Comparison of high and low affinity serotonin 1A receptors by PET in vivo in nonhuman primates

Ventral prefrontal serotonin 1A receptor binding: a neural marker of vulnerability for mood disorder and suicidal behavior?

Mood disorders and suicidal behavior have moderate heritability and are associated with altered corticolimbic serotonin 1A receptor (5-HT1A) brain binding. However, it is unclear whether this reflects genetic effects or epigenetic effects of childhood adversity, compensatory mechanisms, or illness stress-related changes. We sought to separate such effects on 5-HT1A binding by examining high familial risk individuals (HR) who have passed through the age of greatest risk for psychopathology onset with and without developing mood disorder or suicidal behavior. PET imaging quantified 5-HT1A binding potential BPND using [11C]CUMI-101 in healthy volunteers (HV, N = 23) and three groups with one or more relatives manifesting early-onset mood disorder and suicide attempt: 1. unaffected HR (N = 23); 2. HR with lifetime mood disorder and no suicide attempt (HR-MOOD, N = 26); and 3. HR-MOOD with previous suicide attempt (HR-MOOD + SA, N = 20). Findings were tested in an independent cohort not selected for family history (HV, MOOD, and MOOD + SA, total N = 185). We tested for regional BPND differences and whether brain-wide patterns distinguished between groups. Low ventral prefrontal 5-HT1A BPND was associated with lifetime mood disorder diagnosis and suicide attempt, but only in subjects with a family history of mood disorder and suicide attempt. Brain-wide 5-HT1A BPND patterns including low ventral prefrontal and mesiotemporal cortical binding distinguished HR-MOOD + SA from HV. A biological endophenotype associated with resilience was not observed. Low ventral prefrontal 5-HT1A BPND may reflect familial mood disorder and suicide-related pathology. Further studies are needed to determine if higher ventral prefrontal 5-HT1A BPND confers resilience, reducing risk of suicidal behavior in the context of familial risk, and thereby offer a potential prevention target.

Is There a Role for GPCR Agonist Radiotracers in PET Neuroimaging?

Positron emission tomography (PET) is a molecular imaging modality that enables in vivo exploration of metabolic processes and especially the pharmacology of neuroreceptors. G protein-coupled receptors (GPCRs) play an important role in numerous pathophysiologic disorders of the central nervous system. Thus, they are targets of choice in PET imaging to bring proof concept of change in density in pathological conditions or in pharmacological challenge. At present, most radiotracers are antagonist ligands. In vitro data suggest that properties differ between GPCR agonists and antagonists: antagonists bind to receptors with a single affinity, whereas agonists are characterized by two different affinities: high affinity for receptors that undergo functional coupling to G-proteins, and low affinity for those that are not coupled. In this context, agonist radiotracers may be useful tools to give functional images of GPCRs in the brain, with high sensitivity to neurotransmitter release. Here, we review all existing PET radiotracers used from animals to humans and their role for understanding the ligand-receptor paradigm of GPCR in comparison with corresponding antagonist radiotracers.

Hunting for the high-affinity state of G-protein-coupled receptors with agonist tracers: Theoretical and practical considerations for positron emission tomography imaging

The concept of the high‐affinity state postulates that a certain subset of G‐protein‐coupled receptors is primarily responsible for receptor signaling in the living brain. Assessing the abundance of this subset is thus potentially highly relevant for studies concerning the responses of neurotransmission to pharmacological or physiological stimuli and the dysregulation of neurotransmission in neurological or psychiatric disorders. The high‐affinity state is preferentially recognized by agonists in vitro. For this reason, agonist tracers have been developed as tools for the noninvasive imaging of the high‐affinity state with positron emission tomography (PET). This review provides an overview of agonist tracers that have been developed for PET imaging of the brain, and the experimental paradigms that have been developed for the estimation of the relative abundance of receptors configured in the high‐affinity state. Agonist tracers appear to be more sensitive to endogenous neurotransmitter challenge than antagonists, as was originally expected. However, other expectations regarding agonist tracers have not been fulfilled. Potential reasons for difficulties in detecting the high‐affinity state in vivo are discussed.

Oxytocin and Serotonin Brain Mechanisms in the Nonhuman Primate

Oxytocin (OT) is increasingly studied for its therapeutic potential in psychiatric disorders, which are associated with the deregulation of several neurotransmission systems. Studies in rodents demonstrated that the interaction between OT and serotonin (5-HT) is critical for several aspects of social behavior. Using PET scan in humans, we have recently found that 5-HT 1A receptor (5-HT_1AR) function is modified after intranasal oxytocin intake. However, the underlying mechanism between OT and 5-HT remains unclear. To understand this interaction, we tested 3 male macaque monkeys using both [¹¹C]DASB and [¹⁸F]MPPF, two PET radiotracers, marking the serotonin transporter and the 5-HT_1AR, respectively. Oxytocin (1 IU in 20 μl of ACSF) or placebo was injected into the brain lateral ventricle 45 min before scans. Additionally, we performed postmortem autoradiography. Compared with placebo, OT significantly reduced [¹¹C]DASB binding potential in right amygdala, insula, and hippocampus, whereas [¹⁸F]MPPF binding potential increased in right amygdala and insula. Autoradiography revealed that [¹¹C]DASB was sensitive to physiological levels of 5-HT modification, and that OT does not act directly on the 5-HT_1AR. Our results show that oxytocin administration in nonhuman primates influences serotoninergic neurotransmission via at least two ways: (1) by provoking a release of serotonin in key limbic regions; and (2) by increasing the availability of 5-HT_1AR receptors in the same limbic areas. Because these two molecules are important for social behavior, our study sheds light on the specific nature of their interaction, therefore helping to develop new mechanisms-based therapies for psychiatric disorders.SIGNIFICANCE STATEMENT Social behavior is largely controlled by brain neuromodulators, such as oxytocin and serotonin. While these are currently targeted in the context of psychiatric disorders such as autism and schizophrenia, a new promising pharmaceutical strategy is to study the interaction between these systems. Here we depict the interplay between oxytocin and serotonin in the nonhuman primate brain. We found that oxytocin provokes the release of serotonin, which in turn impacts on the serotonin 1A receptor system, by modulating its availability. This happens in several key brain regions for social behavior, such as the amygdala and insula. This novel finding can open ways to advance treatments where drugs are combined to influence several neurotransmission networks.

Lack of association between the serotonin transporter and serotonin 1A receptor: an in vivo PET imaging study in healthy adults

The serotonin neurotransmitter system is modulated in part by the uptake of synaptically released serotonin (5-HT) by the serotonin transporter (5-HTT), and by specific serotonin autoreceptors such as the somatodendritic 5-HT1A receptor, which can limit serotonin neuron depolarization. However, little is known about how 5-HTT and 5-HT1A are related in vivo. To study this question, we reanalyzed positron emission tomography (PET) data obtained earlier in 40 healthy participants (21 females) using [(11)C]WAY-100635 for quantification of 5-HT1A binding and [(11)C](+)-McN-5652 for quantification of 5-HTT binding. We hypothesized negative correlations between 5-HT1A binding in the raphe nuclei (RN) and 5-HTT binding in RN terminal field regions. Controlling for sex, no significant correlations were found (all p>0.05). Similarly, an exploratory analysis correlating whole-brain voxel-wise 5-HTT binding with 5-HT1A binding in RN identified no significant clusters meeting our a priori statistical threshold. The lack of correlation between 5-HT1A and 5-HTT binding observed in the current study may be due to the different temporal responsiveness of regulatory processes controlling the somatodendritic 5-HT1A receptor and 5-HTT in response to changing availability of intrasynaptic serotonin.

Marmoset Serotonin 5-HT1A Receptor Mapping with a Biased Agonist PET Probe 18F-F13714: Comparison with an Antagonist Tracer 18F-MPPF in Awake and Anesthetized States

BACKGROUND

In vivo mapping by positron emission tomography of the serotonin 1A receptors has been hindered by the lack of suitable agonist positron emission tomography probes. ¹⁸F-labeled F13714 is a recently developed biased agonist positron emission tomography probe that preferentially targets subpopulations of serotonin 1A receptors in their "active state," but its brain labeling pattern in nonhuman primate has not been described. In addition, a potential confound in the translatability of PET data between nonhuman animal and human arise from the use of anesthetics that may modify the binding profiles of target receptors.

METHODS

Positron emission tomography scans were conducted in a cohort of common marmosets (n=4) using the serotonin 1A receptor biased agonist radiotracer, ¹⁸F-F13714, compared with a well-characterized ¹⁸F-labeled antagonist radiotracer, ¹⁸F-MPPF. Experiments on each animal were performed under both consciousness and isoflurane-anesthesia conditions.

RESULTS

¹⁸F-F13714 binding distribution in marmosets by positron emission tomography differs markedly from that of the ¹⁸F-MPPF. Whereas ¹⁸F-MPPF showed highest binding in hippocampus and amygdala, ¹⁸F-F13714 showed highest labeling in other regions, including insular and cingulate cortex, thalamus, raphe, caudate nucleus, and putamen. The binding potential values of ¹⁸F-F13714 were about one-third of those observed with ¹⁸F-MPPF, with marked individual- and region-specific differences under isoflurane-anesthetized vs conscious conditions.

CONCLUSIONS

These findings highlight the importance of investigating the brain imaging of serotonin 1A receptors using agonist probes such as ¹⁸F-F13714, which may preferentially target subpopulations of serotonin 1A receptors in specific brain regions of nonhuman primate as a biased agonist.

Autoradiographic Evaluation of [(18)F]FECUMI-101, a High Affinity 5-HT1AR Ligand in Human Brain

[(18)F]FECUMI-101 ([(18)F]1) is a 5HT1AR ligand demonstrating specific binding in brain regions corresponding to the distribution of 5-HT1AR in baboons. However, we detected moderate uptake of [(18)F]1 in baboon thalamus, a brain region lacking 5-HT1AR. We sought to investigate the relative binding of [(18)F]1 to 5-HT1AR, α1R, and 5-HT7R in vitro. Using autoradiography in human brain sections, specific binding of [(18)F]1 to 5-HT1AR was confirmed. However, [(18)F]1 also showed 26% binding to α1R in PFC. The hippocampal formation exhibited 51% and 92% binding of [(18)F]1 to α1R and 5-HT1AR, respectively. Thalamus and cerebellum showed very little binding. There is no measurable specific binding of [(18)F]1 to 5-HT7R and no effect of temperature on [(18)F]1 specific binding to 5-HT1AR or α1R. These results indicate that, while [(18)F]FECUMI-101 is not a completely selective 5-HT1AR ligand for receptor quantification, it may be useful for occupancy measurements of drugs acting at 5-HT1AR in vivo.

PET tracers for serotonin receptors and their applications

Serotonin receptors (5-HTRs) are implicated in the pathophysiology of a variety of neuropsychiatric and neurodegenerative disorders and are also targets for drug therapy. In the CNS, most of these receptors are expressed in high abundance in specific brain regions reflecting their role in brain functions. Quantifying binding to 5-HTRs in vivo may permit assessment of physiologic and pathologic conditions, and monitoring disease progression, evaluating treatment response, and for investigating new treatment modalities. Positron emission tomography (PET) molecular imaging has the sensitivity to quantify binding of 5-HTRs in CNS disorders and to measure drug occupancy as part of a process of new drug development. Although research on PET imaging of 5-HTRs have been performed more than two decades, the successful radiotracers so far developed for human studies are limited to 5-HT₁AR, 5-HT₁BR, 5-HT₂AR, 5-HT₄R and 5-HT₆R. Herein we review the development and application of radioligands for PET imaging of 5-HTRs in living brain.

Positron emission tomography quantification of serotonin(1A) receptor binding in suicide attempters with major depressive disorder

IMPORTANCE

Serotonergic system dysfunction has been associated with increased lethal suicide attempts and suicide. Dysfunction includes higher binding of serotonin(1A) autoreceptor in the brainstem raphe of individuals who die by suicide.

OBJECTIVES

To determine the relationships between brain serotonin(1A) binding and suicidal behavior in vivo in major depressive disorder (MDD) using positron emission tomography and the serotonin(1A) antagonist radiotracer carbon C 11 [11C]-labeled WAY-100635.

DESIGN, SETTING, AND PARTICIPANTS

Cross-sectional positron emission tomography study at an academic medical center from 1999 through 2009. We compared serotonin(1A) binding between individuals with MDD who did not attempt suicide (nonattempters) (n = 62) and those who attempted suicide (attempters) (n = 29). We subdivided the attempters into those with lower (n = 16) and higher (n = 13) levels of lethality.

MAIN OUTCOMES AND MEASURES

The binding potential (BPF) of [11C]WAY-100635 (calculated as the number of receptors available divided by affinity) in the prefrontal cortex (PFC) and brainstem, estimated by kinetic modeling with an arterial input function; the severity of suicidal behaviors, including lethality and intent of suicide attempts; and suicidal ideation.

RESULTS

Using a linear mixed-effects model, we found no difference between attempters and nonattempters with MDD in serotonin(1A) BPF in the PFC regions (F1,88 = 0.03; P = .87) or in the raphe nuclei (F1,88 = 0.29; P = .59). Raphe nuclei serotonin(1A) BPF was 45.1% greater in higher-lethality attempters compared with lower-lethality attempters (F1,25 = 7.33; P = .01), whereas no difference was observed in the PFC regions (F1,25 = 0.12; P = .73). Serotonin(1A )BPF in the raphe nuclei of suicide attempters was positively correlated with the lethality rating (F1,25 = 10.56; P = .003) and the subjective lethal intent factor (F1,25 = 10.63; P = .003; R2 = 0.32) based on the most recent suicide attempt. Suicide ideation in participants with MDD was positively correlated with serotonin(1A) BPF in the PFC regions (F1,88 = 5.19; P = .03) and in the raphe nuclei (F1,87 = 7.38; P = .008; R2 = 0.12).

CONCLUSIONS AND RELEVANCE

Higher brainstem raphe serotonin(1A)BPF observed in higher-lethality suicide attempters with MDD is in agreement with findings in suicide studies and also with the finding of low cerebrospinal fluid levels of 5-hydroxyindoleacetic acid in higher-lethality suicide attempters. Higher brainstem raphe serotonin(1A) BPF would be consistent with lower levels of serotonin neuron firing and release and supports a model of impaired serotonin signaling in suicide and higher-lethality suicidal behavior. Severity of suicidal ideation in MDD is related to brainstem and prefrontal serotonin(1A) BPF, suggesting a role for both regions in suicidal ideation. Lower levels of serotonin release at key brain projection sites, such as the prefrontal regions, may favor more severe suicidal ideation and higher-lethality suicide attempts.

Synthesis and evaluation of arylpiperazines derivatives of 3,5-dioxo-(2H,4H)-1,2,4-triazine as 5-HT1AR ligands

5-HT1AR agonist or partial agonists are established drug candidates for psychiatric and neurological disorders. We have reported the synthesis and evaluation of a series of high affinity 5-HT1AR partial agonist PET imaging agents with greater selectivity over α-1AR. The characteristic of these molecules are 3,5-dioxo-(2H,4H)-1,2,4-triazine skeleton tethered to an arylpiperazine unit through an alkyl side chain. The most potent 5-HT1AR agonistic properties were found to be associated with the molecules bearing C-4 alkyl group as the linker. Therefore development of 3,5-dioxo-(2H,4H)-1,2,4-triazine bearing arylpiperazine derivatives may provide high affinity selective 5-HT1AR ligands. Herein we describe the synthesis and evaluation of the binding properties of a series of arylpiperazine analogues of 3,5-dioxo-(2H,4H)-1,2,4-triazine.

Synthesis and in vivo evaluation of [(18)F]2-(4-(4-(2-(2-fluoroethoxy)phenyl)piperazin-1-yl)butyl)-4-methyl-1,2,4-triazine-3,5(2H,4H)-dione ([(18)F]FECUMI-101) as an imaging probe for 5-HT1A receptor agonist in nonhuman primates

The 5-HT1AR partial agonist PET radiotracer, [(11)C]CUMI-101, has advantages over an antagonist radiotracer as it binds preferentially to the high affinity state of the receptor and thereby provides more functionally meaningful information. The major drawback of C-11 tracers is the lack of cyclotron facility in many health care centers thereby limiting widespread clinical or research use. We identified the fluoroethyl derivative, 2-(4-(4-(2-(2-fluoroethoxy)phenyl)piperazin-1-yl)butyl)-4-methyl-1,2,4-triazine-3,5(2H,4H)dione (FECUMI-101) (Ki=0.1nM; Emax=77%; EC50=0.65nM) as a partial agonist 5-HT1AR ligand of the parent ligand CUMI-101. FECUMI-101 is radiolabeled with F-18 by O-fluoroethylation of the corresponding desmethyl analogue (1) with [(18)F]fluoroethyltosylate in DMSO in the presence of 1.6equiv of K2CO3 in 45±5% yield (EOS). PET shows [(18)F]FECUMI-101 binds specifically to 5-HT1AR enriched brain regions of baboon. The specificity of [(18)F]FECUMI-101 binding to 5-HT1AR was confirmed by challenge studies with the known 5-HT1AR ligand WAY100635. These findings indicate that [(18)F]FECUMI-101 can be a viable agonist ligand for the in vivo quantification of high affinity 5-HT1AR with PET.

Autoradiographic evaluation of [3H]CUMI-101, a novel, selective 5-HT1AR ligand in human and baboon brain

[11C]CUMI-101 is the first selective serotonin receptor (5-HT1AR) partial agonist radiotracer for positron emission tomography (PET) tested in vivo in nonhuman primates and humans. We evaluated specific binding of [3H]CUMI-101 by quantitative autoradiography studies in postmortem baboon and human brain sections using the 5-HT1AR antagonist WAY-100635 as a displacer. The regional and laminar distributions of [3H]CUMI-101 binding in baboon and human brain sections matched the known distribution of [3H]8-OH-DPAT and [3H]WAY-100635. Prazosin did not measurably displace [3H]CUMI-101 binding in baboon or human brain sections, thereby ruling out [3H]CUMI-101 binding to α1-adrenergic receptors. This study demonstrates that [11C]CUMI-101 is a selective 5-HT1AR ligand for in vivo and in vitro studies in baboon and human brain.