Pre-reproductive maternal enrichment influences rat maternal care and offspring developmental trajectories: behavioral performances and neuroplasticity correlates

Abstract

Environmental enrichment (EE) is a widely used paradigm for investigating the influence of complex stimulations on brain and behavior. Here we examined whether pre-reproductive exposure to EE of female rats may influence their maternal care and offspring cognitive performances. To this aim, from weaning to breeding age enriched females (EF) were reared in enriched environments. Females reared in standard conditions were used as controls. At 2.5 months of age all females were mated and reared in standard conditions with their offspring. Maternal care behaviors and nesting activity were assessed in lactating dams. Their male pups were also behaviorally evaluated at different post-natal days (pnd). Brain BDNF, reelin and adult hippocampal neurogenesis levels were measured as biochemical correlates of neuroplasticity. EF showed more complex maternal care than controls due to their higher levels of licking, crouching and nest building activities. Moreover, their offspring showed higher discriminative (maternal odor preference T-maze, pnd 10) and spatial (Morris Water Maze, pnd 45; Open Field with objects, pnd 55) performances, with no differences in social abilities (Sociability test, pnd 35), in comparison to controls. BDNF levels were increased in EF frontal cortex at pups' weaning and in their offspring hippocampus at pnd 21 and 55. No differences in offspring reelin and adult hippocampal neurogenesis levels were found. In conclusion, our study indicates that pre-reproductive maternal enrichment positively influences female rats' maternal care and cognitive development of their offspring, demonstrating thus a transgenerational transmission of EE benefits linked to enhanced BDNF-induced neuroplasticity.

Keywords: BDNF; cognition; environmental enrichment; maternal behavior; neurogenesis; reelin.

Figure 1

Timeline of the experimental procedure. Experimental groups of female rats according to different pre-reproductive rearing conditions (EF, Enriched Females; SF, Standard Females. Duration: ≈7 weeks). Experimental groups of male pups (EF-p, Enriched Females' pups; SF-p, Standard Females' pups), behavioral testing (maternal odor preference; Sociability test; Morris Water Maze, MWM; Open Field with objects, OF) and biochemical analyses (■, neurotrophin determination; Δ, reelin quantification; •, hippocampal neurogenesis analyses).

Figure 2

Nest building activity. Results of pre-reproductive maternal rearing condition on nest building activity are depicted. Quantitative (cotton weight, A; latency, B) and qualitative indexes (nest quality, C; position, D; position change, E; height, F; texture, G; additional structures, H) of nest building activity analyzed in EF and SF from the day of delivery (ppd 0) to ppd 13 (^*p < 0.05; ^**p < 0.01). Results are reported as mean ± SEM.

Figure 3

Maternal behavior observation. Results of pre-reproductive maternal rearing condition on maternal behavior are depicted. Sum of duration of the different kind of maternal behaviors (pup-directed, non pup-directed and other behaviors) in each of the three 10 min-blocks of observation. (^*p < 0.05). Results are reported as mean ± SEM.

Figure 4

Maternal behaviors. Results of pre-reproductive maternal rearing condition on specific maternal behaviors are depicted. Line graphs represent duration and frequency of single pup-directed (A–D) and non pup-directed (E–F) maternal behaviors analyzed in EF and SF on ppd 1 (^*p < 0.05; ^**p < 0.01). Results are reported as mean ± SEM.

Figure 5

Maternal odor preference T-maze test. Results of pre-reproductive maternal rearing condition on pup's discriminative performances are depicted. Histograms show time spent in maternal and neutral arm (A), and time spent in direct contact with the maternal and neutral olfactory stimulus (B) analyzed in EF-p and SF-p on pnd 10 (^*p < 0.05; ^**p < 0.01). Results are reported as mean ± SEM.

Figure 6

Sociability test. Results of pre-reproductive maternal rearing condition on pup's sociability are depicted. Histograms show time spent in the three compartment (A), and in the stranger and empty targets (B) analyzed in EF-p and SF-p on pnd 35 (^*p < 0.05; ^**p = 0.01; ^***p < 0.001). Results are reported as mean ± SEM.

Figure 7

Morris Water Maze test. Results of pre-reproductive maternal rearing condition on pup's spatial and procedural performances are depicted. Latency (A), total and peripheral distances (B,C), velocity (D), and navigational strategies put into action (E) throughout MWM test. Percentage of time spent in the four quadrants during Probe phase (F) analyzed in EF-p and SF-p on pnd 45 (^*p = 0.05; ^**p = 0.01; ^***p < 0.001). Results are reported as mean ± SEM. C, Circling; ES, Extended Searching; RS, Restricted Searching; F, Finding; NE, North-East (previously rewarded) quadrant; NW, North-West quadrant; SE, South-East quadrant; SW, South-West quadrant.

Figure 8

Open Field with Objects. Results of pre-reproductive maternal rearing condition on pup's spatial and novelty recognition performances are depicted. Histograms represent discrimination index of spatial change (A) and novelty (B) analyzed in EF-p and SF-p on pnd 55 (^*p < 0.05). Results are reported as mean ± SEM.

Figure 9

BDNF protein levels. Results of pre-reproductive maternal rearing condition on BDNF expression are depicted. Histograms show: BDNF expression levels in frontal cortex, hippocampus and cerebellum of EF and SF at ppd 21 (A); BDNF expression levels in cortex and hippocampus of EF-p and SF-p at birth (pnd 0, B1); BDNF expression levels in frontal cortex, hippocampus and cerebellum of EF-p and SF-p at weaning (pnd 21, B2) and early adulthood (pnd 55, B3) (^*p = 0.05). Results are reported as mean ± SEM. FC, Frontal Cortex; Hp, Hippocampus; Cb, Cerebellum; C, Cortex.