Toll-Like Receptor 4 Activation Reduces Adrenal Chromaffin Cell Excitability Through a Nuclear Factor-κB-Dependent Pathway

Abstract

The adrenal medulla contains fenestrated capillaries that allow catecholamines and neuropeptides secreted by adrenal chromaffin cells (ACCs) to readily access the circulation. These capillaries may also allow bacterial products to enter the adrenal medulla and interact with ACCs during infection. One potential mediator of this interaction is toll-like receptor 4 (TLR-4), a pattern-recognition receptor that detects lipopolysaccharide (LPS) from Gram-negative bacteria. Evidence suggests that excitable cells can express TLR-4 and that LPS can modulate important neuronal and endocrine functions. The present study was therefore performed to test the hypothesis that TLR-4 activation by LPS affects ACC excitability and secretory output. RT-PCR revealed that TLR-4, cluster of differentiation 14, myeloid differentiation protein-2, and myeloid-derived factor 88 are expressed within mouse adrenal medullae. TLR-4 immunoreactivity was observed within all tyrosine hydroxylase immunoreactive ACCs. Incubation of isolated ACCs in LPS dose dependently hyperpolarized the resting membrane potential and enhanced large conductance (BK) Ca(2+)-activated K(+) currents. LPS (10 μg/ml) also increased rheobase, decreased the number of action potentials fired at rheobase, and reduced the percentage of ACCs exhibiting spontaneous and anodal break action potentials. Although catecholamine release was unaltered, LPS significantly reduced high-K(+)-stimulated neuropeptide Y release from isolated ACCs. LPS did not alter the excitability of ACCs from TLR-4(-/-) mice. Inhibition of nuclear factor-κB signaling with SC-514 (20 μm) abolished the effects of LPS on ACC excitability. Our findings suggest that LPS acts at TLR-4 to reduce ACC excitability and neuropeptide Y release through an nuclear factor-κB-dependent pathway.

Endotoxemia Enhances Catecholamine Secretion From Male Mouse Adrenal Chromaffin Cells Through an Increase In Ca2+ Release From the Endoplasmic Reticulum

“…We therefore examined the contribution of each of these processes to the sepsis-induced changes in ACC function. Similar to previous reports (35,39), control ACCs exhibited large input resistances and fired APs in response to small current injections. Endotoxemia had no effect on resting membrane potential, input resistance, rheobase, the number of APs fired at 2ϫ rheobase, AP amplitude, AP half-width, the proportion of ACCs exhibiting spontaneous APs or the proportion of ACCs exhibiting anodal break APs (Table 1).…”

“…ACCs were then stimulated for 5 minutes with a 20-mM K ϩ solution, prepared by equimolar substitution of KCl for NaCl, and the supernatant was collected. In previous studies, 20 mM high-K ϩ elicited an approximately 20-mV depolarization in the ACC membrane potential, which was sufficient to elevate [Ca 2ϩ ] i and promote catecholamine secretion (35,39). As a result, high-K ϩ was the primary stimulus used to characterize the changes in ACC function during sepsis in the present study.…”

“…Lipopolysaccharide is also unlikely to contribute to the effect of septic serum on Ca 2ϩ signaling because we have recently reported that lipopolysaccharide decreases ACC excitability without altering high-K ϩ -stimulated [Ca 2ϩ ] i transients (39). The factor responsible for mediating enhanced Ca 2ϩ signaling during sepsis therefore remains to be identified.…”

Lipopolysaccharide induces catecholamine production in mesenteric adipose tissue of rats previously exposed to immobilization stress

“…It is possible that LPS-mediated attenuation of anti-inflammatory status may consequently affect CA production. Furthermore, Lukewich et al (2013) reported a decreased CA response in adrenal medulla after LPS administration, where LPS via tolllike receptors (TLR-4) activation and nuclear factor kappa B-dependent pathway reduced adrenal chromaffin cell excitability. However, increased NE turnover following LPS treatment reduced the pro-inflammatory while enhanced the anti-inflammatory response (Elenkov et al, 1995;Suberville et al, 1996), indicating a physiological mechanism to limit the magnitude of an inflammatory response.…”

Divergent neuroendocrine responses to localized and systemic inflammation

“…In the context of the sympathetic-immune network, LPS has been shown to directly activate DRG and vagal afferent neurons [33;66]. Chronic exposure to LPS also reduces ACC excitability and inhibits neuropeptide Y release through the activation of nuclear factor (NF)-κB [67]. …”