Neuroinflammation is a pathological feature of several neurodegenerative conditions including amyotrophic lateral sclerosis (ALS), which involves the selective degeneration of motor neurons in the brain and spinal cord. There is controversy over the role of neuroinflammation in the disease process. Microglia become activated and proliferate in areas of neurodegeneration with disease progression in human patients and in animal models of the disease [1, 2]. Anti-inflammatory therapies have shown efficacy in mouse models of ALS [3–6], and activated microglia can have cytotoxic effects on motor neurons in culture [7–10]. Microglial activation is a response to damage signals from neurons and astrocytes. This activation is seen morphologically as a transition from a resting, ramified state to an active, amoeboid state through a "primed" intermediate state .
Once activated, microglia have cytotoxic and phagocytic potential. However, the role of primed microglia in the early response to damage signals remains unclear. Primed microglia express major histocompatibility class (MHC) II molecules and have antigen presenting capabilities. Priming results in intensification of local surveillance and production of pro- or anti-inflammatory cytokines . It is possible that microglia perform different tasks depending on whether they are "primed" or "activated". We predict that the function of primed microglia is to protect the damaged neuron from further injury and enable recovery, while activated microglia serve to remove the damaged neuron in order to preserve proper function of surrounding cells. In order to examine this, we have developed a model in which the function of primed microglia can be studied. Ultimately, we intend to use this model to examine the effects of microglial priming on healthy motor neurons and on motor neurons predisposed to develop ALS-like pathology.
The induction of cerebral ischemia in animals is a commonly used method to investigate the pathophysiology of stroke. A relatively non-invasive procedure has been developed in rats and involves the insertion of an intraluminal suture into the Circle of Willis to occlude the middle cerebral artery (MCA) . The suture can be removed after a period of occlusion resulting in reperfusion of the Circle of Willis and the production of a marked region of infarct. This method has been modified for use in mice [14, 15]. However, this latter model suffers from a high rate of mortality and inconsistencies in stroke outcome, including high variability of lesion size. Because variability in animal size, strain, and cerebrovascular anatomy can directly affect the consistency of stroke outcome in mice, modifications that include coating the suture material with poly-L-lysine to enhance adhesion of the suture to the vascular endothelium  and increasing occlusion time to maximize infarct volume [17, 18] have been introduced. When coupled with a neurobehavioural assessment, an accurate prediction of lesion severity can be made during MCAo , thus allowing for the exclusion of animals that are unlikely to harbour the necessary infarct from further study. MCAo of 60 minutes or less leads to a substantial recovery of function within 24 hours . While longer occlusion times result in persistence of behavioural symptoms including altered reflexes and contralateral weakness, they are also associated with increased mortality [15, 19].
Most of the studies examining the cellular effects of cerebral ischemia have focused on the primary lesion in the brain. However, neuropathological changes occur far removed from the focal lesion epicentre in studies carried out in rats [20, 21]. These changes include a glial inflammatory response in the contralateral lumbar spinal cord 24 hours following permanent MCAo in which ventral horn motor neurons that appear to be undergoing degeneration are engulfed by phagocytic microglia . This is accompanied by an increase in expression of pro-inflammatory cytokines and markers of oxidative stress 24–72 hours following permanent MCAo [21, 22]. The mechanism by which this inflammatory response is induced is unknown. However, it has been suggested that it may be due to transsynaptic degeneration mediated by ischemic degeneration of the descending supraspinal (upper motor neuron) pathways and an associated deafferentation of the ventral spinal motor neurons . This in turn is postulated to give rise to a glutamatergic injury of the postsynaptic lower motor neuron through release of presynaptic glutamate, the activation of postsynaptic N-methyl-D-aspartate (NMDA) receptors, and an accompanying increased calcium influx . The MCAo model has therefore provided a unique opportunity to observe the effects of upper motor neuron injury on lower motor neurons and surrounding glia in the rat.
In these experiments, we have developed a model of murine MCAo that is easily quantifiable, reproducible in terms of upper motor neuron involvement regardless of the overall size of the cerebral infarct, and which gives rise to microglial priming in the contralateral lumbar ventral horn 24 hours and 72 hours post-reperfusion in mice. We have observed that a 30 minute occlusion time was associated with 100% survival at the 24 hour post-reperfusion time interval. We propose that this model, easily performed in mice, will allow for the study of microglial priming in the lumbar spinal cord in a reproducible manner that will be instrumental in determining the exact roles of early microglial priming and activation, an event that can apparently produce two very opposing outcomes following neuronal injury.