TROPHIC FACTOR INDUCED SYNAPTIC REGROWTH IN THE CNS

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The long term objectives of the present proposal are to determine the limits of trophic factor-induced synaptic remodelling in the cerebral cortex of lesioned animals. The proposed research addresses two key issues: (l) the extent of functional restoration in the damaged CNS and (2) the possible therapeutic use of neurotrophic factors. This research group demonstrated that, alter cortical infarct, the administration of nerve growth factor causes a regrowth of the atrophied cholinergic input to the cortex as manifested by an increase in the number of cortical cholinergic varicosities, in the size of the presynaptic elements and in the number of synaptic contacts. In the proposed research, we will address the following questions: how widespread are these changes in the cortical mantle; are they specific for the cholinergic system; does synaptic remodelling persist alter the end of therapy; can these events also be induced in the injured CNS of aged animals. These studies will be carried out in young adult and aged Fischer 344 rats. Cortical devascularizing lesions will be performed on anesthetized animals. An Alzet minipump containing either vehicle or putative neurotrophic agents (nerve growth factor and/or GM1) will be connected to a permanent cannula placed in the contralateral lateral ventricle. After one week of therapy (or longer for aged animals) the minipumps will be removed and the animals allowed to survive for various times. The cortical cholinergic fiber network will be studied by means of light and electron microscopy quantitative immunocytochemistry, with the help of image analysis. Parameters to be assessed are the density of the fiber network, the number and size of varicosities and number of synapses. These studies will be carried out in the remaining cortex, at several distances from the edge of the lesion. Furthermore, the noradrenergic and the somatostatin-immunoreactive networks will be studied in order to assess the neurochemical specificity of the neurotrophic factor induced effects on cortical synaptic circuits. Studies of the postsynaptic cholinergic targets in the remaining cortex will be carried out using double-labeling light and electron microscopic immunocytochemistry. For this purpose, choline acetyltransferase immunocytochemistry will be combined to the immunocytochemical detection of glutamate, GABA, somatostatin and VIP and the relative percentage of cholinergic synapses on each transmitter specific postsynaptic target will be assessed by quantitative electron microscopy. These studies should provide important information relevant to the issue of the potential use of neurotrophic agents to treat neurodegenerative diseases on a long term basis, on the assumption that besides the recovery of damaged perikarya, the stimulation and maintenance of neoformed synapses, as a replacement for lost circuits, is a desirable effect.