ESPE Abstracts

Introduction: The circadian rhythmicity of the hypothalamic–pituitary–adrenal (HPA) axis depends on the synchronization of the clock molecular systems in the suprachiasmatic nucleus and in the adrenals. When and how this process occurs in the adrenal is unknown.

Objective: To assess the ontogeny of daily variation of the expression of the adrenal clock genes (Clock, Arntl, Per1, Per2, Per3, Cry1, Cry2, Rora, and Nr1d1), steroidogenesis-related genes (Star and Mc2r), and plasma corticosterone (B).

Material and methods: Male Wistar rats were kept under a 12 h light:12 h darkness cycle (lights on at 0700 h, Zeitgeber time, ZT0). Plasma and adrenal tissue samples obtained every 4 h over a 24 h period on postnatal days P3, P6, P14, P16, P21, and P24 were used for plasma B measurement (RIA) and mRNA expression by qPCR. The results were analysed using the Kruskal–Wallis test.

Results: From P14 until P24 there was a progressive nocturnal increase of B concentrations, with peak at ZT20 and nadir at ZT0 (P<0.01), characterising the establishment of the circadian rhythm of the HPA. There was a daily variation in the mRNA expression of Clock, Arntl, Per2, Per3, Cry1, Nr1d1, and Star since P3 (P<0.05), with attenuation between nadir and peak at P6 and reversal of these parameters from P14, reaching adult patterns at P24. Synchronisation between the expression of the clock genes and adrenal steroidogenesis was observed from P14 and thereafter when the mRNA expression pattern of Per2, Per3, and Cry1 genes became concordant with that of the Star gene.

Conclusions: In the adrenal, there is a gradual synchronization of the molecular mechanisms modulating the ontogeny of the HPA axis circadian rhythm. From P14, the expression of the adrenal clock genes and the genes involved in adrenal steroidogenesis are synchronised resulting in the appearance of the circadian rhythm of the HPA axis.

Funding: Supported by FAPESP (Säo Paulo State Research Agency) grant number 2012/22164-2.