Hisashi Ohta

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

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.

RELEASE OF GLUTAMATE IN THE NUCLEUS TRACTUS SOLITARII IN RESPONSE TO BAROREFLEX ACTIVATION IN RATS

Release of endogenous aspartate and glutamate from the region of the nucleus tractus solitarii was measured in vitro by perfusion methods and in vivo by microdialysis. Stimulation of the nucleus tractus solitarii with 35 mM potassium in vitro significantly increased extracellular concentrations of aspartate and glutamate. Glutamate and aspartate concentrations also increased with dialysis of 100 mM KCl into the nucleus tractus solitarii in vivo, but only changes in glutamate were significant. Experiments in vivo revealed that activation of the baroreflex by intravenous infusion of phenylephrine significantly increased glutamate in dialysates, while hypoventilation that accompanies baroreceptor activation and may activate chemoreceptors tended to increase aspartate but not glutamate. The demonstration that glutamate, but not aspartate, is released with activation of the baroreflex further supports the hypothesis that glutamate is a neurotransmitter of baroreceptor afferents terminating in the nucleus tractus solitarii.

Nitroxidergic Influences on Cardiovascular Control by NTS: A Link with Glutamate

Abstract: Glutamate (GLU) receptor activation, which is important in cardiovascular reflex transmission through the nucleus tractus solitarii (NTS), leads to release of nitric oxide (NO·) from central nitroxidergic neurons. Therefore, we hypothesized that GLU and NO· are linked in cardiovascular control by NTS. We first sought to determine if NO· released into NTS led to cardiovascular changes like those produced by GLU and found that the nitrosothiol S‐nitrosocysteine, but not NO· itself or other NO· donors, elicited such responses in anesthetized rats. The responses were dependent on activation of soluble guanylate cyclase but, not being affected by a scavenger of NO·, likely did not depend on release of NO· into the extracellular space. Responses to ionotropic GLU agonists in NTS, like those to S‐nitrosocysteine, were inhibited by inhibition of soluble guanylate cyclase. Inhibition of neuronal NO· synthase (nNOS) also inhibited responses to ionotropic GLU agonists. The apparent physiologic link between GLU and NO· mechanisms in NTS was further supported by anatomical studies that demonstrated frequent association between GLU‐containing nerve terminals and neurons containing nNOS. Furthermore, GLU receptors were often found on NTS neurons that were immunoreactive for nNOS. The anatomical relationships between GLU and nNOS and GLU receptors and nNOS were more pronounced in some subnuclei of NTS than in others. While seen in subnuclei that are known to receive cardiovascular afferents, the association was even more prominent in subnuclei that receive gastrointestinal afferents. These studies support a role for nitroxidergic neurons in mediating cardiovascular and other visceral reflex responses that result from release of GLU into the NTS.

Glycine elicits release of acetylcholine from the nucleus tractus solitarii in rat

Previous studies have suggested that cardiovascular responses elicited by injection of glycine into the nucleus tractus solitarii (NTS) depend upon interactions between glycinergic and cholinergic neuronal elements in NTS. Release of acetylcholine in response to glycine is one such interaction that has been shown in slices of hippocampus and striatum. In this study we sought to test the hypothesis that glycine causes release of acetylcholine from neurotransmitter stores in NTS. We compared release from NTS with that from adjacent hypoglossal nucleus and from caudate nucleus. Release of radiolabeled acetylcholine was determined in vitro after incubating NTS with [3H]choline. Exposure of NTS and caudate nucleus, but not hypoglossal nucleus, to glycine caused release of acetylcholine in a calcium-dependent manner that varied with concentration of glycine in the incubation medium. The maximally effective concentration (1 mM) of glycine elicited 136% increases over basal levels. Glycine did not elicit release of [3H]acetylcholine from tissue when calcium ion had been removed from the bath. Acetylcholine also was not released if tissue was incubated with either strychnine (10 microM) or hemicholinium-3 (1 mM) prior to exposure to glycine (1 mM). Thus, glycine, acting at strychnine-sensitive receptors in NTS, elicits release of acetylcholine from a portion of locally synthesized neurotransmitter stores.

Spermidine and cardiovascular control in nucleus tractus solitarii in rat

Cardiovascular effects elicited by unilateral microinjection of polyamines into the nucleus tractus solitarii (NTS) were tested in anesthetized rats. Spermidine (50-900 pmol) produced dose-dependent depressor and bradycardiac responses that were similar to those elicited by N-methyl-D-aspartate (NMDA; 0.1-1.2 pmol). The polyamines, putrescine and spermine, did not produce significant cardiovascular responses. The cardiovascular responses induced by spermidine and NMDA were almost eliminated by prior injection of the noncompetitive NMDA antagonist MK801 into NTS. On the other hand, a competitive antagonist 2-amino-5-phosphonovaleric acid (APV) blocked responses to NMDA but only attenuated those to spermidine. These findings support an action of the polyamine spermidine at the NMDA receptor complex in NTS where it could participate in modulating cardiovascular activity.

Cardiovascular Responses Produced by the Systemic Injection of Kainic Acid in Conscious Wky and Sh Rats

This study examined the cardiovascular effects of systemic injections of the excitatory amino acid analogue (EAA) kainic acid (KA) in conscious normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SH) rats. The injection of KA (100-1000 micrograms/kg, i.v.) produced dose-dependent but transient pressor and tachycardic responses followed by a prolonged period of bradycardia in both strains. Higher doses of KA also produced a secondary prolonged depressor response in WKY but not SH rats. Pharmacological studies demonstrated that the pressor and tachycardic responses are due mainly to increased sympathetic drive to the vasculature and heart. The KA-induced depressor response in the WKY is due principally to cardiovagal activation. After ganglionic blockade, KA produced a prolonged pressor response which was due to the release of vasopressin. These results indicate that the systemic administration of EAAs may produce their cardiovascular effects by activation of the autonomic nervous and endocrine systems and that the absence of the KA-induced hypotension in the SH rats may be related to an abnormality of cardiovagal function in this strain.

Baroreceptors in the carotid sinus contribute to arterial baroreceptor reflexes in normotensive rats

1. Baroreceptor reflex influences on renal sympathetic nerve activity (RSNA) were estimated with non‐linear regression analysis (sigmoidal curve fitting) before and after transection of carotid sinus nerves (CSN) and aortic depressor nerves (ADN) in rats.

The Region of Medial Forebrain Bundle is Involved in Tonic Maintenance of Arterial Pressure and Heart Rate

The role of the medial forebrain bundle (MFB) in central regulation of cardiovascular system was studied in freely moving rats. Injection of lidocaine into the ventral, but not other portions of the MFB, produced transient (approximately ten minutes) pressor and tachycardic responses. These responses resulted from an increase in sympathetic tone, since they were abolished by ganglionic blockade. The pressure response was attenuated by alpha-adrenoceptor blockade and both pressor and tachycardic responses were diminished by beta-adrenoceptor blockade. Electrolytic lesion of the same site caused long lasting pressor and tachycardic responses. Injection into the MFB of the excitatory amino acids, kainate (KA) and N-methyl-D,L-aspartate (NMDA) produced pressor and tachycardic responses accompanied by behavioral excitation, whereas no effects were produced by L-glutamate. The cardiovascular responses induced by KA and NMDA were abolished by ganglionic blockade. These results suggest that a selective region of MFB contains fibers of passage and/or cell bodies, with receptor specificity for excitatory amino acids, that are capable of influencing sympathetic sympathetic control of arterial pressure.