A series of studies evaluating recovery of function in a rodent model of stroke has recently begun. These investigations utilize functional imaging (structural and functional MRI, evoked potentials), behavioral and immunocytochemical methods to evaluate the correspondence of functional, structural and neurochemical changes in the course of recovery following stroke. These studies will clarify the interpretation of data emerging from human imaging studies and provide a model in which we will evaluate novel clinical interventions. One of the principal motivations for these studies is that the neurochemical processes that underlie changes in the BOLD fMRI signal (which are interpreted as cortical map reorganization in humans) are unknown. This animal model will make it possible to investigate these processes at the cellular and molecular level, using the immunocytochemical markers as indices of neurochemical changes that occur in the recovering brain, and provide a useful means of testing drugs that facilitate these processes prior to their introduction into clinical trials. Prior to sacrifice animals are scanned using the same imaging protocols that are used in patients so that MR and immunocytochemical data can be compared. Lesions are induced in adult rats using the photothrombosis method, in which in which a dye (rose bengal) is injected intravenously and the brain is exposed to light transcranially, producing discrete infarcts in stereotaxically specified brain areas (in this case in the somatosensory cortex). Imaging is performed on 7T/ Bruker Avance scanner and includes high resolution SPGR images of the whole brain, quantitative T2 and diffusion weighted images, and cerebral blood flow, investigated with arterial spin labeling. BOLD fMRI and EEG/ERP (somatosensory evoked potential) responses following ipsilesional and contralesional forepaw stimulation are conducted following acquisition of structural data. Since the lesion is within the somatosensory cortex, stimulating the contralateral paw may produce both perilesional and contralesional activition (similar to the results of patient studies) and this preparation should make it possible to investigate the relevance of contralesional activation in a controlled fashion. Additional animals will be studied using intracortical recording methods to evaluate responses to forepaw stimulation in perilesional and contralesional hemispheres. Following in vivo imaging studies, cohorts of lesioned and control animals are sacrificed and brains are perfusion fixed and sectioned and routine histological and immunocytochemical staining is performed on contiguous sections using GFAP (as an index of glial reactivity), synaptophysin (as a measure of synaptic density), MAP2 (as a cytoskeletal marker), Nestin and Doublecortin (as indices of endogenous neural stem cell activation and migration). Additional tissues will be prepared for Western blot analysis of AMPA, NMDA and GABA-A receptor subunits in perilesional and contralesional tissue. In a followup series of studies, animals will be divided into two groups following lesion induction, one group housed in an enriched, one in a standard environment. On the basis of previous research, we expect enrichment to increase measures such as cortical thickness, dendritic arborization, and synaptogensis, which should be reflected in our immunocytochemical indices. If we are able to demonstrate the effects of enrichment in this way, we will administer drugs to subsets of animals housed in both environments to identify those that selectively augment the effects of enrichment. Drugs that appear useful in this model will be considered for clinical trials.
