Acute oral dexamethasone administration reduces levels of orphan GPCR glucocorticoid-induced receptor (GIR) mRNA in rodent brain: potential role in HPA-axis function.

Glucocorticoid-induced receptor (GIR) is an orphan G-protein-coupled receptor (GPCR) with predominant expression in brain and thymus. More specifically, high levels of GIR expression have been described in brain regions of mouse, rat and human including limbic forebrain and hypothalamic regions, suggesting a role for GIR in memory, cognition, stress, reward or the control of emotion. Previous in vitro studies performed in murine thymocytes demonstrated an induction of GIR following dexamethasone treatment, suggesting the potential in vivo regulation of GIR by glucocorticoids. Glucocorticoids have been implicated in a number of disorders. In this study we employed in situ hybridisation with semi-quantitative image analysis to assess the level of GIR expression in mouse brain following acute dexamethasone administration. GIR was highly expressed in the nucleus accumbens, striatum, olfactory tubercle and nuclei of the hypothalamus. Three hours following acute dexamethasone treatment (0.05 mg/kg, p.o.), levels of GIR mRNA were found to be significantly decreased in striatum (25%, P<0.05), nucleus accumbens (19%, P<0.05), olfactory tubercle (19%, P<0.05) and CA2 sub-region of the hippocampus (30%, P<0.05) compared with vehicle. Significant decreases in GIR expression were also observed in hypothalamic nuclei including the dorsomedial (48%, P<0.05) and ventrolateral (58%, P<0.05) part of the ventromedial hypothalamic nuclei, dorsomedial hypothalamic nuclei (39%, P<0.01) and arcuate nucleus (54%, P<0.05), compared with vehicle. These data demonstrate the in vivo regulation of GIR in response to glucocorticoids and suggest a potential role for GIR in mediating the response to altered levels of glucocorticoids in disease states.