Age effects on serotonin receptor 1B as assessed by PET

The regional pattern of age-related synaptic loss in the human brain differs from gray matter volume loss: in vivo PET measurement with [11C]UCB-J

PURPOSE

Aging is a major societal concern due to age-related functional losses. Synapses are crucial components of neural circuits, and synaptic density could be a sensitive biomarker to evaluate brain function. [11C]UCB-J is a positron emission tomography (PET) ligand targeting synaptic vesicle glycoprotein 2A (SV2A), which can be used to evaluate brain synaptic density in vivo.

METHODS

We evaluated age-related changes in gray matter synaptic density, volume, and blood flow using [11C]UCB-J PET and magnetic resonance imaging (MRI) in a wide age range of 80 cognitive normal subjects (21-83 years old). Partial volume correction was applied to the PET data.

RESULTS

Significant age-related decreases were found in 13, two, and nine brain regions for volume, synaptic density, and blood flow, respectively. The prefrontal cortex showed the largest volume decline (4.9% reduction per decade: RPD), while the synaptic density loss was largest in the caudate (3.6% RPD) and medial occipital cortex (3.4% RPD). The reductions in caudate are consistent with previous SV2A PET studies and likely reflect that caudate is the site of nerve terminals for multiple major tracts that undergo substantial age-related neurodegeneration. There was a non-significant negative relationship between volume and synaptic density reductions in 16 gray matter regions.

CONCLUSION

MRI and [11]C-UCB-J PET showed age-related decreases of gray matter volume, synaptic density, and blood flow; however, the regional patterns of the reductions in volume and SV2A binding were different. Those patterns suggest that MR-based measures of GM volume may not be directly representative of synaptic density.

Sex and the serotonergic underpinnings of depression and migraine

Most psychiatric disorders demonstrate sex differences in their prevalence and symptomatology, and in their response to treatment. These differences are particularly pronounced in mood disorders. Differences in sex hormone levels are among the most overt distinctions between males and females and are thus an intuitive underpinning for these clinical observations. In fact, treatment with estrogen and testosterone was shown to exert antidepressant effects, which underscores this link. Changes to monoaminergic signaling in general, and serotonergic transmission in particular, are understood as central components of depressive pathophysiology. Thus, modulation of the serotonin system may serve as a mechanism via which sex hormones exert their clinical effects in mental health disorders. Over the past 20 years, various experimental approaches have been applied to identify modes of influence of sex and sex hormones on the serotonin system. This chapter provides an overview of different molecular components of the serotonin system, followed by a review of studies performed in animals and in humans with the purpose of elucidating sex hormone effects. Particular emphasis will be placed on studies performed with positron emission tomography, a method that allows for human in vivo molecular imaging and, therefore, assessment of effects in a clinically representative context. The studies addressed in this chapter provide a wealth of information on the interaction between sex, sex hormones, and serotonin in the brain. In general, they offer evidence for the concept that the influence of sex hormones on various components of the serotonin system may serve as an underpinning for the clinical effects these hormones demonstrate.

Synaptic Changes in Parkinson Disease Assessed with in vivo Imaging

OBJECTIVE

Parkinson disease is characterized by motor and nonmotor symptoms, reduced striatal dopamine signaling, and loss of dopamine neurons in the substantia nigra. It is now known that the pathological process in Parkinson disease may begin decades before the clinical diagnosis and include a variety of neuronal alterations in addition to the dopamine system.

METHODS

This study examined the density of all synapses with synaptic vesicle glycoprotein 2A (SV2A) in Parkinson disease subjects with mild bilateral disease (n = 12) and matched normal controls (n = 12) using in vivo high-resolution positron emission tomographic imaging as well as postmortem autoradiography in an independent sample with Parkinson disease (n = 15) and normal controls (n = 13) in the substantia nigra and putamen.

RESULTS

A group-by-brain region interaction effect (F10, 22 = 3.52, p = 0.007) was observed in the primary brain areas with in vivo SV2A binding. Post hoc analyses revealed that the Parkinson disease group exhibited lower SV2A in the substantia nigra (-45%; p < 0.001), red nucleus (-31%; p = 0.03), and locus coeruleus (-17%; p = 0.03). Exploratory analyses also revealed lower SV2A binding in clinically relevant cortical areas. Using autoradiography, we confirmed lower SV2A in the substantia nigra (-17%; p < 0.005) and nonsignificant findings in the putamen (-4%; p = 0.06).

INTERPRETATION

This work provides the first evidence of synaptic loss in brainstem nuclei involved in the pathogenesis of Parkinson disease in living patients. SV2A imaging holds promise for understanding synaptic changes central to the disease. Ann Neurol 2020;87:329-338.

Metabolic, Molecular, and Behavioral Effects of Western Diet in Serotonin Transporter-Deficient Mice: Rescue by Heterozygosity?

Reduced function of the serotonin transporter (SERT) is associated with increased susceptibility to anxiety and depression and with type-2 diabetes, which is especially true in older women. Preference for a "Western diet" (WD), enriched with saturated fat, cholesterol, and sugars, may aggravate these conditions. In previous studies, decreased glucose tolerance, central and peripheral inflammation, dyslipidemia, emotional, cognitive, and social abnormalities were reported in WD-fed young female mice. We investigated the metabolic, molecular, and behavioral changes associated with a 3-week-long dietary regime of either the WD or control diet in 12-month-old female mice with three different Sert genotypes: homozygous (Slc6a4) gene knockout (Sert ^-/-: KO), heterozygous (Sert ^+/-: HET), or wild-type mice (Sert ^+/+: WT). In the WT-WD and KO-WD groups, but not in HET-WD-fed mice, most of changes induced by the WD paralleled those found in the younger mice, including brain overexpression of inflammatory marker Toll-like receptor 4 (Tlr4) and impaired hippocampus-dependent performance in the marble test. However, the 12-month-old female mice became obese. Control diet KO mice exhibited impaired hippocampal-dependent behaviors, increased brain expression of the serotonin receptors Htr2c and Htr1b, as well as increased Tlr4 and mitochondrial regulator, peroxisome proliferator-activated receptor gamma-coactivator-1a (Ppargc1a). Paradoxically, these, and other changes, were reversed in KO-WD mutants, suggesting a complex interplay between Sert deficiency and metabolic factors as well as potential compensatory molecular mechanisms that might be disrupted by the WD exposure. Most, but not all, of the changes in gene expression in the brain and liver of KO mice were not exhibited by the HET mice fed with either diet. Some of the WD-induced changes were similar in the KO-WD and HET-WD-fed mice, but the latter displayed a "rescued" phenotype in terms of diet-induced abnormalities in glucose tolerance, neuroinflammation, and hippocampus-dependent performance. Thus, complete versus partial Sert inactivation in aged mice results in distinct metabolic, molecular, and behavioral consequences in response to the WD. Our findings show that Sert ^+/- mice are resilient to certain environmental challenges and support the concept of heterosis as evolutionary adaptive mechanism.

Social status and demographic effects of the kappa opioid receptor: a PET imaging study with a novel agonist radiotracer in healthy volunteers

Kappa opioid receptors (KORs) have been characterized as an aversive system in the brain and implicated in social behavior in preclinical models. This work investigated the effect of social status on the KOR system in humans using positron emission tomography (PET) imaging with the novel KOR agonist radiotracer [¹¹C]EKAP. Eighteen healthy participants (mean age 41.2 ± 9.3) completed the Barratt Simplified Measure of Social Status (BSMSS), an MRI and an [¹¹C]EKAP PET scan on the High Resolution Research Tomograph. Arterial blood sampling and metabolite analysis were conducted to obtain the input function. Regions of interest were based upon an MR template and included the reward/aversion areas of the brain. The multilinear analysis-1 (MA1) method was applied to the regional time-activity curves (TACs) to calculate [¹¹C]EKAP regional volume of distribution (V_T). Mixed models and Pearson correlation coefficients were used for body mass index (BMI), gender and age, with age being dropped in subsequent analyses because of nonsignificance. An overall effect of primary ROIs (F_7, 112 7.43, p < 0.0001), BSMSS score (F_1, 13 7.45, p = 0.02), BMI (F_1, 13 23.5, p < 0.001), and gender (F_1, 13 23.75, p < 0.001), but not age (F_1, 13 1.12, p = 0.35) was observed. Regional [¹¹C]EKAP V_T and BSMSS were found to be negatively correlated in the amygdala (r = -0.69, p < 0.01), anterior cingulate cortex (r = -0.56, p = 0.02), caudate (r = -0.66, p < 0.01), frontal cortex (r = -0.52, p = 0.04), hippocampus (r = -0.60, p = 0.01), pallidum (r = -0.59, p = 0.02), putamen (r = -0.62, p = 0.01), and ventral striatum (r = -0.66, p < 0.01). In secondary (non-reward) regions, correlations of [¹¹C]EKAP V_T and BSMSS were nonsignificant with the exception of the insula. There was an inverse correlation between social status and KOR levels that was largely specific to the reward/aversion (e.g., saliency) areas of the brain. This finding suggests the KOR system may act as a mediator for the negative effects of social behaviors in humans.

Gi/o-Protein Coupled Receptors in the Aging Brain

Cells translate extracellular signals to regulate processes such as differentiation, metabolism and proliferation, via transmembranar receptors. G protein-coupled receptors (GPCRs) belong to the largest family of transmembrane receptors, with over 800 members in the human species. Given the variety of key physiological functions regulated by GPCRs, these are main targets of existing drugs. During normal aging, alterations in the expression and activity of GPCRs have been observed. The central nervous system (CNS) is particularly affected by these alterations, which results in decreased brain functions, impaired neuroregeneration, and increased vulnerability to neuropathologies, such as Alzheimer's and Parkinson diseases. GPCRs signal via heterotrimeric G proteins, such as G_o, the most abundant heterotrimeric G protein in CNS. We here review age-induced effects of GPCR signaling via the G_i/o subfamily at the CNS. During the aging process, a reduction in protein density is observed for almost half of the G_i/o-coupled GPCRs, particularly in age-vulnerable regions such as the frontal cortex, hippocampus, substantia nigra and striatum. G_i/o levels also tend to decrease with aging, particularly in regions such as the frontal cortex. Alterations in the expression and activity of GPCRs and coupled G proteins result from altered proteostasis, peroxidation of membranar lipids and age-associated neuronal degeneration and death, and have impact on aging hallmarks and age-related neuropathologies. Further, due to oligomerization of GPCRs at the membrane and their cooperative signaling, down-regulation of a specific G_i/o-coupled GPCR may affect signaling and drug targeting of other types/subtypes of GPCRs with which it dimerizes. G_i/o-coupled GPCRs receptorsomes are thus the focus of more effective therapeutic drugs aiming to prevent or revert the decline in brain functions and increased risk of neuropathologies at advanced ages.

Reduced serotonin receptors and transporters in normal aging adults: a meta-analysis of PET and SPECT imaging studies

Alterations in serotonin (5-HT) function have been hypothesized to underlie a range of physiological, emotional, and cognitive changes in older age. Here, we conducted a quantitative synthesis and comparison of the effects of age on 5-HT receptors and transporters from cross-sectional positron emission tomography and single-photon emission computed tomography imaging studies. Random-effects meta-analyses of 31 studies including 1087 healthy adults yielded large negative effects of age in 5-HT-2A receptors (largest in global cortex), moderate negative effects of age in 5-HT transporters (largest in thalamus), and small negative effects of age in 5-HT-1A receptors (largest in parietal cortex). Presynaptic 5-HT-1A autoreceptors in raphe/midbrain, however, were preserved across adulthood. Adult age differences were significantly larger in 5-HT-2A receptors compared with 5-HT-1A receptors. A meta-regression showed that 5-HT target, radionuclide, and publication year significantly moderated the age effects. The findings overall identify reduced serotonergic signal transmission in healthy aging. The evidence for the relative preservation of 5-HT-1A compared with 5-HT-2A receptors may partially explain psychological age differences, such as why older adults use more emotion-focused rather than problem-focused coping strategies.

Age-Related Change in 5-HT6 Receptor Availability in Healthy Male Volunteers Measured with 11C-GSK215083 PET

Serotonin receptor 6 (5-hydroxytrypamine-6, or 5-HT₆) is a potential therapeutic target given its distribution in brain regions that are important in depression, anxiety, and cognition. This study sought to investigate the effects of age on 5-HT₆ receptor availability using ¹¹C-GSK215083, a PET ligand with affinity for 5-HT₆ in the striatum and 5-HT_2A in the cortex. Methods: Twenty-eight healthy male volunteers (age range, 23-52 y) were scanned with ¹¹C-GSK215083 PET. Time-activity curves in regions of interest were fitted using a multilinear analysis method. Nondisplaceable binding potential (BP_ND) was calculated using the cerebellum as the reference region and corrected for partial-volume effects. Results: In 5-HT₆-rich areas, regional ¹¹C-GSK215083 showed a negative correlation between BP_ND and age in the caudate (r = -0.41, P = 0.03) (14% change per decade) and putamen (r = -0.30, P = 0.04) (11% change per decade) but not in the ventral striatum or pallidum. A negative correlation with age was also seen in cortical regions (r = -0.41, P = 0.03) (7% change per decade), consistent with the literature on 5-HT_2A availability. Conclusion: To our knowledge, this was the first in vivo study on humans to examine the effect of age on 5-HT₆ receptor availability. The study demonstrated a significant age-related decline in 5-HT₆ availability (BP_ND) in the caudate and putamen.

Distribution and levels of 5-HT1B receptors in anterior cingulate cortex of patients with bipolar disorder, major depressive disorder and schizophrenia - An autoradiography study

The serotonin 1B receptor has recently received more interest as a possible new target for pharmacological treatment of psychiatric disorders. However, the exact mechanisms of action remain unclear. This study aimed to examine the binding distribution and levels of the serotonin 1B receptor in-depth in the anterior cingulate cortex (ACC) and provide more insight in its functional role in bipolar disorder (BD), major depressive disorder (MDD) and schizophrenia (SZ). Serotonin 1B receptor binding distribution was visualized with high resolution autoradiography (ARG), using the radioligand [³H]AZ10419369, in postmortem ACC tissue from patients diagnosed with BD (n=14), MDD (n=12), SZ (n=13) and healthy subjects (n=13). Moreover, a quantification of receptor binding was made with ARG, in relation to patient group, age and gender. In all subject groups a significantly higher specific binding of serotonin 1B receptor was measured in the outer ACC layers compared to the inner ACC layers. Correlation analysis with ARG binding patterns of several radioligands resulted in a significant correlation with glutamatergic N-methyl-D-aspartate receptor binding in the outer layers. No significant difference was found between subject groups in binding levels and distribution. In female subjects a significantly lower receptor binding was found than in male subjects, which was most profound in patients diagnosed with MDD. The binding distribution of the serotonin 1B receptor found in this study supports a role in glutamate transmission in the ACC and was not shown to be significantly altered in BD, MDD or SZ. A gender difference in serotonin 1B receptor binding was found.

Altered behavioral and neural responsiveness to counterfactual gains in the elderly

Counterfactual information processing refers to the consideration of events that did not occur in comparison to those actually experienced, in order to determine optimal actions, and can be formulated as computational learning signals, referred to as fictive prediction errors. Decision making and the neural circuitry for counterfactual processing are altered in healthy elderly adults. This experiment investigated age differences in neural systems for decision making with knowledge of counterfactual outcomes. Two groups of healthy adult participants, young (N = 30; ages 19-30 years) and elderly (N = 19; ages 65-80 years), were scanned with fMRI during 240 trials of a strategic sequential investment task in which a particular strategy of differentially weighting counterfactual gains and losses during valuation is associated with more optimal performance. Elderly participants earned significantly less than young adults, differently weighted counterfactual consequences and exploited task knowledge, and exhibited altered activity in a fronto-striatal circuit while making choices, compared to young adults. The degree to which task knowledge was exploited was positively correlated with modulation of neural activity by expected value in the vmPFC for young adults, but not in the elderly. These findings demonstrate that elderly participants' poor task performance may be related to different counterfactual processing.

Low 5-HT1B receptor binding in the migraine brain: A PET study

Background The pathophysiology of migraine may involve dysfunction of serotonergic signaling. In particular, the 5-HT_1B receptor is considered a key player due to the efficacy of 5-HT_1B receptor agonists for treatment of migraine attacks. Aim To examine the cerebral 5-HT_1B receptor binding in interictal migraine patients without aura compared to controls. Methods Eighteen migraine patients, who had been migraine free for >48 hours, and 16 controls were scanned after injection of the 5-HT_1B receptor specific radioligand [¹¹C]AZ10419369 for quantification of cerebral 5-HT_1B receptor binding. Patients who reported migraine <48 hours after the PET examination were excluded from the final analysis. We defined seven brain regions involved in pain modulation as regions of interest and applied a latent variable model (LVM) to assess the group effect on binding across these regions. Results Our data support a model wherein group status predicts the latent variable ( p = 0.038), with migraine patients having lower 5-HT_1B receptor binding across regions compared to controls. Further, in a whole-brain voxel-based analysis, time since last migraine attack correlated positively with 5-HT_1B receptor binding in the dorsal raphe and in the midbrain. Conclusion We report here for the first time that migraine patients have low 5-HT_1B receptor binding in pain modulating regions, reflecting decreased receptor density. This is either a primary constitutive trait of the migraine brain or secondary to repeated exposure to migraine attacks. We also provide indirect support for the dorsal raphe 5-HT_1B receptors being temporarily downregulated during the migraine attack, presumably in response to higher cerebral serotonin levels in the ictal phase.

The serotonergic system and cognitive function

Symptoms of cognitive dysfunction like memory loss, poor concentration, impaired learning and executive functions are characteristic features of both schizophrenia and Alzheimer’s disease (AD). The neurobiological mechanisms underlying cognition in healthy subjects and neuropsychiatric patients are not completely understood. Studies have focused on serotonin (5-hydroxytryptamine, 5-HT) as one of the possible cognitionrelated biomarkers. The aim of this review is to provide a summary of the current literature on the role of the serotonergic (5-HTergic) system in cognitive function, particularly in AD and schizophrenia.

The role of the 5-HTergic system in cognition is modulated by the activity and function of 5-HT receptors (5-HTR) classified into seven groups, which differ in structure, action, and localization. Many 5-HTR are located in the regions linked to various cognitive processes. Preclinical studies using animal models of learning and memory, as well as clinical in vivo (neuroimaging) and in vitro (post-mortem) studies in humans have shown that alterations in 5-HTR activity influence cognitive performance.

The current evidence implies that reduced 5-HT neurotransmission negatively influences cognitive functions and that normalization of 5-HT activity may have beneficial effects, suggesting that 5-HT and 5-HTR represent important pharmacological targets for cognition enhancement and restoration of impaired cognitive performance in neuropsychiatric disorders.

Age-related changes in binding of the D2/3 receptor radioligand [(11)C](+)PHNO in healthy volunteers

OBJECTIVE

Previous imaging studies with positron emission tomography (PET) have reliably demonstrated an age-associated decline in the dopamine system. Most of these studies have focused on the densities of dopamine receptor subtypes D2/3R (D2R family) in the striatum using antagonist radiotracers that are largely nonselective for D2R vs. D3R subtypes. Therefore, less is known about any possible age effects in D3-rich extrastriatal areas such as the substantia nigra/ventral tegmental area (SN/VTA) and hypothalamus. This study sought to investigate whether the receptor availability measured with [(11)C](+)PHNO, a D3R-preferring agonist radiotracer, also declines with age.

METHODS

Forty-two healthy control subjects (9 females, 33 males; age range 19-55 years) were scanned with [(11)C](+)PHNO using a High Resolution Research Tomograph (HRRT). Parametric images were computed using the simplified reference tissue model (SRTM2) with cerebellum as the reference region. Binding potentials (BPND) were calculated for the amygdala, caudate, hypothalamus, pallidum, putamen, SN/VTA, thalamus, and ventral striatum and then confirmed at the voxel level with whole-brain parametric images.

RESULTS

Regional [(11)C](+)PHNO BPND displayed a negative correlation between receptor availability and age in the caudate (r=-0.56, corrected p=0.0008) and putamen (r=-0.45, corrected p=0.02) in healthy subjects (respectively 8% and 5% lower per decade). No significant correlations with age were found between age and other regions (including the hypothalamus and SN/VTA). Secondary whole-brain voxel-wise analysis confirmed these ROI findings of negative associations and further identified a positive correlation in midbrain (SN/VTA) regions.

CONCLUSION

In accordance with previous studies, the striatum (an area rich in D2R) is associated with age-related declines of the dopamine system. We did not initially find evidence of changes with age in the SN/VTA and hypothalamus, areas previously found to have a predominantly D3R signal as measured with [(11)C](+)PHNO. A secondary analysis did find a significant positive correlation in midbrain (SN/VTA) regions, indicating that there may be differential effects of aging, whereby D2R receptor availability decreases with age while D3R availability stays unchanged or is increased.

Reductions in brain 5-HT1B receptor availability in primarily cocaine-dependent humans

BACKGROUND

Preclinical evidence implicates the serotonin receptor 5-hydroxytryptamine 1B (5-HT1B) in the effects of cocaine. This study explores 5-HT1B in humans by examining receptor availability in vivo in subjects whose primary addiction is cocaine dependence (CD) using positron emission tomography.

METHODS

Study participants included 14 medically healthy subjects with CD (mean age = 41 ± 6 years) who were compared with 14 age-matched healthy control subjects (mean age = 41 ± 8 years) with no past or current history of cocaine or other illicit substance abuse. Participants underwent magnetic resonance imaging followed by positron emission tomography with the highly selective 5-HT1B tracer, [(11)C]P943, for purposes of quantifying regional binding potential. Voxel-based morphometry and gray matter masking also were employed to control for potential partial volume effects.

RESULTS

The [(11)C]P943 positron emission tomography imaging data in nine candidate regions (amygdala, anterior cingulate cortex, caudate, frontal cortex, hypothalamus, pallidum, putamen, thalamus, and ventral striatum) showed significant or nearly significant reductions of regional binding potential in subjects with CD in three regions: anterior cingulate (-16%, p < .01), hypothalamus (-16%, p = .03), and frontal cortex (-7%, p = .08). Voxel-based morphometry showed significant gray matter reductions in the frontal cortex of subjects with CD. After gray matter masking, statistically significant reductions in the [(11)C]P943 regional binding potential were either retained (anterior cingulate, -14%, p = .01; hypothalamus, -20%, p < .01) or achieved (frontal cortex, -14%, p < .01). Whole-brain voxel-wise parameter estimation confirmed these results. Secondary analyses were also significant in some regions for years of cocaine and daily tobacco use.

CONCLUSIONS

The reductions found in this study suggest that 5-HT1B receptors may contribute to the etiology or expression of CD and potentially represent a target for medication development.

Distinct regional age effects on [11C]AZ10419369 binding to 5-HT1B receptors in the human brain

PURPOSE

Age-related changes in the serotonin system have been described, and proposed to be associated with behavioral changes observed particularly in the elderly population. The 5-HT1B receptor is thought to have a regulatory role in a number of physiological functions, and has been implicated in several age-related diseases. The purpose of the present study was to examine if the availability of 5-HT1B receptors is decreasing with age in healthy subjects.

METHODS

Data from five previous studies were reanalyzed and pooled, generating data from fifty-one healthy subjects, age 20 to 70, that had been examined with positron emission tomography (PET) and the 5-HT1B specific radioligand [11C]AZ10419369 at baseline conditions. The binding potential (BPND) in cortical and subcortical areas was calculated using the simplified reference tissue model (SRTM). After correction for partial volume effects (PVEc), the correlation between age and regional BPND was examined.

RESULTS

A statistically significant negative correlation between age and BPND was obtained for neocortical regions and the ventral striatum (VST). The average reduction in BPND per decade was 8% in cortex and 4% in VST. The BPND in the caudate nucleus and the putamen was mainly unaffected by age.

CONCLUSION

The 5-HT1B receptor availability decreases by age in cortical regions, whereas it remains stable in the caudate nucleus and putamen. By consequence, age-matching of control subjects will be necessary in future clinical studies.

Transitions in the transcriptome of the serotonergic and dopaminergic systems in the human brain during adolescence

Adolescence is a period of profound neurophysiological, behavioral, cognitive and psychological changes, but not much is known about the underlying molecular neural mechanisms. The aim of this study was to systematically analyze expression levels of the genes forming serotonergic and dopaminergic synapses during adolescence. We analyzed the mRNA expression profiles of genes that code for all components of serotonergic and dopaminergic synapses, in 16 brain areas from human and non-human primates from public domain databases, to detect genes whose expression changes during adolescence. Two serotonin receptors, HTR1E and HTR1B had expression levels that exhibit a sharp transition in the prefrontal cortex in adolescence, but we found no similar transition in the dopaminergic system. A similar but smoother rise in expression levels is observed in HTR4 and HTR5A, and in HTR1E and HTR1B in three other expression datasets published. An earlier rise is observed in HTR1A, and a smooth and significant rise with age is observed in the expression of HTR1E in microarray measurements in macaque monkeys. The expression of HTR1E and HTR1B is correlated across subjects within each age group, suggesting that they are controlled by common mechanisms. These results point to HTR1E and HTR1B as major candidate genes involved in adolescence maturation processes, and to their operation through common control mechanisms. The maturation profiles may also involve several other 5-HT receptors, including the genes HTR5A, HTR4 and HTR1A.