Ranking of memories and behavioral strategies in the radial maze.

New features of actual choice behavior and effortful information processing in rats were demonstrated in an eight-arm radial maze through modifications of a matching-to-sample task. Two attempts were allowed for a squad of hooded Sprague-Dawley rats (n=7) for finding a reward in a testing phase of the task. The results showed flexibility and sooner learning to matching rule on the second testing attempt that was only later followed by an improvement of choice accuracy on the first attempt. "Hidden learning" on the second attempt could reflect memories and behavioral strategy, which were present, but not expressed on the first choice. The hypothesis was advanced that learning expressed on the second attempt reflects encoding of a matching rule, whereas improvement on the first choice reflects changes in the rank of acquired memories and behavioral strategy. A second experiment on the same squad of rats tested the ability of trained animals to rank already acquired memories. Following the introduction of the second sample to the study phase of the task, the rats learned to prefer to match the first sample in the testing phase, rearranging ranks of stored memories under the internal control of win-stay strategy. Alternative explanations of interference, trace decay and ranking were compared in order to account for the present results.

Development of thermal nociception in rats.

In the present study, we show that hypersensitivity to noxious thermal stimulation can be seen clearly in developing rats. Rats, at postnatal days 3, 6, 9, 12, 15, 21 and 90 were tested for reflex responsiveness to noxious heat, using tail withdrawal from hot water as the assay. Thermal nociceptive thresholds are considerably lowered, relative to adults, up to postnatal day 12. Thresholds were 39, 37.5, 40.8, 43.3, 46.5, 45.2 and 47.2 degrees C for the respective age groups. Enhanced sensitivity to suprathreshold noxious stimuli is seen in neonates up to postnatal day 15 (but not on day 9). Starting on day 21, sensitivity to noxious stimuli decreases with increasing age, as can be seen by the decrease in the slope of the temperature-response curve (system gain). Spinal transections at postnatal days 13, 17, 20, 60, or 100 did not produce a change in nociceptive thresholds in any of the age groups. In contrast, sensitivity to noxious stimulation (system gain) was enhanced by spinalization in rats 20 days of age or older. Based on these results we suggest that threshold elevation with increasing age most probably reflects changes in local spinal properties, while changes in responsiveness to suprathreshold noxious stimuli involves maturation of both spinal and descending supraspinal structures.