Doug Hussey

Lower dopamine transporter binding potential in striatum during depression.

Previous studies suggest that there is a dopamine lowering process during major depressive episodes (MDE). To investigate this, we measured the dopamine transporter binding potential (DAT BP) in the striatum of depressed and healthy subjects using [11C]RTI-32 PET. The DAT, a predominantly presynaptic receptor, decreases in density after chronic dopamine depletion and the BP is proportional to receptor density. In all striatal regions, subjects with MDE had significantly lower DAT BP. Low striatal DAT BP in MDE is consistent with a downregulation of DAT in response to a dopamine lowering process. There was also a strong, highly significant, inverse correlation between striatal DAT BP and neuropsychological tests of dopamine-implicated symptoms in patients (i.e. patients with lower DAT BP performed better). Lower DAT BP itself reduces extracellular clearance of dopamine. Patients who did not decrease their striatal DAT BP failed to compensate for low dopamine and showed greater impairment on dopamine related tests.

Bupropion occupancy of the dopamine transporter is low during clinical treatment

AbstractRationale. Bupropion is thought to treat major depression by blocking the dopamine transporter (DAT) because bupropion appears to have a selective affinity for the DAT. The validity of this mechanism has been questioned because the affinity of bupropion for the DAT is quite low. Objective. To determine the occupancy of bupropion for the DAT during clinical treatment of patients with depression. Methods. Positron emission tomography with [11C]-RTI32 was used to determine the striatal DAT binding potential (BP) of eight depressed patients before and during treatment with bupropion. BP is proportional to available receptor density (receptors not blocked by drug). Occupancy is the percent change in BP. Eight healthy subjects were similarly studied in a test-retest design. Results. No significant difference in DAT BP was found after bupropion treatment in comparison to the test-retest data. The occupancy after bupropion treatment was 14% (confidence interval 6–22%) as compared to 7% in the test-retest condition. Conclusions. Bupropion treatment occupies less than 22% of DAT sites. This raises the question as to whether a DAT occupancy of less than 22% is therapeutic or whether there is another mechanism involved during treatment with bupropion.

High-affinity states of human brain dopamine D2/3 receptors imaged by the agonist [11C]-(+)-PHNO

BACKGROUND The high-affinity states of dopamine D2-receptors (D2(high)) are postulated to be functionally responsible for signal transduction. At present, no useful in vivo method exists to selectively measure D2(high) in humans, as current D2 radioligands for positron emission tomography (PET) are either not D2-selective or do not differentiate between D2 high- and low-affinity states. METHODS The D2-agonist (+)-PHNO [(+)4-propyl-9-hydroxynaphthoxazine] was labeled with carbon-11 and studied with PET. Eight [11C]-(+)-PHNO scans were acquired in four healthy volunteers. RESULTS We observed greatest [11C]-(+)-PHNO accumulation in caudate, putamen, and globus pallidus [binding potentials (BPs): 3.00 +/- .4, 3.10 +/- .2, and 4.17 +/- 1.2]. Small but detectable binding was identified in the substantia nigra/ventral tegmental area. Preliminary test-retest data in two subjects suggests BP-estimates to be reliable. Pre-treatment with haloperidol reduced BPs in regions showing specific binding with no detectable changes in cerebellum. Parallel imaging with [11C]-raclopride showed substantial differences in the globus pallidus. CONCLUSIONS [11C]-(+)-PHNO proved to be a D2/3-receptor agonist-radioligand with good brain uptake and favorable kinetics for PET in humans. [11C]-(+)-PHNO delineated D2/3-receptor rich brain regions with high signal-to-noise ratio. This is the first demonstration of a viable agonist-radioligand for D2 receptors in humans and opens the door for investigating D2(high) in health and disease.

Serotonin 5-HT1A receptor binding potential declines with age as measured by [11C]WAY-100635 and PET

Positron emission tomography (PET) and [11C]WAY-100635 were used to examine the effect of age on serotonin-1A (5-HT1A) receptor binding potential (BP) in 19 healthy subjects. Regions of interest (ROI) were drawn on the co-registered magnetic resonance imaging (MRI) in orbitofrontal (OFC), dorsolateral prefrontal (DLPFC), anterior cingulate (ACC), lateral (LTC), and mediotemporal (MTC), parietal, occipital and cerebellar cortex, and the raphe nuclei. BP values were calculated using a simplified reference tissue method. In addition, a voxelwise analysis was performed using SPM99. Voxelwise analysis revealed a significant global decrease of 5-HT1A BP with age (set level < .001). ROI analysis revealed significant age-related 5-HT1A BP decreases in DLPFC (r = −0.56), ACC (r = −0.44), OFC (r = −0.42), LTC (r = −0.40), parietal (r = −0.65), and occipital cortex (r = −0.43), but not in MTC or raphe nuclei. Overall, cortical 5-HT1A BP declined by approximately 10% per decade, except for the MTC, where we did not find a significant age effect. Hence, careful age matching may be recommended for future studies using PET and [11C]WAY-100635 to examine 5-HT1A receptors.

Synthesis and in vivo evaluation of novel radiotracers for the in vivo imaging of the norepinephrine transporter.

The (R,R) and (S,S) enantiomers of 2-[(2-methoxyphenoxy)phenylmethyl]morpholine (MeNER) have been radiolabelled with carbon-11 in good yield and at high specific activity. These radiotracers are close analogues of reboxetine, a potent and selective ligand for the norepinephrine transporter (NET). They were examined as potential ligands for imaging NET in vivo by positron emission tomography (PET). The in vivo brain distribution of both [(11)C]-labeled enantiomers were evaluated in rats. Following tail-vein injection of the (R,R)-enantiomer regional brain uptake and washout of radioactivity was homogeneous at all time points examined (5-60 min). In contrast, administration of the (S,S)-enantiomer produced a heterogeneous distribution of radioactivity in brain with highest uptake in the hypothalamus, a NET rich region, and lowest uptake in the striatum, a brain region devoid of NET. Hypothalamus to striatum ratios of 2.5 to one were achieved at 60 min post injection of (S,S)-[(11)C]-MeNER. Pre-injection of the norepinephrine reuptake inhibitors, reboxetine or desipramine, reduced hypothalamus to striatum ratios to near unity while reuptake inhibitors of dopamine and serotonin had no significant effect on binding. In vitro autoradiography studies (rat brain slices) with (S,S)-[(11)C]-MeNER produced a regional distribution pattern that was consistent with the reported distribution of NET. (S,S)-[(11)C]-MeNER has the potential to be the first successful PET ligand to image NET.

Positron Emission Tomography Quantification of [11C]-Harmine Binding to Monoamine Oxidase-A in the Human Brain

This article describes the kinetic modeling of [11C]-harmine binding to monoamine oxidase A (MAO-A) binding sites in the human brain using positron emission tomography (PET). Positron emission tomography studies were performed in healthy volunteers at placebo conditions and after treatment with clinical doses of moclobemide. In either condition, a two-tissue compartment model (2CM) provided better fits to the data than a one-tissue model. Estimates of k3/k4 values from an unconstrained 2CM were highly variable. In contrast, estimates of the specifically bound radioligand distribution volume (DVB) from an unconstrained 2CM were exceptionally stable, correlated well with the known distribution of MAO-A in the brain (cerebellum <frontal cortex ≈ putamen <temporal cortex ≈ cingulate <thalamus) and thus provided reliable indices of MAO-A density. Total distribution volume (DV) values were also highly stable and not different from those estimated with the Logan approach. Fixing the DV of free and nonspecifically bound radiotracer (DVF + NS) or coupling DVF + NS between brain regions enabled more stable estimates of k3/k4 as compared with an unconstrained 2CM. Moclobemide treatment leads to a 64% to 79% MAO-A blockade across brain regions, a result that supports the specificity of [11C]-harmine binding to MAO-A. The stability and reliability of DVB values obtained from an unconstrained 2CM, together with the computational simplicity associated with this method, support the use of DVB as an appropriate outcome measure for [11C]-harmine. These results indicate the suitability of using [11C]-harmine for quantitative evaluation of MAO-A densities using PET and should enable further studies of potential MAO-A dysregulation in several psychiatric and neurologic illnesses.

Stimulation of the subthalamic nucleus in Parkinson's disease does not produce striatal dopamine release.

OBJECTIVEThe subthalamic nucleus (STN) is a target in the surgical treatment of Parkinson’s disease (PD). The mechanism by which electrical stimulation of the STN ameliorates symptoms of PD remains unknown. One consistent aspect of STN stimulation is the ability to reduce the dosage of dopaminergic medications; sometimes they can be eliminated altogether. Furthermore, nigrostriatal projection axons are apposed to the dorsal surface of the STN and are likely affected by the application of current in this region. We sought to determine whether STN stimulation could release endogenous striatal dopamine. METHODSFive patients with PD, who had previously undergone surgical implantation of bilateral STN stimulators, underwent [11C]raclopride positron emission tomographic scanning. l-dopa was withheld for 12 hours, and both stimulators were turned off 9 hours before scanning. We assayed for striatal dopamine release by measuring radioligand displacement as a consequence of turning on the right STN stimulator after 45 minutes of a 90-minute [11C]raclopride infusion. Patients were evaluated with the motor section of the Unified Parkinson’s Disease Rating Scale before and after the studies. RESULTSComparisons between the right and left striata, before and after right STN stimulation, demonstrated no significant differences in [11C]raclopride binding, despite significant improvements in Unified Parkinson’s Disease Rating Scale motor scores with unilateral stimulation (mean improvement, 26.0 ± 16.4%; P < 0.05). This finding was also noted when the striatum was partitioned into dorsal and ventral caudate and putamen and the four regions were analyzed separately. CONCLUSIONOur results suggest that STN stimulation does not mediate its anti-PD effects via the release of dopamine, as assessed with [11C]raclopride displacement.

Effects of catecholamine depletion on D2 receptor binding, mood, and attentiveness in humans: a replication study

The effect of catecholamine depletion, achieved by per-oral administration of 5250 mg alpha-methyl-para-tyrosine (AMPT) given in the 29 h prior to [11C]raclopride positron emission tomography (PET) was studied on measures of dopamine (DA) release, mood, and attention. Neostriatal DA levels in vivo were estimated by comparing the neostriatal DA D(2) receptor binding potential (D(2)RBP) before and after catecholamine depletion using PET and the radiotracer [11C]raclopride. Six healthy subjects completed the protocol. The AMPT treatment increased D(2)RBP significantly by 13.3+/-5.9% (average+/-standard deviation) and decreased plasma levels of the DA metabolite homovanillic acid (HVA) by 62+/-17%, and levels of the norepinephrine (NE) metabolite 3-methoxy-4-hydroxyphenethyleneglycol (MHPG) by 66+/-5%. Catecholamine depletion resulted in decreased happiness, euphoria, energy, talkativeness, vigor, and attentiveness, and in increased sleepiness, fatigue, sedation, and eye blink rate (EBR). These changes were not correlated with the D(2)RBP increments. The results of this study are overall consistent with previous findings by our group using the same methodology in a different cohort of six healthy subjects.

D2-receptor upregulation is dependent upon temporal course of D2-occupancy: a longitudinal [11C]-raclopride PET study in cats.

Long-term occupancy of dopamine D2-receptors, as achieved by chronic treatment with antipsychotics, leads to D2-receptor upregulation, and this upregulation is thought to be responsible for loss of efficacy and development of tardive dyskinesia. However, little is known about the parameters of D2-receptor blockade (duration and percentage of blockade) that lead to upregulation. In this study, we investigated the effects of different degrees (60 vs >80%) and durations (a transient peak vs 24 h/day) of D2-receptor blockade on inducing this upregulation. These different patterns of D2-receptor occupancy kinetics were produced in cats using bolus vs constant infusion of haloperidol for 4 weeks. D2-receptors were measured using positron emission tomography and Scatchard analyses of [11C]raclopride binding, before and after withdrawal of treatment. Continuously high (80% for 24 h/day) D2-receptor blockade led to a robust upregulation of striatal D2-receptors that was maximal at 1-week withdrawal (35±5%) and still detectable at 2-week withdrawal (20±3%). This pattern of D2-receptor blockade also induced behavioral tolerance to the effect of haloperidol on spontaneous locomotor activity. Continuously moderate (60% for 24 h/day) or transiently high (80% for a few hours/day) D2-receptor blockade did not produce any of these effects. The long-term effect of haloperidol on D2-receptor density and behavioral tolerance thus appears to be dependent not only on a critical threshold of D2-receptor blockade but also on the daily duration of D2-receptors blockade. This suggests that as far as antipsychotics are concerned, not only dose but disbursment throughout the day have an impact on eventual pharmacodynamic and behavioral outcomes.

Task-Independent Effect of Time on rCBF

Positron emission tomography was used to identify brain regions that showed general increase or decrease in regional cerebral blood flow (rCBF) across time that was task-independent. Twelve male subjects were scanned eight times: the first and last scans were taken while subjects performed a baseline fixation task and the middle six scans were taken while subjects performed a visuomotor activation task. To determine whether there was a consistency across different studies in the regions that showed this time-related change in rCBF two additional datasets were analyzed. There were similarities across all three studies in the regions that showed a monotonic task-independent change in activity. In all three studies there was a general bilateral decrease in rCBF of occipital and temporal areas across scans that might be related to habituation in the visual domain. Increases in rCBF were found in anterior cingulate, postcentral gyrus, and precentral gyrus across studies. It is likely that these changes reflect motor learning and motor program retrieval. This implies that, unless the experimenter controls for time-dependent changes in brain activity, the interpretation of task-related changes in rCBF may be confounded by these monotonic changes in rCBF. We present analytic strategies to identify experimental effects that are independent of nonspecific time effects, which can be used when it is not possible to control these effects through counterbalancing the experimental design. Nonspecific confounds are particularly relevant in functional MRI studies in which the number of scans acquired per study is much larger.