MDMA-evoked changes in cerebral blood flow in living porcine brain: correlation with hyperthermia

3,4-Methylenedioxymethamphetamine (MDMA) acutely releases intraneuronal dopamine and serotonin and evokes hyperthermia which is linked to toxicity for serotonin fibers. The acute effects of MDMA on cerebral blood flow (CBF) in living brain have not been described in an animal model of MDMA intoxication. We predicted that MDMA-induced hyperthermia should correlate with increased CBF in the hypothalamus, a serotonin-rich brain region subserving thermoregulation. To test this prediction, we used positron emission tomography with statistical parametric mapping for exploratory analysis of the focal changes in the magnitude of CBF in the anesthetized female Landrace pig (n = 9) at 30 and 150 min after acute challenge with MDMA-HCl (1 mg/kg, i.v.). The MDMA treatment was followed by increased CBF in the occipital cortex and in the medial mesencephalon overlapping the dorsal raphé nucleus, and reduced CBF in the cerebellar vermis and in a cluster in the medulla encompassing the left locus coeruleus. The individual increase of body temperature correlated positively with increased CBF in the vicinity of the raphé nucleus, in the hypothalamus (regions linked to thermoregulation), and also in the medial frontal cortex, which together comprise the regions receiving the most dense serotonin innervations in pig brain. Thus, individual differences in the susceptibility to MDMA-induced hyperthermia in this population correlated with the magnitude of focal increases in CBF within specific brain regions endowed with a dense serotonin innervation, including regions linked to thermoregulation.     

Effect of monoamine oxidase inhibition on amphetamine-evoked changes in dopamine receptor availability in the living pig: a dual tracer PET study with [11C]harmine and [11C]raclopride

The activity of both isozymes of monoamine oxidase (MAO) is reduced by 50% in the brain of human smokers. We hypothesized that this is not an epiphenomenon, but should bring about potentiation of the action of psychostimulant drugs. To test this hypothesis, we carried out serial positron emission tomography (PET) studies in G?ttingen miniature pigs to measure the binding of the MAO-A ligand [11C]harmine and to measure the changes in [11C]raclopride binding evoked by a low dose of amphetamine (0.7 mg/kg as free base, i.v.), first in a baseline condition, and, one month later, after acute treatment with pargyline (2 x 3 mg/kg as free base, i.m.). In the baseline, the distribution volume of [11C]harmine relative to the arterial input (V(d), ml g(-1)) ranged from 74 ml g(-1) in cerebellum to 139 ml g(-1) in the medial hypothalamus. Pargyline treatment reduced the magnitude of V(d) globally to 34-54 ml g(-1). Nearly complete displacement of [11C]harmine binding was detected in neocortex and striatum, but there was evidence for pargyline-resistant binding in the pituitary gland and diencephalon. In the baseline condition, the low dose of amphetamine evoked a 14% decline in the binding potential (BP) (pB) of [11C]raclopride in striatum (P = 0.026). After pargyline treatment, the amphetamine effect was of similar magnitude (-11%), although not statistically significant (P = 0.054). However, the second amphetamine challenge evoked a 24% reduction in [11C]raclopride pB relative to the original baseline condition (P = 0.018). Present results do not strongly support our hypothesis that MAO inhibition should potentiate the amphetamine-evoked dopamine release as measured in the [11C]raclopride competition paradigm.     

Differential effects of stress on [11C]raclopride and [11C]MNPA binding to striatal D2/D3 dopamine receptors: A PET study in conscious monkeys

It has been reported that stress and facilitation of dopamine neuronal system are closely related. In the present study, the effects of stress on the binding of antagonist‐based [¹¹C]raclopride and agonist‐based (R)‐2‐CH3O‐N‐n‐ propylnorapomorphine ([¹¹C]MNPA) to D₂/D₃ receptors were evaluated in the striatum of conscious monkey brain. The stress state assessed from plasma cortisol level was negatively correlated with [¹¹C]raclopride binding as expected. It was noteworthy that [¹¹C]MNPA binding exhibited a positive correlation with stress state; thus, the animals with higher cortisol levels showed higher binding to D₂/D₃ receptors. Synapse 65:84–89, 2011. © 2010 Wiley‐Liss, Inc.     

Comparative effects of methamphetamine and nicotine on the striatal [(11)C]raclopride binding in unanesthetized monkeys

Although a very large literature exists on the in vitro, ex vivo, and in vivo effects of nicotine on dopamine release in rodents, similar data in primates are scant. This study was initiated to compare methamphetamine to nicotine given i.v. to normal unanesthetized monkeys using positron emission tomography (PET) techniques. Release of dopamine in the striatum using [¹¹C]raclopride was determined indirectly in four nicotine‐naïve adult Macaca mulatta monkeys under conscious and isoflurane‐anesthetized conditions using high‐resolution PET. [¹¹C]Raclopride was given i.v. as a bolus injection followed by continuous infusion with steady state over 30–45 min. Nicotine bitartrate was then given as a bolus plus infusion for 30 min in doses of 32 μg/kg + 0.8 μg/kg/min or 100 μg/kg + 2.53 μg/kg/min as base. The larger doses of nicotine caused significant cardiovascular effects; these doses did not displace [¹¹C]raclopride binding in either dorsal or ventral striatum under the anesthetized conscious condition. In contrast, isoflurane‐anesthesia induced a slight but significant dose‐dependent reduction of [¹¹C]raclopride binding by nicotine even at the same doses used in the anesthetized condition. Methamphetamine in bolus doses of 0.1, 0.3, and 1.0 mg/kg i.v. under conscious condition caused a significant displacement of [¹¹C]raclopride and isoflurane‐anesthesia facilitated the displacement induced by nicotine. These results indicate that nicotine, in high tobacco‐smoking‐related doses, does not release sufficient brain dopamine to displace [¹¹C]raclopride in the striatum in the awake and fully conscious state, in contrast to small doses of methamphetamine. Synapse 45:207–212, 2002. © 2002 Wiley‐Liss, Inc.     

11C]-NS 4194 versus [11C]-DASB for PET imaging of serotonin transporters in living porcine brain

In vitro, the novel diazabicyclononane NS 4194 has several thousand-fold selectivity for blocking the transport into rat brain synaptosomes of [(3)H]-serotonin in comparison to [(3)H]-dopamine or [(3)H]-noradrenaline. We have prepared [(11)C]-NS 4194 in order to test its properties for PET imaging of brain serotonin transporters in comparison with the well-documented tracer [(11)C]-DASB. Both compounds had rapid clearance from blood to brain of living pigs. The apparent equilibrium distribution volumes in cerebellum were 35 ml g(-1) for [(11)C]-NS 4194 and 11 ml g(-1) for [(11)C]-DASB. Pretreatment of pigs with citalopram did not reduce the uptake of either tracer in cerebellum, validating the use of that tissue as a nonbinding reference tissue for kinetic analysis of specific binding. The binding potential (pB) calculated for [(11)C]-NS 4194 using arterial input models was close to 0.5 in the telencephalon, and was 60% displaced by citalopram. However, the reference tissue method of Lammertsma was unsuited to calculate pB for this tracer, apparently due to its excessive nonspecific binding. In contrast to the relatively homogeneous binding of [(11)C]-NS 4194, the pB of [(11)C]-DASB ranged from 0.6 in frontal cortex to 2 in the mesencephalon when calculated by the method of Lammertsma. Parametric maps of the pB of [(11)C]-DASB showed a pattern consistent with the known distribution of serotonin transporters in pig brain in vitro, and there was a uniform displacement of 80% of the specific binding after citalopram treatment in vivo. In conclusion, [(11)C]-DASB is in several respects superior to [(11)C]-NS 4194 for the detection of serotonin uptake sites by PET.     

Difference in in vivo receptor binding between [3 H]N-methylspiperone and [3 H]raclopride in reserpine-treated mouse brain

Summary The in vivo binding of [³ H]N-methylspiperone (NMSP) and [³ H]raclopride was compared in mice treated with reserpine (5 mg/kg, 24 hr prior to the tracer injection). With both radioligands, selective accumulation of radioactivity in the striatum following intravenous injection was observed, whereas a relatively low accumulation and a rapid decline in radioactivity in the cerebellum was seen. Reserpine significantly decreased [³ H]NMSP binding in vivo, however it increased [³ H]raclopride binding. By compartment model analysis, it was found that the decrease in [³ H]NMSP binding was primarily due to the decrease in the association rate (K3) and the increase in [³ H]raclopride was due to the decrease in the dissociation rate (K4) in vivo. As both Kd and Bmax of dopamine D2 receptors have been reported to be unaltered by reserpine, these results suggested that some unknown factors except Kd and Bmax which influence on in vivo binding of receptors might be changed by reserpine. These results revealed that it is of importance to measure kinetics of ligand-receptor binding in vivo rather than static analysis. These two different types of radioligands can be combined to reveal functional roles of dopamine receptor in vivo, especially in the study of the human brain with positron emission tomography (PET).     

Time-course of change in [11C]carfentanil and [11C]raclopride binding potential after a nonpharmacological challenge

Positron Emission Tomography (PET) with appropriate radiotracers and quantification methods allows the detection of changes in endogenous neurotransmission by determine the reduction in the binding potential (BP) of receptors before and after experimental challenges. These have typically employed psychostimulants and PET with dopamine (DA) receptor radiotracers. However, reductions in BP persist far beyond the increases in the release of the endogenous neurotransmitter, an effect ascribed to receptor internalization and recycling, a possible confound in repeated studies. Here we examined the time-course of changes in BP during a nonpharmacological challenge, moderate levels of sustained pain, shown to induce robust reductions in micro-opioid and DA D2 BP, as measured with [(11)C]carfentanil and [(11)C]raclopride. It was hypothesized that, contrary to pharmacological probes, the use of a more "physiological" stimulus would not be associated with persistent changes in the BP measures. The pain challenge was associated with reductions in micro-opioid receptor BP in several cortical and subcortical regions. These did not persist in a subsequent scan. Similar results were obtained for DA D2 receptor BP, where the pain challenge induced significant reductions in the caudate nucleus. These data demonstrate that changes in receptor BP induced by a nonpharmacological challenge did not persist into subsequent scans. They further suggest differences in the effect of pharmacological and nonpharmacological probes on PET BP measures. These may reflect varying levels of change in receptor affinity, receptor internalization, and recycling depending on the type of challenge employed.     

Reproducibility of striatal and thalamic dopamine D2 receptor binding using [11C]raclopride with high-resolution positron emission tomography

Positron emission tomography (PET) imaging of small striatal brain structures such as the ventral striatum (VST) has been hampered by low spatial resolution causing partial-volume effects. The high-resolution research tomograph (HRRT) is a brain-dedicated PET scanner that has considerably better spatial resolution than its predecessors. However, its superior spatial resolution is associated with a lower signal-to-noise ratio. We evaluated the test–retest reliability of the striatal and thalamic dopamine D₂ receptor binding using the HRRT scanner. Seven healthy male volunteers underwent two [¹¹C]raclopride PET scans with a 2.5-hour interval. Dopamine D₂ receptor availability was quantified as binding potential (BP_ND) using the simplified reference tissue model. To evaluate the reproducibility of repeated BP_ND estimations, absolute variability (VAR) and intraclass correlation coefficients (ICCs) were calculated. VAR values indicated fairly good reproducibility and were 3.6% to 4.5% for the caudate nucleus and putamen and 4.5% to 6.4% for the lateral and medial part of the thalamus. In the VST, the VAR value was 5.8% when the definition was made in the coronal plane. However, the ICC values were only moderate, in the range of 0.34 to 0.66, for all regions except the putamen (0.87). Experimental signal processing methods improved neither ICC nor VAR values significantly.     

Effects of single and repeated administration of 1,2,3,4-tetrahydroisoquinoline analogs on the binding of [11C]raclopride to dopamine D2 receptors in the mouse brain

Summary. We investigated the effects of intraperitoneal injection of 1,2,3,4-tetrahydroisoquinoline (TIQ) analogs and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on the binding of [¹¹C]raclopride to striatal dopamine D₂ receptors in mice. The binding of [¹¹C]raclopride, but not of [¹¹C]N-methylspiperone or [¹¹C]nemonapride with higher affinity, to the receptors was significantly decreased immediately after TIQ injection. Neither a dopamine transporter blocker induced such effect nor TIQ affected the dopamine transporter-radioligand binding. Among the compounds investigated, including parkinsonism-inducing TIQ and (R/S)-1-benzyl-TIQ, parkinsonism-preventing (R)- and (S)-1-methyl-TIQ, and probable N-methylated metabolites of TIQ and 1-methyl-TIQ, TIQ and (S)-1-methyl-TIQ had the strongest effect on the binding of [¹¹C]raclopride, and N-methylated derivatives showed less of an effect than the respective parent compounds. The decrease in the binding of [¹¹C]raclopride continued for 7 hours and was followed by an increase until 10 days after the single and subchronic administration of TIQ. These findings suggest that TIQ analogs profoundly stimulated dopamine release which resulted in the competitive inhibition of the binding of [¹¹C]raclopride to dopamine D₂ receptors, but did not induce degeneration of the receptors. Received April 11, 2001; accepted June 25, 2001     

Temporal characterisation of amphetamine-induced dopamine release assessed with [11C]raclopride in anaesthetised rodents

Competition between endogenous neurotransmitters and radiolabelled tracers, as measured by positron emission tomography (PET), may provide a measure of endogenous neurotransmitter flux in vivo. For example, carbon-11 labelled raclopride has been effectively used to monitor dopamine release following pharmacological and behavioural manipulations. The current study describes a rodent model of amphetamine-induced [11C]raclopride reduction, which allowed the characterisation of the dose-response and temporal dynamics of this reduction over a 24-h time course. Over the range studied, a monotonic dose-response relationship between amphetamine dose and [11C]raclopride reduction was observed. When compared with previously published microdialysis data, an approximate 16% reduction in [11C]raclopride binding potential was associated with a approximately 25-fold increase in extracellular dopamine. A reduction of 20-30% in raclopride binding was observed 30 min after amphetamine injection (4 mg/kg i.p.). This reduction in [11C]raclopride binding persisted for 4 h but returned to baseline by 8 h. The data suggest a persistent amphetamine-induced raclopride displacement in rodents and reinforce findings from nonhuman primates that a simple competitive occupancy model may not adequately explain the temporal characteristics of the amphetamine-induced decrease in radiotracer binding.     

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 D_2/3 receptors and lower amphetamine‐induced dopamine (DA) release in cocaine abusers relative to healthy controls. A limitation of these studies that were performed with D_2/3 antagonist radiotracers such as [¹¹C]raclopride is the failure to provide information that is specific to D_2/3 receptors configured in a state of high affinity for the agonists (i.e., D_2/3 receptors coupled to G‐proteins, D_{2/3 HIGH}). As the endogenous agonist DA binds with preference to D_{2/3 HIGH} relative to D_{2/3 LOW} receptors (i.e., D_2/3 receptors uncoupled to G‐proteins) it is critical to understand the in vivo status of D_{2/3 HIGH} receptors in cocaine dependence. Thus, we measured the available fraction of D_{2/3 HIGH} receptors in 10 recently abstinent cocaine abusers (CD) and matched healthy controls (HC) with the D_2/3 antagonist and agonist PET radiotracers [¹¹C]raclopride and [¹¹C]NPA. Methods: [¹¹C]raclopride and [¹¹C]NPA binding potential (BP) (BP_ND) in the striatum were measured with kinetic analysis using the arterial input function. The available fraction of D_{2/3 HIGH} receptors, i.e., % R_HIGH available = D_{2/3 HIGH}/(D_{2/3 HIGH} + D_{2/3 LOW}) was then computed as the ratio of [¹¹C]NPA BP_ND/[¹¹C]raclopride BP_ND. Results: No differences in striatal [¹¹C]NPA BP_ND (HC = 1.00 ± 0.17; CD = 0.97 ± 0.17, P = 0.67) or available % R_HIGH (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 D_{2/3 HIGH} binding in the striatum in cocaine dependence. Synapse, 2011. © 2011 Wiley‐Liss, Inc.     

The effect of nicotine on striatal dopamine release in man: A [11C]raclopride PET study

In common with many addictive substances and behaviors nicotine activates the mesolimbic dopaminergic system. Brain microdialysis studies in rodents have consistently shown increases in extrasynaptic DA levels in the striatum after administration of nicotine but PET experiments in primates have given contradicting results. A recent PET study assessing the effect of smoking in humans showed no change in [(11)C]raclopride binding in the brain, but did find that "hedonia" correlated with a reduction in [(11)C]raclopride binding suggesting that DA may mediate the positive reinforcing effects of nicotine. In this experiment we measured the effect of nicotine, administered via a nasal spray, on DA release using [(11)C]raclopride PET, in 10 regular smokers. There was no overall change in [(11)C]raclopride binding after nicotine administration in any of the striatal regions examined. However, the individual change in [(11)C]raclopride binding correlated with change in subjective measures of "amused" and "happiness" in the associative striatum (AST) and sensorimotor striatum (SMST). Nicotine concentration correlated negatively with change in BP in the limbic striatum. Nicotine had significant effects on cardiovascular measures including pulse rate, systolic blood pressure (BPr), and diastolic BPr. Baseline [(11)C]raclopride binding potential (BP) in the AST correlated negatively with the Fagerstr?m score, an index of nicotine dependence. These results support a role for the DA system in nicotine addiction, but reveal a more complex relationship than suggested by studies in animals.     

Within-subject comparison of [11C]-(+)-PHNO and [11C]raclopride sensitivity to acute amphetamine challenge in healthy humans

[¹¹C]PHNO is a D₂/D₃ agonist positron emission tomography radiotracer, with higher in vivo affinity for D₃ than for D₂ receptors. As [¹¹C]-(+)-PHNO is an agonist, its in vivo binding is expected to be more affected by acute fluctuations in synaptic dopamine than that of antagonist radiotracers such as [¹¹C]raclopride. In this study, the authors compared the effects of an oral dose of the dopamine releaser amphetamine (0.3 mg/kg) on in vivo binding of [¹¹C]-(+)-PHNO and [¹¹C]raclopride in healthy subjects, using a within-subjects, counterbalanced, open-label design. In the dorsal striatum, where the density of D₃ receptors is negligible and both tracers predominantly bind to D₂ receptors, the reduction of [¹¹C]-(+)-PHNO binding potential (BP_ND) was 1.5 times larger than that of [¹¹C]raclopride. The gain in sensitivity associated with the agonist [¹¹C]-(+)-PHNO implies that ∼65% of D₂ receptors are in the high-affinity state in vivo. In extrastriatal regions, where [¹¹C]-(+)-PHNO predominantly binds to D₃ receptors, the amphetamine effect on [¹¹C]-(+)-PHNO BP_ND was even larger, consistent with the higher affinity of dopamine for D₃. This study indicates that [¹¹C]-(+)-PHNO is superior to [¹¹C]raclopride for studying acute fluctuations in synaptic dopamine in the human striatum. [¹¹C]-(+)-PHNO also enables measurement of synaptic dopamine in D₃ regions.     

Inhibition of [11C]mirtazapine binding by alpha2-adrenoceptor antagonists studied by positron emission tomography in living porcine brain

We have developed [(11)C]mirtazapine as a ligand for PET studies of antidepressant binding in living brain. However, previous studies have determined neither optimal methods for quantification of [(11)C]mirtazapine binding nor the pharmacological identity of this binding. To obtain that information, we have now mapped the distribution volume (V(d)) of [(11)C]mirtazapine relative to the arterial input in the brain of three pigs, in a baseline condition and after pretreatment with excess cold mirtazapine (3 mg/kg). Baseline V(d) ranged from 6 ml/ml in cerebellum to 18 ml/ml in frontal cortex, with some evidence for a small self-displaceable binding component in the cerebellum. Regional binding potentials (pBs) obtained by a constrained two-compartment model, using the V(d) observation in cerebellum, were consistently higher than pBs obtained by other arterial input or reference tissue methods. We found that adequate quantification of pB was obtained using the simplified reference tissue method. Concomitant PET studies with [(15)O]-water indicated that mirtazapine challenge increased CBF uniformly in cerebellum and other brain regions, supporting the use of this reference tissue for calculation of [(11)C]mirtazapine pB. Displacement by mirtazapine was complete in the cerebral cortex, but only 50% in diencephalon, suggesting the presence of multiple binding sites of differing affinities in that tissue. Competition studies with yohimbine and RX 821002 showed decreases in [(11)C]mirtazapine pB throughout the forebrain; use of the multireceptor version of the Michaelis-Menten equation indicated that 42% of [(11)C]mirtazapine binding in cortical regions is displaceable by yohimbine. Thus, PET studies confirm that [(11)C]mirtazapine affects alpha(2)-adrenoceptor binding sites in living brain.     

Imaging endogenous dopamine competition with [11C]raclopride in the human brain

This study images dopamine release in response to a neurochemically specific challenge with the psychostimulant drug methylphenidate. Changes in synaptic dopamine induced by methylphenidate were evaluated with positron emission tomography and [¹¹C]raclopride, a D₂ receptor radioligand that is sensitive to endogenous dopamine. Methylphenidate significantly decreased striatal [¹¹C]raclopride binding. The decrease was variable and was negatively correlated with age. Mood and anxiety at baseline, were also correlated with methylphenidate-induced DA changes. This strategy provides a tool to investigate the responsiveness of the dopamine system in the normal and diseased human brain and to investigate the neurochemical correlates of behavior. © 1994 Wiley-Liss, Inc.     

Therapeutic effect of repetitive transcranial magnetic stimulation in Parkinson's disease: Analysis of [11C] raclopride PET study

Repetitive transcranial magnetic stimulation (rTMS) has been used as a potential therapeutic tool in Parkinson's disease (PD). However, the therapeutic value and/or the placebo effect of rTMS on PD remain to be elucidated. To investigate the therapeutic value and/or placebo effect of rTMS in PD, we compared the motor section of unified PD rating scale (UPDRS III) and the amount of extracellular dopamine concentration using [¹¹C] raclopride PET before and after two sessions of rTMS in 9 PD patients. During a consecutive 2 days while off‐medication, two series of 15 trains of 5 Hz‐frequency rTMS (intensity, 90% of the resting motor threshold) were applied to the hand area of more severely symptomatic motor cortex (MC). After unilateral rTMS of MC, mean raclopride binding potentials (BPs) were reduced not only in putaminal and caudate areas on the stimulated side (?4.9% and ?6.5%, respectively) (P > 0.05) but also in putaminal and caudate areas of nonstimulated hemispheres (?6.6%, P > 0.05 and ?12.1%, P = 0.049, respectively). UPDRS III scores were significantly decreased (35.0 ± 14.1 to 32.0 ± 13.4, P = 0.049). A reduction of raclopride BP in nonstimulated ventral striatum by unilateral rTMS supports the placebo response during rTMS. © 2007 Movement Disorder Society     

Intravenous ethanol increases dopamine release in the ventral striatum in humans: PET study using bolus-plus-infusion administration of [11C]raclopride

Ethanol increases the interstitial dopamine (DA) concentration in the nucleus accumbens (NAcc) of experimental animals, but positron emission tomography (PET) studies using the single-bolus protocol of the [¹¹C]-raclopride competition paradigm have yielded conflicting results in humans. To resolve disparate previous findings, we utilized the bolus-plus-infusion (B/I) method, allowing both baseline and intervention quantification of [¹¹C]raclopride binding during a single 105-minute PET scan, to investigate possible ethanol-induced DA release in nine healthy male subjects. A 25-minute intravenous ethanol (7.6%) infusion, resulting in a 1.3 g/L mean blood ethanol concentration, was administered using masked timing during the PET scan. Automated region-of-interest analysis testing the difference between baseline (40–50 minutes) and intervention (60–85 minutes) revealed an average 12.6% decrease in [¹¹C]raclopride binding in the ventral striatum (VST, P=0.003) including the NAcc. In addition, a shorter time interval from the start of ethanol infusion to the first subjective effect was associated with a greater binding potential decrease bilaterally in the VST (r=0.92, P=0.004), and the feeling of pleasure was associated with a decrease in binding potential values in both the caudate nucleus (r=−0.87, P=0.003) and putamen (r=−0.74; P=0.02). These results confirm that ethanol induces rapid DA release in the limbic striatum, which can be reliably estimated using the B/I method in one imaging session.