Neurochemical and Behavioral Outcomes of Intranasal Orexin Administration in Young and Aged Animals

Cognitive function represents the most important determinative factor for independent functioning in the elderly. As the existing pharmacotherapeutic tactics for treating Alzheimer’s disease (AD) provide only modest benefits, novel treatment options are urgently needed. The hypothalamic orexin (hypocretin) system, a central integrator of physiological function, is anatomically and functionally positioned to modulate cognition. Indeed, mounting evidence suggests that a loss of orexin neurons and/or their peptides and receptors may underlie the cognitive decline observed during aging and AD. The current paucity of orexin receptor agonists has hindered the ability to study their potential as cognitive enhancers. Intranasal administration of orexin peptides circumvents these issues and others involving peptide delivery into the CNS. To investigate the overarching hypothesis that intranasal orexin administration improves the anatomical, neurochemical, and behavioral substrates of age-related cognitive dysfunction, the proceeding studies utilized a rodent model of aging in combination with acute intranasal administration of saline, orexin-A or the modified orexin-B analog [Ala11, D-Leu15]- orexin-B. These results highlight that intranasal OxA (5nmol) increases c-Fos expression, a marker for neuronal activation, in various telencephalic brain regions, including various subdivisions of the frontal cortex and the basal forebrain cholinergic system. Importantly, these brain regions are known to modulate attention and learning and memory in both young and aged subjects. Conversely, administration of intranasal [Ala11, D-Leu15]- orexin-B (5nmol), an orexin-2 receptor (Ox2R) selective peptide, elicited increases in c-Fos expression that were far less reaching, suggesting that the orexin-1 receptor may provide a more important mechanistic role in this context. To ensure that intranasal orexin administration was affecting neurotransmission in-vivo, a separate cohort of animals were utilized for a prefrontal cortex (PFC) in-vivo microdialysis study, where counterbalanced animals received both intranasal saline and intranasal orexin-A over two separate sessions. Acetylcholine and glutamate in PFC dialysates were measured by high- performance liquid chromatography. These results show that intranasal orexin-A increases PFC acetylcholine and glutamate efflux, two putative neurochemical correlates of attentional function, in young and aged rats. Importantly, deficits in in-vivo cholinergic and glutamatergic neurotransmission play an essential role in the attentional dysfunction observed during aging. Therefore, to test whether intranasal orexin-A administration improves attentional capacity, a separate cohort of animals were trained and tested in an attentional set-shifting (ASST) behavioral paradigm. These results show that vehicle- treated aged animals are impaired in the extradimensional shift (EDS) stage compared to vehicle-treated young animals, highlighting that aging impairs PFC-mediated attentional function. Importantly, intranasal orexin-A in aged animals abolished the age-related EDS impairment relative to young control animals treated with saline. These results indicate that intranasal orexin-A may restore age-related PFC dysfunction to levels on par with their cognitively intact younger counterparts. Together, these studies highlight the putative mechanisms that underlie cognitive dysfunction during aging and provide insights into the possible therapeutic benefit of intranasal orexin-A in treating age-related cognitive dysfunction