Wylie Vale and colleagues first isolated CRF nearly 30 years after Selye suspected the existence of a molecule in the brain responsible for pituitary corticotropin release (Vale et al., 1981). Subsequently, investigations have established the intimate role of CRF and its family of related molecules in stress processing, and strongly implicated it in mood disorder-related neuropathology. The CRF system consists of four ligands, CRF and three peptides belonging to the urocortin family (Ucn1, Ucn2, and Ucn3), and two CRF receptor subtypes (CRF1 and CRF2). CRF and the urocortin family of ligands bind with varying affinities to CRF receptors. CRF binds CRF1 with high affinity, while the urocortins bind to CRF2 with high affinity and CRF1 with low affinity (Heinrichs et al., 1997; Reyes et al., 2001). Distribution patterns of CRF receptors in the brain are discrete and mostly non-overlapping. CRF1 is largely expressed in sensory processing and motor control areas such as the neocortex, cerebellum, and thalamus, as well as in subcortical regions including the amygdala, hippocampus, and anterior pituitary (De Souza et al., 1985; Reul and Holsboer, 2002). CRF2 is predominantly found in subcortical and limbic structures such as the lateral septum, hippocampus, amygdala, BNST, and hypothalamus (Chalmers et al., 1995; Refojo and Holsboer, 2009; Reul and Holsboer, 2002).
CRF is a particularly effective stress-signaling molecule due to its ability to function as a neurohormone in the HPA axis, as well as a neurotransmitter, allowing widespread and
anatomically diverse stress effects throughout the brain. Beyond that, CRF coordinates autonomic and behavioral aspects of the stress response (Owens and Nemeroff, 1991; Raux- Demay and Girard, 1985; Reul and Holsboer, 2002; Vazquez et al., 2003). The PVN and CeA are the major sites of CRF synthesis in the brain. Furthermore, CRF neurotransmission, unlike that of most metabotropic neurotransmitters, activates receptors in a paracrine fashion,
simultaneously activating many neurons, potentially promoting widespread responses to stressors that are particularly intense or traumatic.
Central administration of CRF in rats results in the stimulation of hypothalamic and limbic brain regions that influence the HPA axis, activates the autonomic nervous system, increases stress-related behaviors such as anxiety, fear, despair, and emotionality, and decreases food intake, sexual activity, exploration, and social interaction (Campbell et al., 2004; Dunn and Swiergiel, 2008; Imaki et al., 2001; Rivest et al., 1989; Sutton et al., 1982). Specifically, the activation of CRF1 receptors by CRF reliably activates the HPA axis and elevates circulating stress hormone levels (Arborelius et al., 1999; Heinrichs and Koob, 2004). CRF1 knockout mice exhibit reduced stress-induced ACTH and corticosterone release and adrenal medulla atrophy (Timpl et al., 1998). CRF2 knockout mice display hypersensitive corticosterone responses that are slower to recover from acute stress (Bale and Vale, 2003). Once thought to work in
opposition to CRF1 receptors by decreasing HPA axis responses to stress, CRF2 receptors have since been observed to increase, decrease, and not affect HPA axis responsivity based on evidence from three separate lines of CRF2 knockout mice (Bale et al., 2000; Contarino et al., 2000; Coste et al., 2000; Jamieson et al., 2006). Mice lacking both CRF1 and CRF2 receptors exhibit severe HPA axis dysfunction consisting of elevated CRF levels in the PVN, blunted basal ACTH and corticosterone levels, and atrophy of the adrenal cortex (Bale et al., 2002). A similar
phenotype is observed in CRF1 deficiency alone, indicating that CRF2 cannot overcome the loss of CRF1 (Preil et al., 2001).
1.4.1 Sex differences in the CRF system
Some stress-sensitive neural targets of CRF receptor signaling are sexually dimorphic, making them candidates for mediating sex-specific stress effects in the brain. In rats, females typically express higher CRF levels than males in regions including the PVN, CeA, and the BNST (Iwasaki-Sekino et al., 2009; Sterrenburg et al., 2012; Victor et al., 2005). Recently, CRF receptor signaling was found to be sexually dimorphic at the sub-cellular level. In rat cortical neurons, CRF1 receptors more readily couple to stimulatory G proteins to increase cellular activity more in females than males (Bangasser et al., 2010; Bangasser and Valentino, 2012). Because CRF1 receptors stimulate the HPA axis, greater female CRF1 receptor activation could underlie dimorphic HPA axis sensitivity, rendering females more susceptible to stress-related psychopathologies. In stressed adult males rats, but not females, cortical CRF1 receptors interact
with βarrestin2, which facilitates receptor internalization and inhibits G protein-coupled second messenger activity. This suggests a mechanism by which stress induces CRF1 receptor
desensitization more in males than females, rendering them less sensitive to CRF and able to recover faster from stress. The potential contribution of molecular level sex differences in CRF receptor function to mood disorder development have not been identified.
Additionally, chronic mild variable stress has been shown to induce sex-specific methylation of the CRF gene promoter, decreasing methylation in the male and female BNST, while increasing methylation in the PVN of females only (Sterrenburg et al., 2012). This finding may reflect a priming effect in females whereby experiencing one stressor alters the ability to mount a subsequent stress response. Urocortin 2 (Ucn 2) selectively binds and activates CRF2
and is expressed among stress-sensitive brain regions including the PVN, supraoptic and arcuate nuclei of the hypothalamus, and locus ceruleus (Reyes et al., 2001). Interestingly, Ucn 2 null mice sex-specifically express stress-related phenotypes such that females, but not males, exhibit elevated basal ACTH and corticosterone levels and reduced depression-like behavior (Chen et al., 2006). Furthermore, although both male and female CRF2-lacking mice display increased depression-like behavior that decreases following treatment with the CRF1 antagonist
antalarmin, and this decrease lasts significantly longer in females (Bale and Vale, 2003). Sexually dimorphic components of the CRF system may be neural substrates involved in sex- specific expression of mood disorder susceptibility.
1.4.2 Human studies: mood disorders are associated with CRF system abnormalities
Stress-related mood disorders are often associated with abnormalities in the HPA axis response to stress (Ehlert et al., 2001; Heim et al., 2000; Heim et al., 2002; Holsboer, 2001; Yehuda, 2001). Most commonly, mood disorder patients express CRF overactivity, attenuated negative feedback, and either hyper or hypocortisolism (Carroll et al., 2007; Gold and Chrousos, 2002; Parker et al., 2003; Paslakis et al., 2011). Initial experimental findings identified a subset of depressed patients displaying elevated cerebrospinal CRF levels (Nemeroff et al., 1984), an observation also reported for combat veterans suffering from PTSD (Bremner et al., 1997). More recent postmortem findings have reported elevated CRF levels in the brains of depressed
patients, as well as in suicide victims (Merali et al., 2004). Similarly, disrupted CRF1 receptor expression may be associated with psychopathology. The same postmortem study reporting elevated CRF levels in the brains of suicide victims found a reduction in CRFR1 transcripts (Merali et al., 2004), a finding replicated in additional studies (de Kloet et al., 2005b; Regev and Baram, 2014).