Endocrine Abstracts

Endogenous glucocorticoids are secreted in hourly pulses, establishing a characteristic ultradian rhythm. Accordingly, the glucocorticoid receptor (GR) is activated in distinct pulses, causing target-gene pulsing. In contrast, constant glucocorticoid treatment causes aberrant target-gene overexpression in cell culture models, however this has not yet been described in vivo. Therefore, here we assess the effect of altering the glucocorticoid ultradian pattern on transcriptional output in the rat hippocampus.

Male Sprague–Dawley rats were adrenalectomised and replaced with i.v. infusion of pulsatile or constant glucocorticoid at either physiological levels or double concentration to produce large-mass pulses. For the low physiological level infusions, we found differential pattern-dependent regulation of hippocampal glucocorticoid-targets including Per1, KLF15, and SGK1. When the pulse mass was increased, the output of these three targets was not different between constant and pulsatile patterns. Therefore the large-mass pulses caused a similar aberrant transcriptional output of hippocampal target genes to the highly abnormal constant infusion.

Chromatin immunoprecipitation assays for GR, phospho-Ser5 Pol2 and phospho-Ser2 Pol2 during the peak and trough of the pulsatile glucocorticoid infusion indicated that the large-mass pulses resulted in a significant disruption of the normal ultradian rhythm at the DNA template. Notably, we detected a significantly raised nadir-phase binding for GR at regulatory regions and phospho-Ser2 Pol2 within the gene body. Therefore, GR-dependent transcriptional cyclical activity was dysregulated by the large-mass pulses.

Our findings with the larger amplitude pulses may be of clinical relevance as large-mass pulses have been reported in patients with obstructive sleep apnoea. These patients also present with metabolic, cognitive, and affective dysfunction. After treatment, their glucocorticoid profiles normalise, along with an improvement in their clinical profile. Our data suggests that large-mass pulses disrupt the timing of ultradian GR-dependent transcriptional rhythms in the hippocampus, and therefore may contribute to the associated cognitive and affective dysfunction in these patients.