PACAP regulates immediate catecholamine release from adrenal chromaffin cells in an activity‐dependent manner through a protein kinase C‐dependent pathway

Abstract

Adrenal medullary chromaffin cells are a major peripheral output of the sympathetic nervous system. Catecholamine release from these cells is driven by synaptic excitation from the innervating splanchnic nerve. Acetylcholine has long been shown to be the primary transmitter at the splanchnic‐chromaffin synapse, acting through ionotropic nicotinic acetylcholine receptors to elicit action potential‐dependent secretion from the chromaffin cells. This cholinergic stimulation has been shown to desensitize under sustained stimulation, yet catecholamine release persists under this same condition. Recent evidence supports synaptic chromaffin cell stimulation through alternate transmitters. One candidate is pituitary adenylate cyclase activating peptide (PACAP), a peptide transmitter present in the adrenal medulla shown to have an excitatory effect on chromaffin cell secretion. In this study we utilize native neuronal stimulation of adrenal chromaffin cells in situ and amperometric catecholamine detection to demonstrate that PACAP specifically elicits catecholamine release under elevated splanchnic firing. Further data reveal that the immediate PACAP‐evoked stimulation involves a phospholipase C and protein kinase C‐dependant pathway to facilitate calcium influx through a Ni2+ and mibefradil‐sensitive calcium conductance that results in catecholamine release. These data demonstrate that PACAP acts as a primary secretagogue at the sympatho‐adrenal synapse under the stress response.

Pituitary Adenylate Cyclase-activating Peptide (PACAP) Recruits Low Voltage-activated T-type Calcium Influx under Acute Sympathetic Stimulation in Mouse Adrenal Chromaffin Cells

“…Thus, a non-cholinergic splanchnic-adrenal transmitter was predicted to elicit chromaffin cell catecholamine release under elevated stimulation (14). Previous work identified the peptidergic transmitter pituitary adenylate cyclase-activating peptide (PACAP) 2 as a likely candidate (15)(16)(17) to substitute for cholinergic adrenal stimulation under the sympathoadrenal stress response.…”

“…Previous studies have shown that acute PACAP stimulation elicits a voltage-dependent influx of external calcium that is independent of the major high voltage-activated Ca 2ϩ channels (15,44). Moreover, PACAP stimulation leads to an up-regulation of protein kinase C activity (16,45,46). PKC has been shown to positively modulate T-type channel conductance in other cell types (47).…”

“…Because PACAP does not cause action potential firing, calcium entry through a high voltage-activated calcium channel seems unlikely during PACAP signaling. Furthermore, studies have shown that PACAP-evoked secretion is blocked by micromolar concentrations of nickel chloride as well as mibefradil, implicating the possible role of low voltageactivated T-type calcium channels in acute sympathetic stress (16).…”

“…The predominant PACAP target receptor in the adrenal medulla is the high affinity PAC 1 -R, a G s -linked receptor that activates adenylate cyclase to increase cyclic adenosine monophosphate (cAMP) (24,25). Recent studies have shown that PACAP stimulation activates the stimulatory exchange protein activated by cAMP (Epac) pathway through the elevation in cAMP (26,27), triggering phospholipase C and a protein kinase C (PKC)-dependent subthreshold depolarization that is necessary for catecholamine release from chromaffin cells (16). PACAP-mediated exocytosis is independent of sodium-based action potentials (28) or nicotinic acetylcholine receptor function (29,30), thus bypassing key sites of nicotinic secretory desensitization.…”

“…PACAP stimulation leads to an array of systemic functions, including regulation of cell differentiation (18,19), cell division (20), and transcription of stress-associated genes (21). As a neurotransmitter, PACAP is present in the splanchnic nerve terminal and released preferentially under elevated sym-pathetic firing to elicit adrenal medullary catecholamine release (16,22,23). The predominant PACAP target receptor in the adrenal medulla is the high affinity PAC 1 -R, a G s -linked receptor that activates adenylate cyclase to increase cyclic adenosine monophosphate (cAMP) (24,25).…”