Parametric and regional maps of free serotonin 5HT1A receptor sites in human brain as function of age in healthy humans

Poorer aging trajectories are associated with elevated serotonin synthesis capacity

The dorsal raphe nucleus (DRN) is one of the earliest targets of Alzheimer's disease-related tau pathology and is a major source of brain serotonin. We used [18F]Fluoro-m-tyrosine ([18F]FMT) PET imaging to measure serotonin synthesis capacity in the DRN in 111 healthy adults (18-85 years-old). Similar to reports in catecholamine systems, we found elevated serotonin synthesis capacity in older adults relative to young. To establish the structural and functional context within which serotonin synthesis capacity is elevated in aging, we examined relationships among DRN [18F]FMT net tracer influx (Ki) and longitudinal changes in cortical thickness using magnetic resonance imaging, longitudinal changes in self-reported depression symptoms, and AD-related tau and β-amyloid (Aβ) pathology using cross-sectional [18F]Flortaucipir and [11C]Pittsburgh compound-B PET respectively. Together, our findings point to elevated DRN [18F]FMT Ki as a marker of poorer aging trajectories. Older adults with highest serotonin synthesis capacity showed greatest temporal lobe cortical atrophy. Cortical atrophy was associated with increasing depression symptoms over time, and these effects appeared to be strongest in individuals with highest serotonin synthesis capacity. We did not find direct relationships between serotonin synthesis capacity and AD-related pathology. Exploratory analyses revealed nuanced effects of sex within the older adult group. Older adult females showed the highest DRN synthesis capacity and exhibited the strongest relationships between entorhinal cortex tau pathology and increasing depression symptoms. Together these findings reveal PET measurement of the serotonin system to be a promising marker of aging trajectories relevant to both AD and affective changes in older age.

Chronic unpredictable stress induces depression/anxiety-related behaviors and alterations of hippocampal monoamine receptor mRNA expression in female mice at different ages

Depression and anxiety are the most common mental health disorders. Though they affect people at any age and occur more often in females, the pathophysiological changes under these conditions are less investigated. In the present study, we examined the effects of age and stress on depression- and anxiety-related behaviors in female mice. Saccharin preference and the open field test were carried out before and after chronic unpredictable stress in 4-, 14- and 25-month-old female mice. After behavioral tests, mRNA levels of monoamine receptors in the hippocampus were measured by real-time RT-PCR. Chronic unpredictable stress decreased saccharin preference in 4-, 14- and 25-month-old mice and the time spent in the center in the open field test in 25-month-old mice. For monoamine receptors, analysis of variance revealed significant effects of age on mRNA levels of Htr1a, Htr2a, Htr6, Adra1a, Adrb2, and Adrb3, significant effects of stress on mRNA levels of Htr4, Adra2c, Adrb1, and Adrb2, and interactions of age × stress on mRNA levels of Htr1a, Htr5b, Adra1d, Adra2a, Adra2c, and Adrb1. Chronic unpredictable stress decreased mRNA levels of Htr4, Htr5b, Adra2c, and Adrb1 in 4-month-old female mice. Correlations were observed between saccharin preference and mRNA levels of Htr4, Htr5b, Htr6, Adra1d, Adra2a, and Adra2c in 4-month-old mice and between the time spent in the center in the open field test and mRNA levels of Htr1b in 4-month-old mice, Htr3a, Htr7, and Adrb2 in 14-month-old mice, and Drd2 in 4- and 14-month-old mice. Our findings support that stress induces depression- and anxiety-related behaviors and the expression of hippocampal monoamine receptors in an age-dependent manner in female mice.

Age-Related Changes in Central Nervous System 5-Hydroxytryptamine Signalling and Its Potential Effects on the Regulation of Lifespan

Serotonin or 5-hydroxytryptamine (5-HT) is an important neurotransmitter in the central nervous system and the periphery. Most 5-HT (~99%) is found in the periphery where it regulates the function of the gastrointestinal (GI) tract and is an important regulator of platelet aggregation. However, the remaining 1% that is found in the central nervous system (CNS) can regulate a range of physiological processes such as learning and memory formation, mood, food intake, sleep, temperature and pain perception. More recent work on the CNS of invertebrate model systems has shown that 5-HT can directly regulate lifespan.This chapter will focus on detailing how CNS 5-HT signalling is altered with increasing age and the potential consequences this has on its ability to regulate lifespan.

Dopamine D2/3 Receptor Availabilities in Striatal and Extrastriatal Regions of the Adult Human Brain: Comparison of Four Methods of Analysis

Values of binding potentials (BP_ND) of dopamine D_2/3 receptors differ in different regions of the brain, but we do not know with certainty how much of this difference is due either to different receptor numbers, or to different affinities of tracers to the receptors, or to both. We tested the claim that both striatal and extrastriatal dopamine D_2/3 receptor availabilities vary with age in vivo in humans by determining the values of BP_ND of the specific radioligand [¹¹C]raclopride. We determined values of BP_ND in striatal and extrastriatal volumes-of-interest (VOI) with the same specific receptor radioligand. We estimated values of BP_ND in individual voxels of brains of healthy volunteers in vivo, and we obtained regional averages of VOI by dynamic positron emission tomography (PET). We calculated average values of BP_ND in caudate nucleus and putamen of striatum, and in frontal, occipital, parietal, and temporal cortices of the forebrain, by means of four methods, including the ERLiBiRD (Estimation of Reversible Ligand Binding and Receptor Density) method, the tissue reference methods of Logan and Logan-Ichise, respectively, and the SRTM (Simplified Reference Tissue Method). Voxelwise generation of parametric maps of values of BP_ND used the multi-linear regression version of SRTM. Age-dependent changes of the binding potential presented with an inverted U-shape with peak binding potentials reached between the ages of 20 and 30. The estimates of BP_ND declined significantly with age after the peak in both striatal and extrastriatal regions, as determined by all four methods, with the greatest decline observed in posterior (occipital and parietal) cortices (14% per decade) and the lowest decline in caudate nucleus (3% per decade). The sites of the greatest declines are of particular interest because of the clinical implications.

Positron Emission Tomography (PET) Imaging Tracers for Serotonin Receptors

Serotonin (5-hydroxytryptamine, 5-HT) and 5-HT receptors (5-HTRs) have crucial roles in various neuropsychiatric disorders and neurodegenerative diseases, making them attractive diagnostic and therapeutic targets. Positron emission tomography (PET) is a noninvasive nuclear molecular imaging technique and is an essential tool in clinical diagnosis and drug discovery. In this context, numerous PET ligands have been developed for "visualizing" 5-HTRs in the brain and translated into human use to study disease mechanisms and/or support drug development. Herein, we present a comprehensive repertoire of 5-HTR PET ligands by focusing on their chemotypes and performance in PET imaging studies. Furthermore, this Perspective summarizes recent 5-HTR-focused drug discovery, including biased agonists and allosteric modulators, which would stimulate the development of more potent and subtype-selective 5-HTR PET ligands and thus further our understanding of 5-HTR biology.

Non-Peptide Opioids Differ in Effects on Mu-Opioid (MOP) and Serotonin 1A (5-HT1A) Receptors Heterodimerization and Cellular Effectors (Ca2+, ERK1/2 and p38) Activation

The importance of the dynamic interplay between the opioid and the serotonin neuromodulatory systems in chronic pain is well recognized. In this study, we investigated whether these two signalling pathways can be integrated at the single-cell level via direct interactions between the mu-opioid (MOP) and the serotonin 1A (5-HT1A) receptors. Using fluorescence cross-correlation spectroscopy (FCCS), a quantitative method with single-molecule sensitivity, we characterized in live cells MOP and 5-HT1A interactions and the effects of prolonged (18 h) exposure to selected non-peptide opioids: morphine, codeine, oxycodone and fentanyl, on the extent of these interactions. The results indicate that in the plasma membrane, MOP and 5-HT1A receptors form heterodimers that are characterized with an apparent dissociation constant Kdapp = (440 ± 70) nM). Prolonged exposure to all non-peptide opioids tested facilitated MOP and 5-HT1A heterodimerization and stabilized the heterodimer complexes, albeit to a different extent: Kd, Fentanylapp = (80 ± 70) nM), Kd,Morphineapp = (200 ± 70) nM, Kd, Codeineapp = (100 ± 70) nM and Kd, Oxycodoneapp = (200 ± 70) nM. The non-peptide opioids differed also in the extent to which they affected the mitogen-activated protein kinases (MAPKs) p38 and the extracellular signal-regulated kinase (Erk1/2), with morphine, codeine and fentanyl activating both pathways, whereas oxycodone activated p38 but not ERK1/2. Acute stimulation with different non-peptide opioids differently affected the intracellular Ca2+ levels and signalling dynamics. Hypothetically, targeting MOP−5-HT1A heterodimer formation could become a new strategy to counteract opioid induced hyperalgesia and help to preserve the analgesic effects of opioids in chronic pain.

Serotonin-1A receptor C-1019G polymorphism affects brain functional networks

The serotonin-1A (5-HT1A) receptor is strongly implicated in major depression and other affective disorders due to its negative regulation of serotonin neurone firing rates. Behavioural and clinical studies have repeatedly reported that the −1019G allele carries a high susceptibility for affective disorders. However, the underlying pathophysiology remains unknown. Here, we employed a genetic neuroimaging strategy in 99 healthy human subjects to explore the effect of serotonin-1A receptor polymorphism on brain resting-state functional connectivity (FC). We used functional magnetic resonance imaging, along with a seed-based approach, to identify three main brain networks: the default mode network (DMN), the salience network (SN) and the central executive network. We observed a significant decrease in the FC of the DMN within the dorsolateral and ventromedial prefrontal cortices in G-carriers. Furthermore, compared with the C-homozygote group, we observed decreased FC of the SN within the ventromedial prefrontal cortex and subgenual anterior cingulate cortex in the G-carrier group. Our results indicate that 5-HT1A receptor genetic polymorphism modulates the activity of resting-state FC within brain networks including the DMN and SN. These genotype-related alterations in brain networks and FC may provide novel insights into the neural mechanism underlying the predisposition for affective disorders in G allele carriers.

Hippocampal monoamine receptor complex levels linked to spatial memory decline in the aging C57BL/6J

Although a large series of reports on monoamine receptor (MAR) biochemistry and pharmacology in aging are available, work on MAR complexes rather than subunits is limited. It was the aim of the study to determine MAR complexes in hippocampi of three different age groups (3-12 and 18 months) in the mouse and to link MAR changes to spatial memory retrieval in the Morris water maze (MWM). MAR complexes were separated by blue native electrophoresis. Immunohistochemistry was performed in order to show the pattern of dopamine receptors and its colocalizations. D1R, D2R and 5-HT7R containing receptor complex levels were decreasing with age while 5-HT1AR-containing complex levels were increasing with age. D1R, 5-HT7R and 5-HT1AR were significantly correlating with the time spent in the target quadrant, representing retrieval in the MWM. D1R and D2R immunoreactivity was decreasing in an area-dependent pattern and D1R and D2R were colocalized. Individual monoamine receptors are linked to spatial memory retrieval and are modulated by age. The findings are relevant for interpretation of previous and design of future work on brain receptors, spatial memory and aging.

Memory deficits in aging and neurological diseases

Memory is central to our ability to perform daily life activities and correctly function in society. Improvements in public health and medical treatment for a variety of diseases have resulted in longer life spans; however, age-related memory impairments have been significant sources of morbidity. Loss in memory function is not only associated with aging population but is also a feature of neurodegenerative diseases such as Alzheimer's disease and other psychiatric and neurological disorders. Here, we focus on current understanding of the impact of normal aging on memory and what is known about its mechanisms, and further review pathological mechanisms behind the cause of dementia in Alzheimer's disease. Finally, we discuss schizophrenia and look into abnormalities in circuit function and neurotransmitter systems that contribute to memory impairment in this illness.

Molecular Neurobiology of Depression: PET Findings on the Elusive Correlation with Symptom Severity

Molecular mechanisms in the brain are assumed to cause the symptoms and severity of neuropsychiatric disorders. This review concerns the elusive nature of relationships between the severity of depressive disorders and neuromolecular processes studied by positron emission tomography (PET). Recent PET studies of human depression have focused on serotonergic, dopaminergic, muscarinic, nicotinic, and GABAergic receptors, as well as central processes dependent on monoamine oxidase, phosphodiesterase type 4, amyloid plaques, neurofibrillar tangles, and P-glycoprotein. We find that reliable causal links between neuromolecular mechanisms and relief from depressive disorders have yet to be convincingly demonstrated. This situation may contribute to the currently limited use of PET for exploring the neuropathways that are currently viewed as being responsible for beneficial effects of antidepressant treatment regimes.

The relation of developmental changes in brain serotonin transporter (5HTT) and 5HT1A receptor binding to emotional behavior in female rhesus monkeys: effects of social status and 5HTT genotype

The goal of the present study was to examine how social subordination stress and 5HTT polymorphisms affect the development of brain serotonin (5HT) systems during the pubertal transition in female rhesus monkeys. We also examined associations with developmental changes in emotional reactivity in response to a standardized behavioral test, the Human Intruder (HI). Our findings provide the first longitudinal evidence of developmental increases in 5HT1A receptor and 5HTT binding in the brain of female primates from pre- to peripuberty. The increase in 5HT1A BP(ND) in these socially housed female rhesus monkeys is a robust finding, occurring across all groups, regardless of social status or 5HTT genotype, and occurring in the left and right hemispheres of all prefrontal regions studied, as well as the amygdala, hippocampus, hypothalamus, and raphe nuclei. 5HTT BP(ND) also showed an increase with age in raphe, anterior cingulate cortex, and dorsolateral prefrontal cortex. These changes in brain 5HT systems take place as females establish more adult-like patterns of social behavior, as well as during the HI paradigm. Indeed, the main developmental changes in behavior during the HI (increase in freezing and decrease in submission/appeasement) were related to neurodevelopmental increases in 5HT1A receptors and 5HTT, because the associations between these behaviors and 5HT endpoints emerge at peripuberty. We detected an effect of social status on 5HT1A BP(ND) in the hypothalamus and on 5HTT BP(ND) in the orbitofrontal cortex, with subordinates showing higher BP(ND) than dominants in both cases during the pubertal transition. No main effects of 5HTT genotype were observed for 5HT1A or 5HTT BP(ND). Our findings indicate that adolescence in female rhesus monkeys is a period of central 5HT reorganization, partly influenced by exposure to the social stress of subordination, that likely functions to integrate adrenal and gonadal systems and shape the behavioral response to emotionally challenging social situations.

Serotonin modulation of cerebral glucose metabolism: sex and age effects

The serotonin system is implicated in a variety of psychiatric disorders whose clinical presentation and response to treatment differ between males and females, as well as with aging. However, human neurobiological studies are limited. Sex differences in the cerebral metabolic response to an increase in serotonin concentrations were measured, as well as the effect of aging, in men compared to women. Thirty-three normal healthy individuals (14 men/19 women, age range 20-79 years) underwent two resting positron emission tomography studies with the radiotracer [18F]-2-deoxy-2-fluoro-D-glucose ([(18)F]-FDG) after placebo and selective serotonin reuptake inhibitor (SSRI, citalopram) infusions on two separate days. Results indicated that women demonstrated widespread areas of increased cortical glucose metabolism with fewer areas of decrease in metabolism in response to citalopram. Men, in contrast, demonstrated several regions of decreased cortical metabolism, but no regions of increased metabolism. Age was associated with greater increases in women and greater decreases in men in most brain regions. These results support prior studies indicating that serotonin function differs in men and women across the lifespan. Future studies aimed at characterizing the influences of age and sex on the serotonin system in patients with psychiatric disorders are needed to elucidate the relationship between sex and age differences in brain chemistry and associated differences in symptom presentation and treatment response.

The cognitive neuroscience of ageing

The availability of neuroimaging technology has spurred a marked increase in the human cognitive neuroscience literature, including the study of cognitive ageing. Although there is a growing consensus that the ageing brain retains considerable plasticity of function, currently measured primarily by means of functional MRI, it is less clear how age differences in brain activity relate to cognitive performance. The field is also hampered by the complexity of the ageing process itself and the large number of factors that are influenced by age. In this Review, current trends and unresolved issues in the cognitive neuroscience of ageing are discussed.

Positron emission tomography assessment of 8-OH-DPAT-mediated changes in an index of cerebral glucose metabolism in female marmosets

As part of a larger experiment investigating serotonergic regulation of female marmoset sexual behavior, this study was designed to (1) advance methods for PET imaging of common marmoset monkey brain, (2) measure normalized FDG uptake as an index of local cerebral metabolic rates for glucose, and (3) study changes induced in this index of cerebral glucose metabolism by chronic treatment of female marmosets with a serotonin 1A receptor (5-HT(1A)) agonist. We hypothesized that chronic treatment with the 5-HT(1A) agonist 8-OH-DPAT would alter the glucose metabolism index in dorsal raphe (DR), medial prefrontal cortex (mPFC), medial preoptic area of hypothalamus (mPOA), ventromedial nucleus of hypothalamus (VMH), and field CA1 of hippocampus. Eight adult ovariectomized female common marmosets (Callithrix jacchus) were studied with and without estradiol replacement. In a crossover design, each subject was treated daily with 8-OH-DPAT (0.1mg/kg SC daily) or saline. After 42-49 days of treatment, the glucose metabolism radiotracer FDG was administered to each female immediately prior to 30 min of interaction with her male pairmate, after which the subject was anesthetized and imaged by PET. Whole brain normalized PET images were analyzed with anatomically defined regions of interest (ROI). Whole brain voxelwise mapping was also used to explore treatment effects and correlations between alterations in the glucose metabolism index and pairmate interactions. The rank order of normalized FDG uptake was VMH/mPOA>DR>mPFC/CA1 in both conditions. 8-OH-DPAT did not induce alterations in the glucose metabolism index in ROIs. Voxelwise mapping showed a significant reduction in normalized FDG uptake in response to 8-OH-DPAT in a cluster in medial occipital cortex as well as a significant correlation between increased rejection of mount attempts and reduced normalized FDG uptake in an overlapping cluster. In conclusion, PET imaging has been used to measure FDG uptake relative to whole brain in marmoset monkeys. Voxelwise mapping shows that 8-OH-DPAT reduces this index of glucose metabolism in medial occipital cortex, consistent with alterations in female sexual behavior.

Neuromodulation of reward-based learning and decision making in human aging

In this paper, we review the current literature to highlight relations between age-associated declines in dopaminergic and serotonergic neuromodulation and adult age differences in adaptive goal-directed behavior. Specifically, we focus on evidence suggesting that deficits in neuromodulation contribute to older adults' behavioral disadvantages in learning and decision making. These deficits are particularly pronounced when reward information is uncertain or the task context requires flexible adaptations to changing stimulus-reward contingencies. Moreover, emerging evidence points to age-related differences in the sensitivity to rewarding and aversive outcomes during learning and decision making if the acquisition of behavior critically depends on outcome processing. These age-related asymmetries in outcome valuation may be explained by age differences in the interplay of dopaminergic and serotonergic neuromodulation. This hypothesis is based on recent neurocomputational and psychopharmacological approaches, which suggest that dopamine and serotonin serve opponent roles in regulating the balance between approach behavior and inhibitory control. Studying adaptive regulation of behavior across the adult life span may shed new light on how the aging brain changes functionally in response to its diminishing resources.

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-HT(4) receptor activation influence cognitive function, affective symptoms, and the development of Alzheimer's 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-HT(4) receptor-binding potentials in striatum, the limbic system, and neocortex. Positron-emission tomographic scans were conducted using the radioligand [(11)C]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, BP(ND), 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-HT(4) receptor binding in the limbic system. The lower limbic 5-HT(4) receptor binding in women supports a role for 5-HT(4) 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-HT(4) receptor binding with aging contrasts others in subtypes of receptors, which generally decrease with aging.

Modeling treatment-resistant depression

Depression is a polygenic and highly complex psychiatric disorder that is currently a major burden on society. Depression is highly heterogeneous in presentation and frequently exhibits high comorbidity with other psychiatric and somatic disorders. Commonly used treatments, such as selective serotonin reuptake inhibitors (SSRIs), are not ideal since only a subset of patients achieve remission. In addition, the reason why some individuals respond to SSRIs while others don't are unknown. Here we begin to ask what the basis of treatment resistance is, and propose new strategies to model this phenomenon in animals. We focus specifically on animal models that offer the appropriate framework to study treatment resistance with face, construct and predictive validity.

The postictal state: effects of age and underlying brain dysfunction

There is relatively little information on the underlying parameters that affect clinical features of the postictal period. Age-related physiological changes, including alterations in cerebral blood flow and metabolism, neurotransmitter function, and responses of the brain to seizure activity may affect postictal clinical phenomena. Some conclusions can be drawn. Elderly adults and children, particularly in the presence of diffuse cerebral dysfunction, may have more prolonged postictal confusion. Postictal dysphasia strongly suggests a dominant hemisphere focus, more often temporal, and Todd's paralysis is always contralateral to the epileptogenic zone. Much additional information could be derived from the vast amount of video/EEG monitoring data available.

Neuroendocrine effects of citalopram, a selective serotonin re-uptake inhibitor, during lifespan in humans

OBJECTIVE

Serotonergic system contributes to the regulation of hypothalamus-pituitary-adrenal axis. In humans, serotonergic agonists increase PRL, ACTH, and cortisol, while serotonin (5HT) influence on GH is controversial. Central 5HT activity and neuroendocrine function change during lifespan.

DESIGN

To clarify the neuroendocrine response to 5HT across lifespan, we assessed ACTH, cortisol, DHEA, PRL, and GH responses to citalopram (CT) in young adults (YA) (no.=12, 29.2±1.7 yr mean±SEM), middle aged (MA) (no.=12, 54.3±0.9 yr), and elderly (ES) (no.=12, 69.3±0.9 yr) males. All the subjects received placebo (saline iv over 120 min) or CT (20 mg iv over 120 min). Blood samples were taken every 15 min up to 240 min.

RESULTS

During placebo, ACTH, cortisol, GH, and PRL were similar in all groups while DHEA showed an age-dependent reduction from middle age (p<0.001). During CT, ACTH, and cortisol were higher than during placebo in YA (p<0.05) and even more in MA (p<0.01 vs placebo, p<0.05 vs YA); in ES, the increase of both ACTH and cortisol (p<0.05 vs placebo) was lower than in MA (p<0.05) and higher than in YA (p<0.05 for cortisol only). No changes were observed for DHEA, GH, and PRL in any group.

CONCLUSIONS

Corticotrope response to CT is age-dependent in normal men, being amplified starting from middle age, suggesting precocious changes in the serotonergic neuroendocrine control during lifespan. CT is a useful tool to evaluate the age-dependent serotonergic function in humans.

5-HT1A gene promoter polymorphism and [18F]MPPF binding potential in healthy subjects: a PET study

Background

Previous Positron Emission Tomography (PET) studies of 5-HT_1A receptors have shown an influence of several genetic factors, including the triallelic serotonin transporter gene-linked polymorphic region on the binding potential (BP_ND) of these receptors. The aim of our study was to investigate the relationship between a 5-HT_1A promoter polymorphism and the binding potential of another selective 5-HT_1A receptor antagonist, [¹⁸F]MPPF, in healthy subjects.

Methods

Thirty-five volunteers, including 23 women, underwent an [¹⁸F]MPPF scan and were genotyped for both the C(-1019)G 5-HT_1A promoter polymorphism and the triallelic serotonin transporter gene-linked polymorphic region. We used a simplified reference tissue model to generate parametric images of BP_ND. Whole brain Statistical Parametric Mapping and raphe nuclei region of interest analyses were performed to look for an association of [¹⁸F]MPPF BP_ND with the C(-1019)G 5-HT_1A promoter polymorphism.

Results

Among the 35 subjects, 5-HT_1A promoter genotypes occurred with the following frequencies: three G/G, twenty-one G/C, and eleven C/C. No difference of [¹⁸F]MPPF BP_ND between groups was observed, except for two women who were homozygote carriers for the G allele and showed greater binding potential compared to other age-matched women over the frontal and temporal neocortex. However, the biological relevance of this result remains uncertain due to the very small number of subjects with a G/G genotype. These findings were not modified by excluding individuals carrying the S/S genotype of the serotonin transporter gene-linked polymorphic region.

Conclusions

We failed to observe an association between the C(-1019)G 5-HT_1A promoter polymorphism and [¹⁸F]MPPF binding in healthy subjects. However our data suggest that the small number of women homozygote for the G allele might have greater [¹⁸F]MPPF BP_ND relative to other individuals. This finding should be confirmed in a larger sample.

Altered function of ventral striatum during reward-based decision making in old age

Normal aging is associated with a decline in different cognitive domains and local structural atrophy as well as decreases in dopamine concentration and receptor density. To date, it is largely unknown how these reductions in dopaminergic neurotransmission affect human brain regions responsible for reward-based decision making in older adults. Using a learning criterion in a probabilistic object reversal task, we found a learning stage by age interaction in the dorsolateral prefrontal cortex (dlPFC) during decision making. While young adults recruited the dlPFC in an early stage of learning reward associations, older adults recruited the dlPFC when reward associations had already been learned. Furthermore, we found a reduced change in ventral striatal BOLD signal in older as compared to younger adults in response to high probability rewards. Our data are in line with behavioral evidence that older adults show altered stimulus–reward learning and support the view of an altered fronto-striatal interaction during reward-based decision making in old age, which contributes to prolonged learning of reward associations.

Elevated serotonin 1A binding in remitted major depressive disorder: evidence for a trait biological abnormality

Several biological abnormalities in major depressive disorder (MDD) persist during episode remission, including altered serotonin neurotransmission, and may reflect underlying pathophysiology. We previously described elevated brain serotonin 1A (5-HT(1A)) receptor binding in antidepressant-naive (AN) subjects with MDD within a major depressive episode (MDE) compared with that in healthy controls using positron emission tomography (PET). In this study, we measured 5-HT(1A) receptor binding in unmedicated subjects with MDD during sustained remission, hypothesizing higher binding compared with that in healthy controls, and binding comparable with currently depressed AN subjects, indicative of a biological trait. We compared 5-HT(1A) binding potential (BP(F)) assessed through PET scanning with [(11)C]WAY-100635 in 15 subjects with recurrent MDD in remission for >or=12 months and off antidepressant medication for >or=6 months, 51 healthy controls, and 13 AN MDD subjects in a current MDE. Metabolite-corrected arterial input functions were acquired for the estimation of BP(F). Remitted depressed subjects had higher 5-HT(1A) BP(F) compared with healthy controls; this group difference did not vary significantly in magnitude across brain regions. 5-HT(1A) BP(F) was comparable in remitted and currently depressed subjects. Elevated 5-HT(1A) BP(F) level among subjects with remitted MDD appears to be a trait abnormality in MDD, which may underlie recurrent MDEs. Future studies should evaluate the role of genetic and environmental factors in producing elevated 5-HT(1A) BP(F) and MDD, and should examine whether 5-HT(1A) BP(F) is a vulnerability factor to MDEs that could have a role in screening high-risk populations for MDD.

Parametric mapping of 5HT1A receptor sites in the human brain with the Hypotime method: theory and normal values

The radioligand [carbonyl-(11)C]WAY-100635 ((11)C-WAY) is a PET tracer of the serotonin 5HT(1A) receptors in the human brain. It is metabolized so rapidly in the circulation that it behaves more as a chemical microsphere than as a tracer subject to continuous exchange between the circulation and brain tissue. Although reference tissue methods are useful as analyses of uptake of some radioligands with indeterminate arterial input functions, their use to analyze (11)C-WAY uptake and binding is challenged by the rapid plasma metabolism, which violates the assumption that regions of interest and reference regions continue to exchange radioligand with the circulation during the entire uptake period. Here, we proposed a method of calculation (Hypotime) that specifically uses the washout rather than the accumulation of (11)C-WAY to determine binding potentials (BP(ND)), without the use of regression analysis.

METHODS

A total of 19 healthy volunteers (age range, 23-73 y) underwent PET to test the Hypotime application of the chemical microsphere properties of (11)C-WAY to identify regions of binding and nonbinding on the exclusive basis of the rate of washout of (11)C-WAY.

RESULTS

The results of the Hypotime method were compared with the simplified but multilinearized reference tissue method (MLSRTM). The distribution of receptor BP(ND) obtained with Hypotime was consistent with previous autoradiography of postmortem brain tissue, with the highest values of BP(ND) recorded in the medial temporal lobe and decline of receptor availability with age. The values in the basal ganglia and cerebellum were negligible. The MLSRTM, in contrast, yielded lower BP(ND) in all regions and only weakly revealed the decline with age.

CONCLUSION

The simple and computationally efficient Hypotime method gave reliable values of BP(ND) without the use of regression. The MLSRTM, on the other hand, appeared to be affected by the early disappearance of the radioligand from the circulation and the associated uncertain late presence of (11)C-WAY in the circulation.

Serotonin transporter genotype and neuroanatomy in autism spectrum disorders

There is increasing evidence that people with autism spectrum disorders (ASDs) have abnormalities in the serotonergic system. For example, a functional polymorphism of the serotonin transporter gene promoter region (5HTTLPR long/short polymorphism) has been reported to confer risk for ASDs, and to affect cortical grey matter volume in young children. However, the persistence of this association later in development is unknown. Hence, we investigated whether variation in the 5HTTLPR long/short polymorphism modulates brain anatomy in older people with ASD. We related 5HTTLPR long/short polymorphism in 43 adolescents and adults with ASD to brain anatomy using structural magnetic resonance imaging and voxel-based morphometry. There were no significant associations between brain anatomy and genotype. When considered alongside evidence of a relationship between 5HTTLPR genotype and brain volume amongst children with autism, our findings raise the possibility that the relationship between 5HTTLPR polymorphism and brain anatomy in ASDs anatomy may differ as a function of age and/or ASD subdiagnosis.

Association between triallelic polymorphism of the serotonin transporter and [18F]MPPF binding potential at 5-HT1A receptors in healthy subjects

Previous [(11)C]WAY100-635 PET studies have demonstrated that the short (S) and long (L) alleles of the serotonin transporter gene-linked polymorphic region (5-HTTLPR) were associated with distinct patterns of 5-HT(1A) receptor distribution in human. However, these studies reported discordant findings and did not take into account the recent description of two functional variants of the L allele (L(A)/L(G)). To further explore this issue, we investigated the triallelic functional polymorphism of the 5-HTTLPR in 38 healthy volunteers who underwent a [(18)F]MPPF PET study of 5-HT1A receptors. We used a simplified reference tissue model to generate parametric images of [(18)F]MPPF binding potential (BP(ND)), and compared these data among the different genotypes using statistical parametric mapping and region of interest of the raphe nuclei. Homozygote carriers of the S allele demonstrated greater [(18)F]MPPF BP(ND) than carriers of the L(A) allele, but this association was only found in women. Differences in [(18)F]MPPF BP(ND) between women with and without L(A) allele were observed over large clusters encompassing the right and left temporal lobes, cingulate and perisylvian regions, as well as the right precuneus and frontal dorso-lateral cortex, and the left orbitofrontal cortex. In contrast, no difference was found between groups in the raphe nuclei. The greater [(18)F]MPPF BP(ND) observed in women homozygote carriers of the S allele could either reflect a greater 5-HT1A receptor density or a lower extracellular concentration of 5-HT. Our data suggest that any future PET studies of 5-HT1A receptors should incorporate the 5-HTTLPR polymorphism status of the population studied.

Monoaminergic modulation of emotional impact in the inferomedial prefrontal cortex

People assess the impact of emotionally loaded images differently. We define this impact as the average difference between individual ratings of standardized "pleasant" and "unpleasant" images. To determine the neuroanatomical correlate of a hypothetical interaction between emotional impact and cerebral excitability, we first determined the individual effect on cerebral blood flow of a pharmacological challenge with the monoamine reuptake inhibitor clomipramine in nine healthy volunteers. In a later, independent, session the nine volunteers rated pleasant, neutral, and unpleasant images of the standard Empathy Picture System on a scale from +3 to -3. We then used regression analysis to identify sites in the ventromedial prefrontal cortex at which the two separately acquired measures, blood flow change and emotional impact of images, correlated significantly. The regression analysis identified a locus in Brodmann's area 11 of the inferomedial prefrontal cortex (IMPC) at which these two separate measures had significant inverse correlation. Thus, under the specific circumstance of positron emission tomography (PET) of a pharmacological challenge, a key region of the inferomedial prefrontal cortex underwent deactivation in proportion to a separately rated emotional impact of a stimulus. We propose a specific pharmacodynamic mechanism that explains the correlation between the emotional impact and the effect of a serotonin-noradrenaline reuptake inhibitor on cerebral blood flow.

A genetic rat model of depression, Flinders sensitive line, has a lower density of 5-HT(1A) receptors, but a higher density of 5-HT(1B) receptors, compared to control rats

Deficiencies in brain serotonergic neurotransmission, which is in part associated with the alteration of brain serotonin (5-HT) receptors, have been proposed as part of a neurochemical imbalance in affective disorders, including depression. The drugs used for the treatment of these disorders generally act through and/or on the serotonergic system. Different animal models of depression have provided researchers with tools to obtain a better understanding of drug actions and possibilities to obtain insight into the neurochemical bases of these disorders. The measurements of the 5-HT(1A) and 5-HT(1B) receptor densities in a rat model of depression, Flinders sensitive line (FSL) rats, and comparisons with Sprague-Dawley (SPD) and Flinders resistant line (FRL) rats, are reported here. The receptor sites were quantified by autoradiography in more than 25 distinct brain regions known to have relatively large densities of respective sites. Some brain regions (e.g., dental gyrus, septal nucleus) were divided into several parts, according to previously known subdivisions, because of a substantial heterogeneity of these receptors. The densities in the FSL rats ("depressed" rats) were compared statistically to those in the SPD rats. In addition, comparisons were made to the densities in the FRL rats (rats not showing depressive symptoms). Comparisons were performed with the SPD and FRL rats because both of these strains have been used as control animals in studies of FSL rats. The results show that the densities of 5-HT(1A) receptors are not significantly different between the FSL and SPD rats, but they are significantly different from the FRL rats. 5-HT(1A) receptor density is significantly higher in the FRL rats than the SPD rats. The 5-HT(1B) receptors were significantly greater in the FSL rats than in either the SPD or FRL rats. In addition, the FRL rats have 5-HT(1B) receptor densities significantly lower in many brain regions than the SPD rats. The data presented here, in addition to previously reported differences in regional synthesis between these strains and the effect of acute citalopram on synthesis, suggest that SPD rats are likely a more appropriate control than FRL rats, when studies of FSL rats are performed with drugs acting directly or indirectly on, or through, the brain serotonergic system. However, comparisons, particularly of neurochemical and/or biological parameters in FRL rats, may reveal new insight into the alterations of 5-HT neurotransmission in this animal model of depression and possibly human depression, as well as the elevation of symptoms with treatments. The data also suggest that there could be a different fraction of 5-HT(1A) receptors in high and low affinity states in these strains, as well as the possibility of different intracellular signalling.

The serotonin-1A receptor distribution in healthy men and women measured by PET and [carbonyl-11C]WAY-100635

PURPOSE

The higher prevalence rates of depression and anxiety disorders in women compared to men have been associated with sexual dimorphisms in the serotonergic system. The present positron emission tomography (PET) study investigated the influence of sex on the major inhibitory serotonergic receptor subtype, the serotonin-1A (5-HT(1A)) receptor.

METHODS

Sixteen healthy women and 16 healthy men were measured using PET and the highly specific radioligand [carbonyl-(11)C]WAY-100635. Effects of age or gonadal hormones were excluded by restricting the inclusion criteria to young adults and by controlling for menstrual cycle phase. The 5-HT(1A) receptor BP(ND) was quantified using (1) the 'gold standard' manual delineation approach with ten regions of interest (ROIs) and (2) a newly developed delineation method using a PET template normalized to the Montreal Neurologic Institute space with 45 ROIs based on automated anatomical labeling.

RESULTS

The 5-HT(1A) receptor BP(ND) was found equally distributed in men and women applying both the manual delineation method and the automated delineation approach. Women had lower mean BP(ND) values in every region investigated, with a borderline significant sex difference in the hypothalamus (p = 0.012, uncorrected). There was a high intersubject variability of the 5-HT(1A) receptor BP(ND) within both sexes compared to the small mean differences between men and women.

CONCLUSIONS

To conclude, when measured in the follicular phase, women do not differ from men in the 5-HT(1A) receptor binding. To explain the higher prevalence of affective disorders in women, further studies are needed to evaluate the relationship between hormonal status and the 5-HT(1A) receptor expression.