Odor-aversion learning and retention span in neonatal mouse pups

One hundred and sixty-four litters of Swiss CD-1 random-bred mice were used to assess learning and retention capacities during the first postnatal week. In Experiment 1, whole 7-day litters were exposed for 65 min to commercial extracts of either mint or lemon sprinkled over wood shavings. Five minutes after the beginning of the exposure, half of the litters were injected ip with the illness-inducing agent lithium chloride (LiCl; 0.20 M, 2% of body weight); the other half was treated with saline solution (8% NaCl). On Postnatal Day 10, the animals were singly introduced in a warmed arena for a 180-s preference test, and the time spent in the mint- and lemon-scented areas of the apparatus was recorded. When compared with saline-injected pups, mice that experienced lemon-LiCl pairings showed a significant aversion for the lemon-scented area, while the mint aversion in the mint-LiCl group just missed statistical significance. Three additional control groups (unhandled on Day 7, or only LiCl- or saline-injected) did not show significant preferences for either the mint or the lemon odor. In Experiment 2, litters of 3, 5, or 7 days were similarly exposed to lemon-scented shavings for either 5 or 20 min, injected with LiCl or saline, and then exposed for an additional 60 min to the shavings. On Postnatal Day 10, tests like those of Experiment 1 showed a significant odor-aversion in animals conditioned on Day 7, but not in those conditioned on Day 3 or 5. In Experiment 3, 3- and 5-day old pups were exposed to lemon odor-LiCl or -NaCl pairings, and tested for aversion after 3 or 7 days (CS duration 5 min before injection and either 30 or 60 min after injection). Only when the conditioning-testing interval was limited to 3 days did LiCl-injected groups show a significant aversion, which did not depend on duration of CS exposure.