Spontaneous partial recovery of striatal dopaminergic uptake despite nigral cell loss in asymptomatic MPTP-lesioned female minipigs.

The Göttingen minipig is a large animal with a gyrencephalic brain that expresses -complex behavior, making it an attractive model for Parkinson's disease research. Here, we investigate the temporal evolution of presynaptic dopaminergic function for 14 months after injections of 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) into the minipig using a multi-tracer longitudinal positron emission tomography (PET) design. We injected seven sedated minipigs with 1-2 mg/kg of MPTP, and two with saline, three times a week over four weeks. We monitored behavioral deficits using a validated motor scale and walking mat. Brains were imaged with (+)-⍺-[11C]-dihydrotetrabenazine ([11C]-DTBZ) and [18F]-dihydroxyphenylalanine ([18F]-FDOPA) PET at baseline and 1, 3, 10 and 14 months after MPTP injection, and immunohistochemistry was used to assess nigral cell loss. The minipigs showed mild bradykinesia and impaired coordination at early timepoints after MPTP. PET revealed decreases of striatal [11C]-DTBZ and [18F]-FDOPA uptake post-MPTP with partial spontaneous recovery of [18F]-FDOPA after 10 months. Postmortem analysis estimated an MPTP-induced nigral loss of 57% tyrosine hydroxylase+ and 43% Nissl-stained cells. Normal motor function despite substantial damage to the dopaminergic system is consistent with prodromal Parkinson's disease, and offers an opportunity for testing disease-modifying therapies. However, partial spontaneous recovery of dopamine terminal function must be taken into account in future studies.

Preclinical PET Studies of [11C]UCB-J Binding in Minipig Brain.

PURPOSE: Loss of neuronal synapse function is associated with a number of brain disorders. The [11C]UCB-J positron emission tomography (PET) tracer allows for in vivo examination of synaptic density, as it binds to synaptic vesicle glycoprotein 2A (SV2A) expressed in presynaptic terminals. Here, we characterise [11C]UCB-J imaging in Göttingen minipigs. PROCEDURES: Using PET imaging, we examined tracer specificity and compared kinetic models. We explored the use of a standard blood curve and centrum semiovale white matter as a reference region. We compared in vivo [11C]UCB-J PET imaging to in vitro autoradiography, Western blotting and real-time quantitative polymerase chain reaction. RESULTS: The uptake kinetics of [11C]UCB-J could be described using a 1-tissue compartment model and blocking of SV2A availability with levetiracetam showed dose-dependent specific binding. Population-based blood curves resulted in reliable [11C]UCB-J binding estimates, while it was not possible to use centrum semiovale white matter as a non-specific reference region. Brain [11C]UCB-J PET signals correlated well with [3H]UCB-J autoradiography and SV2A protein levels. CONCLUSIONS: [11C]UCB-J PET is a valid in vivo marker of synaptic density in the minipig brain, with binding values close to those reported for humans. Minipig models of disease could be valuable for investigating the efficacy of putative neuroprotective agents for preserving synaptic function in future non-invasive, longitudinal studies.

Type of Anaesthetic Influences [11C]MDL100,907 Binding to 5HT2A Receptors in Porcine Brain.

PURPOSE: Anaesthesia routinely is used in animal neuroimaging in order to reduce head motion artefacts and minimize the influence of stress. However, anaesthetics can modify radioligand binding profiles at receptor targets studied by positron emission tomography (PET). Here, we determined the effects of two routine anaesthetics on the binding of a tracer of the serotonin 5HT2A receptors. PROCEDURES: Isoflurane- and propofol-anesthetised Göttingen minipigs were imaged with [11C]MDL100,907 PET and analysed using regions of interest and statistical non-parametric mapping. RESULTS: The binding potentials of the tracer in striatum under isoflurane anaesthesia significantly exceeded those obtained under propofol anaesthesia, an effect we attribute to the higher blood flow in brain induced by the former. CONCLUSIONS: Interactions between radioligands and anaesthesia must be carefully evaluated in the design of in vivo neuroimaging and interpretation of data.

Sucrose intake lowers µ-opioid and dopamine D2/3 receptor availability in porcine brain.

Excessive sucrose consumption elicits addiction-like craving that may underpin the obesity epidemic. Opioids and dopamine mediate the rewarding effects of drugs of abuse, and of natural rewards from stimuli such as palatable food. We investigated the effects of sucrose using PET imaging with [11C]carfentanil (µ-opioid receptor agonist) and [11C]raclopride (dopamine D2/3 receptor antagonist) in seven female anesthetized Göttingen minipigs. We then gave minipigs access to sucrose solution for one hour on 12 consecutive days and performed imaging again 24 hours after the final sucrose access. In a smaller sample of five minipigs, we performed an additional [11C]carfentanil PET session after the first sucrose exposure. We calculated voxel-wise binding potentials (BPND) using the cerebellum as a region of non-displaceable binding, analyzed differences with statistical non-parametric mapping, and performed a regional analysis. After 12 days of sucrose access, BPND of both tracers had declined significantly in striatum, nucleus accumbens, thalamus, amygdala, cingulate cortex and prefrontal cortex, consistent with down-regulation of receptor densities. After a single exposure to sucrose, we found decreased binding of [11C]carfentanil in nucleus accumbens and cingulate cortex, consistent with opioid release. The lower availability of opioid and dopamine receptors may explain the addictive potential associated with intake of sucrose.

Cerebral water content mapping in cirrhosis patients with and without manifest HE.

Hepatic encephalopathy (HE) is a frequent and debilitating complication of cirrhosis and its pathogenesis is not definitively clarified. Recent hypotheses focus on the possible existence of low-grade cerebral edema due to accumulation of osmolytes secondary to hyperammonemia. In the present study we investigated increases in cerebral water content by a novel magnetic resonance impedance (MRI) technique in cirrhosis patients with and without clinically manifest HE. We used a 3 T MRI technique for quantitative cerebral water content mapping in nine cirrhosis patients with an episode of overt HE, ten cirrhosis patients who never suffered from HE, and ten healthy aged-matched controls. We tested for differences between groups by statistical non-parametric mapping (SnPM) for a voxel-based spatial evaluation. The patients with HE had significantly higher water content in white matter than the cirrhosis patients (0.6%), who in turn, had significantly higher content than the controls (1.7%). Although the global gray matter water content did not differ between the groups, the patients with HE had markedly higher thalamic water content than patients who never experienced HE (6.0% higher). We found increased white matter water content in cirrhosis patients, predominantly in those with manifest HE. This confirms the presence of increasing degrees of low-grade edema with exacerbation of pathology. The thalamic edema in manifest HE may lead to compromised basal ganglia-thalamo-cortical circuits, in accordance with the major clinical symptoms of HE. The identification of the thalamus as particularly inflicted in manifest HE is potentially relevant to the pathophysiology of HE.

Electroconvulsive stimulation differentially affects [11C]MDL100,907 binding to cortical and subcortical 5HT2A receptors in porcine brain.

BACKGROUND: Electroconvulsive therapy is an effective therapy of depression. We hypothesized that the beneficial effects are mediated partly by decreased serotonin receptor availability in the cortex. AIMS: We used positron emission tomography with the serotonin 5HT2A receptor radioligand [11C]MDL100,907 to determine serotonin receptor availability in response to electroconvulsive stimulation (ECS). METHODS: Seven Göttingen minipigs were deeply anaesthetized and imaged at baseline before the onset of ECS, and at 1-2 and 8-10 days after the end of a clinical course of ECS, consisting of 10 sessions over 3.5 weeks, and post-ECS values were compared to baseline. One additional minipig was anaesthetized over 10 sessions without ECS, as a control. We analysed images with the Ichise model for binding in cortex and hippocampus, followed by whole-brain analysis by statistical non-parametric mapping. RESULTS: We found significantly increased binding potential of [11C]MDL100,907 in the cortex and hippocampus 1-2 days after ECS, consistent with increased serotonin receptor availability compared to baseline. By 8-10 days after the final ECS, the average tracer binding had returned towards baseline. However, we also found significantly decreased tracer binding in the subcortical regions of olfactory bulb, pons, thalamus and striatum. CONCLUSIONS: With ECS, minipigs, unlike primates but like rodents, have higher availability at cortical and hippocampal 5HT2A receptors. Decreased tracer binding is consistent with reduced serotonin receptor availability as a differential effect of ECS on 5HT2A receptors in subcortical regions of minipig brain.

Longitudinal monoaminergic PET imaging of chronic proteasome inhibition in minipigs.

Impairment of the ubiquitin proteasome system has been implicated in Parkinson's disease. We used positron emission tomography to investigate longitudinal effects of chronic intracerebroventricular exposure to the proteasome inhibitor lactacystin on monoaminergic projections and neuroinflammation. Göttingen minipigs were implanted in the cisterna magna with a catheter connected to a subcutaneous injection port. Minipigs were imaged at baseline and after cumulative doses of 200 and 400 µg lactacystin, respectively. Main radioligands included [11C]-DTBZ (vesicular monoamine transporter type 2) and [11C]-yohimbine (α2-adrenoceptor). [11C]-DASB (serotonin transporter) and [11C]-PK11195 (activated microglia) became available later in the study and we present their results in a smaller subset of animals for information purposes only. Striatal [11C]-DTBZ binding potentials decreased significantly by 16% after 200 µg compared to baseline, but the decrease was not sustained after 400 µg (n = 6). [11C]-yohimbine volume of distribution increased by 18-25% in the pons, grey matter and the thalamus after 200 µg, which persisted at 400 µg (n = 6). In the later subset of minipigs, we observed decreased [11C]-DASB (n = 5) and increased [11C]-PK11195 (n = 3) uptake after 200 µg. These changes may mimic monoaminergic changes and compensatory responses in early Parkinson's disease.