Following cerebral ischemia, perilesional astrocytes and activated microglia form a glial scar that hinders the genesis of new axons and blood vessels in the infarcted region. Since glial reactivity is chronically augmented in the normal aging brain, the authors hypothesized that postischemic gliosis would be temporally abnormal in aged rats compared to young rats. Focal cerebral ischemia was produced by reversible occlusion of the right middle cerebral artery in 3- and 20-month-old male Sprague Dawley rats. The functional outcome was assessed in neurobehavioral tests at 3, 7, 14, and 28 days after surgery. Brain tissue was immunostained for microglia, astrocytes, oligodendrocytes, and endothelial cells. Behaviorally, aged rats were more severely impaired by stroke and showed diminished functional recovery compared with young rats. Histologically, a gradual activation of both microglia and astrocytes that peaked by days 14 to 28 with the formation of a glial scar was observed in young rats, whereas aged rats showed an accelerated astrocytic and microglial reaction that peaked during the first week after stroke. Oligodendrocytes were strongly activated at early stages of infarct development in all rats, but this activation persisted in aged rats. Therefore, the development of the glial scar was abnormally accelerated in aged rats and coincided with the stagnation of recovery in these animals. These results suggest that a temporally anomalous gliotic reaction to cerebral ischemia in aged rats leads to the premature formation of scar tissue that impedes functional recovery after stroke.
Old age is associated with a deficient recovery from stroke, but the cellular mechanisms underlying such phenomena are poorly understood. To address this issue, focal cerebral ischemia was produced by reversible occlusion of the right middle cerebral artery in 3- and 20-month-old male Sprague-Dawley rats. Aged rats showed a delayed and suboptimal functional recovery in the post-stroke period. Using BrdU-labeling, quantitative immunohistochemistry and 3-D reconstruction of confocal images, we found that aged rats are predisposed to rapidly develop an infarct within the first few days after ischemia. The emergence of the necrotic zone is associated with a high rate of cellular degeneration, premature accumulation of proliferating BrdU-positive cells that appear to emanate from capillaries in the infarcted area, and a large number of apoptotic cells. With double labeling techniques, we were able to identify, for the first time, over 60% of BrdU-positive cells either as reactive microglia (45%), oligodendrocyte progenitors (17%), astrocytes (23%), CD8+ lymphocytes (4%), or apoptotic cells (<1%). Paradoxically, despite a robust reactive phenotype of microglia and astrocytes in aged rats, at 1-week post-stroke, the number of proliferating microglia and astrocytes was lower in aged rats than in young rats. Our data indicate that aging is associated with rapid infarct development and a poor prognosis for full recovery from stroke that is correlated with premature cellular proliferation and increased cellular degeneration and apoptosis in the infarcted area.
The age-related decline in plasticity of the brain may be one factor underlying poor functional recovery after stroke. In the present work we tested the hypothesis that the attenuation of neural plasticity in old age could be the result of an altered temporal relationship between factors promoting brain plasticity [microtubule-associated protein 1B (MAP1B)] and neurotoxic factors such as C-terminal betaAPP. Focal cerebral ischemia was produced by reversible occlusion of the right middle cerebral artery in 3- and 20-month-old male Sprague-Dawley rats. The functional outcome was assessed in neurobehavioral tests at 3, 7, 14 and 28 days after surgery. At the indicated timepoints, brains were removed and immunostained for C- and N-terminal betaAPP and MAP1B. At 2 weeks poststroke, we found an age-related increase in the amount of the C-terminal fragment of betaAPP in the peri-infarcted area and the infarct core as well as an early, vigorous incorporation of N-terminal betaAPP into the developing astroglial scar. The recovery of the plasticity-associated protein MAP1B following stroke was delayed in both age groups and became prominent between days 14 and 28. As aged rats showed diminished functional recovery compared with young rats, these results suggest that the accumulation of C-terminal betaAPP, together with the early incorporation of N-terminal betaAPP into the glial scar, may over-ride the beneficial role of plasticity factors such as MAP1B.
Background and Purpose: The age-related decline in plasticity of the brain may be one factor underlying the poor functional recovery after stroke. In the present work we tested the hypothesis that the attenuation of neural plasticity could be the result of an age-related reduction in the upregulation of factors promoting brain plasticity (microtubule-associated protein 1B [MAP1B], β-amyloid precursor protein [βAPP]), and an age-related increase in glial reactivity and the accumulation of Aβ, a proteolytic cleavage product of βAPP with neurotoxic properties. Methods: Focal cerebral ischemia was produced by reversible occlusion of the right middle cerebral artery in 3- and 20-month-old male Sprague-Dawley rats. The functional outcome was assessed in neurobehavioral tests 3, 7, 14 and 28 days after surgery. At the indicated time points, brains were removed and immunostained for glial cells. Aβ, as well as the markers of brain plasticity, βAPP and MAP1B. Results: Histologically, in young rats there was a gradual activation of both microglia and astrocytes that peaked by days 14–28 with the formation of a glial scar. In contrast, aged rats showed an accelerated astrocytic and microglial reaction that peaked in the first week after stroke. The expression patterns of a growth-associated phenotype of βAPP as well as with MAP1B accumulation in varicosities along axons in cortical areas affected by stroke peaked between days 14 and 28 in young animals. In aged rats their expression was both delayed (28 days) and reduced. In addition the carboxy terminal fragment of βAPP steadily accumulated over time and reached a maximum by day 14 in aged rats as compared to 28 days in young rats. Conclusions: These results suggest that a temporally anomalous gliotic reaction to cerebral ischemia in aged rats in conjunction with a late and limited upregulation of neuronal plasticity proteins as well as a diminished neurogenesis potential lead to the prevalence of scar tissue that impedes functional recovery from stroke.
Kindled seizures are widely used as a model for epileptogenesis. Although the achievement of kindling criterion is known to require time to develop, the precise developmental period has not been identified. We now report that optimal achievement of the kindling criterion in the Sprague-Dawley rat is associated with a critical inter-stimulus interval of 24 to 26 days. We show that highly efficient kindling can be achieved with only two subconvulsive doses of pentylenetetrazole so long as they are given 25 days apart. Using Northern blot hybridization we show that the increased seizure susceptibility at 25 days coincides with an increased expression of the plasticity-associated proteins, growth-associated protein-43 (GAP-43), microtubule-associated protein 1B (MAP1B), and tissue plasminogen activator (tPA) mRNAs in the hippocampus. By in situ hybridization and immunocytochemistry on tissue sections, we also show an increased expression for GAP-43 in the polymorphic layer of the dentate gyrus, mossy fibers, and pyramidal cells in the CA3 region of the hippocampus. The demonstration of a long, defined developmental interval for inducing the kindling criterion should enable a dissection of the cellular and genetic events underlying this phenomenon in the rat.
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