This study showed that in rats limited access to nesting material (ELS paradigm) altered the relationship between dam and pups, and was associated with anxiety and increased peripheral BDNF levels in adult life. Moreover, the degree of pure contact between dams and pups (time spent in contact with pups without LG or nursing, that is, without actually caring for them) correlated positively with plasma BDNF levels in the adult rat offspring. Interestingly, similar associations were found in our sample of adolescent humans, especially for BDNF Met carriers.
Regarding maternal care, the control group showed the well-described normal distribution of the LG score.28
On the other hand, the model of ELS applied seems to have affected the variance of the LG score, concentrating it at a certain level, and was associated with an increased time spent in lower arched-back postures, which are less efficient in terms of milk letdown.39
Classic animal models (such as handling and maternal separation) show that variations in maternal behavior, particularly in the LG scores, are implicated in some aspects of developmental regulation in the rat. Neonatal handling increases LG scores and is associated with reduced behavioral and endocrine stress responses in the adult offspring.6, 7, 8, 9
On the other hand, separation that deprives pups from active maternal care, induces increased stress and fear responses in the adult offspring.10, 11, 40
The ELS applied in our study wasn't able to alter the mean LG score, but affected its variance. This finding suggests that maternal behavior in the ELS group could be interpreted as ‘stereotyped'.
Nevertheless, the most interesting result around the maternal behavior in this animal model seems to be the longest time spent in pure contact, which could suggest a lower quality of maternal care in the ELS dams. These peculiar findings regarding maternal care could be attributed to some differences between our animal model and classic models: first, our study is based on an environmental challenge rather than in maternal separations; second, we studied the full spectrum of LG scores without selecting the dams based on a pre-established LG cutoff,9, 41
because this selection could be less natural than analyzing the entire cohort.
The ELS paradigm used here replicates a model described for other authors12, 13
and, because it is based on an environmental change rather than maternal separations, it provides an interesting means of studying the dam/pup relationship without the confounding metabolic consequences of being deprived from maternal milk and warmth. The period chosen for the intervention follows that used in other studies,12
which has been shown to be a sensitive period for epigenetic programming of the hypothalamic–pituitary–adrenal axis by variations in maternal care.41, 42
Interestingly, the ELS applied was able to alter the dam/pup relationship, differently from other classical animal models, yet been associated with the development of adult anxiety in the rat offspring. Similarly, in the human cohort described here low maternal care scores in the PBI predicted higher levels of anxiety symptoms as measured by the SCARED, similarly to what was originally described by Parker et al.43
Another recent clinical study using the PBI also showed that, whereas low and high maternal care groups exhibit similarly low responses to acute stress, there are increased levels of depression and anxiety in the low care group.45
Furthermore, Parker and Lipscombe37
also proposes that maternal overprotection is associated with low, rather than high maternal care, which appears to be consistent with our findings. The increased time spent in ‘pure contact' in the ELS group could be interpreted as a behavior related to maternal focus on controlling the nest's physical needs (for example, temperature maintenance),46, 47
rather than actual caring for pups. It bears noting that this behavior correlates with peripheral measures of BDNF in adulthood, as did PBI overprotection scores in the human cohort of BDNF Met carriers. That is a rigidly controlled type of mothering seems to be associated with increased peripheral BDNF levels and anxiety later in life.
Similar to what was described here, a very recent study showed that decreased maternal care, as assessed by the PBI, correlates with increased harm avoidance and decreased self-directedness in healthy adult subjects and for both personality traits. The partial correlation coefficient was highest in the Met/Met genotype group, suggesting that the BDNF Val66Met polymorphism modulates the effects of parental rearing on these personality traits.48
Others have shown that BDNF Met carriers exposed to ELS have elevated neuroticism and higher anxiety, as well as smaller hippocampal and amygdala volumes.49
Therefore, as this study reports that the effect of maternal overprotection in the peripheral BDNF levels is evident only in BDNF Met carriers, it is in accordance with the literature, which suggests an increased vulnerability of this gene variant to environmental influences.50
Likewise, in rodents, it has been shown that a null mutation in the mouse BDNF gene moderates the long-term effect of maternal care on innate anxiety behavior,51
supporting the role of this neurotrophic factor in the development of adult anxiety.
The increased peripheral BDNF levels found in response to early adversity may constitute a way to compensate for a functional deficit in this system, as BDNF has a key role in the maintenance of neuronal integrity. In animal models, however, some studies describe that intra-hippocampal infusions of BDNF induces anxiogenic-like activity.52
Interestingly, in our animal model, although ELS was associated with anxiety in the EPM paradigm, there were no differences in central BDNF levels (hippocampus, amygdala and periaqueductal gray). We did not find associations between brain and plasma BDNF levels, so in this animal model peripheral BDNF levels do not seem to directly reflect central BDNF levels, at least in the structures studied. Nevertheless, we cannot rule out a possible pathway dysfunction through the modulation of the number/activity of BDNF receptors or in the induction of the intracellular signaling cascade in our model. Other study using a similar animal model, in which neonatal rats were exposed to either a stressed-abusive mother (maltreatment) or positive caregiving mother (cross-fostered care) only for 30
min daily during the first postnatal week, shows that early maltreatment in rodents produces persisting epigenetic changes in BDNF DNA, altering BDNF gene expression in the adult prefrontal cortex.53
In addition, deletion of TrkB in the newborn neuron results in enhanced anxiety-like behavior in adult mice54
and mice overexpressing this receptor exhibit attenuated anxiety-like behavior.55
All these findings on central BDNF/TrkB expression lead us to believe that the differences found in studies about early stress can be attributed to the type of stressors, duration of stress, mouse strain, as well as brain pathway examined.
In both the experimental and the clinical studies reported here, early adversity leading to altered maternal care was associated with increased peripheral BDNF levels. This may be due to an adaptive response associated to the exposure to an acute stressor in a vulnerable period of life,56, 57, 58
persistently altering baseline BDNF production and increasing BDNF levels. It is possible that, as these individuals age or face other stressful situations or diseases during the life course, their resilience will be dampened due to a chronic ‘waste' of resources or due to a diminished biological capacity to respond to these insults. Therefore, it would make sense that peripheral BDNF levels are classically decreased in various psychiatric disorders, such as major depression59
and bipolar disorder,60
when these diseases are associated with childhood trauma.
This paper adds to the growing literature that aims to find biological correlates for the effects of early environmental factors on mental health. Although other authors have studied the relationship between early-life adversity, BDNF and anxiety,51, 53
this paper offers an interesting opportunity to provide links between clinical and experimental research, as one of the first studies to undertake a translational approach considering the impact of early adversity on anxiety and peripheral BDNF levels in adult life. We recognize this to be a strength of our study, as well as the consistency of the animal model (different cohorts were statistically comparable in the several components of maternal behavior) and the careful to consistently check dams' estrous cycles before the elevated plus maze test, as it is well known that female hormonal changes can interfere with anxiety-like behavior.29
Nevertheless, this study also has some limitations. For instance, litter size standardization was not done in the first days of life to avoid effects on maternal care. The variable nutritional status of pups may have influenced study outcomes, however, we were careful to adjust the analysis for litter size in an attempt to minimize this problem. Moreover, the correlations observed between maternal care and anxiety symptoms in humans are significant, though modest. Interestingly, they are mostly consistent with the animal data. In addition, the SCARED is a screening tool for anxiety, not a diagnostic instrument, and we could not take into account potential confounders in the human analysis because of the small sample size. Finally, the human cohort was also oversampled for girls, although there were no sex-related differences in mean SCARED scores. Despite these limitations, we were still able to highlight similarities between the two species, strengthening the validity of our findings. It should be noted that we are still very far from understanding the specificities and complexities of anxiety-like behaviors and a complete inter-species overlap was in no way expected a priori
. On the other hand, similar correlates with similar measures provide a promising framework for experimentation in clinically posed research questions.
In conclusion, our study contributes to the body of evidence showing that early trauma that impacts the mother/offspring relationship is associated with anxiety and increased peripheral BDNF levels later in life, both in a rodent model and in a human cohort. The translational approach employed in this study was successful and we propose the rodent model described herein as a useful tool to investigate other mechanisms whereby early adversity altering maternal behavior is linked to adult psychopathology.