Fear conditioning in C57/BL/6 and DBA/2 mice: variability in nucleus accumbens function according to the strain predisposition to show contextual- or cue-based responding.

The contribution of the nucleus accumbens shell, the dorsal hippocampus, and the basolateral amygdala to contextual and explicit cue fear conditioning was assessed in C57BL/6 (C57) and DBA/2 (DBA) mice showing differences in processing contextual information associated with consistent but non-pathological variations in hippocampal functionality. Mice from both strains with bilateral ibotenic acid or sham lesions located in each area were introduced in a conditioning chamber and exposed twice to the pairing of a tone (2 x 8 s, 2000 Hz, 80 dB) with a shock (2 s, 0.7 mA). On the following day, mice were first exposed to the training context then to the tone in a different context. Freezing behaviour was scored in all situations. C57 showed more freezing to the context than to the tone whereas DBA showed more freezing to the tone than to the context. In C57, both nucleus accumbens and hippocampal lesions impaired acquisition of contextual fear conditioning but paradoxically improved acquisition of cue fear conditioning, whereas amygdala lesions disrupted performance in every task. In DBA, nucleus accumbens lesions, like amygdala lesions, impaired acquisition of both contextual and cue fear conditioning, whereas hippocampal lesions did not produce any effect. The parallelism between the effect of nucleus accumbens and hippocampus lesions in C57, and between the effect of nucleus accumbens and amygdala lesions in DBA points to a variability in nucleus accumbens function according to the strain specialization to develop context- or cue-based responding.

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, rats 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) delivered 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 one series of ten CS-US pairings, all rats showed freezing in the shock compartment, with aged rats exhibiting the stronger response. Adult rats also avoided the shock compartment during place preference tests in contrast to aged rats, that spent an equivalent time - with an intense freezing reaction - in both the shock and the safe compartments. After 60 CS-US pairings, contextual freezing in the shock compartment decreased in both groups, but, contrary to adults, aged rats were still not avoiding that compartment. In Experiment 2, radial maze performance was studied under distinct quantitative extra-maze cueing conditions (poor versus rich) and successive context shifts. Compared to adults, aged rats were impaired when trained initially under 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 affects more the utilization than the processing of contextual information.

Posterior parietal cortex lesions severely disrupt spatial learning in DBA mice characterized by a genetic hippocampal dysfunction.

C57BL/6 (C57) and DBA/2 (DBA) inbred mice with posterior parietal cortex or sham lesions were tested in a radial eight-arm maze task with all the paths baited. In the high learner C57 strain, parietal lesions produced a limited impairment of performance without affecting maze-running strategies while the same lesions were found to affect more severely performance in the poor learner DBA strain. Because (1) the processing of spatial information has been found to depend on the conjunctive participation of the hippocampus and the posterior parietal cortex, and (2) DBA mice represent a genetic model of hippocampal dysfunction, the fact that parietal lesions impair spatial performance more severely in the DBA strain suggests that the contribution of the posterior parietal cortex to spatial learning depends on the degree of functionality of the hippocampus.

Spatial and visual discrimination learning in CD1 mice: partial analogy between the effect of lesions to the hippocampus and the amygdala.

CD1 mice with dorsal hippocampal and central amygdaloid lesions were submitted to three radial eight-arm maze tasks requiring various degrees of spatial and nonspatial information processing. The results show that, on the standard version of the radial maze, only hippocampal lesions disrupted performance. In the spatial four-baited path task, both hippocampal and amygdaloid lesions increased the errors and modified the patterns of arm choice. Finally, on the visually cued four-baited path task, the two lesions did not affect any quantitative aspect of performance but a strong and unexpected effect of amygdaloid lesions on the patterns of arm choice was found. Taken together, the results indicate i) an effect of hippocampal lesions in all situations that require performing a spatial discrimination, and ii) an effect of amygdaloid lesions in all situations that require distinguishing between baited and unbaited arms, whatever the modality--spatial or visual--being relevant for discriminating the two sets of arms. The analogy between the effects of hippocampal and amygdaloid lesions specifically observed in the spatial four-baited path task suggests that these two limbic areas can exert a similar control on performance in tasks involving mapping operations based upon the discrimination of rewarded and nonrewarded reinforced locations.