Abstract
GABAergic neurons of the medial septum of the basal forebrain make up a substantial portion of the septo-hippocampal pathway fibers, and are known to modulate hippocampal amino acid neurotransmission and support cognitive function. Importantly, these neurons are also implicated in age-related cognitive decline. Hypothalamic orexin/hypocretin neurons innervate and modulate the activity of these basal forebrain neurons and also provide direct inputs to the hippocampus. However, the precise role of orexin inputs in modulating hippocampal amino acid neurotransmission--as well as how these interactions are altered in aging--has not been defined. Here, orexin A (OxA) was administered to CA1 and the medial septum of young (3-4 months) and aged (27-29 months) Fisher 344 Brown Norway rats, and hippocampal GABA and glutamate efflux was analyzed by in vivo microdialysis. Following CA1 infusion of OxA, extracellular GABA and glutamate efflux was increased, but the magnitude of orexin-mediated efflux was not altered as a function of age. However, medial septum infusion of OxA did not impact hippocampal efflux in young rats, while aged rats exhibited a significant enhancement in GABA and glutamate efflux compared to young counterparts. Furthermore, immunohistochemical characterization of the medial septum revealed a significant decrease in parvalbumin (PV)-positive cell bodies in aged animals, and a significant reduction in orexin fiber innervation to the remaining GABAergic cells within the septum, while orexin innervation to the hippocampus was unaltered by the aging process. These findings indicate that: (1) OxA directly modulates hippocampal amino acid neurotransmission in young animals, (2) Aged animals show enhanced responsivity to exogenous OxA activation of the septo-hippocampal pathway, and (3) Aged animals undergo an intrinsic reduction in medial septum PV-immunoreactivity and a decrease in orexin innervation to remaining septal PV neurons. Alterations in orexin regulation of septo-hippocampal activity may contribute to age-related dysfunctions in arousal, learning, and memory.