Age-related changes in muscarinic cholinergic receptors in the living brain: a PET study using N-[C]methyl-4-piperidyl benzilate combined with cerebral blood flow measurement in conscious monkeys

The effects of changes in regional cerebral blood flow (rCBF) with aging on muscarinic cholinergic receptor binding were evaluated with [¹⁵O]H₂O and N-[¹¹C]methyl-4-piperidyl benzilate (4-MPB) in the living brains of young (5.9±1.8 years old) and aged (19.0±3.3 years old) monkeys (Macaca mulatta) in the conscious state using high-resolution positron emission tomography (PET). For quantitative analysis of receptor binding in vivo with [¹¹C]4-MPB, metabolite-corrected arterial plasma radioactivity curves were obtained as an input function into the brain, and graphical Patlak plot analysis was applied. In addition, two-compartment model analysis using the radioactivity curve in the cerebellum as an input function (reference analysis) was also applied to determine the distribution volume (DV=K₁/k₂′) for [¹¹C]4-MPB. With metabolite-corrected arterial input, Patlak plot analysis of [¹¹C]4-MPB indicated a regionally specific decrease in muscarinic cholinergic receptor binding in vivo in the frontal and temporal cortices as well as the striatum in aged compared with young animals, showing no correlation with the degree of reduced rCBF. In contrast, on the reference analysis with cerebellar input of [¹¹C]4-MPB, all regions assayed except the pons showed a significant age-related decrease of DV, and the degree of reduction of DV was correlated with that of rCBF. These results demonstrated the usefulness of kinetic analysis of [¹¹C]4-MPB with metabolite-corrected arterial input, not with reference region’s input, as an indicator of the aging process of cortical muscarinic cholinergic receptors in vivo measured by PET with less blood flow dependency.     

Regional distribution and behavioral correlates of 5-HT2A receptors in Alzheimer's disease with [F]deuteroaltanserin and PET

Postmortem studies show reductions in brain serotonin 2A (5-HT_2A) receptors in Alzheimer's disease (AD). Converging evidence also suggests that serotonergic dysregulation may contribute to behavioral symptoms that frequently occur in AD. This study aimed to define regional reductions in 5-HT_2A binding in AD patients and to examine their behavioral correlates. Nine patients with probable AD and eight elderly controls were studied using a constant infusion paradigm for equilibrium modeling of [¹⁸F]deuteroaltanserin with positron emission tomography (PET). Region of interest analyses were performed on PET images coregistered to MRI scans. The outcome measures BP_P (ratio of specific brain uptake to total plasma parent concentration) and BP_ND (ratio of specific to nondisplaceable uptake) were obtained for pertinent cortical and subcortical regions. AD patients showed a statistically significant decrease in the anterior cingulate in both BP_P and BP_ND, but in no other region. Within the AD patient sample, no significant correlations were observed between regional 5-HT_2A binding and behavioral measures, including depressive and psychotic symptoms. These results confirm a reduction in cortical 5-HT_2A receptors in AD, specifically in the anterior cingulate. However, in a limited AD patient sample, they fail to demonstrate a relationship between regional 5-HT_2A binding and major behavioral symptoms.     

The 5-HT2A receptor and serotonin transporter in Asperger's Disorder: A PET study with [C]MDL 100907 and [C]DASB

Evidence from biochemical, imaging, and treatment studies suggest abnormalities of the serotonin system in autism spectrum disorders, in particular in frontolimbic areas of the brain. We used the radiotracers [¹¹C]MDL 100907 and [¹¹C]DASB to characterize the 5-HT_2A receptor and serotonin transporter in Asperger's Disorder. Seventeen individuals with Asperger's Disorder (age = 34.3 ± 11.1 years) and 17 healthy controls (age = 33.0 ± 9.6 years) were scanned with [¹¹C]MDL 100907. Of the 17 patients, eight (age = 29.7 ± 7.0 years) were also scanned with [¹¹C]DASB, as were eight healthy controls (age = 28.7 ± 7.0 years). Patients with Asperger's Disorder and healthy control subjects were matched for age, gender, and ethnicity, and all had normal intelligence. Metabolite-corrected arterial plasma inputs were collected and data analyzed by two-tissue compartment modeling. The primary outcome measure was regional binding potential BP_ND. Neither regional [¹¹C]MDL 100907 BP_ND nor [¹¹C]DASB BP_ND was statistically different between the Asperger's and healthy subjects. This study failed to find significant alterations in binding parameters of 5-HT_2A receptors and serotonin transporters in adult subjects with Asperger's Disorder.     

Autoradiographic localization of 5-HT1A receptors in the post-mortem human brain using [H]WAY-100635 and [C]WAY-100635

The distribution of 5-HT_1A receptors was examined in the post-mortem human brain using whole hemisphere autoradiography and the selective 5-HT_1A receptor antagonist [³H]WAY-100635 ([O-methyl-³H]-N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridinyl)cyclohexanecarboxamide trihydrochloride). The autoradiograms showed very dense binding to hippocampus, raphe nuclei and neocortex. The labeling in neocortex was slightly lower than in the hippocampus and was mainly at superficial layers, although a faintly labeled band could be seen in deeper neocortical layers. Other regions, such as the amygdala, septum and claustrum, showed low densities of [³H]WAY-100635 binding, reflecting low densities of 5-HT_1A receptors. The labeling was very low in basal ganglia, such as nucleus caudatus and putamen, in cerebellum or in structures of the brain stem except in the raphe nuclei. The labeling of human 5-HT_1A receptors with [³H]WAY-100635 was antagonized by the addition of the 5-HT_1A receptor ligands, 5-HT, buspirone, pindolol or 8-OH-DPAT (10 μM), leaving a very low background of non-specific binding. Saturation analysis of semiquantitative data from several human regions indicated that [³H]WAY-100635 has a K_d of approximately 2.5 nM. The selective labeling of 5-HT_1A receptors with [³H]WAY-100635 clearly show that this compound is useful for further studies of the human 5-HT_1A receptor subtype in vitro. [¹¹C]WAY-100635 is used for the characterization of 5-HT_1A receptors with positron emission tomography (PET). WAY-100635 was also radiolabeled with the short-lived positron-emitting radionuclide carbon-11 (t_1/2=20 min) and used for in vitro autoradiography on human whole hemisphere cryosections. [¹¹C]WAY-100635 gave images qualitatively similar to those of [³H]WAY-100635, although with a lower resolution. Thus, the hippocampal formation was densely labeled, with lower density in the neocortex. Buspirone, pindolol or 8-OH-DPAT (10 μM), blocked all binding of [¹¹C]WAY-100635. The in vitro autoradiography of the distribution of 5-HT_1A receptors obtained with radiolabeled WAY-100635 provide detailed qualitative and quantitative information on the distribution of 5-HT_1A-receptors in the human brain. Moreover, the studies give reference information for the interpretation of previous initial results at much lower resolution in humans with PET and [¹¹C]WAY-100635. These data provide a strong basis for expecting [¹¹C]WAY-100635 to behave as a highly selective radioligand in vivo.     

Autoradiographic mapping of 5-HT1, 5-HT1A, 5-HT1B and 5-HT2 receptors in the rat spinal cord

The distribution of 5-HT₁, 5-HT_1A, 5-HT_1B and 5-HT₂ receptors in the rat spinal cord was investigated with quantitative autoradiography. Receptors were labeled respectively with [³H]serotonin (5-[³H]HT],8-hydroxy-2-[N-dipropylamino-³H]tetralin (8-OH-[³H]DPAT), [¹²⁵I]iodocyanopindolol and [³H]ketanserin. It is shown that 5-HT₁, 5-HT_1A and 5-HT_1B receptors are distributed within the spinal cord according to a rostro-caudal gradient. Both 5-HT₁ and 5-HT_1A receptors are mainly present in the dorsal horn and 5-HT_1B is present throughout the spinal cord, exhibiting high densities in the caudal-most part of the dorsal in lamina X and in the sacral parasympathetic area. On the other hand, 5-HT₂ receptors are shown mostly in the thoracic sympathetic area and in the thoracic ventral horn; the dorsal horn exhibits few 5-HT₂ receptors. The differential involvement of 5-Ht through different receptors in nociception, autonomous nervous system control and motility are discussed.     

Differential effects of stress on [11C]raclopride and [11C]MNPA binding to striatal D2/D3 dopamine receptors: A PET study in conscious monkeys

It has been reported that stress and facilitation of dopamine neuronal system are closely related. In the present study, the effects of stress on the binding of antagonist‐based [¹¹C]raclopride and agonist‐based (R)‐2‐CH3O‐N‐n‐ propylnorapomorphine ([¹¹C]MNPA) to D₂/D₃ receptors were evaluated in the striatum of conscious monkey brain. The stress state assessed from plasma cortisol level was negatively correlated with [¹¹C]raclopride binding as expected. It was noteworthy that [¹¹C]MNPA binding exhibited a positive correlation with stress state; thus, the animals with higher cortisol levels showed higher binding to D₂/D₃ receptors. Synapse 65:84–89, 2011. © 2010 Wiley‐Liss, Inc.     

Autoradiographic distribution of α2 adrenoceptors, NAIBS, and 5-HT1A receptors in human brain using [H]idazoxan and [H]rauwolscine

The regional distribution of [³H]idazoxan and [³H]rauwolscine was studied autoradiographically in human brain. [³H]Idazoxan binds with high affinity to α₂ adrenoceptors as well as to non-adrenergic sites (NAIBS). [³H]Rauwolscine, besides binding to α₂ adrenoceptors, also binds to 5-HT_1A receptors. Both radioligands labelled the same population of α₂ adrenoceptors, defined as the epinephrine-displaceable binding component. The highest densities of α₂ adrenoceptors occur in the leptomeninges, cerebral cortex and claustrum; lower densities were visualised in the basal ganglia, thalamus, pons, substantia nigra, cerebellum and medulla oblongata; no α₂ adrenoceptors were detected in amygdala and nucleus ruber. NAIBS were present in all the examined brain areas, with the highest densities found in the basal ganglia and substantia nigra. The finding that certain brain regions, such as the amygdala, contained NAIBS but no detectable α₂ adrenoceptors, suggests that the binding sites are independent from each other. The regional distribution of 5-HT_1A receptors labelled by [³H]rauwolscine is in agreement with previous studies using [³H]8-OH-DPAT.     

Effect of 6R- -erythro-5,6,7,8-tetrahydrobiopterin and infusion of -tyrosine on the in vivo l-[β-C]DOPA disposition in the monkey brain

The effect of 6R- -erythro-5,6,7,8-tetrahydrobiopterin (6R-BH₄) and -tyrosine infusion on [¹¹C]dopamine synthesis was analyzed in the striatum of Rhesus monkey using positron emission tomography (PET). The rate for decarboxylation from -[β-¹¹C]DOPA to [¹¹C]dopamine was calculated using a graphical method with cerebellum as a reference region. Although the peripheral administration of 6R-BH₄ at low dose (2 mg/kg) did not provide a significant increase in the rate of dopamine biosynthesis, a high dose of 6R-BH₄ (20 mg/kg) induced an elevation of the rate. This 6R-BH₄-induced elevation of the dopamine synthesis rate was further dose-dependently enhanced by the continuous infusion of -tyrosine (0.2 and 1.0 μmol/min/kg). -Tyrosine infusion with a rate of 1.0 μmol/min/kg caused an enhancement of the rate even during low dose administration of 6R-BH₄ (2 mg/kg). -Tyrosine infusion alone did not induce any elevation of the dopamine biosynthesis rate. The analysis of plasma indicated that the metabolic ratios of -[β-¹¹C]DOPA to each metabolite were not affected by 6R-BH₄ and/or -tyrosine infusion. The results suggest that the low dose loading of tyrosine facilitates the activity of 6R-BH₄ on the presynaptic dopamine biosynthesis, and also that the combined effects can be monitored by PET using -[β-¹¹C]DOPA as a biochemical probe.     

Ketamine increases the striatal N-[C]methylspiperone binding in vivo: positron emission tomography study using conscious rhesus monkey

A system for positron emission tomography study of conscious monkeys was newly developed. By use of this system in combination with a microdialysis technique, the effect of ketamine on the binding and release of dopamine was investigated. The administration of ketamine (5 mg/kg) caused sedation accompanied by psychotic symptoms such as nystagmus and stereotyped movements of extremities. During this psychotomimetic period produced by ketamine, a significant increase in the accumulation of the dopamine D₂ receptor ligand N-[¹¹C]methylspiperone was observed in the striatum compared with the level in the conscious state, while no significant change was observed in the frontal cortex and cerebellum. In contrast to the use of ketamine as the anesthetic, pentobarbital (25 mg/kg), which produced deeper anesthesia but no psychotic symptoms, caused a decrease in the accumulation of N-[¹¹C]methylspiperone in the striatum. Kinetic analysis, conducted by a graphical method, revealed that the value of the association constant (K3) for N-[¹¹C]methylspiperone binding in the striatum was increased to approximately 130% by ketamine and decreased to approximately 70% by pentobarbital compared with the control values. Furthermore, the release of dopamine from the striatum measured by microdialysis was not affected by ketamine anesthesia. These results indicate that ketamine facilitates striatal dopaminergic neurotransmission through increasing the binding activity of dopamine D₂ receptors in the striatum, and suggest that these changes may be related to the psychotomimetic behavioral symptoms of this drug.     

A PET study of 5-HT1A receptors at different phases of the menstrual cycle in women with premenstrual dysphoria

The cause of premenstrual dysphoric disorder (PMDD) is largely unknown. It has been hypothesized that normal ovarian function triggers PMDD-related biochemical events within the brain and that serotonin plays an important role. In the present study, positron emission tomography (PET) and [carbonyl-¹¹C]WAY-100635 were used to examine serotonin 5-HT_1A receptors in a control group of women and in a group of women with PMDD. Two PET examinations were performed in each subject, one before (follicular phase) and one after ovulation (luteal phase). Each subject's menstrual cycle was confirmed by ultrasonography of the ovaries as well as with hormone levels in blood and urine. The 5-HT_1A binding potential was measured in six regions of interest and calculated according to the simplified reference tissue model. In the raphe nuclei, the 5-HT_1A binding potential changed from the follicular to the luteal phase of the menstrual cycle in asymptomatic controls. In women with PMDD, the observed change between phases was significantly smaller. The results are in concordance with previously reported challenge studies of 5-HT_1A receptor-mediated effects indicating different serotonergic responses between women with PMDD and controls. The study principally provides new support, in vivo, for a serotonergic dysregulation in women with PMDD.     

Autoradiographic mapping of 5-HT1B and 5-HT1D receptors in the post mortem human brain using [H]GR 125743

The distribution of 5-HT_1B and 5-HT_1D receptors in the human post mortem brain was examined using whole hemisphere autoradiography and the radioligand [³H]GR 125743. [³H]GR 125743 binding was highest in the substantia nigra and the globus pallidus. Lower levels were detected in the striatum, with the highest densities in the ventromedial parts. In the amygdala, the hippocampus, the septal region and the hypothalamus, lower [³H]GR 125743 binding was observed, reflecting low densities of 5-HT_1B/1D receptors. In the cerebral cortex, binding was similar in most regions, although restricted parts of the medial occipital cortex were markedly more densely labeled. Binding densities were very low in the cerebellar cortex and in the thalamus. Two methods were used to distinguish between the two receptor subtypes, the first using ketanserin to block 5-HT_1D receptors and the second using SB 224289 to inhibit 5-HT_1B receptor binding. The autoradiograms indicated that in the human brain, the 5-HT_1B receptor is much more abundant than the 5-HT_1D receptor, which seemed to occur only in low amounts mainly in the ventral pallidum. Although [³H]GR 125743 is a suitable radioligand to examine the distribution of 5-HT_1B receptors in the human brain in vitro, the selectivities of ketanserin and SB 224289 are not sufficiently high to give definite evidence for the occurrence of the 5-HT_1D receptor in the human brain.     

Increase in thalamic binding of [C]PE2I in patients with schizophrenia: A positron emission tomography study of dopamine transporter

Previous in vivo imaging studies reported no difference in dopamine transporter (DAT) bindings in the striatum between control subjects and patients with schizophrenia. However, as the signals of radioligands with moderate affinity were insufficient for allowing the evaluation of small amounts of DAT, DAT binding in extrastriatal regions has not been determined. Positron emission tomography scanning using [¹¹C]PE2I was performed on eight patients with schizophrenia and twelve normal control subjects. Binding potential (BP_ND) for DAT in the caudate, putamen, thalamus and substantia nigra was calculated, using the cerebellum as reference region. In patients with schizophrenia, clinical symptoms were evaluated by Positive and Negative Syndrome Scale (PANSS). BP_ND in the thalamus of patients with schizophrenia was significantly higher than in control subjects (P = 0.044). In patients with schizophrenia, there were significantly positive correlations between BP_ND in the thalamus and total (r = 0.75), positive (r = 0.78) and negative PANSS scores (r = 0.82). Altered DAT in the thalamus might be related to the pathophysiology and clinical symptoms of schizophrenia.     

Serotonin transporter availability in patients with schizophrenia: a positron emission tomography imaging study with [11C]DASB.

BACKGROUND: Postmortem studies have reported several alterations in serotonin transporter (SERT) binding parameters in patients with schizophrenia. The aim of this study was to compare SERT availability in vivo in patients with schizophrenia and matched control subjects. METHODS: Ten medication-free patients with schizophrenia and 10 healthy subjects underwent positron emission tomography (PET) scans for 90 min after 11C-3-amino-4-(2-dimethylaminomethylphenylthio)benzonitrile ([11C]DASB) injection. Metabolite-corrected arterial input function was measured. Regional distribution volumes (mL/g) were derived with a two tissue compartment kinetic model. Outcome measures for SERT availability included binding potential (BP) and the specific-to-nonspecific equilibrium partition coefficient (V3'). Ten brain regions with high density of SERT and where SERT availability can be reliably quantified with [11C]DASB were included in the analysis. RESULTS: No significant differences were observed in regional BP or V3' between patients and control subjects. No significant relationships were observed between regional SERT availability and severity of positive, negative, and depressive symptoms. CONCLUSIONS: This study failed to detect alterations of SERT availability in patients with schizophrenia; however, this study does not rule out the possibility that schizophrenia might be associated with alterations of SERT density in the cortical regions, where the [11C]DASB-specific binding signal is too low for reliable quantification of SERT.     

Measurement of serotonin transporter binding with PET and [11C]MADAM: a test-retest reproducibility study

[(11)C]MADAM, or [(11)C]N,N-dimethyl-2-(2-amino-4-methylphenyl thio)benzylamine, is a radioligand suitable for positron emission tomography (PET) studies of the serotonin transporter (5-HTT) in man. The purpose of this study was to examine the test-retest reproducibility using a design tailored for future applied studies. Nine healthy male subjects were examined with PET and [(11)C]MADAM under baseline conditions at two occasions 4-8 weeks apart. The subjects participated in a Phase 1 trial to which the present study was an addendum. Eight regions of interest were studied, including frontal cortex, hippocampal complex, and the raphe nuclei. All regions, but the raphe nuclei, were defined on MR-images to which the PET-images were coregistered using SPM2. Binding potentials were calculated using the simplified reference tissue model, with cerebellum as reference region. Test-retest data were calculated from the binding potentials, and included binding potential (BP) quotient, BP difference, and the intraclass correlation coefficient. The quotient was about one in all regions, and the mean difference varied between 0 and 11%. The intraclass correlation coefficient varied between 0.96 and 0.51 in the raphe nuclei and averaged bilateral regions. [(11)C]MADAM was shown to have good to excellent reliability in measurements of 5-HTT binding in brain regions of interest in research on psychiatric disorders.     

Age-related assessment of central 5-HT1A receptors following irreversible inactivation by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ)

Age-dependent differences in the ability of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) to irreversibly inactivate 5-HT_1A receptors were investigated in female Fischer 344 rats (ages 3 and 22 months). In the hippocampus, frontal cortex and amygdala, EEDQ reduced 5-HT_1A receptor density (33–70%) and drug affinity (2.3–6.2 fold) as determined by Scatchard analyses using [³H]8-hydroxy-2-(di-N-propylamino)tetralin. In the frontal cortex, the reduction in B_max values was significantly greater in 3 months vs. 22 months groups. These region-specific and age-dependent alterations in 5-HT_1A receptors may be of pathophysiological significance in age-related cognitive decline and Alzheimer's disease.