Although the higher incidence of stress-related psychiatric disorders in females is well documented, its basis is unknown. Here we demonstrate that the receptor for corticotropin-releasing factor (CRF), the neuropeptide that orchestrates the stress response, signals and is trafficked differently in female rats in a manner that could result in a greater response and decreased adaptation to stressors. Most cellular responses to CRF in the brain are mediated by CRF receptor (CRFr) association with the GTP-binding protein, Gs. Receptor immunoprecipitation studies revealed enhanced CRFr-Gs coupling in cortical tissue of unstressed female rats. Previous stressor exposure abolished this sex difference by increasing CRFr-Gs coupling selectively in males. These molecular results mirrored the effects of sex and stress on sensitivity of locus ceruleus (LC)-norepinephrine neurons to CRF. Differences in CRFr trafficking were also identified that could compromise stress adaptation in females. Specifically, stress-induced CRFr association with β-arrestin2, an integral step in receptor internalization, occurred only in male rats. Immunoelectron microscopy confirmed that stress elicited CRFr internalization in LC neurons of male rats exclusively, consistent with reported electrophysiological evidence for stress-induced desensitization to CRF in males. Together, these studies identified two aspects of CRFr function, increased cellular signaling and compromised internalization, which render CRF-receptive neurons of females more sensitive to low levels of CRF and less adaptable to high levels of CRF. CRFr dysfunction in females may underlie their increased vulnerability to develop stress-related pathology, particularly that related to increased activity of the LC-norepinephrine system, such as depression or post-traumatic stress disorder.

Stress-related psychiatric disorders are more prevalent in females than males, and this has been attributed to differences in stress sensitivity. As activation of the locus coeruleus (LC)-norepinephrine (NE) system is an important component of the stress response, this study compared LC responses to stress in female and male rats under different hormonal conditions in the halothane-anesthetized state. The mean basal LC discharge rate was similar between groups. However, the magnitude of LC activation elicited by hypotensive stress was substantially greater in females, regardless of hormonal status. The difference in stress sensitivity could be attributed to the differential postsynaptic sensitivity of LC neurons to corticotropin-releasing factor (CRF), which mediates LC activation by hypotension. CRF was 10-30 times more potent in activating LC neurons in female vs male rats. Interestingly, previous exposure to swim stress differentially regulated LC responses to CRF by sensitizing LC neurons of male, but not female, rats to CRF. The net effect of this was to abolish sex differences in LC sensitivity. Finally, CRF receptor (CRF-R) protein levels in the LC were greater in ovarectomized female vs male rats. This is the first study to demonstrate sex differences in the stress responsiveness of the brain noradrenergic system. Substantial sex differences were apparent in postsynaptic sensitivity to CRF and stress-induced regulation of postsynaptic sensitivity to CRF. These sex differences in the CRF regulation of the LC-NE system translate to a differential response to stress and may play a role in the increased vulnerability of females to stress-related psychiatric disorders.

The Wistar Kyoto (WKY) rat strain is a putative genetic model of comorbid depression and anxiety. Previous research showing increased kappa opioid receptor (KOR) gene expression in the brains of WKY rats, combined with studies implicating the KOR in animal models of depression and anxiety, suggest that alterations in the KOR system could play a role in the WKY behavioral phenotype. Here, the effects of KOR antagonists in the forced swim test (FST) were compared in the WKY and the Sprague Dawley (SD) rat strains. As previously reported, WKY rats displayed more immobility behavior than SD rats. The KOR antagonists selectively produced antidepressant-like effects in the WKY rats. By contrast, the antidepressant desipramine reduced immobility in both strains. Brain regions potentially underlying the strain-specific effects of KOR antagonists in the FST were identified using c-fos expression as a marker of neuronal activity. The KOR antagonist nor-binaltorphimine produced differential effects on the number of c-fos positive profiles in the piriform cortex and nucleus accumbens shell between SD and WKY rats. The piriform cortex and nucleus accumbens also contained higher levels of KOR protein and dynorphin A peptide, respectively, in the WKY strain. In addition, local administration of nor-BNI directly into the piriform cortex produced antidepressant-like effects in WKY rats further implicating this region in the antidepressant-like response to KOR antagonists. These results support the use of the WKY rat as a model of affective disorders potentially involving KOR overactivity and provide more evidence that KOR antagonists could potentially be utilized as novel antidepressants.

Chronic opiate use produces persistent changes in brain neurons that are expressed as adverse effects, including physical dependence and compulsive drug-seeking behavior. Dysregulation of the hypothalamic-pituitary-adrenal response to stress also occurs with chronic opiate administration and has been implicated as a contributing factor to continued substance abuse. This study provides the first evidence for dysregulation of the central noradrenergic response to stress by chronic opiates. Chronic morphine selectively sensitized locus ceruleus (LC)-norepinephrine (NE) neurons to corticotropin-releasing factor (CRF), an integral mediator of the stress response. CRF doses that were inactive in vehicle-treated rats produced a near-maximal activation of LC neurons of rats chronically administered morphine. LC sensitization to CRF was not solely a pharmacological phenomenon but was expressed as hyperresponsivity to physiological stress. Finally, opiate-induced LC sensitization translated to a change in the behavioral repertoire in response to environmental stress (swim stress) such that NE-mediated hyperactive behaviors predominated. The opiate-induced sensitization of the central NE response to stress predicts that chronic opiate administration increases vulnerability to certain stress-related symptoms (e.g., hyperarousal, attentional dysfunction), and this may contribute to the maintenance of opiate-seeking behavior.

This schematic illustrates sex differences in corticotropin-releasing factor receptor (CRFr) function. CRFr coupling to the GTP-binding protein, G s is greater in the female compared to the male neuron, an effect that can result in increased cellular signaling in response to stress in females. In the male neuron (right) stress induces CRFr association with b-arrestin2 and subsequent receptor internalization into endosomes (yellow). Internalized receptors can be trafficked back the membrane or degraded in multivesicular bodies (shown in purple). Stress-induced association of CRFr with b-arrestin2 and internalization is compromised in females, suggesting a decreased ability to adapt to high levels of CRF, as are hypothesized to be secreted in depression. For more information on this topic, please refer to the article by Bangasser et al. on pages 896-904.

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Depression, a common stress‐related mental illness, affects twice as many women as men. Dysfunctions in corticotropin‐releasing factor (CRF) have been linked to depression, but sex differences in the CRF system are not well characterized. Previous studies from our laboratory demonstrated that the postsynaptic sensitivity of locus coeruleus (LC) neurons to CRF was greater in female vs. male rats. Moreover, prior stress sensitized LC neurons of male but not female rats. These findings suggested the existence of sex differences in CRF receptor (CRFr) coupling and regulation. To test this, the CRFr was immunoprecipitated from cortical samples of stressed (15min swim) or handled male and female rats. Under unstressed conditions, the CRFr is more highly coupled to Gs in females vs. males. Stress increases coupling of CRFr to Gs in male but not female rats. Additionally, we found that following stress, CRFr association with βarrestin2— which targets the CRFr for desensitization and internalization—was increased in male but not female rats. Thus, sex differences in CRFr coupling and/or desensitization may contribute to differential neuronal sensitivity of females to CRF and stress. This may translate to increased vulnerability of females to depression. Finally, sex differences in CRFr should be considered in the development of CRF antagonists as therapeutic agents. Supported by PHS Grants MH 40008 and MH 63267 to RJV.