The purpose of this study was to determine behavioural changes that occur prior to parturition as a result of gestational, prenatal and transgenerational stress. Using tail chasing as a new measure of antepartum maternal behaviour, we show that prenatal and transgenerational stress, but not gestational stress, alters this behaviour. We observed a significant decrease in the amount of tail chasing over that of controls during the antepartum period in F1 and F2 mothers that had experienced prenatal stress. We show that the effects of prenatal stress as expressed by reduced tail chasing are passed on to the next generation and its progeny. The effects of prenatal stress persist in the absence of stress in the filial generations, suggesting physiological and behavioural programming in the offspring with possibly lifelong consequences.
To date there has been very little investigation of rodent antepartum behaviours. We propose that maternal behaviour during the antepartum period may reflect preparatory activities, such as nest building. A previous study showed that nest building activities of the dam undergoes a significant increase during the 24 hours prior to parturition [17
], which is in agreement with the time course of tail chasing behaviour. In the present study, however, dams were not provided with nest building material for better visibility during video analysis and thus no unequivocal correlations between tail chasing and nest building activities are possible. It is possible that the antepartum increase in tail chasing behaviour is indicative of post-partum maternal care. Maternal care, including licking and grooming as a form of tactile stimulation, has been shown to reduce the behavioural and endocrine consequences of preterm birth or early environmental adversity in rodents and human infants [18
]. Tail chasing may also be indicative of other forms of maternal care, such as the retrieval of pups. The observation that the pregnant dam typically engages with the tail outside of her core nest area and completes the tail chasing bout by carrying it back to her nest would supports this hypothesis. Although the specific function of maternal tail chasing behaviour remains to be determined, the present findings suggest that antepartum maternal behaviour may represent a valid indicator of post-partum maternal care.
Periparturitional maternal behaviours may be particularly sensitive to the effects of stress. Prenatal stress may permanently alter brain development, which may manifest in altered nest building behaviour and behavioural simplification when a prenatally stressed rat matures and becomes pregnant [22
]. Furthermore, corticosterone levels in pregnant rats peak on gestational day 18 and remain high until parturition [23
]. The intricate endocrine changes of gestation and the rise in antepartum corticosterone levels in particular may stimulate central dopaminergic systems and lead to greater locomotor activity [24
]. Thus, greater engagement in tail chasing behaviour in dams may be causally related to enhanced hypothalamo-pituitary-adrenal (HPA) axis activity preparing for parturition. It is possible, however, that HPA axis programming by prenatal and transgenerational stress reduces overall motor activity and leads to reduced tail chasing behaviour [25
]. Furthermore, prenatal stress may alter basal activity of the HPA axis and the response to stress in adulthood [1
]. The resulting imbalance of glucocorticoid-regulated endocrine factors participating in parturition may contribute to altering antepartum maternal behaviours.
Altered maternal behaviour during gestation may also reflect changes in profiles of progesterone levels in rodents. In rats, parturition is associated with a decrease in progesterone production, also termed progesterone withdrawal [27
]. Progesterone plasma levels usually begin to decline on gestational day 19 [28
]. In the present study, tail chasing behaviour was analyzed on gestational day 22, 23, 19 and 15 hours prior to parturition, at a time of low progesterone levels. The time course of tail chasing behaviour in the 24 hours leading to parturition suggest that endocrine changes may mediate an increase or decrease in this activity. While gestational stress in the parental generation did not affect tail chasing behaviour and likely did not affect progesterone levels, prenatal and transgenerational stress may have diminished the engagement in tail chasing through interference with progesterone regulation and other components. This notion is supported by a study showing that the onset of maternal nest building at or about the end of pregnancy is associated with a fall in circulating levels of progesterone in rabbits [29
]. Furthermore, inadequacy or absence of a nest area can adversely affect maternal care [29
]. Notably, gestational stress can dysregulate progesterone formation in juvenile offspring [30
], a change that may persist into adulthood in female F1 and F2 animals to perturb physiological and behavioural adjustments to pregnancy. If any of these changes contributed to the present behavioural observations, our data show that the underlying endocrine processes were not associated with profound maternal weight or litter size effects.
The influence of transgenerational programming by prenatal stress was evident in the F2-SNN generation. Although an F3 generation would be necessary to confirm truly epigenetic effects [31
], our findings suggest that programming by prenatal stress disrupts tail chasing behaviour in the grand-offspring and great-grand-offspring. These effects may be due to direct germ line exposure to maternal stress in the womb. Moreover, the exposure to multi-generational stress in F1-SS and F2-SSS rats indicates that prenatal stress has cumulative, context-dependent consequences. These findings suggest that epigenetic mechanisms may mediate a gradually altering physiological response to recurrent stress in each generation. The formation of an epigenetic memory to a single or recurrent adverse event within a family history may assist in adjusting physiological and/or behavioural patterns to a stressful environment. Epigenetic memory refers to transgenerationally stable, yet dynamic re-programming of the germline epigenome that transfers information across generations in the absence of changes in DNA sequence [33
]. Through this kind of memory, the trait of altered maternal behaviour may be passed on to the subsequent generation [13
] via, for example, a heritable pattern of hypermethylation of the gene encoding brain-derived neurotrophic factor (BDNF) [37
]. In the offspring, the resulting reduction in BDNF expression and low levels of this growth factor in the prefrontal cortex during development may have drastic consequences for cognitive and affective abilities and the response to stress in adulthood.
In conclusion, our findings show that antepartum maternal behaviour may have particularly predictive value of an activated stress response, parturition and post-partum maternal care towards her offspring. Importantly, prenatal stress may program physiological and behavioural responses to pregnancy and postpartum maternal care in subsequent generations and their progeny.