(ND), whilst the relative cerebellar uptake was unchanged. The ID(50) was 85.4±30.2 μg, and the upper mass dose limit was 4.5±1.6 μg, which does not require ultrahigh specific activity. The estimated in vivo K(D) was 2.8 nM (range 1.0-4.8), without any regional differences. CONCLUSION The presented method for estimating the upper mass dose limit is suggested as part of validation of PET radioligands.

The 5-HT4 receptor levels in hippocampus correlates inversely with memory test performance in humans.

The cerebral serotonin (5‐HT) system is involved in cognitive functions such as memory and learning and animal studies have repeatedly shown that stimulation of the 5‐HT type 4 receptor (5‐HT4R) facilitates memory and learning and further that the 5‐HT4R modulates cellular memory processes in hippocampus. However, any associations between memory functions and the expression of the 5‐HT4R in the human hippocampus have not been investigated. Using positron emission tomography with the tracer [11C]SB207145 and Reys Auditory Verbal Learning Test we aimed to examine the individual variation of the 5‐HT4R binding in hippocampus in relation to memory acquisition and consolidation in healthy young volunteers. We found significant, negative associations between the immediate recall scores and left and right hippocampal BPND, (p = 0.009 and p = 0.010 respectively) and between the right hippocampal BPND and delayed recall (p = 0.014). These findings provide evidence that the 5‐HT4R is associated with memory functions in the human hippocampus and potentially pharmacological stimulation of the receptor may improve episodic memory. Hum Brain Mapp 34:3066–3074, 2013.

5-HTTLPR status predictive of neocortical 5-HT4 binding assessed with [11C]SB207145 PET in humans

Serotonin (5-HT) is a neuromodulator affecting myriad aspects of personality and behavior and has been implicated in the pathophysiology of affective disorders including depression and anxiety. The 5-HTTLPR is a common genetic polymorphism within the promoter region of the gene coding for the serotonin transporter such that the S allele is associated with reduced transcriptional efficacy compared to the L allele, potentially contributing to increased serotonin levels. In humans, this genetic variant has been linked to inter-individual variability in risk for affective disorders, related aspects of personality and brain function including response to threat. However, its effects on aspects of serotonin signaling in humans are not fully understood. Studies in animals suggest that the 5-HT 4 receptor (5-HT(4)) shows a monotonic inverse association with long-term changes in serotonin levels indicating that it may be a useful measure for identifying differences in serotonergic neurotransmission. In 47 healthy adults we evaluated the association between 5-HTTLPR status and in vivo 5-HT(4) receptor binding assessed with [(11)C]SB207145 positron emission tomography (PET). We observed a significant association within the neocortex where [(11)C]SB207145 binding was 9% lower in S carriers compared to LL homozygotes. We did not find evidence for an effect of season or a season-by-5-HTTLPR interaction effect on regional [(11)C]SB207145 binding. Our findings are consistent with a model wherein the 5-HTTLPR S allele is associated with relatively increased serotonin levels. These findings provide novel evidence supporting an effect of 5-HTTLPR status on serotonergic neurotransmission in adult humans. There were no indications of seasonal effects on serotonergic neurotransmission.