Maternal inexperience as a risk factor of innate fear and PTSD-like symptoms in mice.

In laboratory rats and mice, differences in maternal care during the first week of life have been shown to exert long-lasting consequences on cognitive functioning and stress processing of the offspring. Such epigenetic programming is also assumed to play an important role in the transgenerational transmission of PTSD in humans. Here we studied whether even subtle within-subject differences in maternal care - caused by increasing mothering experience from the first to the second litter - can determine subsequent vulnerability for PTSD-like behaviour. To assess the influence of maternal experience on different components of fear, we analysed the adult male offspring of two subsequent litters (offspring 1, 2) from the same parental C57BL/6NCrl (B6N) and C57BL/6JOla (B6JOla) mice for (i) their innate anxiety behaviour on a modified hole board and (ii) their vulnerability to develop long-lasting PTSD-like fear symptoms ("hyperarousal", contextually conditioned fear) following perception of an inescapable foot shock. Increasing maternal experience reduced the animals' innate fear on the modified hole board (more exploration, less inhibition), the acute stress reaction to the shock and - one month after trauma - the levels of hyperarousal-like behaviour in the PTSD-prone B6N strain. In contrast, both acquisition and extinction of contextually conditioned fear were increased in the second offspring, representing cognitive flexibility. A factor analysis showed that innate fear, "hyperarousal" and conditioned fear represent independent behavioural dimensions. In conclusion, the present study identifies maternal inexperience as a risk factor for the development of PTSD-like symptoms. This effect - occurring in inbred mice on an almost identical genetic background - emphasizes the impact of epigenetic factors in PTSD-like behaviour.