Gradients of dopamine D1- and D2/3-binding sites in the basal ganglia of pig and monkey measured by PET

Canonical and Non-Canonical Antipsychotics' Dopamine-Related Mechanisms of Present and Next Generation Molecules: A Systematic Review on Translational Highlights for Treatment Response and Treatment-Resistant Schizophrenia

Schizophrenia is a severe psychiatric illness affecting almost 25 million people worldwide and is conceptualized as a disorder of synaptic plasticity and brain connectivity. Antipsychotics are the primary pharmacological treatment after more than sixty years after their introduction in therapy. Two findings hold true for all presently available antipsychotics. First, all antipsychotics occupy the dopamine D2 receptor (D2R) as an antagonist or partial agonist, even if with different affinity; second, D2R occupancy is the necessary and probably the sufficient mechanism for antipsychotic effect despite the complexity of antipsychotics' receptor profile. D2R occupancy is followed by coincident or divergent intracellular mechanisms, implying the contribution of cAMP regulation, β-arrestin recruitment, and phospholipase A activation, to quote some of the mechanisms considered canonical. However, in recent years, novel mechanisms related to dopamine function beyond or together with D2R occupancy have emerged. Among these potentially non-canonical mechanisms, the role of Na²⁺ channels at the dopamine at the presynaptic site, dopamine transporter (DAT) involvement as the main regulator of dopamine concentration at synaptic clefts, and the putative role of antipsychotics as chaperones for intracellular D2R sequestration, should be included. These mechanisms expand the fundamental role of dopamine in schizophrenia therapy and may have relevance to considering putatively new strategies for treatment-resistant schizophrenia (TRS), an extremely severe condition epidemiologically relevant and affecting almost 30% of schizophrenia patients. Here, we performed a critical evaluation of the role of antipsychotics in synaptic plasticity, focusing on their canonical and non-canonical mechanisms of action relevant to the treatment of schizophrenia and their subsequent implication for the pathophysiology and potential therapy of TRS.

A distributional code for value in dopamine-based reinforcement learning

Since its introduction, the reward prediction error theory of dopamine has explained a wealth of empirical phenomena, providing a unifying framework for understanding the representation of reward and value in the brain^1-3. According to the now canonical theory, reward predictions are represented as a single scalar quantity, which supports learning about the expectation, or mean, of stochastic outcomes. Here we propose an account of dopamine-based reinforcement learning inspired by recent artificial intelligence research on distributional reinforcement learning^4-6. We hypothesized that the brain represents possible future rewards not as a single mean, but instead as a probability distribution, effectively representing multiple future outcomes simultaneously and in parallel. This idea implies a set of empirical predictions, which we tested using single-unit recordings from mouse ventral tegmental area. Our findings provide strong evidence for a neural realization of distributional reinforcement learning.

Hypoxia-Ischemia and Hypothermia Independently and Interactively Affect Neuronal Pathology in Neonatal Piglets with Short-Term Recovery

Therapeutic hypothermia is the standard of clinical care for moderate neonatal hypoxic-ischemic encephalopathy. We investigated the independent and interactive effects of hypoxia-ischemia (HI) and temperature on neuronal survival and injury in basal ganglia and cerebral cortex in neonatal piglets. Male piglets were randomized to receive HI injury or sham procedure followed by 29 h of normothermia, sustained hypothermia induced at 2 h, or hypothermia with rewarming during fentanyl-nitrous oxide anesthesia. Viable and injured neurons and apoptotic profiles were counted in the anterior putamen, posterior putamen, and motor cortex at 29 h after HI injury or sham procedure. Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) identified genomic DNA fragmentation to confirm cell death. Though hypothermia after HI preserved viable neurons in the anterior and posterior putamen, hypothermia prevented neuronal injury in only the anterior putamen. Hypothermia initiated 2 h after injury did not protect against apoptotic cell death in either the putamen or motor cortex, and rewarming from hypothermia was associated with increased apoptosis in the motor cortex. In non-HI shams, sustained hypothermia during anesthesia was associated with neuronal injury and corresponding viable neuron loss in the anterior putamen and motor cortex. TUNEL confirmed increased neurodegeneration in the putamen of hypothermic shams. Anesthetized, normothermic shams did not show abnormal neuronal cytopathology in the putamen or motor cortex, thereby demonstrating minimal contribution of the anesthetic regimen to neuronal injury during normothermia. We conclude that the efficacy of hypothermic protection after HI is region specific and that hypothermia during anesthesia in the absence of HI may be associated with neuronal injury in the developing brain. Studies examining the potential interactions between hypothermia and anesthesia, as well as with longer durations of hypothermia, are needed.

Ex Vivo and In Vivo Characterization of Interpolymeric Blend/Nanoenabled Gastroretentive Levodopa Delivery Systems

One approach for delivery of narrow absorption window drugs is to formulate gastroretentive drug delivery systems. This study was undertaken to provide insight into in vivo performances of two gastroretentive systems (PXLNET and IPB matrices) in comparison to Madopar® HBS capsules. The pig model was used to assess gastric residence time and pharmacokinetic parameters using blood, cerebrospinal fluid (CSF), and urine samples. Histopathology and cytotoxicity testing were also undertaken. The pharmacokinetic parameters indicated that levodopa was liberated from the drug delivery systems, absorbed, widely distributed, metabolized, and excreted. C_max were 372.37, 257.02, and 461.28 ng/mL and MRT were 15.36, 14.98, and 13.30 for Madopar HBS capsules, PXLNET, and IPB, respectively. In addition, X-ray imaging indicated that the gastroretentive systems have the potential to reside in the stomach for 7 hours. There was strong in vitro-in vivo correlation for all formulations with r² values of 0.906, 0.935, and 0.945 for Madopar HBS capsules, PXLNET, and IPB, respectively. Consequently, PXLNET and IPB matrices have pertinent potential as gastroretentive systems for narrow absorption window drugs (e.g., L-dopa) and, in this application specifically, enhanced the central nervous system and/or systemic bioavailability of such drugs.

Subcortical neuromorphometry in schizophrenia spectrum and bipolar disorders

Background

Disorders within the schizophrenia spectrum genetically overlap with bipolar disorder, yet questions remain about shared biological phenotypes. Investigation of brain structure in disease has been enhanced by developments in shape analysis methods that can identify subtle regional surface deformations. Our study aimed to identify brain structure surface deformations that were common across related psychiatric disorders, and characterize differences.

Methods

Using the automated FreeSurfer-initiated Large Deformation Diffeomorphic Metric Mapping, we examined volumes and shapes of seven brain structures: hippocampus, amygdala, caudate, nucleus accumbens, putamen, globus pallidus and thalamus. We compared findings in controls (CON; n = 40), and those with schizophrenia (SCZ; n = 52), schizotypal personality disorder (STP; n = 12), psychotic bipolar disorder (P-BP; n = 49) and nonpsychotic bipolar disorder (N-BP; n = 24), aged 15–35. Relationships between morphometric measures and positive, disorganized and negative symptoms were also investigated.

Results

Inward deformation was present in the posterior thalamus in SCZ, P-BP and N-BP; and in the subiculum of the hippocampus in SCZ and STP. Most brain structures however showed unique shape deformations across groups. Correcting for intracranial size resulted in volumetric group differences for caudate (p < 0.001), putamen (p < 0.01) and globus pallidus (p < 0.001). Shape analysis showed dispersed patterns of expansion on the basal ganglia in SCZ. Significant clinical relationships with hippocampal, amygdalar and thalamic volumes were observed.

Conclusions

Few similarities in surface deformation patterns were seen across groups, which may reflect differing neuropathologies. Posterior thalamic contraction in SCZ and BP suggest common genetic or environmental antecedents. Surface deformities in SCZ basal ganglia may have been due to antipsychotic drug effects.

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Shape analysis identified structural abnormalities in psychiatric disorders, where volume analysis did not

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Few similarities in surface deformation patterns were seen across diagnostic groups

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Posterior thalamic contraction was seen in both schizophrenia and bipolar patients

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Expansion of basal ganglia regions were seen in schizophrenia patients

Miniature Swine for Preclinical Modeling of Complexities of Human Disease for Translational Scientific Discovery and Accelerated Development of Therapies and Medical Devices

Noncommunicable diseases, including cardiovascular disease, diabetes, chronic respiratory disease, and cancer, are the leading cause of death in the world. The cost, both monetary and time, of developing therapies to prevent, treat, or manage these diseases has become unsustainable. A contributing factor is inefficient and ineffective preclinical research, in which the animal models utilized do not replicate the complex physiology that influences disease. An ideal preclinical animal model is one that responds similarly to intrinsic and extrinsic influences, providing high translatability and concordance of preclinical findings to humans. The overwhelming genetic, anatomical, physiological, and pathophysiological similarities to humans make miniature swine an ideal model for preclinical studies of human disease. Additionally, recent development of precision gene-editing tools for creation of novel genetic swine models allows the modeling of highly complex pathophysiology and comorbidities. As such, the utilization of swine models in early research allows for the evaluation of novel drug and technology efficacy while encouraging redesign and refinement before committing to clinical testing. This review highlights the appropriateness of the miniature swine for modeling complex physiologic systems, presenting it as a highly translational preclinical platform to validate efficacy and safety of therapies and devices.

Balanced modulation of striatal activation from D2 /D3 receptors in caudate and ventral striatum: Disruption in cannabis abusers

Proper communication between dorsal caudate (CD) and ventral striatum (VS) is likely to be crucial for on-time responses and its disruption might result in impulsivity. Here, we used functional magnetic resonance imaging (fMRI) with a sensorimotor reaction time task and positron emission tomography (PET) with [(11)C]raclopride in 14 healthy controls and 18 cannabis abusers to contrast the modulation of striatal fMRI responses by dopamine receptors (D2 /D3 R) in CD and VS. In controls, we show that the fMRI signals in VS that occurs concomitantly with on-time responses showed opposite modulation from D2 /D3 R in CD (inhibitory) and D2 /D3 R in VS (stimulatory). In contrast, this modulation was not significant in cannabis abusers. Findings suggest that action speed requires balanced VS-inhibition from D2 /D3 R in CD and VS-facilitation from D2 /D3 R in VS.

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.

Rostrocaudal gradients of dopamine D2/3 receptor binding in striatal subregions measured with [(11)C]raclopride and high-resolution positron emission tomography

The human striatum has structural and functional subdivisions, both dorsoventrally and rostrocaudally. To date, the gradients of dopamine D2/3 receptor binding in the human striatum have not been measured with positron emission tomography (PET). Seven healthy male subjects aged 24.5 ± 3.5 years were scanned with brain-dedicated high-resolution research tomography (HRRT, Siemens Medical Solutions, Knoxville, TN, USA) and [(11)C]raclopride. Coronally defined regions of interest (ROIs) of the caudate nucleus, putamen and ventral striatum (VST) were sampled plane-by-plane, 1.5mm apart, on spatially normalized binding potential (BPND) images. Regional [(11)C]raclopride BPND values were calculated using the simplified reference tissue model (SRTM) from a total of 25 coronal planes. An increasing rostrocaudal gradient of the D2/3 receptor binding was detected in the putamen, which is consistent with the known distribution of D2/3 dopamine receptors. In the caudate nucleus, there was an initial increase in the BPND values in the most anterior planes, suggesting that the highest D2/3 receptor binding occurred in the head; however, there was an overall descending gradient. A declining trend was also observed in the VST. The novelty of this study lies in the presentation, for the first time, of the D2/3 receptor binding gradients in each striatal subregion in the brains of living healthy humans. The high spatial resolution provided by HRRT enables frequent sampling of BPND along the longitudinal extent of striatum; this method is superior to the sectioning used in previous post mortem studies. Regarding the functional organization of the striatum, our findings can inform future investigations of normal neurophysiology as well as efforts to differentiate neuropsychiatric disorders affecting the brain dopamine (DA) system. Furthermore, the average distribution of D2/3 receptor binding revealed in this study could serve as a basis for a database that includes distributions of various DA markers as a function of healthy aging.

Haloperidol induces higher Homer1a expression than risperidone, olanzapine and sulpiride in striatal sub-regions

Homer1a and Yotiao are two post-synaptic density proteins at the crossroad of dopamine-glutamate neurotransmission. Homer1a has been implicated in the pathophysiology of schizophrenia and is differentially induced by typical and atypical antipsychotics, perhaps according to their dopaminergic profile. Yotiao has been involved in glutamate and dopamine post-synaptic signalling. Here, we seek to determine whether Homer1a and Yotiao might be implicated in post-synaptic response to antipsychotics with affinity to different dopamine D(2) receptors: haloperidol (0.8mg kg(-1)), risperidone (3mg kg(-1)), olanzapine (2.5mg kg(-1)) and (-)-sulpiride (50mg kg(-1)). Homer1a expression was significantly induced by haloperidol compared to vehicle and to atypical antipsychotics in almost all striatal sub-regions. Atypical antipsychotics induced the gene in the lateral putamen and in the core of the accumbens only. All antipsychotics, with the exclusion of sulpiride, elicited a dorsolateral-to-ventromedial distribution pattern of Homer1a expression. No significant induction was detected for Yotiao. These results suggest that the quantitative and topographical pattern of Homer1a expression may putatively be related to antipsychotics affinity and/or occupancy at dopamine D(2) receptors.

Dopamine D1 Receptor Imaging in the Rodent and Primate Brain Using the Isoquinoline (+)-[11C]A-69024 and Positron Emission Tomography

In vivo pharmacokinetic and brain binding characteristics of (+)-[(11)C]A-69024, a high-affinity-D1-selective dopamine receptor antagonist, were assessed with micro-PET and beta-microprobes in the rat and PET in the baboon. The biodistribution of (+)-[(11)C]A-69024 in rats and baboons showed a rapid brain uptake (reaching a maximal value at 5 and 15 min postinjection in rats and baboons, respectively), followed by a slow wash out. The region/cerebellum concentration ratio was characterized by a fourfold higher uptake in striatum and a twofold higher uptake in cortical regions, consistent with in vivo specific binding of the radiotracer in these cerebral regions. Furthermore, this specific (+)-[(11)C]A-69024 binding significantly correlated with the reported in vitro distribution of dopamine D1-receptors. Finally, the specific uptake of the tracer in the striatum and cortical regions was completely prevented by either a pretreatment with large doses of nonradioactive (+/-)A-69024 or of the D1-selective antagonist SCH23390, resulting in a similar uptake in the reference region (cerebellum) and in other brain regions. Thus, (+)-[(11)C]A-69024 appears to be a specific and enantioselective radioligand to visualize and quantify brain dopamine D1 receptors in vivo using positron emission tomography.

Brain region binding of the D2/3 agonist [11C]-(+)-PHNO and the D2/3 antagonist [11C]raclopride in healthy humans

The D(2) receptors exist in either the high- or low-affinity state with respect to agonists, and while agonists bind preferentially to the high-affinity state, antagonists do not distinguish between the two states. [(11)C]-(+)-PHNO is a PET D(2) agonist radioligand and therefore provides a preferential measure of the D(2) (high) receptors. In contrast, [(11)C]raclopride is an antagonist radioligand and thus binds with equal affinity to the D(2) high- and low-affinity states. The aim was to compare the brain uptake, distribution and binding characteristics between [(11)C]-(+)-PHNO and [(11)C]raclopride in volunteers using a within-subject design. Both radioligands accumulated in brain areas rich in D(2)/D(3)-receptors. However, [(11)C]-(+)-PHNO showed preferential uptake in the ventral striatum and globus pallidus, while [(11)C]raclopride showed preferential uptake in the dorsal striatum. Mean binding potentials were higher in the putamen (4.3 vs. 2.8) and caudate (3.4 vs 2.1) for [(11)C]raclopride, equal in the ventral-striatum (3.4 vs. 3.3), and higher in the globus pallidus for [(11)C]-(+)-PHNO (1.8 vs. 3.3). Moreover [(11)C]-(+)-PHNO kinetics in the globus pallidus showed a slower washout than other regions. One explanation for the preferential binding of [(11)C]-(+)-PHNO in the globus pallidus and ventral-striatum could be the presence of a greater proportion of high- vs. low-affinity receptors in these areas. Alternatively, the observed distribution could also be explained by a preferential binding of D(3)-over-D(2) with [(11)C]-(+)-PHNO. This differential binding of agonist vs. antagonist radioligand, especially in the critically important region of the limbic striatum/pallidum, offers new avenues to investigate the role of the dopamine system in health and disease.

FDG-PET in never-previously medicated psychotic adolescents treated with olanzapine or haloperidol

We acquired Positron emission tomography with 18-F-deoxyglucose (FDG-PET) and anatomical MRI in 30 never-previously medicated psychotic adolescents (ages 13-20). (FDG-PET) was obtained at baseline and after 8-9 weeks of a randomized double-blind trial of either olanzapine or haloperidol. Neuropsychological tests of executive function were also obtained. Patients carried out the serial verbal learning task, a modification of the California Verbal Learning Test, during the uptake of the FDG. PET scans were coregistered with spoiled gradient MRI (TR=24, TE=5, flip angle 40 degrees, slice thickness 1.2 mm, field of view 230 mm) for accurate anatomical identification of regions of interest traced on the MRI. Twenty-two of the thirty patients completed the second PET and clinical evaluation. Individuals treated with olanzapine increased relative metabolic rates in the frontal lobe more than the occipital lobe while patients treated with haloperidol failed to increase frontal metabolic rates and did not show an anteroposterior gradient in medication response. Haloperidol increased striatal metabolic rate more than olanzapine. Both drugs increased thalamic metabolic rates and this increase was significantly larger in younger (age 13-15) than older (16-21) patients.

Raclopride-induced motor consolidation impairment in primates: role of the dopamine type-2 receptor in movement chunking into integrated sequences

Results obtained in patients with schizophrenia have shown that antipsychotic drugs may induce motor learning deficits correlated with the striatal type-2 dopamine receptors (D(2)R) occupancy. Other findings suggest that the role of the striatum in motor learning could be related to a process of "chunking" discrete movements into motor sequences. We therefore hypothesized that a D(2)R blocking substance, such as raclopride, would affect motor learning by specifically disrupting the grouping of movements into sequences. Two monkeys were first trained to perform a baseline-overlearned sequence (Seq. A) drug free. Then, a new sequence was learned (Seq. B) and the overlearned sequence was recalled OFF-drug (Seq. A recall OFF-drug). The effect of raclopride was then assessed on the learning of a third sequence (Seq. C), and on the recall of the overlearned sequence (Seq. A recall ON-drug). Results showed that performance related to the overlearned sequence remained the same in the three experimental conditions (Seq. A, Seq. A recall OFF-drug, Seq. A recall ON-drug), whether the primates received raclopride or not. On the other hand, new sequence learning was significantly affected during raclopride treatment (Seq. C), when compared with new sequence learning without the effect of any drug (Seq. B). Raclopride-induced disturbances consisted in performance fluctuations, which persisted even after many days of trials, and prevented the monkeys from reaching a stable level of performance. Further analyses also showed that these fluctuations appeared to be related to monkeys' inability to group movements into single flowing motor sequences. The results of our study suggest that dopamine is involved in the stabilization or consolidation of motor performances, and that this function would involve a chunking of movements into well-integrated sequences.

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.

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.

Structural analysis of the basal ganglia in schizophrenia

Increases in the total volume of basal ganglia structures have been reported in schizophrenia. However, patterns of basal ganglia shape change, which can reveal localized changes in substructure volumes, have not been investigated. In this study, the total volume and shape of several basal ganglia structures were compared in subjects with and without schizophrenia. T(1)-weighted magnetic resonance scans were collected in 54 schizophrenia and 70 comparison subjects. High-dimensional (large-deformation) brain mapping was used to assess the shape and volume of several basal ganglia structures. The relationships of shape and volume measures with psychopathology, cognition and motor function were also assessed. Left and right volumes of the caudate and putamen, as well as the right globus pallidus volume, were significantly increased in subjects with schizophrenia as compared to comparison subjects after total brain volume was included as a covariate. Significant differences in shape accompanied these volume changes in the caudate, putamen and globus pallidus, after their total volumes were included as covariates. There were few significant correlations between volume or shape measures and either cognitive function or clinical symptoms, other than a positive correlation between an attention/vigilance cognitive dimension and the volume of the caudate and putamen, and a negative correlation between nucleus accumbens volume and delusions. In conclusion, basal ganglia volumes relative to total brain volume were larger in schizophrenia subjects than healthy comparison subjects. Specific patterns of shape change accompanied these volume differences.

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 age on markers for monoaminergic neurons of normal and MPTP-lesioned rhesus monkeys: A multi-tracer PET study

The binding of three tracers for monoaminergic terminals was mapped in the brain of healthy young (N=6) and healthy old rhesus monkeys (N=4), aged monkeys with mild unilateral intracarotid MPTP lesions (N=3), and monkeys of intermediate age with severe systemic MPTP lesions (N=6). The ligand for monoaminergic vesicles (+)-[(11)C]dihydrotetrabenazine (+DTBZ) had a mean binding potential (pB) of 1.4 in striatum of the healthy young monkeys, which was reduced by 20% in putamen of the old monkeys. The catecholamine transporter ligand (+)-[(11)C]methylphenidate (+MP) had a mean pB of 1.3 in striatum of the young monkeys, which was reduced by 40% in caudate and putamen of the old monkeys. The DOPA decarboxylase substrate [(18)F]fluoro-l-DOPA (FDOPA) had a mean decarboxylation coefficient (k(3)(S)) of 0.4 h(-1) in striatum of the young group, and was not significantly reduced in the aged group. Of the three ligands, only +DTBZ pB was significantly reduced in striatum of the small group of animals with mild unilateral lesions. In the group with systemic MPTP lesions, the mean reduction of the binding of the three ligands was 80% in the caudate and putamen. However, the decline in +MP pB in the ventral striatum (-75%) exceeded the declines of +DTBZ pB and FDOPA k(3)(S) in that region (-65%), suggesting that compensatory down-modulation of uptake sites may occur in the striatal regions with the least dopamine depletion. Binding of all three ligands was reduced by 50% in the anterior cingulate cortex and in the thalamus, suggesting toxicity of MPTP for extrastriatal catecholamine innervations. +DTBZ binding in the hypothalamus, presumably mainly in serotonin fibers, was unaffected by systemic MPTP treatment. Of the three tracers, +DTBZ was most sensitive for detecting MPTP-induced dopamine depletion in monkey striatum.

Striatal dopamine D1 and D2 receptor balance in twins at increased genetic risk for schizophrenia

The dopamine hypothesis of schizophrenia postulates that a dysfunctional dopaminergic system is a major pathophysiological mechanism in the disease. Most studies have focused on striatal dopamine D2 receptors, but a disturbed link between dopamine D1 and D2 receptors has also been proposed. Schizophrenia is highly heritable, and recent evidence suggests that alterations in the dopaminergic system confer susceptibility for schizophrenia instead of being solely related to the to overt expression of the disease. To explore the impact of genetic vulnerability for schizophrenia on the balance of striatal dopamine D1 and D2 receptors, we studied monozygotic (MZ) and dizygotic (DZ) unaffected co-twins from twin pairs discordant for schizophrenia as well as healthy control twins using positron emission tomography (PET). Both [(11)C]SCH 23390 and [(11)C]raclopride were used to quantitate D1 and D2 receptor binding, respectively, in the same individuals during the same day. The association between D1 and D2 receptor binding was analyzed using conventional region of interests as well as voxel-wise D1/D2 ratio maps. All levels of analyses failed to show any differences in D1/D2 ratio between the unaffected MZ or DZ co-twins and control twins. We noted rostrocaudally declining and dorsoventrally increasing gradients in D1/D2 ratio in the striatum, with no differences between groups in these gradients. In this sample, we did not find evidence for an association between increased genetic risk for schizophrenia and altered D1/D2 receptor balance in the striatum.