Regional-specific average time courses of spontaneous fluctuations in blood oxygen level dependent (BOLD) MRI contrast at 9.4T in lightly anesthetized resting rat brain are formed, and correlation coefficients between time course pairs are interpreted as measures of connectivity. A hierarchy of regional pairwise correlation coefficients (RPCCs) is observed, with the highest values found in the thalamus and cortex, both intra-and interhemisphere, and lower values between the cortex and thalamus. Independent sensory networks are distinguished by two methods: data driven, where task activation defines regions of interest (ROI), and hypothesis driven, where regions are defined by the rat histological atlas. Success in these studies is attributed in part to the use of medetomidine hydrochloride (Domitor) for anesthesia. Functional connectivity in the resting human brain using blood oxygen level dependent (BOLD) contrast is revealed by analysis of a series of MRI echo-planar images acquired over a period of several minutes with the subject at rest (1). In the present work, functional connectivity experiments are extended from human brain to rat brain and our central hypothesis is that the underlying physiology is conserved across all mammalian species.A reference time course obtained from a reference voxel (or, alternatively, an average over a cluster of voxel time courses in a region of interest [ROI]) is formed. Cross correlation of the reference time course with all voxel time courses in the slice provides a functional connectivity map. A strategy is required for selection of the reference time course, and performance of a task is commonly used to define ROIs in resting brain that can be used to form reference time courses.We have discovered that electrical stimulation using an implanted electrode on the radial nerve of the brachial plexus of a rat (2) results in activation of a network of sensorimotor brain regions, each of which is a suitable candidate for formation of a reference time course when analyzing resting-state data. Experiments not only in the sensorimotor system but also in the visual system provide further support for our central hypothesis. Functional connectivity was studied using reference waveforms obtained from areas that were found to be activated by light incident on the retina that was turned on and off in a block-trial functional MRI (fMRI) experiment.Anatomic images acquired in this experiment are of high quality, and it is possible to define anatomic regions purely by reference to the rat histological atlas (3). One can develop a reference waveform from each of these regions and test a specific hypothesis that functional connectivity to a second region is consistent with a known connectivity. We report here success in this hypothesis-driven approach to analysis of resting-state data. A total of 22 sensorimotor regions were identified, and connectivities between each of these regions and the other 21 regions were determined.Most functional connectivity studies have been in the awake human b...

The α2-adrenoreceptor agonist, medetomidine, which exhibits dose-dependent sedative effects and is gaining acceptance in small-animal functional magnetic resonance imaging (fMRI), has been studied. Rats were examined on the bench using the classic tail-pinch method with three infusion sequences: 100 μg/kg/hr, 300 μg/kg/hr, or 100 μg/kg/hr followed by 300 μg/kg/hr. Stepping the infusion rate from 100 to 300 μg/kg/hr after 2.5 hours resulted in a prolonged period of approximately level sedation that cannot be achieved by a constant infusion of either 100 or 300 μg/kg/hr. By stepping the infusion dosage, experiments as long as six hours are possible. Functional MRI experiments were carried out on rats using a frequency dependent electrical stimulation protocol—namely, forepaw stimulation at 3, 5, 7, and 10 Hz. Each rat was studied for a four-hour period, divided into two equal portions. During the first portion, rats were started at a 100 μg/kg/hr constant infusion. During the second portion, four secondary levels of infusion were used: 100, 150, 200, and 300 μg/kg/hr. The fMRI response to stimulation frequency was used as an indirect measure of modulation of neuronal activity through pharmacological manipulation. The frequency response to stimulus was attenuated at the lower secondary infusion dosages 100 or 150 μg/kg/hr but not at the higher secondary infusion dosages 200 or 300 μg/kg/hr. Parallel experiments with the animal at rest were carried out using both electroencephalogram (EEG) and functional connectivity MRI (fcMRI) methods with consistent results. In the secondary infusion period using 300 μg/kg/hr, resting-state functional connectivity is enhanced.

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Functional connectivity magnetic resonance imaging (fcMRI) studies in rat brain show brain reorganization following peripheral nerve injury. Subacute neuroplasticity was observed two weeks following transection of the four major nerves of the brachial plexus. Direct functional magnetic resonance imaging (fMRI) stimulation of the intact radial nerve reveals an activation pattern in the forelimb regions of the sensory and motor cortices that is significantly different from that observed in normal rats. Results of this fMRI experiment were used to determine seed voxel regions for fcMRI analysis. Intrahemispheric connectivities in the sensorimotor forelimb representations in both hemispheres are largely unaffected by deafferentation, whereas substantial disruption of interhemispheric sensorimotor cortical connectivity occurs. In addition, significant intra-and interhemispheric changes in connectivities of thalamic nuclei were found. These are the central findings of the study. They could not have been obtained from fMRI studies alone-both fMRI and fcMRI are needed. The combination provides a general marker for brain plasticity. The rat visual system was studied in the same animals as a control. No neuroplastic changes in connectivities were found in the primary visual cortex upon forelimb deafferentation. Differences were noted in regions responsible for processing multisensory visual-motor information. This incidental discovery is considered to be significant. It may provide insight into phantom limb epiphenomena.

The response of the rat visual system to flashes of blue light has been studied by blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI). The BOLD temporal response is dependent on the number of flashes presented and demonstrates a refractory period that depends on flash frequency. Activated brain regions included the primary and secondary visual cortex, superior colliculus (SC), dorsal lateral geniculate (DLG), and lateral posterior nucleus (LP), which were found to exhibit differing temporal responses. To explain these differences, the BOLD neurovascular response function was modeled. A second-order differential equation was developed and solved numerically to arrive at region-specific response functions. Included in the model are the light input from the diode (duty cycle), a refractory period, a transient response following onset and cessation of stimulus, and a slow adjustment to changes in the average level of the signal. Constants in the differential equation were evaluated for each region by fitting the model to the experimental BOLD response from a single flash, and the equation was then solved for multiple flashes. The simulation mimics the major features of the data; however, remaining differences in the frequency dependence of the response between the cortical and subcortical regions were unexplained. We hypothesized that these discrepancies were due to regional-specific differences in neuronal response to flash frequency. To test this hypothesis, cortical visual evoked potentials (VEPs) were recorded using the same stimulation protocol as the fMRI. Cortical VEPs were more suppressed than subcortical VEPs as flash frequency increased, supporting our hypothesis. This is the first report that regional differences in neuronal activation to the same stimulus lead to differential BOLD activation.