Mechanisms governing dendritic release of oxytocin in the CNS

About This Grant

PROJECT SUMMARY Oxytocin (OT) is a nine amino acid peptide synthesized almost exclusively in hypothalamic neurons of the supraoptic and paraventricular nuclei (SON, PVN). Despite the highly localized nature of OT synthesizing neurons, OT receptors (OTRs) are distributed widely throughout the CNS. Significant evidence indicates an important modulatory role for central oxytocinergic signaling in a wide variety of processes including fear conditioning, stress responding, anxiety-related behaviors, maternal behavior, sexual behavior, and social recognition. Oxytocinergic magnocellular neurons (OT-MCNs) in both the SON and PVN have a fascinating ability to release peptide not only from their axon terminals, but also from their dendrites. It is clear that dendritic release of OT creates a local paracrine signal within the hypothalamus and there is substantial reason to expect that OT released in this manner enters the ventricular system and ultimately fuels endogenous activation of OTRs throughout many cortical and limbic areas via volume transmission. Understanding how dendritic release is regulated is important because current efforts to therapeutically target central OTRs are hampered by low permeability of exogenous OTR agonists to the blood brain barrier, and by limited ability to deliver such agonists in a way that mimics natural concentration, kinetic, or contextual profiles. Thus, a primary goal of this project is to further advance our understanding of the mechanisms governing dendritic release of endogenous OT from the dendrites of OT-MCNs. Existing evidence suggests that there exists a complex relationship between cellular activity (action potential firing initiated in the soma), and calcium (Ca2+)- dependent dendritic release of OT into the CNS. Recently published work from our group indicates that previously unidentified dendritic osmosensors in OT-MCNs modulate dendritic conductivity, and in so doing, play an important role in regulating the relationship between somatic firing and dendritic Ca2+ influx. Aim 1 of the current proposal will build on these findings by evaluating the ability of other endogenously available signaling molecules to modulate activity-induced Ca2+ influx in OT-MCN dendrites by either increasing or decreasing dendritic conductivity, and will further determine which signaling molecules can directly modulate dendritic Ca2+ homeostasis. In Aim 2 we will map expression and explore function of extrasynaptic NMDA receptors on OT-MCN dendrites, testing the core hypothesis that activation of these receptors by ambient glutamate enhances dendritic Ca2+ influx produced by back propagating action potentials. In Aim 3 we will use novel sniffer cell technology developed by our group, as well as a new oxytocin biosensor, to directly evaluate the relationship between dendritic Ca2+ influx and dendritic release of OT. Successful completion of this work will substantially advance our understanding of the mechanisms governing dendritic release of oxytocin into the CNS, and may ultimately provide information necessary to advance new therapeutic approaches for modulation of central OTRs that seek to modulate endogenous release mechanisms, rather than to bring exogenous agonists.