Effect of the Glucocorticoid Receptor Antagonist Org 34850 on Basal and Stress‐Induced Corticosterone Secretion

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Abstract
The activity of the hypothalamic‐pituitary‐adrenal (HPA) axis is characterised both by an ultradian pulsatile pattern of glucocorticoid secretion and an endogenous diurnal rhythm. Glucocorticoid feedback plays a major role in regulating HPA axis activity and this mechanism occurs via two different receptors: mineralocorticoid (MR) and glucocorticoid receptors (GR). In the present study, the effects of both acute and subchronic treatment with the GR antagonist Org 34850 on basal and stress‐induced HPA axis activity in male rats were evaluated. To investigate the effect of Org 34850 on basal diurnal corticosterone rhythm over the 24‐h cycle, an automated blood sampling system collected samples every 10 min. Acute injection of Org 34850 (10 mg/kg, s.c.) did not affect basal or stress‐induced corticosterone secretion, but was able to antagonise the inhibitory effect of the glucocorticoid agonist methylprednisolone on stress‐induced corticosterone secretion. However, 5 days of treatment with Org 34850 (10 mg/kg, s.c., two times a day), compared to rats treated with vehicle (5% mulgofen in 0.9% saline, 1 ml/kg, s.c.), increased corticosterone secretion over the 24‐h cycle and resulted in changes in the pulsatile pattern of hormone release, but had no significant effect on adrenocorticotrophic hormone secretion or on stress‐induced corticosterone secretion. Subchronic treatment with Org 34850 did not alter GR mRNA expression in the hippocampus, paraventricular nucleus of the hypothalamus or anterior‐pituitary, or MR mRNA expression in the hippocampus. Our data suggest that a prolonged blockade of GRs is required to increase basal HPA axis activity. The changes observed here with ORG 34850 are consistent with inhibition of GR‐mediated negative feedback of the HPA axis. In light of the evidence showing an involvement of dysfunctional HPA axis in the pathophysiology of depression, Org 34850 could be a potential treatment for mood disorders.

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