Regional vulnerability of the hippocampus to repeated motor activity deprivation.

Spontaneous vertical and horizontal exploratory movements are integral components of rodent behavior. Little is known, however, about the structural and functional consequences of restricted spontaneous exploration. Here, we report two experiments to probe whether restriction in vertical activity (rearing) in rats could induce neuro-hormonal and behavioral disturbances. Rearing movements in rats were deprived for 3h/day for 30 consecutive days by placing the animal into a circular tunnel task. Rats temporarily deprived of rearing behavior showed elevated plasma corticosterone levels but no detectable psychological distress and/or anxiety-related behavior within an elevated plus maze. However, rats emitted a greater number of 22-kHz ultrasonic vocalizations and spent significantly more time vocalizing than controls when deprived of their rearing behavior. Despite intact spatial performance within wet- and dry-land spatial tasks, rearing-deprived rats also exhibited a significant alteration in search strategies within both spatial tasks along with reduced volume and neuron number in the hippocampal subregion CA2. These data suggest a new approach to test the importance of free exploratory behavior in endocrine and structural manifestations. The results support a central role of the CA2 in spontaneous exploratory behavior and vulnerability to psychological stress.

Stress enhances return-based behaviors in Wistar rats during spatial navigation without altering spatial performance: improvement or deficit?

Stress is frequently reported to be deleterious to spatial learning and memory. However, there are many instances in which spatial performance is not affected by stress. This discrepancy observed across different studies, in addition to the animals' strain and gender, may be caused by the type of the task employed to assess stress-related behavioral changes. The present experiments set out to investigate the effects of repeated restraint stress (3h/21 days) on spatial performance within the two wet-land (Morris water task; MWT) and dry-land (the ziggurat task; ZT) tasks for spatial learning and memory in adult male Wistar rats. All rats were tested before and after stress treatment. Stressed rats gained less weight than controls. Stress also enhanced circulating corticosterone (CORT). We did not observe a deleterious effect of stress on spatial learning and memory in either of the tasks: both groups successfully performed the wet- and dry-land tasks across all spatial testing days, indicating intact spatial cognition in control and stress rats. However, daily restraint stress for 21 days significantly caused enhancement in rats' memory-dependent returns during the goal-directed investigation in the ZT. The number of returns on learning days was not affected by repeated restraint stress. Return-based spatial investigation induced by stress only on memory days in the dry-land task, not only emphasize on the task-dependent nature of stress-related alterations, it may reveal one of the silent, but arguably deleterious effects of stress on spatial memory in Wistar rats.