Brain-derived neurotrophic factor (BDNF) is thought to be involved in neuronal survival, migration, morphological and biochemical differentiation, and modulation of synaptic function in the CNS. In the rodent cortex, postnatal BDNF expression is initially low but subsequently increases to reach maximal levels around weaning. Thus, BDNF expression peaks at a time when both structural and functional maturation of cortical circuitry occurs. Although the function of BDNF has been probed using many approaches, its requirements during this phase of life have not previously been examined genetically. To test the in vivo requirements for BDNF during this important phase of development we generated early-onset forebrain-specific BDNF mutant mice. Although these mice undergo forebrain-restricted deletion of BDNF by Cre-mediated recombination during embryogenesis, they are healthy, and we did not detect the loss of specific cortical excitatory or inhibitory neurons. However, the neocortex of 5-week-old mice was thinner, attributable at least partly to neuronal shrinkage. Importantly, although visual cortical layer 2/3 neurons in the mutants initially developed normal dendrite structure, dendritic retraction became apparent by 3 weeks of age. Thus, our observations suggest that cortically expressed BDNF functions to support the maintenance of cortical neuron size and dendrite structure rather than the initial development of these features. This is consistent with a role for BDNF in stabilizing the "survival" of circuitry during the phase of activity-dependent reorganization of cortical connectivity.

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Purpose of review Recovery after stroke can occur either via reductions in impairment or through compensation. Studies in humans and non-human animal models show that most recovery from impairment occurs in the first 1 to 3 months after stroke as a result of both spontaneous reorganization and increased responsiveness to enriched environments and training. Improvement from impairment is attributable to a short-lived sensitive period of post-ischemic plasticity defined by unique genetic, molecular, physiological and structural events. In contrast, compensation can occur at any time after stroke. Here we address both the biology of the brain's post-ischemic sensitive period and the difficult question of what kind of training (task-specific vs. a stimulating environment for self-initiated exploration of various natural behaviors) best exploits this period. Recent findings Data suggest that three important variables determine the degree of motor recovery from impairment: (i) the timing, intensity, and approach to training with respect to stroke onset, (ii) the unique post-ischemic plasticity milieu, and (iii) the extent of cortical reorganization. Summary Future work will need to further characterize the unique interaction between types of training and post-ischemic plasticity, and find ways to augment and prolong the sensitive period using pharmacological agents or non-invasive brain stimulation.

Purpose To characterize intracranial plaque inflammation in vivo by using three-dimensional (3D) high-spatial-resolution contrast material–enhanced black-blood (BB) magnetic resonance (MR) imaging and to investigate the relationship between intracranial plaque inflammation and cerebrovascular ischemic events. Materials and Methods The study was approved by the institutional review board and was HIPAA compliant. Twenty-seven patients (19 men; mean age, 56.8 years ± 12.4 [standard deviation]) with cerebrovascular ischemic events (acute stroke, n = 20; subacute stroke, n = 2; chronic stroke, n = 3; transient ischemic attack, n = 2) underwent 3D time-of-flight MR angiography and contrast-enhanced BB 3-T MR imaging for intracranial atherosclerotic disease. Each identified plaque was classified as either culprit (the only or most stenotic lesion upstream from a stroke), probably culprit (not the most stenotic lesion upstream from a stroke), or nonculprit (not within the vascular territory of a stroke). Plaque contrast enhancement was categorized on BB MR images (grade 0, enhancement less than or equal to that of normal arterial walls seen elsewhere; grade 1, enhancement greater than grade 0 but less than that of the pituitary infundibulum; grade 2, enhancement greater than or equal to that of the pituitary infundibulum), and degree of contrast enhancement was calculated. Associations of the likelihood of being a culprit lesion with both plaque contrast-enhancement and plaque thickness were estimated with ordinal logistic regression. Results Seventy-eight plaques were identified in 20 patients with acute stroke (21 [27%] culprit, 12 [15%] probably culprit, and 45 [58%] nonculprit plaques). In these patients, grade 2 contrast enhancement was associated with culprit plaques (odds ratio 34.6; 95% confidence interval: 4.5, 266.5 compared with grade 0) when adjusted for plaque thickness. Grade 0 was observed in only nonculprit plaques. Culprit plaques had a higher degree of contrast enhancement than did nonculprit plaques (25.9% ± 13.4 vs 13.6% ± 12.3, P = .003). Conclusion Contrast enhancement of intracranial atherosclerotic plaque is associated with its likelihood to have caused a recent ischemic event and may serve as a marker of its stability, thereby providing important insight into stroke risk.

Background and Purpose Preliminary studies suggest ntracranial arteries are capable of accommodating plaque formation by remodeling. We sought to study the ability and extent of intracranial arteries to remodel using 3D high-resolution black blood MRI (BBMRI) and investigate its relation to ischemic events. Methods 42 patients with cerebrovascular ischemic events underwent 3D time-of-flight MRA and contrast-enhanced BBMRI examinations at 3T for intracranial atherosclerotic disease. Each plaque was classified by location (e.g., posterior vs. anterior circulation) and its likelihood to have caused a stroke identified on MRI (culprit, indeterminate, or non-culprit). Lumen area (LA), outer wall area (OWA), and wall area (WA) were measured at the lesion and reference sites. Plaque burden was calculated as WA divided by OWA. The arterial remodeling ratio (RR) was calculated as OWA at the lesion site divided by OWA at the reference site, after adjusting for vessel tapering. Arterial remodeling was categorized as positive if RR >1.05, intermediate if 0.95≤RR ≤ 1.05, and negative if RR <0.95. Results 137 plaques were identified in 42 patients (37% [50] posterior, 63% [87] anterior). Compared with anterior circulation plaques, posterior circulation plaques had a larger plaque burden (77.7±15.7 vs. 69.0±14.0, p=0.008), higher RR (1.14±0.38 vs. 0.95±0.32, p=0.002), and more often exhibited positive remodeling (54.0% vs.29.9%, p=0.011). Positive remodeling was marginally associated with downstream stroke presence when adjusted for plaque burden (OR 1.34, 95% CI: 0.99–1.81). Conclusions Intracranial arteries remodel in response to plaque formation, and posterior circulation arteries have a greater capacity for positive remodeling and, consequently, may more likely elude angiographic detection. Arterial remodeling may provide insight into stroke risk.