Neonatal brain resting-state functional connectivity imaging modalities

Infancy is the most critical period in human brain development. Studies demonstrate that subtle brain abnormalities during this state of life may greatly affect the developmental processes of the newborn infants. One of the rapidly developing methods for early characterization of abnormal brain development is functional connectivity of the brain at rest. While the majority of resting-state studies have been conducted using magnetic resonance imaging (MRI), there is clear evidence that resting-state functional connectivity (rs-FC) can also be evaluated using other imaging modalities. The aim of this review is to compare the advantages and limitations of different modalities used for the mapping of infants’ brain functional connectivity at rest. In addition, we introduce photoacoustic tomography, a novel functional neuroimaging modality, as a complementary modality for functional mapping of infants’ brain

Photoacoustic Neuroimaging - Perspectives on a Maturing Imaging Technique and its Applications in Neuroscience

A prominent goal of neuroscience is to improve our understanding of how brain structure and activity interact to produce perception, emotion, behavior, and cognition. The brain’s network activity is inherently organized in distinct spatiotemporal patterns that span scales from nanometer-sized synapses to meter-long nerve fibers and millisecond intervals between electrical signals to decades of memory storage. There is currently no single imaging method that alone can provide all the relevant information, but intelligent combinations of complementary techniques can be effective. Here, we thus present the latest advances in biomedical and biological engineering on photoacoustic neuroimaging in the context of complementary imaging techniques. A particular focus is placed on recent advances in whole-brain photoacoustic imaging in rodent models and its influential role in bridging the gap between fluorescence microscopy and more non-invasive techniques such as magnetic resonance imaging (MRI). We consider current strategies to address persistent challenges, particularly in developing molecular contrast agents, and conclude with an overview of potential future directions for photoacoustic neuroimaging to provide deeper insights into healthy and pathological brain processes

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

Investigation of neuronal activity in a murine model of Alzheimer’s disease using in vivo two-photon calcium imaging

Alzheimer’s disease (AD) is one of the biggest challenges for biomedical research nowadays as with the growth of life span more and more people are affected by this disorder. Etiology of AD is unknown, yet growing evidence identifies alterations in neuronal activity as of the great importance for pathology. Although several significant studies of neuronal activity alteration in AD were done during the last decade, none of them addressed the question of the time course of these changes over the disease progression. Alzheimer’s disease (AD) is characterized by impairments of brain neurons that are responsible for the storage and processing of information. Studies have revealed decrease in the activity of neurons (Silverman et al., 2001; Prvulovic et al., 2005) and it was proposed that generalized hypoactivity and silencing of brain circuits takes place as formulated in the synaptic failure hypothesis (Selkoe, 2002). However, more recent studies also reported opposite effects – hyperexcitability and hyperactivity of neurons in the AD models (Busche et al., 2008; Sanchez et al., 2012; Liebscher et al., 2016). It still remains unclear if these are two sides of the same coin or if these are two stages, that follow each other. Moreover, it is not clear if observed neuronal activity alterations are caused by the dysfunction of individual neurons or if overall circuitry is disturbed because the crucial “activity controllers” (most probably - inhibitory neurons) alter their activity. This project aimed to examine spontaneous neuronal activity in the murine model of AD at the early stages of disease progression using chronic in vivo imaging to address the character and the stability of neuronal activity alterations as well relation of the activity alterations to amyloid plaque proximity. Compared to earlier studies the approach of in vivo awake calcium imaging used in the current study has many benefits for brain research. The main advantage is that brain activity can be measured without artifacts generated by anesthesia, which can exaggerate or mitigate experimental readouts. In this project, I used genetically encoded calcium indicator GCaMP6 that enables prolonged repetitive imaging of the same neurons in an intact environment. Recording of calcium transients in cell bodies of neurons was accompanied by in vivo imaging of Aβ plaques and followed by immunohistochemical staining of GCaMP6-expressing neurons to investigate how activity changes are correlated with proximity to the plaque. All the experiments were done in awake mice to ensure the absence of anesthesia-derived impact on spontaneous neuronal activity. My results support previously published reports of the increased proportion of hyperactive excitatory neurons in the AD mouse model. Importantly, my results also demonstrate that this increased activity is present in the awake state, is stable over a longer period of time (one month) and does not depend on the distance to the closest plaque. These findings support the hypothesis of permanent network alterations driving aberrant activity patterns that appear early in the disease progression, resulting in a chronic excitation/inhibition disbalance. Another important finding of my project is that individual neurons do not stay in the silent state and most of them remain functional demonstrating normal activity at the later time points. This finding requires further research as it has important implication for the development of the AD treatment, as in case many neurons remain functional and their normal neuronal activity can be recovered by addressing the cause of the circuit dysfunction with treatment. To summarize, the study presented in this PhD thesis is the first longitudinal study of neuronal activity changes in an AD mouse model, and while it provides important insight into pathology, it also emphasizes the importance of chronic in vivo studies to investigate neuronal activity and its role in the disease progression

Investigation of neuronal activity in a murine model of Alzheimer’s disease using in vivo two-photon calcium imaging

Alzheimer’s disease (AD) is one of the biggest challenges for biomedical research nowadays as with the growth of life span more and more people are affected by this disorder. Etiology of AD is unknown, yet growing evidence identifies alterations in neuronal activity as of the great importance for pathology. Although several significant studies of neuronal activity alteration in AD were done during the last decade, none of them addressed the question of the time course of these changes over the disease progression. Alzheimer’s disease (AD) is characterized by impairments of brain neurons that are responsible for the storage and processing of information. Studies have revealed decrease in the activity of neurons (Silverman et al., 2001; Prvulovic et al., 2005) and it was proposed that generalized hypoactivity and silencing of brain circuits takes place as formulated in the synaptic failure hypothesis (Selkoe, 2002). However, more recent studies also reported opposite effects – hyperexcitability and hyperactivity of neurons in the AD models (Busche et al., 2008; Sanchez et al., 2012; Liebscher et al., 2016). It still remains unclear if these are two sides of the same coin or if these are two stages, that follow each other. Moreover, it is not clear if observed neuronal activity alterations are caused by the dysfunction of individual neurons or if overall circuitry is disturbed because the crucial “activity controllers” (most probably - inhibitory neurons) alter their activity. This project aimed to examine spontaneous neuronal activity in the murine model of AD at the early stages of disease progression using chronic in vivo imaging to address the character and the stability of neuronal activity alterations as well relation of the activity alterations to amyloid plaque proximity. Compared to earlier studies the approach of in vivo awake calcium imaging used in the current study has many benefits for brain research. The main advantage is that brain activity can be measured without artifacts generated by anesthesia, which can exaggerate or mitigate experimental readouts. In this project, I used genetically encoded calcium indicator GCaMP6 that enables prolonged repetitive imaging of the same neurons in an intact environment. Recording of calcium transients in cell bodies of neurons was accompanied by in vivo imaging of Aβ plaques and followed by immunohistochemical staining of GCaMP6-expressing neurons to investigate how activity changes are correlated with proximity to the plaque. All the experiments were done in awake mice to ensure the absence of anesthesia-derived impact on spontaneous neuronal activity. My results support previously published reports of the increased proportion of hyperactive excitatory neurons in the AD mouse model. Importantly, my results also demonstrate that this increased activity is present in the awake state, is stable over a longer period of time (one month) and does not depend on the distance to the closest plaque. These findings support the hypothesis of permanent network alterations driving aberrant activity patterns that appear early in the disease progression, resulting in a chronic excitation/inhibition disbalance. Another important finding of my project is that individual neurons do not stay in the silent state and most of them remain functional demonstrating normal activity at the later time points. This finding requires further research as it has important implication for the development of the AD treatment, as in case many neurons remain functional and their normal neuronal activity can be recovered by addressing the cause of the circuit dysfunction with treatment. To summarize, the study presented in this PhD thesis is the first longitudinal study of neuronal activity changes in an AD mouse model, and while it provides important insight into pathology, it also emphasizes the importance of chronic in vivo studies to investigate neuronal activity and its role in the disease progression