Dale Corbett

Plasticity during stroke recovery: from synapse to behaviour

Reductions in blood flow to the brain of sufficient duration and extent lead to stroke, which results in damage to neuronal networks and the impairment of sensation, movement or cognition. Evidence from animal models suggests that a time-limited window of neuroplasticity opens following a stroke, during which the greatest gains in recovery occur. Plasticity mechanisms include activity-dependent rewiring and synapse strengthening. The challenge for improving stroke recovery is to understand how to optimally engage and modify surviving neuronal networks, to provide new response strategies that compensate for tissue lost to injury.

Delayed and prolonged post-ischemic hypothermia is neuroprotective in the gerbil

Global ischemia, in the gerbil, produces profound hippocampal CA1 loss which leads to functional abnormalities (e.g. habituation impairment). In experiment 1, gerbils were subjected to 3 or 5 min of normothermic (brain) ischemia. Hypothermic groups were cooled to 32 degrees C for 12 h beginning 1 h after ischemia, while control groups (no hypothermia) regulated their own temperature. Exploration in a novel open field was assessed on days 3, 7 and 10 following ischemia and CA1 neurons were counted after 10- or 30-day survival. Both ischemia durations produced severe CA1 necrosis which resulted in increased open field activity. Hypothermia attenuated this behavioral pattern and substantially reduced CA1 necrosis against 3 min of ischemia when assessed at 10 and 30 days, but was only partially effective against a 5 min occlusion where, in addition, some cell death appeared to be delayed rather than prevented. In experiment 2, gerbils were occluded for 5 min and survived for 30 days. Twenty-four hours of hypothermia initiated 1 h after ischemia resulted in near total preservation of CA1 neurons. Thus, increasing the duration of post-ischemic hypothermia from 12 to 24 h produced much greater neuroprotection against severe ischemia. Prolonged post-ischemic hypothermia may be a valuable intervention in stroke patients.

POSTISCHEMIC HYPOTHERMIA: A CRITICAL APPRAISAL WITH IMPLICATIONS FOR CLINICAL TREATMENT

The use of hypothermia to mitigate cerebral ischemic injury is not new. From early studies, it has been clear that cooling is remarkably neuroprotective when applied during global or focal ischemia. In contrast, the value of postischemic cooling is typically viewed with skepticism because of early clinical difficulties and conflicting animal data. However, more recent rodent experiments have shown that a protracted reduction in temperature of only a few degrees Celsius can provide sustained behavioral and histological neuroprotection. Conversely, brief or very mild hypothermia may only delay neuronal damage. Accordingly, protracted hypothermia of 32–34°C may be beneficial following acute clinical stroke.A thorough mechanistic understanding of postischemic hypothermia would lead to a more selective and effective therapy. Unfortunately, few studies have investigated the mechanisms by which postischemic cooling conveys its beneficial effect. The purpose of this article is to evaluate critically the effects of postischemic temperature changes with a comparison to some current drug therapies. This article will stimulate new research into the mechanisms of lengthy postischemic hypothermia and its potential as a therapy for stroke patients.

The problem of assessing effective neuroprotection in experimental cerebral ischemia.

In animal models of global and focal ischemia neuroprotection is typically determined by quantifying the degree of cell loss or reduction in infarct volume shortly after the ischemic insult. These methods are unable to reliably detect more subtle forms of neuronal death and dysfunction that arise from injury to non-homogeneous cell populations (e.g. hilar and striatal neurons), or to dendrites (e.g. loss of structural proteins or decreased synaptic transmission). It is argued that this type of covert injury contributes to a wide range of functional impairments (e.g. decreased working memory, altered field potentials, loss of forelimb dexterity) that are rarely used as outcome measures in experimental studies even though they are of paramount importance clinically. The limitations of a purely histological approach in assessing neuroprotection are clearly illustrated using examples of protective drug therapies, mild hypothermia and ischemic preconditioning. An alternative strategy that incorporates behavioural, electrophysiological and histological endpoints is put forth as a more powerful method for gauging neuroprotection. The strength of this approach will be increased if these assessments are performed on the same animals. By incorporating functional measures and longer postischemic survival into their experimental protocols, investigators will increase the validity of their models and hopefully reduce the likelihood of advancing ineffective therapies into costly clinical trials.

Brain-Derived Neurotrophic Factor Contributes to Recovery of Skilled Reaching After Focal Ischemia in Rats

Background and Purpose— Brain-derived neurotrophic factor (BDNF) is involved in neuronal survival, synaptic plasticity, learning and memory, and neuroplasticity. Further, exogenous treatment with BDNF or exposing animals to enrichment and exercise regimens, which also increase BDNF, enhances behavioral recovery after brain injury. Thus, the beneficial effects of rehabilitation in promoting recovery after stroke may also depend on BDNF. We tested this hypothesis by evaluating the contribution of BDNF to motor skill relearning after endothelin-1–induced middle cerebral artery occlusion in rats. Methods— Antisense BDNF oligonucleotide, which blocks the expression of BDNF (or saline vehicle) was infused into the contralateral lateral ventricle for 28 days after ischemia. Animals received either a graduated rehabilitation program, including running exercise and skilled reaching training, which simulates clinical practice, or no rehabilitation. Functional recovery was assessed with a battery of tests that measured skilled reaching, forelimb use asymmetry, and foraging ability. Results— Rehabilitation significantly improved skilled reaching ability in the staircase task. Antisense BDNF oligonucleotide effectively blocked BDNF mRNA, and negated the beneficial effects of rehabilitation on recovery of skilled reaching. Importantly, antisense BDNF oligonucleotide did not affect reaching with the unaffected limb, body weight, infarct size, or foraging ability, indicating the treatment was specific to relearning of motor skill after ischemia. Conclusions— This study is the first to identify a critical role for BDNF in rehabilitation-induced recovery after stroke, and our results suggest that new treatments to enhance BDNF would constitute a promising therapy for promoting recovery of function after stroke.

Getting neurorehabilitation right: what can be learned from animal models?

Animal models suggest that a month of heightened plasticity occurs in the brain after stroke, accompanied by most of the recovery from impairment. This period of peri-infarct and remote plasticity is associated with changes in excitatory/inhibitory balance and the spatial extent and activation of cortical maps and structural remodeling. The best time for experience and training to improve outcome is unclear. In animal models, very early (<5 days from onset) and intense training may lead to increased histological damage. Conversely, late rehabilitation (>30 days) is much less effective both in terms of outcome and morphological changes associated with plasticity. In clinical practice, rehabilitation after disabling stroke involves a relatively brief period of inpatient therapy that does not come close to matching intensity levels investigated in animal models and includes the training of compensatory strategies that have minimal impact on impairment. Current rehabilitation treatments have a disappointingly modest effect on impairment early or late after stroke. Translation from animal models will require the following: (1) substantial increases in the intensity and dosage of treatments offered in the first month after stroke with an emphasis on impairment; (2) combinational approaches such as noninvasive brain stimulation with robotics, based on current understanding of motor learning and brain plasticity; and (3) research that emphasizes mechanistic phase II studies over premature phase III clinical trials.

Prolonged but Delayed Postischemic Hypothermia: A Long-term Outcome Study in the Rat Middle Cerebral Artery Occlusion Model

Delayed but prolonged hypothermia persistently decreases cell death and functional deficits after global cerebral ischemia in rodents. Postischemic hypothermia also reduces infarction after middle cerebral artery occlusion (MCAO) in rat. Because initial neuroprotection is sometimes transient and may not subserve functional recovery, especially on demanding tasks, the authors examined whether postischemic cooling would persistently reduce infarction and forelimb reaching deficits after MCAO. Male spontaneously hypertensive rats were trained to retrieve food pellets in a staircase test that measures independent forelimb reaching ability. Later, rats underwent 90 minutes of normothermic MCAO, through a microclip, or sham operation. In some rats, prolonged cooling (33°C for 24 hours and then 35°C for 24 hours) began 2.5 hours after the onset of ischemia (60 minutes after the start of reperfusion; n = 17 with subsequently 1 death) or sham procedures (n = 4), whereas untreated sham (n = 4) and ischemic (n = 16 with subsequently 1 death) rats maintained normothermia. An indwelling abdominal probe continually measured core temperature, and an automated fan and water spray system was used to produce hypothermia. One month later rats were reassessed in the staircase test over five days and then killed. The contralateral limb impairment in food pellet retrieval was completely prevented by hypothermia (P = 0.0001). Hypothermia reduced an infarct volume of 67.5 mm3 after untreated ischemia to 35.8 mm3 (P < 0.0001). These findings of persistent benefit encourage the clinical assessment of hypothermia.

MK-801 reduced cerebral ischemic injury by inducing hypothermia

The non-competitive N-methyl-D-aspartate (NMDA) antagonist, MK-801, has been reported to prevent or attenuate ischemic brain damage in various animal models. In halothane-anesthetized gerbils it was found that an optimal dose of MK-801 (3.0 mg/kg) for providing cerebral protection also produced hypothermia (31.1 +/- 0.62 degrees C) relative to control animals (34.2 +/- 0.77 degrees C, P less than 0.01). This degree of hypothermia alone was sufficient to provide complete histological and functional protection (spatial memory) against 5 min of carotid artery occlusion. In gerbils made ischemic, but maintained at normal body temperature, a dose of 3.0 mg/kg of MK-801 provided no protection against hippocampal cell loss or spatial memory impairment. These data suggest that the protective actions of MK-801 may be due entirely to drug-induced hypothermia.

Persistent Neuroprotection with Prolonged Postischemic Hypothermia in Adult Rats Subjected to Transient Middle Cerebral Artery Occlusion

Postischemic hypothermia provides long-lasting neuroprotection against global cerebral ischemia in adult rats and gerbils. Studies indicate that hypothermia must be prolonged (e.g., 24 h) to indefatigably salvage hippocampal CA1 neurons. Delayed hypothermia also reduces focal ischemic injury. However, no study has examined long-term outcome following postischemic hypothermia in adult animals. Furthermore, most studies examined only brief hypothermia (e.g., 3 h). Since previous studies may have overestimated long-term benefit and have likely used suboptimal durations of hypothermia, we examined whether prolonged cooling would attenuate infarction at a 2-month survival time following middle cerebral artery occlusion (MCAo) in rats. Adult male Wistar rats were implanted with telemetry brain temperature probes and later subjected to 30 min of normothermic MCAo (contralateral to side of probe placement) or sham operation. Ischemia was produced by the insertion of an intraluminal suture combined with systemic hypotension (60 mm Hg). Sham rats and one ischemic group controlled their own postischemic temperature while another ischemic group was cooled to 34 degrees C for 48 h starting at 30 min following the onset of reperfusion. The infarct area was quantified after a 2-month survival time. Normothermic MCAo resulted in almost complete striatal destruction (91% loss +/- 12 SD) with extensive cortical damage (36% +/- 16 SD). Delayed hypothermia treatment significantly reduced cortical injury to 10% +/- 10 SD (P < 0.001) while striatal injury was marginally reduced to 79% loss +/- 17 SD (P < 0.05). Delayed hypothermia of only 34 degrees C provided long-lasting cortical and striatal protection in adult rats subjected to a severe MCAo insult. These results strongly support the clinical assessment of hypothermia in acute stroke.

Neuroprotection After Several Days of Mild, Drug-Induced Hypothermia

Stroke trials are initiated after demonstrated pharmacological protection in animal models. NBQX protects CA1 neurons against global ischemia; however, this glutamate antagonist induces a period of subnormal temperature (e.g., a decrease of only 1.0–1.5°C) lasting several days. In this study, NBQX (3 × 30 mg/kg, i.p.) was administered starting 60 min after reperfusion, and brain temperature had declined significantly below vehicle-treated animals by 2 h after reperfusion. When the postischemic brain temperature of NBQX-treated gerbils was regulated, no neuronal protection was found. Mimicking an NBQX-induced temperature profile for 28 h postischemia yielded histological protection 4 days later comparable to that of NBQX. However, both the NBQX and temperature simulation groups showed decreased protection after 10-day survival. Our data suggest that a protracted period of subnormal temperature during the postischemic period can obscure the interpretation of preclinical drug studies.