Pre- and postsynaptic pathways underlying the stress response in the adrenal medulla

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ABSTRACT The “fight-or-flight” response refers to the state of heightened physiological and mental arousal triggered by a physical threat, emotionally charged event, or metabolic disturbance. Although the precise reaction to each stressor may vary, all share some basic characteristics of sympathetic nervous system activation. Adrenomedullary chromaffin cells are core effectors of the sympathetic response in the periphery. When activated during stress, they discharge a cocktail of hormones into the suprarenal vein for circulation throughout the body. These hormones modulate cardiac, pulmonary, and metabolic functions in ways that favor survival or maintain internal conditions when they are threatened. Secretion from the adrenal medulla is dependent on input from preganglionic splanchnic fibers, which release acetylcholine (ACh) and pituitary adenylate cyclase-activating polypeptide (PACAP) onto chromaffin cells. What remains poorly understood is how ACh and PACAP tune hormone release to accommodate the range of splanchnic firing frequencies associated with variations in sympathetic tone. Three Specific Aims are proposed to fill this gap in our understanding of the stress response pathway. The Aims will test the overall hypothesis that variations in presynaptic splanchnic input are translated by different receptor-coupled pathways in chromaffin cells to dynamically regulate the amount of hormone output. Aim 1 builds on recent preliminary data that shows, for the first time, a role for any synaptotagmin (Syt) at the splanchnic-chromaffin cell synapse. Planned studies, using in situ slice electrophysiology, will define the role of Ca2+ sensing at this synapse, and evaluate the idea that synaptic facilitation, regulated by Syt7, amplifies hormone discharge from chromaffin cells when splanchnic fibers discharge at high frequencies. Aim 2 is motivated by preliminary data that shows Phospholipase C-epsilon (PLC) is required for transducing PACAP stimulation into Ca2+ signals in chromaffin cells that cause exocytosis. PACAP is thought to underlie chromaffin cell secretion during stress. This has prompted us to posit that with increased splanchnic firing frequencies that produce facilitation, PLC activity in chromaffin cells must be “turned on” to sustain increases in hormone output. Aim 3 builds on data which shows that PACAP stimulates exocytosis in chromaffin cells, but does so while paradoxically restricting the release of peptide hormones packaged within a chromaffin granule. Relevant for the differential release of biogenic amines and hormone peptides, we will characterize the properties of PACAP-stimulated fusion pores and investigate mechanisms by which they are constrained. We expect these studies will provide a coherent molecular and physiological framework for understanding how presynaptic activity, postsynaptic receptor- coupled signaling pathways, and exocytosis are mechanistically linked to regulate the stress response.