Brain changes due to hypoxia during light anaesthesia can be prevented by deepening anaesthesia:a study in rats

In anaesthetic practice the risk of cerebral ischemic/hypoxic damage is thought to be attenuated by deep anaesthesia. The rationale is that deeper anaesthesia reduces cerebral oxygen demand more than light anaesthesia, thereby increasing the tolerance to ischemia or hypoxia. However, evidence to support this is scarce. We thus investigated the influence of light versus deep anaesthesia on the responses of rat brains to a period of hypoxia. In the first experiment we exposed adult male Wistar rats to deep or light propofol anaesthesia and then performed [18F]- Fludeoxyglucose (FDG) Positron Emission Tomography (PET) scans to verify the extent of cerebral metabolic suppression. In subsequent experiments, rats were subjected to light/deep propofol anaesthesia and then exposed to a period of hypoxia or ongoing normoxia (n = 9-11 per group). A further 5 rats, not exposed to anaesthesia or hypoxia, served as controls. Four days later a Novel Object Recognition (NOR) test was performed to assess mood and cognition. After another 4 days, the animals were sacrificed for later immunohistochemical analyses of neurogenesis/neuroplasticity (Doublecortin; DCX), Brain Derived Neurotrophic Factor (BDNF) expression and neuroinflammation (Ionized calcium-binding adaptor protein-1; Iba-1) in hippocampal and piriform cortex slices. The hippocampi of rats subjected to hypoxia during light anaesthesia showed lower DCX positivity, and therefore lower neurogenesis, but higher BDNF levels and microglia hyper-ramification. Exploration was reduced, but no significant effect on NOR was observed. In the piriform cortex, higher DCX positivity was observed, associated with neuroplasticity. All these effects were attenuated by deep anaesthesia. Deepening anaesthesia attenuated the brain changes associated with hypoxia. Hypoxia during light anaesthesia had a prolonged effect on the brain, but no impairment in cognitive function was observed. Although reduced hippocampal neurogenesis may be considered unfavourable, higher BDNF expression, associated with microglia hyper-ramification may suggest activation of repair mechanisms. Increased neuroplasticity observed in the piriform cortex supports this, and might reflect a prolonged state of alertness rather than damage

PET imaging of the anticancer drug candidate [11C]trimebutine in a rat glioma model.

PURPOSE: Preclinical studies suggest that trimebutine could be a potential treatment for glioblastoma. The aim of this study was to investigate the distribution, kinetics and tumor accumulation of [ 11C]trimebutine. METHOD: A proliferation assay and cell scratch healing assay were performed to confirm the antitumor effects of trimebutine on C6 glioma cells in-vitro. Trimebutine was subsequently labeled with 11C. The distribution and kinetics of [ 11C]trimebutine in health rats and rats with an orthotopic C6 glioma were evaluated by ex-vivo gamma counting and positron emission tomography, respectively. Blocking experiments with an excess of unlabeled trimebutine or the μ-opioid receptor ligand cyprodime were employed to determine if trimebutine exhibits saturable binding in the brain. In addition, plasma stability of the tracer was assessed. RESULTS: The proliferation assay and cell scratch healing assay confirmed that trimebutine has anti-tumor effects in-vitro. [ 11C]Trimebutine with a radiochemical purity &gt;98 % was synthesized in 15 ± 5 % radiochemical yield. In peripheral organs, the highest accumulation of the tracer was detected in excretion organs. In the brain, the highest tracer uptake was observed in the brainstem and the lowest in the hypothalamus, although differences between regions were small. PET imaging showed rapid brain uptake of [ 11C]trimebutine, followed by a gradual washout. Administration of an intravenous dose of trimebutine (10 mg/kg) significantly decreased the uptake in all brain regions (p &lt; 0.05), except midbrain. Likewise, administration of cyprodime (2 mg/kg) significantly reduced [ 11C]trimebutine uptake in the brain (p &lt; 0.01). However, uptake of [ 11C]trimebutine in the tumor was not significantly different from its brain uptake in rats bearing an orthotopic C6 glioma. The percentage of intact [ 11C]trimebutine at 60 min post injection was only 1.7 ± 0.6 %. CONCLUSION: Trimebutine exhibits inhibitory effects on the growth and migration of glioma cells in a dose- and time-dependent manner. [ 11C]Trimebutine was able to penetrate the blood-brain barrier in rats and tracer uptake could be significantly reduced by administration of a μ-opioid receptor antagonist. However, [ 11C]trimebutine failed to selectively accumulate in orthotopic C6 glioma, which could be caused by low expression levels of the drug target in these tumors, or by fast metabolism of the tracer. </p

Evaluation of [C-11]rofecoxib as PET tracer for cyclooxygenase 2 overexpression in rat models of inflammation

Background: Overexpression of cyclooxygenase type 2 (COX-2) is triggered by inflammatory stimuli, but it also plays a prominent role in the initiation and progression of various diseases. This study aims to investigate [C-11]rofecoxib as a positron emission tomography (PET) tracer for COX-2 expression.Methods: [C-11]Rofecoxib was prepared by methylation of its sulphinate precursor. Regional brain distribution and specific binding of [C-11] rofecoxib in healthy rats was studied by ex vivo biodistribution and autoradiography. Regional brain distribution and PET imaging studies were also performed on rats with severe encephalitis, caused by nasal infection with herpes simplex virus (HSV). Finally, ex vivo biodistribution and blocking studies were carried in rats with a sterile inflammation, induced by intramuscular turpentine injection.Results: [C-11]rofecoxib brain uptake in control animals corresponded with the known distribution of COX-2. Pretreatment with NS398 significantly reduced tracer uptake in the cingulate/frontopolar cortex, whereas the reduction in hippocampus approached significance. Ex vivo autoradiography also revealed preferential tracer uptake in hippocampus and cortical areas that could be blocked by NS398. In HSV-infected animals, [C-11]rofecoxib uptake was moderately increased in all brain regions, but it could not be blocked with indomethacin. Yet, some PET images revealed increased tracer uptake in brain areas with microglia activation. In turpentine-injected animals, [C-11]rofecoxib uptake in inflamed muscle was not higher than in control muscle and could not be blocked with NS398. Indomethacin caused a slight reduction in muscle uptake.Conclusions: Despite the apparent correlation between [C-11]rofecoxib uptake and COX-2 distribution in healthy rats, [C-11]rofecoxib could not unambiguously detect COX-2 overexpression in two rat models of inflammation. (C) 2008 Elsevier Inc. All rights reserved.</p

Is cyclooxygenase-1 involved in neuroinflammation?

Purpose: Reactive microglia are an important hallmark of neuroinflammation. Reactive microglia release various inflammatory mediators, such as cytokines, chemokines, and prostaglandins, which are produced by enzymes like cyclooxygenases (COX). The inducible COX‐2 subtype has been associated with inflammation, whereas the constitutively expressed COX‐1 subtype is generally considered as a housekeeping enzyme. However, recent evidence suggests that COX‐1 can also be upregulated and may play a prominent role in the brain during neuroinflammation. In this review, we summarize the evidence that supports this involvement of COX‐1. Methods: Five databases were used to retrieve relevant studies that addressed COX‐1 in the context of neuroinflammation. The search resulted in 32 articles, describing in vitro, in vivo, post mortem, and in vivo imaging studies that specifically investigated the COX‐1 isoform under such conditions. Results: Reviewed literature generally indicated that the overexpression of COX‐1 was induced by an inflammatory stimulus, which resulted in an increased production of prostaglandin E2. The pharmacological inhibition of COX‐1 was shown to suppress the induction of inflammatory mediators like prostaglandin E2. Positron emission tomography (PET) imaging studies in animal models confirmed the overexpression of COX‐1 during neuroinflammation. The same imaging method, however, could not detect any upregulation of COX‐1 in patients with Alzheimer's disease. Conclusion: Taken together, studies in cultured cells and living rodents suggest that COX‐1 is involved in neuroinflammation. Most postmortem studies on human brains indicate that the concentration of COX‐1‐expressing microglial cells is increased near sites of inflammation. However, evidence for the involvement of COX‐1 in neuroinflammation in the living human brain is still largely lacking

Assessing brain immune activation in psychiatric disorders:Clinical and preclinical PET imaging studies of the 18-kDa translocator protein

Accumulating evidence from different lines of research suggests an involvement of the immune system in the pathophysiology of several psychiatric disorders. During recent years, a series of positron emission tomography (PET) studies have been published using radioligands for the translocator protein (TSPO) to study microglia activation in schizophrenia, bipolar I disorder, major depression, autism spectrum disorder, and drug abuse. The results have been somewhat conflicting, which could be due to differences both in patient sample characteristics and in PET methods. In particular, further work is needed to address both methodological and biological sources of variability in TSPO levels, a process in which the use of animal models and small animal PET systems can be a valuable tool. Given this development, PET studies of immune activation have the potential to further increase our understanding of disease mechanisms in psychiatric disorders, which is a requisite in the search for new treatment approaches. Furthermore, molecular imaging could become an important clinical tool for identifying specific subgroups of patients or disease stages that would benefit from treatment targeting the immune system

The dual hit hypothesis of schizophrenia:evidence from animal models

Schizophrenia is a heterogeneous psychiatric disorder, which can severely impact social and professional functioning. Epidemiological and clinical studies show that schizophrenia has a multifactorial aetiology comprising genetic and environmental risk factors. Although several risk factors have been identified, it is still not clear how they result in schizophrenia. This knowledge gap, however, can be investigated in animal studies. In this review, we summarise animal studies regarding molecular and cellular mechanisms through which genetic and environmental factors may affect brain development, ultimately causing schizophrenia. Preclinical studies suggest that early environmental risk factors can affect the immune, GABAergic, glutamatergic, or dopaminergic system and thus increase the susceptibility to another risk factor later in life. A second insult, like social isolation, stress, or drug abuse, can further disrupt these systems and the interactions between them, leading to behavioural abnormalities. Surprisingly, first insults like maternal infection and early maternal separation can also have protective effects. Single gene mutations associated with schizophrenia did not have a major impact on the susceptibility to subsequent environmental hits

The Acute and Early Effects of Whole-Brain Irradiation on Glial Activation, Brain Metabolism, and Behavior:a Positron Emission Tomography Study

Purpose: Radiotherapy is a frequently applied treatment modality for brain tumors. Concomitant irradiation of normal brain tissue can induce various physiological responses. The aim of this study was to investigate whether acute and early-delayed effects of brain irradiation on glial activation and brain metabolism can be detected with positron emission tomography (PET) and whether these effects are correlated with behavioral changes. Procedures: Rats underwent 0-, 10-, or 25-Gy whole-brain irradiation. At 3 and 31 days post irradiation, 1-(2-chlorophenyl)-N-[11C]methyl-(1-methylpropyl)-3-isoquinoline carboxamide ([11C]PK11195) and 2-deoxy-2-[18F]fluoro-d-glucose ([18F]FDG) PET scans were acquired to detect changes in glial activation (neuroinflammation) and glucose metabolism, respectively. The open-field test (OFT) was performed on days 6 and 27 to assess behavioral changes. Results: Twenty-five-gray-irradiated rats showed higher [11C]PK11195 uptake in most brain regions than controls on day 3 (striatum, hypothalamus, accumbens, septum p < 0.05), although some brain regions had lower uptake (cerebellum, parietal association/retrosplenial visual cortex, frontal association/motor cortex, somatosensory cortex, p < 0.05). On day 31, several brain regions in 25-Gy-irradiated rats still showed significantly higher [11C]PK11195 uptake than controls and 10-Gy-irradiated group (p < 0.05). Within-group analysis showed that [11C]PK11195 uptake in individual brain regions of 25-Gy treated rats remained stable or slightly increased between days 3 and 31. In contrast, a significant reduction (p < 0.05) in tracer uptake between days 3 and 31 was found in all brain areas of controls and 10-Gy-irradiated animals. Moreover, 10-Gy treatment led to a significantly higher [18F]FDG uptake on day 3 (p < 0.05). [18F]FDG uptake decreased between days 3 and 31 in all groups; no significant differences between groups were observed anymore on day 31, except for increased uptake in the hypothalamus in the 10-Gy group. The OFT did not show any significant differences between groups. Conclusions: Non-invasive PET imaging indicated that brain irradiation induces neuroinflammation and a metabolic flare, without causing acute or early-delayed behavioral changes

Optimization of the k(2)' Parameter Estimation for the Pharmacokinetic Modeling of Dynamic PIB PET Scans Using SRTM2

Background: This study explores different approaches to estimate the clearance rate of the reference tissue (k2 ') parameter used for pharmacokinetic modeling, using the simplified reference tissue model 2 (SRMT2) and further explores the effect on the binding potential (BPND) of C-11-labeled Pittsburgh Compound B (PIB) PET scans. Methods: Thirty subjects underwent a dynamic PIB PET scan and were classified as PIB positive (+) or negative (-). Thirteen regions were defined from where to estimate k2 ': the whole brain, eight anatomical region based on the Hammer's atlas, one region based on a SPM comparison between groups on a voxel level, and three regions using different BPNDSRTM thresholds. Results: The different approaches resulted in distinct k2 ' estimations per subject. The median value of the estimated k2 ' across all subjects in the whole brain was 0.057. In general, PIB+ subjects presented smaller k2 ' estimates than this median, and PIB-, larger. Furthermore, only threshold and white matter methods resulted in non-significant differences between groups. Moreover, threshold approaches yielded the best correlation between BPNDSRTM and BPNDSRTM2 for both groups (R-2 = 0.85 for PIB+, and R-2 = 0.88 for PIB-). Lastly, a sensitivity analysis showed that overestimating k2 ' values resulted in less biased BPNDSRTM2 estimates. Conclusion: Setting a threshold on BPNDSRTM might be the best method to estimate k2 ' in voxel-based modeling approaches, while the use of a white matter region might be a better option for a volume of interest based analysis

Tracer-specific PET and SPECT templates for automatic co-registration of functional rat brain images

Objectives: Template based spatial co-registration of PET and SPECT data is an important first step in its semi- automatic processing, facilitating VOI- and voxel-based analysis. Although this procedure is standard in human, using corresponding MRI images, these systems are often not accessible for preclinical research. Alternatively, manual co-registration of images to a MRI template is often performed. However, this is operator dependent and can introduce bias. Therefore, we constructed several tracer-specific PET and SPECT rat brain templates for automatic co-registration, spatially aligned with a widely used MRI-based template in Paxinos stereotactic space [1]. Methods: PET (18F-FDG, 11C-PK11195, and 11C-MeDAS) and SPECT (99mTc-HMPAO) brain scans were acquired from healthy male Sprague-Dawley and Wistar rats. Symmetrical left-right templates were constructed by averaging the scans. Within-modality registration was performed by minimizing the sum of squared difference and template to MRI registration by normalized mutual information maximization algorithm. For validation purposes, PET scans were acquired from a rat model of multiple sclerosis (MS) where focal demyelination was induced by injection of lysolecithin (or control saline) in right corpus callosum and striatum. Parametric SUV images were created for automatic co-registration. The validity of the templates was assessed by estimation of registration accuracy errors, inter-subject variability, right-to-left asymmetry indices, and voxel-based analysis of the MS model [2]. Results: The obtained mean registration errors were 0.097-1.277mm for PET, and 0.059-0.477mm for SPECT. These values are below spatial resolution of the cameras (1.4mm and 0.8mm, respectively) and in agreement with human literature [3]. Results from voxel-based analyses (Figure 1) correspond with those previously reported using VOI-based analysis [4], and correlate with the regions where lesion was induced. Conclusion: The constructed tracer-specific templates allow accurate registration of functional rat brain data, using automatic normalization algorithms available in standard packages (e.g., SPM, FSL), supporting either VOI- or voxel-based analysis. The templates will be made freely available for the research community

Delayed Effects of a Single Dose Whole-Brain Radiation Therapy on Glucose Metabolism and Myelin Density:a Longitudinal PET Study

Purpose: Radiotherapy is an important treatment option for brain tumors, but the unavoidable irradiation of normal brain tissue can lead to delayed cognitive impairment. The mechanisms involved are still not well explained and, therefore, new tools to investigate the processes leading to the delayed symptoms of brain irradiation are warranted. In this study, positron emission tomography (PET) is used to explore delayed functional changes induced by brain irradiation. Materials and methods: Male Wistar rats were subjected to a single 25-Gy dose of whole brain X-ray irradiation, or sham-irradiation. To investigate delayed effects of radiation on cerebral glucose metabolism and myelin density, 18F-fluorodeoxyglucose (18F-FDG) PET scans were performed at baseline and on day 64 and 94, whereas N-11C-methyl-4,4′-diaminostilbene (11C-MeDAS) PET scans were performed at baseline and on day 60 and 90 post-irradiation. In addition, the open field test (OFT) and novel spatial recognition (NSR) test were performed at baseline and on days 59 and 89 to investigate whether whole brain irradiation induces behavioral changes. Results: Whole-brain irradiation caused loss of bodyweight and delayed cerebral hypometabolism, with 18F-FDG uptake in all brain regions being significantly decreased in irradiated rat on day 64 while it remained unchanged in control animals. Only amygdala and cortical brain regions of irradiated rats still showed reduced 18F-FDG uptake on day 94. 11C-MeDAS uptake in control animals was significantly lower on days 60 and 90 than at the baseline, suggesting a reduction in myelin density in young adults. In irradiated animals, 11C-MeDAS uptake was similarly reduced on day 60, but on day 90 tracer uptake was somewhat increased and not significantly different from baseline anymore. Behavioral tests showed a similar pattern in control and irradiated animals. In both groups, the OFT showed significantly reduced mobility on days 59 and 89, whereas the NSR did not reveal any significant changes in spatial memory over time. Interestingly, a positive correlation between the NSR and 11C-MeDAS uptake was observed in irradiated rats. Conclusions: Whole-brain irradiation causes delayed brain hypometabolism, which is not accompanied by white matter loss. Irradiated animals showed similar behavioral changes over time as control animals and, therefore, cerebral hypometabolism could not be linked to behavioral abnormalities. However, spatial memory seems to be associated with myelin density in irradiated rats