A systematic review of physiological methods in rodent pharmacological MRI studies

Rationale: Pharmacological magnetic resonance imaging (phMRI) provides an approach to study effects of drug challenges on brain processes. Elucidating mechanisms of drug action helps us to better understand the workings of neurotransmitter systems, map brain function or facilitate drug development. phMRI is increasingly used in preclinical research employing rodent models; however, data interpretation and integration are complicated by the use of different experimental approaches between laboratories. In particular, the effects of different anaesthetic regimes upon neuronal and haemodynamic processes and baseline physiology could be problematic. Objectives: This paper investigates how differences in phMRI research methodologies are manifested and considers associated implications, placing particular emphasis on choice of anaesthetic regimes. Methods: A systematic review of rodent phMRI studies was conducted. Factors such as those describing anaesthetic regimes (e.g. agent, dosage) and parameters relating to physiological maintenance (e.g. ventilatory gases) and MRI method were recorded. Results: We identified 126 eligible studies and found that the volatile agents isoflurane (43.7 %) and halothane (33.3 %) were most commonly used for anaesthesia, but dosage and mixture of ventilatory gases varied substantially between laboratories. Relevant physiological parameters were usually recorded, although 32 % of studies did not provide cardiovascular measures. Conclusions: Anaesthesia and animal preparation can influence phMRI data profoundly. The variation of anaesthetic type, dosage regime and ventilatory gases makes consolidation of research findings (e.g. within a specific neurotransmitter system) difficult. Standardisation of a small(er) number of preclinical phMRI research methodologies and/or increased consideration of approaches that do not require anaesthesia is necessary to address these challenges

Normothermic mouse functional MRI of acute focal thermostimulation for probing nociception

Combining mouse genomics and functional magnetic resonance imaging (fMRI) provides a promising tool to unravel the molecular mechanisms of chronic pain. Probing murine nociception via the blood oxygenation level-dependent (BOLD) effect is still challenging due to methodological constraints. Here we report on the reproducible application of acute noxious heat stimuli to examine the feasibility and limitations of functional brain mapping for central pain processing in mice. Recent technical and procedural advances were applied for enhanced BOLD signal detection and a tight control of physiological parameters. The latter includes the development of a novel mouse cradle designed to maintain whole-body normothermia in anesthetized mice during fMRI in a way that reflects the thermal status of awake, resting mice. Applying mild noxious heat stimuli to wildtype mice resulted in highly significant BOLD patterns in anatomical brain structures forming the pain matrix, which comprise temporal signal intensity changes of up to 6% magnitude. We also observed sub-threshold correlation patterns in large areas of the brain, as well as alterations in mean arterial blood pressure (MABP) in response to the applied stimulus

The role of ultrasound as a diagnostic and therapeutic tool in experimental animal models of stroke: A review

Ultrasound is a noninvasive technique that provides real-time imaging with excellent resolution, and several studies demonstrated the potential of ultrasound in acute ischemic stroke monitoring. However, only a few studies were performed using animal models, of which many showed ultrasound to be a safe and effective tool also in therapeutic applications. The full potential of ultrasound application in experimental stroke is yet to be explored to further determine the limitations of this technique and to ensure the accuracy of translational research. This review covers the current status of ultrasound applied to monitoring and treatment in experimental animal models of stroke and examines the safety, limitations, and future perspectives.This research was funded by the Carlos III Health Institute Health Care Research Fund grant number FIS PI16/01052 and cofunded by the European Regional Development Fund (ERDF)–Miguel Servet (CP15/00069, CPII20/00002 to María Gutiérrez–Fernández; CP20/00024 to Laura Otero–Ortega) and predoctoral fellowship (FI17/00188 to Mari Carmen Gómez–de Frutos, FI18/00026 to Fernando Laso–García) and the INVICTUS PLUS Spanish Network (RD16/0019/0005) of the Carlos III Health Institute (ISCIII)

GLUTAMATE REGULATION IN THE HIPPOCAMPAL TRISYNAPTIC PATHWAY IN AGING AND STATUS EPILEPTICUS

A positive correlation exists between increasing age and the incidence of hippocampal-associated dysfunction and disease. Normal L-glutamate neurotransmission is absolutely critical for hippocampal function, while abnormal glutamate neurotransmission has been implicated in many neurodegenerative diseases. Previous studies, overwhelmingly utilizing ex vivo methods, have filled the literature with contradicting reports about hippocampal glutamate regulation during aging. For our studies, enzyme-based ceramic microelectrode arrays (MEA) were used for rapid (2 Hz) measurements of extracellular glutamate in the hippocampal trisynaptic pathway of young (3-6 months), late-middle aged (18 mo.) and aged (24 mo.) urethane-anesthetized Fischer 344 rats. Compared to young animals, glutamate terminals in cornu ammonis 3 (CA3) showed diminished potassium-evoked glutamate release in aged rats. In late-middle aged animals, terminals in the dentate gyrus (DG) showed increased evoked release compared to young rats. The aged DG demonstrated an increased glutamate clearance capacity, indicating a possible age-related compensation to deal with the increased glutamate release that occurred in late-middle age. To investigate the impact of changes in glutamate regulation on the expression of a disease process, we modified the MEA technology to allow recordings in unanesthetized rats. These studies permitted us to measure glutamate regulation in the hippocampal formation without anesthetic effects, which showed a significant increase in basal glutamatergic tone during aging. Status epilepticus was induced by local application of 4-aminopyridine. Realtime glutamate measurements allowed us to capture never-before-seen spontaneous and highly rhythmic glutamate release events during status epilepticus. A significant correlation between pre-status tonic glutamate and the quantity of status epilepticus-associated convulsions and glutamate release events was determined. Taken together, this body of work identifies the DG and CA3 as the loci of age-associated glutamate dysregulation in the hippocampus, and establishes elevated levels of glutamate as a key factor controlling severity of status epilepticus in aged animals. Based upon the potential ability to discriminate brain areas experiencing seizure (i.e. synchronized spontaneous glutamate release) from areas not, we have initiated the development of a MEA for human use during temporal lobe resection surgery. The final studies presented here document the development and testing of a human microelectrode array prototype in non-human primates

Brain Injury

The present two volume book "Brain Injury" is distinctive in its presentation and includes a wealth of updated information on many aspects in the field of brain injury. The Book is devoted to the pathogenesis of brain injury, concepts in cerebral blood flow and metabolism, investigative approaches and monitoring of brain injured, different protective mechanisms and recovery and management approach to these individuals, functional and endocrine aspects of brain injuries, approaches to rehabilitation of brain injured and preventive aspects of traumatic brain injuries. The collective contribution from experts in brain injury research area would be successfully conveyed to the readers and readers will find this book to be a valuable guide to further develop their understanding about brain injury

Contribution of Intravital Neuroimaging to Study Animal Models of Multiple Sclerosis

Multiple sclerosis (MS) is a complex and long-lasting neurodegenerative disease of the central nervous system (CNS), characterized by the loss of myelin within the white matter and cortical fbers, axonopathy, and infammatory responses leading to consequent sensory-motor and cognitive defcits of patients. While complete resolution of the disease is not yet a reality, partial tissue repair has been observed in patients which ofers hope for therapeutic strategies. To address the molecular and cellular events of the pathomechanisms, a variety of animal models have been developed to investigate distinct aspects of MS disease. Recent advances of multiscale intravital imaging facilitated the direct in vivo analysis of MS in the animal models with perspective of clinical transfer to patients. This review gives an overview of MS animal models, focusing on the current imaging modalities at the microscopic and macroscopic levels and emphasizing the importance of multimodal approaches to improve our understanding of the disease and minimize the use of animals

MULTI-MODAL OPTICAL NEUROIMAGING AND APPLICATIONS

Optical imaging tools provide superior details than MRI, PET in monitoring the physiological and pathological state of brain in preclinical models. By combining different optical imaging modalities, a variety of physiological parameters (e.g. cerebral hemodynamics, metabolism and neuronal activity) could be detected simultaneously; such simultaneous imaging is expected to profoundly enhance our understanding of normal brain regulation and its disruption from neurovascular disorders. As a part of this thesis, I designed a multi-modal optical imaging system that could perform simultaneous laser speckle contrast imaging, wide-field fluorescence imaging and optical intrinsic signal imaging. The principles, processing methods and applications of these imaging modalities are presented in Chapter 1. The system was first applied to study a new atherothrombotic stroke model and to evaluate the recovery of stroke from different treatment protocols in mice (Chapter 2). Cerebral blood flow changes and thrombus formations were imaged by laser speckle contrast imaging and wide-field fluorescence imaging, respectively. We concluded that the combination treatment of tissue plasminogen activator and cathepsin G inhibitor improved the neurological outcomes of ischemic brain injury from induced atherothrombotic stroke. To investigate brain activity in high-resolution by optical imaging tools, cranial window preparation is an essential procedure to allow optical access to the brain. We also employed the optical imaging system to investigate the effects of cranial windows on monitoring neurovasculature by laser speckle contrast imaging (Chapter 3). Open-skull and thin-skull cranial window procedures were performed in separate experiments, and the neurovasculature underlying the cranial windows were monitored for fourteen days. The differences between two window types were systematically compared by parameters such as contrast-to-noise ratio and microvessel density. Finally, the last part of my thesis was to miniaturize the multi-modal bench-top imaging system to a head-mounted microscope, which allows imaging on awake freely moving animals. The natural physiological state of brain activities can be detected without the confounding effects of anesthetics. The current version of the microscope weighs less than 5 g and is able to perform laser speckle contrast imaging, wide-field fluorescence imaging and optical intrinsic signal imaging simultaneously. We are currently testing the miniaturized microscope to study a brain tumor murine model. Finally, I describe the current progress of miniaturized optical neuroimaging systems on awake moving animals in Chapter 4 of this thesis

The (un)conscious mouse as a model for human brain functions: key principles of anesthesia and their impact on translational neuroimaging

In recent years, technical and procedural advances have brought functional magnetic resonance imaging (fMRI) to the field of murine neuroscience. Due to its unique capacity to measure functional activity non-invasively, across the entire brain, fMRI allows for the direct comparison of large-scale murine and human brain functions. This opens an avenue for bidirectional translational strategies to address fundamental questions ranging from neurological disorders to the nature of consciousness. The key challenges of murine fMRI are: (1) to generate and maintain functional brain states that approximate those of calm and relaxed human volunteers, while (2) preserving neurovascular coupling and physiological baseline conditions. Low-dose anesthetic protocols are commonly applied in murine functional brain studies to prevent stress and facilitate a calm and relaxed condition among animals. Yet, current mono-anesthesia has been shown to impair neural transmission and hemodynamic integrity. By linking the current state of murine electrophysiology, Ca(2+) imaging and fMRI of anesthetic effects to findings from human studies, this systematic review proposes general principles to design, apply and monitor anesthetic protocols in a more sophisticated way. The further development of balanced multimodal anesthesia, combining two or more drugs with complementary modes of action helps to shape and maintain specific brain states and relevant aspects of murine physiology. Functional connectivity and its dynamic repertoire as assessed by fMRI can be used to make inferences about cortical states and provide additional information about whole-brain functional dynamics. Based on this, a simple and comprehensive functional neurosignature pattern can be determined for use in defining brain states and anesthetic depth in rest and in response to stimuli. Such a signature can be evaluated and shared between labs to indicate the brain state of a mouse during experiments, an important step toward translating findings across species