Pre and post-synaptic pathways underlying the stress response in the adrenal medulla

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.

Grant Number: 5R01NS122534-10
NIH Institute/Center: NIH
Principal Investigator: Arun Anantharam