Synaptic reorganization in the hippocampal formation of alcohol-fed rats may compensate for functional deficits related to neuronal loss

We have examined the behavioral and neuroanatomical effects of long-term al cohol intake in rats ingesting a 20% solution of ethanol for 30 weeks. Prev ious studies have shown that this treatment provokes neuronal degeneration in the hippocampal formation: which occurs in parallel with remodeling proc esses. Spatial reference and working memory of alcohol-fed rats were evalua ted during last 4 weeks of treatment by comparison of their performance wit h age-matched controls on the Morris water maze. Alcohol consumption did no t affect the performance of rats in the reference memory task as indicated by the measures derived from the acquisition trials and from the probe-tria l, which were highly similar for alcohol-fed and control animals. Also, per formance in the working memory task was not significantly altered in alcoho l-treated animals. No treatment-related changes in swim speed or impairment s of sensorimotor abilities, tested in the visible platform task, were dete cted. Stereological methods were applied to evaluate the damage inflicted b y alcohol intake in the structure of the hippocampal formation. In the alco hol-treated animals, there was a noticeable cell loss in the granular layer of the dentate gyrus (10%), and in CA3 (18%) and CA1 (19%) hippocampal sub divisions. In spite of the neuronal loss, the total number of synapses betw een mossy fibers and CA3 pyramids was unaffected by alcohol treatment sugge sting that new synaptic contacts were formed between the surviving neurons. We show that, regardless the marked hippocampal cell loss in rats exposed to chronic alcohol intake, the reorganization that takes place at the synap tic level may alleviate the expected functional deficits. (C) 2000 Elsevier Science Inc. All rights reserved.