Major depressive disorder is one of the most prevalent mental health disorders, posing a global socioeconomic burden. Conventional antidepressant treatments have a slow onset of action, and 30% of patients show no clinically significant treatment response. The recently approved fast-acting antidepressant S-ketamine, an N-methyl-D-aspartate receptor antagonist, provides a new approach for treatment-resistant patients. However, knowledge of S-ketamine's mechanism of action is still being established. Depressed human subjects have lower striatal dopamine transporter (DAT) availability compared to healthy controls. Rodent studies report increased striatal dopamine concentration in response to acute ketamine administration. In vivo [18F]FE-PE2I ([18F]-(E)-N-(3-iodoprop-2-enyl)-2β-carbofluoroethoxy-3β-(4'-methyl-phenyl) nortropane) positron emission tomography (PET) imaging of the DAT has not previously been applied to assess the effect of acute subanesthetic S-ketamine administration on DAT availability. We applied translational in vivo [18F]FE-PE2I PET imaging of the DAT in healthy female rats to evaluate whether an acute subanesthetic intraperitoneal dose of 15 mg/kg S-ketamine alters DAT availability. We also performed [3H]GBR-12935 autoradiography on postmortem brain sections. We found no effect of acute S-ketamine administration on striatal DAT binding using [18F]FE-PE2I PET or [3H]GBR-12935 autoradiography. This negative result does not support the hypothesis that DAT changes are associated with S-ketamine's rapid antidepressant effects, but additional studies are warranted.

The N-methyl-D-aspartate receptor (NMDA-R) antagonist S-ketamine has been approved as a rapid-acting antidepressant for treatment-resistant depression (TRD). The antidepressant mechanisms have not fully been elucidated; however, alterations of synaptic proteins and mechanisms may play a vital role. Here, we study the effect of a single subanaesthetic dose of 15 mg/kg S-ketamine vs saline 1 h after administration in the Wistar Kyoto rat model of depression on the density of synaptic vesicle glycoprotein 2A (SV2A) and the metabotropic glutamate receptor 5 (mGluR5) using [3H]UCB-J and [3H]MPEP gamma autoradiography, respectively, compared with control Wistar Hannover rats. In a separate cohort of Wistar Kyoto rats, we investigate the transcriptional regulation of presynaptic markers Sv2a, Syn 1-3, Syt 1-3, Synaptophysin, Vamp1, 2, 5, and 7, postsynaptic markers Homer1-3, Nrg 1, Nlgn 2, Nlgn 3, Psd95, NMDA receptor subunits Nr2a, Nr2b, AMPA receptor subunits Gria1-3, GABA type A receptor-associated protein (Gabarap), glutamate metabotropic receptor subtype 5 (Grm5), and brain-derived neurotrophic factor (Bdnf) using real-time quantitative polymerase chain reaction (qPCR) in hippocampus in response to S-ketamine vs saline injection. In Wistar Kyoto rats, S-ketamine increases [3H]UCB-J binding to SV2A compared to saline-injected controls in the nucleus accumbens and dorsal and ventral hippo-campus, an effect absent in the Wistar Hannover strain. No changes were observed in [3H]MPEP gamma binding to mGluR5, nor in gene regulation. S-ketamine can regulate presynaptic SV2A density in brain areas relevant to depression in the Wistar Kyoto model, but not in controls, suggesting a role for SV2A in the antidepressant effects of S-ketamine.

Despite well-documented dysregulation in central serotonergic signaling in Alzheimer’s disease (AD), knowledge about the potential involvement of the serotonin-2B receptor (5-HT2BR) subtype remains sparse. Here, we assessed the levels of 5-HT2BRs in brain tissue from APPswe/PS1dE9 transgenic (TG) mice, AD patients, and adult microglial cells. 5-HT2BR mRNA was measured by RT-qPCR in ageing TG and wild-type (WT) mice, in samples from the middle frontal gyrus of female, AD and control subjects, and in microglia from the cerebral cortex of WT mice. The density of 5-HT2BRs was measured by autoradiography using [3H]RS 127445. Both mouse and human brains had low levels of 5-HT2BR mRNA. In whole-brain mouse samples, mRNA expression was significantly lower in TG mice compared to WT at > 18 months of age. In the Aβ-plaque-burdened neocortex and hippocampus of old TG mice, however, levels of 5-HT2BR mRNA were two-fold higher over control, with similar elevations observed in the Aβ-plaque-burdened frontal cortex of human AD patients. 5-HT2BR mRNA expression varied widely in adult microglia and was higher compared to other cortical cell subtypes. In mice, specific [3H]RS-127445 binding in the cortex was first detected after 3 months of age. The density of 5-HT2BRs was low and overall reduced in TG, compared to WT mice. Binding was detectable but too low to be reliably quantified in the human cortex. Our results document Aβ-associated increases in 5-HT2BR mRNA expression and suggest reduced receptor binding in the context of AD. Studies investigating the functional involvement of microglial 5-HT2BRs in AD are considered relevant.

Diets high in sucrose and fat are becoming more prevalent the world over, accompanied by a raised prevalence of cardiovascular diseases, cancers, diabetes, obesity, and metabolic syndrome. Clinical studies link unhealthy diets with the development of mental health disorders, particularly depression. Here, we investigate the effects of 12 days of sucrose consumption administered as two liters of 25% sucrose solution daily for 12 days in Göttingen minipigs on the function of brain receptors involved in reward and motivation, regulating feeding, and pre- and post-synaptic mechanisms. Through quantitative autoradiography of cryostat sections containing limbic brain regions, we investigated the effects of sucrose restricted to a 1-hour period each morning, on the specific binding of [3H]raclopride on dopamine D2/3 receptors, [3H]UCB-J at synaptic vesicle glycoprotein 2A (SV2A), [3H]MPEPγ at metabotropic glutamate receptor subtype 5 (mGluR5) and [3H]SR141716A at the cannabinoid receptor 1 (CB1). Compared to control diet animals, the sucrose group showed significantly lower [3H]UCB-J and [3H]MPEPγ binding in the prefrontal cortex. The sugar-consuming minipigs showed higher hippocampal CB1 binding, but unaltered dopamine D2/3 binding compared to the control group. We found that the sucrose diet reduced the synaptic density marker while increasing CB1 binding in limbic brain structures, which may subserve maladaptive changes in appetite regulation and feeding. Further studies of the effects of diets and lifestyle habits on brain neuroreceptor and synaptic density markers are warranted.

OBJECTIVE:Cocaine is a highly addictive psychostimulant that affects synaptic activity with structural and functional adaptations of neurons. The transmembrane synaptic vesicle glycoprotein 2A (SV2A) of pre-synaptic vesicles is commonly used to measure synaptic density, as a novel approach to the detection of synaptic changes. We do not know if a single dose of cocaine suffices to affect pre-synaptic SV2A density, especially during adolescence when synapses undergo intense maturation. Here, we explored potential changes of pre-synaptic SV2A density in target brain areas associated with the cocaine-induced boost of dopaminergic neurotransmission, specifically testing if the effects would last after the return of dopamine levels to baseline.METHODS:We administered cocaine (20 mg/kg i.p.) or saline to rats in early adolescence, tested their activity levels and removed the brains 1 hour and 7 days after injection. To evaluate immediate and lasting effects, we did autoradiography with [3H]UCB-J, a specific tracer for SV2A, in medial prefrontal cortex, striatum, nucleus accumbens, amygdala, and dorsal and ventral areas of hippocampus. We also measured the striatal binding of [3H]GBR-12935 to test cocaine's occupancy of the dopamine transporter at both times of study.RESULTS:We found a significant increase of [3H]UCB-J binding in the dorsal and ventral sections of hippocampus 7 days after the cocaine administration compared to saline-injected rats, but no differences 1 hour after the injection. The [3H]GBR-12935 binding remained unchanged at both times.CONCLUSION:Cocaine provoked lasting changes of hippocampal synaptic SV2A density after a single exposure during adolescence.

Background & Aims: Cognitive dysfunction is an increasingly recognised manifestation of metabolic dysfunction -associated steatotic liver disease (MASLD), but the mechanistic link remains unclear. The aim of this study was to investigate the hypothesis that experimental MASLD leads to cognitive dysfunction via systemic inflammation and neuroinflammation. Methods: Twenty male Sprague Dawley rats were randomised to a high -fat high -cholesterol (HFHC) diet to induce MASLD, or a standard diet (n = 10/group), for 16 weeks. Assessments included: MASLD severity (histology), neurobehaviour, inflammation (liver, plasma and cerebrospinal fluid), brain microglia and astrocyte activation, and synaptic density. Results: The HFHC diet induced MASLD with extensive steatosis and lobular inflammation without fibrosis. Several plasma cytokines were elevated (CXCL1, IL -6, IL -17, MIP-1a, MCP -1, IL -10; all p <0.05) and correlated with increases in hepatic chemokine gene expression. Cerebrospinal fluid concentrations of CXCL1 were elevated (p = 0.04). In the prefrontal brain cortex, we observed a 19% increase in microglial activation confirmed by Iba1 immunohistochemistry (p = 0.03) and H-3-PK11195 autoradiography (p <0.01). In parallel, synaptic density was reduced to 92%, assessed by H-3-UCB-J autoradiography (p <0.01). MASLD animals exhibited impaired memory to previously encountered objects in the novel object recognition test (p = 0.047) and showed depression -like behaviour evidenced by increased immobility time (p <0.01) and reduced swimming time (p = 0.03) in the forced swim test. Conclusions: Experimental non -fibrotic MASLD, as a model to reflect the early stage of human disease, results in cognitive impairment and depression -like behaviour. This is associated with an inflammatory phenotype not only in the liver but also in the plasma and brain, which together with diminished synaptic density, provides a pathophysiological link between liver disease and cognitive dysfunction in MASLD. Impact and implications: Cognitive dysfunction is an increasingly recognised comorbidity in patients with metabolic dysfunction -associated steatotic liver disease (MASLD), yet the underlying mechanisms remain unclear. This study provides evidence of impaired memory and depression -like symptoms in early experimental MASLD and indicates that hepatic inflammation may drive a systemic inflammatory response, resulting in neuroinflammation and reduced brain synaptic density. The evidence of impaired memory in MASLD and establishing its underlying pathophysiological link provides insights that could guide the development of potential new treatments for this increasingly common condition in people of working age. The study also emphasises the need to develop better tools for clinical cognitive testing, which will enable physicians to assess and manage brain dysfunction early in MASLD. (c) 2023 The Authors. Published by Elsevier B.V. on behalf of European Association for the Study of the Liver (EASL). This is an open access article under the CC BY -NC -ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Carboxypeptidase II (CBPII) in brain metabolizes the neuroactive substance N‐acetyl‐L‐aspartyl‐L‐glutamate (NAGG) to yield the elements of glutamate and N‐acetyl‐aspartate (NAA). In peripheral organs, CBPII is known as prostrate specific membrane antigen (PSMA), which presents an important target for nuclear medicine imaging in prostate cancer. Available PSMA ligands for PET imaging do not cross the blood–brain barrier, and there is scant knowledge of the neurobiology of CBPII, despite its implication in the regulation of glutamatergic neurotransmission. In this study we used the clinical PET tracer [18F]‐PSMA‐1007 ([18F]PSMA) for an autoradiographic characterization of CGPII in rat brain. Ligand binding and displacement curves indicated a single site in brain, with KD of about 0.5 nM, and Bmax ranging from 9 nM in cortex to 19 nM in white matter (corpus callosum and fimbria) and 24 nM in hypothalamus. The binding properties of [18F]PSMA in vitro should enable its use for autoradiographic investigations of CBPII expression in animal models of human neuropsychiatric conditions.

Parkinson's disease (PD) is a debilitating neurodegenerative multisystem disorder leading to motor and non-motor symptoms in millions of individuals. Despite intense research, there is still no cure, and early disease biomarkers are lacking. Animal models of PD have been inspired by basic elements of its pathogenesis, such as dopamine dysfunction, alpha-synuclein accumulation, neuroinflammation and disruption of protein degradation, and these have been crucial for a deeper understanding of the mechanisms of pathology, the identification of biomarkers, and evaluation of novel therapies. Imaging biomarkers are non-invasive tools to assess disease progression and response to therapies; their discovery and validation have been an active field of translational research. Here, we highlight different considerations of animal models of PD that can be applied to future research, in terms of their suitability to answer different research questions. We provide the reader with important considerations of the best choice of model to use based on the disease features of each model, including issues related to different species. In addition, positron emission tomography studies conducted in PD animal models in the last 5 years are presented. With a variety of different species, interventions and genetic information, the choice of the most appropriate model to answer research questions can be daunting, especially since no single model recapitulates all aspects of this complex disorder. Appropriate animal models in conjunction with in vivo molecular imaging tools, if selected properly, can be a powerful combination for the assessment of novel therapies and developing tools for early diagnosis.

•This study provided evidence for memory loss and altered behaviour in a NAFLD animal model•Cognitive dysfunction was associated with systemic and neuroinflammation•Diminished synaptic density was observed in the prefrontal cortex•The study suggests a pathophysiological link between NAFLD and cognitive dysfunction

The noradrenaline system attracts attention for its role in mood disorders and neurodegenerative diseases but the lack of well-validated methods impairs our understanding when assessing its function and release in vivo. This study combines simultaneous positron emission tomography (PET) and microdialysis to explore if [11C]yohimbine, a selective antagonist radioligand of the α2 adrenoceptors, may be used to assess in vivo changes in synaptic noradrenaline during acute pharmacological challenges. Anesthetised Göttingen minipigs were positioned in a head holder in a PET/CT device. Microdialysis probes were placed in the thalamus, striatum and cortex and dialysis samples were collected every 10 min. Three 90 min [11C]yohimbine scans were acquired: at baseline and at two timepoints after the administration of amphetamine (1–10 mg/kg), a non-specific releaser of dopamine and noradrenaline, or nisoxetine (1 mg/kg), a specific noradrenaline transporter inhibitor. [11C]yohimbine volumes of distribution (VT) were obtained using the Logan kinetic model. Both challenges induced a significant decrease in yohimbine VT, with time courses reflecting their different mechanisms of action. Dialysis samples revealed a significant increase in noradrenaline extracellular concentrations after challenge and an inverse correlation with changes in yohimbine VT. These data suggest that [11C]yohimbine can be used to evaluate acute variations in synaptic noradrenaline concentrations after pharmacological challenges.

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.

The number of functionally active synapses provides a measure of neural integrity, with reductions observed in neurodegenerative disorders. [11C]UCB-J binds to synaptic vesicle 2A (SV2A) transmembrane protein located in secretory vesicles. We aimed to assess [11C]UCB-J PET as an in vivo biomarker of regional cerebral synaptic SV2A density in rat lesion models of neurodegeneration. Healthy anesthetized rats had [11C]UCB-J PET and arterial blood sampling. We compared different models describing [11C]UCB-J brain uptake kinetics to determine its regional distribution. Blocking studies were performed with levetiracetam (LEV), an antiepileptic SV2A antagonist. Tracer binding was measured in rodent unilateral acute lesion models of Parkinsonism and Huntington's disease, induced with 6-hydroxydopamine (6-OHDA) and quinolinic acid (QA), respectively. [3H]UCB-J autoradiography was performed in postmortem tissue. Rat brain showed high and fast [11C]UCB-J uptake and washout with up to 80% blockade by LEV. [11C]UCB-J PET showed a 6.2% decrease in ipsilateral striatal SV2A binding after 6-OHDA and 39.3% and 55.1% decreases after moderate and high dose QA confirmed by autoradiography. In conclusion, [11C]UCB-J PET provides a good in vivo marker of synaptic SV2A density which can potentially be followed longitudinally along with synaptic responses to putative neuroprotective agents in models of neurodegeneration.

Alpha-synuclein (a-syn) can aggregate and form toxic oligomers and insoluble fibrils which are the main component of Lewy bodies. Intra-neuronal Lewy bodies are a major pathological characteristic of Parkinson's disease (PD). These fibrillar structures can act as seeds and accelerate the aggregation of monomeric a-syn. Indeed, recent studies show that injection of preformed a-syn fibrils (PFF) into the rodent brain can induce aggregation of the endogenous monomeric a-syn resulting in neuronal dysfunction and eventual cell death.We injected 8 μg of murine a-syn PFF, or soluble monomeric a-syn into the right striatum of rats. The animals were monitored behaviourally using the cylinder test, which measures paw asymmetry, and the corridor task that measures lateralized sensorimotor response to sugar treats. In vivo PET imaging was performed after 6, 13 and 22 weeks using [11C]DTBZ, a marker of the vesicular monoamine 2 transporter (VMAT2), and after 15 and 22 weeks using [11C]UCB-J, a marker of synaptic SV2A protein in nerve terminals. Histology was performed at the three time points using antibodies against dopaminergic markers, aggregated a-syn, and MHCII to evaluate the immune response.While the a-syn PFF injection caused only mild behavioural changes, [11C]DTBZ PET showed a significant and progressive decrease of VMAT2 binding in the ipsilateral striatum. This was accompanied by a small progressive decrease in [11C]UCB-J binding in the same area. In addition, our histological analysis revealed a gradual spread of misfolded a-syn pathology in areas anatomically connected to striatum that became bilateral with time. The striatal a-syn PFF injection resulted in a progressive unilateral degeneration of dopamine terminals, and an early and sustained presence of MHCII positive ramified microglia in the ipsilateral striatum and substantia nigra.Our study shows that striatal injections of a-syn fibrils induce progressive pathological synaptic dysfunction prior to cell death that can be detected in vivo with PET. We confirm that intrastriatal injection of a-syn PFFs provides a model of progressive a-syn pathology with loss of dopaminergic and synaptic function accompanied by neuroinflammation, as found in human PD.

Progressive degeneration of dopaminergic neurons, immune activation, and α-synuclein pathology characterize Parkinson’s disease (PD). We previously reported that unilateral intranigral injection of recombinant adeno-associated viral (rAAV) vectors encoding wild-type human α-synuclein produced a rat model of early PD with dopamine terminal dysfunction. Here we tested the hypothesis that decreases in dopamine result in increased postsynaptic dopamine D2/D3 receptor expression, neuroinflammation, and reduced synaptic vesicle glycoprotein 2A (SV2A) density. Rats were injected with rAAV encoding α-synuclein or green fluorescent protein and subjected to non-pharmacological motor tests, before euthanization at 12 weeks post-injection. We performed: (1) in situ hybridization of nigral tyrosine hydroxylase mRNA, (2) HPLC of striatal dopamine content, and (3) autoradiography with [3H]raclopride, [3H]DTBZ, [3H]GBR12935, [3H]PK11195, and [3H]UCB-J to measure binding at D2/3 receptors, vesicular monoamine transporter 2, dopamine transporters, mitochondrial translocator protein, and SV2A, respectively. rAAV-α-synuclein induced motor asymmetry and reduced tyrosine hydroxylase mRNA and dopamine content in ipsilateral brain regions. This was paralleled by elevated ipsilateral postsynaptic dopamine D2/3 receptor expression and immune activation, with no changes to synaptic SV2A density. In conclusion, α-synuclein overexpression results in dopaminergic degeneration that induced compensatory increases in D2/3 binding and immune activation, recapitulating many of the pathological characteristics of PD.

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.

Supplemental material, sj-pdf-1-jcb-10.1177_0271678X20931140 for In vivo imaging of synaptic SV2A protein density in healthy and striatal-lesioned rats with [11C]UCB-J PET by Majken B Thomsen, Jan Jacobsen, Thea P Lillethorup, Anna C Schacht, Mette Simonsen, Marina Romero-Ramos, David J Brooks and Anne M Landau in Journal of Cerebral Blood Flow & Metabolism

Epilepsy is a debilitating seizure disorder that affects approximately 50 million people. Noradrenaline reduces neuronal excitability, has anticonvulsant effects and is protective against seizure onset.