Distribution and retrograde transport of trophic factors in the central nervous system: functional implications for the treatment of neurodegenerative diseases.

Neurotrophins play a crucial role in the maintenance, survival and selective vulnerability of various neuronal populations within the normal and diseased brain. Several families of growth promoting substances have been identified within the central nervous system (CNS) including the superfamily of nerve growth factor related neurotrophin factors, glial derived neurotrophic factor (GDNF) and ciliary neurotrophic factor (CNTF). In addition, other non-neuronal growth factors such as fibroblast growth factor (FGF) have also been identified. This article reviews the trophic anatomy of these factors within the CNS. Intraventricular and intraparenchymal injections of exogenous nerve growth factor result in retrograde labeling mainly within the cholinergic basal forebrain. Distribution of brain derived neurotrophic factor (BDNF) following intraventricular injection is minimal due to the binding to the trkB receptor along the ventricular wall. In contrast, intraparenchymal injections of BDNF results in widespread retrograde transport throughout the CNS. BDNF has also been shown to be transported anterogradely within the CNS. Infusion of GDNF into the CNS results in retrograde transport limited to the nigrostriatal pathway. Hippocampal injections of NT-3 retrogradely label mainly basal forebrain neurons. Retrograde transport of radiolabeled CNTF has only been observed in sensory neurons of the sciatic nerve. Following intraventricular and intraparenchymal infusion of radiolabeled bFGF, retrograde neuronal labeling was found in the telecephalon, diencephalon, mesencephalon and pons. In contrast retrograde labeling for aFGF was found only in the hypothalamus and midbrain. Since select neurotrophins traffic anterogradely and retrogradely within the nervous system, these proteins could be used to treat neurological diseases such as Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis.