Differential Preexposure Effects on Eyeblink Conditioning over a One-Week Retention Interval for Pre- vs. Post-Weanling Rat Pups

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Classical eyeblink conditioning (EBC) in rats becomes robust around postnatal day (PND) 24 with dramatic development occurring between PND 17 (pre-weanling) and PND 24 (post-weanling). Past developmental studies of conditioning at PND 17 showed little or no behavioral evidence of learning. However, facilitation of learning occurred when the same animals were re-tested on Day 20, compared to age-matched controls being conditioned for the first time (Stanton et al., 1998). This suggests a carry-over effect from Day 17 to 20. The present study explores the nature of this carry-over in pre-weanling rats while also extending the findings over a one week retention interval for both pre- and post-weanling rat pups.
Groups of pre- and post-weanling rats (PND 17 & 24) were randomly assigned into one of 3 preexposure conditions: Paired, Shock, or Context. Paired exposure consisted of 90 trials of a tone conditioned stimulus (CS) followed by a periorbital shock unconditioned stimulus (US), Shock exposure consisted of 90 shock-only presentations, and Context exposure consisted of being put in the conditioning chamber with no stimulus presentations. One week later (PND 24 & 31, respectively) all animals received paired CS-US presentations and learning was assessed.
As expected, there was some evidence of conditioned responding in the Paired group at PND 17, relative to the Shock and Context groups. At PND 24, all three groups of animals showed statistically similar rates of conditioning, suggesting no savings over the one-week retention interval. The facilitation previously observed for paired training between PND 17 and 20 was not present between PND 17 and 24. Although not statistically significant, there was a marked trend for inhibition of conditioning in the Shock-preexposure group relative to Context controls, suggesting a possible US preexposure effect. In contrast, the post-weanling Paired group showed asymptotic conditioned responding at PND 24 with little change upon retesting at PND 31, indicating strong retention over a one week interval. Interestingly, the Shock group performed just as well as the Paired group from the outset of paired training on PND 31, while it took 40 trials of paired training for the Context group to reach asymptotic performance. This suggests there is shock-enhanced facilitation of eyeblink conditioning over a one-week interval in post-weanling pups, contrasting the inhibitory trend observed in younger animals. Context preexposed animals demonstrated the same rate of learning at PND 31 as did both naïve and context -preexposed animals on PND 24, indicating this group is a valid control group.


Abstract from the 42nd Annual Meeting of the Society for Neuroscience, New Orleans, LA, October 13-17, 2012.