Endocrine Profile and Neuroendocrine Challenge Tests in Transgenic Mice Expressing Antisense RNA against the Glucocorticoid Receptor

Abstract

A transgene expressing antisense RNA complementary to a fragment of the glucocorticoid receptor cDNA was incorporated into the mouse genome and resulted in a transgenic animal that has decreased glucocorticoid receptor function. The transgenic mice showed basal plasma ACTH and corticosterone levels similar to those of the normal control animals. We have further investigated changes in HPA axis regulation by use of different neuroendocrine challenge tests including a dexamethasone suppression test (DST). In comparison to normal mice, a tenfold higher dose of dexamethasone (i.e. 20 µg/100 g body weight) was required to suppress the basal corticosterone levels of transgenic mice. Dexamethasone (2 µg/100 g body weight) produced a long-lasting suppression of plasma ACTH and corticosterone levels in control mice, whereas in transgenic animals only a short-lasting decrease in ACTH levels was apparent. Corticotropin-releasing hormone (CRH) administration resulted in an enhanced response in plasma ACTH levels in transgenic mice, whereas the corticosterone response was markedly reduced. The discrepancy between ACTH and corresponding corticosterone secretions in transgenic mice could be attributed, in part, to a reduced sensitivity of the adrenal gland to stimulation by ACTH. Pituitaries of transgenic mice contained about 50% less proopiomelanocortin (POMC) mRNA than those of control animals. No significant differences were noted in the ACTH or protein contents of normal and transgenic mice pituitary glands although a slight increase in protein content of the transgenic mouse adrenal gland was apparent. In conclusion, transgenic mice with impaired GR function show major disturbances in HPA axis regulation which seem to be caused by the primary defect in conjunction with secondary modifications in, amongst others, pituitary CRH receptor system(s), sympathetic output and adrenal development. This mouse is therefore a useful model to study the consequences of life-long defective GR function and HPA axis regulation in general.

Stress and the Aging Hippocampus

“…Although the DST is most likely working at the pituitary level (99), the underlying disregulation is undoubtedly of CNS origin and reflects increased drive upon the CRH and AVP systems of the hypothalamus (45) and constitutes a form of endogenously driven stress. A lessening of the adrenal steroid feedback effects on the hippocampus might be a contributing factor to elevated HPA activity in depression, and recent studies of Barden and co-workers with a transgenic mouse strain (7,56) have suggested that decreased forebrain Type II receptor expression might be a contributing factor to depression and a potential target of antidepressant therapy (56,102). It is not clear, however, how much hippocampal Type II receptor expression, as opposed to receptor expression in other brain regions, is a key factor.…”

Mechanisms of action of antidepressants: from neurotransmitter systems to signaling pathways

“…Mice with decreased glucocorticoid receptor gene expression (by transgenic expression of antisense mRNA directed against the receptor) exhibit reduced hypothalamic CRH expression [20], enhanced stress-associated ACTH response [21][22][23], and impaired efficiency of glucocorticoid-mediated negative feedback [21,[23][24][25]. These mice exhibit deficits in spatial learning, enhanced responses to novelty and decreased locomotion in familiar environments [6].…”

Altered Regulation of CREB by Chronic Antidepressant Administration in the Brain of Transgenic Mice with Impaired Glucocorticoid Receptor Function

“…These transgenic mice displayed impaired endogenous GR function and were characterized by dysfunctional glucocorticoid inhibitory feedback and disturbed ACTH and corticosterone secretory responses Karanth et al 1997;Dijkstra et al 1998). Long-term treatment of the transgenic mice with antidepressant drugs caused an increase in GR gene expression and produced significant normalization of the HPA system hyperactivity through restoration of functional glucocorticoid inhibitory feedback (Pepin et al 1992b;Barden et al 1995;Barden et al 1997).…”

“…These transgenic mice displayed impaired endogenous GR function and were characterized by dysfunctional glucocorticoid inhibitory feedback and disturbed ACTH and corticosterone secretory responses Karanth et al 1997;Dijkstra et al 1998). Long-term treatment of the transgenic mice with antidepressant drugs caused an increase in GR gene expression and produced significant normalization of the HPA system hyperactivity through restoration of functional glucocorticoid inhibitory feedback (Pepin et al 1992b;Barden et al 1995;Barden et al 1997).Because the second messenger cyclic AMP plays a central role in signaling within the HPA axis, long-term adaptations in cAMP regulated gene expression are critical to chronic HPA dysfunction (Hatalski and Baram 1997). Given that depressed patients display reduced cAMP protein kinase activity (Shelton et al 1996) and that the cAMP/PKA cascade is involved in mediating the action of antidepressant drugs, GR dysfunction and disruption of proper cAMP/PKA signaling both observed in depression are highly interdependent.…”