Interaction does Count: A Cross-Fostering Study on Transgenerational Effects of Pre-reproductive Maternal Enrichment

Abstract

Pre-reproductive environmental enrichment of female rats influences sensorimotor development and spatial behavior of the offspring, possibly through the changed maternal nurturing. Nevertheless, maternal care could be not the solely responsible for changing offspring developmental trajectories. To disentangle the specific contribution to the transgenerational inheritance of pre- and post-natal factors, a cross-fostering study was performed. Female rats were reared in an enriched environment from weaning to sexual maturity, while control female rats were reared under standard conditions. Following mating with standard-reared males, all females were housed individually. Immediately after delivery, in- or cross-fostering manipulations were performed so that any foster dams received pups born to another dam of the same (in-fostering) or the opposite (cross-fostering) pre-reproductive rearing condition. In lactating dams maternal care and nesting activities were assessed, while in their male pups spatial abilities were assessed through Morris Water Maze (MWM) test at post-natal day 45. Moreover, the expression of Brain-Derived-Neurotrophic-Factor (BDNF) was evaluated in the hippocampus and frontal cortex of dams and pups at weaning. Pre-reproductive maternal environmental enrichment, followed by adoption procedures, loosened its potential in modifying maternal care and offspring developmental trajectories, as indicated by the lack of differences between in-fostered groups of dams and pups. In addition, enriched dams rearing standard pups showed the least complex maternal repertoire (the highest sniffing duration and the lowest nest quality), and their pups showed a reduced spatial learning in the MWM. Nevertheless, pre-reproductive maternal enrichment kept influencing neurotrophic pattern, with enriched dams expressing increased frontal BDNF levels (regardless of the kind of fostered pups), and their offspring expressing increased hippocampal BDNF levels. The present findings enlighten the crucial importance of the early mother-pups interactions in influencing maternal care and offspring phenotype, with the enriched dam-standard pups couple resulting in the most maladaptive encounter. Our study thus sustains that the bidirectional interactions between mother and pups are able to deeply shape offspring phenotype.

Keywords: BDNF; cognition; cross-fostering; environmental enrichment; maternal care; rats.

FIGURE 1

Experimental timing: female rats reared according to different pre-reproductive conditions (EF: enriched females; SF: standard females); cross- or in- fostering procedures (○); behavioral testing (Nesting activity and Maternal care observations); biochemical analyses (■, BDNF determination). Groups of male pups according to pre-reproductive conditions of foster and biological mothers (EeP: pups reared by an EF and born to another EF; EsP: pups reared by an EF but born to a SF; SsP: pups reared by a SF and born to another SF; SeP: pups reared by a SF but born to an EF); behavioral testing (MWM, Morris Water Maze). EE, environmental enrichment; SC, standard condition; pnd, post-natal day; ppd, post-partum day.

FIGURE 2

Results of pre-reproductive maternal enrichment and fostering manipulations on litter size, sex ratio and body weight: histograms show litter size (A), sex ratio (B), and body weight at birth (C) of pups from EF (eP) and SF (sP), and the body weight at pnd 1, 21, and 45 (D) analyzed in EeP, SsP, EsP, and SeP. In this and in the following figures, data are reported as mean ± SEM.

FIGURE 3

Results of pre-reproductive maternal enrichment and fostering manipulations on nest building activity: histograms show quantitative (latency, A; used cotton, B) and qualitative (nest quality, C) indexes of nest building activity of EeF, EsF, SeF, and SsF dams (^∗p < 0.05, ^∗∗p < 0.01).

FIGURE 4

Results of pre-reproductive maternal enrichment and fostering manipulations on maternal behaviors: pie charts show sum of different kind of maternal behaviors (pup-directed, non-pup-directed and other behaviors) (A). Histograms show total duration of Licking (B), and Sniffing (C) behaviors of EeF, EsF, SeF, and SsF dams (^∗p < 0.05, ^∗∗p < 0.01).

FIGURE 5

Results of pre-reproductive maternal enrichment and fostering manipulations on offspring spatial performances: histograms show latency (A,C), total distance (B,D) swimming velocity (E), and navigational strategies put into action (Searching, F; Finding, G) throughout MWM test, and percentage of time spent in the previously rewarded quadrant during Probe phase (H) of EeP, SsP, EsP, and SeP rats (^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p < 0.001).

FIGURE 6

Results of pre-reproductive maternal enrichment and fostering manipulations on BDNF expression: histograms show BDNF expression levels analyzed in frontal cortex and hippocampus of EeF, SsF, EsF, and SeF dams (A), and of EeP, SsP, EsP, and SeP offspring (B) (^∗p < 0.05, ^∗∗p < 0.01).