Classical conditioning in the rat fetus: Temporal characteristics and behavioral correlates of the conditioned response

This study examined the temporal characteristics and behavioral correlates of the conditioned response (CR) following classical conditioning of the embryonic Day 20 (E20 rat fetus). The conditioning procedure involved pairing of an artificial nipple (the CS) with an infusion of milk (the US) to establish classical conditioning. The test for classical conditioning involved measurement of a stimulus-evoked facial wiping response in a classical conditioning test. Experiment 1 compared the effectiveness of one- and three-trial procedures to establish classical conditioning. Experiment 2, 3, and 4 described the time course for the CR following one- and three-trial conditioning procedures. Experiments 3b and 4b describe the behavioral responses to (a) presentation of the CS at the time of conditioning, (b) infusion of the milk US at the time of conditioning, and (c) reexposure to the CS before the test for classical conditioning. Experiments 5 and 6 exposed the fetus to manipulations that either increased or decreased stretching (a behavior found to be associated with the CR). The results are discussed in terms of the temporal characteristics and behavioral correlates of conditioned and unconditioned responses and their mediation by activity in endogenous mu and kappa opioid systems.