In vivo imaging of neuromodulatory synaptic transmission using PET: A review of relevant neurophysiology

Effects of deep brain stimulation on dopamine D2 receptor binding in patients with treatment-refractory depression

BACKGROUND

Depression is a chronic psychiatric disorder related to diminished dopaminergic neurotransmission. Deep brain stimulation (DBS) has shown effectiveness in treating patients with treatment-refractory depression (TRD). This study aimed to evaluate the effect of DBS on dopamine D2 receptor binding in patients with TRD.

METHODS

Six patients with TRD were treated with bed nucleus of the stria terminalis (BNST)-nucleus accumbens (NAc) DBS were recruited. Ultra-high sensitivity [11C]raclopride dynamic total-body positron emission tomography (PET) imaging was used to assess the brain D2 receptor binding. Each patient underwent a [11C]raclopride PET scan for 60-min under DBS OFF and DBS ON, respectively. A simplified reference tissue model was used to generate parametric images of binding potential (BPND) with the cerebellum as reference tissue.

RESULTS

Depression and anxiety symptoms improved after 3-6 months of DBS treatment. Compared with two-day-nonstimulated conditions, one-day BNST-NAc DBS decreased [11C]raclopride BPND in the amygdala (15.9 %, p < 0.01), caudate nucleus (15.4 %, p < 0.0001) and substantia nigra (10.8 %, p < 0.01).

LIMITATIONS

This study was limited to the small sample size and lack of a healthy control group.

CONCLUSIONS

Chronic BNST-NAc DBS improved depression and anxiety symptoms, and short-term stimulation decreased D2 receptor binding in the amygdala, caudate nucleus, and substantia nigra. The findings suggest that DBS relieves depression and anxiety symptoms possibly by regulating the dopaminergic system.

Detecting and classifying neurotransmitter signals from ultra-high sensitivity PET data: the future of molecular brain imaging

Efforts to build the next generation of brain PET scanners are underway. It is expected that a new scanner (NS) will offer anorder-of-magnitude improvementin sensitivity to counts compared to the current state-of-the-art, Siemens HRRT. Our goal was to explore the use of the anticipated increased sensitivity in combination with the linear-parametric neurotransmitter PET (lp-ntPET) model to improve detection and classification of transient dopamine (DA) signals. We simulated striatal [¹¹C]raclopride PET data to be acquired on a future NS which will offer ten times the sensitivity of the HRRT. The simulated PET curves included the effects of DA signals that varied in start-times, peak-times, and amplitudes. We assessed the detection sensitivity of lp-ntPET to various shapes of DA signal. We evaluated classification thresholds for their ability to separate 'early'- versus 'late'-peaking, and 'low'- versus 'high'-amplitude events in a 4D phantom. To further refine the characterization of DA signals, we developed a weighted k-nearest neighbors (wkNN) algorithm to incorporate information from the neighborhood around each voxel to reclassify it, with a level of certainty. Our findings indicate that the NS would expand the range of detectable neurotransmitter events to 72%, compared to the HRRT (31%). Application of wkNN augmented the detection sensitivity to DA signals in simulated NS data to 92%. This work demonstrates that the ultra-high sensitivity expected from a new generation of brain PET scanner, combined with a novel classification algorithm, will make it possible to accurately detect and classify short-lived DA signals in the brain based on their amplitude and timing.

Model Comparison Metrics Require Adaptive Correction if Parameters Are Discretized: Proof-of-Concept Applied to Transient Signals in Dynamic PET

Linear parametric neurotransmitter PET (lp-ntPET) is a novel kinetic model that estimates the temporal characteristics of a transient neurotransmitter component in PET data. To preserve computational simplicity in estimation, the parameters of the nonlinear term that describe this transient signal are discretized, and only a limited set of values for each parameter are allowed. Thus, linear estimation can be performed. Linear estimation is implemented using predefined basis functions that incorporate the discretized parameters. The implementation of the model using discretized parameters poses unique challenges for significance testing. Significance testing employs model comparison metrics to determine the significance of the improvement of the fit accomplished by including a basis function, i.e. it determines the presence of a transient signal in the PET data. A false positive occurs when the bases overfit data that do not contain a transient component. The number of parameters in a model, p, is necessary to determine the degrees of freedom in the model. In turn, p is crucial for the calculation of model selection metrics and controlling the false positive rate (FPR). In this work, we first explore the effect of parameter discretization on FPR by fitting simulated null data with varying numbers of bases. We demonstrate the dependence of FPR on number of bases. Then, we propose a correction to the number of parameters in the model, p^eff , which adapts to the number of bases used. Implementing model selection with p^eff maintains a stable FPR independent of number of bases.

Bayesian Estimation of the ntPET Model in Single-Scan Competition PET Studies

This paper proposes an innovative method, named b-ntPET, for solving a competition model in PET. The model is built upon the state-of-the-art method called lp-ntPET. It consists in identifying the parameters of the PET kinetic model relative to a reference region that rule the steady state exchanges, together with the identification of four additional parameters defining a displacement curve caused by an endogenous neurotransmitter discharge, or by a competing injected drug targeting the same receptors as the PET tracer. The resolution process of lp-ntPET is however suboptimal due to the use of discretized basis functions, and is very sensitive to noise, limiting its sensitivity and accuracy. Contrary to the original method, our proposed resolution approach first estimates the probability distribution of the unknown parameters using Markov-Chain Monte-Carlo sampling, distributions from which the estimates are then inferred. In addition, and for increased robustness, the noise level is jointly estimated with the parameters of the model. Finally, the resolution is formulated in a Bayesian framework, allowing the introduction of prior knowledge on the parameters to guide the estimation process toward realistic solutions. The performance of our method was first assessed and compared head-to-head with the reference method lp-ntPET using well-controlled realistic simulated data. The results showed that the b-ntPET method is substantially more robust to noise and much more sensitive and accurate than lp-ntPET. We then applied the model to experimental animal data acquired in pharmacological challenge studies and human data with endogenous releases induced by transcranial direct current stimulation. In the drug challenge experiment on cats using [¹⁸F]MPPF, a serotoninergic 1A antagonist radioligand, b-ntPET measured a dose response associated with the amount of the challenged injected concurrent 5-HT1A agonist, where lp-ntPET failed. In human [¹¹C]raclopride experiment, contrary to lp-ntPET, b-ntPET successfully detected significant endogenous dopamine releases induced by the stimulation. In conclusion, our results showed that the proposed method b-ntPET has similar performance to lp-ntPET for detecting displacements, but with higher resistance to noise and better robustness to various experimental contexts. These improvements lead to the possibility of detecting and characterizing dynamic drug occupancy from a single PET scan more efficiently.

Multineuromodulator measurements across fronto-striatal network areas of the behaving macaque using solid-phase microextraction

Different neuromodulators rarely act independent from each other to modify neural processes but are instead coreleased, gated, or modulated. To understand this interdependence of neuromodulators and their collective influence on local circuits during different brain states, it is necessary to reliably extract local concentrations of multiple neuromodulators in vivo. Here we describe results using solid-phase microextraction (SPME), a method providing sensitive, multineuromodulator measurements. SPME is a sampling method that is coupled with mass spectrometry to quantify collected analytes. Reliable measurements of glutamate, dopamine, acetylcholine, and choline were made simultaneously within frontal cortex and striatum of two macaque monkeys (Macaca mulatta) during goal-directed behavior. We find glutamate concentrations several orders of magnitude higher than acetylcholine and dopamine in all brain regions. Dopamine was reliably detected in the striatum at tenfold higher concentrations than acetylcholine. Acetylcholine and choline concentrations were detected with high consistency across brain areas within monkeys and between monkeys. These findings illustrate that SPME microprobes provide a versatile novel tool to characterize multiple neuromodulators across different brain areas in vivo to understand the interdependence and covariation of neuromodulators during goal-directed behavior. Such data would be important to better distinguish between different behavioral states and characterize dysfunctional brain states that may be evident in psychiatric disorders.NEW & NOTEWORTHY Our paper reports a reliable and sensitive novel method for measuring the absolute concentrations of glutamate, acetylcholine, choline, dopamine, and serotonin in brain circuits in vivo. We show that this method reliably samples multiple neurochemicals in three brain areas simultaneously while nonhuman primates are engaged in goal-directed behavior. We further describe how the methodology we describe here may be used by electrophysiologists as a low-barrier-to-entry tool for measuring multiple neurochemicals.

Development of a population pharmacokinetic model to predict brain distribution and dopamine D2 receptor occupancy of raclopride in non-anesthetized rat

BACKGROUND

Raclopride is a selective antagonist of the dopamine D2 receptor. It is one of the most frequently used in vivo D2 tracers (at low doses) for assessing drug-induced receptor occupancy (RO) in animals and humans. It is also commonly used as a pharmacological blocker (at high doses) to occupy the available D2 receptors and antagonize the action of dopamine or drugs on D2 in preclinical studies. The aims of this study were to comprehensively evaluate its pharmacokinetic (PK) profiles in different brain compartments and to establish a PK-RO model that could predict the brain distribution and RO of raclopride in the freely moving rat using a LC-MS based approach.

METHODS

Rats (n=24) received a 10-min IV infusion of non-radiolabeled raclopride (1.61μmol/kg, i.e. 0.56mg/kg). Plasma and the brain tissues of striatum (with high density of D2 receptors) and cerebellum (with negligible amount of D2 receptors) were collected. Additional microdialysis experiments were performed in some rats (n=7) to measure the free drug concentration in the extracellular fluid of the striatum and cerebellum. Raclopride concentrations in all samples were analyzed by LC-MS. A population PK-RO model was constructed in NONMEM to describe the concentration-time profiles in the unbound plasma, brain extracellular fluid and brain tissue compartments and to estimate the RO based on raclopride-D2 receptor binding kinetics.

RESULTS

In plasma raclopride showed a rapid distribution phase followed by a slower elimination phase. The striatum tissue concentrations were consistently higher than that of cerebellum tissue throughout the whole experimental period (10-h) due to higher non-specific tissue binding and D2 receptor binding in the striatum. Model-based simulations accurately predicted the literature data on rat plasma PK, brain tissue PK and D2 RO at different time points after intravenous or subcutaneous administration of raclopride at tracer dose (RO <10%), sub-pharmacological dose (RO 10%-30%) and pharmacological dose (RO >30%).

CONCLUSION

For the first time a predictive model that could describe the quantitative in vivo relationship between dose, PK and D2 RO of raclopride in non-anesthetized rat was established. The PK-RO model could facilitate the selection of optimal dose and dosing time when raclopride is used as tracer or as pharmacological blocker in various rat studies. The LC-MS based approach, which doses and quantifies a non-radiolabeled tracer, could be useful in evaluating the systemic disposition and brain kinetics of tracers.

Dopamine cross-reacts with adrenoreceptors in cortical astrocytes to induce BDNF expression, CREB signaling and morphological transformation

Expression of brain-derived neurotrophic factor (BDNF) is induced in cultured rat cortical astrocytes by catecholamines norepinephrine and dopamine as well as selective α1 and β adrenergic agonists. However, it has remained unknown which receptors mediate dopamine-induced BDNF upregulation in astrocytes. Here, we demonstrate that β adrenoreceptors are the main mediators of this effect in cultured cortical astrocytes, while α1 adrenoreceptors and D1 dopamine receptors contribute to a lesser extent. We show that in cortical astrocytes BDNF exon IV and exon VI containing mRNAs are induced by dopamine and norepinephrine via CREB-dependent signaling and that this regulation is mediated by a mechanism that is distinct from activity-dependent CREB-mediated activation of BDNF transcription in neurons. We also show that regulation of BDNF promoters IV and VI by catecholamines requires a distal regulatory element in the BDNF locus. Finally, we demonstrate that dopamine-induced astrocyte stellation and induction of CREB signaling are mediated by cross-reaction of dopamine with β adrenoreceptors.

Sexual dimorphism in striatal dopaminergic responses promotes monogamy in social songbirds

In many songbird species, males sing to attract females and repel rivals. How can gregarious, non-territorial songbirds such as zebra finches, where females have access to numerous males, sustain monogamy? We found that the dopaminergic reward circuitry of zebra finches can simultaneously promote social cohesion and breeding boundaries. Surprisingly, in unmated males but not in females, striatal dopamine neurotransmission was elevated after hearing songs. Behaviorally too, unmated males but not females persistently exchanged mild punishments in return for songs. Song reinforcement diminished when dopamine receptors were blocked. In females, we observed song reinforcement exclusively to the mate’s song, although their striatal dopamine neurotransmission was only slightly elevated. These findings suggest that song-triggered dopaminergic activation serves a dual function in social songbirds: as low-threshold social reinforcement in males and as ultra-selective sexual reinforcement in females. Co-evolution of sexually dimorphic reinforcement systems can explain the coexistence of gregariousness and monogamy.

While monogamy is rare within the animal kingdom, some species – including humans and many birds – can be highly social and yet sustain monogamous relationships. Zebra finches, for example, are among a number of species of songbirds in which numerous males and females live closely together but maintain monogamous partnerships. Male songbirds use their songs to attract females, who do not themselves sing. But if female birds are attracted to any male song, how do they manage to remain monogamous when surrounded by potential suitors?

In songbirds, and in humans too, a region of the brain called the striatum regulates both social and sexual behaviors. It does this by modulating the release of a molecule called dopamine, which is the brain’s reward signal. Tokarev et al. show that hearing songs triggers dopamine release within the striatum of unattached male zebra finches, but has no such effect in single females. Unattached male songbirds will also put up with irritating puffs of air in exchange for being able to watch videos of singing birds, whereas unattached females will not. Female songbirds with partners will tolerate the air puffs, but only if the videos are accompanied with the songs of their own mate.

These findings suggest that song serves as a generic social stimulus for zebra finch males, helping large numbers of birds to live together. By contrast, for a female zebra finch, the song of her partner is a highly selective sexual stimulus. These sex-specific responses to the same socially-relevant stimuli may explain how gregarious animals are able to maintain monogamous pair bonds. More generally, these results are a step towards understanding how brain reward systems regulate social interactions. Studying these mechanisms in songbird species with different social and mating systems could ultimately provide insights into social and sexual behavior in people.

Noninvasive visualization of human dopamine dynamics from PET images

We recently introduced strategies for extracting temporal patterns of brain dopamine fluctuations from dynamic positron emission tomography (PET) data using the tracer [11C]-raclopride. Each of our methods yields a collection of time-concentration curves for endogenous dopamine. Given a spatially dense collection of curves (i.e., one at every voxel in a region of interest), we produce image volumes of dopamine (DA) concentration, DA(X, t), at multiple voxel locations and each time-frame. The volume over time-frames constitutes a 4D dataset that can be thought of as a DA "movie". There are a number of ways to visualize such data. Viewing cine loops of a slice through the DA volume is one way. Creating images of dopamine peak-time, Tpeak(X), derived from a movie, is another. Each visualization may reveal spatio-temporal patterns of neurotransmitter activity heretofore unobservable. We conducted an initial validation experiment in which identical DA responses were induced by an identical task, initiated at different times by the same subject, in two separate PET scans. A comparison of the resulting Tpeak(X) images revealed a large contiguous cluster of striatal voxels, on each side, whose DA timing was consistent with the relative timing of the tasks. Hence, the DA movies and their respective peak-time images were shown to be new types of functional images that contain bonafide timing information about a neurotransmitter's response to a stimulus.

What were they thinking? Cognitive states may influence [11C]raclopride binding potential in the striatum

[(11)C]Raclopride ([(11)C]RAC) is a selective dopamine D(2)/D(3) antagonist that is commonly used in positron emission tomography (PET) studies to assess both basal levels of receptor availability and changes in availability caused by alterations in striatal dopamine concentration. When designing [(11)C]RAC studies, it is important to understand what variables may affect the results. Here, we examined differences in baseline striatal [(11)C]RAC binding potential (BP(ND)) under two different "rest" conditions. Thirteen subjects received [(11)C]RAC scans. Eight subjects were aware prior to initiation of scanning that they would receive a "baseline" scan, and that no additional procedures would take place during the scan ("certain rest" group, CER). Five subjects were informed that they might or might not receive an i.v. alcohol infusion during the scan ("uncertain rest" group, UNC). This group was informed five min after scan start that they would not receive alcohol. Voxel-wise analyses of binding potential (BP(ND)) images generated for both "rest" conditions indicated that receptor availability was higher in UNC than in CER. This result was confirmed by a region-of-interest analysis, which indicated that the average BP(ND) in right and left putamen was statistically higher in UNC. There were no differences in groups with respect to age or raclopride mass dose that could account for the difference in D(2)/D(3) receptor availability. Our findings suggest that even slight differences in cognitive states between groups can have an effect on BP(ND), presumably mediated by changes in endogenous dopamine concentration.

Estimating neurotransmitter kinetics with ntPET: a simulation study of temporal precision and effects of biased data

We recently introduced neurotransmitter PET (ntPET), an analysis technique that estimates the kinetics of stimulus-induced neurotransmitter (NT) release. Here, we evaluate two formulations of ntPET. The arterial (ART) approach measures the tracer input function (TIF) directly. The reference (REF) approach derives the TIF from reference region data. Arterial sampling is considered the gold standard in PET modeling but reference region approaches are preferred for reduced cost and complexity. If simulated PET data with unbiased TIFs were analyzed using ART or REF, temporal precision was better than 3 min provided NT concentration peaked less than 30 min into the scanning session. The consequences of biased TIFs or stimulus-induced changes in tracer delivery were also evaluated. ART TIFs were biased by the presence of uncorrected radiometabolites in the plasma whereas REF TIFs were biased by specific binding in the reference region. Simulated changes in tracer delivery emulated ethanol-induced blood flow alterations observed previously with PET. ART performance deteriorated significantly if metabolites amounted to 50% of plasma radioactivity by 60 min. The accuracy and precision of REF were preserved even if the reference region contained 40% of the receptor density of the target region. Both methods were insensitive to blood flow alterations (proportional changes in K(1) and k(2)). Our results suggest that PET data contain information--heretofore not extracted--about the timing of NT release. The REF formulation of ntPET proved to be robust to many plausible model violations and under most circumstances is an appropriate alternative to ART.

Positron emission tomography displacement sensitivity: predicting binding potential change for positron emission tomography tracers based on their kinetic characteristics

There is great interest in positron emission tomography (PET) as a noninvasive assay of fluctuations in synaptic neurotransmitter levels, but questions remain regarding the optimal choice of tracer for such a task. A mathematical method is proposed for predicting the utility of any PET tracer as a detector of changes in the concentration of an endogenous competitor via displacement of the tracer (a.k.a., its 'vulnerability' to competition). The method is based on earlier theoretical work by Endres and Carson and by the authors. A tracer-specific predictor, the PET Displacement Sensitivity (PDS), is calculated from compartmental model simulations of the uptake and retention of dopaminergic radiotracers in the presence of transient elevations of dopamine (DA). The PDS predicts the change in binding potential (DeltaBP) for a given change in receptor occupancy because of binding by the endogenous competitor. Simulations were performed using estimates of tracer kinetic parameters derived from the literature. For D(2)/D(3) tracers, the calculated PDS indices suggest a rank order for sensitivity to displacement by DA as follows: raclopride (highest sensitivity), followed by fallypride, FESP, FLB, NMSP, and epidepride (lowest). Although the PDS takes into account the affinity constant for the tracer at the binding site, its predictive value cannot be matched by either a single equilibrium constant, or by any one rate constant of the model. Values for DeltaBP have been derived from published studies that employed comparable displacement paradigms with amphetamine and a D(2)/D(3) tracer. The values are in good agreement with the PDS-predicted rank order of sensitivity to displacement.

Nonparametric extraction of transient changes in neurotransmitter concentration from dynamic PET data

We have developed a nonparametric approach to the analysis of dynamic positron emission tomography (PET) data for extracting temporal characteristics of the change in endogenous neurotransmitter concentration in the brain. An algebraic method based on singular value decomposition (SVD) was applied to simulated data under both rest (neurotransmitter at baseline) and activated (transient neurotransmitter release) conditions. The resulting signals are related to the integral of the change in free neurotransmitter concentration in the tissue. Therefore, a specially designed minimum mean-square error (MMSE) filter must be applied to the signals to recover the desired temporal pattern of neurotransmitter change. To test the method, we simulated sets of realistic time activity curves representing uptake of [11C]raclopride, a dopamine (DA) receptor antagonist, in brain regions, under baseline and dopamine-release conditions. Our tests considered two scenarios: 1) a spatially homogeneous pattern with all voxels in the activated state presenting an identical DA signal; 2) a spatially heterogeneous pattern in which different DA signals were contained in different families of voxels. In the first case, we demonstrated that the timing of a single DA peak can be accurately identified to within 1 min and that two distinct neurotransmitter peaks can be distinguished. In the second case, separate peaks of activation separated by as little as 5 min can be distinguished. A decrease in blood flow during activation could not account for our findings. We applied the method to human PET data acquired with [11C]raclopride in the presence of transiently elevated DA due to intravenous (IV) alcohol. Our results for an area of the nucleus accumbens-a region relevant to alcohol consumption-agreed with a model-based method for estimating the DA response. SVD-based analysis of dynamic PET data promises a completely noninvasive and model-independent technique for determining the dynamics of a neurotransmitter response to cognitive or pharmacological stimuli. Our results indicate that the method is robust enough for application to voxel-by-voxel data.

ntPET: A New Application of PET Imaging for Characterizing the Kinetics of Endogenous Neurotransmitter Release

We present a new application of positron emission tomography (“ntPET” or “neurotransmitter PET”) designed to recover temporal patterns of neurotransmitter release from dynamic data. Our approach employs an enhanced tracer kinetic model that describes uptake of a labeled dopamine D2/D3 receptor ligand in the presence of a time-varying rise and fall in endogenous dopamine. Data must be acquired during both baseline and stimulus (transient dopamine release) conditions. Data from a reference region in both conditions are used as an input function, which alleviates the need for any arterial blood sampling. We use simulation studies to demonstrate the ability of the method to recover the temporal characteristics of an increase in dopamine concentration that might be expected following a drug treatment. The accuracy and precision of the method—as well as its potential for false-positive responses due to noise or changes in blood flow—were examined. Finally, we applied the ntPET method to small-animal imaging data in order to produce the first noninvasive assay of the time-varying release of dopamine in the rat striatum following alcohol.

Effect of sensory stimulus on striatal dopamine release in humans and cats: a [11C]raclopride PET study

BACKGROUND

Sensory stimulation of the forelimb extremities constitutes a well-established experimental model that has consistently shown to activate dopamine (DA) neurotransmission in the mammals' forebrain.

OBJECTIVES

To visualize in vivo this modification of striatal DA release in healthy human volunteers using Positron Emission Tomography (PET) and [(11)C]raclopride. Experiments in humans were paralleled by experiments in anesthetized cats. Changes in endogenous DA release were assessed through its competition with [(11)C]raclopride binding (BP(raclo)), a radioligand probing DA D2-receptors.

RESULTS

In humans no significant difference of BP(raclo) in caudate (with sensory stimulation: 2.0 +/- 0.3 versus without sensory stimulation: 2.2 +/- 0.3; P = 0.3) or putamen (2.6 +/- 0.3 versus 2.6 +/- 0.2; P = 0.9) ipsilateral to the stimulus was disclosed as a result of sensory stimulation. Similarly, no change of BP(raclo) was observed contralaterally to the stimulation in the caudate nucleus (with sensory stimulation: 2.0 +/- 0.4 versus without sensory stimulation: 2.1 +/- 0.2; P = 0.5) and the putamen (2.5 +/- 0.4 versus 2.6 +/- 0.2; P = 0.4). In cats the same results were obtained in the ipsilateral to stimulation striatum (with sensory stimulation: 2.5 +/- 0.03 versus without sensory stimulation: 2.4 +/- 0.05; P = 0.7). No change was also observed contralaterally to the stimulation (2.4 +/- 0.04 versus 2.5 +/- 0.06; P = 0.6). The [(11)C]raclopride binding remained unchanged by sensory stimuli in both humans and cats.

CONCLUSION

This suggests that the DA release induced by sensory stimulus is mostly extrasynaptic whereas the synaptic DA release is probably small, which fits well with the absence of [(11)C]raclopride displacement. The mechanism of this extrasynaptic DA release could be related to a local action of glutamate on dopaminergic terminals via a thalamo-cortico-striatal loop. Present results also underline homology between cat and human responses to sensory stimuli and validate the use of cat brain to find physiological concepts in humans.

Depletion and restoration of endogenous monoamines affects β-CIT binding to serotonin but not dopamine transporters in non-human primates

The radioligand [123I]beta-CIT binds to dopamine transporters in striatum and to serotonin transporters in brainstem. Endogenous dopamine or serotonin may compete with radioligand binding at monoamine transporters. We used alpha-methyl-p-tyrosine (AMPT) to block dopamine production and measured [123I]beta-CIT binding before and after endogenous dopamine was restored by IV administration of the dopamine precursor L-dihydroxyphenylalanine (L-DOPA) in rhesus monkeys. P-chlorophenylalanine (pCPA) was used to inhibit serotonin production, and [123I]beta-CIT binding was assessed before and after IV administration of the serotonin precursor 5-hydroxy-L-tryptophan (L-5-HTP) restored endogenous serotonin. Pretreatment with benserazide blocked peripheral decarboxylization in both paradigms. Serotonin restoration measurably displaced [123I]beta-CIT binding to brainstem serotonin transporters but not to striatal dopamine transporters. Restoration of dopamine apparently did not affect [123I] beta-CIT binding to striatal dopamine transporters. However, dopamine restoration reduced radioligand binding to brainstem serotonin transporters, most likely due to dopamine release from serotonin neurons following L-DOPA administration. The higher striatal density of dopamine transporters relative to dopamine concentrations may explain why [123I] beta-CIT displacement by endogenous dopamine was not observed. This study indicates that [123I]beta-CIT binding in brainstem (raphe area) is affected by endogenous serotonin release in vivo and that L-DOPA treatment may cause serotonin neurons in the brainstem to corelease dopamine.

Striatal dopamine release during unrewarded motor task in human volunteers

SUMMARY

Striatal dopamine is associated with the processing of rewarded motor tasks. Its involvement in mediating unrewarded tasks is, however, unclear. We used a recently developed PET technique to dynamically measure the rate of displacement of a dopamine receptor ligand raclopride in healthy volunteers performing a finger opposition task. Rapid displacement of the ligand from the posterior putamen and the caudate immediately after the task initiation suggested striatal dopamine release during task performance. Since dopamine release was observed in the striatal areas that are implicated in unrewarded tasks by neuroimaging studies, the results demonstrate that the PET method can be used to extend the findings of conventional neuroimaging techniques, that do not provide information about signal transduction.

A novel method for noninvasive detection of neuromodulatory changes in specific neurotransmitter systems

Over the last decade, it has become possible to study theories of cognition using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). These methods yield statistical parametric maps of changes in cerebral blood flow (CBF) elicited by cognitive tasks. A limitation of these studies is that they provide no information about the underlying neurochemistry. However, it is possible to extend the concept of activation studies to include measurements targeting neurotransmitters and specific receptor populations. Cognitive activation increases neuronal firing rate, increasing the endogenous neurotransmitter level. The increased neurotransmitter level can be used to alter the kinetics of specifically bound radioligands. We describe a new approach to the design and analysis of neuromodulation experiments. This approach uses PET, a single-scan session design, and a linear extension of the simplified reference region model (LSSRM) that accounts for changes in ligand binding induced by cognitive tasks or drug challenge. In the LSSRM, an "activation" parameter is included that represents the presence or absence of change in apparent dissociation rate. Activation of the neurotransmitter is detected statistically when the activation parameter is shown to violate the null hypothesis. Simulation was used to explore the properties of the LSSRM with regard to model identifiability, effect of statistical noise, and confounding effects of CBF-related changes. Simulation predicted that it is possible to detect and map neuromodulatory changes in single-subject designs. A human study was conducted to confirm the predictions of simulation using (11)C-raclopride and a motor planning task. Parametric images of transport, binding potential, areas of significant dopamine release, and statistical parameters were computed. Examination of the kinetics of activation demonstrated that maximum dopamine release occurred immediately following task initiation and then decreased with a half-time of about 3 min. This method can be extended to explore neurotransmitter involvement in other behavioral and cognitive domains.

Absolute quantification by positron emission tomography of the endogenous ligand

The results of several recent papers have shown a significant influence of the endogenous neurotransmitters on the exogenous ligand kinetics measured by positron emission tomography. For example, several groups found that the percentage of D2 receptor sites occupied by the endogenous dopamine ranged from 25% to 40% at basal level. An obvious consequence of this significant occupancy is that the ligand-receptor model parameters, usually estimated by a model that does not take into account the endogenous ligand (EL) kinetics, can be significantly biased. In the current work, the authors studied the biases obtained by using the multiinjection approach. The results showed that in the classical ligand-receptor model, the receptor concentration is correctly estimated and that only the apparent affinity is biased by not taking the EL into account. At present, all absolute quantifications of the EL have been obtained through pharmacologic manipulation of the endogenous transmitter concentration, which is often too invasive a method to be used in patients. A theoretical reasoning showed that a noninvasive approach is necessarily based on both the apparent affinity measurement and on a multiregion approach. The correlation between the receptor concentration and the apparent affinity, previously observed with some ligands, verifies these two conditions; thus, the authors suggest that this correlation could be the result of the EL effect. To test this assumption experimentally, the effect of reserpine-induced dopamine depletion on the interactions between the D2 receptor sites and the FLB 457 is studied. With untreated baboons, the apparent FLB 457 affinity was smaller in the receptor-rich regions (striatum) than in the receptor-poor regions. This discrepancy disappeared after dopamine depletion, strongly suggesting that this affinity difference was related to the EL effect. Therefore, the purpose of the current study was to test the ability to quantify the EL based on the observed correlation between the receptor concentration and the apparent affinity. This approach offers a method for estimating the percentage of receptor sites occupied by the EL and, if its affinity is known, the free EL concentration. From the data obtained using FLB 457 with baboons, the authors found that approximately 53% of the D2 receptor sites are occupied by dopamine in the striatum and that the free dopamine concentration is approximately 120 nmol/L at basal level. This approach is transferable to patients, because the experimental data are obtained without pharmacologically induced modification of the EL.

Elevated hypothalamic/midbrain serotonin (monoamine) transporter availability in depressive drug-naive children and adolescents

Cumulative data suggest depression in adulthood being connected to reduced availability of brain serotonin while the role of dopamine remains less specific. Prospective studies have shown a continuity of depressive episodes from childhood to adulthood, combined with poor social function and excess mortality. The object of this study was to examine whether alterations in brain serotonin and/or dopamine transporter levels are already present in depressive children and adolescents. We examined 41 drug-naive patients (aged 7-17) by single photon emission tomography (SPET) using iodine-123-labelled 23-carbomethoxy-3P3(iodophenyl) tropane [123I]beta-CIT as a tracer for monoamine transporters. In addition to the ordinary clinical examination, the patients were given a structured interview and information was gathered from teachers and parents with questionnaires. The diagnoses were established by consensus evaluation between three child psychiatrists. To test the serotonin hypothesis and the dopamine hypothesis regarding depression in children and adolescents, the series was divided into groups with depression present (31) and no depression present (10). In this study, the depressive child and adolescent patients had significantly higher serotonin transporter availability (P < 0.02) in the hypothalamic/midbrain area. Age did not correlate to the hypothalamic/midbrain serotonin transporter binding ratio. No significant difference in dopamine transporter availability in striatum was found between the depressive and the nondepressive children and adolescents.

A relationship between serotonin transporter genotype and in vivo protein expression and alcohol neurotoxicity

BACKGROUND

Genetic variation of the promoter for the serotonin transporter (5-HTT) gene has been associated with its functional capacity. In vitro, carriers of a short allele (s-carriers) of the 5-HTT promoter display significant reduction in 5-HTT capacity. Dysfunction of 5-HTT has been observed in alcoholic individuals. We assessed whether the allelic constitution of the 5-HTT gene is associated with reduced serotonin transporter availability among alcoholic individuals.

METHODS

We genotyped the 5-HTT promoter region and measured the availability of serotonin transporter protein with [I-123]beta-CIT SPECT in the raphe area in 14 abstinent male alcoholic subjects and 8 age-matched control subjects of European American descent.

RESULTS

Among control subjects, the ratio of in vivo 5-HTT availability for ll-homozygous individuals relative to s-carriers was comparable to serotonin uptake ratios measured in vitro. There was a significant interaction of diagnosis and 5-HTT promoter genotype on 5-HTT availability (p <.01). Among controls, ll-homozygous individuals displayed a significant increase as compared with s-carriers. The availability of raphe 5-HTT was significantly reduced in ll-homozygous alcoholic individuals and was negatively correlated with their amount of alcohol consumption. Among s-carriers, 5-HTT availability did not differ significantly between control and alcoholic subjects.

CONCLUSIONS

Our preliminary findings suggest an association between 5-HTT allelic constitution and in vivo measurements of human serotonin transporter availability, and a potentially selective susceptibility of ll-homozygous individuals to the neurotoxic effects of chronic excessive alcohol consumption.

Influence of acetylcholine on binding of 4-[125I]iododexetimide to muscarinic brain receptors

The distribution of nicotinic and muscarinic cholinergic receptors in the human brain in vivo has been successfully characterized using radiolabeled tracers and emission tomography. The effect of acetylcholine release into the synaptic cleft on receptor binding of these tracers has not yet been investigated. The present study examined the influence of acetylcholine on binding of 4-[125I]iododexetimide to muscarinic cholinergic receptors of porcine brain synaptosomes in vitro. 4-Iododexetimide is a subtype-unspecific muscarinic receptor antagonist with high affinity. Acetylcholine competed with 4-[125I]iododexetimide in a dose-dependent manner. A concentration of 500 microM acetylcholine inhibited 50% of total specific 4-[125I]iododexetimide binding to synaptosomes when both substances were given simultaneously. An 800 microM acetylcholine solution reduced total specific 4-[125I]iododexetimide binding by about 35%, when acetylcholine was given 60 min after incubation of synaptosomes with 4-[125I]iododexetimide. Variations in the synaptic acetylcholine concentration might influence muscarinic cholinergic receptor imaging in vivo using 4-[123I]iododexetimide. Conversely, 4-[123I]iododexetimide might be an appropriate molecule to investigate alterations of acetylcholine release into the synaptic cleft in vivo using single photon emission computed tomography.

In vivo neurochemistry of the brain in schizophrenia as revealed by magnetic resonance spectroscopy

Magnetic resonance spectroscopy (MRS), an application of the methods of nuclear magnetic resonance (NMR), is a functional imaging modality that provides a view of localized biochemistry in vivo. A number of studies applying MRS to the neurochemistry of schizophrenia have been reported, which encompass a range of patient populations, states of medication, anatomic regions, nuclear species, and MRS techniques. A brief review of the history and methodology of NMR and MRS is presented. Comparison is made of MRS capabilities with other functional imaging modalities. Aspects of the neurochemistry of schizophrenia relevant to MRS studies are reviewed, as are the reported MRS studies involving patients with schizophrenia. Areas of consistent findings include decreased phosphomonoesters and increased phosphodiesters in frontal lobes, and decreases in the putative neuronal cell marker, N-acetylaspartate, in temporal lobes. Studies of neurotransmitters such as glutamate, gamma-aminobutyric acid, and glutamine have generated inconsistent results. New insights into alterations in neurochemistry in schizophrenia have been provided by MRS. Studies of neurotransmitters have future potential with improvements in field strength and in spectral editing techniques. MRS has the potential to measure brain medication levels and simultaneous effects on neurochemistry. MRS may assist in characterizing high-risk populations, and ultimately guide medication use.

Evidence for striatal dopamine release during a video game

Dopaminergic neurotransmission may be involved in learning, reinforcement of behaviour, attention, and sensorimotor integration. Binding of the radioligand 11C-labelled raclopride to dopamine D2 receptors is sensitive to levels of endogenous dopamine, which can be released by pharmacological challenge. Here we use 11C-labelled raclopride and positron emission tomography scans to provide evidence that endogenous dopamine is released in the human striatum during a goal-directed motor task, namely a video game. Binding of raclopride to dopamine receptors in the striatum was significantly reduced during the video game compared with baseline levels of binding, consistent with increased release and binding of dopamine to its receptors. The reduction in binding of raclopride in the striatum positively correlated with the performance level during the task and was greatest in the ventral striatum. These results show, to our knowledge for the first time, behavioural conditions under which dopamine is released in humans, and illustrate the ability of positron emission tomography to detect neurotransmitter fluxes in vivo during manipulations of behaviour.

Within-study repeated measurements to increase sensitivity for positron emission tomography activation studies

A novel data collection strategy was examined for positron emission tomography activation studies. After an injection of H2(15)O, data were collected in multiple 10-second frames and analyzed with a blocked analysis of variance design in which blocking was performed across frames. An estimate of residual error based on a larger number of statistically independent measurements was hence obtained and the statistical significance of detected differences increased. The feasibility of the suggested scheme was demonstrated on phantom data, where higher significance was achieved when dividing the same data into more frames. The method was further used for single-subject analysis of data from eight human subjects participating in a study on visceral sensation. The results show agreement with the group-based analysis and indicate that it is possible to detect areas with changes of 10 mL/(min x 100 mL) or more in single subjects. The residuals from the statistical analysis were analyzed and did not indicate any violations of the assumptions of statistical independence between frames, normal distribution of errors, and homoscedasticity across blocks. The specificity was worse than the theoretically expected 0.05, but this may have resulted from lack of complete control over the experimental situation rather than the statistical method per se.

Effect of reserpine-induced depletion of synaptic dopamine on [11C]raclopride binding to D2-dopamine receptors in the monkey brain

Positron emission tomography was used to examine the in vivo binding of [11C]raclopride to D2-dopamine (DA) receptors in the striatum of two Cynomolgus monkeys after a single dose of reserpine (1 mg/kg, i.v.). A Scatchard procedure was repeated five times to follow D2 receptor density and apparent affinity for 7 weeks after reserpine. Reserpine-induced depletion of DA lead to a marked increase in [11C]raclopride binding, which was still detectable 20 days after treatment. Scatchard analyses indicated that the measured increase in [11C]raclopride binding reflected an increase in receptor affinity but no evident change in receptor density (Bmax). Thus, the increase in [11C]raclopride binding after reserpine should correspond to a reduced competition with endogenous DA for binding to D2 receptors. The results were used to estimate the DA-induced D2 occupancy to be about 40% at physiological conditions.