Age-related modifications of contextual information processing in rats: role of emotional reactivity, arousal and testing procedure

Two experiments were conducted to examine contextual information processing in adult (7 months) and aged (22 months) Wistar rats. In Experiment 1, rat s were tested for contextual fear conditioning when exposed to six series, one per day, of ten pairings of a tone (CS) with a foot-shock (US) delivere d in one of a two-compartment apparatus. Conditioned fear was estimated by recording: (1) the amount of freezing in the shock compartment; and (2) the time spent avoiding the shock compartment. Results show that, after only o ne series of ten CS-US pairings, all rats showed freezing in the shock comp artment, with aged rats exhibiting the stronger response. Adult rats also a voided the shock compartment during place preference tests in contrast to a ged rats, that spent an equivalent time - with an intense freezing reaction - in both the shock and the safe compartments. After 60 CS-US pairings, co ntextual freezing in the shock compartment decreased in both groups, but, c ontrary to adults, aged rats were still not avoiding that compartment. In E xperiment 2, radial maze performance was studied under distinct quantitativ e extra-maze cueing conditions (poor versus rich) and successive context sh ifts. Compared to adults, aged rats were impaired when trained initially un der poor cueing conditions. No group difference was evident when rats were transferred to a context involving more cues (rich cueing conditions), but age-related impairments re-emerged when rats were returned to the original poor cueing conditions. Thus, the fact that performance deficits in a given task were restricted to certain testing procedures suggests that aging aff ects more the utilization than the processing of contextual information. (C ) 2000 Elsevier Science B.V. All rights reserved.