Differential targeting of brain stress circuits with a selective glucocorticoid receptor modulator

Abstract

Glucocorticoid receptor (GR) antagonism may be of considerable therapeutic value in stress-related psychopathology such as depression. However, blockade of all GR-dependent processes in the brain will lead to unnecessary and even counteractive effects, such as elevated endogenous cortisol levels. Selective GR modulators are ligands that can act both as agonist and as antagonist and may be used to separate beneficial from harmful treatment effects. We have discovered that the high-affinity GR ligand C108297 is a selective modulator in the rat brain. We first demonstrate that C108297 induces a unique interaction profile between GR and its downstream effector molecules, the nuclear receptor coregulators, compared with the full agonist dexamethasone and the antagonist RU486 (mifepristone). C108297 displays partial agonistic activity for the suppression of hypothalamic corticotropin-releasing hormone (CRH) gene expression and potently enhances GR-dependent memory consolidation of training on an inhibitory avoidance task. In contrast, it lacks agonistic effects on the expression of CRH in the central amygdala and antagonizes GR-mediated reduction in hippocampal neurogenesis after chronic corticosterone exposure. Importantly, the compound does not lead to disinhibition of the hypothalamus-pituitary-adrenal axis. Thus, C108297 represents a class of ligands that has the potential to more selectively abrogate pathogenic GR-dependent processes in the brain, while retaining beneficial aspects of GR signaling.

Keywords: HPA axis; NCoA1; neuroendocrinology; steroid pharmacology; transcription regulation.

Fig. 1.

C108297 behaves like a selective modulator in vitro and in vivo. (A) Ligand-induced interactions between the GR-LBD and coregulator motifs. DEX induced many interactions compared with DMSO. RU486 induced modest interactions with corepressor motifs (black arrow: NCoR1). C108297 showed an intermediate profile. GR-LBD interactions with the central motifs from SRC-1 were much weaker or absent (boxed), but others were retained (white arrow indicates SRC-1 motif IV). (B) Hippocampal Drd1a mRNA was regulated by corticosterone after vehicle but not C108298 treatment. (C) BDNF mRNA was down-regulated by both corticosterone and C108297. Asterisks indicate significant differences from the control group (*P < 0.05; **P < 0.01).

Fig. 2.

SCR-1 splice variant 1E is selectively recruited by GR-C108297. (A) Protein structure of SRC-1 harboring three NR central boxes (roman numerals). SRC-1A harbors a repressor function (RF) and the additional NR-box IV. Protein fragments marked by dotted lines refer to C. (B) MARCoNI quantification showed that unlike DEX, C108297 induced interactions only between GR and NR-box IV. (C) In a two-hybrid assay only DEX induced interaction with the SRC-1 fragment common to both splice variants. The SRC-1A–specific protein fragment was also recruited by GR-C108297. A fragment of corepressor NCoR1 only interacted after incubation with RU486. Asterisks indicate significant difference from the control condition (P < 0.001).

Fig. 3.

Selective GR modulation in the stress system: C108297-agonism in ADX rats after subchronic treatment compared with the prototypic agonist DEX. (A) In the PVN, where SRC-1A is expressed at high levels, DEX led to strong down-regulation of CRH mRNA (P < 0.001). C108297 had a modest agonist effect that reached significance in the stressed group (P < 0.05). (B) In the CeA, DEX up-regulated CRH mRNA in nonstressed rats (P < 0.05), but C108297 was without effect. (C) The acute response of the Crh gene in response to restraint stress was strongly attenuated both by pretreatment with DEX and C108297. (D) DEX led to a complete blockade of the HPA axis (P < 0.001), whereas C108297 leads to a very weak attenuation of the adrenocortical stress response (P < 0.05).

Fig. 4.

Selective GR modulation in the stress system: antagonism in adrenally intact rats after subchronic treatment compared with the prototypic antagonist RU486. (A) The acute c-fos response to stress in the PVN was enhanced both by pretreatment with RU486 and C108297. (B) RU486 treatment led to increased circadian peak levels of plasma corticosterone. C108297 does not have this effect. *P < 0.05; **P < 0.01.

Fig. 5.

C108297 acts as GR antagonist in neurogenesis and as agonist in memory retention. (A) Chronic corticosterone suppressed the number of BrdU-positive cell, and 4 d of C108297 treatment increased this number. BrdU scores were significantly higher in animals that received C108297 in combination with chronic corticosterone, compared with corticosterone-treated animals that did not receive C108297. (B) Total DCX-positive cells were significantly fewer after 3 wk of corticosterone treatment but not in animals that also received C108297. (C) Acute posttraining C108297 (20 mg/kg) or corticosterone (1 mg/kg) led to long 48-h retention test latencies in the inhibitory avoidance task, and these effects were blocked by pretreatment with RU486. Significant differences: *P < 0.05; **P < 0.01; ***P < 0.001.