Date of Award

Summer 2021

Document Type

Open Access Dissertation

Department

Biomedical Science

First Advisor

Jim Fadel

Abstract

The basal forebrain (BF) is comprised of several nuclei including the substantia innominata, medial septum, nucleus basalis and diagonal band of broca which are involved in cognitive functions including attention, motivation, and arousal. BF neurons are particularly vulnerable to dysfunction and degeneration in aged humans and, more dramatically, in diseases such as Alzheimer's disease (AD). Age-related BF dysfunction may reflect diminished afferent regulation as well as an altered local glial environment. Our lab has previously shown reduced orexin/hypocretin innervation of BF in aging, a phenomenon that may link afferent dysfunction with altered microglial homeostasis. There is little research examining these relationships involving afferent neuronal and glial cell populations in the BF as it relates to aging. The purpose of this study is to compare specific neuronal and glial populations to identify anatomical factors susceptible to agerelated homeostatic dysfunction in the BF.

Several lines of evidence demonstrate the responsiveness of the BF to homeostatic stimuli including food related stimuli. Using a food-paired stimulus to examine effects of aging on physiologically-relevant afferent stimulation of this area, we deposited the retrograde neuronal tracer, cholera toxin B (CTb) in BF of aged (26-28 months) and young (2-3 months) F344/Brown Norway F1 hybrid rats and trained them for 7 days. We then combined neural tract-tracing with functional and phenotypic markers of activation to elucidate neural circuits that may underlie age-related loss of activation of BF neurons using immunohistochemistry.

Previous research in our lab has shown that a homeostatically- relevant circuits involving the orexin neuropeptide, show age-related degredation which may be linked to BF dysregulation. Additionally, orexin loss is linked to conditions such as narcolepsy, anorexia nervosa, age-related cognitive decline, and age-related neurodegenerative diseases, such as AD. To show that loss of orexin afferents affects inflammation in the BF via microglial dysregulation, we administered a miRNA-expressing lentivirus designed to knock down orexin expression (LV-PPOX) in the BF in young rats. We then analyzed phenotypic changes in microglia using immunohistochemistry and ELISA against a panel of pro- and anti-inflammatory cytokines. Changes in morphological and cytokine correlates of microglial activation following orexin loss can be seen in animals administered LV-PPOX. Together, these studies compare specific neuronal and glial populations of young and aged rats to identify anatomical factors susceptible to agerelated dysfunction.

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