Actions of S-nitrosocysteine in the nucleus tractus solitarii are unrelated to release of nitric oxide

doi:10.1016/S0006-8993(96)01188-2

Brain Research

Volume 746, Issues 1–2, 23 January 1997, Pages 98-104

https://doi.org/10.1016/S0006-8993(96)01188-2 Get rights and content

Abstract

Cardiovascular effects elicited by microinjection of l-S-nitrosocysteine in the nucleus tractus solitarii (NTS) were compared and contrasted with those produced by the dextroisomer, other nitric oxide donors and nitric oxide itself. l-S-nitrosocysteine produced dose-related decreases of arterial pressure and heart rate. In contrast, d-S-nitrosocysteine, S-nitrosoglutathione, glyceryl trinitrate, and sodium nitroprusside produced minimal responses that were not dose-related. Likewise, injection of cystine and nitric oxide, two products of S-nitrosocysteine breakdown, produced no significant response. Headspace analysis using chemiluminescence revealed that l- and d-S-nitrosocysteine released identical amounts of nitric oxide when exposed to homogenates of whole rat brain. Responses to l-S-nitrosocysteine were not affected by local injection of oxyhemoglobin or the nitric oxide synthase inhibitor l-nitroarginine methylester. Although injection of l-cysteine into the NTS produced responses similar to those seen with injection of l-S-nitrosocysteine, blockade of excitatory amino acid receptors with kynurenic acid inhibited responses to cysteine but not those to the nitrosothiol. The study demonstrates that S-nitrosocysteine is biologically active in the NTS. Its action is independent of release of nitric oxide from the nitrosothiol but may be mediated through stereoselective sites on target neurons.

Introduction

Description of an endothelium derived relaxing factor [3]led to evidence supporting the role of a similar compound in central neurotransmission [7]. Nitric oxide (NO^⋅) has been considered that centrally active agent [1]. Many studies have suggested that NO^⋅ may be synthesized by central neurons, freely diffuse across membranes, and act on soluble guanylate cyclase in target cells to effect neuronal responses [40]. In such a situation the actions of NO^⋅ would not depend on binding to a classic receptor. Instead, NO^⋅ binds to the allosteric site of guanylate cyclase and elicits conformational changes in the enzyme, which may thus act as a `receptor' for NO^⋅[33]. Although many studies support a role for authentic NO^⋅ as a central messenger others suggest that S-nitrosylated thiols such as S-nitrosocysteine (SNC) may be equally or more effective than NO^⋅ in activating soluble guanylate cyclase [28]and may share many features with NO^⋅[30]. These compounds are synthesized by brain through reactions that would depend upon NO^⋅ synthesis [16]. Although some suggest that S-nitrosothiols simply provide a pool of NO^⋅ and that their biological actions depend upon their decomposition to NO^⋅[9], recent studies have challenged that idea and proposed that S-nitrosothiols may be more potent in their actions than NO^⋅ itself 8, 17, 28. We have previously shown that microinjection of S-nitrosocysteine into the nucleus tractus solitarii (NTS), the site of termination of cardiovascular afferent nerves, leads to reductions of arterial pressure (AP) and heart rate (HR) and that the cardiovascular responses are blocked by methylene blue, an inhibitor of soluble guanylate cyclase 21, 22. Therefore, we have used the NTS as a model system to test the hypothesis that SNC is independently active and may elicit central responses through stereoselective actions in the brain.

Section snippets

Physiological studies

Rats were prepared for experiments as described previously [22]. In brief, adult male Sprague–Dawley rats (275–350 g) were anesthetized with halothane and instrumented for recording AP, mean AP (MAP), and HR. The rats were placed in a stereotaxic frame (DKI, Tujunga, CA), a partial occipital craniotomy was performed, the dura and arachnoid incised, and the cerebellum retracted to expose the dorsal surface of the brain stem at the level of the obex. The calamus scriptorius was identified and

Results

Relative potencies were determined for l-SNC and d-SNC unilaterally microinjected into the NTS of anesthetized rats. Microinjection of 25–250 pmol of l-SNC produced dose-dependent hypotension and bradycardia (Fig. 1). Higher doses elicited progressively weaker effects. Microinjection of d-SNC (25–250 pmol) produced substantially smaller responses. Responses to the maximally effective dose (250 pmol) of l-SNC were approximately 300% greater than those to the same dose of d-SNC. The dose-response

Discussion

This study makes four pertinent observations with regard to possible participation of S-nitrosothiols in signal transduction at the level of the NTS. First, it supports previous studies that showed dose-dependent reductions of both AP and HR when SNC was microinjected into the NTS of anesthetized [22]and conscious rats [26]. Second, it shows that those responses cannot be explained by extracellular release of NO^⋅ from the nitrosothiol or by generation of NO^⋅ within the target cell through

Acknowledgements

All studies were approved by institutional animal care and use review committees. The work was supported by a Veterans Affairs Clinical Investigatorship (WTT) and Merit Review and in part by NIH HL23305 and 14388 and an American Heart Association Grant in Aid.

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Research reportActions of S-nitrosocysteine in the nucleus tractus solitarii are unrelated to release of nitric oxide