MDMA-evoked changes in [11C]raclopride and [11C]NMSP binding in living pig brain

Dopamine D2/3 Receptor Availabilities in Striatal and Extrastriatal Regions of the Adult Human Brain: Comparison of Four Methods of Analysis

Values of binding potentials (BP_ND) of dopamine D_2/3 receptors differ in different regions of the brain, but we do not know with certainty how much of this difference is due either to different receptor numbers, or to different affinities of tracers to the receptors, or to both. We tested the claim that both striatal and extrastriatal dopamine D_2/3 receptor availabilities vary with age in vivo in humans by determining the values of BP_ND of the specific radioligand [¹¹C]raclopride. We determined values of BP_ND in striatal and extrastriatal volumes-of-interest (VOI) with the same specific receptor radioligand. We estimated values of BP_ND in individual voxels of brains of healthy volunteers in vivo, and we obtained regional averages of VOI by dynamic positron emission tomography (PET). We calculated average values of BP_ND in caudate nucleus and putamen of striatum, and in frontal, occipital, parietal, and temporal cortices of the forebrain, by means of four methods, including the ERLiBiRD (Estimation of Reversible Ligand Binding and Receptor Density) method, the tissue reference methods of Logan and Logan-Ichise, respectively, and the SRTM (Simplified Reference Tissue Method). Voxelwise generation of parametric maps of values of BP_ND used the multi-linear regression version of SRTM. Age-dependent changes of the binding potential presented with an inverted U-shape with peak binding potentials reached between the ages of 20 and 30. The estimates of BP_ND declined significantly with age after the peak in both striatal and extrastriatal regions, as determined by all four methods, with the greatest decline observed in posterior (occipital and parietal) cortices (14% per decade) and the lowest decline in caudate nucleus (3% per decade). The sites of the greatest declines are of particular interest because of the clinical implications.

Recent progress of imaging agents for Parkinson's disease

Parkinson's disease (PD) is a common progressive, neurodegenerative brain disease that is promoted by mitochondrial dysfunction, oxidative stress, protein aggregation and proteasome dysfunction in the brain. Compared with computer tomography (CT) or magnetic resonance imaging (MRI), non-invasive nuclear radiopharmaceuticals have great significance for the early diagnosis of PD due to their high sensitivity and specificity in atypical and preclinical cases. Based on the development of coordination chemistry and chelator design, radionuclides may be delivered to lesions by attaching to PD-related transporters and receptors, such as dopamine, serotonin, and others. In this review, we comprehensively detailed the current achievements in radionuclide imaging in Parkinson's disease.

The domestic piglet: an important model for investigating the neurodevelopmental consequences of early life insults

Insults in the prenatal and early postnatal period increase the risk for behavioral problems later in life. One hypothesis is that pre- and postnatal stressors influence structural and functional brain plasticity. Understanding the mechanisms is important, but progress has lagged because certain studies in human infants are impossible, while others are extremely difficult. Furthermore, results from popular rodent models are difficult to translate to human infants owing to the substantial differences in brain development and morphology. Because it overcomes some of these obstacles, the domestic piglet has emerged as an important model. Piglets have a gyrencephalic brain that develops similar to the human brain and that can be assessed in vivo by using clinical-grade neuroimaging instruments. Furthermore, owing to their precocial nature, piglets can be weaned at birth and used in behavioral testing paradigms to assess cognitive behavior at an early age. Thus, the domestic piglet represents an important translational model for investigating the neurodevelopmental consequences of early life insults.

Effects of anesthesia and species on the uptake or binding of radioligands in vivo in the Göttingen minipig

Progress in neuroscience research often involves animals, as no adequate alternatives exist to animal models of living systems. However, both the physiological characteristics of the species used and the effects of anesthesia raise questions of common concern. Here, we demonstrate the confounding influences of these effects on tracer binding in positron emission tomography (PET). We determined the effects of two routinely used anesthetics (isoflurane and propofol) on the binding of two tracers of monoamine function, [¹¹C]SCH23390, a tracer of the dopamine D1 and D5 receptors, and the alpha2-adrenoceptor antagonist, [¹¹C]yohimbine, in Göttingen minipigs. The kinetics of SCH23390 in the pigs differed from those of our earlier studies in primates. With two different graphical analyses of uptake of SCH23390, the initial clearance values of this tracer were higher with isoflurane than with propofol anesthesia, indicative of differences in blood flow, whereas no significant differences were observed for the volumes of distribution of yohimbine. The study underscores the importance of differences of anesthesia and species when the properties of radioligands are evaluated under different circumstances that may affect blood flow and tracer uptake. These differences must be considered in the choice of a particular animal species and mode of anesthesia for a particular application.

Surrogate markers for cerebral blood flow correlate with [¹⁸F]-fallypride binding potential at dopamine D(2/3) receptors in human striatum

Positron emission tomography (PET) with the high affinity dopamine D(2/3) receptor ligand [¹⁸F]-fallypride affords estimates of the binding potential (BP(ND) ) in extra-striatal regions of low receptor abundance, but the sufficient recording time for accurate measurements in striatum has been called into question. We have earlier argued that transient equilibrium measurements are obtained in striatum with [¹⁸F]-fallypride PET recordings of 3 h duration, which may be the practical limit for clinical investigations without interrupted scanning. However, the high extraction fraction of [¹⁸F]-fallypride predicts flow-dependence of tracer delivery to brain, which may be a source of variance of the apparent BP(ND) in regions of high binding. To test this prediction, we conducted a retrospective analysis of [¹⁸F]-fallypride PET data from a group of 50 healthy volunteers (age 18-58 years [mean ± SD: 32.6 ± 10.6), who had participated in clinical studies without arterial input measurements. We used the initial 120-s integral (AUC) of the venous confluence (VC) as a surrogate marker for cerebral blood flow (CBF) and tested for correlations between regional estimates of BP(ND) calculated by the simplified reference tissue model (SRTM) and the individual VC-AUC. The magnitude of BP(ND) in a high binding region (putamen), but not in a low binding region (thalamus) correlated positively with VC-AUC, suggesting that approximately 9% of the variance in the [¹⁸F]-fallypride BP(ND) in putamen can be attributed to individual differences in this surrogate marker for CBF, a contribution equal in magnitude to the effects of age on BP(ND) in putamen of the present healthy control group.

Simultaneous polysubstance use among Danish 3,4-methylenedioxymethamphetamine and hallucinogen users: combination patterns and proposed biological bases

OBJECTIVE

To describe patterns of simultaneous polysubstance use (SPU) among Danish 3,4-methylenedioxymethamphetamine (MDMA) ("Ecstasy") and hallucinogen users.

METHODS

A cross-sectional survey of 98 active MDMA and/or hallucinogen users recruited through homepage advertisements, flyers, and word of mouth in Denmark. Lifetime and recent substance use and SPU at last recalled use was described by structured interviews. Hair samples from a subset of participants were analyzed for MDMA.

RESULTS

The participants had used an average of 12.6 (95% confidence interval: 11.7-13.4) psychoactive substances during their lifetime. SPU was prevalent among MDMA, d-lysergic acid diethylamide (LSD), and psilocybin users, in particular with alcohol and cannabis. Among MDMA users, 69% had combined MDMA with amphetamines, 56% with hallucinogens, and 47% with cocaine. At last recalled use, MDMA was taken with 2.1 ± 1.2 substances in 32 different combinations. The participants preferred specific drug combinations and named several, which in their experience enhanced or counteracted each other. Alcohol and cannabis were typically used before, during, and after MDMA, LSD, and psilocybin, whereas amphetamines were predominantly taken before these substances. When LSD was combined with MDMA, the majority took MDMA after LSD.

CONCLUSIONS

Simultaneous polysubstance use was common among Danish MDMA and hallucinogen users, and patterns of preferred substance combinations were evident.

What have positron emission tomography and 'Zippy' told us about the neuropharmacology of drug addiction?

Translational molecular imaging with positron emission tomography (PET) and allied technologies offer unrivalled applications in the discovery of biomarkers and aetiological mechanisms relevant to human disease. Foremost among clinical PET findings during the past two decades of addiction research is the seminal discovery of reduced dopamine D(2/3) receptor expression in the striatum of drug addicts, which could indicate a predisposing factor and/or compensatory reaction to the chronic abuse of stimulant drugs. In parallel, recent years have witnessed significant improvements in the performance of small animal tomographs (microPET) and a refinement of animal models of addiction based on clinically relevant diagnostic criteria. This review surveys the utility of PET in the elucidation of neuropharmacological mechanisms underlying drug addiction. It considers the consequences of chronic drug exposure on regional brain metabolism and neurotransmitter function and identifies those areas where further research is needed, especially concerning the implementation of PET tracers targeting neurotransmitter systems other than dopamine, which increasingly have been implicated in the pathophysiology of drug addiction. In addition, this review considers the causal effects of behavioural traits such as impulsivity and novelty/sensation-seeking on the emergence of compulsive drug-taking. Previous research indicates that spontaneously high-impulsive rats--as exemplified by 'Zippy'--are pre-disposed to escalate intravenous cocaine self-administration, and subsequently to develop compulsive drug taking tendencies that endure despite concurrent adverse consequences of such behaviour, just as in human addiction. The discovery using microPET of pre-existing differences in dopamine D(2/3) receptor expression in the striatum of high-impulsive rats suggests a neural endophenotype that may likewise pre-dispose to stimulant addiction in humans.

Lateralization and gender differences in the dopaminergic response to unpredictable reward in the human ventral striatum

Electrophysiological studies have shown that mesostriatal dopamine (DA) neurons increase activity in response to unpredicted rewards. With respect to other functions of the mesostriatal dopaminergic system, dopamine's actions show prominent laterality effects. Whether changes in DA transmission elicited by rewards also are lateralized, however, has not been investigated. Using [¹¹C]raclopride-PET to assess the striatal DA response to unpredictable monetary rewards, we hypothesized that such rewards would induce an asymmetric reduction in [¹¹C]raclopride binding in the ventral striatum, reflecting lateralization of endogenous dopamine release. In 24 healthy volunteers, differences in the regional D₂/₃ receptor binding potential (ΔBP) between an unpredictable reward condition and a sensorimotor control condition were measured using the bolus-plus-constant-infusion [¹¹C]raclopride method. During the reward condition subjects randomly received monetary awards while performing a 'slot-machine' task. The ΔBP between conditions was assessed in striatal regions-of-interest and compared between left and right sides. We found a significant condition × lateralization interaction in the ventral striatum. A significant reduction in binding potential (BP(ND) ) in the reward condition vs. the control condition was found only in the right ventral striatum, and the ΔBP was greater in the right than the left ventral striatum. Unexpectedly, these laterality effects appeared to be partly accounted for by gender differences, as our data showed a significant bilateral BP(ND) reduction in women while in men the reduction reached significance only in the right ventral striatum. These data suggest that DA release in response to unpredictable reward is lateralized in the human ventral striatum, particularly in males.

Selective serotonin reuptake inhibitor (SSRI) modulation of striatal dopamine measured with [11C]-raclopride and positron emission tomography

The effect of a pharmacologic increase in serotonin concentrations on striatal dopamine (D2) receptor availability has been measured in several studies using positron emission tomography (PET) and the radiotracer [11C]-raclopride as a method for the in vivo imaging of serotonin modulation of striatal dopamine in human subjects. These studies have shown that an acute increase in serotonin concentrations produced a decrease in striatal D2 receptor availability. The current study was undertaken to measure the effects of a more pharmacologically selective serotonergic agent compared to previous studies, the serotonin reuptake inhibitor, citalopram, on striatal D2 receptor availability. Twelve healthy control subjects underwent two PET scans performed on the same day following i.v. administration of saline (Scan 1) and citalopram (Scan 2, 40 mg, i.v.). The [11C]-raclopride data were analyzed with a graphical analysis method using the cerebellum as the input function. Plasma levels of citalopram, cortisol, and prolactin were measured. The citalopram concentrations peaked at the end of infusion (EOI) and remained relatively consistent from 30 min to 3 h postinfusion. An increase in cortisol and prolactin concentrations was observed from the EOI until 60 min after the EOI. A significant decrease in striatal D2 receptor availability was observed after citalopram infusion (-5%), presumably due to an increase in endogenous dopamine concentrations. In summary, i.v. administration of the selective serotonin reuptake inhibitor, citalopram, produced modest reductions in striatal D2 receptor availability, consistent with other human [11C]-raclopride studies using less pharmacologically selective serotonergic agents.

Differences in aggressive behavior and in the mesocorticolimbic DA system between A/J and BALB/cJ mice

The neurotransmitters DA and serotonin are known to be important modulators of aggression, but endogenous differences in these systems between aggressive and nonaggressive animals are poorly understood. To examine this issue, the mesocorticolimbic DA and serotonin systems of two mouse strains that differ in aggressive behavior, BALB/cJ and A/J, were analyzed using high performance liquid chromatography and quantitative receptor autoradiography. Significant differences in both serotonergic and dopaminergic systems were found between aggressive and nonaggressive mice. The nonaggressive A/J mice exhibited higher DA utilization in the nucleus accumbens and prefrontal cortex, higher D1 receptor expression in the rostral pole of the accumbens, and lower D2 receptor expression throughout the accumbens, as compared with aggressive BALB/cJ mice. Although correlative in nature, these data suggest that differences in mesocorticolimbic DA and serotonin systems may contribute to endogenous differences in aggression.

Parametric and regional maps of free serotonin 5HT1A receptor sites in human brain as function of age in healthy humans

Serotonin 5HT(1A)-binding sites can be detected in living human brain with the positron-emitting antagonist [(11)C]WAY-100635. Previous measurements of the availability of [(11)C]WAY-binding sites in normal aging are equivocal, in part because of the greatly variable binding of this ligand. To test the null hypothesis that the binding potential (pB) of 5HT(1A) sites remains constant with age; 19 healthy volunteers aged 23-73 years (8 women, 11 men) underwent positron emission tomography. To determine pBs, we applied a novel tissue reference method of analysis, Estimation of Reversible Ligand Binding and Receptor Density (ERLiBIiRD) (Gjedde, 2003; Rosa-Neto et al, 2004), which extrapolates measures of specific binding to an estimated steady-state. We compared these estimates in the two age groups with results obtained with the conventional Logan Plot and Simplified Reference Tissue Method (SRTM) applied to both regions of interest-based as parametric analyses. The regional distribution of specific binding of free sites [(11)C]WAY-100635 was similar to that reported in previous studies, with the highest pBs in limbic structures and the raphé nuclei. Although the results of the three methods differed, pBs in the elderly subjects consistently were lower than those of young subjects. Thus, the correlation between pB and age applied to regions-of-interest revealed significant decline of pB at the rate of 3 or 4% per decade, and a 10% decline of the global mean 5HT(1A) receptor-pB in elderly relative to young subjects. The results demonstrate that the number of available 5HT(1A)-binding sites declines with age.

Serotonin 5HT1A receptor availability and pathological crying after stroke

OBJECTIVES

Post-stroke depression and pathological crying (PC) implicate an imbalance of serotonergic neurotransmission. We claim that PC follows serotonin depletion that raises the binding potential (p(B)) of the 5-HT(1A) receptor antagonist [carbonyl-(11)C]WAY-100635, which is reversible by selective serotonin re-uptake inhibitor (SSRI) treatment.

MATERIALS AND METHODS

We PET scanned patients with acute stroke and PC and age-matched control subjects. Maps of receptor availability were generated from the images of eight cortical regions and raphe nuclei.

RESULTS

The maps showed highest binding in limbic areas and raphe nuclei, while binding in basal ganglia and cerebellum was negligible. Baseline binding potentials of patients were lower than that of control subjects (3.7 +/- 0.6 vs 4.2 +/- 0.2). Treatment with SSRI markedly reduced free receptor sites, whereas placebo administration led to a global increase.

DISCUSSION

The study is the first suggestion of changes of serotonergic neurotransmission in the early phase of stroke and the modulation of these changes with SSRI treatment.

Mapping the amphetamine-evoked changes in [11C]raclopride binding in living rat using small animal PET: Modulation by MAO-inhibition

The performance of small animal PET for neuroreceptor studies in a psychopharmacological challenge paradigm is not yet well-described. Therefore, we used microPET and [(11)C]raclopride to map the availability of dopamine D(2/3) receptors in brain of anesthetized rats, first in a baseline condition, and again after challenge with saline or d-amphetamine. Parametric maps of the specific binding (binding potential, pB) were calculated using a reference tissue input from cerebellum, and spatially normalized to a digitized stereotaxic coordinate system for rat brain. In volumes of interest (VOIs), the mean baseline pB (n=6) was 2.05 in dorsal striatum (caudate-putamen), and 1.34 in ventral striatum (nucleus accumbens), and did not significantly differ upon retest 2 h later. The availability of [(11)C]raclopride binding sites at baseline was 8% higher in the right striatum. Challenge with amphetamine sulfate (1 mg/kg, i.v., n=4) decreased pB by 19% in both ventral and dorsal striatum. We have earlier predicted that blockade of monoamine oxidase (MAO) should potentiate the amphetamine-evoked dopamine release, thus enhancing the displacement of [(11)C]raclopride binding in vivo. However, pretreatment of rats with pargyline hydrochloride (4 mg/kg, n=4; 20 mg/kg, n=4) 1 day prior to PET did not potentiate the amphetamine-evoked reduction in dopamine receptor availability within the extended striatum. We conclude that small animal PET can be used to investigate stimulant-induced dopamine release, but that the spatial resolution is insufficient to detect differences between relative changes in dorsal vs. ventral divisions of the rat striatum. Furthermore, the present results do not reveal potentiation of the amphetamine-evoked release of dopamine in rats with MAO inhibition.

A PET study of effects of chronic 3,4-methylenedioxymethamphetamine (MDMA, “ecstasy”) on serotonin markers in Göttingen minipig brain

The psychostimulant 3,4-methylendioxymethamphetamine (MDMA, "ecstasy") evokes degeneration of telencephalic serotonin innervations in rodents, nonhuman primates, and human recreational drug users. However, there has been no alternative to nonhuman primates for studies of the cognitive and neurochemical consequences of serotonin depletion in a large-bodied animal. Therefore, we used positron emission tomography (PET) with [(11)C]DASB to map the distribution of plasma membrane serotonin transporters in brain of Göttingen minipigs, first in a baseline condition, and again at 2 weeks after treatment with MDMA (i.m.), administered at a range of doses. In parallel PET studies, [(11)C]WAY-100635 was used to map the distribution of serotonin 5HT(1A) receptors. The acute MDMA treatment in awake pigs evoked 1 degrees C of hyperthermia. MDMA at total doses greater than 20 mg/kg administered over 2-4 days reduced the binding potential (pB) of [(11)C]DASB for serotonin transporters in porcine brain. A mean total dose of 42 mg/kg MDMA in four animals evoked a mean 32% decrease in [(11)C]DASB pB in mesencephalon and diencephalon, and a mean 53% decrease in telencephalic structures. However, this depletion of serotonin innervations was not associated with consistent alterations in the binding of [(11)C]WAY-100635 to serotonin 5HT(1A) receptors. Stereological cell counting of serotonin-positive neurons, which numbered 95,000 in the dorsal raphé nucleus of normal animals, was unaffected in MDMA-treated group. group.

The use of pigs in neuroscience: Modeling brain disorders

The use of pigs in neuroscience research has increased in the past decade, which has seen broader recognition of the potential of pigs as an animal for experimental modeling of human brain disorders. The volume of available background data concerning pig brain anatomy and neurochemistry has increased considerably in recent years. The pig brain, which is gyrencephalic, resembles the human brain more in anatomy, growth and development than do the brains of commonly used small laboratory animals. The size of the pig brain permits the identification of cortical and subcortical structures by imaging techniques. Furthermore, the pig is an increasingly popular laboratory animal for transgenic manipulations of neural genes. The present paper focuses on evaluating the potential for modeling symptoms, phenomena or constructs of human brain diseases in pigs, the neuropsychiatric disorders in particular. Important practical and ethical aspects of the use of pigs as an experimental animal as pertaining to relevant in vivo experimental brain techniques are reviewed. Finally, current knowledge of aspects of behavioral processes including learning and memory are reviewed so as to complete the summary of the status of pigs as a species suitable for experimental models of diverse human brain disorders.

Statistical mapping of effects of middle cerebral artery occlusion (MCAO) on blood flow and oxygen consumption in porcine brain

The volume of cerebral tissue perturbed in experimental models of middle cerebral artery occlusion (MCAO) can be highly variable. Thus, the territories of reduced cerebral blood flow (CBF) or oxygen consumption (CMRO(2)) following MCAO might properly be defined using statistical parametric mapping within a population. In order to establish such a method, we mapped CBF and CMRO(2) in 18 pigs with acute MCAO. Parametric maps were flipped about the axis of symmetry, and CBF and CMRO(2) in the infarcted hemisphere were calculated as percentages of the magnitudes in mirror-image pixels. There were log-linear relationships between the volumes of affected tissue and the percentages of normal CFB or CMRO(2). This graphical analysis showed that the volume of the core deficit was smaller for CBF that for CMRO(2), but expanded more rapidly with decreasing CBF deficit than did the corresponding volumes of reduced CMRO(2). Thus, acute changes in CBF and CMRO(2) following MCAO in the pig can be defined as probabilistic volumes.

Synthesis, radiolabeling and in vivo evaluation of [11C]RAL-01, a potential phosphodiesterase 5 radioligand

Very few tracers are available for imaging studies of second messenger systems. We developed a radiosynthesis for the phosphodiesterase (PDE) 5 inhibitor [(11)C]RAL-01. Whole body distribution studies using positron emission tomography (PET) revealed a time-dependant passage through the liver and accumulation of radioactivity in the bile of the Landrace pig. Displaceable binding was readily discerned in the myocardium, and traces of binding were seen in pulmonary tissue, consistent with the use of this class of drug in the treatment of pulmonary hypertension and heart failure. [(11)C]RAL-01 readily entered the brain and obtained an equilibrium distribution volume of 4-5 ml g(-1). Mean parametric images suggested the presence of a small displaceable binding component, but this binding was not significant in the present group of three pigs. Thus, [(11)C]RAL-01 shows considerable promise for PET studies of biliary elimination and of PDE5 binding in the cardiovascular system. However, analogues of higher affinity may be required for investigations of central nervous system binding sites.

Distribution of PK11195 binding sites in porcine brain studied by autoradiography in vitro and by positron emission tomography

The cerebral distribution of peripheral-type benzodiazepine binding sites (PBBS) in human brain has been investigated by positron emission tomography (PET) with the specific radioligand [11C]PK11195 in diverse neuropathological conditions. However, little is known about the pattern of PK11195 binding sites in healthy brain. Therefore, we used quantitative autoradiography to measure the saturation binding parameters for [3H]PK11195 in cryostat sections from young Landrace pigs. Specific binding was lowest in the cerebellar white matter (85 fmol mg(-1)) and highest in the caudate nucleus (370 fmol mg(-1)), superior colliculus (400 fmol mg(-1)), and anterior thalamic nucleus (588 fmol mg(-1)). The apparent affinity was in the range of 2-6 nM in vitro, predicting high specific binding in PET studies of living brain. However, the distribution volume (V(d), ml g(-1)) of high specific activity [11C]PK11195 was nearly homogeneous (3 ml g(-1)) throughout brain of healthy Landrace pigs, and was nearly identical in studies with lower specific activity, suggesting that factors in vivo disfavor the detection of PBBS in Landrace pigs with this radioligand. In young, adult Göttingen minipig brain, the magnitude of V(d) for [11C]PK11195 was in the range 5-10 ml g(-1), and had a heterogeneous distribution resembling the in vitro findings in Landrace pigs. There was a trend toward globally increased V(d) in a group of minipigs with acute MPTP-induced parkinsonism, but no increase in V(d) was evident in the same pigs rescanned at 2 weeks after grafting of fetal mesencephalon to the partially denervated striatum. Thus, [11C]PK11195 binding was not highly sensitive to constituitively expressed PBBS in brain of young Landrace pigs, and did not clearly demonstrate the expected microglial activation in the MPTP/xenograft model of minipigs.

Quantitative autoradiography of ligands for dopamine receptors and transporters in brain of Göttingen minipig: comparison with results in vivo

The pig has been used as animal model for positron emission tomography (PET) studies of dopamine (DA) receptors and pharmacological perturbations of DA neurotransmission. However, the binding properties of DA receptors and transporters in pig brain have not been characterized in vitro. Therefore, the saturation binding parameters of [3H]SCH 23390 for DA D1 receptors and [3H]raclopride for DA D2/3 receptors were measured by quantitative autoradiography in cryostat sections from brain of groups of 8 week old and adult female Göttingen minipigs. The magnitudes of Bmax and Kd for these ligands were similar in young and old pigs, and were close to those reported for rat and human brain. Furthermore, gradients in the concentrations of D1 and D2/3 sites in striatum measured in vitro agreed with earlier findings in PET studies. However, the dopamine transporter (DAT) ligand [3H]GBR12935 did not bind in pig brain cryostat sections. Whereas the tropane derivative [125I]RTI-55 labeled serotonin transporters (serotonin transporter (SERT)) in pig brain, use of the same ligand under conditions specific for DAT, revealed a pattern of binding similar to that observed for SERT conditions. Parallel studies revealed the presence of DAT in rat and ferret brain. The distribution volume (Vd) of the selective DAT ligand [11C]NS2214 ([11C]Brasofensine) was mapped in groups of normal and MPTP-lesioned Göttingen miniature pigs. The in vivo pattern of Vd matched the distribution of SERT in vitro, and did not differ between the normal pigs and the lesioned animals with documented 60% DA depletions. However, the pattern of specific binding of the selective noradrenaline transporter ligand (S,S)-[11C]MeNER in a single Landrace pig showed that, of the three monoamine transporters, only DAT could not be detected in pig brain. We conclude that the pig is a suitable model for PET studies of DA D1 and D2/3 binding sites, which are fully developed on the eighth postnatal week. However, well-characterized piperazine and tropane radioligands failed to recognize DAT in pig brain; the two tropane radioligands lacked pharmacological specificity for DAT and SERT in pig brain in vitro and in vivo.

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 Göttingen 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.

Pig brain stereotaxic standard space: Mapping of cerebral blood flow normative values and effect of MPTP-lesioning

The analysis of physiological processes in brain by position emission tomography (PET) is facilitated when images are spatially normalized to a standard coordinate system. Thus, PET activation studies of human brain frequently employ the common stereotaxic coordinates of Talairach. We have developed an analogous stereotaxic coordinate system for the brain of the Gottingen miniature pig, based on automatic co-registration of magnetic resonance (MR) images obtained in 22 male pigs. The origin of the pig brain stereotaxic space (0, 0, 0) was arbitrarily placed in the centroid of the pineal gland as identified on the average MRI template. The orthogonal planes were imposed using the line between stereotaxic zero and the optic chiasm. A series of mean MR images in the coronal, sagittal and horizontal planes were generated. To test the utility of the common coordinate system for functional imaging studies of minipig brain, we calculated cerebral blood flow (CBF) maps from normal minipigs and from minipigs with a syndrome of parkisonism induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-poisoning. These maps were transformed from the native space into the common stereotaxic space. After global normalization of these maps, an undirected search for differences between the groups was then performed using statistical parametric mapping. Using this method, we detected a statistically significant focal increase in CBF in the left cerebellum of the MPTP-lesioned group. We expect the present approach to be of general use in the statistical parametric mapping of CBF and other physiological parameters in living pig brain.

Mapping the amphetamine-evoked dopamine release in the brain of the Göttingen minipig

The availability of dopamine D(2/3) binding sites in brain of six male and six female Göttingen minipigs was measured in a baseline condition and after challenge with amphetamine sulfate (1mg/kg, i.v.) in PET studies with [(11)C]raclopride. Maps of the binding potential (pB; B(max)/K(d)) of [(11)C]raclopride were spatially normalized and co-registered to a common stereotaxic coordinate system for pig brain. The pB maps were then analyzed by volume of interest and voxel-wise comparisons of gender and condition. The mean baseline pB tended to be 10-20% higher in striatum of the female group, but this gender difference was not significant. Variance of the mean baseline pB was higher in the males (44%) than in females (30%), but there was no correlation between pB and individual plasma cortisol or testosterone concentrations. Using statistical parametric mapping, we detected a focus in the right posterior putamen where the magnitude of the amphetamine-evoked decrease in pB was greater in the male than in the female group. Thus, the spatial pattern of reactivity of dopamine D(2/3) receptor availability to amphetamine challenge is not identical in male and female pigs. Within the entire population, the decline in pB evoked by amphetamine (Delta pB) was greater in the ventral striatum (-28%) than in the caudate nucleus (-17%), consistent with earlier reports in monkeys and humans. The magnitude of Delta pB correlated highly with the baseline pB values in all divisions of the striatum. Based upon the principles of competitive binding, the slope of this empirical relationship, f(i), is equal to the fraction of [(11)C]raclopride binding sites sensitive to endogenous dopamine; the magnitude of this fraction ranged from 0.29 in the caudate to 0.36 in the ventral striatum.

Interaction between LSD and dopamine D2/3 binding sites in pig brain

The psychoactive properties of the hallucinogen LSD have frequently been attributed to high affinity interactions with serotonin 5HT2 receptors in brain. Possible effects of LSD on dopamine D2/3 receptor availability have not previously been investigated in living brain. Therefore, we used PET to map the binding potential (pB) of [11C]raclopride in brain of three pigs, first in a baseline condition, and again at 1 and 4 h after administration of LSD (2.5 microg/kg, i.v.). There was a progressive treatment effect in striatum, where the pB was significantly reduced by 19% at 4 h after LSD administration. Concomitant maps of cerebral blood flow did not reveal significant changes in perfusion during this interval. Subsequent in vitro studies showed that LSD displaced [3H]raclopride (2 nM) from pig brain cryostat sections with an IC50 of 275 nM according to a one-site model. Fitting of a two-site model to the data suggested the presence of a component of the displacement curves with a subnanomolar IC50, comprising 20% of the total [3H]raclopride binding. In microdialysis experiments, LSD at similar and higher doses did not evoke changes in the interstitial concentration of dopamine or its acidic metabolites in rat striatum. Together, these results are consistent with a direct interaction between LSD and a portion of dopamine D2/3 receptors in pig brain, possibly contributing to the psychopharmacology of LSD.