EVIDENCE OF REORGANIZATION IN THE HIPPOCAMPAL MOSSY FIBER SYNAPSES OFADULT-RATS REHABILITATED AFTER PROLONGED UNDERNUTRITION

We have previously demonstrated that prolonged low-protein diet leads to irreversible cell loss in the hippocampal formation of the adult ra t. Because the extent of the resulting hippocampal synaptic alteration s is not well characterized, we studied the contacts between messy fib ers and the dendritic excrescences of CA3 pyramidal cells (MF-CA3 syna pses) using quantitative methods. Moreover, we investigated whether re habilitation from undernutrition would influence the morphology of hip pocampal synapses. To address these issues, three groups of adult rats were compared: (a) rats fed with a normal diet for 12 months (control rats); (b) rats treated during the same period with low-protein diet (undernourished rats); and (c) rats undernourished for 6 months and th en switched to normal diet for 6 months (recovery rats). Timm staining and electron microscopy were employed to estimate the volume of the m essy fiber system and the number and related quantitative features of MF-CA3 synapses. The volume of the suprapyramidal bundle of the messy fiber system and its total number of synapses were smaller in undernou rished rats than in control and recovery animals. These parameters did not differ between the latter two groups. The size of messy fiber ter minals and dendritic excrescences and the surface area of synapses wer e smaller in undernourished than in control and recovery groups. Conve rsely, in recovery animals, the volume of the suprapyramidal bundle of the messy fiber system, the size of messy fiber terminals and dendrit ic excrescences, and the total number and surface area of synapses wer e similar to those of controls. These findings indicate that, followin g rehabilitation, the pre- and postsynaptic compartments of MF-CA3 syn apses undergo structural alterations which compensate for the neuronal loss induced by undernutrition.