Purpose
To determine optimal constrained tissue parameters and off‐resonance sequence parameters for single‐point macromolecular proton fraction (SP‐MPF) mapping based on a comprehensive quantitative ...magnetization transfer (qMT) protocol in healthy and demyelinated living mice at 7T.
Methods
Using 3D spoiled gradient echo‐based sequences, a comprehensive qMT protocol is performed by sampling the Z‐spectrum of mice brains, in vivo. Provided additional T1, B1+ and B0 maps allow for the estimation of qMT tissue parameters, among which three will be constrained, namely the longitudinal and transverse relaxation characteristics of the free pool (R1,fT2,f), the cross‐relaxation rate (R) and the bound pool transverse relaxation time (T2,r). Different sets of constrained parameters are investigated to reduce the bias between the SP‐MPF and its reference based on the comprehensive protocol.
Results
Based on a whole‐brain histogram analysis about the constrained parameters, the optimal experimental parameters that minimize the global bias between reference and SP‐MPF maps consist of a 600° and 6 kHz off‐resonance irradiation pulse. Following a Bland‐Altman analysis over regions of interest, optimal constrained parameters were R1,fT2,f = 0.0129, R = 26.5 s−1, and T2,r = 9.1 µs, yielding an overall MPF bias of 10−4 (limits of agreement −0.0068;0.0070) and a relative variation of 0.64% ± 5.95% between the reference and the optimal single‐point method across all mice.
Conclusion
The necessity of estimating animal model‐ and field‐dependent constrained parameters was demonstrated. The single‐point MPF method can be reliably applied at 7T, as part of routine preclinical in vivo imaging protocol in mice.
Preclinical applications of resting-state functional magnetic resonance imaging (rsfMRI) offer the possibility to non-invasively probe whole-brain network dynamics and to investigate the determinants ...of altered network signatures observed in human studies. Mouse rsfMRI has been increasingly adopted by numerous laboratories worldwide. Here we describe a multi-centre comparison of 17 mouse rsfMRI datasets via a common image processing and analysis pipeline. Despite prominent cross-laboratory differences in equipment and imaging procedures, we report the reproducible identification of several large-scale resting-state networks (RSN), including a mouse default-mode network, in the majority of datasets. A combination of factors was associated with enhanced reproducibility in functional connectivity parameter estimation, including animal handling procedures and equipment performance. RSN spatial specificity was enhanced in datasets acquired at higher field strength, with cryoprobes, in ventilated animals, and under medetomidine-isoflurane combination sedation. Our work describes a set of representative RSNs in the mouse brain and highlights key experimental parameters that can critically guide the design and analysis of future rodent rsfMRI investigations.
•Multi-centre rsfMRI revealed a set of representative FC networks in the mouse brain.•The mouse default-mode network converges toward spatially delineated structures.•Seed-based maps converge in 70% of the datasets.•Enhanced FC detection is associated with specific animal handling protocols and higher SNR.
Connectome genetics seeks to uncover how genetic factors shape brain functional connectivity; however, the causal impact of a single gene’s activity on whole-brain networks remains unknown. We tested ...whether the sole targeted deletion of the mu opioid receptor gene (Oprm1) alters the brain connectome in living mice. Hypothesis-free analysis of combined resting-state fMRI diffusion tractography showed pronounced modifications of functional connectivity with only minor changes in structural pathways. Fine-grained resting-state fMRI mapping, graph theory, and intergroup comparison revealed Oprm1-specific hubs and captured a unique Oprm1 gene-to-network signature. Strongest perturbations occurred in connectional patterns of pain/aversion-related nodes, including the mu receptor-enriched habenula node. Our data demonstrate that the main receptor for morphine predominantly shapes the so-called reward/aversion circuitry, with major influence on negative affect centers.
Alcohol acts as an addictive substance that may lead to alcohol use disorder. In humans, magnetic resonance imaging showed diverse structural and functional brain alterations associated with this ...complex pathology. Single magnetic resonance imaging modalities are used mostly but are insufficient to portray and understand the broad neuroadaptations to alcohol. Here, we combined structural and functional magnetic resonance imaging and connectome mapping in mice to establish brain-wide fingerprints of alcohol effects with translatable potential.
Mice underwent a chronic intermittent alcohol drinking protocol for 6 weeks before being imaged under medetomidine anesthesia. We performed open-ended multivariate analysis of structural data and functional connectivity mapping on the same subjects.
Structural analysis showed alcohol effects for the prefrontal cortex/anterior insula, hippocampus, and somatosensory cortex. Integration with microglia histology revealed distinct alcohol signatures, suggestive of advanced (prefrontal cortex/anterior insula, somatosensory cortex) and early (hippocampus) inflammation. Functional analysis showed major alterations of insula, ventral tegmental area, and retrosplenial cortex connectivity, impacting communication patterns for salience (insula), reward (ventral tegmental area), and default mode (retrosplenial cortex) networks. The insula appeared as a most sensitive brain center across structural and functional analyses.
This study demonstrates alcohol effects in mice, which possibly underlie lower top-down control and impaired hedonic balance documented at the behavioral level, and aligns with neuroimaging findings in humans despite the potential limitation induced by medetomidine sedation. This study paves the way to identify further biomarkers and to probe neurobiological mechanisms of alcohol effects using genetic and pharmacological manipulations in mouse models of alcohol drinking and dependence.
Connectomics of brain disorders seeks to reveal how altered brain function emerges from the architecture of cerebral networks; however the causal impact of targeted cellular damage on the whole brain ...functional and structural connectivity remains unknown. In the central nervous system, demyelination is typically the consequence of an insult targeted at the oligodendrocytes, the cells forming and maintaining the myelin. This triggered perturbation generates cascades of pathological events that most likely alter the brain connectome. Here we induced oligodendrocyte death and subsequent demyelinating pathology via cuprizone treatment in mice and combining mouse brain resting state functional Magnetic Resonance Imaging and diffusion tractography we established functional and structural pathology-to-network signatures. We demonstrated that demyelinated brain fundamentally reorganizes its intrinsic functional connectivity paralleled by widespread damage of the structural scaffolding. We evidenced default mode-like network as core target of demyelination-induced connectivity modulations and hippocampus as the area with strongest connectional perturbations.
The authors demonstrate via non-invasive resting state fMRI that intrinsic functional connectivity is fundamentally reorganized in demyelinated mouse brain, paralleled by widespread damage of structural scaffolding and strong modulations of default mode and hippocampal networks. Display omitted
•Cuprizone pathology disrupts the antagonistic relationship between DMN and TPN.•The DMN is the core target of demyelination-induced connectivity modulations.•HPF emerged as area with strong functional and structural connectivity perturbations.•Small-worldness hallmarks imply preserved functional network efficiency.
Alcohol use disorder (AUD) is devastating and poorly treated, and innovative targets are actively sought for prevention and treatment. The orphan G protein–coupled receptor GPR88 is enriched in ...mesocorticolimbic pathways, and Gpr88 knockout mice show hyperactivity and risk-taking behavior, but a potential role for this receptor in drug abuse has not been examined.
We tested Gpr88 knockout mice for alcohol-drinking and -seeking behaviors. To gain system-level understanding of their alcohol endophenotype, we also analyzed whole-brain functional connectivity in naïve mice using resting-state functional magnetic resonance imaging.
Gpr88 knockout mice showed increased voluntary alcohol drinking at both moderate and excessive levels, with intact alcohol sedation and metabolism. Mutant mice also showed increased operant responding and motivation for alcohol, while food and chocolate operant self-administration were unchanged. Alcohol place conditioning and alcohol-induced dopamine release in the nucleus accumbens were decreased, suggesting reduced alcohol reward in mutant mice that may partly explain enhanced alcohol drinking. Seed-based voxelwise functional connectivity analysis revealed significant remodeling of mesocorticolimbic centers, whose hallmark was predominant weakening of prefrontal cortex, ventral tegmental area, and amygdala connectional patterns. Also, effective connectivity from the ventral tegmental area to the nucleus accumbens and amygdala was reduced.
Gpr88 deletion disrupts executive, reward, and emotional networks in a configuration that reduces alcohol reward and promotes alcohol seeking and drinking. The functional connectivity signature is reminiscent of alterations observed in individuals at risk for AUD. The Gpr88 gene, therefore, may represent a vulnerability/resilience factor for AUD, and a potential drug target for AUD treatment.
Mu opioid receptors (MORs) are central to pain control, drug reward, and addictive behaviors, but underlying circuit mechanisms have been poorly explored by genetic approaches. Here we investigate ...the contribution of MORs expressed in gamma-aminobutyric acidergic forebrain neurons to major biological effects of opiates, and also challenge the canonical disinhibition model of opiate reward.
We used Dlx5/6-mediated recombination to create conditional Oprm1 mice in gamma-aminobutyric acidergic forebrain neurons. We characterized the genetic deletion by histology, electrophysiology, and microdialysis; probed neuronal activation by c-Fos immunohistochemistry and resting-state functional magnetic resonance imaging; and investigated main behavioral responses to opiates, including motivation to obtain heroin and palatable food.
Mutant mice showed MOR transcript deletion mainly in the striatum. In the ventral tegmental area, local MOR activity was intact, and reduced activity was only observed at the level of striatonigral afferents. Heroin-induced neuronal activation was modified at both sites, and whole-brain functional networks were altered in live animals. Morphine analgesia was not altered, and neither was physical dependence to chronic morphine. In contrast, locomotor effects of heroin were abolished, and heroin-induced catalepsy was increased. Place preference to heroin was not modified, but remarkably, motivation to obtain heroin and palatable food was enhanced in operant self-administration procedures.
Our study reveals dissociable MOR functions across mesocorticolimbic networks. Thus, beyond a well-established role in reward processing, operating at the level of local ventral tegmental area neurons, MORs also moderate motivation for appetitive stimuli within forebrain circuits that drive motivated behaviors.
Mapping brain structural and functional connectivity (FC) became an essential approach in neuroscience as network properties can underlie behavioral phenotypes. In mouse models, revealing ...strain-related patterns of brain wiring is crucial, since these animals are used to answer questions related to neurological or neuropsychiatric disorders. C57BL/6 and BALB/cJ strains are two of the primary “genetic backgrounds” for modeling brain disease and testing therapeutic approaches. However, extensive literature describes basal differences in the behavioral, neuroanatomical and neurochemical profiles of the two strains, which raises questions on whether the observed effects are pathology specific or depend on the genetic background of each strain. Here, we performed a systematic comparative exploration of brain structure and function of C57BL/6 and BALB/cJ mice using Magnetic Resonance Imaging (MRI). We combined deformation-based morphometry (DBM), diffusion MRI and high-resolution fiber mapping (hrFM) along with resting-state functional MRI (rs-fMRI) and demonstrated brain-wide differences in the morphology and “connectome” features of the two strains. Essential inter-strain differences were depicted regarding the size and the fiber density (FD) within frontal cortices, along cortico-striatal, thalamic and midbrain pathways as well as genu and splenium of corpus callosum. Structural dissimilarities were accompanied by specific FC patterns, emphasizing strain differences in frontal and basal forebrain functional networks as well as hubness characteristics. Rs-fMRI data further indicated differences of reward-aversion circuitry and default mode network (DMN) patterns. The inter-hemispherical FC showed flexibility and strain-specific adjustment of their patterns in agreement with the structural characteristics.
Recent studies correlate chronic Toxoplasma gondii (T. gondii) infection with behavioral changes in rodents; additionally, seropositivity in humans is reported to be associated with behavioral and ...neuropsychiatric diseases. In this study we investigated whether the described behavioral changes in a murine model of chronic toxoplasmosis are associated with changes in synaptic plasticity and brain neuronal circuitry. In mice chronically infected with T. gondii, magnetic resonance imaging (MRI) data analysis displayed the presence of heterogeneous lesions scattered throughout all brain areas. However, a higher density of lesions was observed within specific regions such as the somatosensory cortex (SSC). Further histopathological examination of these brain areas indicated the presence of activated resident glia and recruited immune cells accompanied by limited alterations of neuronal viability. In vivo diffusion-tensor MRI analysis of neuronal fiber density within the infected regions revealed connectivity abnormalities in the SSC. Altered fiber density was confirmed by morphological analysis of individual, pyramidal and granule neurons, showing a reduction in dendritic arbor and spine density within the SSC, as well as in the hippocampus. Evaluation of synapse efficacy revealed diminished levels of two key synaptic proteins, PSD95 and synaptophysin, within the same brain areas, indicating deficits in functionality of the synaptic neurotransmission in infected mice. Our results demonstrate that persistent T. gondii infection in a murine model results in synaptic deficits within brain structures leading to disturbances in the morphology of noninfected neurons and modified brain connectivity, suggesting a potential explanation for the behavioral and neuropsychiatric alterations.
Growing evidence supports the implication of DYRK1A in the development of cognitive deficits seen in Down syndrome (DS) and Alzheimer's disease (AD). We here demonstrate that pharmacological ...inhibition of brain DYRK1A is able to correct recognition memory deficits in three DS mouse models with increasing genetic complexity Tg(
), Ts65Dn, Dp1Yey, all expressing an extra copy of
Overexpressed DYRK1A accumulates in the cytoplasm and at the synapse. Treatment of the three DS models with the pharmacological DYRK1A inhibitor leucettine L41 leads to normalization of DYRK1A activity and corrects the novel object cognitive impairment observed in these models. Brain functional magnetic resonance imaging reveals that this cognitive improvement is paralleled by functional connectivity remodelling of core brain areas involved in learning/memory processes. The impact of
trisomy and L41 treatment on brain phosphoproteins was investigated by a quantitative phosphoproteomics method, revealing the implication of synaptic (synapsin 1) and cytoskeletal components involved in synaptic response and axonal organization. These results encourage the development of DYRK1A inhibitors as drug candidates to treat cognitive deficits associated with DS and AD.
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