Identifying reproducible resting state networks and functional connectivity alterations following chronic restraint stress in anaesthetized rats

Background

Resting-state functional MRI (rs-fMRI) in rodent models have the potential to bridge invasive experiments and observational human studies, increasing our understanding of functional alterations in the brains of patients with depression. A major limitation in current rodent rs-fMRI studies is that there has been no consensus on healthy baseline resting-state networks (RSNs) that are reproducible in rodents. Therefore, the present study aimed to construct reproducible RSNs in a large dataset of healthy rats and then evaluate functional connectivity changes within and between these RSNs following a chronic restraint stress (CRS) model within the same animals.

Methods

A combined MRI dataset of 109 Sprague Dawley rats at baseline and after two weeks of CRS, collected during four separate experiments conducted by our lab in 2019 and 2020, was re-analysed. The mICA and gRAICAR toolbox were first applied to detect optimal and reproducible ICA components and then a hierarchical clustering algorithm (FSLNets) was applied to construct reproducible RSNs. Ridge-regularized partial correlation (FSLNets) was used to evaluate the changes in the direct connection between and within identified networks in the same animals following CRS.

Results

Four large-scale networks in anesthetised rats were identified: the DMN-like, spatial attention-limbic, corpus striatum, and autonomic network, which are homologous across species. CRS decreased the anticorrelation between DMN-like and autonomic network. CRS decreased the correlation between amygdala and a functional complex (nucleus accumbens and ventral pallidum) in the right hemisphere within the corpus striatum network. However, a high individual variability in the functional connectivity before and after CRS within RSNs was observed.

Conclusion

The functional connectivity changes detected in rodents following CRS differ from reported functional connectivity alterations in patients with depression. A simple interpretation of this difference is that the rodent response to CRS does not reflect the complexity of depression as it is experienced by humans. Nonetheless, the high inter-subject variability of functional connectivity within networks suggests that rats demonstrate different neural phenotypes, like humans. Therefore, future efforts in classifying neural phenotypes in rodents might improve the sensitivity and translational impact of models used to address aetiology and treatment of psychiatric conditions including depression.

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.

Accelerated low-intensity rTMS does not rescue anxiety behaviour or abnormal connectivity in young adult rats following chronic restraint stress

Currently approved repetitive transcranial magnetic stimulation (rTMS) protocols for the treatment of major depressive disorder (MDD) involve once-daily (weekday) stimulation sessions, with 10 Hz or intermittent theta burst stimulation (iTBS) frequencies, over 4–6 weeks. Recently, accelerated treatment protocols (multiple daily stimulation sessions for 1–2 weeks) have been increasingly studied to optimize rTMS treatments. Accelerated protocols might confer unique advantages for adolescents and young adults but there are many knowledge gaps related to dosing in this age group. Off-label, clinical practice frequently outpaces solid evidence as rigorous clinical trials require substantial time and resources. Murine models present an opportunity for high throughput dose finding studies to focus subsequent clinical trials in humans. This project investigated the brain and behavioural effects of an accelerated low-intensity rTMS (LI-rTMS) protocol in a young adult rodent model of chronic restraint stress (CRS). Depression and anxiety-related behaviours were induced in young adult male Sprague Dawley rats using the CRS model, followed by the 3-times-daily delivery of 10 Hz LI-rTMS, for two weeks. Behaviour was assessed using the Elevated Plus Maze and Forced Swim Test, and functional, chemical, and structural brain changes measured using magnetic resonance imaging techniques. CRS induced an agitated depression-like phenotype but therapeutic effects from the accelerated protocol were not detected. Our findings suggest that the age of rodents may impact response to CRS and LI-rTMS. Future studies should also examine higher intensities of rTMS and accelerated theta burst protocols.

A Preclinical Study of Standard Versus Accelerated Transcranial Magnetic Stimulation for Depression in Adolescents

Objective: Ongoing studies are focused on adapting transcranial magnetic stimulation (TMS) for the treatment of major depressive disorder in adolescent humans. Most protocols in adolescent humans to date have delivered daily 10 Hz prefrontal stimulation with mixed results. Novel TMS dosing strategies such as accelerated TMS have recently been considered. There are knowledge gaps related to the potential clinical and pragmatic advantages of accelerated TMS. This pilot study compared the behavioral effects of a standard daily and accelerated low-intensity TMS (LI-TMS) protocol in an adolescent murine model of depression. Methods: Male adolescent Sprague Dawley rats were placed in transparent plexiglass tubes for 2.5 hours daily for 13 days as part of a study to validate the chronic restraint stress (CRS) protocol. Rats subsequently received 10 minutes of active or sham 10 Hz LI-TMS daily for 2 weeks (standard) or three times daily for 1 week (accelerated). Behavior was assessed using the elevated plus maze and forced swim test (FST). Hippocampal neurogenesis was assessed by injection of the thymidine analogue 5-ethynyl-2'-deoxyuridine at the end of LI-TMS treatment (2 weeks standard, 1 week accelerated), followed by postmortem histological analysis. Results: There were no significant differences in behavioral outcomes among animals receiving once-daily sham or active LI-TMS treatment. However, animals treated with accelerated LI-TMS demonstrated significant improvements in behavioral outcomes compared with sham treatment. Specifically, animals receiving active accelerated treatment showed greater latency to the first immobility behavior (p < 0.05; active: 130 ± 46 seconds; sham: 54 ± 39 seconds) and increased climbing behaviors (p < 0.05; active: 16 ± 5; sham: 9 ± 5) during FST. There were no changes in hippocampal neurogenesis nor any evidence of cell death in histological sections. Conclusions: An accelerated LI-TMS protocol outperformed the standard (once-daily) protocol in adolescent male animals with depression-like behaviors induced by CRS and was not accompanied by any toxicity or tolerability concerns. These preliminary findings support the speculation that novel TMS dosing strategies should be studied in adolescent humans and will inform future clinical protocols.

Improved detection of rare CALLA-positive cells in peripheral blood using multiparameter flow cytometry

A major limitation to the detection of rare cell types in the peripheral blood using monoclonal antibodies is nonspecific binding of the antibody reagent to normal cells. Detection of rare common acute lymphoblastic leukemia antigen (CALLA)-positive cells in peripheral blood is significantly improved by using multiple flow cytometric parameters to exclude a variety of mature blood cells which may nonspecifically bind the antibody reagent. Monocytes and granulocytes are excluded by gating out cells with high 90 degrees light scatter. By gating on red fluorescence, a variety of mature cell types binding to phycoerythrin (PE)-conjugated Leu 3, Leu 2, and M3 monoclonal antibodies are also excluded. CALLA-positive lymphoblasts from 6 consecutive patients were not excluded on the basis of these parameters. Gating on log 90 degrees light scatter and log red fluorescence in this fashion reduced the incidence of nonspecific binding to peripheral blood mononuclear cells of a fluorescein-conjugated irrelevant monoclonal antibody by 98% from 308 cells per million to 5 cells per million. One CALLA-positive lymphoblast per 100,000 peripheral blood mononuclear cells could be detected in mixture experiments using this method. The normal range of CALLA-positive cells in adults is less than 16 cells per million peripheral blood mononuclear cells. This low background of CALLA-positive peripheral blood cells may permit the detection of early leukemic relapse in acute lymphoblastic leukemia by analysis of the peripheral blood. This methodology can be applied to the detection of any rare cell type by using phycoerythrin-conjugated antibodies to markers that the cell type does not possess.