A 29-year-old male combat veteran presents with hypervigilance, exaggerated startle response, intrusive memories of combat, emotional numbing, and chronic insomnia for the past 14 months following deployment. Laboratory studies reveal low baseline cortisol levels with an enhanced negative feedback response on the dexamethasone suppression test. His PMHNP is reviewing the neuroendocrine profile characteristic of PTSD. Which of the following best explains the HPA axis findings in this patient?