Conditioning of aversion to an odor paired with peripheral shock in the developing rat.

Four experiments examined an apparent inability to associate, or severe deficiency in associating, an odor and a footshock during the first 2 weeks of life in the rat, a cue-to-consequence relationship that had formerly seemed age-dependent. With a particular classical conditioning procedure, however, significant conditioning occurred on postnatal Days 6 and 10 with relatively few conditioning trials; the procedure employed an odor explicitly unpaired with footshock (CS-), as well as an odor paired with footshock (CS+) (Experiment I). Experiment II assessed the contribution of a CS- exposure in the conditioning of rats 8, 15, or 50 days of age. For 8-day-olds, exposure to both the CS+ and CS- resulted in conditioned aversion to the CS+ after eight but not one conditioning trials, but neither 1 nor eight trials with only a CS+ produced conditioning. For 15- and 50-day-olds, conditioning to the CS+ odor was significant after one trial with, but not without, a specific CS-; with eight trials, however, conditioning was significant with or without the specific CS-. It was verified with the 50-day-olds in Experiment III that aversion to the CS+ was conditioned with a single trial only if a CS- had been presented, with a slight trend toward superior conditioning if the CS- preceded rather than followed the CS+ during conditioning. Experiment IV tested the hypothesis that exposure to the distinctive CS- odor sensitizes the animal to the specific properties of the CS+ odor. Fifteen and 50-day-old rats were given one conditioning trial with a CS+ odor that was either unaccompanied by a CS- or that was presented with a CS- odor in the same context as the CS+ or in a different context. For both 15- and 50-day-old rats, conditioning to the CS+ occurred only for animals given the CS- in the same context as the CS+, indicating that the hypothesis should be rejected. The results generally indicate rapid and substantial odor-footshock conditioning in rats as young as 6 days of age, with CS- exposure established as perhaps especially significant for conditioning early in life, but important for rats of all ages tested, from infancy to adulthood.