Ovarian Hormone Modulation of Learning Strategy Preferences: A Role for Hippocampal Disinhibition
McElroy, Molly
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Permalink
https://hdl.handle.net/2142/82514
Description
Title
Ovarian Hormone Modulation of Learning Strategy Preferences: A Role for Hippocampal Disinhibition
Author(s)
McElroy, Molly
Issue Date
2007
Doctoral Committee Chair(s)
Korol, Donna L.
Department of Study
Neuroscience
Discipline
Neuroscience
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biology, Neuroscience
Language
eng
Abstract
High levels of circulating ovarian hormones are related to use of the hippocampus-sensitive place learning strategy whereas low hormone levels are related to use of the striatum-sensitive response strategy. Other studies demonstrate that estrogen suppresses hippocampal GABAergic function and that progesterone's agonistic actions at GABAA receptors may enhance hippocampal inhibition. It is possible that the balance between estrogen-induced disinhibition and progesterone-induced inhibition contribute to the estrous cycle-related strategy differences. In this work, hippocampal GABA was pharmacologically suppressed and enhanced to evaluate the contribution of inhibition to strategy preferences across the estrous cycle. Intrahippocampal infusions of the GABA A antagonist bicuculline failed to shift strategy use from response to place in estrous rats (Chapter 2), while intrahippocampal infusions of the GABAA agonist muscimol caused proestrous rats to shift from a place to a response strategy preference (Chapter 3). To evaluate the independent role of estrogen in the muscimol-induced response strategy preference at proestrus, strategy preferences in ovariectomized rats treated with oil, estrogen or estrogen+progesterone were measured. Estrogen treatment induced a place strategy bias, which was reversed by progesterone (Chapter 4, Experiment 1). Furthermore, muscimol treatment produced shifts to response in rats treated with estradiol only, suggesting the presence of progesterone is not necessary to observe muscimol's effects (Chapter 4, Experiment 2). These experiments provide evidence for the emerging theory that ovarian hormones work, at least in part, through hippocampal activity to modulate learning strategies. Further, these studies use a unique approach to the study of cognitive neural systems, a field dominated by studies using pharmacological manipulations in male rats. In contrast, these studies demonstrate how ovarian hormones change the balance of neural systems such that changes in learning strategies are observed.
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