Transient focal cerebral ischemia induces sensorimotor deficits in mice

Abstract

Rodents have been extensively used for experimental stroke research with rat and gerbil the preferred species. With the advent of transgenesis and gene targeting the number of mutant mouse strains is rapidly increasing. Thus, mouse models of stroke will be of great importance in the analysis of genetic factors affecting stroke. Demonstrating long-term functional recovery is of paramount importance for the pharmacological evaluation of putative stroke therapies. In the present paper we induce mild focal cerebral ischemia by tandem occlusion of the right middle cerebral artery (MCA), via craniotomy, together with the common carotid artery for 45 min in C57BL/6 strain of mice. The effects of ischemia were evaluated acutely by MRI and long-term (>3 weeks) sensorimotor functional deficits were analyzed using a number of behavioral paradigms including the rotorod, wire hang, horizontal surface approach, eye-closure reflex, and T-maze tests. Although the induced brain damage is mild we show that it leads to clearly detectable and significant sensorimotor defects associated with fine motor coordination, balance, and postural and sensory reflexes. We conclude that the applied behavioral tests will be useful in the analysis of stroke in mutant mice.

Introduction

Occlusion of the middle cerebral artery (MCA) in the rat brain has become an established model for focal ischemia and is believed to mimic certain aspects of the pathophysiology seen in clinical stroke [17], [18]. The gerbil, due to its unique vascular anatomy, provides a convenient model for global ischemia (for examples see review by Hunter et al. [18]). There is, however, little genetic information available on either the rat or gerbil compared to another rodent species, the house mouse. Gene targeting and the over expression of foreign genes are carried out in mice and a large number of inbred mouse strains are available that exhibit distinct phenotypes; many of which have specific traits relevant for stroke research [2], [8]. As a result, the mouse has become the primary mammalian model organism in genetic studies. The availability of transgenic and knockout mice provides an invaluable tool for studying the molecular mechanisms underlying the pathophysiology of cerebral ischemia, and traits thought to be pertinent to the clinical condition can be revealed [5], [6], [21], [27].

No animal model will precisely mimic the clinical condition but occlusion of the MCA has become the model of choice for stroke experimental researchers [7], [17]. In the present paper we induce a transient focal cerebral ischemia by tandem occlusion of the MCA (via a cranial window) and the common carotid artery for 45 min in mice [7]. In this model of focal ischemia-reperfusion injury the vasogenic edema is largely restricted to the cortex and evolves gradually over the first 24 h following reperfusion. Ultimately, a discrete unilateral infarction in the cortex is produced.

Clinical trials of novel stroke therapies use functional measures as the primary endpoint, and restoration of behavioral function needs to be demonstrated in the pre-clinical evaluation of any putative stroke therapy. The ability to measure functional impairment and thus potential recovery following therapeutic intervention is an essential requirement for stroke research. To this end, we have adopted and modified a number of sensorimotor tasks and, in a model of transient focal ischemia in the mouse, show that these behavioral tests reliably detect significant impairments associated with the ischemic damage. We focus our attention on the long-term (3 weeks post-ischemia) functional consequences of stroke, i.e. the chronic effects. Behavioral alterations at this time point, although mild, represent important measures of the long-term outcome of ischemic brain damage and therefore are of a valuable endpoint [19].