Disruption of estrogen regulation of orexin neurons: a novel mechanism in the pathophysiology of respiratory manifestations of panic disorder in females

Abstract

Panic disorder (PD) is an anxiety disorder that affects ~5% of the population. A significant group of PD patients are characterised by excessive behavioral and physiological responses to CO2 inhalation. The prevalence of PD is 2 to 3 times more frequent in women yet, the origins of this sexual dimorphism are unknown; the fact that excessive responsiveness to CO2 inhalation peaks during the pre‐menstrual phase suggest a role of ovarian hormones. Early life stress is a significant risk factor for PD and there is growing evidence indicating that stress‐related increase in the function of orexin‐producing neurons (ORX) is key to PD. Based on data suggesting that 17b‐estradiol (E2) inhibits ORX, we tested the hypothesis that insufficient E2‐regulation of ORX neurons contributes to stress‐related respiratory manifestations of PD in females. Experiments were performed on adult female rats previously subjected to neonatal maternal separation (NMS; 3h/day postnatal days 3 to 12) or raised under standard conditions. In this model, NMS females show an excessive hyperventilatory response to CO2 by comparison with controls; this effect is greatest during the proestrus phase. Inactivation of ORX1 receptors by SB 33486 (15 mg/kg; ip) prevents this effect. Whole cell recordings (voltage clamp) were performed on GFP‐labeled ORX neurons by AAV microinjection. Excitatory post‐synaptic currents (EPSCs) converging onto ORX neurons were recorded under basal conditions and following E2 bath application (100nM; 10 min). Monitoring of the estrus cycle showed that in control females, EPSC frequency was inversely proportional to circulating E2 levels associated with each phase. This relationship was not observed in NMS females; EPSC frequency of NMS females was greater than controls during the proestrus phase. E2 application reduced EPSC frequency (but not amplitude); however, this effect was not affected by stress or the estrus cycle phase. We conclude that NMS disrupts E2‐mediated inhibition of ORX neurons. This is a novel mechanism in the pathophysiology of PD.Support or Funding InformationSupported by an operating grant from the Canadian Institutes of Health Research