Approaches in the laboratory are twofold: First, we have initiated studies examining the endogenous expression of growth factors after brain injury or ischemia, in an effort to understand the role that these factors might play in wound healing and functional reorganization. Second, we have initiated studies of the exogenous administration of trophic factors to examine their neuroprotective properties in vivo after brain injury or ischemia. As an example of the first aim, in recent studies we have examined the expression of basic gibroblast growth factor (bFGF) in animal models of global and focal cerebral ischemia. Following transient global forebrain ischemia in gerbils there is a delayed death of selected neuronal populations (esp. in CA1 hippocampus) occurring at 2-4 days after ischemia. Using this model, we found a biphasic pattern of bFGF gene expression in vulnerable brain regions, with the first peak occurring before neuronal death, and a second peak occurring well after neuronal death has occurred. The first peak may represent a protective response of surviving neurons, whereas the second peak, associated with reactive astroglia, may participate in processes of cellular and synaptic reorganization after ischemia. In other studies, we have used a model of focal ischemia due to occlusion of the middle cerebral artery in the rat, which results in the development of a focal zone of necrosis (infarct) within 24 hours. In this circumstance, we found a monophasic increase of bFGF gene expression occurring in tissue surrounding infarcts, and peaking at 24 hours, possibly representing a protective response of surviving cells. Further studies will be required to fully elucidate the role of the endogenous expression of bFGF after ischemia, including studies using neutralizing antisera and in vivo antisense methods to block bFGF expression, and use of over- and underexpressing genetic mutants.
In other studies, we have directly administered recombinant bFGF to rodents in attempt to limit the extent of neural damage after ischemia. An increasing literature shows that several neurotrophic growth factors, including bFGF, greatly enhance the survival of cultured neurons in the presence of a number of toxins and insults, including excitatory amino acids, intracellular calcium, free radicals, nitric oxide, hypoglycemia, and anoxia. The mechanism of protection against these toxins appears to depend on new neuronal gene expression and protein synthesis induced by the growth factors. Since many of these toxins are implicated i the pathogenesis of neuronal death after ischemia, we hypothesized that the exogenous administration of trophic factors might 'rescue' some vulnerable neurons after ischemia in vivo. Indeed, in recent studies we found that constant infusion of bFGF into the lateral cerebral ventricles of mature rats, beginning at 3 d before focal ischemia, resulted in a 25-30% reduction in infarct size following ischemia. More recently, we found an equivalent or greater (40-50%) reduction in infarct size when bFGF was administered intravenously, starting at times as long as 2 hours after ischemia. Animals showed little or no side effects at the low dosages used. We also found that systemically administered bFGF readily crosses the damaged blood brain barrier after ischemia, suggesting that this factor may enter ischemic brain to directly exert cytoprotective effects. We have also found that bFGF is a potent cerebral vasodilator, suggesting that effects on cerebral blood flow may be another important mechanism of reduction in infarct size by bFGF. Based on these results, collaboration with a pharmaceutical company has begun with the aim of bringing bFGF to clinical trials in human stroke patients.