Serotonin and the serotonin transporter in the adrenal gland

The adrenal glands are key components of the mammalian endocrine system, helping maintain physiological homeostasis and the coordinated response to stress. Each adrenal gland has two morphologically and functionally distinct regions, the outer cortex and inner medulla. The cortex is organized into three concentric zones which secrete steroid hormones, including aldosterone and cortisol. Neural crest-derived chromaffin cells in the medulla are innervated by preganglionic sympathetic neurons and secrete catecholamines (epinephrine, norepinephrine) and neuropeptides into the bloodstream, thereby functioning as the neuroendocrine arm of the sympathetic nervous system. In this article we review serotonin (5-HT) and the serotonin transporter (SERT; SLC6A4) in the adrenal gland. In the adrenal cortex, 5-HT, primarily sourced from resident mast cells, acts as a paracrine signal to stimulate aldosterone and cortisol secretion through 5-HT4/5-HT7 receptors. Medullary chromaffin cells contain a small amount of 5-HT due to SERT-mediated uptake and express 5-HT1A receptors which inhibit secretion. The atypical mechanism of the 5-HT1A receptors and interaction with SERT fine tune this autocrine pathway to control stress-evoked catecholamine secretion. Receptor-independent signaling by SERT/intracellular 5-HT modulates the amount and kinetics of transmitter release from single vesicle fusion events. SERT might also influence stress-evoked upregulation of tyrosine hydroxylase transcription. Transient signaling via 5-HT3 receptors during embryonic development can limit the number of chromaffin cells found in the mature adrenal gland. Together, this emerging evidence suggests that the adrenal medulla is a peripheral hub for serotonergic control of the sympathoadrenal stress response.

Chronic restraint stress induces serotonin transporter expression in the rat adrenal glands

Chronic restraint stress (CRS) magnifies restraint-induced corticosterone secretion through a mechanism involving increased adrenocortical 5-HT content and turnover. We analysed the impact of CRS on serotonin transporter (SERT) expression and distribution in rat adrenal glands. Male Wistar rats were submitted to CRS (20 min/day) or undisturbed control conditions for 14 days. Exposure to CRS induced a remarkable increase in SERT-like immunoreactivity in the adrenal cortex, which closely matched that of chromogranin A immunostaining, along with a significant increase in SERT protein and mRNA levels in whole adrenals as determined by immunohistochemistry, Western blot and RT-PCR assays, respectively; all these CRS-induced changes occurred almost exclusively in left adrenals. Closely similar results were obtained in animals that received a 14-day chronic corticosterone treatment. These results unravel an interesting association between chronic stress exposure and SERT expression in adrenocortical chromogranin A-positive cells, which seems to be a glucocorticoid-dependent phenomenon.

Attenuation of stress-induced behavioral changes by activation of serotonin type 7 receptors in the median raphe nucleus of rats

BACKGROUND

Exposure to stressful aversive situations induces physiological and behavioral changes. Serotonin has been suggested to mediate such changes, as well as adaptation to stressful events. Serotoninergic projections arising from the median raphe nucleus to the dorsal hippocampus have been suggested to promote adaptation to chronic aversive stimuli. Such pathway may involve serotonin type 1a receptor-mediated neurotransmission. However, the serotonin 7 receptor can also be found in the median raphe nucleus and may be involved in mechanisms underlying response to stress.

AIMS

In this work we sought to investigate if activation of serotonin type 7 receptors would attenuate stress-induced deficits in different animal models of depression.

METHODS

Male Wistar rats with a guide-cannula aimed to the median raphe nucleus were submitted to restraint or forced swim stress and were tested in an elevated plus maze or forced swim test, respectively, 24 h later. SB 258741 (serotonin type 7 receptor antagonist) and/or LP 44 (serotonin type 7 receptor agonist) were administered intra-median raphe nucleus immediately before or after exposure to stress or before test. Control groups received intra-median raphe nucleus treatment 24 h or immediately before test in the elevated plus maze or forced swim test.

RESULTS

LP 44 attenuated restraint-induced exploratory deficits independently of the moment it was administered. Similar results were observed in the forced swim test, with the exception on post-stress condition. These effects on adaptation to stress induced by serotonin type 7 receptor activation were prevented by previous treatment with SB 258741.

CONCLUSIONS

Our data support the idea that activation of median raphe nucleus serotonin 7 receptor is important to the development of adaptation to stress.