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Using electroencephalographic methods, rats learning or not learning a two-way active avoidance task were found to differ significantly in the structure of sleep determined the day before training. The main differences concerned (i) synchronized sleep episodes followed by wakefulness, which were longer and fewer in learning rats; (ii) paradoxical sleep episodes, which were longer in learning rats. Significant correlations were present between the number and/or the average duration of synchronized sleep episodes followed by wakefulness or by paradoxical sleep and the number of avoidances or escapes scored in the training session. Power spectral analysis indicated that the relative output in the 6-7-Hz region was higher in learning rats, notably during short episodes of synchronized sleep followed by paradoxical sleep. As two-way active avoidance training induces comparable modifications in postacquisition sleep (Ambrosini et al., Physiol. Behav., 51, 217-226, 1992), the features of preacquisition sleep which prevail in learning rats might directly determine their capacity to learn. Alternatively, they might reflect the existence of a genetic determinant independently conditioning the ability to learn.

The aim of our study was to assess the pattern of copper and zinc-containing superoxide dismutase (Cu, ZnSOD) and manganese-containing superoxide dismutase (MnSOD) activity from embryonic life to senescence in rat brain and liver. The two isoenzymes showed different profiles in the two organs examined. In particular, the cerebral MnSOD activity profile suggests a primary role during differentiation of this enzymatic form.

Male and female Wistar rats, aged 3 months, were massively trained (3.5 h) for a two-way avoidance task and tested for retention (30 min) the following day. Some of them were subjected to a brief test (30 min) at 5, 10, 15, and 20 months. At 3 months, male rats scored more avoidances and a lower number of escapes, freezings, and intertrial crossings than females; in the test session, both groups improved their performance. Over time, male rats maintained the avoidance score displayed at 3 months. Female rats improved their performance at 5 and 10 months and then, at 20 months, they returned to the level attained at 3 months. These data suggest that active avoidance learning in young animals is fixed as permanent functional changes in the neuronal network of the brain which make aged rats, as well as young adult rats, able to master the task.