Neuronal survival, differentiation, and maturation are influenced by soluble trophic agents. The importance of trophic factors has been best demonstrated in the case of nerve growth factor (NGF) and its responsive peripheral neurons. Sequestration of endogenous NGF by appropriately timed injection of NGF antibodies destroys sympathetic and sensory neurons. Identification and characterization of central nervous system (CNS) trophic agents has been hampered by the tiny amounts present in nervous and other tissues. It appears that this problem may be circumvented in the case of basal forebrain cholinergic neurons. NGF injected into the cerebral ventricle of neonatal rats produces dramatic and selective increases in the activity of choline acetyltransferase (ChAT), the neurotransmitter synthetic enzyme for cholinergic neurons. It has been postulated that NGF is the endogenous trophic factor for these cells. In this proposal, experiments will be conducted to directly test this hypothesis. First, it will be proven that NGF (and not an impurity) is responsible for the effect on ChAT activity. NGF will be purified by high-performance, reverse-phase liquid chromatography. To determine the nature and pattern of its activities, the effect of NGF administration on several neurochemical markers for cholinergic and other neurons will be examined. These studies will record the response of septohippocampal cholinergic neurons before and during the period of synapse formation. The physiological role of endogenous NGF will be examined during the same developmental stages by intracerebroventricular injections of affinity-purified NGF antibodies. These studies will indicate whether cholinergic neurochemical markers are selectively depressed by the presence of these antibodies. If so, this will indicate that NGF does function as an endogenous trophic factor for basal forebrain cholinergic neurons. It is expected that the proposed studies will lead to an improved understanding of the growth and development of these neurons. It is hoped that they will provide insights regarding the role that trophic factors generally play in the developing CNS. Furthermore, they may indicate the potential consequences of altered synthesis or release of trophic factors and suggest means for evaluating the contribution of such disturbances to neurological disease.
