Stress Neurobiology and Corticotropin-Releasing Factor

In response to stress, this neural circuit becomes activated, thereby releasing CRF from median eminence nerve terminals into the hypothalamohypophyseal portal system, where it activates CRF receptors on corticotrophs in the anterior pituitary to promote the synthesis of pro-opiomelanocortin and the release of its major posttranslation products, ACTH and .-endorphin. ACTH, released from the anterior pituitary, stimulates the production and release of cortisol from the adrenal cortex. These same hypothalamic CRF neurons also project to the spinal cord¹⁶ and brainstem nuclei,¹⁷ including the locus caeruleus (LC), the major noradrenergic nucleus in the brain.¹⁸

Shortly after the isolation and characterization of CRF, a standardized intravenous CRF stimulation test was developed to assess HPA axis activity. In this paradigm, CRF is administered intravenously (usually at a dose of 1 µg/kg or a fixed dose of 100 µg) and the ACTH and cortisol responses are measured at 30-minute intervals over a 2- to 3-hour period.¹⁹ Numerous studies have now documented a blunted ACTH and ß-endorphin response to exogenously administered ovine CRF or human CRF in depressed patients compared with nondepressed persons; the cortisol response in depressed patients and nondepressed control subjects did not consistently differ.^20-24

It has been hypothesized that the attenuated ACTH response to CRF is due to chronic hypersecretion of CRF from nerve terminals in the median eminence,which results in down-regulation of CRF receptors in the anterior pituitary, and/or to chronic hypercortisolemia and its associated negative feedback. CRF receptor down-regulation results in a reduced responsivity of the anterior pituitary to CRF, as repeatedly demonstrated in laboratory animals.^25-29
Two CRF receptor subtypes, CRF₁ and CRF₂, with distinct anatomic localization and receptor pharmacology, have been identified in rats and humans.^11,14,15 Both receptors are G-protein coupled receptors (GPCRs) and are positively coupled to adenylyl cyclase via the protein G_s. The CRF₁ receptor is predominantly expressed in the pituitary, cerebellum, and neocortex in the rat.³⁰ Considerable evidence from laboratory animal studies has shown that CRF₁ receptors may specifically mediate some of the anxiogenic-like behaviors observed after administration of CRF.^31-34

In agreement with these findings, mice with targeted knockouts of the CRF₁ receptor were found to have an impaired stress response.³⁵ The CRF1 receptor knockout mice were less anxious than their wild-type litter mates when tested in the elevated plus maze, a paradigm commonly used to assess anxiety-like behavior. In addition, data in these transgenic mice showed a significant reduction in stressinduced release of ACTH and corticosterone.

CRF₂ receptor knockout mice have also been generated.^36,37 Deletion of the CRF₂ receptor gene during development provided an ambiguous profile, showing increased anxiety in some but not all anxiety tasks^36,37: in males, but not females³⁷; in males and females³⁶; or not at all.³⁸ Thus these studies suggest CRF₂ receptor blockade may lead to states of increased anxiety, although it is likely that both the environment and the genetic background on which the knockouts were bred significantly contribute to the behavioral phenotype of these animals.

Research using selective CRF₂ receptor agonists and antagonists has been even more inconsistent. Several studies have used the selective CRF₂ receptor antagonist antisauvagine-30 (ASV-30),³⁹ which has been reported to be between 100- and 1000- fold selective for the CRF₂ receptor, depending on whether the radiolabeled ligand is sauvagine³⁹ or ASV-^30,40 respectively. Intraseptal administration of ASV-30 was shown to reduce anxious behavior induced by immobilization stress in the plus maze task or by previous association with foot shock in mice.⁴¹ These behavioral data were corroborated in rats, where intracerebroventricular ASV-30 reduced anxious behavior in the plus maze, defensive withdrawal, and a conditioned anxiety paradigm.^32,42

Selective agonists at the CRF₂ receptor have also been discovered. The peptides urocortin II and urocortin III are structurally and ancestrally related to CRF but show between 100- and 1000-fold selectivity at the CRF₂ receptor versus the CRF₁ receptor.^43,44 Urocortin III has been shown to mildly suppress locomotion and has an anxiolytic-like profile in mice.⁴⁵ However, another study from the same group demonstrated that urocortin II was inactive in the mice in the plus maze after acute administration but increased their exploratory behavior in the plus maze 4 hours later. Thus, compounds reported to be both selective agonists and antagonists at the CRF₂ receptor have shown anxiolytic-like effects, making the exact role of this receptor in modulating stress-induced behaviors ambiguous.

Extrahypothalamic CRF circuits and depression

Although initially investigated for its role as one of the key modulators of the HPA axis, further research has revealed that CRF controls not only the neuroendocrine but also the autonomic, immune, and behavioral responses to stress in mammals. Results from both clinical studies and a rich body of literature conducted primarily in rodents and lower primates has highlighted the importance of extrahypothalamic CRF neurons.^12,46,47 In rodents, primates, and humans, CRF and its receptors have been heterogeneously localized in a variety of regions, including the amygdala, thalamus, hippocampus, and prefrontal cortex, among others.^48-51 These brain regions are important in regulating many aspects of the mammalian stress response and affect.