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

A kainic acid-induced status epilepticus model of epileptogenesis in the C57BL/6J mouse. Interventions targeting nitric oxide and NMDA receptor-mediated pathophysiology

In this thesis, the behavioral, electrographic and neurobiological effects of a period of kainic acid-induced status epilepticus (SE) on the C57BL/6J inbred mouse strain are characterised. The severity of epileptic behaviour was scored, used immunohistochemistry to investigate the anatomical distribution of c-Fos expression in the hippocampal formation following SE and recorded EEG during and after SE using an implantable, wireless telemetry device. Further to assessing the severity of SE, changes subsequent to seizures related to the emergence of chronic epilepsy were investigated, including reactive gliosis and synaptogenesis and epileptiform discharges in the EEG trace. I investigated the potential of a range of pharmacological agents for modulating the severity of induced seizures and disease progression. These included drugs targetting the NR2B subunit of the NMDA receptor (RO 25-6981), neuronal nitric oxide synthase (L-NPA), The post synaptic density protein 95 (Tat-NR2B9a) and inducible nitric oxide synthase (1400W). L-NPA, when administered prior to the induction of SE was found to profoundly suppress the emergence of epileptiform activity, including behavioural, electrographic and neurobiological indicators. Further, L-NPA’s modulation of the precipitating event lead to a decrease in neurobiological changes associated with epileptogenesis, such as reactive gliosis in the CA3 region of the hippocampus and 5 elevated synaptogenesis in th molecular layer of the hippocampus. This correlated with a marked decrease in epileptiform discharges in the EEG trace. A novel method of kainic acid administration was trialed, involving multiple small doses of the drug, titrated by the severity of behaviour. This method led to a decrease in mortality and an increase in the severity and inter-individual uniformity of SE, assessed by the analysis of behaviour, EEG and c-Fos expression in the hippocampus. Furthermore, this method induced neurobiological changes associated with epileptogenesis 3 days following SE and was associated with an increased frequency of epileptiform discharges for 7 days post SE

Coupled Oscillators Model of Hyperexcitable Neuroglial Networks

Presented in this thesis is an oscillator-based neuroglial model capable of generating Spontaneous Electrical Discharges (SEDs) in hyperexcitable conditions. The network is composed of 16 coupled Cognitive Rhythm Generators (CRGs), which are mathematical constructs that represent one of the following populations of excitable cells: an excitatory pyramidal cell population, an inhibitory interneuron population, an astrocyte population, or a microglial population. Various mechanisms leading to hyperexcitability were investigated, and results suggest an important role for astrocytes and microglia in the generation of SEDs of various durations. Analysis of the resultant SEDs revealed two underlying distributions with differing properties. Particularly, short and long SEDs are associated with deterministic and random processes, respectively. Furthermore, there was evidence of theta-HFO phase-amplitude cross-frequency coupling (CFC) in the short SEDs, and delta-HFO CFC in the long SEDs, as was previously reported in a mouse model of Seizure-Like Events (SLEs) and in human patients with epilepsy.M.A.S

The microbiota-gut-brain axis

The importance of the gut-brain axis in maintaining homeostasis has long been appreciated. However, the past 15 yr have seen the emergence of the microbiota (the trillions of microorganisms within and on our bodies) as one of the key regulators of gut-brain function and has led to the appreciation of the importance of a distinct microbiota-gut-brain axis. This axis is gaining ever more traction in fields investigating the biological and physiological basis of psychiatric, neurodevelopmental, age-related, and neurodegenerative disorders. The microbiota and the brain communicate with each other via various routes including the immune system, tryptophan metabolism, the vagus nerve and the enteric nervous system, involving microbial metabolites such as short-chain fatty acids, branched chain amino acids, and peptidoglycans. Many factors can influence microbiota composition in early life, including infection, mode of birth delivery, use of antibiotic medications, the nature of nutritional provision, environmental stressors, and host genetics. At the other extreme of life, microbial diversity diminishes with aging. Stress, in particular, can significantly impact the microbiota-gut-brain axis at all stages of life. Much recent work has implicated the gut microbiota in many conditions including autism, anxiety, obesity, schizophrenia, Parkinson's disease, and Alzheimer's disease. Animal models have been paramount in linking the regulation of fundamental neural processes, such as neurogenesis and myelination, to microbiome activation of microglia. Moreover, translational human studies are ongoing and will greatly enhance the field. Future studies will focus on understanding the mechanisms underlying the microbiota-gut-brain axis and attempt to elucidate microbial-based intervention and therapeutic strategies for neuropsychiatric disorders