The study of the neurobiological mechanisms of anxiety has been facilitated by the discovery of specific antianxiety compounds and animal models of anxiety-related behavior that are sensitive to the effects of these drugs. In the last 5 years, a large body of evidence has demonstrated that 3alpha-hydroxy pregnane steroids potentiate the inhibitory actions of gamma-aminobutyric acid (GABA) at the GABA/benzodiazepine (BDZ) receptor complex (GABAA), a substrate thought to be a major neural mediator of stress responses and anxiolytic BDZ effects. One of the most potent GABAergic pregnane steroid metabolites, 3alpha-hydroxy5alpha-pregnan-20-one (allopregnanolone), is formed in the ovaries, adrenal glands, and brain by the bioconversion of progesterone. To date, few studies have addressed the role of neuroactive steroid metabolites on behavior. An area of inquiry that has been neglected is the putative effect that fluctuating levels of ovarian steroids have on the GABAA receptor. The goals of this research project are to delineate whether functional indices of the GABAA receptor are affected by the changing hormonal milieu that characterizes altered reproductive states. We will study female rats In different phases of the estrous cycle, ovariectomized rats with and without ovarian hormone replacement treatment, and aged females in reproductive senescence. Spontaneous and drug-induced changes in ambulation in an automated activity chamber, and behavior in 2 animal models of anxiety, the elevated plusmaze and the defensive burying paradigm will be evaluated. Thus, the anxiolytic and sedative effects of diazepam (DZ), a prototypical BDZ, and pregnanolone, a structurally-related isomer of allopregnanolone will be determined. Following the behavioral assays, neurochemical correlates of function of the GABAA receptor will be evaluated in the following: chloride ion (Cl-) enhancement of 3H-flunitrazepam binding (3H-FLU), pregnanolone enhancement of 3H-FLU binding, GABA-stimulated Cl- influx, DZ facilitation of GABA-mediated Cl-influx, and pregnanolone facilitation of GABA-stimulated Cl-influx. Cortex, hippocampus, and cerebellum, regions thought to be involved in mediating anxiolytic and sedative effects of BDZ and pragnanolone, will be chosen for neurochemical assay. Our findings indicate that neuroendocrine conversions of progesterone result in psychoactive pregnane metabolites that influence GABAergic neurotransmission. It is therefore hypothesized that ovarian status, which characterizes different reproductive states, gives rise to fluctuations in brain levels of progesterone and reduced metabolites that alter function of the GABAA receptor. Furthermore, we anticipate that behavioral manifestations of these hormonal and neurochemical changes in animal models of anxiety will be observed. Data generated by the proposed experiments may provide insight into potential neuroendocrine mechanisms of catamenial epilepsy and psychiatric conditions of premenstrual syndrome and postpartum depression.
