Acute mechanical sensitization of peripheral nociceptors by aldosterone through non-genomic activation of membrane bound mineralocorticoid receptors in naive rats

Abstract

Recently, there is increasing interest in the role of peripheral mineralocorticoid receptors (MR) to modulate pain, but their localization in neurons and glia of the periphery and their distinct involvement in pain control remains elusive. In naive Wistar rats our double immunofluorescence confocal microscopy of the spinal cord, dorsal root ganglia, sciatic nerve and innervated skin revealed that MR predominantly colocalized with calcitonin-gene-related peptide (CGRP)- and trkA-immunoreactive (IR) nociceptive neurons and only marginally with myelinated trkB-IR mechanoreceptive and trkC-IR proprioreceptive neurons underscoring a pivotal role for MR in the modulation of pain. MR could not be detected in Schwann cells, satellite cells, and astrocytes and only scarcely in spinal microglia cells excluding a relevant functional role of glia-derived MR at least in naïve rats. Intrathecal (i.t.) and intraplantar (i.pl.) application of increasing doses of the MR selective agonist aldosterone acutely increased nociceptive behavior which was reversible by a MR selective antagonist and most likely due to non-genomic effects. This was further substantiated by the first identification of membrane bound MR specific binding sites in sensory neurons of dorsal root ganglia and spinal cord. Therefore, a crucial role of MR on nociceptive neurons but not on glia cells and their impact on nociceptive behavior most likely due to immediate non-genomic effects has to be considered under normal but more so under pathological conditions in future studies.

Comparative Expression Analyses of Pro- versus Anti-Inflammatory Mediators within Synovium of Patients with Joint Trauma, Osteoarthritis, and Rheumatoid Arthritis

“…Finally, the tissues were washed in PBS, and nuclei were stained with DAPI (4′,6-diamidino-2-phenylindole) and then mounted in Vectashield (Vector Laboratories) and imaged on a confocal laser scanning microscope, LSM510, as described previously [19]. Then, the photographs were taken by a confocal microscope (633 nm; Carl Zeiss, Göttingen, Germany) as described in our previous study [20]. To demonstrate specificity of staining, the following controls were included: omission of the primary antisera or the secondary antibodies, as described in our previous studies [19].…”

Aldosterone Synthase in Peripheral Sensory Neurons Contributes to Mechanical Hypersensitivity during Local Inflammation in Rats

“…2,15 More recently, we identified mineralocorticoid receptors predominantly on the subpopulation of peptidergic unmyelinated nociceptive C-fibers, strongly indicating a role for the modulation of nociception. 16 Indeed, local application of the mineralocorticoid receptor agonist aldosterone at the peripheral and central nerve terminals of nociceptive neurons elicited a rapid increased mechanical sensitivity. 16 Consistent with this pronociceptive effect, previous studies have shown that aldosterone increased the number of action potentials that were evoked in acutely dissociated dorsal root ganglion neurons.…”

“…16 Indeed, local application of the mineralocorticoid receptor agonist aldosterone at the peripheral and central nerve terminals of nociceptive neurons elicited a rapid increased mechanical sensitivity. 16 Consistent with this pronociceptive effect, previous studies have shown that aldosterone increased the number of action potentials that were evoked in acutely dissociated dorsal root ganglion neurons. 2,15,17 These effects occurred rapidly, were most likely due to nongenomic mechanisms, and were mediated by membrane-bound mineralocorticoid receptors which were identified by membrane saturation binding.…”

“…2,15,17 These effects occurred rapidly, were most likely due to nongenomic mechanisms, and were mediated by membrane-bound mineralocorticoid receptors which were identified by membrane saturation binding. 16 Using the animal model of unilateral Freund's complete adjuvant hind paw inflammation, this study investigated whether: (1) the antagonism of mineralocorticoid receptors on peripheral sensory neurons will result in the attenuation of mechanical hyperalgesia, suggesting an intrinsic tone of endogenous aldosterone; (2) peripheral sensory neurons are a potential source for local aldosterone production; (3) aldosterone is coexpressed with its processing enzyme aldosterone synthase and if their expression is upregulated during inflammatory conditions; (4) there is a functional link between endogenous aldosterone and sensory neuron mineralocorticoid receptors; and (5) the inhibition of aldosterone's production in peripheral sensory neurons by local administration of a specific aldosterone synthase inhibitor will result in the attenuation of mechanical hypersensitivity. The overall hypothesis, therefore, is that antagonism of sensory neuron mineralocorticoid receptors attenuates inflammation-induced mechanical hyperalgesia most likely due to the inhibition of an endogenous tonic activation through aldosterone derived from peripheral sensory neurons.…”

“…Dorsal root ganglia and spinal cords from adult rats were solubilized and extracted for immunoblotting investigations as previously described. 16,24 After blotting, membranes were blocked in 3% bovine serum albumin for 2 h and incubated with mouse anti-aldosterone synthase (1:1,000 in 3% BSA; anti-CYP11B2; Merck Millipore, Germany) or rabbit anti-11β-hydroxysteroid-dehydrogenase 2 (1:500 in 3% BSA; St. John's Laboratory Ltd., United Kingdom) or sheep anti-11β-hydroxysteroid-dehydrogenase 2 (1:200 in 3% BSA; private gift from Dr. Elise Gomez-Sanchez, Department of Pharmacology and Toxicology, University of Mississippi Medical Center, Jackson, Mississippi). After incubation with the secondary antibody (peroxidase-conjugated goat anti-rabbit, 1:40,000; Jackson ImmunoResearch, USA) for 2 h at ambient temperature, reactive protein bands were digitally visualized using enhanced chemiluminescence solutions (SuperSignal West Pico; Thermo Fisher Scientific) in ChemiDoc MP Imager.…”