Limited effects of dehydration on object discrimination in the novel object recognition paradigm in young and middle-aged male and female rats

Abstract

Dehydration is associated with impaired cognitive function in humans. Limited animal research also suggests that disruptions in fluid homeostasis impair performance in cognitive tasks. We previously demonstrated that extracellular dehydration impaired performance in the novel object recognition memory test in a sex and gonadal hormone specific manner. The experiments in this report were designed to further characterize the behavioral effects of dehydration on cognitive function in male and female rats. In Experiment 1, we tested whether dehydration during the training trial in the novel object recognition paradigm would impact performance, while euhydrated, in the test trial. Regardless of hydration status during training, all groups spent more time investigating the novel object during the test trial. In Experiment 2, we tested whether aging exacerbated dehydration-induced impairments on test trial performance. Although aged animals spent less time investigating the objects and had reduced activity levels, all groups spent more time investigating the novel object, compared to the original object, during the test trial. Aged animals also had reduced water intake after water deprivation and, unlike the young adult rats, there was no sex difference in water intake. Together these results, in combination with our previous findings, suggest that disruptions in fluid homeostasis have limited effects on performance in the novel object recognition test and may only impact performance after specific types of fluid manipulations.

Keywords: aging; memory; sex differences; water deprivation.

Figure 1.

Dehydration during training had no effect on euhydrated performance in the novel object recognition test. (A) Regardless of group or hydration status, time spent investigating the novel object was significantly greater than time spent investigating the original object. (B) There were no differences in the discrimination index between any group. *Greater than original object, p < 0.05.

Figure 2.

Activity and water intake measures. (A) Both groups of females were more active, as indicated by the number of lines crossed in the open field, compared to males. Dehydration during the training phase had no effect on activity levels during the test phase. (B) Dehydrated rats consumed more water in the 4 h period between training and testing that hydrated rats. *Greater than males, p < 0.05. +Greater than hydrated rats, p < 0.05.

Figure 3.

Dehydration and age had no effect on performance in the novel object recognition test. (A) Regardless of age, sex, or hydration status, time spent investigating the novel object was significantly greater than time spent investigating the original object. Object exploration time was reduced in the MA females, compared the young male and young female groups. Object exploration time in the MA males was less than all other groups. (B) There were no differences in the discrimination index between any group. *Greater than original object, p < 0.05. ^aLess than young males and young females, p < 0.05. ^bLess than young males, young females, and MA females, p < 0.05.

Figure 4.

Activity and water intake measures. (A) Young females were more active, as indicated by the number of lines crossed in the open field, compared to all other groups. Young males were more active than the MA males and females. MA females were more active than the MA males. Dehydration had no effect on activity levels during the test phase. (B) Dehydrated rats consumed more water after the test trial than hydrated rats. Young rats consumed more water than MA rats. In young rats, dehydrated males consumed more water than dehydrated females, but this sex difference was not detected in MA rats. ^aLess than young females, p < 0.05. ^bLess than young females and males, p < 0.05. ^cLess than all other groups, p < 0.05. *Greater than hydrated rats, p < 0.05. +Greater than MA rats, < 0.05. #Less than dehydrated young male rats, p < 0.05.