The impact of maternal neglect on genetic hyperactivity

Abstract

Early environmental conditions are increasingly appreciated as critical in shaping behavior and cognition. Evidence suggests that stressful rearing environments can have an enduring impact on behaviors in adulthood, but few studies have explored the possibility that rearing environment could exacerbate genetic hyperactivity disorders. Uncovering a strong environmental influence on the transmission of hyperactivity could provide novel avenues for translational research. Recently we developed a selectively bred High-Active line of mice to model ADHD, providing a unique resource to address the question of environmental transmission. The High-Active line demonstrates transgenerational hyperactivity, but the influence of the postnatal environment (i.e. maternal care provided by dams) on hyperactivity had not been systemically quantified. This study employed a cross-fostering method to simultaneously address 1) whether High-Active and Control pups are provided with similar levels of care in the early environment, and 2) whether any differences in rearing environment influence hyperactive behavior. High-Active dams demonstrated impairment in all measures of maternal competence relative to Controls, which reduced survival rates and significantly reduced the body mass of offspring in early life and at weaning. While the deteriorated postnatal environment provided by High-Active dams was ultimately sufficient to depress Control activity, the hyperactivity of High-Active offspring remained unaffected by fostering condition. These data not only confirm the power of genetics to influence hyperactivity across generations, but also provide evidence that early rearing environments may not have a significant impact on the extreme end of hyperactive phenotypes.

Keywords: Genetic hyperactivity; Maternal care; Postnatal stress; Pup retrieval; Selective breeding.

Figure 1. Transgenerational data

A. Each data point reflects the percentage of paired mice each generation which successfully contributed offspring to propagate the lines. Typically 14 High-Active and 14 Control pairs were made during each breeding cycle. B. Average body mass in grams (±SEM) at PNDs 21 (weaning) and 60 (adulthood phenotyping) are represented for each generation. During Generations 13 and 14, mice were not phenotyped, therefore adulthood body mass was not recorded (as indicated by the boxed “relaxed selective pressure”). C. Data reflect the average locomotor activity in the home cage in km/day (±SEM) of adult High-Active and Control mice. Each data point reflects the phenotype of between 100–200 High-Active and Control mice per generation. Mice underwent distance tracking in the home cage for six consecutive days; the 24-hour activity levels on days 5 and 6 of a six-day test were averaged to assess phenotype.

Figure 2. Generation 18 parental demographics

An asterisk (*) denotes statistical significance (P≤0.05) between High-Active and Control groups. A. Data represent the average locomotor activity in the home cage in km/day (±SEM) of Generation 18 parents on days 5 and 6 of a six-day test. These phenotypic data reflect the genetic pressure contributed by dams and sires to their offspring. B. Data represent the average number of pups (±SEM) birthed by dams of the Control and High-Active lines. C. Data reflect the total number of pups cannibalized by PND 6. One Control dam cannibalized 1 pup, while five High-Active dams cannibalized a total of 21 pups.

Figure 3. Pup retrieval testing

An asterisk (*) denotes statistical significance (P≤0.05) between High-Active and Control groups. A. The average latency in seconds (±SEM) for dams to retrieve a displaced pup from the corner and return it to the nest across the four days of retrieval sessions. B. The percentage (±SEM) of dams with nests constructed in the home cage across the four days of pup retrieval tests. C. The average number of Control vs High-Active dams (±SEM) which failed to retrieve pups across the four days. These dams were removed from all subsequent pup retrieval analyses. D. On the final day of pup retrieval testing (Day 4), the total time in minutes (±SEM) for dams to return all pups back into the nest. Litter sizes were not significantly different between Control and High-Active dams. E. Average number of trampling events (±SEM) during pup retrieval across all four days. A trampling event was defined as the dam running over one or more pups. F. Average number of approaches without retrieving pups (±SEM) across all four days. Approaches were defined as a dam investigating a distressed pup with its nose for one or more seconds, but not returning it to the nest. G. Average number of digging events (±SEM) performed during pup retrieval across all four days. A digging event was defined as focused bedding displacement for one or more seconds.

Figure 4. Maternal and non-maternal dam behaviors

A total of 24 observations were made during each session, and dam activities were classified broadly into maternal versus non-maternal behaviors. Bar graph data represent the average number of maternally-related behaviors performed by Control versus High-Active dams (±SEM). An asterisk (*) denotes statistical significance (P≤0.05) between High-Active and Control groups. Pie charts summarize dam behavior both in detail and more broadly, as maternal behavior is shaded while non-maternal behavior is unshaded. Activities performed less than 1% of the time were not labeled with a percentage in pie charts. A. Maternal and non-maternal behavior recorded for dams 2–3 hours into the dark cycle (active phase) on pups’ PND 8. B. Maternal and non-maternal behavior recorded for dams 2–3 hours into the light cycle (inactive phase) on pups’ PND 8. C. Maternal and non-maternal behavior recorded for dams 2–3 hours into the dark cycle (active phase) on pups’ PND 14.

Figure 5. Environmental impact on phenotypic home cage activity

Data represent the average home cage locomotor activity in km/day (±SEM) of Generation 19 cross-fostered offspring during adulthood. An asterisk (*) denotes statistical significance (P≤0.05) between Control pups raised by High-Active versus Control dams. A. Phenotype of female mice. B. Phenotype of male mice. C. Phenotype of all mice collapsed across sex.