94. The involvement of orexin (hypocretin) in the formalin-induced inflammatory pain response in adulthood following a neonatal bacterial immune challenge

Abstract

Early life physiological stressors have been implicated in the onset of psychopathology in adulthood. In preclinical models, the neonatal bacterial immune challenge, lipopolysaccharide (LPS), has been shown to alter anxiety-like behaviour, neuroendocrine function and behavioural pain responses in adulthood. Interestingly, recent evidence suggests a role for the lateral hypothalamic peptide orexin in stress, arousal and nociceptive processing. How a neonatal LPS exposure affects the reactivity of the orexin system to formalin-induced inflammatory pain in adulthood remains to be determined. Male Wistar rat pups (n = 12) were exposed to either LPS or saline (0.05 mg/kg, IP) on postnatal days (PND) 3 and 5. On PND 82–97, all rats were exposed to a subcutaneous hindpaw injection of 2.25% formalin (50 μL). 1.5 h following behavioural testing, animals were perfused and brains processed for Fos-protein and orexin immunohistochemistry. LPS-treated rats exhibited increased flinching but not licking responses in adulthood compared to saline controls (p