Neuronal death after trophic factor deprivation.

Since the discovery of nerve growth factor (NGF) nearly 40 years ago, extensive studies have elaborated the trophic relationship between a neuron and its end organ. Much evidence has accumulated that depicts the crucial role of neurotrophic factors in determining neuronal survival both during development and after injury. The molecular events by which trophic factors determine whether an individual neuron lives or dies are poorly understood. The function of active protein synthesis as a prerequisite for neuronal death has led to the hypothesis that all neurons contain a mechanism to self-destruct or a phenomenon of programmed cell death. The loss of trophic support may, therefore, lead to the initiation of this so called "suicide" program within individual cells. Recently, multiple other trophic factors have been discovered. At least two of these factors, brain-derived neurotrophic factor and neurotrophin-3, appear related to members of the same gene family as the prototypic neurotrophic factor, nerve growth factor. Other metabolic factors, such as intracellular calcium, may be instrumental in determining the dependence of individual neurons on specific trophic support. A "set-point" hypothesis has been described concerning the relationship between intracellular calcium and the NGF dependency of embryonic neurons in cell culture. Changes in calcium metabolism may be an important determinant in altering trophic dependence and naturally occurring cell death. Neurotrophic factors also influence the reaction of the neuron to axotomy and subsequent axonal regeneration. Exogenously supplied trophic factors may prevent neuronal loss after axotomy when supplied at the site of injury. Both NGF and FGF have been shown to enhance early regeneration across gaps within silicone chambers.(ABSTRACT TRUNCATED AT 250 WORDS)