Posttraining handling facilitates memory for auditory-cue fear conditioning in rats

A large number of studies have indicated that stress exposure or the administration of stress hormones and other neuroactive drugs immediately after a learning experience modulates the consolidation of long-term memory. However, there has been little investigation into how arousal induced by handling of the animals in order to administer these drugs affects memory. Therefore, the present study examined whether the posttraining injection or handling procedure per se affects memory of auditory-cue classical fear conditioning. Male Sprague-Dawley rats, which had been pre-handled on three days for 1 min each prior to conditioning, received three pairings of a single-frequency auditory stimulus and footshock, followed immediately by either a subcutaneous injection of a vehicle solution or brief handling without injection. A control group was placed back into their home cages without receiving any posttraining treatment. Retention was tested 24 h later in a novel chamber and suppression of ongoing motor behavior during a 10-s presentation of the auditory-cue served as the measure of conditioned fear. Animals that received posttraining injection or handling did not differ from each other but showed significantly less stimulus-induced movement compared to the non-handled control group. These findings thus indicate that the posttraining injection or handling procedure is sufficiently arousing or stressful to facilitate memory consolidation of auditory-cue classical fear conditioning.