Ovarian hormones remodel the lattice of dendrites and oxytocinergic innervation of the female rat hypothalamic ventromedial nucleus

Ovarian hormones regulate energy balance, affect, cognition, and mating through their actions to alter the physiology, chemistry, and structure of neurons. However, most of this work has been restricted to estradiol. Estradiol and progesterone modulate some behaviors in distinct manners. The goal of this thesis was to test the hypothesis that estradiol and progesterone differentially affect neural circuitry in the relevant brain regions. Estradiol has a modest effect on sexual receptivity; whereas, progesterone is required for its full expression in the female rat. An important brain region for the control of sexual receptivity is the ventrolateral subdivision of the ventromedial nucleus of the hypothalamus (vlVMH). In Chapter Two, analysis of Golgi impregnated neurons showed that estradiol treatment reduces the length of dendrites in the vlVMH. Within four hours, progesterone reversed this effect. These actions were specific to dendrites that extended in a ventrolateral direction towards the fiber plexus adjacent to the VMH. The neuropeptide oxytocin (OT) is found in the fiber plexus lateral to the VMH and is critical for the full expression of sexual receptivity. Yet, the manner by which OT reaches its receptors in the VMH is unknown. In Chapter Three, immunogold electron microscopy revealed that OT is localized to myelinated axons, dendrites and boutons. The boutons that contain OT also express a marker of excitatory synapses, vesicular glutamate transporter-2. The results of the ultrastructural experiments also corroborated the Golgi impregnation study. That is, estradiol decreased the density of dendrites in the fiber plexus; progesterone reversed this reduction. To ascertain the molecular intermediates OT might employ to regulate sexual behavior, an in vitro study was performed in Chapter Four. The results showed that OT activates mitogen activated protein kinase (MAPK) via the epidermal growth factor receptor (EGFR), a mechanism that may amplify the actions of the neuropeptide. Overall, this thesis reveals plasticity within the lattice of dendrites and oxytocinergic fibers lateral to the VMH. In concert, estradiol and progesterone regulate both the presence of specific dendrites in this lattice, some of which come into synaptic contact with OT