Knowledge of mechanisms involved in vulnerability/resilience to stress disorders is crucial for prevention and treatment schemes. We previously documented that insulin-like growth factor I (IGF-I) is associated to vulnerability to stress both in mice and humans. Since hypothalamic orexin neurons express IGF-I receptors and are involved in responses to stress, we analyzed their role in the modulatory actions of IGF-I on stress. Anxiolytic actions of IGF-I after exposure to a predator were absent in mice lacking IGF-I receptors in orexin neurons (Firoc mice). Based on these observations we speculated that Firoc mice may be prone to develop fear-related disturbances, including post-traumatic stress disorder (PTSD)-like symptoms when confronted to fear learning, a process that is postulated to be altered in PTSD. Firoc mice submitted to fear conditioning showed increased freezing responses, suggesting aberrant fear learning. Exaggerated freezing was accompanied by increased levels of orexin, together with enhanced c-fos staining of these neurons –an indicator of increased cell activity, and of noradrenergic neurons of the locus coeruleus nucleus, a region downstream of orexinergic activation. After fear conditioning, Firoc mice developed PTSD-like behavioral traits such as prolonged context-dependent fear and post-stress anhedonia. Since abnormal fear learning was ameliorated by chemogenetic (DREADD) inhibition of orexin neurons, reduced IGF-I input to orexin neurons in Firoc mice seems to enhance their excitability to fear-related inputs. Collectively, these results suggest that IGF-I input to orexin neurons is an important determinant of vulnerability to stress disorders, which provides additional targets for therapy of these high social impact conditions.
