Research paperContribution of maternal oxygenic state to the effects of chronic postnatal hypoxia on mouse body and brain development
Introduction
1–2% of all live births are to very low birth weight (VLBW), premature infants, typically less than 32 weeks gestational age and weighing under 1 kg. VLBW infants face a difficult developmental trajectory, plagued with psychological and neurological sequelae, including decreased cortical volume, ventriculomegaly, significant developmental delays and an increased incidence of psychiatric disorders such as schizophrenia, autism and anxiety disorders [1], [3], [17], [18], [21], [24], [25], [26], [27], [38], [39], [40]. It is thought that the pathophysiology of these sequelae is, at least in part, due to chronic hypoxia experienced in VLBW infants during the early neonatal period as a consequence of poor oxygen exchange due to immature lung development [41]. There is also a documented deleterious effect of chronic hypoxia or simulated high altitude conditions on fetal and perinatal growth [10], [34], [35]. This strengthens the importance of oxygenation in achieving optimal fetal and perinatal growth and highlights the importance of chronic metabolic adaptations. Several rodent models have been developed that induce postnatal hypoxia to mimic both the neurological and psychological findings of children exposed to sub-optimal oxygenation conditions, documenting effects on the brain that include decreased brain weight, cortical volume and ventriculomegaly [33]. Using a model in which mice are reared in 10% oxygen during the early postnatal period, we have examined recovery from chronic hypoxic injury and the factors that may mediate recovery, including environmental enrichment and growth factor signaling [7], [12], [13], [14], [16], [19], [30]. We have demonstrated that, similar to the findings in VLBW children, there is heterogeneity in recovery among brain regions and cell types. In this model, cortical volume and excitatory neuron number recover by adulthood, whereas cortical interneurons fail to achieve complete maturation of their protein markers by adulthood. Indeed, we believe there is a delay in cortical maturation following chronic postnatal hypoxia, perhaps linked to an extended period of plasticity [29], [31]. While delayed maturation may increase the potential for recovery, it may also increase the likelihood of missing critical developmental windows.
Despite the considerable evidence generated from the use of chronic postnatal hypoxia, a potential difficulty in the mechanistic interpretation of these studies is the inseparable interaction between the pups and dam during the period of low oxygen exposure (i.e., from postnatal day 3 (P3) to P11). During hypoxic exposure, dams and pups (typically in a C57/B6 genetic background) are housed together in the hypoxic chamber and therefore the dams are also subjected to the same hypoxic conditions as the pups. In order to minimize the adverse effects of maternal stress due to hypoxic exposure on the pups, we include a CD1 foster dam along with each C57 dam and her pups. These litters are culled to 8–10 pups to achieve homogeneity of total “maternal workload”. Nevertheless, the question remains as to the relative contribution of hypoxia directly on the pups as opposed to the indirect contribution mediated by the effects of hypoxia and potential alterations in the dam’s care of the pups. To disentangle these effects, we examined whether reducing the dams exposure to hypoxia may significantly increase pup outcomes on measures that we have found consistently changed immediately following chronic hypoxia exposure (i.e., body weight, brain weight, cortical volume and cortical neuronal number). In order to achieve this, we rotated dams between normoxic and hypoxic conditions, leaving the litters untouched in their respective conditions and compared gross anatomical measures of normoxic and hypoxic pups with non-rotating or rotating mothers.
Section snippets
Mice
Ten C57/B6 mice pregnant dams and ten CD1 pregnant fosters were ordered from Charles River and housed in single cages until birth. All litters were fostered to CD1 mothers, culling litters to a total of 8 pups for all dams on Postnatal Day 2 (P2). Dams and litters were housed in hypoxic (approx. 10% O2) or normoxic control conditions from P3–P11 as previously described [17], [18], [20], [21]. The mean O2 level during a typical 8 day hypoxic exposure period was 10%, with a range from 10.2 to
Results
Two litters of C57/B6 normoxic control pups were weighed at P3, P7 and P11, and compared to two litters of pups weighed prior to starting hypoxic exposure on P3, during exposure on P7 and immediately after being removed from hypoxia on P11. In order to understand the additional impact, if any, of maternal exposure to hypoxic conditions on pup outcome, we examined two additional cohorts of 3 litter (6 litter total) in which 3 dams were sequentially rotated every 12 h between two normoxic cages
Discussion
In the current analyses we find that, corresponding with our previous studies, hypoxic-rearing decreased pup body weight, brain weight and cortical volume [7], [9], [14], [19]. However, no significant decrease was seen in the total number of NeuN+ cortical neurons. Importantly, rotating mothers to reduce the dam’s exposure to hypoxic conditions not only did not reverse the effects of hypoxia on body and brain weight, but actually amplified the effects of hypoxia on body weight, such that
Acknowledgements
Supported by USPHS grant P01 NS062686 and a Canadian Institute of Health Research Fellowship to N.S.
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2020, Experimental NeurologyCitation Excerpt :Dams must remain with the pups during hypoxia treatment. Attempts to minimize dam exposure to hypoxia by rotating dams between groups of normoxic and hypoxic pups illustrate that it might be best to leave the dams with their pups (Salmaso et al., 2015). Rotation of the dams has counterproductive effects on pups’ development that are attributed to potential decreases in nutrition, feeding behaviour, and maternal care due to the stress of rotations.
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Current address: Department of Neuroscience, Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6, Canada.