Michael L. Himes

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.

Amphetamine induced dopamine release increases anxiety in individuals recovered from anorexia nervosa

OBJECTIVE Genetic, pharmacologic, and physiological data suggest that individuals with anorexia nervosa (AN) have altered striatal dopamine (DA) function. METHOD We used an amphetamine challenge and positron emission tomography [(11) C]raclopride paradigm to explore DA striatal transmission in 10 recovered (REC) AN compared with 9 control women (CW). RESULTS REC AN and CW were similar for baseline, postamphetamine [(11) C]raclopride binding potential (BP(ND) ) and change (Δ) in BP(ND) for all regions. In CW, ventral striatum Δ BP(ND) was associated with euphoria (r = -0.76; p = 0.03), which was not found for REC AN. Instead, REC AN showed a significant relationship between anxiety and Δ BP(ND) in the precommissural dorsal caudate (r = -0.62, p = 0.05). DISCUSSION REC AN have a positive association between endogenous DA release and anxiety in the dorsal caudate. This finding could explain why food-related DA release produces anxiety in AN, whereas feeding is pleasurable in healthy participants.

In vivo evidence for low striatal vesicular monoamine transporter 2 (VMAT2) availability in cocaine abusers.

OBJECTIVE Positron emission tomography (PET) imaging studies in cocaine abusers have shown that low dopamine release in the striatum following an amphetamine challenge is associated with higher relapse rates. One possible mechanism that might lead to lower amphetamine-induced dopamine release is low availability of dopamine storage vesicles in the presynaptic terminals for release. Consistent with this hypothesis, postmortem studies have shown low levels of vesicular monoamine transporter, type 2 (VMAT2), the membrane protein that regulates the size of the vesicular dopamine pool, in cocaine abusers relative to healthy subjects. To confirm the postmortem findings, the authors used PET and the VMAT2 radioligand [¹¹C]-(+)-dihydrotetrabenazine (DTBZ) to assess the in vivo VMAT2 availability in a group of 12 recently abstinent cocaine-dependent subjects and matched healthy comparison subjects. METHOD [¹¹C]DTBZ nondisplaceable binding potential (BP(ND)) was measured by kinetic analysis using the arterial input function or, if arterial input was unavailable, by the simplified reference tissue method. RESULTS [¹¹C]DTBZ BP(ND) was significantly lower in the cocaine abusers than in the comparison subjects in the limbic striatum (10.0% lower), associative striatum (-13.4%), and sensorimotor striatum (-11.5%). CONCLUSIONS The results of this in vivo PET study confirm previous in vitro reports of low VMAT2 availability in the striatum of cocaine abusers. It also suggests a compensatory down-regulation of the dopamine storage vesicles in response to chronic cocaine abuse and/or a loss of dopaminergic terminals. Further research is necessary to understand the clinical relevance of this observation to relapse and outcome in abstinent cocaine abusers.

Decreased Prefrontal Cortical Dopamine Transmission in Alcoholism

OBJECTIVE Basic studies have demonstrated that optimal levels of prefrontal cortical dopamine are critical to various executive functions such as working memory, attention, inhibitory control, and risk/reward decisions, all of which are impaired in addictive disorders such as alcoholism. Based on this and imaging studies of alcoholism that have demonstrated less dopamine in the striatum, the authors hypothesized decreased dopamine transmission in the prefrontal cortex in persons with alcohol dependence. METHOD To test this hypothesis, amphetamine and [11C]FLB 457 positron emission tomography were used to measure cortical dopamine transmission in 21 recently abstinent persons with alcohol dependence and 21 matched healthy comparison subjects. [11C]FLB 457 binding potential, specific compared to nondisplaceable uptake (BPND), was measured in subjects with kinetic analysis using the arterial input function both before and after 0.5 mg kg-1 of d-amphetamine. RESULTS Amphetamine-induced displacement of [11C]FLB 457 binding potential (ΔBPND) was significantly smaller in the cortical regions in the alcohol-dependent group compared with the healthy comparison group. Cortical regions that demonstrated lower dopamine transmission in the alcohol-dependent group included the dorsolateral prefrontal cortex, medial prefrontal cortex, orbital frontal cortex, temporal cortex, and medial temporal lobe. CONCLUSIONS The results of this study, for the first time, unambiguously demonstrate decreased dopamine transmission in the cortex in alcoholism. Further research is necessary to understand the clinical relevance of decreased cortical dopamine as to whether it is related to impaired executive function, relapse, and outcome in alcoholism.

[11C]flumazenil binding is increased in a dose-dependent manner with tiagabine-induced elevations in GABA levels.

Evidence indicates that synchronization of cortical activity at gamma-band frequencies, mediated through GABA-A receptors, is important for perceptual/cognitive processes. To study GABA signaling in vivo, we recently used a novel positron emission tomography (PET) paradigm measuring the change in binding of the benzodiazepine (BDZ) site radiotracer [11C]flumazenil associated with increases in extracellular GABA induced via GABA membrane transporter (GAT1) blockade with tiagabine. GAT1 blockade resulted in significant increases in [11C]flumazenil binding potential (BPND) over baseline in the major functional domains of the cortex, consistent with preclinical studies showing that increased GABA levels enhance the affinity of GABA-A receptors for BDZ ligands. In the current study we sought to replicate our previous results and to further validate this approach by demonstrating that the magnitude of increase in [11C]flumazenil binding observed with PET is directly correlated with tiagabine dose. [11C]flumazenil distribution volume (VT) was measured in 18 healthy volunteers before and after GAT1 blockade with tiagabine. Two dose groups were studied (n = 9 per group; Group I: tiagabine 0.15 mg/kg; Group II: tiagabine 0.25 mg/kg). GAT1 blockade resulted in increases in mean (± SD) [11C]flumazenil VT in Group II in association cortices (6.8±0.8 mL g−1 vs. 7.3±0.4 mL g−1;p = 0.03), sensory cortices (6.7±0.8 mL g−1 vs. 7.3±0.5 mL g−1;p = 0.02) and limbic regions (5.2±0.6 mL g−1 vs. 5.7±0.3 mL g−1;p = 0.03). No change was observed at the low dose (Group I). Increased orbital frontal cortex binding of [11C]flumazenil in Group II correlated with the ability to entrain cortical networks (r = 0.67, p = 0.05) measured via EEG during a cognitive control task. These data provide a replication of our previous study demonstrating the ability to measure in vivo, with PET, acute shifts in extracellular GABA.

Cocaine Abuse in Humans Is Not Associated with Increased Microglial Activation: An 18-kDa Translocator Protein Positron Emission Tomography Imaging Study with [11C]PBR28

Basic science investigations have consistently shown that repeated exposure to psychostimulant drugs, such as cocaine, activate the immune response and lead to inflammatory changes in the brain. No previous in vivo studies have confirmed this observation in chronic cocaine-abusing humans. To test this hypothesis, we used positron emission tomography imaging to measure the binding of [11C]PBR28 to the 18 kDa translocator protein (TSPO), a marker for microglial activation in a group of 15 recently abstinent cocaine abusers and 17 matched healthy controls. [11C]PBR28 volumes of distribution expressed relative to total plasma ligand concentration (VT) were measured in subjects with kinetic analysis using the arterial input function. Subjects were also genotyped for the TSPO alanine147 threonine (Ala147Thr, rs6971) polymorphism that has been shown to influence the in vivo binding of PBR28 to TSPO. Consistent with previous reports, the TSPO Ala147Thr genotype predicted the in vivo binding of [11C]PBR28. No significant differences in [11C]PBR28 VT were observed in the cortical and subcortical regions in cocaine abusers compared with healthy controls. The results of this in vivo study do not support increased TSPO expression and, by extension, microglial activation in chronic cocaine-abusing humans. Further research with more direct markers of microglial activation is necessary to conclusively rule out neuroinflammation in cocaine dependence.

Imaging of dopamine D2/3 agonist binding in cocaine dependence: a [11C]NPA positron emission tomography study.

Objective: Positron emission tomography (PET) studies performed with [11C]raclopride have consistently reported lower binding to D2/3 receptors and lower amphetamine‐induced dopamine (DA) release in cocaine abusers relative to healthy controls. A limitation of these studies that were performed with D2/3 antagonist radiotracers such as [11C]raclopride is the failure to provide information that is specific to D2/3 receptors configured in a state of high affinity for the agonists (i.e., D2/3 receptors coupled to G‐proteins, D2/3 HIGH). As the endogenous agonist DA binds with preference to D2/3 HIGH relative to D2/3 LOW receptors (i.e., D2/3 receptors uncoupled to G‐proteins) it is critical to understand the in vivo status of D2/3 HIGH receptors in cocaine dependence. Thus, we measured the available fraction of D2/3 HIGH receptors in 10 recently abstinent cocaine abusers (CD) and matched healthy controls (HC) with the D2/3 antagonist and agonist PET radiotracers [11C]raclopride and [11C]NPA. Methods:[11C]raclopride and [11C]NPA binding potential (BP) (BPND) in the striatum were measured with kinetic analysis using the arterial input function. The available fraction of D2/3 HIGH receptors, i.e., % RHIGH available = D2/3 HIGH/(D2/3 HIGH + D2/3 LOW) was then computed as the ratio of [11C]NPA BPND/[11C]raclopride BPND. Results:No differences in striatal [11C]NPA BPND (HC = 1.00 ± 0.17; CD = 0.97 ± 0.17, P = 0.67) or available % RHIGH (HC = 39% ± 5%; CD = 41% ± 5%, P = 0.50) was observed between cocaine abusers and matched controls. Conclusions: The results of this [11C]NPA PET study do not support alterations in D2/3 HIGH binding in the striatum in cocaine dependence. Synapse, 2011.

Evaluation of dopamine D2/3 specific binding in the cerebellum for the PET radiotracer [11C]FLB 457: implications for measuring cortical dopamine release

In a recent positron emission tomography (PET) study, we demonstrated the ability to measure amphetamine‐induced dopamine (DA) release in the human cortex with the DA D2/3 radioligand [11C]FLB 457. As previous studies in animals have shown that a relatively high fraction of the [11C]FLB 457 signal in the cerebellum represents specific binding to D2/3 receptors, there was concern that the use of the cerebellum as a measure of nonspecific binding (i.e., reference region) to derive [11C]FLB 457 binding potential (BP) (BPND) would bias cortical DA release measurements. Thus, we evaluated the fractional contribution of specific binding to D2/3 receptors in the human cerebellum for [11C]FLB 457. Six healthy human subjects (5M/1F) were studied twice with [11C]FLB 457, once at baseline and again following a single oral dose of 15 mg of aripiprazole, a D2/3 partial agonist. [11C]FLB 457 distribution volume (VT) was estimated using kinetic analysis in the cortical regions of interest and potential reference regions. The change in [11C]FLB 457 VT following aripiprazole ranged from −33 to −42% in the cortical regions of interest (ROIs). The aripiprazole‐induced change in [11C]FLB 457 VT in three potential reference regions suggests significant specific binding the cerebellum (CER, –17 ± 12%), but not pons (PON, –10 ± 10%) and centrum semiovale (CESVL, –3 ± 12%). Nevertheless, a reanalysis of the published [11C]FLB 457 test–retest and amphetamine studies suggests that the use of the PON VT and CESVL VT as an estimate of nonspecific binding to derive [11C]FLB 457 BPND in DA release studies is unlikely to be successful because it leads to less reproducible outcome measures, which in turn diminishes the ability to measure DA release in the cortex. D2/3 blocking studies with aripiprazole and [11C]FLB 457 suggest specific binding to D2/3 receptors in the cerebellum. These data also suggest that the contribution of specific binding to D2/3 receptors in the cerebellum is lower than that in the cortical ROIs and that CER VT is mostly representative of nonspecific binding. Nevertheless, caution is advised when using reference tissue methods that rely solely on the cerebellum signal as an input function to quantify [11C]FLB 457 BPND. Synapse, 2011.

Evaluation of dopamine D₂/₃ specific binding in the cerebellum for the positron emission tomography radiotracer [¹¹C]FLB 457: implications for measuring cortical dopamine release

In a recent positron emission tomography (PET) study, we demonstrated the ability to measure amphetamine‐induced dopamine (DA) release in the human cortex with the DA D2/3 radioligand [11C]FLB 457. As previous studies in animals have shown that a relatively high fraction of the [11C]FLB 457 signal in the cerebellum represents specific binding to D2/3 receptors, there was concern that the use of the cerebellum as a measure of nonspecific binding (i.e., reference region) to derive [11C]FLB 457 binding potential (BP) (BPND) would bias cortical DA release measurements. Thus, we evaluated the fractional contribution of specific binding to D2/3 receptors in the human cerebellum for [11C]FLB 457. Six healthy human subjects (5M/1F) were studied twice with [11C]FLB 457, once at baseline and again following a single oral dose of 15 mg of aripiprazole, a D2/3 partial agonist. [11C]FLB 457 distribution volume (VT) was estimated using kinetic analysis in the cortical regions of interest and potential reference regions. The change in [11C]FLB 457 VT following aripiprazole ranged from −33 to −42% in the cortical regions of interest (ROIs). The aripiprazole‐induced change in [11C]FLB 457 VT in three potential reference regions suggests significant specific binding the cerebellum (CER, –17 ± 12%), but not pons (PON, –10 ± 10%) and centrum semiovale (CESVL, –3 ± 12%). Nevertheless, a reanalysis of the published [11C]FLB 457 test–retest and amphetamine studies suggests that the use of the PON VT and CESVL VT as an estimate of nonspecific binding to derive [11C]FLB 457 BPND in DA release studies is unlikely to be successful because it leads to less reproducible outcome measures, which in turn diminishes the ability to measure DA release in the cortex. D2/3 blocking studies with aripiprazole and [11C]FLB 457 suggest specific binding to D2/3 receptors in the cerebellum. These data also suggest that the contribution of specific binding to D2/3 receptors in the cerebellum is lower than that in the cortical ROIs and that CER VT is mostly representative of nonspecific binding. Nevertheless, caution is advised when using reference tissue methods that rely solely on the cerebellum signal as an input function to quantify [11C]FLB 457 BPND. Synapse, 2011.

Decreased vesicular monoamine transporter, type 2 availability in the striatum following chronic cocaine self-administration in nonhuman primates

BACKGROUND Consistent with postmortem data, in a recent positron emission tomography study, we demonstrated less [(11)C]-(+)-dihydrotetrabenazine ([(11)C]DTBZ) binding to striatal vesicular monoamine transporter type 2 (VMAT2) in cocaine abusers compared with control subjects. A major limitation of these between-group comparison human studies is their inability to establish a causal relationship between cocaine abuse and lower VMAT2. Furthermore, studies in rodents that evaluated VMAT2 binding before and after cocaine self-administration do not support a reduction in VMAT2. METHODS To clarify these discrepant VMAT2 findings and attribute VMAT2 reduction to cocaine abuse, we imaged four rhesus monkeys with [(11)C]DTBZ positron emission tomography before and after 16 months of cocaine self-administration. [(11)C]DTBZ binding potential in the striatum was derived using the simplified reference tissue method with the occipital cortex time activity curve as an input function. RESULTS Chronic cocaine self-administration led to a significant (25.8 ± 7.8%) reduction in [(11)C]DTBZ binding potential. CONCLUSIONS In contrast to the cocaine rodent investigations that do not support alterations in VMAT2, these results in nonhuman primates clearly demonstrated a reduction in VMAT2 binding following prolonged exposure to cocaine. Lower VMAT2 implies that fewer dopamine storage vesicles are available in the presynaptic terminals for release, a likely factor contributing to decreased dopamine transmission in cocaine dependence. Future studies should attempt to clarify the clinical significance of lower VMAT2 in cocaine abusers, for example, its relationship to relapse and vulnerability to mood disorders.