Increased Callosal Connectivity in Reeler Mice Revealed by Brain-Wide Input Mapping of VIP Neurons in Barrel Cortex

The neocortex is composed of layers. Whether layers constitute an essential framework for the formation of functional circuits is not well understood. We investigated the brain-wide input connectivity of vasoactive intestinal polypeptide (VIP) expressing neurons in the reeler mouse. This mutant is characterized by a migration deficit of cortical neurons so that no layers are formed. Still, neurons retain their properties and reeler mice show little cognitive impairment. We focused on VIP neurons because they are known to receive strong long-range inputs and have a typical laminar bias toward upper layers. In reeler, these neurons are more dispersed across the cortex. We mapped the brain-wide inputs of VIP neurons in barrel cortex of wild-type and reeler mice with rabies virus tracing. Innervation by subcortical inputs was not altered in reeler, in contrast to the cortical circuitry. Numbers of long-range ipsilateral cortical inputs were reduced in reeler, while contralateral inputs were strongly increased. Reeler mice had more callosal projection neurons. Hence, the corpus callosum was larger in reeler as shown by structural imaging. We argue that, in the absence of cortical layers, circuits with subcortical structures are maintained but cortical neurons establish a different network that largely preserves cognitive functions

Spatial signatures of anesthesia-induced burst-suppression differ between primates and rodents

During deep anesthesia, the electroencephalographic (EEG) signal of the brain alternates between bursts of activity and periods of relative silence (suppressions). The origin of burst-suppression and its distribution across the brain remain matters of debate. In this work, we used functional magnetic resonance imaging (fMRI) to map the brain areas involved in anesthesia-induced burst-suppression across four mammalian species: humans, long-tailed macaques, common marmosets, and rats. At first, we determined the fMRI signatures of burst-suppression in human EEG-fMRI data. Applying this method to animal fMRI datasets, we found distinct burst-suppression signatures in all species. The burst-suppression maps revealed a marked inter-species difference: in rats, the entire neocortex engaged in burst-suppression, while in primates most sensory areas were excluded-predominantly the primary visual cortex. We anticipate that the identified species-specific fMRI signatures and whole-brain maps will guide future targeted studies investigating the cellular and molecular mechanisms of burst-suppression in unconscious states

Lack of astrocytes hinders parenchymal oligodendrocyte precursor cells from reaching a myelinating state in osmolyte-induced demyelination

Demyelinated lesions in human pons observed after osmotic shifts in serum have been referred to as central pontine myelinolysis (CPM). Astrocytic damage, which is prominent in neuroinflammatory diseases like neuromyelitis optica (NMO) and multiple sclerosis (MS), is considered the primary event during formation of CPM lesions. Although more data on the effects of astrocyte-derived factors on oligodendrocyte precursor cells (OPCs) and remyelination are emerging, still little is known about remyelination of lesions with primary astrocytic loss. In autopsy tissue from patients with CPM as well as in an experimental model, we were able to characterize OPC activation and differentiation. Injections of the thymidine-analogue BrdU traced the maturation of OPCs activated in early astrocyte-depleted lesions. We observed rapid activation of the parenchymal NG2+ OPC reservoir in experimental astrocyte-depleted demyelinated lesions, leading to extensive OPC proliferation. One week after lesion initiation, most parenchyma-derived OPCs expressed breast carcinoma amplified sequence-1 (BCAS1), indicating the transition into a pre-myelinating state. Cells derived from this early parenchymal response often presented a dysfunctional morphology with condensed cytoplasm and few extending processes, and were only sparsely detected among myelin-producing or mature oligodendrocytes. Correspondingly, early stages of human CPM lesions also showed reduced astrocyte numbers and non-myelinating BCAS1+ oligodendrocytes with dysfunctional morphology. In the rat model, neural stem cells (NSCs) located in the subventricular zone (SVZ) were activated while the lesion was already partially repopulated with OPCs, giving rise to nestin+ progenitors that generated oligodendroglial lineage cells in the lesion, which was successively repopulated with astrocytes and remyelinated. These nestin+ stem cell-derived progenitors were absent in human CPM cases, which may have contributed to the inefficient lesion repair. The present study points to the importance of astrocyte-oligodendrocyte interactions for remyelination, highlighting the necessity to further determine the impact of astrocyte dysfunction on remyelination inefficiency in demyelinating disorders including MS

A consensus protocol for functional connectivity analysis in the rat brain

Task-free functional connectivity in animal models provides an experimental framework to examine connectivity phenomena under controlled conditions and allows for comparisons with data modalities collected under invasive or terminal procedures. Currently, animal acquisitions are performed with varying protocols and analyses that hamper result comparison and integration. Here we introduce StandardRat, a consensus rat functional magnetic resonance imaging acquisition protocol tested across 20 centers. To develop this protocol with optimized acquisition and processing parameters, we initially aggregated 65 functional imaging datasets acquired from rats across 46 centers. We developed a reproducible pipeline for analyzing rat data acquired with diverse protocols and determined experimental and processing parameters associated with the robust detection of functional connectivity across centers. We show that the standardized protocol enhances biologically plausible functional connectivity patterns relative to previous acquisitions. The protocol and processing pipeline described here is openly shared with the neuroimaging community to promote interoperability and cooperation toward tackling the most important challenges in neuroscience

Functional imaging of the anesthetized brain in primates and rodents

Understanding the brain is aided by visualizing neural activity over time. The most popular method for doing so in humans is functional magnetic resonance imaging (fMRI)—a method that tracks blood oxygenation as a proxy for neural activity. fMRI relies on neurovascular coupling, the brain’s capacity to increase its blood supply locally and on demand. Apart from humans, fMRI can be also applied to experimental animals and thereby plays an essential role in translating findings across species. Additionally, the combination of animal fMRI with electrophysiological and optical methods is crucial for uncovering the neural correlates of the observed blood-oxygen-level-dependent (BOLD) fMRI signal. Since fMRI necessitates immobility, animals must be either restrained or anesthetized. Most researchers take the latter approach, for both practical and ethical reasons. However, anesthesia confounds the results of fMRI experiments by profoundly altering neural activity and by interfering with neurovascular coupling. This conundrum, which can be viewed both as a challenge and an opportunity, motivated the three studies presented in this thesis. The challenge lies in choosing the right anesthesia for animal fMRI experiments. The ideal anesthetic protocol must provide sufficient sedation, guarantee immobility, and crucially, preserve a degree of neural responsiveness and neurovascular coupling. Anesthetic protocols based on the continuous infusion of the sedative medetomidine exhibit these qualities and have thus become a popular choice for rats—the most widely used animal fMRI model. Despite this, it has not yet been established how fMRI readouts evolve over several hours of medetomidine anesthesia and how they are affected by variations in timing, dose, and route of administration. In my first study (Chapter 2), I used four different protocols of medetomidine administration to anesthetize rats for up to six hours and repeatedly evaluated stimulus-evoked responses and fMRI measures of functional connectivity. I found that the temporal evolution of fMRI readouts varied between administration schemes. Based on the results, I made recommendations regarding the administration of medetomidine and the timing of fMRI experiments. These factors are important for obtaining reproducible results and should be considered for the design and interpretation of future rat fMRI studies. The opportunity lies in exploiting anesthesia’s effects on fMRI to better understand large-scale phenomena in the anesthetized brain. The case in point is burst-suppression, a poorly understood pattern of neural activity that appears in deep anesthesia and coma. In animals anesthetized with isoflurane, burst-suppression has been associated with the widespread synchronization of brain areas. In the second study (Chapter 3), I used fMRI data from four species—humans, macaques, marmosets, and rats—to precisely describe the fMRI signatures of anesthesia-induced burst-suppression and to map their distribution across the brain. I discovered a marked difference between primates and rodents. In rats the entire neocortex engaged in burst-suppression, while in the three primate species certain cortical areas were excluded—most notably the visual cortex. Based on the fMRI data alone, I could not determine the underlying cause of this exclusion. I concluded that answering this question would necessitate direct recordings of neural activity in the visual cortex of both primates and rodents. In the third study (Chapter 4), I aimed to develop the methods required for such direct neural recordings. Specifically, I conducted a series of pilot experiments in isoflurane-anesthetized rats and demonstrated the feasibility of in vivo two-photon calcium imaging through chronically implanted cranial windows. I was able to record the activity of hundreds of layer 2/3 neurons in the rat somatosensory and visual cortex and confirm my previous findings regarding the pancortical distribution of burst-suppression. I also examined the effects of varying the isoflurane dose on spontaneous activity and stimulus-evoked responses, thereby reproducing several known properties of burst-suppression in rodents. The developed methods can be easily adapted to record from the marmoset visual cortex, with the aim of understanding the primate-rodent difference described in Chapter 3. The above studies showcase that anesthetizing animals for functional neuroimaging experiments should not be viewed as a necessary evil. Anesthetic protocols can be optimized to allow for a host of neuroscientific questions to be asked. Moreover, such experiments can shed light on the functional organization of the anesthetized brain and on elusive anesthetic mechanisms of actions.2022-07-1

Temporal stability of fMRI in medetomidine-anesthetized rats

Medetomidine has become a popular choice for anesthetizing rats during long-lasting sessions of blood-oxygen-level dependent (BOLD) functional magnetic resonance imaging (fMRI). Despite this, it has not yet been thoroughly established how commonly reported fMRI readouts evolve over several hours of medetomidine anesthesia and how they are affected by the precise timing, dose, and route of administration. We used four different protocols of medetomidine administration to anesthetize rats for up to six hours and repeatedly evaluated somatosensory stimulus-evoked BOLD responses and resting state functional connectivity. We found that the temporal evolution of fMRI readouts strongly depended on the method of administration. Intravenous administration of a medetomidine bolus (0.05 mg/kg), combined with a subsequent continuous infusion (0.1 mg/kg/h), led to temporally stable measures of stimulus-evoked activity and functional connectivity throughout the anesthesia. Deviating from the above protocol-by omitting the bolus, lowering the medetomidine dose, or using the subcutaneous route-compromised the stability of these measures in the initial two-hour period. We conclude that both an appropriate protocol of medetomidine administration and a suitable timing of fMRI experiments are crucial for obtaining consistent results. These factors should be considered for the design and interpretation of future rat fMRI studies

Lack of astrocytes hinders parenchymal oligodendrocyte precursor cells from reaching a myelinating state in osmolyte-induced demyelination

Demyelinated lesions in human pons observed after osmotic shifts in serum have been referred to as central pontine myelinolysis (CPM). Astrocytic damage, which is prominent in neuroinflammatory diseases like neuromyelitis optica (NMO) and multiple sclerosis (MS), is considered the primary event during formation of CPM lesions. Although more data on the effects of astrocyte-derived factors on oligodendrocyte precursor cells (OPCs) and remyelination are emerging, still little is known about remyelination of lesions with primary astrocytic loss. In autopsy tissue from patients with CPM as well as in an experimental model, we were able to characterize OPC activation and differentiation. Injections of the thymidine-analogue BrdU traced the maturation of OPCs activated in early astrocyte-depleted lesions. We observed rapid activation of the parenchymal NG

A collaborative resource platform for non-human primate neuroimaging

International audienceNeuroimaging non-human primates (NHPs) is a growing, yet highly specialized field of neuroscience. Resources that were primarily developed for human neuroimaging often need to be significantly adapted for use with NHPs or other animals, which has led to an abundance of custom, in-house solutions. In recent years, the global NHP neuroimaging community has made significant efforts to transform the field towards more open and collaborative practices. Here we present the PRIMatE Resource Exchange (PRIME-RE), a new collaborative online platform for NHP neuroimaging. PRIME-RE is a dynamic community-driven hub for the exchange of practical knowledge, specialized analytical tools, and open data repositories, specifically related to NHP neuroimaging. PRIME-RE caters to both researchers and developers who are either new to the field, looking to stay abreast of the latest developments, or seeking to collaboratively advance the field

A collaborative resource platform for non-human primate neuroimaging

Neuroimaging non-human primates (NHPs) is a growing, yet highly specialized field of neuroscience. Resources that were primarily developed for human neuroimaging often need to be significantly adapted for use with NHPs or other animals, which has led to an abundance of custom, in-house solutions. In recent years, the global NHP neuroimaging community has made significant efforts to transform the field towards more open and collaborative practices. Here we present the PRIMatE Resource Exchange (PRIME-RE), a new collaborative online platform for NHP neuroimaging. PRIME-RE is a dynamic community-driven hub for the exchange of practical knowledge, specialized analytical tools, and open data repositories, specifically related to NHP neuroimaging. PRIME-RE caters to both researchers and developers who are either new to the field, looking to stay abreast of the latest developments, or seeking to collaboratively advance the field