Romina Mizrahi

Role of Translocator Protein Density, a Marker of Neuroinflammation, in the Brain During Major Depressive Episodes

IMPORTANCE The neuroinflammatory hypothesis of major depressive disorder is supported by several main findings. First, in humans and animals, activation of the immune system causes sickness behaviors that present during a major depressive episode (MDE), such as low mood, anhedonia, anorexia, and weight loss. Second, peripheral markers of inflammation are frequently reported in major depressive disorder. Third, neuroinflammatory illnesses are associated with high rates of MDEs. However, a fundamental limitation of the neuroinflammatory hypothesis is a paucity of evidence of brain inflammation during MDE. Translocator protein density measured by distribution volume (TSPO VT) is increased in activated microglia, an important aspect of neuroinflammation. OBJECTIVE To determine whether TSPO VT is elevated in the prefrontal cortex, anterior cingulate cortex (ACC), and insula in patients with MDE secondary to major depressive disorder. DESIGN, SETTING, AND PARTICIPANTS Case-control study in a tertiary care psychiatric hospital from May 1, 2010, through February 1, 2014. Twenty patients with MDE secondary to major depressive disorder and 20 healthy control participants underwent positron emission tomography with fluorine F 18-labeled N-(2-(2-fluoroethoxy)benzyl)-N-(4-phenoxypyridin-3-yl)acetamide ([18F]FEPPA). Patients with MDE were medication free for at least 6 weeks. All participants were otherwise healthy and nonsmokers. MAIN OUTCOMES AND MEASURES Values of TSPO VT in the prefrontal cortex, ACC, and insula. RESULTS In MDE, TSPO VT was significantly elevated in all brain regions examined (multivariate analysis of variance, F15,23 = 4.5 [P = .001]). The magnitude of TSPO VT elevation was 26% in the prefrontal cortex (mean [SD] TSPO VT, 12.5 [3.6] in patients with MDE and 10.0 [2.4] in controls), 32% in the ACC (mean [SD] TSPO VT, 12.3 [3.5] in patients with MDE and 9.3 [2.2] in controls), and 33% in the insula (mean [SD] TSPO VT, 12.9 [3.7] in patients with MDE and 9.7 [2.3] in controls). In MDE, greater TSPO VT in the ACC correlated with greater depression severity (r = 0.63 [P = .005]). CONCLUSIONS AND RELEVANCE This finding provides the most compelling evidence to date of brain inflammation, and more specifically microglial activation, in MDE. This finding is important for improving treatment because it implies that therapeutics that reduce microglial activation should be promising for MDE. The correlation between higher ACC TSPO VT and the severity of MDE is consistent with the concept that neuroinflammation in specific regions may contribute to sickness behaviors that overlap with the symptoms of MDE.

Increased Stress-Induced Dopamine Release in Psychosis

BACKGROUND A pathologic response to common life stressors, in which a hyperresponsive dopaminergic system is thought to play a key role, is a potential etiologic factor in the triggering and relapse of psychosis. However, there is no direct evidence that brain dopaminergic response to stress is exaggerated in psychosis. METHODS Using the ability of endogenous dopamine (DA) to compete with [(11)C]-(+)-PHNO binding, as measured with positron emission tomography, we examined stress-induced DA release in response to a validated psychosocial stress task. We studied 12 clinical high-risk (CHR), 10 antipsychotic-naive subjects with schizophrenia (SCZ), and 12 matched healthy volunteers (HV). Stress-induced DA release was estimated as the percent change in binding potential between conditions (stress and control scan) in the striatal subdivisions: limbic striatum (LST), associative striatum (AST), and sensorimotor striatum (SMST). RESULTS We found a significant difference between groups in the AST (F = 8.13, df = 2,31, p = .001), and at the SMST (F = 3,64, df = 2,31, p = .03) but not in the LST (F = .43, df = 2,31, p = .40) with CHR and SCZ having larger [(11)C]-(+)-PHNO displacement in response to the stress. Bonferroni-corrected comparisons confirmed that HV displacement (-2.86%) in the AST was significantly different in CHR (6.97%) and SCZ (11.44%) (with no significant difference between CHR and SCZ). CONCLUSIONS This study reveals a sensitized dopaminergic response to stress in a psychiatric condition and may have important theoretical and clinical implications regarding efforts to abort or delay relapse and/or conversion to psychosis.

Binding characteristics and sensitivity to endogenous dopamine of [11C]-(+)-PHNO, a new agonist radiotracer for imaging the high-affinity state of D2 receptors in vivo using positron emission tomography

[11C]‐(+)‐PHNO (4‐propyl‐9‐hydroxynaphthoxazine) is a new agonist radioligand that provides a unique opportunity to measure the high‐affinity states of the D2 receptors (D2‐high) using positron emission tomography (PET). Here we report on the distribution, displaceablity, specificity and modeling of [11C]‐(+)‐PHNO and compare it with the well characterized antagonist D2 radioligand, [11C]raclopride, in cat. [11C]‐(+)‐PHNO displayed high uptake in striatum with a mean striatal binding potential (BP) of 3.95 ± 0.85. Pre‐treatment with specific D1 (SCH23390), D2 (raclopride, haloperidol) and D3 receptor (SB‐277011) antagonists indicated that [11C]‐(+)‐PHNO binding in striatum is specific to D2 receptors. Within‐subject comparisons showed that [11C]‐(+)‐PHNO BP in striatum was almost 2.5‐fold higher than that measured with [11C]‐(–)‐NPA ([11C]‐(–)‐N‐propyl‐norapomorphine). Comparison of the dose‐effect of amphetamine (0.1, 0.5 and 2 mg/kg; i.v.) showed that [11C]‐(+)‐PHNO was more sensitive to the dopamine releasing effect of amphetamine than [11C]raclopride. Amphetamine induced up to 83 ± 4% inhibition of [11C]‐(+)‐PHNO BP and only up to 56 ± 8% inhibition of [11C]raclopride BP. Scatchard analyses of [11C]‐(+)‐PHNO and [11C]raclopride bindings in two cats showed that the Bmax obtained with the agonist (29.6 and 32.9 pmol/mL) equalled that obtained with the antagonist (30.6 and 33.4 pmol/mL). The high penetration of [11C]‐(+)‐PHNO in brain, its high signal‐to‐noise ratio, its favorable in vivo kinetics and its high sensitivity to amphetamine shows that [11C]‐(+)‐PHNO has highly suitable characteristics for probing the D2‐high with PET.

Imaging Neuroinflammation in Gray and White Matter in Schizophrenia: An In-Vivo PET Study With [18F]-FEPPA

Neuroinflammation and abnormal immune responses have been implicated in schizophrenia (SCZ). Past studies using positron emission tomography (PET) that examined neuroinflammation in patients with SCZ in vivo using the translocator protein 18kDa (TSPO) target were limited by the insensitivity of the first-generation imaging agent [(11)C]-PK11195, scanners used, and the small sample sizes studied. Present study uses a novel second-generation TSPO PET radioligand N-acetyl-N-(2-[(18)F]fluoroethoxybenzyl)-2-phenoxy-5-pyridinamine ([(18)F]-FEPPA) to evaluate whether there is increased neuroinflammation in patients with SCZ. A cross-sectional study was performed using [(18)F]-FEPPA and a high-resolution research tomograph (HRRT). Eighteen patients with SCZ with ongoing psychotic symptoms and 27 healthy volunteers (HV) were recruited from a tertiary psychiatric clinical setting and the community, respectively. All participants underwent [(18)F]-FEPPA PET and magnetic resonance imaging, and PET data were analyzed to obtain [(18)F]-FEPPA total volume of distribution (VT) using a 2-tissue compartment model with an arterial plasma input function, as previously validated. All subjects were classified as high-, medium- or low-affinity [(18)F]-FEPPA binders on the basis of rs6971 polymorphism, and genotype information was incorporated into the analyses of imaging outcomes. No significant differences in neuroinflammation indexed as [(18)F]-FEPPA VT were observed between groups in either gray (F(1,39) = 0.179, P = .674) or white matter regions (F(1,38) = 0.597, P = .445). The lack of significant difference in neuroinflammation in treated patients with SCZ in the midst of a psychotic episode and HV suggests that neuroinflammatory processes may take place early in disease progression or are affected by antipsychotic treatment.

The dopamine D2 receptors in high-affinity state and D3 receptors in schizophrenia: a clinical [11C]-(+)-PHNO PET study.

The dopamine D2 receptors exist in two states: a high-affinity state (D2high) that is linked to second messenger systems, is responsible for functional effects, and exhibits high affinity for agonists; and a low-affinity state that is functionally inert and exhibits lower affinity for agonists. The dopamine D3 receptors have high-affinity for agonist (eg dopamine) and the existence of the two affinity states is controversial. Although preclinical studies in animal models of psychosis have shown a selective increase of D2high as the common pathway to psychosis, the D3 has been suggested to be involved in the pathophysiology of psychosis. We report the first study of the D2high and D3 in schizophrenia using the novel PET radiotracer, [11C]-(+)-PHNO. We recruited 13 patients with schizophrenia-spectrum disorder amidst an acute psychotic episode, drug free for at least 2 weeks, and 13 age–sex-matched healthy controls. The binding potential no-displaceable (BPND) was examine in the main regions of interest (caudate, putamen, ventral striatum, globus pallidus, substantia nigra, and anterior thalamus) and in a voxel-wise analysis. The BPND between patients and controls was not different in any of the regions. The voxel-wise analysis did not reveal any difference and no correlations were found between the BPND and positive and negative syndrome scale subscales. Our results do not find support for the hypothesis linking psychosis to a selective increase in D2high and/or D3 in schizophrenia. It is possible that receptors with high affinity are not accessible by [11C]-(+)-PHNO because they are occupied by endogenous dopamine, a possibility that can be ruled out in future experiments.

Brain region binding of the D2/3 agonist [11C]-(+)-PHNO and the D2/3 antagonist [11C]raclopride in healthy humans.

The D2 receptors exist in either the high‐ or low‐affinity state with respect to agonists, and while agonists bind preferentially to the high‐affinity state, antagonists do not distinguish between the two states. [11C]‐(+)‐PHNO is a PET D2 agonist radioligand and therefore provides a preferential measure of the D2high receptors. In contrast, [11C]raclopride is an antagonist radioligand and thus binds with equal affinity to the D2 high‐ and low‐affinity states. The aim was to compare the brain uptake, distribution and binding characteristics between [11C]‐(+)‐PHNO and [11C]raclopride in volunteers using a within‐subject design. Both radioligands accumulated in brain areas rich in D2/D3‐receptors. However, [11C]‐(+)‐PHNO showed preferential uptake in the ventral striatum and globus pallidus, while [11C]raclopride showed preferential uptake in the dorsal striatum. Mean binding potentials were higher in the putamen (4.3 vs. 2.8) and caudate (3.4 vs 2.1) for [11C]raclopride, equal in the ventral‐striatum (3.4 vs. 3.3), and higher in the globus pallidus for [11C]‐(+)‐PHNO (1.8 vs. 3.3). Moreover [11C]‐(+)‐PHNO kinetics in the globus pallidus showed a slower washout than other regions. One explanation for the preferential binding of [11C]‐(+)‐PHNO in the globus pallidus and ventral‐striatum could be the presence of a greater proportion of high‐ vs. low‐affinity receptors in these areas. Alternatively, the observed distribution could also be explained by a preferential binding of D3‐over‐D2 with [11C]‐(+)‐PHNO. This differential binding of agonist vs. antagonist radioligand, especially in the critically important region of the limbic striatum/pallidum, offers new avenues to investigate the role of the dopamine system in health and disease. Hum Brain Mapp 2008.

The Effect of Antipsychotics on the High-Affinity State of D2 and D3 Receptors: A Positron Emission Tomography Study With [11C]-(+)-PHNO

CONTEXT Most antipsychotics are thought to have an effect on D(2) and D(3) receptors. The development of carbon 11-labeled (+)-4-propyl-9-hydroxynaphthoxazine ([(11)C]-(+)-PHNO), the first agonist radioligand with higher affinity for D(3) than D(2) receptors, allows one to differentiate the effects of antipsychotics on high-affinity vs low-affinity sites of the D(2) receptor and on D(3) vs D(2) receptor subtypes. OBJECTIVES To examine the effects of antipsychotics (clozapine, risperidone, or olanzapine) on the high- vs high- + low-affinity sites of the D(2) and D(3) receptors by comparing the [(11)C]-(+)-PHNO and [(11)C]raclopride binding in the D(3) receptor-rich (globus pallidus and ventral striatum) and D(2) receptor-rich (caudate and putamen) regions. DESIGN Two sequential studies with different participants and appropriate controls were performed. The first compared the occupancy produced by 3 antipsychotics as measured with [(11)C]-(+)-PHNO and [(11)C]raclopride. The second was a double-blind, placebo-controlled experiment to compare the effect of pramipexole (a D(3) receptor-preferring agonist) vs placebo on the increased [(11)C]-(+)-PHNO signal observed in the globus pallidus of patients. SETTING Positron Emission Tomography Centre, Centre for Addiction and Mental Health, Toronto, Ontario, Canada. PARTICIPANTS Twenty-three patients with schizophrenia and 23 healthy controls. MAIN OUTCOME MEASURES Antipsychotic occupancies as measured with [(11)C]-(+)-PHNO and [(11)C]raclopride. RESULTS The antipsychotic-treated patients showed high occupancies with both [(11)C]-(+)-PHNO and [(11)C]raclopride in the dorsal striatum, with [(11)C]raclopride occupancies about 20% higher. Most strikingly, patients did not show any occupancy with [(11)C]-(+)-PHNO in the globus pallidus as compared with normal controls or with their own scans using [(11)C]raclopride. This unblocked [(11)C]-(+)-PHNO signal was displaced by a single dose of pramipexole. CONCLUSIONS Antipsychotics block both the high- and low-affinity states of the D(2) receptors across the brain, but antipsychotic treatment does not block the [(11)C]-(+)-PHNO signal in the D(3) receptor-rich regions, despite the ongoing D(2) receptor blockade. This [(11)C]-(+)-PHNO signal in regions such as the globus pallidus seems increased despite antipsychotic treatment and is displaceable by a D(3) receptor-preferring agonist. The radiotracer [(11)C]-(+)-PHNO and the data open up new avenues for exploring the potential therapeutic significance of the D(3) receptor in schizophrenia.

Depression in Alzheimer's disease: Phenomenology, clinical correlates and treatment

Depression is one of the most frequent comorbid psychiatric disorders in Alzheimers disease and other dementias, and is associated with worse quality of life, greater disability in activities of daily living, a faster cognitive decline, a high rate of nursing home placement, relatively higher mortality, and a higher frequency of depression and burden in caregivers. Depression in Alzheimers disease is markedly under-diagnosed, and most patients with depression are either not treated or are on subclinical doses of antidepressants. This is related to the lack of validated diagnostic criteria and specific instruments to assess depression in dementia. Apathy and pathological affect-crying are the main differential diagnoses of depression in Alzheimers disease. Left untreated, major depression in Alzheimers disease may last for about 12 months. Recent randomized controlled trials demonstrated the efficacy of sertraline and moclobemide to treat depression in Alzheimers disease. Other psychoactive compounds may be useful as well, but careful consideration must be given to potentially serious side-effects.

Translocator Protein (18 kDa) Polymorphism (rs6971) Explains in-vivo Brain Binding Affinity of the PET Radioligand [18F]-FEPPA:

[18F]-FEPPA binds to the 18-kDa translocator protein (TSPO) and is used in positron emission tomography (PET) to detect microglial activation. However, quantitative interpretations of the PET signal with new generation TSPO PET radioligands are confounded by large interindividual variability in binding affinity. This presents as a trimodal distribution, reflecting high-affinity binders (HABs), low-affinity binder (LAB), and mixed-affinity binders (MABs). Here, we show that one polymorphism (rs6971) located in exon 4 of the TSPO gene, which results in a nonconservative amino-acid substitution from alanine to threonine (Ala147Thr) in the TSPO protein, predicts [18F]-FEPPA total distribution volume in human brains. In addition, [18F]-FEPPA exhibits clearly different features in the shape of the time activity curves between genetic groups. Testing for the rs6971 polymorphism may allow quantitative interpretation of TSPO PET studies with new generation of TSPO PET radioligands.

How antipsychotics work-from receptors to reality.

How does a small molecule blocking a few receptors change a patients’ passionately held paranoid belief that the FBI is out to get him? To address this central puzzle of anti-psychotic action, we review a framework linking dopamine neurochemistry to psychosis, and then link this framework to the mechanism of action of antipsychotics. Normal dopamine transmission has a role in predicting novel rewards and in marking and responding to motivationally salient stimuli. Abnormal dopamine transmission alters these processes and results in an aberrant sense of novelty and inappropriate assignment of salience leading to the experience of psychosis. Antipsychotics improve psychosis by diminishing this abnormal transmission by blocking the dopamine D2/3 receptor (not D1 or D4), and although several brain regions may be involved, it is suggested that the ventral striatal regions (analog of the nucleus accumbens in animals) may have a particularly critical role. Contrary to popular belief, the antipsychotic effect is not delayed in its onset, but starts within the first few days. There is more improvement in the first 2 weeks, than in any subsequent 2-week period thereafter. However, a simple organic molecule cannot target the complex phenomenology of the individual psychotic experience. Antipsychotics diminish dopamine transmission and thereby dampen the salience of the preoccupying symptoms. Therefore, in the initial stage of an antipsychotic response, the patients experience a detachment from symptoms, a relegation of the delusions and hallucinations to the back of their minds, rather than a complete erasure of the symptoms. Only with time, and only in some, via the mediation of new learning and plasticity, is there a complete resolution of symptoms. The implications of these findings for clinical care, animal models, future target discovery and drug development are discussed.