A protocol for use of medetomidine anesthesia in rats for extended studies using task-induced BOLD contrast and resting-state functional connectivity

MRI of neuronal plasticity in rodent models

Modifications in the behavior and architecture of neuronal networks are well documented to occur in association with learning and memory, as well as following injury. These plasticity mechanisms are crucial to ensure adequate processing of stimuli, and they also dictate the degree of recovery following peripheral or central nervous system injury. Nevertheless, the underlying neuronal mechanisms that determine the degree of plasticity of neuronal pathways are not fully understood. Recent developments in animal-dedicated magnetic resonance imaging (MRI) scanners and related hardware afford a high spatial and temporal resolution, making functional MRI and manganese-enhanced MRI emerging tools for studying reorganization of neuronal pathways in rodent models. Many of the observed changes in neuronal functions in rodent's brains following injury discussed here agree with clinical human fMRI findings. This demonstrates that animal model imaging can have a significant clinical impact in the neuronal plasticity and rehabilitation arenas.

Functional connectivity fMRI of the rodent brain: comparison of functional connectivity networks in rat and mouse

At present, resting state functional MRI (rsfMRI) is increasingly used in human neuropathological research. The present study aims at implementing rsfMRI in mice, a species that holds the widest variety of neurological disease models. Moreover, by acquiring rsfMRI data with a comparable protocol for anesthesia, scanning and analysis, in both rats and mice we were able to compare findings obtained in both species. The outcome of rsfMRI is different for rats and mice and depends strongly on the applied number of components in the Independent Component Analysis (ICA). The most important difference was the appearance of unilateral cortical components for the mouse resting state data compared to bilateral rat cortical networks. Furthermore, a higher number of components was needed for the ICA analysis to separate different cortical regions in mice as compared to rats.

Longitudinal functional magnetic resonance imaging in animal models

Functional magnetic resonance imaging (fMRI) has had an essential role in furthering our understanding of brain physiology and function. fMRI techniques are nowadays widely applied in neuroscience research, as well as in translational and clinical studies. The use of animal models in fMRI studies has been fundamental in helping elucidate the mechanisms of cerebral blood-flow regulation, and in the exploration of basic neuroscience questions, such as the mechanisms of perception, behavior, and cognition. Because animals are inherently non-compliant, most fMRI performed to date have required the use of anesthesia, which interferes with brain function and compromises interpretability and applicability of results to our understanding of human brain function. An alternative approach that eliminates the need for anesthesia involves training the animal to tolerate physical restraint during the data acquisition. In the present chapter, we review these two different approaches to obtaining fMRI data from animal models, with a specific focus on the acquisition of longitudinal data from the same subjects.

Functional and pharmacological MRI in understanding brain function at a systems level

Functional magnetic resonance imaging (fMRI) methods have been extensively applied to study the human brain and its functional organization in healthy and disease states. A strong rationale exists for the extension of this approach to animal models as a translational tool to bridge clinical and preclinical research. Specifically, the development of pharmacological MRI (phMRI), i.e., the use of fMRI to map spatiotemporal patterns of brain activity induced by pharmacological agents, has provided a robust and flexible tool to resolve brain circuits and mechanism-specific functional changes produced by selective intervention in different neurotransmitter systems in vivo. This chapter describes the methodological aspects of fMRI and phMRI in preclinical species, and some of the key findings, with a special emphasis on the translational potential of these methods in neuropharmacological research.

Functional connectivity in the rat at 11.7T: Impact of physiological noise in resting state fMRI

Resting state functional MRI (rs-fMRI) of the brain has the potential to elicit networks of functional connectivity and to reveal changes thereof in animal models of neurological disorders. In the present study, we investigate the contribution of physiological noise and its impact on assessment of functional connectivity in rs-fMRI of medetomidine sedated, spontaneously breathing rats at ultrahigh field of 11.7 Tesla. We employed gradient echo planar imaging (EPI) with repetition times of 3s and used simultaneous recordings of physiological parameters. A model of linear regression was applied to quantify the amount of BOLD fMRI signal fluctuations attributable to physiological sources. Our results indicate that physiological noise - mainly originating from the respiratory cycle -dominates the rs-fMRI time course in the form of spatially complex correlation patterns. As a consequence, these physiological fluctuations introduce severe artifacts into seed-based correlation maps and lead to misinterpretation of corresponding connectivity measures. We demonstrate that a scheme of motion correction and linear regression can significantly reduce physiological noise in the rs-fMRI time course, remove artifacts, and hence improve the reproducibility of functional connectivity assessment. In conclusion, physiological noise can severely compromise functional connectivity MRI (fcMRI) of the rodent at high fields and must be carefully considered in design and interpretation of future studies. Motion correction should be considered the primary strategy for reduction of apparent motion related to respiratory fluctuations. Combined with subsequent regression of physiological confounders, this strategy has proven successful in reducing physiological noise and related artifacts affecting functional connectivity analysis. The proposed new and rigorous protocol now opens the potential of fcMRI to elicit the role of brain connectivity in pathological processes without concerns of confounding contributions from physiological noise.

Functional connectivity in the rat brain: a complex network approach

Functional connectivity analyses of fMRI data can provide a wealth of information on the brain functional organization and have been widely applied to the study of the human brain. More recently, these methods have been extended to preclinical species, thus providing a powerful translational tool. Here, we review methods and findings of functional connectivity studies in the rat. More specifically, we focus on correlation analysis of pharmacological MRI (phMRI) responses, an approach that has enabled mapping the patterns of connectivity underlying major neurotransmitter systems in vivo. We also review the use of novel statistical approaches based on a network representation of the functional connectivity and their application to the study of the rat brain functional architecture.