The Effects of Sex and Chronic Restraint on Instrumental Learning in Rats

Abstract

Chronic stress has been shown to impact learning, but studies have been sparse or nonexistent examining sex or task differences. We examined the effects of sex and chronic stress on instrumental learning in adult rats. Rats were tested in an aversive paradigm with or without prior appetitive experience, and daily body weight data was collected as an index of stress. Relative to control animals, reduced body weight was maintained across the stress period for males (-7%, P ≤ .05) and females (-5%, P ≤ .05). For males, there were within-subject day-by-day differences after asymptotic transition, and all restrained males were delayed in reaching asymptotic performance. In contrast, stressed females were facilitated in appetitive and aversive-only instrumental learning but impaired during acquisition of the aversive transfer task. Males were faster than females in reaching the appetitive shaping criterion, but females were more efficient in reaching the appetitive tone-signaled criterion. Finally, an effect of task showed that while females reached aversive shaping criterion at a faster rate when they had prior appetitive learning, they were impaired in tone-signaled avoidance learning only when they had prior appetitive learning. These tasks reveal important nuances on the effect of stress and sex differences on goal-directed behavior.

Figure 1

The left side compares the body weights of males across 21 days of restraint with the weights of litter-matched controls. Restraint attenuated weight gain by approximately 7% across the three weeks. The right side compares the body weights of females across 21 days of restraint with the weights of litter-matched controls. Restraint attenuated weight gain by approximately 5% over the three weeks.

Figure 2

The left side compares the number of days to reach criterion in FR-1 and FR-4 appetitive shaping for control versus restrained males. There were no differences between restrained and control males. The right side compares the number of days to reach criterion in FR-1 and FR-4 appetitive shaping for control versus restrained females. The data show that restrained females reached criterion significantly faster than did control animals and that males reached all criterions faster than females.

Figure 3

The upper left graph compares the number of days to reach criterion in appetitive training for control versus restrained males. There were no differences between groups. The upper right graph compares the number of days to reach criterion in appetitive training for control versus restrained females. Restrained animals reached appetitive criterion faster than control animals. The lower left graph compares appetitive response latencies for control and restrained males. There were no differences between groups. The lower right graph compares appetitive response latencies for control and restrained females. There were no differences between groups.

Figure 4

It compares escape shaping performance for transfer and AV-only male and female restrained and control rats. The data show two main effects of sex and task and one interaction of sex by treatment by task: (1) males reached criterion slower than females, (2) transfer animals reached criterion faster than AV-only animals, and (3) female restrained animals reached criterion of the fastest only when they had prior appetitive training. All other restrained animals were slower than their control counterparts.

Figure 5

The upper left graph compares avoidance % for restrained transfer males versus control males. Restrained males were not impaired relative to control animals during acquisition but had less stable response patterns after asymptotic training days. The upper right graph compares avoidance % for restrained transfer females versus control females. Restrained females were impaired relative to control animals during acquisition and at the asymptotic transition point. The lower left graph compares avoidance % for restrained aversive-only males versus control males. Restrained males were delayed in reaching asymptotic levels relative to control animals and also had less stable responding patterns. The lower right graph compares avoidance % for restrained aversive-only females versus control females. Restrained females were facilitated in reaching the asymptotic transition point relative to control animals.

Figure 6

Shows the timeline for the restrained and control animals' progressions into each learning paradigm for male and female animals.