Life-long environmental enrichment counteracts spatial learning, reference and working memory deficits in middle-aged rats subjected to perinatal asphyxia

Abstract

Continuous environmental stimulation induced by exposure to enriched environment (EE) has yielded cognitive benefits in different models of brain injury. Perinatal asphyxia results from a lack of oxygen supply to the fetus and is associated with long-lasting neurological deficits. However, the effects of EE in middle-aged rats suffering perinatal asphyxia are unknown. Therefore, the aim of the present study was to assess whether life-long exposure to EE could counteract the cognitive and behavioral alterations in middle-aged asphyctic rats. Experimental groups consisted of rats born vaginally (CTL), by cesarean section (C+), or by C+ following 19 min of asphyxia at birth (PA). At weaning, rats were assigned to standard (SE) or enriched environment (EE) for 18 months. During the last month of housing, animals were submitted to a behavioral test battery including Elevated Plus Maze, Open Field, Novel Object Recognition and Morris water maze (MWM). Results showed that middle-aged asphyctic rats, reared in SE, exhibited an impaired performance in the spatial reference and working memory versions of the MWM. EE was able to counteract these cognitive impairments. Moreover, EE improved the spatial learning performance of middle-aged CTL and C+ rats. On the other hand, all groups reared in SE did not differ in locomotor activity and anxiety levels, while EE reduced locomotion and anxiety, regardless of birth condition. Recognition memory was altered neither by birth condition nor by housing environment. These results support the importance of environmental stimulation across the lifespan to prevent cognitive deficits induced by perinatal asphyxia.

Keywords: aging; anxiety; environmental enrichment; habituation; perinatal asphyxia; recognition memory; spatial reference memory; spatial working memory.

Figure 1

Schematic diagram depicting study design and photograph of two of the enriched environment cages. Cesarean section and perinatal asphyxia procedures were performed on gestational day 22 (PND day 0). At weaning (PND 21) rats from each birth condition (CTL, C+, and PA) were randomly assigned to standard (SE) or enriched environment (EE). After seventeen months, rats were assessed in a behavioral test battery including Elevated Plus Maze (EPM), Open Field (OF), Novel Object Recognition Test (NORT) and Morris water maze (MWM).

Figure 2

Enriched environment was able to counteract the spatial learning and reference memory impairment displayed by middle-aged asphyctic rats. (A) Escape latency; (B) Path length; (C) Time spent in the target quadrant. CTL SE, vaginal delivery rats reared in standard environment; C+ SE, rats born by cesarean section reared in standard environment; PA SE, rats born by cesarean section + asphyxia reared in standard environment; CTL EE, vaginal delivery rats reared in enriched environment; C+ EE, rats born by cesarean section reared in enriched environment; PA EE, rats born by cesarean section + asphyxia reared in enriched environment; CTL, vaginal delivery rats; C+, rats born by cesarean section; PA, rats born by cesarean section + asphyxia. In (C) data are expressed as the mean + s.e.m. of n = 12. In (A,B) error bars are omitted for clarity. ^*p < 0.05 vs. time expected by chance (C); ^**p < 0.01 PA SE vs. all other groups (A,B); ^***p ≤ 0.001 PA SE vs. all other groups (A,B) or vs. time expected by chance (C); ^δp < 0.05 PA SE vs. CTL, C+ and PA reared in EE (A,B); ⁺p < 0.05 CTL EE or C+ EE vs. all other groups except PA reared in EE (p = 0.065) (A,B); ^#p < 0.05 CTL EE or C+ EE vs. all other groups (a and b); ^##p < 0.01 CTL EE or C+ EE vs. all other groups (A,B).

Figure 3

Spatial working memory impairment displayed by middle-aged asphyctic rats was counteracted by life-long exposure to environmental enrichment. (A) Escape latency; (B) Path length. CTL, vaginal delivery rats; C+, rats born by cesarean section; PA, rats born by cesarean section + asphyxia. Each bar represents the mean + s.e.m. of n = 12. ^*p < 0.05; ^**p ≤ 0.01; ^***p ≤ 0.001 compared to sample trials.

Figure 4

Middle-aged rats reared in an enriched environment showed reduced anxiety-related behaviors in the Elevated Plus Maze test. (A) Total distance traveled; (B) number of closed arm entries; (C) percentage of open arms entries; (D) percentage of time spent in open arms. CTL, vaginal delivery rats; C+, rats born by cesarean section; PA, rats born by cesarean section + asphyxia; SE, standard environment; EE, enriched environment. Each bar represents the mean + s.e.m. of n = 12. ^**p < 0.01; ^***p < 0.001.

Figure 5

Middle-aged rats reared in an enriched environment displayed rapid habituation to the Open Field test. (A) Distance traveled over time, insert: total distance traveled; (B) number of rears over time, insert: total number of rears; (C) time spent in the central area over time, insert: total time spent in the central area; (D) ratio central / total distance traveled over time, insert: Ratio central / total distance traveled during the entire OF session. CTL SE, vaginal delivery rats reared in standard environment; C+ SE, rats born by cesarean section reared in standard environment; PA SE, rats born by cesarean section + asphyxia reared in standard environment; CTL EE, vaginal delivery rats reared in enriched environment; C+ EE, rats born by cesarean section reared in enriched environment; PA EE, rats born by cesarean section + asphyxia reared in enriched environment; CTL, vaginal delivery rats; C+, rats born by cesarean section; PA, rats born by cesarean section + asphyxia; SE, standard environment; EE, enriched environment. Data are expressed as the mean (A–D) or the mean + s.e.m. [inserts] of n = 12. In (A–D) error bars are omitted for clarity. ^***p < 0.001 and ^**p < 0.01 compared to groups reared in SE, ^##p < 0.01 and ⁺p < 0.05 compared to the first 5 min time bin.

Figure 6

Neither environment nor birth condition did affect the performance of rats in the Novel Object Recognition test. (A) discrimination index (d1); (B) discrimination ratio (d2). All groups showed preference for the novel object, spending more time exploring it. CTL: vaginal delivery rats; C+: rats born by cesarean section; PA, rats born by cesarean section + asphyxia. Each bar represents the mean + s.e.m. of n = 12.