N. Scott Mason

Identification of extrastriatal dopamine D2 receptors in post mortem human brain with [125I]epidepride

The regional distribution of striatal and extrastriatal dopamine D2 receptors in human brain was studied in vitro with (S)-N-[(1-ethyl-2- pyrrolidinyl)methyl]-5-[125I]iodo-2,3-dimethoxybenzamide, [125I]epidepride, using post mortem brain specimens from six subjects. Scatchard analysis of the saturation equilibrium binding in twenty-three regions of post mortem brain revealed highest levels of binding in the caudate (16.5 pmol/g tissue) and putamen (16.6 pmol/g tissue) with lower levels seen in the globus pallidus (7.0 pmol/g tissue), nucleus accumbens (7.2 pmol/g tissue), hypothalamus (1.8 pmol/g tissue), pituitary (1.3 pmol/g tissue), substantia innominata (1.0 pmol/g tissue), and amygdala (0.87 pmol/g tissue). Of note was the presence of dopamine D2 receptors in the four thalamic nuclei studied, i.e. anterior nucleus (1.0 pmol/g tissue), dorsomedial nucleus (0.96 pmol/g tissue), ventral nuclei (0.72 pmol/g tissue), and pulvinar (0.86 pmol/g tissue), at levels comparable to the amygdala (0.87 pmol/g tissue) and considerably higher than levels seen in anterior cingulate (0.26 pmol/g tissue) or anterior hippocampus (0.36 pmol/g tissue). The frontal cortex had very low levels of dopamine D2 receptors (0.17-0.20 pmol/g tissue) while the inferior and medial temporal cortex had relatively higher levels (0.31-0.46 pmol/g tissue). Inhibition of [125I]epidepride binding by a variety of neurotransmitter ligands to striatal, ventral thalamic and inferior temporal cortical homogenates demonstrated that [125I]epidepride binding was potently inhibited only by dopamine D2 ligands. The present study demonstrates that dopamine D2 receptors are present in basal ganglia, many limbic regions, cortex and in the thalamus. The density of thalamic D2 receptors is comparable to many limbic regions and is considerably higher than in cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

Gender-specific aging effects on the serotonin 1A receptor.

The effects of age on serotonergic function have been hypothesized to underlie age-related changes in mood and behaviors such as sleep and eating. Of particular interest is the serotonin type-1A (5-HT1A) receptor, due to its putative role in mediating the therapeutic efficacy of antidepressant treatment. Using positron emission tomography (PET) and [11C--carbonyl] WAY100635, we assessed 5-HT1A receptor binding in 21 healthy subjects (10 men, 11 women) ranging in age from 21 to 80 years. Regional binding potential values were generated using a reference tissue model and corrected for partial volume effects. We observed an inverse relationship between age and binding of [11C--carbonyl] WAY100635 to the 5-HT1A receptor in men, but not women. This finding is in accord with observations reported in the postmortem literature. Gender-specific effects of age on central serotonergic function may relate to differences between men and women in behavior, mood, and susceptibility to neuropsychiatric disease across the adult lifespan.

Development of Positron Emission Tomography β-Amyloid Plaque Imaging Agents

For 100 years, β-amyloid (Aβ) plaques and neurofibrillary tangles (NFTs) have been recognized as the neuropathological hallmarks of Alzheimers disease (AD), and their presence or absence could only be assessed postmortem using stains and dyes that identified these microscopic structures. Approximately 10 years ago, the first successful Aβ plaque-specific positron emission tomography (PET) imaging study was conducted in a living human subject clinically diagnosed with probable AD using the (11)C-labeled radiopharmaceutical Pittsburgh Compound B (PiB). Laboratory studies and preclinical evaluations to design PiB began a decade earlier than the first human PiB PET study and involved chemical modifications of different well-known dyes that bound specifically to the extended β-pleated sheets that comprise the fibrils of amyloid proteins such as Aβ plaques, NFTs, α-synuclein deposits, and prions. These preclinical studies were conducted in our laboratories at the University of Pittsburgh, starting with Congo red derivatives, followed by Chrysamine G derivatives, followed by X-series compounds, and finally with neutral derivatives of thioflavin-T. The in vitro and in vivo evaluations of the different derivatives as candidate PET radioligands for imaging Aβ plaques and neurofibrillary tangles in human brain are described in this review, along with the specific evaluation criteria by which the candidate radioligands were judged. Out of these studies came PiB, a PET radioligand that binds selectively and with high affinity to only fibrillar forms of Aβ. PiB has been used in many different human research protocols throughout the world and has demonstrated the usefulness of assessing the Aβ plaque status of subjects many years before the clinical diagnosis of probable AD. Recently, longer-lived (18)F-radiolabeled Aβ-selective radiopharmaceuticals have been developed. It is likely that the full clinical impact of these imaging agents will be realized by identifying presymptomatic subjects who would benefit from early drug treatments with future disease-modifying AD therapeutics.

Positron emission tomography imaging of amphetamine‐induced dopamine release in the human cortex: A comparative evaluation of the high affinity dopamine D2/3 radiotracers [11C]FLB 457 and [11C]fallypride

The use of PET and SPECT endogenous competition binding techniques has contributed to the understanding of the role of dopamine in several neuropsychiatric disorders. An important limitation of these imaging studies is the fact that measurements of acute changes in synaptic dopamine have been restricted to the striatum. The ligands previously used, such as [11C]raclopride and [123I]IBZM, do not provide sufficient signal to noise ratio to quantify D2 receptors in extrastriatal areas, such as cortex, where the concentration of D2 receptors is much lower than in the striatum. Given the importance of cortical DA function in cognition, a method to measure cortical dopamine function in humans would be highly desirable. The goal of this study was to compare the ability of two high affinity DA D2 radioligands [11C]FLB 457 and [11C]fallypride to measure amphetamine‐induced changes in DA transmission in the human cortex. D2 receptor availability was measured in the cortical regions of interest with PET in 12 healthy volunteers under control and postamphetamine conditions (0.5 mg kg−1, oral), using both [11C]FLB 457 and [11C]fallypride (four scans per subjects). Kinetic modeling with an arterial input function was used to derive the binding potential (BPND) in eight cortical regions. Under controlled conditions, [11C]FLB 457 BPND was 30–70% higher compared with [11C]fallypride BPND in cortical regions. Amphetamine induced DA release led to a significant decrease in [11C]FLB 457 BPND in five out the eight cortical regions evaluated. In contrast, no significant decrease in [11C]fallypride BPND was detected in cortex following amphetamine. The difference between [11C]FLB 457 and [11C]fallypride ability to detect changes in the cortical D2 receptor availability following amphetamine is related to the higher signal to noise ratio provided by [11C]FLB 457. These findings suggest that [11C]FLB 457 is superior to [11C]fallypride for measurement of changes in cortical synaptic dopamine. Synapse 63:447–461, 2009.

Test–retest variability of serotonin 5-HT2A receptor binding measured with positron emission tomography and [18F]altanserin in the human brain

The role of serotonin in CNS function and in many neuropsychiatric diseases (e.g., schizophrenia, affective disorders, degenerative dementias) support the development of a reliable measure of serotonin receptor binding in vivo in human subjects. To this end, the regional distribution and intrasubject test–retest variability of the binding of [18F]altanserin were measured as important steps in the further development of [18F]altanserin as a radiotracer for positron emission tomography (PET) studies of the serotonin 5‐HT2A receptor. Two high specific activity [18F]altanserin PET studies were performed in normal control subjects (n = 8) on two separate days (2–16 days apart). Regional specific binding was assessed by distribution volume (DV), estimates that were derived using a conventional four compartment (4C) model, and the Logan graphical analysis method. For both analysis methods, levels of [18F]altanserin binding were highest in cortical areas, lower in the striatum and thalamus, and lowest in the cerebellum. Similar average differences of 13% or less were observed for the 4C model DV determined in regions with high receptor concentrations with greater variability in regions with low concentrations (16–20%). For all regions, the absolute value of the test–retest differences in the Logan DV values averaged 12% or less. The test–retest differences in the DV ratios (regional DV values normalized to the cerebellar DV) determined by both data analysis methods averaged less than 10%. The regional [18F]altanserin DV values using both of these methods were significantly correlated with literature‐based values of the regional concentrations of 5‐HT2A receptors determined by postmortem autoradiographic studies (r2 = 0.95, P < 0.001 for the 4C model and r2 = 0.96, P < 0.001 for the Logan method). Brain uptake studies in rats demonstrated that two different radiolabeled metabolites of [18F]altanserin (present at levels of 3–25% of the total radioactivity in human plasma 10–120 min postinjection) were able to penetrate the blood–brain barrier. However, neither of these radiolabeled metabolites bound specifically to the 5‐HT2A receptor and did not interfere with the interpretation of regional [18F]altanserin‐specific binding parameters obtained using either a conventional 4C model or the Logan graphical analysis method. In summary, these results demonstrate that the test–retest variability of [18F]altanserin‐specific binding is comparable to that of other PET radiotracers and that the regional specific binding of [18F]altanserin in human brain was correlated with the known regional distribution of 5‐HT2A receptors. These findings support the usefulness of [18F]altanserin as a radioligand for PET studies of 5‐HT2A receptors. Synapse 30:380–392, 1998.

A comparison of the high‐affinity peripheral benzodiazepine receptor ligands DAA1106 and (R)‐PK11195 in rat models of neuroinflammation: implications for PET imaging of microglial activation

Activated microglia are an important feature of many neurological diseases and can be imaged in vivo using 1‐(2‐chlorophenyl)‐N‐methyl‐N‐(1‐methylpropyl)‐3‐isoquinolinecarboxamide (PK11195), a ligand that binds the peripheral benzodiazepine receptor (PBR). N‐(2,5‐dimethoxybenzyl)‐N‐(5‐fluoro‐2‐phenoxyphenyl) acetamide (DAA1106) is a new PBR‐specific ligand that has been reported to bind to PBR with higher affinity compared with PK11195. We hypothesized that this high‐affinity binding of DAA1106 to PBR will enable better delineation of microglia in vivo using positron emission tomography. [3H]DAA1106 showed higher binding affinity compared with [3H](R)‐PK11195 in brain tissue derived from normal rats and the rats injected intrastriatally with 6‐hydroxydopamine or lipopolysaccharide at the site of the lesion. Immunohistochemistry combined with autoradiography in brain tissues as well as correlation analyses showed that increased [3H]DAA1106 binding corresponded mainly to activated microglia. Finally, ex vivo autoradiography and positron emission tomography imaging in vivo showed greater retention of [11C]DAA1106 compared with [11C](R)‐PK11195 in animals injected with either lipopolysaccaride or 6‐hydroxydopamine at the site of lesion. These results indicate that DAA1106 binds with higher affinity to microglia in rat models of neuroinflammation when compared with PK11195, suggesting that [11C]DAA1106 may represent a significant improvement over [11C](R)‐PK11195 for in vivo imaging of activated microglia in human neuroinflammatory disorders.

Visualization of extrastriatal dopamine D2 receptors in the human brain

[123I]Epidepride, a potent and selective dopamine D2 radioligand, was administered to a 27 year old normal male volunteer. Single photon tomography revealed that peak striatal uptake occurred at 4 h after injection with a striatal:cerebellar ratio of 7.8 rising to over 100 at 18 h post injection. Uptake above the levels seen in cerebellum was also noted in the thalamus, pituitary, hypothalamus and temporal lobe, particularly medially. Single photon tomography with [123I]epidepride allows visualization of extrastriatal dopamine D2 receptors in man.

A Comparative Evaluation of the Dopamine D2/3 Agonist Radiotracer [11C](−)-N-Propyl-norapomorphine and Antagonist [11C]Raclopride to Measure Amphetamine-Induced Dopamine Release in the Human Striatum

(−)-N-Propyl-norapomorphine (NPA) is a full dopamine D2/3 receptor agonist, and [11C]NPA is a suitable radiotracer to image D2/3 receptors configured in a state of high affinity for agonists with positron emission tomography (PET). In this study, the vulnerability of the in vivo binding of [11C]NPA to acute fluctuation in synaptic dopamine was assessed with PET in healthy humans and compared with that of the reference D2/3 receptor antagonist radiotracer [11C]raclopride. Ten subjects (eight females and two males) were studied on two separate days, a minimum of 1 week apart, both with [11C]raclopride and [11C]NPA at baseline and after the administration of 0.5 mg · kg−1 oral d-amphetamine. Kinetic modeling with an arterial input function was used to derive the binding potential relative to nonspecific uptake (BPND) in the ventral striatum (VST), caudate (CAD), and putamen (PUT). [11C]Raclopride BPND was significantly reduced by 9.7 ± 4.4, 8.4 ± 4.2, and 14.7 ± 4.8% after amphetamine administration in the VST, CAD, and PUT. [11C]NPA BPND was also reduced significantly, by 16.0 ± 7.0, 16.1 ± 6.1, and 21.9 ± 4.9% after the same dose of amphetamine in the VST, CAD, and PUT. Although these results suggest that [11C]NPA is more vulnerable to endogenous competition by dopamine compared with [11C]raclopride by a factor of 1.49 to 1.90, the same data for a related outcome measure, binding potential relative to plasma concentration, was not significant. Nevertheless, these data add to the growing literature that suggests D2/3 agonist radiotracers are more vulnerable to endogenous competition by dopamine than existing D2/3 antagonist radiotracers.

Tiagabine increases [11C]flumazenil binding in cortical brain regions in healthy control subjects

Accumulating evidence indicates that synchronization of cortical neuronal activity at γ-band frequencies is important for various types of perceptual and cognitive processes and that GABA-A receptor-mediated transmission is required for the induction of these network oscillations. In turn, the abnormalities in GABA transmission postulated to play a role in psychiatric conditions such as schizophrenia might contribute to the cognitive deficits seen in this illness. We measured the ability to increase GABA in eight healthy subjects by comparing the binding of [11C]flumazenil, a positron emission tomography (PET) radiotracer specific for the benzodiazepine (BDZ) site, at baseline and in the presence of an acute elevation in GABA levels through the blockade of the GABA membrane transporter (GAT1). Preclinical work suggests that increased GABA levels enhance the affinity of GABA-A receptors for BDZ ligands (termed ‘GABA shift’). Theoretically, such an increase in the affinity of GABA-A receptors should be detected as an increase in the binding of a GABA-A BDZ-receptor site-specific PET radioligand. GAT1 blockade resulted in significant increases in mean (± SD) [11C]flumazenil-binding potential (BPND) over baseline in brain regions representing the major functional domains of the cerebral cortex: association cortex +15.2±20.2% (p=0.05), sensory cortex +13.5±15.5% (p=0.03) and limbic (medial temporal lobe, MTL) +16.4±20.2% (p=0.03). The increase in [11C]flumazenil-BPND was not accounted for by differences in the plasma-free fraction (fP; paired t-test p=0.24) or changes in the nonspecific binding (pons VT, p=0.73). Moreover, the ability to increase GABA strongly predicted (r=0.85, p=0.015) the ability to entrain cortical networks, measured through EEG γ synchrony during a cognitive control task in these same subjects. Although additional studies are necessary to further validate this technique, these data provide preliminary evidence of the ability to measure in vivo, with PET, acute fluctuations in extracellular GABA levels and provide the first in vivo documentation of a relationship between GABA neurotransmission and EEG γ-band power in humans.

In Vivo Measurement of GABA Transmission in Healthy Subjects and Schizophrenia Patients

OBJECTIVE Postmortem studies in schizophrenia reveal alterations in gene products that regulate the release and extracellular persistence of GABA. However, results of in vivo studies of schizophrenia measuring total tissue GABA with magnetic resonance spectroscopy (MRS) have been inconsistent. Neither the postmortem nor the MRS studies directly address the physiological properties of GABA neurotransmission. The present study addresses this question through an innovative positron emission tomography (PET) paradigm. METHOD The binding of [(11)C]flumazenil, a benzodiazepine-specific PET radiotracer, was measured before and after administration of tiagabine (0.2 mg/kg of body weight), a GABA membrane transporter (GAT1) blocker, in 17 off-medication patients with schizophrenia and 22 healthy comparison subjects. Increased extracellular GABA, through GAT1 blockade, enhances the affinity of GABAA receptors for benzodiazepine ligands, detected as an increase in [(11)C]flumazenil tissue distribution volume (VT). RESULTS [(11)C]Flumazenil VT was significantly increased across all cortical brain regions in the healthy comparison group but not in the schizophrenia group. This lack of effect was most prominent in the antipsychotic-naive schizophrenia group. In this subgroup, [(11)C]flumazenil ΔVT in the medial temporal lobe was correlated with positive symptoms, and baseline [(11)C]flumazenil VT in the medial temporal lobe was negatively correlated with visual learning. In the healthy comparison group but not the schizophrenia group, [(11)C]flumazenil ΔVT was positively associated with gamma-band oscillation power. CONCLUSIONS This study demonstrates, for the first time, an in vivo impairment in GABA transmission in schizophrenia, most prominent in antipsychotic-naive individuals. The impairment in GABA transmission appears to be linked to clinical symptoms, disturbances in cortical oscillations, and cognition.