Neuro-behavioural impact of changes in hippocampal neural activity

Hippocampal metabolic hyperactivity and neural disinhibition, i.e. reduced GABAergic inhibition, have been associated with schizophrenia, although a causal link between disinhibition and metabolic hyperactivity remains to be demonstrated. Regional neural disinhibition might also disrupt neural activation in projection sites, such as the prefrontal cortex and striatum, which may contribute to cognitive impairments and positive symptoms characteristic of schizophrenia. To further examine the brain-wide impact of hippocampal disinhibition and the associated behavioural and cognitive changes, we combined ventral hippocampal infusion of the GABA-A antagonist picrotoxin with translational neural imaging and behavioural methods in rats. First, we used a conditioned emotional response paradigm to assess the impact of hippocampal disinhibition on aversive conditioning and salience modulation in the form of latent inhibition (chapter 2), both of which have been reported to be disrupted in schizophrenia. These experiments demonstrated hippocampal disinhibition caused disrupted cue and contextual fear conditioning, whilst we found no evidence that hippocampal disinhibition affects salience modulation as reflected by latent inhibition of fear conditioning. The disruption of fear conditioning resembles aversive conditioning deficits reported in schizophrenia and may reflect disruption of neural processing at hippocampal projection sites. Second, we used SPECT imaging to map changes in brain-wide activation patterns caused by hippocampal GABA dysfunction (chapter 3). SPECT experiments revealed increased neural activation around the infusion site in the ventral hippocampus, resembling metabolic hippocampal hyperactivity consistently reported in schizophrenia. In contrast, activation in the dorsal hippocampus was significantly reduced. This resembles the finding of anterior hippocampal hyperactivity coupled with reduced posterior hippocampal activation in patients with schizophrenia. Hippocampal disinhibition also caused marked extra-hippocampal activation changes in neocortical and subcortical sites, including sites implicated in fear learning and anxiety such as the medial prefrontal cortex (mPFC), septum, lateral hypothalamus and extended amygdala which may contribute to the disruption of fear conditioning demonstrated in chapter 2. Importantly, increased activation in the mPFC corresponds with previously reported prefrontal-dependent attentional deficits caused by hippocampal disinhibition. Third, to complement these findings we used magnetic resonance spectroscopy (MRS) to determine the effects of hippocampal disinhibition on neuro-metabolites within the mPFC (chapter 4). Using MRS, we demonstrated that hippocampal disinhibition causes metabolic changes in the mPFC, reflected by increased myo-inositol and reduced GABA concentrations. Overall, our results demonstrate ventral hippocampal disinhibition causes regional metabolic hyperactivity, supporting a causal role between GABA dysfunction and increased anterior hippocampal activity. In addition, hippocampal disinhibition causes activation and metabolic changes at distal sites, which may contribute to clinically relevant behavioural deficits, including impaired aversive conditioning, as demonstrated in our behavioural studies

Neuro-behavioural impact of changes in hippocampal neural activity

Hippocampal metabolic hyperactivity and neural disinhibition, i.e. reduced GABAergic inhibition, have been associated with schizophrenia, although a causal link between disinhibition and metabolic hyperactivity remains to be demonstrated. Regional neural disinhibition might also disrupt neural activation in projection sites, such as the prefrontal cortex and striatum, which may contribute to cognitive impairments and positive symptoms characteristic of schizophrenia. To further examine the brain-wide impact of hippocampal disinhibition and the associated behavioural and cognitive changes, we combined ventral hippocampal infusion of the GABA-A antagonist picrotoxin with translational neural imaging and behavioural methods in rats. First, we used a conditioned emotional response paradigm to assess the impact of hippocampal disinhibition on aversive conditioning and salience modulation in the form of latent inhibition (chapter 2), both of which have been reported to be disrupted in schizophrenia. These experiments demonstrated hippocampal disinhibition caused disrupted cue and contextual fear conditioning, whilst we found no evidence that hippocampal disinhibition affects salience modulation as reflected by latent inhibition of fear conditioning. The disruption of fear conditioning resembles aversive conditioning deficits reported in schizophrenia and may reflect disruption of neural processing at hippocampal projection sites. Second, we used SPECT imaging to map changes in brain-wide activation patterns caused by hippocampal GABA dysfunction (chapter 3). SPECT experiments revealed increased neural activation around the infusion site in the ventral hippocampus, resembling metabolic hippocampal hyperactivity consistently reported in schizophrenia. In contrast, activation in the dorsal hippocampus was significantly reduced. This resembles the finding of anterior hippocampal hyperactivity coupled with reduced posterior hippocampal activation in patients with schizophrenia. Hippocampal disinhibition also caused marked extra-hippocampal activation changes in neocortical and subcortical sites, including sites implicated in fear learning and anxiety such as the medial prefrontal cortex (mPFC), septum, lateral hypothalamus and extended amygdala which may contribute to the disruption of fear conditioning demonstrated in chapter 2. Importantly, increased activation in the mPFC corresponds with previously reported prefrontal-dependent attentional deficits caused by hippocampal disinhibition. Third, to complement these findings we used magnetic resonance spectroscopy (MRS) to determine the effects of hippocampal disinhibition on neuro-metabolites within the mPFC (chapter 4). Using MRS, we demonstrated that hippocampal disinhibition causes metabolic changes in the mPFC, reflected by increased myo-inositol and reduced GABA concentrations. Overall, our results demonstrate ventral hippocampal disinhibition causes regional metabolic hyperactivity, supporting a causal role between GABA dysfunction and increased anterior hippocampal activity. In addition, hippocampal disinhibition causes activation and metabolic changes at distal sites, which may contribute to clinically relevant behavioural deficits, including impaired aversive conditioning, as demonstrated in our behavioural studies

Serotonergic modulation of the ventral pallidum by 5HT1A, 5HT5A, 5HT7 AND 5HT2C receptors

Introduction: Serotonin's involvement in reward processing is controversial. The large number of serotonin receptor sub-types and their individual and unique contributions have been difficult to dissect out, yet understanding how specific serotonin receptor sub-types contribute to its effects on areas associated with reward processing is an essential step. Methods: The current study used multi-electrode arrays and acute slice preparations to examine the effects of serotonin on ventral pallidum (VP) neurons. Approach for statistical analysis: extracellular recordings were spike sorted using template matching and principal components analysis, Consecutive inter-spike intervals were then compared over periods of 1200 seconds for each treatment condition using a student’s t test. Results and conclusions: Our data suggests that excitatory responses to serotonin application are pre-synaptic in origin as blocking synaptic transmission with low-calcium aCSF abolished these responses. Our data also suggests that 5HT1a, 5HT5a and 5HT7 receptors contribute to this effect, potentially forming an oligomeric complex, as 5HT1a antagonists completely abolished excitatory responses to serotonin application, while 5HT5a and 5HT7 only reduced the magnitude of excitatory responses to serotonin. 5HT2c receptors were the only serotonin receptor sub-type tested that elicited inhibitory responses to serotonin application in the VP. These findings, combined with our previous data outlining the mechanisms underpinning dopamine's effects in the VP, provide key information, which will allow future research to fully examine the interplay between serotonin and dopamine in the VP. Investigation of dopamine and serotonins interaction may provide vital insights into our understanding of the VP's involvement in reward processing. It may also contribute to our understanding of how drugs of abuse, such as cocaine, may hijack these mechanisms in the VP resulting in sensitization to drugs of abuse

Inhibiting Axon Degeneration in a Mouse Model of Acute Brain Injury Through Deletion of Sarm1

Traumatic brain injury (TBI) is a leading cause of disability worldwide. Annually, 150 to 200/1,000,000 people become disabled as a result of brain trauma. Axonal degeneration is a critical, early event following TBI of all severities but whether axon degeneration is a driver of TBI remains unclear. Molecular pathways underlying the pathology of TBI have not been defined and there is no efficacious treatment for TBI. Despite this significant societal impact, surprisingly little is known about the molecular mechanisms that actively drive axon degeneration in any context and particularly following TBI. Although severe brain injury may cause immediate disruption of axons (primary axotomy), it is now recognized that the most frequent form of traumatic axonal injury (TAI) is mediated by a cascade of events that ultimately result in secondary axonal disconnection (secondary axotomy) within hours to days. Proposed mechanisms include immediate post-traumatic cytoskeletal destabilization as a direct result of mechanical breakage of microtubules, as well as catastrophic local calcium dysregulation resulting in microtubule depolymerization, impaired axonal transport, unmitigated accumulation of cargoes, local axonal swelling, and finally disconnection. The portion of the axon that is distal to the axotomy site remains initially morphologically intact. However, it undergoes sudden rapid fragmentation along its full distal length ~72 h after the original axotomy, a process termed Wallerian degeneration. Remarkably, mice mutant for the Wallerian degeneration slow (Wlds) protein exhibit ~tenfold (for 2–3 weeks) suppressed Wallerian degeneration. Yet, pharmacological replication of the Wlds mechanism has proven difficult. Further, no one has studied whether Wlds protects from TAI. Lastly, owing to Wlds presumed gain-of-function and its absence in wild-type animals, direct evidence in support of a putative endogenous axon death signaling pathway is lacking, which is critical to identify original treatment targets and the development of viable therapeutic approaches. Novel insight into the pathophysiology of Wallerian degeneration was gained by the discovery that mutant Drosophila flies lacking dSarm (sterile a/Armadillo/Toll-Interleukin receptor homology domain protein) cell-autonomously recapitulated the Wlds phenotype. The pro-degenerative function of the dSarm gene (and its mouse homolog Sarm1) is widespread in mammals as shown by in vitro protection of superior cervical ganglion, dorsal root ganglion, and cortical neuron axons, as well as remarkable in-vivo long-term survival (\u3e2 weeks) of transected sciatic mouse Sarm1 null axons. Although the molecular mechanism of function remains to be clarified, its discovery provides direct evidence that Sarm1 is the first endogenous gene required for Wallerian degeneration, driving a highly conserved genetic axon death program. The central goals of this thesis were to determine (1) whether post-traumatic axonal integrity is preserved in mice lacking Sarm1, and (2) whether loss of Sarm1 is associated with improved functional outcome after TBI. I show that mice lacking the mouse Toll receptor adaptor Sarm1 gene demonstrate multiple improved TBI-associated phenotypes after injury in a closed-head mild TBI model. Sarm1-/- mice developed fewer beta amyloid precursor protein (βAPP) aggregates in axons of the corpus callosum after TBI as compared to Sarm1+/+ mice. Furthermore, mice lacking Sarm1 had reduced plasma concentrations of the phosphorylated axonal neurofilament subunit H, indicating that axonal integrity is maintained after TBI. Strikingly, whereas wild type mice exhibited a number of behavioral deficits after TBI, I observed a strong, early preservation of neurological function in Sarm1-/- animals. Finally, using in vivo proton magnetic resonance spectroscopy, I found tissue signatures consistent with substantially preserved neuronal energy metabolism in Sarm1-/- mice compared to controls immediately following TBI. My results indicate that the Sarm1-mediated prodegenerative pathway promotes pathogenesis in TBI and suggest that anti-Sarm1 therapeutics are a viable approach for preserving neurological function after TBI

Investigating the role of Gamma-aminobutyric acid (GABA) in sedation: a combinedelectrophysiological, haemodynamicand spectroscopic study in humans

A better understanding of the mechanisms of anaesthesia and sedation are expected not only to improve the understanding of the neural correlates of consciousness but also to help improve safety from the complications of anaesthesia/ sedation and develop safer drugs and objective brain function monitoring systems. Neuroimaging modalities such as functional MRI, magnetoencephalography and MR spectroscopy provide complimentary information about brain functions and can help interrogate brain activity in a living human brain. Most anaesthetic drugs act by enhancing the inhibitory actions of GABA in the brain. Most neuroimaging research has focused on anaesthetic-induced unconsciousness, with only few investigating the earliest levels of sedation-induced altered consciousness. The work in this thesis used a range of advanced neuroimaging modalities to investigate the role of GABA (through a GABA-ergic drug, propofol), during mild sedation, in humans. This was performed as a series of experiments within two, sequential, scanning sessions, MEG followed by fMRI, in the same participants. Propofol resulted in a dissociation of the visual gamma band response (decreased evoked, increased induced power). This was related to a reduced BOLD fMRI response but there were no changes in MRS detectable GABA concentration. Response to multisensory stimulation also revealed interesting changes with MEG and fMRI. Functional connectivity analyses showed changes in connectivities of the posterior cingulate cortex (key hub of default-mode network) and thalamus with each other and other key brain regions. Resting state networks were identified with MEG too, which revealed interesting increases in connectivity in certain band- limited networks while motor networks showed no change. Perfusion fMRI using arterial spin labelling revealed a global and regional reduction in perfusion, highlighting some of the key regions (frontal cortex, precuenus, PCC and thalamus) involved in sedation

Preclinical MRI of the Kidney

This Open Access volume provides readers with an open access protocol collection and wide-ranging recommendations for preclinical renal MRI used in translational research. The chapters in this book are interdisciplinary in nature and bridge the gaps between physics, physiology, and medicine. They are designed to enhance training in renal MRI sciences and improve the reproducibility of renal imaging research. Chapters provide guidance for exploring, using and developing small animal renal MRI in your laboratory as a unique tool for advanced in vivo phenotyping, diagnostic imaging, and research into potential new therapies. Written in the highly successful Methods in Molecular Biology series format, chapters include introductions to their respective topics, lists of the necessary materials and reagents, step-by-step, readily reproducible laboratory protocols, and tips on troubleshooting and avoiding known pitfalls. Cutting-edge and thorough, Preclinical MRI of the Kidney: Methods and Protocols is a valuable resource and will be of importance to anyone interested in the preclinical aspect of renal and cardiorenal diseases in the fields of physiology, nephrology, radiology, and cardiology. This publication is based upon work from COST Action PARENCHIMA, supported by European Cooperation in Science and Technology (COST). COST (www.cost.eu) is a funding agency for research and innovation networks. COST Actions help connect research initiatives across Europe and enable scientists to grow their ideas by sharing them with their peers. This boosts their research, career and innovation. PARENCHIMA (renalmri.org) is a community-driven Action in the COST program of the European Union, which unites more than 200 experts in renal MRI from 30 countries with the aim to improve the reproducibility and standardization of renal MRI biomarkers

Preclinical MRI of the kidney : methods and protocols

This Open Access volume provides readers with an open access protocol collection and wide-ranging recommendations for preclinical renal MRI used in translational research. The chapters in this book are interdisciplinary in nature and bridge the gaps between physics, physiology, and medicine. They are designed to enhance training in renal MRI sciences and improve the reproducibility of renal imaging research. Chapters provide guidance for exploring, using and developing small animal renal MRI in your laboratory as a unique tool for advanced in vivo phenotyping, diagnostic imaging, and research into potential new therapies. Written in the highly successful Methods in Molecular Biology series format, chapters include introductions to their respective topics, lists of the necessary materials and reagents, step-by-step, readily reproducible laboratory protocols, and tips on troubleshooting and avoiding known pitfalls. Cutting-edge and thorough, Preclinical MRI of the Kidney: Methods and Protocols is a valuable resource and will be of importance to anyone interested in the preclinical aspect of renal and cardiorenal diseases in the fields of physiology, nephrology, radiology, and cardiology. This publication is based upon work from COST Action PARENCHIMA, supported by European Cooperation in Science and Technology (COST). COST (www.cost.eu) is a funding agency for research and innovation networks. COST Actions help connect research initiatives across Europe and enable scientists to grow their ideas by sharing them with their peers. This boosts their research, career and innovation. PARENCHIMA (renalmri.org) is a community-driven Action in the COST program of the European Union, which unites more than 200 experts in renal MRI from 30 countries with the aim to improve the reproducibility and standardization of renal MRI biomarkers