Controlling Stimulus-Secretion Coupling in Adrenal Chromaffin Cells: A Novel Role for the Serotonin Transporter?

Abstract

Adrenal chromaffin cells, an important neuroendocrine component of the sympathetic nervous system, maintain homeostatic and acute stress responses through secretion of catecholamines, neuropeptides, and other hormones. Mounting evidence links depression and cardiovascular diseases, and some depressed patients exhibit elevated catecholamine levels, perhaps suggesting common molecular targets or mechanisms. The serotonin transporter (SERT) mediates reuptake of serotonin (5HT) and is an important target for anti-depressants. Notably, SERT is prominently expressed in adrenal chromaffin cells and 5HT co-localizes with epinephrine in chromaffin granules. However, the effects of 5HT/ SERT on chromaffin cell function remain unclear. Our data show that one role of SERT is to accumulate 5HT in chromaffin cells. The 5HT content of whole adrenal glands isolated from SERT knockout mice was reduced by ∼80% compared to wild-type littermates, but catecholamine content (∼1000 fold higher than 5HT) was unaltered. We used carbon fiber amperometry to investigate the regulation of stimulus-secretion coupling by 5HT/ SERT. Cells were isolated from wild-type or SERT knockout mice, secretion evoked by 30mM KCl, and the number of amperometric spikes (vesicular fusion events) over 60s was quantified. Under control conditions (no 5HT) there was no difference in the number of spikes in wild-type Vs SERT knockout cells. However, extracellular 5HT (25nM-1μM) significantly reduced secretion in SERT knockout compared to wild-type cells. Similarly, when SERT was acutely blocked in wild-type cells using escitalopram (1μM), 5HT inhibited secretion. Escitalopram alone (no 5HT) had no effect, while 5HT alone produced a slight increase in secretion, although this was not statistically significant. Patch clamp experiments demonstrated that neither 5HT nor escitalopram altered Ca2+ entry through voltage-gated calcium channels. Ongoing work will dissect the mechanisms underlying this complex regulation of chromaffin cells by 5HT/ SERT.