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Abstract:
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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 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 while also extending the findings over a one week test- retest period. Groups of pre-weanling (PND 17) and post-weanling (PND 24) rats were randomly assigned into one of three different exposure conditions: Paired, Context, or Shock. Paired exposure consisted of repeated trials of a tone conditioned stimulus (CS) followed by a periorbital shock unconditioned stimulus (US). Context exposure consisted of being put in the test situation with no stimulus presentations, while Shock exposure consisted of 90 shock-only presentations. One week later all animals received paired CS-US presentations. As expected, there was no learning evident in the Paired group at PND 17. When re-tested at PND 24, all three groups of animals showed similar rates of learning. The facilitation previously observed for paired training between PND 17 and 20 was not present between PND 17 and 24, suggesting it is a short-lived phenomenon. In contrast, the post-weanling Paired group showed significant learning at PND 24 with little change upon retesting at PND 31, thus indicating strong retention of learning. The Context and Shock control groups showed no learning at PND 24, but rapid learning at PND 31, matching that of the Paired group. Together, these data suggest that prior exposure to testing context or aversive stimuli does not influence conditioning over a one week retention interval. |
