Impaired functionality of reperfused brain tissue following short transient focal ischemia in rats

Present status and future challenges of electroencephalography- and magnetic resonance imaging-based monitoring in preclinical models of focal cerebral ischemia

Animal models are useful tools for better understanding the mechanisms underlying neurological deterioration after an ischemic insult as well as subsequent evolution of changes and recovery of functions. In response to the updated requirements for preclinical investigations of stroke to include relevant functional measurement techniques and biomarker endpoints, we here review the state of knowledge on application of some translational electrophysiological and neuroimaging methods, and in particular, electroencephalography monitoring and magnetic resonance imaging in rodent models of ischemic stroke. This may lead to improvement of diagnostic methods and identification of new therapeutic targets, which would considerably advance the translational value of preclinical stroke research.

Functional MRI and diffusion tensor imaging of brain reorganization after experimental stroke

The potential of the adult brain to reorganize after ischemic injury is critical for functional recovery and provides a significant target for therapeutic strategies to promote brain repair. Despite the accumulating evidence of brain plasticity, the interaction and significance of morphological and physiological modifications in post-stroke brain tissue remain mostly unclear. Neuroimaging techniques such as functional MRI (fMRI) and diffusion tensor imaging (DTI) enable in vivo assessment of the spatial and temporal pattern of functional and structural changes inside and outside ischemic lesion areas. This can contribute to the elucidation of critical aspects in post-stroke brain remodeling. Task/stimulus-related fMRI, resting-state fMRI, or pharmacological MRI enables direct or indirect measurement of neuronal activation, functional connectivity, or neurotransmitter system responses, respectively. DTI allows estimation of the structural integrity and connectivity of white matter tracts. Together, these MRI methods provide an unprecedented means to (a) measure longitudinal changes in tissue structure and function close by and remote from ischemic lesion areas, (b) evaluate the organizational profile of neural networks after stroke, and (c) identify degenerative and restorative processes that affect post-stroke functional outcome. Besides, the availability of MRI in clinical institutions as well as research laboratories provides an optimal basis for translational research on stroke recovery. This review gives an overview of the current status and perspectives of fMRI and DTI applications to study brain reorganization in experimental stroke models.

Effects of GABA(A) receptor blockade on regional cerebral blood flow and blood-brain barrier disruption in focal cerebral ischemia

In cerebral ischemia, transmission by the inhibitory neurotransmitter, γ-aminobutyric acid (GABA) is altered. This study was performed to determine whether blockade of GABA(A) receptor would affect regional cerebral blood flow (rCBF) and blood-brain barrier (BBB) permeability in a focal ischemic area of the brain. Rats were anesthetized with isoflurane and mechanically ventilated. Fifteen minutes after a permanent middle cerebral artery (MCA) occlusion, one half of the rats were infused with bicuculline 1mg/kg/min iv for 2 min followed by 0.1mg/kg/min iv to the end of the experiment. The other half were infused with normal saline. At one hour after MCA occlusion, rCBF was determined using ¹⁴C-iodoantipyrine and BBB permeability was determined by measuring the transfer coefficient (Ki) of ¹⁴C-α-aminoisobutyric acid. With MCA occlusion, rCBF was decreased in the ischemic cortex (IC) (-70%) in the control rats. In the bicuculline treated rats, the rCBF of the IC was lower (-48%) than the contralateral cortex but higher than the rCBF of the IC of the control rats (+55%). MCA occlusion increased Ki in the IC of the control rats (+72%) and bicuculline administration increased Ki further (+53%) in the IC. Blockade of GABA(A) receptors did not significantly affect rCBF or BBB permeability in the non-ischemic brain regions under isoflurane anesthesia. Our data demonstrated that blockade of GABA(A) receptors increased rCBF and enhanced the BBB disruption in focal cerebral ischemia. Our data suggest that GABA(A) receptors are involved, at least in part, in modulating rCBF and BBB disruption in focal cerebral ischemia.

Assessment of brain responses to innocuous and noxious electrical forepaw stimulation in mice using BOLD fMRI

Functional magnetic resonance imaging (fMRI) using the blood oxygen level-dependent (BOLD) contrast was used to study sensory processing in the brain of isoflurane-anesthetized mice. The use of a cryogenic surface coil in a small animal 9.4T system provided the sensitivity required for detection and quantitative analysis of hemodynamic changes caused by neural activity in the mouse brain in response to electrical forepaw stimulation at different amplitudes. A gradient echo-echo planar imaging (GE-EPI) sequence was used to acquire five coronal brain slices of 0.5mm thickness. BOLD signal changes were observed in primary and secondary somatosensory cortices, the thalamus and the insular cortex, important regions involved in sensory and nociceptive processing. Activation was observed consistently bilateral despite unilateral stimulation of the forepaw. The temporal BOLD profile was segregated into two signal components with different temporal characteristics. The maximum BOLD amplitude of both signal components correlated strongly with the stimulation amplitude. Analysis of the dynamic behavior of the somatosensory 'fast' BOLD component revealed a decreasing signal decay rate constant k(off) with increasing maximum BOLD amplitude (and stimulation amplitude). This study demonstrates the feasibility of a robust BOLD fMRI protocol to study nociceptive processing in isoflurane-anesthetized mice. The reliability of the method allows for detailed analysis of the temporal BOLD profile and for investigation of somatosensory and noxious signal processing in the brain, which is attractive for characterizing genetically engineered mouse models.

Magnetic resonance methods and applications in pharmaceutical research

This review presents an overview of some recent magnetic resonance (MR) techniques for pharmaceutical research. MR is noninvasive, and does not expose subjects to ionizing radiation. Some methods that have been used in pharmaceutical research MR include magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) methods, among them, diffusion-weighted MRI, perfusion-weighted MRI, functional MRI, molecular imaging and contrast-enhance MRI. Some applications of MR in pharmaceutical research include MR in metabonomics, in vivo MRS, studies in cerebral ischemia and infarction, degenerative joint diseases, oncology, cardiovascular disorders, respiratory diseases and skin diseases. Some of these techniques, such as cardiac and joint imaging, or brain fMRI are standard, and are providing relevant data routinely. Skin MR and hyperpolarized gas lung MRI are still experimental. In conclusion, considering the importance of finding and characterizing biomarkers for improved drug evaluation, it can be expected that the use of MR techniques in pharmaceutical research is going to increase in the near future.

Cerebrovascular effects of hemodialysis in chronic kidney disease

Patients with end-stage renal disease (ESRD) undergoing hemodialysis are known to suffer cognitive deficits and stroke of unknown etiology. It has been suspected that the treatment itself may contribute to the syndrome by unknown mechanisms, which we investigated in this study. End-stage renal disease patients on hemodialysis (n=19) or peritoneal dialysis (PD, n=5) were compared with 14 healthy controls. Subjects participated in magnetic resonance imaging (MRI) measurements of cerebral atrophy, cerebral blood flow (CBF) arterial spin labeled-MRI (ASL-MRI), quantitative Doppler blood flow through the internal carotid artery, and cerebral oxymetry. The Doppler and oxymetry procedures were also performed at the beginning and end of a single hemodialysis session. End-stage renal disease patients on hemodialysis showed significant cerebral atrophy, associated with longer hemodialysis duration and cognitive deficits, including focal bilateral lesions in the caudate nucleus and midbrain. Cerebral oxygenation was extremely low before dialysis (rSO(2) 41+/-13, compared with 70+/-2 in controls, P<0.02) and improved only slightly after dialysis. Carotid blood flow was also very low at the start of dialysis (115+/-28 mL/sec, versus 193+/-56 in controls, P<0.005) but normalized at the end of the session (181 mL/sec). The PD patients showed intermediate values, between the hemodialysis and controls. Notably, duration of hemodialysis treatment predicted global gray-matter volume (r=-0.74), change of blood flow during dialysis (r=-0.65), and baseline rSO(2) (r=-0.65). The findings suggest that ESRD patients on hemodialysis suffer low CBF during the interdialytic cycle. Coupled with low cerebral oxygenation levels and atherosclerosis, this may contribute significantly to the etiology of the observed cerebral atrophy, cognitive deficits, and high stroke prevalence.

Current status of functional MRI on small animals: application to physiology, pathophysiology, and cognition

This review aims to make the reader aware of the potential of functional MRI (fMRI) in brain activation studies in small animal models. As small animals generally require anaesthesia for immobilization during MRI protocols, this is believed to be a serious limitation to the type of question that can be addressed with fMRI. We intend to introduce a fresh view with an in-depth overview of the surprising number of fMRI applications in a wide range of important research domains in neuroscience. These include the pathophysiology of brain functioning, the basic science of activity, and functional connectivity of different sensory circuits, including sensory brain mapping, the challenges when studying the hypothalamus as the major control centre in the central nervous system, and the limbic system as neural substrate for emotions and reward. Finally the contribution of small animal fMRI research to cognitive neuroscience is outlined. This review avoids focusing exclusively on traditional small laboratory animals such as rodents, but rather aims to broaden the scope by introducing alternative lissencephalic animal models such as songbirds and fish, as these are not yet well recognized as neuroimaging study subjects. These models are well established in many other neuroscience disciplines, and this review will show that their investigation with in vivo imaging tools will open new doors to cognitive neuroscience and the study of the autonomous nervous system in experimental animals.

Acute ischemic stroke: overview of major experimental rodent models, pathophysiology, and therapy of focal cerebral ischemia

Ischemic stroke is a devastating disease with a complex pathophysiology. Animal modeling of ischemic stroke serves as an indispensable tool first to investigate mechanisms of ischemic cerebral injury, secondly to develop novel antiischemic regimens. Most of the stroke models are carried on rodents. Each model has its particular strengths and weaknesses. Mimicking all aspects of human stroke in one animal model is not possible since ischemic stroke is itself a very heterogeneous disorder. Experimental ischemic stroke models contribute to our understanding of the events occurring in ischemic and reperfused brain. Major approaches developed to treat acute ischemic stroke fall into two categories, thrombolysis and neuroprotection. Trials aimed to evaluate effectiveness of recombinant tissue-type plasminogen activator in longer time windows with finer selection of patients based on magnetic resonance imaging tools and trials of novel recanalization methods are ongoing. Despite the failure of most neuroprotective drugs during the last two decades, there are good chances to soon have effective neuroprotectives with the help of improved preclinical testing and clinical trial design. In this article, we focus on various rodent animal models, pathogenic mechanisms, and promising therapeutic approaches of ischemic stroke.

Fischer-344 rats are unsuitable for the MCAO filament model due to their cerebrovascular anatomy

Middle cerebral artery occlusion (MCAO) in Fischer-344 rats results in a small variance of infarct size. However, complications are frequent especially in aged Fisher-344 rats undergoing endovascular suture occlusion of the middle cerebral artery. Analyzing our experiences with 165 Wistar, 13 Sprague-Dawley and 10 F-344 rats, we compared the incidence of impossible thread advancement and subarachnoid hemorrhage, respectively. Magnetic resonance angiography (MRA) was applied to study the course of the internal carotid artery (ICA) in Fischer and Wistar rats. Finally, we performed a structured review of the literature from 1991 to 2005 evaluating reports on Fischer rats subjected to intraluminal filament MCAO. Complications like fruitless filament advancement or subarachnoid hemorrhage were found to be significantly more frequent in Fischer rats than in other strains. MRA revealed significantly more pronounced kinking of the ICA in F-344 than in Wistar rats. In seven publications available on filament MCAO in F-344 rats, complication rates of 50-100% were reported, corroborating our data. Surgical difficulties accompanied by high complication rates due to their cerebrovascular anatomy make Fischer rats unsuitable for filament MCAO. If the use of Fischer rats for studies on focal cerebral ischemia is indicated, other ischemia models than intraluminal suture occlusion should be chosen.

A role for fMRI in optimizing CNS drug development

Drug development today needs to balance agility, speed and risk in defining the probability of success for molecules, mechanisms and therapeutic concepts. New techniques in functional magnetic resonance imaging (fMRI) promise to be part of a sequence that could transform drug development for disorders of the central nervous system (CNS) by examining brain systems and their functional activation dynamically. The brain is complex and multiple transmitters and intersecting brain circuits are implicated in many CNS disorders. CNS therapeutics are designed against specific CNS targets, many of which are unprecedented. The challenge is to reveal the functional consequences of these interactions to assess therapeutic potential. fMRI can help optimize CNS drug discovery by providing a key metric that can increase confidence in early decision-making, thereby improving success rates and reducing risk, development times and costs of drug development.

Nogo-A antibody improves regeneration and locomotion of spinal cord-injured rats

Spinal cord trauma leads to loss of motor, sensory and autonomic functions below the lesion. Recovery is very restricted, due in part to neurite growth inhibitory myelin proteins, in particular Nogo-A. Two neutralizing antibodies against Nogo-A were used to study recovery and axonal regeneration after spinal cord lesions. Three months old Lewis rats were tested in sensory-motor tasks (open field locomotion, crossing of ladder rungs and narrow beams, the CatWalk(R) runway, reactions to heat and von Frey hairs). A T-shaped lesion was made at T8, and an intrathecal catheter delivered highly purified anti-Nogo-A monoclonal IgGs or unspecific IgGs for 2 weeks. A better outcome in motor behavior was obtained as early as two weeks after lesion in the animals receiving the Nogo-A antibodies. Withdrawal responses to heat and mechanical stimuli were not different between the groups. Histology showed enhanced regeneration of corticospinal axons in the anti-Nogo-A antibody groups. fMRI revealed significant cortical responses to stimulation of the hindpaw exclusively in anti-Nogo-A animals. These results demonstrate that neutralization of the neurite growth inhibitor Nogo-A by intrathecal antibodies leads to enhanced regeneration and reorganization of the injured CNS, resulting in improved recovery of compromised functions in the absence of dysfunctions.

Applications of ultrasmall superparamagnetic iron oxide contrast agents in the MR study of animal models

Ultrasmall superparamagnetic iron oxide nanoparticles have been widely used during the past decade as MR intravascular contrast agents in the study of animal models. Such agents enhance both T1 and T2/T2* relaxation, although for animal studies it is the later type of enhancement that is most commonly exploited. Their strong microscopic intravascular susceptibility effect enables the local blood volume distribution to be mapped in various organs. High spatial resolution and sensitivity can be achieved, because the long half-life of these agents in blood, combined with anesthetization, permits steady-state measurements over extended periods. This capability has been utilized to study the cerebrovascular blood volume distributions and their changes in normal, activated, pathologic and pharmacologically or genetically modified states, particularly in rodent animal models. It has also been applied to study blood volume changes in other tissues, such as the myocardium. The relaxation rate shifts Delta R2 and Delta R2* induced by iron oxide agents may differ depending on certain morphological characteristics of the microvascular network, and sensitive Delta R2 and Delta R2* mapping can potentially provide, in addition to blood volume, measurement of other important microvascular parameters such as blood vessel density and size. This work aims to review the applications of ultrasmall superparamagnetic iron oxide contrast agents in MR animal studies, with an emphasis on the investigation of microvascular parameters.

Magnetic resonance imaging in experimental models of brain disorders

This review gives an overview of the application of magnetic resonance imaging (MRI) in experimental models of brain disorders. MRI is a noninvasive and versatile imaging modality that allows longitudinal and three-dimensional assessment of tissue morphology, metabolism, physiology, and function. MRI can be sensitized to proton density, T1, T2, susceptibility contrast, magnetization transfer, diffusion, perfusion, and flow. The combination of different MRI approaches (e.g., diffusion-weighted MRI, perfusion MRI, functional MRI, cell-specific MRI, and molecular MRI) allows in vivo multiparametric assessment of the pathophysiology, recovery mechanisms, and treatment strategies in experimental models of stroke, brain tumors, multiple sclerosis, neurodegenerative diseases, traumatic brain injury, epilepsy, and other brain disorders. This report reviews established MRI methods as well as promising developments in MRI research that have advanced and continue to improve our understanding of neurologic diseases and that are believed to contribute to the development of recovery improving strategies.