Surprisingly, in our modern 24/7 society, there is scant information on the impact of developmental chronodisruption like the one experienced by shift worker pregnant women on fetal and postnatal physiology. There are important differences between the maternal and fetal circadian systems; for instance, the suprachiasmatic nucleus is the master clock in the mother but not in the fetus. Despite this, several tissues/organs display circadian oscillations in the fetus. Our hypothesis is that the maternal plasma melatonin rhythm drives the fetal circadian system, which in turn relies this information to other fetal tissues through corticosterone rhythmic signaling. The present data show that suppression of the maternal plasma melatonin circadian rhythm, secondary to exposure of pregnant rats to constant light along the second half of gestation, had several effects on fetal development. First, it induced intrauterine growth retardation. Second, in the fetal adrenal in vivo it markedly affected the mRNA expression level of clock genes and clock-controlled genes as well as it lowered the content and precluded the rhythm of corticosterone. Third, an altered in vitro fetal adrenal response to ACTH of both, corticosterone production and relative expression of clock genes and steroidogenic genes was observed. All these changes were reversed when the mother received a daily dose of melatonin during the subjective night; supporting a role of melatonin on overall fetal development and pointing to it as a ‘time giver’ for the fetal adrenal gland. Thus, the present results collectively support that the maternal circadian rhythm of melatonin is a key signal for the generation and/or synchronization of the circadian rhythms in the fetal adrenal gland. In turn, low levels and lack of a circadian rhythm of fetal corticosterone may be responsible of fetal growth restriction; potentially inducing long term effects in the offspring, possibility that warrants further research.

Epidemiological and experimental evidence correlates adverse intrauterine conditions with the onset of disease later in life. For a fetus to achieve a successful transition to extrauterine life, a myriad of temporally integrated humoral/biophysical signals must be accurately provided by the mother. We and others have shown the existence of daily rhythms in the fetus, with peripheral clocks being entrained by maternal cues, such as transplacental melatonin signaling. Among developing tissues, the fetal hippocampus is a key structure for learning and memory processing that may be anticipated as a sensitive target of gestational chronodisruption. Here, we used pregnant rats exposed to constant light treated with or without melatonin as a model of gestational chronodisruption, to investigate effects on the putative fetal hippocampus clock, as well as on adult offspring’s rhythms, endocrine and spatial memory outcomes. The hippocampus of fetuses gestated under light:dark photoperiod (12:12 LD) displayed daily oscillatory expression of the clock genes Bmal1 and Per2, clock-controlled genes Mtnr1b, Slc2a4, Nr3c1 and NMDA receptor subunits 1B-3A-3B. In contrast, in the hippocampus of fetuses gestated under constant light (LL), these oscillations were suppressed. In the adult LL offspring (reared in LD during postpartum), we observed complete lack of day/night differences in plasma melatonin and decreased day/night differences in plasma corticosterone. In the adult LL offspring, overall hippocampal day/night difference of gene expression was decreased, which was accompanied by a significant deficit of spatial memory. Notably, maternal melatonin replacement to dams subjected to gestational chronodisruption prevented the effects observed in both, LL fetuses and adult LL offspring. Collectively, the present data point to adverse effects of gestational chronodisruption on long-term cognitive function; raising challenging questions about the consequences of shift work during pregnancy. The present study also supports that developmental plasticity in response to photoperiodic cues may be modulated by maternal melatonin.

In human and sheep newborns, brown adipose tissue (BAT) accrued during fetal development is used for newborn thermogenesis. Here we explored the role of maternal melatonin during gestation on the amount and functionality of BAT in the neonate. We studied BAT from 6 lambs gestated by ewes exposed to constant light from 63% gestation until delivery to suppress melatonin (LL), 6 lambs gestated by ewes exposed to LL but receiving daily oral melatonin (12mg at 1700 hrs, LL+Mel) and another 6 control lambs gestated by ewes maintained in 12h light:12h dark (LD). Lambs were in-strumented at 2 days of age. At 4-6 days of age, they were exposed to 24ºC (thermal neutrality condi-tions) for 1 hr, 4ºC for 1 hr and 24ºC for 1 hr. Afterward, lambs were euthanized and BAT was dissected for mRNA measurement, histology and ex vivo experiments. LL newborns had lower central BAT and skin temperature under thermal neutrality and at 4°C, and higher plasma norepinephrine concentration than LD newborns. In response to 4°C, they had a pronounced decrease in skin temperature and did not increase plasma glycerol. BAT weight in LL newborns was about half of that of LD newborns. Ex vivo, BAT from LL newborns showed increased basal lipolysis and did not respond to NE. In addition, expression of adipogenic/thermogenic genes (UCP1, ADBR3, PPARγ, PPARα, PGC1α, C/EBPβ, and perilipin) and of the clock genes Bmal1, Clock and Per2 was increased. Remarkably, the effects observed in LL newborns were absent in LL+Mel newborns. Thus, our results support that maternal melatonin during gestation is important in determining amount and normal functionality of BAT in the neonate.

Key points: Light at night is essential to a 24/7 society, but it has negative consequences on health. Basically, light at night induces an alteration of our biological clocks, known as chronodisruption, with effects even when this occurs during pregnancy. Here we explored the developmental impact of gestational chronodisruption (chronic photoperiod shift, CPS) on adult and fetal adrenal biorhythms and function. We found that gestational chronodisruption altered fetal and adult adrenal function, at the molecular, morphological and physiological levels. The differences between control and CPS offspring suggest desynchronization of the adrenal circadian clock and steroidogenic pathway, leading to abnormal stress responses and metabolic adaptation, potentially increasing the risk of developing chronic diseases. Light at night is essential to a 24/7 society, but it has negative consequences on health. Basically, light at night induces an alteration of our biological clocks, known as chronodisruption, with effects even when this occurs during pregnancy. Indeed, an abnormal photoperiod during gestation alters fetal development, inducing long‐term effects on the offspring. Accordingly, we carried out a longitudinal study in rats, exploring the impact of gestational chronodisruption on the adrenal biorhythms and function of the offspring. Adult rats (90 days old) gestated under chronic photoperiod shift (CPS) decrease the time spent in the open arm zone of an elevated plus maze to 62% and increase the rearing time to 170%. CPS adults maintained individual daily changes in corticosterone, but their acrophases were distributed from 12.00 h to 06.00 h. CPS offspring maintained clock gene expression and oscillation, nevertheless no daily rhythm was observed in genes involved in the regulation and synthesis of steroids. Consistent with adult adrenal gland being programmed during fetal life, blunted daily rhythms of corticosterone, core clock gene machinery, and steroidogenic genes were observed in CPS fetal adrenal glands. Comparisons of the global transcriptome of CPS versus control fetal adrenal gland revealed that 1078 genes were differentially expressed (641 down‐regulated and 437 up‐regulated). In silico analysis revealed significant changes in Lipid Metabolism, Small Molecule Biochemistry, Cellular Development and the Inflammatory Response pathway (z score: 48–20). Altogether, the present results demonstrate that gestational chronodisruption changed fetal and adult adrenal function. This could translate to long‐term abnormal stress responses and metabolic adaptation, increasing the risk of developing chronic diseases.

Compelling evidence in rats support the idea that gestational chronodisruption induces major changes in maternal circadian rhythms and fetal development and that these changes impact adult life at many physiological levels. Using a model of chronic photoperiod shifting throughout gestation (CPS), in which pregnant female rats (Sprague–Dawley strain; n = 16 per group) were exposed to lighting schedule manipulation every 3–4 days reversing the photoperiod completely or light/dark photoperiod (12/12; LD), we explored in the adult rat male offspring body weight gain, glucose homeostasis, adipose tissue content, adipose tissue response to norepinephrine (NE), and adipose tissue proteomic in the basal condition with standard diet (SD) and in response to high-fat diet (HFD). In adult CPS male (100–200 days old; n = 8 per group), we found increasing body weight, under SD and adiposity. Also, we found an increased response to intraperitoneal glucose (IGTT). After 12 weeks of HFD, white adipose tissue depots in CPS offspring were increased further, and higher IGTT and lower intraperitoneal insulin tolerance response were found, despite the lack of changes in food intake. In in vitro experiments, we observed that adipose tissue (WAT and BAT) glycerol response to NE from CPS offspring was decreased, and it was completely abolished by HFD. At the proteomic level, in CPS adipose tissue, 275 proteins displayed differential expression, compared with LD animals fed with a standard diet. Interestingly, CPS offspring and LD fed with HFD showed 20 proteins in common (2 upregulated and 18 downregulated). Based on these common proteins, the IPA analysis found that two functional pathways were significantly altered by CPS: network 1 (AKT/ERK) and network 2 (TNF/IL4; data are available via ProteomeXchange with identifier PXD026315). The present data show that gestational chronodisruption induced deleterious effects in adipose tissue recruitment and function, supporting the idea that adipose tissue function was programmed in utero by gestational chronodisruption, inducing deficient metabolic responses that persist into adulthood.