Cliff H. Summers

Social stress, beneficial for evaluating threats and resource opportunities, influences the development of anxious and depressive disorders. Stress responses are initiated via a pro-stress circuit in the anterior region of the basolateral amygdala (aBLA). This circuit contains pyramidal glutamatergic neurons that express genetic markers Camk2α, Rspo2 , and Hcrtr1 , which enhance stress-vulnerable behaviors via Orx ₁ R activity, and are inhibited directly by Orx ₂ R activation on Gad1, Hcrtr2, Cck GABAergic neurons. Our results have suggested stress circuitry in BLA is modulated by Orx ₁ R and Orx ₂ R activity, which counterbalance stress responsivity via Rspo2 -positive glutamatergic neurons in aBLA, but whether these effects are consistent with systemic delivery is unknown. These receptor-dependent biases contribute to behavioral, physiological and molecular outcomes associated with psychiatric disorders, such as anxiety, depression, and post-traumatic stress disorder. Administration of a selective orexin receptor cross-over (SORCO; Orx ₁ R antagonist and Orx ₂ R agonist combination) treatment alleviates anxiogenic behavioral outputs in socially stressed mice, by decreasing escape latency in stress-vulnerable (Stay) mice (phenotypic reversal; vulnerable to resilient), and reducing the time spent freezing in response to the presence of a social aggressor (socially induced) in stress resilient (Escape) mice. Behavioral results coincided with a decrease in Rspo2 and an increase in Hcrtr2 gene expression in aBLA of SORCO-treated Stay mice, and an increase in Akt3 and Mtor transcription in Stay mice treated with an Orx ₂ R agonist (YNT-185). Our findings indicate that a combination of OrxR drugs (SORCO) provides potentially therapeutic outcomes through modifications of stress neurocircuitry, triggered by increasing expression of genes associated with neuroplasticity.

Background: Comprehending sex differences in stress vulnerability and the associated neurocircuitry has been elusive until the development of a method for investigating social defeat in female mice. Methods: The Stress Alternatives Model (SAM) uses conditioning-induced agonistic behavior from novel male aggressors to separate distinct phenotypes characterized by stress-resilient active avoidance (Escape) or vulnerability (no Escape = Stay). Results: Unlike males, females predominantly display Escape behavior, which is reversible (to Stay) by adding more social stress to behavioral environments. Despite this, females exhibit both stress-resilient and vulnerable phenotypic segregation. Stress-vulnerable females exhibit double conditioning-induced corticosterone secretion, more contextual fear conditioning, and reduced social preference. Systemic and intra-BLA injections of Orx ₂ R or α ₂ antagonists resulted in slower escape (Escape ^S ), increased social avoidance, as well as increased cued and contextual freezing in stress-vulnerable females. Neurons in BLA express Orx ₂ R mRNA ( Hcrtr2) predominantly in cholecystokinin-positive GABA neurons. In slower escape (Escape ^S ) females expression of Hcrtr2 and Adra2a in BLA is elevated. Conclusions: Female and male mice exposed to social stress exhibit distinct behavioral adaptations, but similarly, separate into resilient and susceptible subpopulations. Inhibiting Orx ₂ R promotes stress-vulnerable behavior in females, and modifies transcription in BLA microcircuits, suggesting a role for Orx ₂ R defining stress behavior.

Psychological distress, including anxiety or mood disorders, emanates from the onset of chronic/unpredictable stressful events. Symptoms in the form of maladaptive behaviors are learned and difficult to treat. While the origin of stress-induced disorders seems to be where learning and stress intersect, this relationship and molecular pathways involved remain largely unresolved. The hippocampus, studied for its role in learning, is divided into regions that designate the passage of neuronal signaling during memory formation, including dentate gyrus (DG), CA₃, CA₂, and CA₁. Inputs into these hippocampal subregions, like those from hypothalamic orexinergic neurons, may modify learning outcomes. We have previously shown the orexin system to balance stress states, where receptor subtypes prompt opposing actions on behavior. Here, we explore the connection between hippocampal orexin receptors and learning during stress. In a social stress/learning paradigm separating mice into stress resilient and vulnerable populations, hippocampal Orx₁R and Orx₂R transcription is regulated in a phenotype-dependent fashion. We further identified Orx₁R as highly expressed in the hilus of DG, while Orx₂R is abundant in CA₂. Finally, we designed an experiment where mice were provided prior exposure to a stressful environment, which ultimately modified behavior, as well as transcription of hippocampal orexin receptors.

Corticotropin-releasing factor (CRF) is essential for coordinating endocrine and neural responses to stress, frequently facilitated by vasopressin (AVP). Previous work has linked CRF hypersecretion, binding site changes, and dysfunctional serotonergic transmission with anxiety and affective disorders, including clinical depression. Crucially, CRF can alter serotonergic activity. In the dorsal raphé nucleus and serotonin (5-HT) terminal regions, CRF effects can be stimulatory or inhibitory, depending on the dose, site, and receptor type activated. Prior stress alters CRF neurotransmission and CRF-mediated behaviors. Lateral, medial, and ventral subdivisions of the central nucleus of the amygdala (CeA) produce CRF and coordinate stress responsiveness. The purpose of these experiments was to determine the effect of intracerebroventricular (icv) administration of CRF and AVP on extracellular 5-HT as an index of 5-HT release in the CeA, usingin vivomicrodialysis in freely moving rats and high performance liquid chromatography (HPLC) analysis. We also examined the effect of prior stress (1 h restraint, 24 h prior) on CRF- and AVP-mediated release of 5-HT within the CeA. Our results show that icv CRF infusion in unstressed animals had no effect on 5-HT release in the CeA. Conversely, in rats with prior stress, CRF caused a profound dose-dependent decrease in 5-HT release within the CeA. This effect was long-lasting (240 min) and was mimicked by CRF plus AVP infusion without stress. Thus, prior stress and AVP functionally alter CRF-mediated neurotransmission and sensitize CRF-induced inhibition of 5-HT release, suggesting that this is a potential mechanism underlying stress-induced affective reactivity in humans.