Endogenously released dopamine inhibits the binding of dopaminergic PET and SPECT ligands in superfused rat striatal slices

Therapeutic doses of amphetamine or methylphenidate differentially increase synaptic and extracellular dopamine

Methylphenidate (MP) and amphetamine (AMP) are first-line treatments for attention-deficit hyperactivity disorder. Although both drugs have similar therapeutic potencies, the stimulatory effect of AMP on extracellular dopamine (ECF DA) is greater than that of MP. We compared extracellular effects directly against synaptic changes. ECF DA was assessed by microdialysis in freely moving rodents and synaptic dopamine (DA) was measured using PET and [11C]-raclopride displacement in rodents and baboons. Microdialysis data demonstrated that MP (5.0 mg/kg, i.p.) increased ECF DA 360% +/- 31% in striatum, which was significantly less than that by AMP (2.5 mg/kg, i.p.; 1398% +/- 272%). This fourfold difference was not reflected by changes in synaptic DA. In fact, rodent PET studies showed no difference in striatal [11C]-raclopride binding induced by AMP (2.5 mg/kg, i.p.; 25% +/- 4% reduction) compared with that by MP (5.0 mg/kg, i.p.; 21% +/- 4% reduction). Primate PET experiments also showed no differences between AMP (0.5 mg/kg, i.v.; 24% +/- 4% reduction) and MP (1.0 mg/kg, i.v.; 25% +/- 7% reduction) induced changes in [11C]-raclopride binding potential. The similar potencies of MP and AMP to alter synaptic DA, despite their different potencies in raising ECF DA, could reflect their different molecular mechanisms.

Brain dopamine is associated with eating behaviors in humans

OBJECTIVE

Eating behavior in humans is influenced by variables other than just hunger-satiety including cognitive restraint, emotional distress, and sensitivity to food stimuli. We investigate the role of dopamine (DA), a neurotransmitter involved with food motivation, in these variables.

METHODS

We used the Dutch Eating Behavior Questionnaire (DEBQ) to measure Restraint, Emotionality, and Externality in 10 subjects. We correlated DEBQ scores with brain DA levels. Positron emission tomography and [(11)C]raclopride uptake were used to measure baseline D(2) receptors (neutral stimulation) and to assess changes in extracellular DA to food stimulation (display of food).

RESULTS

Restraint was correlated with DA changes with food stimulation (higher restraint, greater responsivity), emotionality was negatively correlated with baseline D(2) receptors (higher emotionality, lower D(2) receptors), whereas externality was not. These correlations were significant in the dorsal but not in the ventral striatum.

DISCUSSION

These results provide evidence that DA in the dorsal striatum is involved with the restraint and emotionality components regulating eating behavior and that these two dimensions reflect different neurobiologic processes.

Distributed versus compartment models for PET receptor studies

Although distributed models are generally accepted as being more realistic than compartment models, use of simpler compartment models is pervasive in nuclear medicine applications, particularly in positron emission tomography (PET). Here, we report on comparisons made between distributed and compartment model outputs to address the question of whether differences between them are sufficient to justify distributed models for analysis of PET receptor experiments. For both two- and three-injection experiments, "data" sets were obtained by simulation using a distributed model and a wide range of parameter values. Optimal fits of the compartment model output to these "data" were achieved with three strategies in which values of different groups of parameter were estimated. Compartment model outputs yielded good fits to all the distributed model outputs and the values of the corresponding parameters were in close agreement. Given the temporal resolution typically available with PET, the use of a distributed model has no advantage over a compartment model for PET receptor quantification.

Demonstration of competition between endogenous dopamine and [11C]raclopride binding in in vitro brain slices using a dynamic autoradiography technique

To elucidate the mechanism of in vivo binding competition between radioligand and endogenously released transmitter, we examined the influence of depolarization-induced dopamine (DA) release on [11C]raclopride-specific binding to D2 receptors in slices of living brain tissues using dynamic positron autoradiography. Rat brain slices were incubated in a chamber with [11C]raclopride in oxygenated medium at 34 degrees C for 150 min. Two-dimensional images of radioactivity in the slices were recorded on a storage phosphor screen and dynamic changes were measured. When the brain slices were exposed to the depolarization agents (25 mM K+, 50 mM K+, and 20 microM veratridine), the percentage inhibition of striatal [11C]raclopride-specific binding was 22 +/- 4%, 44 +/- 8% and 54 +/- 7% of the control, respectively. The percentage inhibition of [11C]raclopride-specific binding during each depolarization treatment agreed proportionally with the amount of DA released into the medium. However, preexposure of brain slices to the same depolarization treatment (50 mM K+) did not affect the [11C]raclopride-specific binding, suggesting that the reduction in receptor density and/or affinity was not involved in the decrease of [11C]raclopride-specific binding. [11C]Raclopride-specific binding decreased dose-dependently in the presence of exogenously added DA (range 0.005-3mM). The synaptic DA concentration during each depolarization treatment estimated using three different methods. These results suggest that the decrease of [11C]raclopride-specific binding to striatal slices following evoked DA release is due to competition between endogenous DA and raclopride. It is unlikely that changes in D(2) receptor density or in affinity of the receptors for raclopride are involved. These results provide supportive evidence for in vivo binding competition between radioligand and endogenous neurotransmitter.

Sensitivity of binding of high-affinity dopamine receptor radioligands to increased synaptic dopamine

PET and SPECT studies have documented that D2 radioligands of moderate affinity, but not radioligands of high affinity, are sensitive to pharmacological challenges that alter synaptic dopamine levels. The objective of this work was to determine whether the brain kinetics of high-affinity radioligands for dopamine D1 ([(3)H]SCH 23390) and D2 ([(123)I]epidepride) receptors were altered by a prolonged elevation of synaptic dopamine induced by the potent cocaine analog RTI-55. Mice were injected intravenously with radioligands either 30 min after or 4 h before intraperitoneal administration of RTI-55 (2 mg/kg). In separate experiments, the pharmacological effects of RTI-55 were assessed biochemically by measuring uptake of dopamine in synaptosomes prepared from RTI-treated mice and behaviorally by monitoring locomotor activity. Consistent with the expected elevation of synaptic dopamine, RTI-55 induced a long-lasting decrement in dopamine uptake measured ex vivo, and a prolonged increase in locomotor activity. RTI-55 injected prior to the radioligands induced a significant (P < 0.05) increase in striatal concentration of [(123)I]epidepride at 15 min, relative to saline-treated controls, but there were no differences between the two groups at later time-points. For [(3)H]SCH 23390, both initial striatal uptake and subsequent clearance were slightly increased by preadministration of RTI-55. Administration of RTI-55 4 h after the radioligands (i.e., when it was presumed that a state of near equilibrium binding of the radioligands had been reached), was associated with a significant reduction of striatal radioactivity for both radiotracers. Our results are consistent with increased competition between dopamine and radioligand for binding to both D1 and D2 receptors after treatment with RTI-55. We suggest that the magnitude of the competition is reduced by failure of the receptor binding of high-affinity radioligands to rapidly attain equilibrium.

Imaging synaptic neurotransmission with in vivo binding competition techniques: a critical review

Several groups have provided evidence that positron emission tomography (PET) and single-photon emission computed tomography (SPECT) neuroreceptor imaging techniques might be applied to measure acute fluctuations in dopamine (DA) synaptic concentration in the living human brain. Competition between DA and radioligands for binding to D2 receptor is the principle underlying this approach. This new application of neuroreceptor imaging provides a dynamic measurement of neurotransmission that is likely to be informative to our understanding of neuropsychiatric conditions. This article reviews and discusses the body of data supporting the feasibility and potential of this imaging paradigm. Endogenous competition studies performed in rodents, nonhuman primates, and humans are first summarized. After this overview, the validity of the model underlying the interpretation of these imaging data is critically assessed. The current reference model is defined as the occupancy model, since changes in radiotracer binding potential (BP) are assumed to be directly caused by changes in occupancy of D2 receptors by DA. Experimental data supporting this model are presented. The evidence that manipulation of DA synaptic levels induces change in the BP of several D2 radiotracers (catecholamines and benzamides) is unequivocal. The fact that these changes in BP are mediated by changes in DA synaptic concentration is well documented. The relationship between the magnitude of BP changes measured with PET or SPECT and the magnitude of changes in DA concentration measured by microdialysis supports the use of these noninvasive techniques to measure changes in neurotransmission. On the other hand, several observations remain unexplained. First, the amphetamine-induced changes in the BP of D2 receptor antagonists [123I]IBZM and [11C]raclopride last longer than amphetamine-induced changes in DA extracellular concentration. Second, nonbenzamide D2 receptor antagonists, such as spiperone and pimozide, are not affected by changes in DA release, or are affected in a direction opposite to that predicted by the occupancy model. Similar observations are reported with D1 radiotracers. These results suggest that the changes in BP following changes in DA concentration might not be fully accounted by a simple occupancy model. Specifically, the data are reviewed supporting that agonist-mediated receptor internalization might play an important role in characterizing receptor-ligand interactions. Finally, it is proposed that a better understanding of the mechanism underlying the effects observed with benzamides is essential to develop this imaging technique to other receptor systems.

Imaging studies on the role of dopamine in cocaine reinforcement and addiction in humans

We summarize our studies with positron emission tomography investigating the role of dopamine (DA) in the reinforcing effects of cocaine and methylphenidate in humans and its involvement in cocaine addiction. These studies have shown that the rate at which cocaine and methylphenidate enter the brain and block the dopamine transporters (DAT) is the variable associated with the 'high', rather than the presence per se of the drug in the brain. Our studies also show that, while the level of DAT blockade is important in predicting the intensity of the 'high' induced by these drugs (DAT blockade > 50% is required for these drugs to induce a 'high'), the rate at which DAT are blocked determines whether the 'high' is perceived or not. Thus, oral methylphenidate, which leads to slow DAT blockade, does not induce a 'high', even at doses which block DAT more than 60%. In cocaine abusers, we have shown significant reductions in DA D2 receptors that are associated with decreased metabolism in cingulate gyrus and in orbitofrontal cortex. We suggest that this is one of the mechanisms by which DA disruption leads to compulsive drug administration in cocaine addiction. Cocaine abusers also show significant decreases in DA release, which coupled with the reduction in D2 receptors may result in decreased activation of reward circuits by physiological reinforcers and may perpetuate cocaine use as a means to compensate for this deficit. Thus, strategies to enhance DA brain function in ways that mimic physiological DA activity may be of help in overcoming cocaine addiction.

PET studies of binding competition between endogenous dopamine and the D1 radiotracer [11C]NNC 756

NNC 756 ((+)-8-chloro-5-(2,3-dihydrobenzofuran-7-yl)-7-hydroxy-3-methyl-2,3,4,5- tetrahydro-1H-3-benzazepine) is a new high affinity dopamine (DA) D1 receptor antagonist. Labeled with C-11, it has been used as a PET radiotracer to visualize D1 receptors both in striatal and extrastriatal areas, such as the prefrontal cortex. The goal of this study was to evaluate several methods for derivation of D1 receptor binding potential (BP) with [11C]NNC 756 in baboons, and to use these methods to assess the vulnerability of [11C]NNC 756 binding to competition by endogenous DA. A three-compartment model provided a good fit to PET data acquired following a single bolus injection. BP values obtained with this analysis were in good agreement with values derived from in vitro studies. BP values measured following injection of the potent DA releaser amphetamine (1 mg/kg, n=2) were similar to values measured under control conditions. Kinetic parameters derived from single bolus experiments were used to design a bolus plus continuous infusion administration protocol aimed at achieving a state of sustained binding equilibrium. Injection of amphetamine during sustained equilibrium did not affect [11C]NNC 756 binding. Similar results were observed with another D1 radiotracer, [11C]SCH 23390. Doses of amphetamine used in this study are known to reduce by 20-40% the binding potential of several D2 receptors radiotracers. Therefore, the absence of displacement of [11C]NNC 756 by an endogenous DA surge may indicate important differences between D1 and D2 receptors in vivo, such as differences in proportion of high affinity states not occupied by DA at baseline. These findings may also imply that a simple binding competition model is inadequate to account for the effects of manipulation of endogenous DA levels on the in vivo binding of radiolabeled antagonists.

Effects of dopamine on the in vivo binding of dopamine D2 receptor radioligands in rat striatum

The effects of moderate changes in extracellular dopamine concentrations on the in vivo binding of specific dopaminergic D2 radioligands with different affinities and kinetics were investigated in rats. Either [125I]NCQ298 (Kd = 19 pM), or [25I]iodolisuride (Kd = 0.27 nM) or [3H]raclopride (Kd = 1.5 nM) were administered intravenously (IV) to animals 1 h after the intraperitoneal (IP) injection of either alpha-methyl-p-tyrosine (AMPT) (250 mg/kg) or nomifensine (15 mg/kg), or saline. The kinetics of radioactivity concentration in the striatum, cerebellum, and plasma were measured for up to 4 h after [125I]NCQ298 or [125I]iodolisuride injection and up to 1.5 h after [3H]raclopride injection. For each tracer, the striatum-to-cerebellum radioactivity concentration ratios (S/C) and the binding potential (BP), calculated as the association to dissociation binding rate constant ratios (k3/k4), were assessed and related to the changes in extracellular dopamine concentration induced by drug treatments. Results show that S/C and BP of [3H]raclopride were significantly diminished by pretreatment with nomifensine, a drug that increases extracellular dopamine concentration. Nomifensine pretreatment induced no changes in the in vivo binding indexes of the high affinity [125I]NCQ298 and a slight but not significant decrease of the binding indexes of 125I]iodolisuride. Treatment with AMPT, which induced a 40% reduction in dopamine concentration, did not change [125I]NCQ298 binding indexes but slightly increased those of [3H]raclopride and [125I]iodolisuride. In conclusion, the change of dopamine concentration induces modification of radiotracer kinetics. Thus, the combined use of tracers with high and low affinities could allow us to obtain information both on receptor density and neurotransmitter release in vivo. However, as indicated by the [3H]raclopride study with AMPT, small changes in the concentration of intrasynaptic dopamine cannot be easily detected.

Comparable changes in synaptic dopamine induced by methylphenidate and by cocaine in the baboon brain

Though the blockade of dopamine transporters (DAT) is associated with cocaine's and methylphenidate's reinforcing effects, it is the stimulation of dopamine (DA) receptors, achieved by increases in synaptic DA, that enables these effects to occur. Positron emission tomography (PET) and [11C]raclopride were used to assess the levels of occupancy of DA D2 receptors by dopamine achieved by doses of cocaine or methylphenidate previously documented to block over 70% of DAT. Studies were performed in five baboons using a paired scan protocol designed to measure DA D2 receptor availability (Bmax/Kd) at baseline conditions and after intravenous administration of either cocaine or methylphenidate. Cocaine (1-2 mg/kg) or methylphenidate (0.5 mg/kg) administered 5 min prior to [11C]raclopride decreased Bmax/Kd by 29+/-3% and 32 + 4%, respectively. Smaller reductions in Bmax/Kd (13% for cocaine given 30 min before [11C]raclopride and 25+/-10% for methylphenidate given 40 min before [11C]raclopride) were seen with longer periods between drug and radioligand. These observations are consistent with the slower striatal clearance kinetics of [11C]methylphenidate than [1C]cocaine observed in previous PET experiments and with the approximately twofold higher potency of methylphenidate than cocaine in in vitro experiments. Though the elevation of synaptic DA induced by >70% occupancy of DAT by these drugs lead to a modest increase in occupancy of D2 receptors (25-30%), further studies are required to assess if this is an underestimation because of differences in D2 receptor binding kinetics between raclopride and DA.

Evaluation of d-amphetamine effects on the binding of dopamine D-2 receptor radioligand, 18F-fallypride in nonhuman primates using positron emission tomography

We have investigated the ability of dopamine to compete with the binding of the high affinity dopamine D2 receptor positron emission tomography (PET) radioligand, 18F-fallypride. In vitro dissociation of 18F-fallypride with dopamine in rat striatal homogenates exhibited a dissociation rate, k(off), of 1.76 x 10(-2) min(-1) while the association rate constant, k(on), was found to be 5.30 x 10(8) M(-1) min(-1). This resulted in a dissociation constant, K(D) of 33 pM for 18F-fallypride. For in vivo studies, we investigated the effects of reserpine and d-amphetamine treatment on 18F-fallypride in an attempt to study competition of endogenous dopamine with the radioligand at the receptor sites in rats and monkeys. PET experiments with 18F-fallypride in two male rhesus monkeys were carried out in a PETT VI scanner. In control experiments, rapid specific uptake of 18F-fallypride in the striata was observed (0.05-0.06% injected dose (ID)/g) while nonspecifically bound tracer cleared from other parts of the brain. Striata/cerebellum ratios for 18F-fallypride were approximately 8 at 80 min postinjection, respectively. The monkeys received various doses (0.25 to 1.50 mg/kg) of d-amphetamine (AMPH) pre- and postinjection of the radioligand. There was a decrease of specifically bound 18F-fallypride as well as evidence of an enhanced clearance of specifically bound 18F-fallypride after administering AMPH in the two monkeys. The dissociation rates, k(off), of 18F-fallypride without AMPH was <10(-4) min(-1) but after 25 min preadministration of AMPH (1 mg/kg), it was 4.1 x 10(-3) min(-1) and after 17, 45 and 90 min postadministration of AMPH (1 mg/kg) it was 3.6 x 10(-3) to 4.0 x 10(-3) min(-1). Lower doses of AMPH (0.25 mg/kg) had a reduced effect on the binding of 18F-fallypride. No effect was seen until about 30 minutes after the injection of AMPH. Studies with various doses indicated that 18F-fallypride has a maximum response at doses of 0.75-1.50 mg/kg, with an approximately 16%/hour reduction in binding. These results indicate that AMPH stimulated release of endogenous dopamine reduces the specific binding of 18F-fallypride.

Decreased striatal dopaminergic responsiveness in detoxified cocaine-dependent subjects

Cocaine blocks the reuptake of dopamine, a neurotransmitter involved in the control of movement, cognition, motivation and reward. This leads to an increase in extracellular dopamine; the reinforcing effect of cocaine is associated with elevated dopamine levels in the nucleus accumbens. But addiction to cocaine involves other effects, such as craving, loss of control and compulsive drug intake; the role of the dopamine system in these effects is less well-understood. We therefore used positron emission tomography (PET) to compare the responses of cocaine addicts and normal controls to intravenous methylphenidate, a drug that, like cocaine, causes an increase in synaptic dopamine. Addicts showed reduced dopamine release in the striatum, the brain region where the nucleus accumbens is located, and also had a reduced 'high' relative to controls. In contrast, addicts showed an increased response to methylphenidate in the thalamus (a region that conveys sensory input to the cortex). This thalamic response was associated with cocaine craving and was not seen in control subjects. Thus, our findings challenge the notion that addiction involves an enhanced striatal dopamine response to cocaine and/or an enhanced induction of euphoria. Moreover, they suggest a participation of thalamic dopamine pathways in cocaine addiction, a possibility that merits further investigation.

Model for estimating dopamine transporter occupancy and subsequent increases in synaptic dopamine using positron emission tomography and carbon-11-labeled cocaine

Although increases in dopamine secondary to the inhibition of the dopamine transporter appear to underlie the reinforcing properties of cocaine, there is presently no model that relates the elevation of synaptic dopamine to the transporter occupancy by cocaine. We propose such a model based on positron emission tomographic (PET) measurements of the brain concentration of cocaine and the assumption of rapid equilibrium between free cocaine and cocaine bound to the dopamine transporter. A euphorigenic dose of cocaine (about 40 mg) is predicted to occupy 80-90% of the transporters, while a perceptible dose (about 5 mg) occupies about 40% of the transporters. If reuptake of dopamine is reduced in proportion to the fraction of transporters occupied by cocaine, our model indicates that synaptic dopamine rises supra-linearly with occupancy, so that 5 and 40 mg doses of cocaine give about 2- and 10-fold increases, respectively. A consequence is that a given dose of cocaine produces a similar degree of elevation of dopamine regardless of the prior level of occupation of the transporters by cocaine. This prediction is supported by recent PET/neuropsychological studies in our laboratory where dopamine transporter occupancy was measured after giving methylphenidate intravenously to volunteers; similarly intense "highs" were reported whether the initial occupancy was zero or 75-85%. It could also explain why attempts to block the psychostimulant-induced "high" by pretreating subjects with drugs that block the dopamine transporter have been unsuccessful, and why the use of methylphenidate to treat cocaine addicts led to increased cocaine consumption.