The interventricular injection of an immunotoxin consisting of a monoclonal antibody to the p75 NGF receptor coupled to the ribosome-inactivating protein, saporin, has recently been reported to selectively destroy cholinergic neurons originating in the basal forebrain which project mainly to the cerebral cortex and hippocampus. The overall hypothesis is that this immunotoxin can be used to define the role of the cholinergic basal forebrain in cognitive function and the relationship of acetylcholine neurotransmission in regions innervated by this system to behavioral performance. This proposal addresses 1) the selectivity of this immunotoxin by histological evaluation of gliotic reaction or changes in neuronal dendritic fields in regions not innervated by cholinergic basal forebrain neurons and therefore hypothesized to be unaffected by the toxin; 2) whether the immunolesion produces deficits in attention (which may underlie the impairment in spatial learning already measured) by means of a visual discrimination task; 3) the changes in extracellular acetylcholine, GABA, glutamate, and aspartate as a function of post- lesion time by microdialysis in frontal cortex and hippocampus; 4) a comparison of the lesion induced changes measured by microdialysis of acetylcholine with changes in in vivo acetylcholine turnover measured by GC/MS; 5) whether the administration of a highly effective, long- acting acetylcholinesterase inhibitor, heptylphysostigmine, improves or reverses the behavioral effects or biochemical changes observed following immunolesioning; 6) whether the chronic administration of nerve growth factor attenuates behavioral deficits and biochemical changes induced by the lesion; 7) whether the peripheral administration of a muscarinic partial agonist, SDZ ENS 116, attenuates behavioral deficits and biochemical consequences of the lesion. Behavioral testing will include measurement of the acoustic startle reflex amplitude and habituation and acquisition, spatial acuity and working memory tasks using the Morris water maze. Biochemical testing will include microdialysis of acetylcholine and amino acids, brain levels of choline acetyltransferase activity, and brain levels of monoamines and amino acids in projection fields of the cholinergic basal forebrain. Amelioration of behavioral effects corresponding to appropriate biochemical measures in response to the pharmacological agents will serve to confirm the cholinergic nature of the behavioral impairment.