Pregnancy-induced adaptations of the central circadian clock and maternal glucocorticoids

Abstract

Maternal physiological adaptations, such as changes to the hypothalamic-pituitary-adrenal (HPA) axis, are central to pregnancy success. Circadian variation of the HPA axis is dependent on clock gene rhythms in the hypothalamus, but it is not known whether pregnancy-induced changes in maternal glucocorticoid levels are mediated via this central clock. We hypothesized that hypothalamic expression of clock genes changes across mouse pregnancy and this is linked to altered HPA activity. The anterior hypothalamus and maternal plasma were collected from C57Bl/6J mice prior to pregnancy and on days 6, 10, 14 and 18 of gestation (termZd19), across a 24-h period (0800, 1200, 1600, 2000, 0000, 0400 h). Hypothalamic expression of clock genes and Crh was determined by qPCR, plasma ACTH concentration measured by Milliplex assay and plasma corticosterone concentration by LC-MS/MS. Expression of all clock genes varied markedly across gestation, most notably at mid-gestation when levels of each gene were elevated. The pregnancy-induced increase in maternal corticosterone levels (by up to 14-fold on day 14) was not accompanied by a parallel shift in plasma ACTH (28% lower on day 14 compared with non-pregnant levels). Moreover, while circadian rhythmicity in corticosterone was maintained up to day 14 of gestation, this was effectively lost by day 18. Overall, our data show that the central circadian clock undergoes marked adaptations throughout mouse pregnancy, changes that are likely to contribute to maternal physiological adaptations. Importantly, however, neither hypothalamic clock genes nor plasma ACTH levels appear to drive the marked increase in maternal corticosterone after mid-gestation.

Diurnal regulation of hypothalamic kisspeptin is disrupted during mouse pregnancy

“…Diurnal rhythmicity in the SCN (as part of the anterior hypothalamus) during pregnancy remains intact, even though the expression of core clock genes (including Bmal1 and Rev-erbα) is altered during pregnancy compared with diestrus (Wharfe et al 2016). The persistence of these rhythms throughout pregnancy suggests that the master pacemaker in the SCN is functioning normally, and thus any changes in the circadian input to the AVPV are likely to occur downstream of the SCN.…”

“…Because diurnal rhythmicity of all the core clock genes in the SCN appears to be intact during pregnancy (Wharfe et al 2016), one possibility is that there is an alteration in the AVP neuronal connections between the SCN and the AVPV, leading to a loss in circadian information. Alternatively, it could be that other hormone(s) that experience an upsurge during pregnancy are overriding the estrogenic and circadian signals that give rise to the increase in kisspeptin in the AVPV.…”

“…7). Bmal1 and Reverbα and other central circadian clock genes also exhibited robust diurnal rhythms during pregnancy, which appeared unchanged despite an apparent upward shift in overall expression from mid-to-late gestation (Wharfe et al 2016).…”

“…The days of pregnancy chosen represent developmental milestones: day 6 is postimplantation, day 10 is the embryonic period, day 14 represents early fetal life, and day 18 late fetal life. We have previously shown that six time points (4 h: intervals) are adequate for the determination of circadian regulation of genes (Wharfe et al 2016). All procedures involving animals were conducted with the approval of the Animal Ethics Committee of the University of Western Australia, Perth, WA, Australia.…”

“…These clock genes are also expressed in numerous peripheral tissues (Boden et al 2010). We have recently shown that the expression of core clock genes in the SCN changes significantly across gestation in the mouse (Wharfe et al 2016), and Wharfe and coworkers have also previously demonstrated that clock gene rhythms in the rat liver are altered by pregnancy (Wharfe et al 2011). Additionally, despite the high estradiol levels during pregnancy, the HPG axis is dormant and ovulation does not occur, suggesting the shutdown of kisspeptin signaling, reduced GnRH neuron or gonadotrope sensitivity, or a combination of each.…”