Brain maps on the go: functional imaging during motor challenge in animals

Cerebral perfusion mapping during retrieval of spatial memory in rats

Studies of brain functional activation during spatial navigation using electrophysiology and immediate-early gene responses have typically targeted a limited number of brain regions. Our study provides the first whole brain analysis of cerebral activation during retrieval of spatial memory in the freely-moving rat. Rats (LEARNERS) were trained in the Barnes maze, an allocentric spatial navigation task, while CONTROLS received passive exposure. After 19 days, functional brain mapping was performed during recall by bolus intravenous injection of [¹⁴C]-iodoantipyrine using a novel subcutaneous minipump triggered by remote activation. Regional cerebral blood flow (rCBF)-related tissue radioactivity was analyzed by statistical parametric mapping from autoradiographic images of the three-dimensionally reconstructed brains. Functional connectivity was examined between regions of the spatial navigation circuit through interregional correlation analysis. Significant rCBF increases were noted in LEARNERS compared to CONTROLS broadly across the spatial navigation circuit, including the hippocampus (anterior dorsal CA1, posterior ventral CA1-3), subiculum, thalamus, striatum, medial septum, cerebral cortex, with decreases noted in the mammillary nucleus, amygdala and insula. LEARNERS showed a significantly greater positive correlation of rCBF of the ventral hippocampus with retrosplenial, lateral orbital, parietal and primary visual cortex, and a significantly more negative correlation with the mammillary nucleus, amygdala, posterior entorhinal cortex, and anterior thalamic nucleus. The complex sensory component of the spatial navigation task was underscored by broad activation across visual, somatosensory, olfactory, auditory and vestibular circuits which was enhanced in LEARNERS. Brain mapping facilitated by an implantable minipump represents a powerful tool for evaluation of mammalian behaviors dependent on locomotion.

Decreased Taurine and Creatine in the Thalamus May Relate to Behavioral Impairments in Ethanol-Fed Mice: A Pilot Study of Proton Magnetic Resonance Spectroscopy

Minimal hepatic encephalopathy (MHE) is highly prevalent, observed in up to 80% of patients with liver dysfunction. Minimal hepatic encephalopathy is defined as hepatic encephalopathy with cognitive deficits and no grossly evident neurologic abnormalities. Clinical management may be delayed due to the lack of in vivo quantitative methods needed to reveal changes in brain neurobiochemical biomarkers. To gain insight into the development of alcoholic liver disease–induced neurological dysfunction (NDF), a mouse model of late-stage alcoholic liver fibrosis (LALF) was used to investigate changes in neurochemical levels in the thalamus and hippocampus that relate to behavioral changes. Proton magnetic resonance spectroscopy of the brain and behavioral testing were performed to determine neurochemical alterations and their relationships to behavioral changes in LALF. Glutamine levels were higher in both the thalamus and hippocampus of alcohol-treated mice than in controls. Thalamic levels of taurine and creatine were significantly diminished and strongly correlated with alcohol-induced behavioral changes. Chronic long-term alcohol consumption gives rise to advanced liver fibrosis, neurochemical changes in the nuclei, and behavioral changes which may be linked to NDF. Magnetic resonance spectroscopy represents a sensitive and noninvasive measurement of pathological alterations in the brain, which may provide insight into the pathogenesis underlying the development of MHE.

Increases in Brain 1H-MR Glutamine and Glutamate Signals Following Acute Exhaustive Endurance Exercise in the Rat

Objective: Proton magnetic resonance spectroscopy (¹H-MRS) in ultra-high magnetic field can be used for non-invasive quantitative assessment of brain glutamate (Glu) and glutamine (Gln) in vivo. Glu, the main excitatory neurotransmitter in the central nervous system, is efficiently recycled between synapses and presynaptic terminals through Glu-Gln cycle which involves glutamine synthase confined to astrocytes, and uses 60–80% of energy in the resting human and rat brain. During voluntary or involuntary exercise many brain areas are significantly activated, which certainly intensifies Glu-Gln cycle. However, studies on the effects of exercise on ¹H-MRS Glu and/or Gln signals from the brain provided divergent results. The present study on rats was performed to determine changes in ¹H-MRS signals from three brain regions engaged in motor activity consequential to forced acute exercise to exhaustion.

Method: After habituation to treadmill running, rats were subjected to acute treadmill exercise continued to exhaustion. Each animal participating in the study was subject to two identical imaging sessions performed under light isoflurane anesthesia, prior to, and following the exercise bout. In control experiments, two imaging sessions separated by the period of rest instead of exercise were performed. ¹H-NMR spectra were recorded from the cerebellum, striatum, and hippocampus using a 7T small animal MR scanner.

Results: Following exhaustive exercise statistically significant increases in the Gln and Glx signals were found in all three locations, whereas increases in the Glu signal were found in the cerebellum and hippocampus. In control experiments, no changes in ¹H-MRS signals were found.

Conclusion: Increase in glutamine signals from the brain areas engaged in motor activity may reflect a disequilibrium caused by increased turnover in the glutamate-glutamine cycle and a delay in the return of glutamine from astrocytes to neurons. Increased turnover of Glu-Gln cycle may be a result of functional activation caused by forced endurance exercise; the increased rate of ammonia detoxification may also contribute. Increases in glutamate in the cerebellum and hippocampus are suggestive of an anaplerotic increase in glutamate synthesis due to exercise-related stimulation of brain glucose uptake. The disequilibrium in the glutamate-glutamine cycle in brain areas activated during exercise may be a significant contributor to the central fatigue phenomenon.

Network Patterns Associated with Navigation Behaviors Are Altered in Aged Nonhuman Primates

The ability to navigate through space involves complex interactions between multiple brain systems. Although it is clear that spatial navigation is impaired during aging, the networks responsible for these altered behaviors are not well understood. Here, we used a within-subject design and [¹⁸F]FDG-microPET to capture whole-brain activation patterns in four distinct spatial behaviors from young and aged rhesus macaques: constrained space (CAGE), head-restrained passive locomotion (CHAIR), constrained locomotion in space (TREADMILL), and unconstrained locomotion (WALK). The results reveal consistent networks activated by these behavior conditions that were similar across age. For the young animals, however, the coactivity patterns were distinct between conditions, whereas older animals tended to engage the same networks in each condition. The combined observations of less differentiated networks between distinct behaviors and alterations in functional connections between targeted regions in aging suggest changes in network dynamics as one source of age-related deficits in spatial cognition.

SIGNIFICANCE STATEMENT

We report how whole-brain networks are involved in spatial navigation behaviors and how normal aging alters these network patterns in nonhuman primates. This is the first study to examine whole-brain network activity in young or old nonhuman primates while they actively or passively traversed an environment. The strength of this study resides in our ability to identify and differentiate whole-brain networks associated with specific navigational behaviors within the same nonhuman primate and to compare how these networks change with age. The use of high-resolution PET (microPET) to capture brain activity of real-world behaviors adds significantly to our understanding of how active circuits critical for navigation are affected by aging.

Complete protection from impending stroke following permanent middle cerebral artery occlusion in awake, behaving rats

Using a rodent model of ischemic stroke [permanent middle cerebral artery occlusion (pMCAO)], our laboratory has previously demonstrated that sensory-evoked cortical activation via mechanical single whisker stimulation treatment delivered under an anesthetized condition within 2 h of ischemic onset confers complete protection from impending infarct. There is a limited time window for this protection; rats that received the identical treatment at 3 h following ischemic onset lost neuronal function and sustained a substantial infarct. Rats in these studies, however, were anesthetized with sodium pentobarbital or isoflurane, whereas most human stroke patients are typically awake. To optimize our animal model, the present study examined, using functional imaging, histological, and behavioral analysis, whether self-induced sensorimotor stimulation is also protective in unrestrained, behaving rats that actively explore an enriched environment. Rats were revived from anesthesia either immediately or at 3 h after pMCAO, at which point they were allowed to freely explore an enriched environment. Rats that explored immediately after ischemic onset maintained normal cortical function and did not sustain infarct, even when their whiskers were clipped. Rats that were revived at 3 h post-pMCAO exhibited eliminated cortical function and sustained cortical infarct. Further, the data suggested that the level of individual active exploration could influence the outcome. Thus, early activation of the ischemic cortical area via unrestrained exploration resulted in protection from ischemic infarct, whereas late activation resulted in infarct, irrespective of the level of arousal or whisker-specific stimulation.

Functional reorganization of motor and limbic circuits after exercise training in a rat model of bilateral parkinsonism

Exercise training is widely used for neurorehabilitation of Parkinson's disease (PD). However, little is known about the functional reorganization of the injured brain after long-term aerobic exercise. We examined the effects of 4 weeks of forced running wheel exercise in a rat model of dopaminergic deafferentation (bilateral, dorsal striatal 6-hydroxydopamine lesions). One week after training, cerebral perfusion was mapped during treadmill walking or at rest using [(14)C]-iodoantipyrine autoradiography. Regional cerebral blood flow-related tissue radioactivity (rCBF) was analyzed in three-dimensionally reconstructed brains by statistical parametric mapping. In non-exercised rats, lesions resulted in persistent motor deficits. Compared to sham-lesioned rats, lesioned rats showed altered functional brain activation during walking, including: 1. hypoactivation of the striatum and motor cortex; 2. hyperactivation of non-lesioned areas in the basal ganglia-thalamocortical circuit; 3. functional recruitment of the red nucleus, superior colliculus and somatosensory cortex; 4. hyperactivation of the ventrolateral thalamus, cerebellar vermis and deep nuclei, suggesting recruitment of the cerebellar-thalamocortical circuit; 5. hyperactivation of limbic areas (amygdala, hippocampus, ventral striatum, septum, raphe, insula). These findings show remarkable similarities to imaging findings reported in PD patients. Exercise progressively improved motor deficits in lesioned rats, while increasing activation in dorsal striatum and rostral secondary motor cortex, attenuating a hyperemia of the zona incerta and eliciting a functional reorganization of regions participating in the cerebellar-thalamocortical circuit. Both lesions and exercise increased activation in mesolimbic areas (amygdala, hippocampus, ventral striatum, laterodorsal tegmental n., ventral pallidum), as well as in related paralimbic regions (septum, raphe, insula). Exercise, but not lesioning, resulted in decreases in rCBF in the medial prefrontal cortex (cingulate, prelimbic, infralimbic). Our results in this PD rat model uniquely highlight the breadth of functional reorganizations in motor and limbic circuits following lesion and long-term, aerobic exercise, and provide a framework for understanding the neural substrates underlying exercise-based neurorehabilitation.

Neonatal +-methamphetamine exposure in rats alters adult locomotor responses to dopamine D1 and D2 agonists and to a glutamate NMDA receptor antagonist, but not to serotonin agonists

Neonatal exposure to (+)-methamphetamine (Meth) results in long-term behavioural abnormalities but its developmental mechanisms are unknown. In a series of experiments, rats were treated from post-natal days (PD) 11-20 (stage that approximates human development from the second to third trimester) with Meth or saline and assessed using locomotor activity as the readout following pharmacological challenge doses with dopamine, serotonin and glutamate agonists or antagonists during adulthood. Exposure to Meth early in life resulted in an exaggerated adult locomotor hyperactivity response to the dopamine D1 agonist SKF-82958 at multiple doses, a high dose only under-response activating effect of the D2 agonist quinpirole, and an exaggerated under-response to the activating effect of the N-methyl-d-aspartic acid (NMDA) receptor antagonist, MK-801. No change in locomotor response was seen following challenge with the 5-HT releaser p-chloroamphetamine or the 5-HT2/3 receptor agonist, quipazine. These are the first data to show that PD 11-20 Meth exposure induces long-lasting alterations to dopamine D1, D2 and glutamate NMDA receptor function and may suggest how developmental Meth exposure leads to many of its long-term adverse effects.

Anxiolytic-like effect of pregabalin on unconditioned fear in the rat: an autoradiographic brain perfusion mapping and functional connectivity study

Clinical and preclinical evidence suggests anxiolytic-like efficacy of pregabalin (PGB, Lyrica). However, its mechanism of action remains under investigation. The current study applied [(14)C]-iodoantipyrine cerebral blood flow (CBF) mapping to examine the effect of PGB on neural substrates underlying unconditioned fear in a rat model of footshock-induced fear. Regional CBF (rCBF) was analyzed by statistical parametric mapping. Functional connectivity and graph theoretical analysis were used to investigate how footshock and PGB affect brain activation at the network level. Pregabalin significantly attenuated footshock-induced ultrasonic vocalization, but showed no significant effect on freezing behavior. Footshock compared to no-shock controls elicited significant increases in rCBF in limbic/paralimbic regions implicated in the processing of unconditioned fear and ultrasonic vocalization, including the amygdala, hypothalamus, lateral septum, dorsal periaqueductal gray, the anterior insular (aINS) and medial prefrontal cortex (mPFC). The activation pattern was similar in vehicle- and PGB-treated subjects, with PGB significantly attenuating activation in the amygdala, hypothalamus, and aINS. The vehicle/no-shock group showed strong, positive intra-structural correlations within the cortex, hypothalamus, amygdala, thalamus, and brainstem. The cortex was negatively correlated with the hypothalamus and brainstem. Footshock reduced the total number of significant correlations, but induced greater intra-cortical connectivity of the aINS and mPFC, and new positive correlations between the hypothalamus and amygdala. In no-shock controls, PGB significantly reduced the positive intra-structural correlations within the cortex and amygdala, as well as the negative cortico-subcortical correlations. Following footshocks, PGB disrupted both the network recruitment of aINS and mPFC, and the positive hypothalamic-amygdaloid correlations. Our findings suggest that PGB may exert anxiolytic effect by attenuating cortico-cortical and cortico-subcortical communication and inhibiting network recruitment of the aINS, mPFC, amygdala, and hypothalamus following a fear-inducing stimulus. Functional brain mapping in rodents may provide new endpoints for preclinical evaluation of anxiolytic drug candidates with potentially improved translational power compared to behavioral measurements alone.

Mapping functional brain activation using [14C]-iodoantipyrine in male serotonin transporter knockout mice

Background

Serotonin transporter knockout mice have been a powerful tool in understanding the role played by the serotonin transporter in modulating physiological function and behavior. However, little work has examined brain function in this mouse model. We tested the hypothesis that male knockout mice show exaggerated limbic activation during exposure to an emotional stressor, similar to human subjects with genetically reduced transcription of the serotonin transporter.

Methodology/Principal Findings

Functional brain mapping using [¹⁴C]-iodoantipyrine was performed during recall of a fear conditioned tone. Regional cerebral blood flow was analyzed by statistical parametric mapping from autoradiographs of the three-dimensionally reconstructed brains. During recall, knockout mice compared to wild-type mice showed increased freezing, increased regional cerebral blood flow of the amygdala, insula, and barrel field somatosensory cortex, decreased regional cerebral blood flow of the ventral hippocampus, and conditioning-dependent alterations in regional cerebral blood flow in the medial prefrontal cortex (prelimbic, infralimbic, and cingulate). Anxiety tests relying on sensorimotor exploration showed a small (open field) or paradoxical effect (marble burying) of loss of the serotonin transporter on anxiety behavior, which may reflect known abnormalities in the knockout animal's sensory system. Experiments evaluating whisker function showed that knockout mice displayed impaired whisker sensation in the spontaneous gap crossing task and appetitive gap cross training.

Conclusions

This study is the first to demonstrate altered functional activation in the serotonin transporter knockout mice of critical nodes of the fear conditioning circuit. Alterations in whisker sensation and functional activation of barrel field somatosensory cortex extend earlier reports of barrel field abnormalities, which may confound behavioral measures relying on sensorimotor exploration.

CNS animal fMRI in pain and analgesia

Animal imaging of brain systems offers exciting opportunities to better understand the neurobiology of pain and analgesia. Overall functional studies have lagged behind human studies as a result of technical issues including the use of anesthesia. Now that many of these issues have been overcome including the possibility of imaging awake animals, there are new opportunities to study whole brain systems neurobiology of acute and chronic pain as well as analgesic effects on brain systems de novo (using pharmacological MRI) or testing in animal models of pain. Understanding brain networks in these areas may provide new insights into translational science, and use neural networks as a "language of translation" between preclinical to clinical models. In this review we evaluate the role of functional and anatomical imaging in furthering our understanding in pain and analgesia.

Detection by voxel-wise statistical analysis of significant changes in regional cerebral glucose uptake in an APP/PS1 transgenic mouse model of Alzheimer's disease

Biomarkers and technologies similar to those used in humans are essential for the follow-up of Alzheimer's disease (AD) animal models, particularly for the clarification of mechanisms and the screening and validation of new candidate treatments. In humans, changes in brain metabolism can be detected by 1-deoxy-2-[(18)F] fluoro-D-glucose PET (FDG-PET) and assessed in a user-independent manner with dedicated software, such as Statistical Parametric Mapping (SPM). FDG-PET can be carried out in small animals, but its resolution is low as compared to the size of rodent brain structures. In mouse models of AD, changes in cerebral glucose utilization are usually detected by [(14)C]-2-deoxyglucose (2DG) autoradiography, but this requires prior manual outlining of regions of interest (ROI) on selected sections. Here, we evaluate the feasibility of applying the SPM method to 3D autoradiographic data sets mapping brain metabolic activity in a transgenic mouse model of AD. We report the preliminary results obtained with 4 APP/PS1 (64+/-1 weeks) and 3 PS1 (65+/-2 weeks) mice. We also describe new procedures for the acquisition and use of "blockface" photographs and provide the first demonstration of their value for the 3D reconstruction and spatial normalization of post mortem mouse brain volumes. Despite this limited sample size, our results appear to be meaningful, consistent, and more comprehensive than findings from previously published studies based on conventional ROI-based methods. The establishment of statistical significance at the voxel level, rather than with a user-defined ROI, makes it possible to detect more reliably subtle differences in geometrically complex regions, such as the hippocampus. Our approach is generic and could be easily applied to other biomarkers and extended to other species and applications.

A low power, on demand electrothermal valve for wireless drug delivery applications

We present a low power, on demand Parylene MEMS electrothermal valve. A novel Omega-shaped thermal resistive element requires low power (approximately mW) and enables rapid valve opening (approximately ms). Using both finite element analysis and valve opening experiments, a robust resistive element design for improved valve opening performance in water was obtained. In addition, a thermistor, as an inrush current limiter, was added into the valve circuit to provide variable current ramping. Wireless activation of the valve using RF inductive power transfer was demonstrated.

Powering an implantable minipump with a multi-layered printed circuit coil for drug infusion applications in rodents

We report the use of a multi-layer printed coil circuit for powering (36-94 mW) an implantable microbolus infusion pump (MIP) that can be activated remotely for use in drug infusion in nontethered, freely moving small animals. This implantable device provides a unique experimental tool with applications in the fields of animal behavior, pharmacology, physiology, and functional brain imaging. Two different designs are described: a battery-less pump usable when the animal is inside a home-cage surrounded by a primary inductive coil and a pump powered by a rechargeable battery that can be used for studies outside the home-cage. The use of printed coils for powering of small devices by inductive power transfer presents significant advantages over similar approaches using hand-wound coils in terms of ease of manufacturing and uniformity of design. The high efficiency of a class-E oscillator allowed powering of the minipumps without the need for close physical contact of the primary and secondary coils, as is currently the case for most devices powered by inductive power transfer.

A Parylene MEMS Electrothermal Valve

The first microelectromechanical-system normally closed electrothermal valve constructed using Parylene C is described, which enables both low power (in milliwatts) and rapid operation (in milliseconds). This low-power valve is well suited for applications in wirelessly controlled implantable drug-delivery systems. The simple design was analyzed using both theory and modeling and then characterized in benchtop experiments. Operation in air (constant current) and water (current ramping) was demonstrated. Valve-opening powers of 22 mW in air and 33 mW in water were obtained. Following integration of the valve with catheters, our valve was applied in a wirelessly operated microbolus infusion pump, and the in vivo functionality for the appropriateness of use of this pump for future brain mapping applications in small animals was demonstrated.

Sex differences in functional brain activation during noxious visceral stimulation in rats

Studies in healthy human subjects and patients with irritable bowel syndrome suggest sex differences in cerebral nociceptive processing. Here we examine sex differences in functional brain activation in the rat during colorectal distention (CRD), a preclinical model of acute visceral pain. [(14)C]-iodoantipyrine was injected intravenously in awake, non-restrained female rats during 60- or 0-mmHg CRD while electromyographic abdominal activity (EMG) and pain behavior were recorded. Regional cerebral blood flow-related tissue radioactivity was analyzed by statistical parametric mapping from autoradiographic images of three-dimensionally reconstructed brains. Sex differences were addressed by comparing the current data with our previously published data collected from male rats. While sex differences in EMG and pain scores were modest, significant differences were noted in functional brain activation. Females showed widespread changes in limbic (amygdala, hypothalamus) and paralimbic structures (ventral striatum, nucleus accumbens, raphe), while males demonstrated broad cortical changes. Sex differences were apparent in the homeostatic afferent network (parabrachial nucleus, thalamus, insular and dorsal anterior cingulate cortices), in an emotional-arousal network (amygdala, locus coeruleus complex), and in cortical areas modulating these networks (prefrontal cortex). Greater activation of the ventromedial prefrontal cortex and broader limbic/paralimbic changes in females suggest greater engagement of affective mechanisms during visceral pain. Greater cortical activation in males is consistent with the concept of greater cortical inhibitory effects on limbic structures in males, which may relate to differences in attentional and cognitive attribution to visceral stimuli. These findings show remarkable similarities to reported sex differences in brain responses to visceral stimuli in humans.