The development of inhibitory learning and memory in hooded and albino rats.
A special kind of passive avoidance was elaborated which made it possible to study inhibitory conditioning in hooded (Long-Evans) and albino (Wistar) rat strains from day of birth to 10 or 11 days of age. Remaining on the safe platform for 60 sec, thus avoiding an electrified grid, for the 1st and 2nd time were the criteria. Passive avoidance was established from the first postnatal hours in both strains. A general developmental trend was demonstrated by a decreasing number of trials to both criteria and increase of average latencies. A temporary inversion of this trend in hooded rats took place at the age of 2 days, and in albinos at the age of 3 days. Habituation or fatigue were excluded by control experiments without noxious stimulation, and the associative character of the reaction was further proved by sensitization experiments. Evidence of memory retention was obtained as early as between the day of birth and the next day. The animals which served on the day of birth as controls needed, on the following day, significantly more trials than naive animals or pups which had been taught, one day before, to avoid the electrified grid. This phenomenon was called 'learned safety' for the previously control animals behaved as if they had learned that descent to the grid was safe. The 24-h memory of the learned inhibitory reaction--withholding movements towards the grid--was first observed in hooded rats between 1 and 2 days of age. Thus the capability to learn an inhibitory reaction has been proved in the neonatal rat which is born less mature than many other altricious mammals. Though learning and memory in both rat strains improved with age, there were small differences in favor of their earlier development in pigmented animals.