Corticotropin releasing factor (CRF) receptor signaling in the central nervous system: new molecular targets

Abstract

Corticotropin-releasing factor (CRF) and the related urocortin peptides mediate behavioral, cognitive, autonomic, neuroendocrine and immunologic responses to aversive stimuli by activating CRF(1) or CRF(2) receptors in the central nervous system and anterior pituitary. Markers of hyperactive central CRF systems, including CRF hypersecretion and abnormal hypothalamic-pituitary-adrenal axis functioning, have been identified in subpopulations of patients with anxiety, stress and depressive disorders. Because CRF receptors are rapidly desensitized in the presence of high agonist concentrations, CRF hypersecretion alone may be insufficient to account for the enhanced CRF neurotransmission observed in these patients. Concomitant dysregulation of mechanisms stringently controlling magnitude and duration of CRF receptor signaling also may contribute to this phenomenon. While it is well established that the CRF(1) receptor mediates many anxiety- and depression-like behaviors as well as HPA axis stress responses, CRF(2) receptor functions are not well understood at present. One hypothesis holds that CRF(1) receptor activation initiates fear and anxiety-like responses, while CRF(2) receptor activation re-establishes homeostasis by counteracting the aversive effects of CRF(1) receptor signaling. An alternative hypothesis posits that CRF(1) and CRF(2) receptors contribute to opposite defensive modes, with CRF(1) receptors mediating active defensive responses triggered by escapable stressors, and CRF(2) receptors mediating anxiety- and depression-like responses induced by inescapable, uncontrollable stressors. CRF(1) receptor antagonists are being developed as novel treatments for affective and stress disorders. If it is confirmed that the CRF(2) receptor contributes importantly to anxiety and depression, the development of small molecule CRF(2) receptor antagonists would be therapeutically useful.

Fig. 1. Diagram of the human CRF₁ and CRF₂ receptors

Depiction of the full-length, wild-type sequence indicates important extracellular amino acids comprising the binding pocket and ligand selectivity domain for both CRF receptors. Serines and threonines (red circles) located in CRF receptor intracellular loops and C terminus represent potential sites for phosphorylation by GRK and PKC isoforms.

Fig. 2. Major intracellular pathways for signal transduction by CRF₁ and CRF₂ receptors

Recent evidence indicates that CRF₁ receptors are regulated by GRK- and PKC-mediated phosphorylation and by interaction with β-arrestins [,–,–235].

Fig. 3

Agonist-stimulated signaling of human CRF₁ and CRF₂ receptors via intracellular cyclic AMP accumulation and transient calcium mobilization. Concentration-response curves were generated for cyclic AMP accumulation (two upper panels) and calcium Ca²⁺ mobilization (two lower panels) stimulated by incubating HEK293 cells stably expressing human CRF₁ and CRF_2(a) receptors with various agonists (0–10 μM) for 10 min at 37^oC. The data points represent mean ± SEM for triplicate determinations of cyclic AMP (pmol/well) or Ca²⁺ mobilization (relative fluorescence units, RFU).

Fig. 4. Comparison of effects on acoustic startle plasticity resulting from manipulations of CRF₁ and CRF₂ receptor signaling

Top Panel: CRF₁ and CRF₂ receptors exhibit opposing actions on prepulse inhibition (PPI) of startle. Bar graph data represent difference scores (%PPI-%PPI of respective vehicle group) after the following in vivo manipulations: (a) intracerebroventricular (ICV) injection of agonist alone, h/rCRF (0.2 nmol); (b) h/rCRF (0.2 nmol ICV) + CRF₁ receptor antagonist NBI-30775 (20 mg/kg intraperitoneal, IP); (c) h/rCRF (0.2 nmol ICV) in CRF₁ receptor knock-out mouse; (d) h/rCRF (0.2 nmol ICV) + CRF₂ receptor antagonist antisauvagine-30 (3 nmol ICV); (e) CRF₂ receptor-selective agonist, urocortin 2 (2 nmol ICV); and CRF₂ receptor-selective agonist, urocortin 3 (2.4 nmol ICV). Bottom Panel: CRF₁ and CRF₂ receptor signaling may exhibit an additive action on acoustic startle magnitude. Bar graph data represents percentage change from respective vehicle startle magnitude [(startle magnitude - startle magnitude of vehicle)/startle magnitude of vehicle) X 100]. The startle data summarized in this graph is described in our two recent publications [254,309].