Takao Kubo

Evidence of N-methyl-d-aspartate receptor-mediated modulation of the aortic baroreceptor reflex in the rat nucleus tractus solitarii

Microinjections of N-methyl-D-aspartate (NMDA) into the medial area of the nucleus tractus solitarii (NTS) of the rat led to a decrease in arterial pressure and heart rate. The NMDA receptor antagonist 2-amino-5-phosphonovalerate (AP5) reduced the cardiovascular responses to NMDA. Depressor and bradycardic responses to aortic nerve stimulation were reduced by AP5 but not by a substance P antagonist, injected into the NTS. High K+ stimulation caused a calcium-dependent release of glutamate and aspartate from tissues in the area of the NTS. These results provide evidence of NMDA receptor-mediated modulation of the aortic baroreceptor reflex in the rat NTS.

Pharmacological characterisation of the α-adrenoceptors responsible for a decrease of blood pressure in the nucleus tractus solitarii of the rat

SummaryThe effects of various α-adrenoceptor agonists, microinjected into the area of the nucleus tractus solitarii (NTS) at the level of the obex, on blood pressure, and the interaction between noradrenaline and some α-antagonists in the area of the NTS were investigated in anaesthetized male rats. A dose-dependent decrease in blood pressure was induced by noradrenaline. The relative potencies of the substances were adrenaline > noradrenaline > α-methylnor-adrenaline > clonidine > tyramine. In contrast, phenylephrine was ineffective. The hypotensive effect of noradrenaline, α-methylnoradrenaline and clonidine was not affected by pretreatment with 6-hydroxydopamine given intraventricularly, while the effect of tyramine was blocked by the pretreatment. Prior application of phentolamine at the same site antagonized the hypotensive response to noradrenaline. Prazosin was about 10 times less potent than yohimbine in antagonizing the noradrenaline-induced hypotension. The present data suggest that the α-adrenoceptors in the area of the NTS responsible for the decrease in blood pressure are the same type as the peripheral presynaptic α2-adrenoceptors but may be located postsynaptically. It appears that the noradrenergic neurons in the NTS can play a role in blood pressure regulation.

Transmitter‐Like Release of Endogenous 3,4‐Dihydroxyphenylalanine from Rat Striatal Slices

Abstract: Biphasic electrical field stimulation (0.5–5 Hz, 2 ms, 25 V, 3 min) and high K+ (10–30 mM, 5 min) released endogenous 3,4‐dihydroxyphenylalanine (DOPA) from superfused rat striatal slices. Characteristics of the DOPA release were compared with those of 3,4‐dihydroxyphenylethylamine (dopamine, DA). Electrical stimulation at 2 Hz evoked DOPA and DA over similar time courses, α‐Methyl‐p‐tyrosine (0.2 mM) markedly reduced release of DOPA but not of DA. Maximal release (0.3 pmol) of DOPA was obtained at 2 Hz and at 15 mM K+. The impulse‐evoked release of DOPA and DA was completely tetrodotoxin (0.3 μM) sensitive and Ca2+ dependent and the 15 mM K+‐evoked release was also Ca2+ dependent. On l‐[3,5‐3H]tyrosine (1 μM) superfusion, high K+ (15 and 60 mM) released DOPA and DA together with concentration‐dependent decreases in tyrosine 3‐monooxygenase (EC 1.14.16.2) activity as indicated by [3H]H2O formation, followed by concentration‐dependent increases after DOPA and DA release ended. These findings suggest that striatal DOPA is released by a Ca2+‐dependent excitation‐secretion coupling process similar to that involved in transmitter release.

Evidence for the presence of GABAergic and glycine-like systems responsible for cardiovascular control in the nucleus tractus solitarii of the rat

Microinjections of gamma-aminobutyric acid (GABA) and glycine, into the medial area of the nucleus tractus solitarii (NTS) of the rat, led to an increase in arterial pressure and heart rate. The GABA receptor antagonist bicuculline and the glycine receptor antagonist strychnine decreased both of these cardiovascular parameters whereas the GABA uptake inhibitor nipecotic acid produced hypertension. High K+ stimulation caused a calcium-dependent release of GABA and glycine from tissues in the area of the NTS. Our results suggest that GABA and glycine may modulate the cardiovascular control within the NTS.

Biphasic actions of L-DOPA on the release of endogenous noradrenaline and dopamine from rat hypothalamic slices.

1 Effects of L‐DOPA on the release of endogenous noradrenaline and dopamine from rat hypothalamic slices evoked by electrical field stimulation at 5 Hz were investigated in the absence and presence of p‐bromobenzyloxyamine (NSD‐1055), a DOPA‐decarboxylase inhibitor. 2 In the absence of NSD‐1055, L‐DOPA produced a facilitation of impulse‐evoked release of noradrenaline at 0.1 μM but not at 1 and 10 μM, and had no effect on the spontaneous release. On the other hand, L‐DOPA 0.1 to 10 μM dose‐dependently increased the spontaneous release of dopamine and the highest concentration only increased the evoked release and tissue content of dopamine. 3 In the presence of NSD‐1055 10 μM, the increase in the spontaneous release of dopamine was prevented and L‐DOPA produced biphasic regulatory effects on the evoked release of noradrenaline and dopamine, a facilitation at 0.1 μM and an inhibition at 1 μM. The facilitation was antagonized by (‐)‐propranolol 0.1 μM, but not by the (+)‐isomer, whereas the inhibition was antagonized by S‐sulpiride 1 nM, but not by the R‐isomer. 4 In conclusion, L‐DOPA appears to produce biphasic actions on the release of endogenous noradrenaline and dopamine from rat hypothalamic slices, not through its conversion to dopamine but through presynaptic regulatory mechanisms, an inhibition via dopamine receptors at a micromolar concentration and a facilitation via β‐adrenoceptors at the lower concentration.

Biphasic actions of l-DOPA on the release of endogenous dopamine via presynaptic receptors in rat striatal slices

In rat striatal slices, 30 nM of L-DOPA increased the impulse (5 Hz)-evoked release of dopamine (DA), without increasing the spontaneous release and tissue content of DA. The minimum dose required to increase spontaneous DA release was 0.1 microM and the dose which led to an accumulation of DA was 100 microM. In the presence of NSD-1055, a DOPA-decarboxylase inhibitor, L-DOPA-induced increases in spontaneous DA release were prevented and L-DOPA produced dual actions on the evoked release of DA, a stereoselective propranolol-sensitive increase at 30 nM and a stereoselective sulpiride-sensitive decrease at 1 microM. L-DOPA produces dual presynaptic regulatory actions on DA release, via facilitatory beta-adrenoceptors at 30 nM and inhibitory DA receptors at 1 microM. The primary action of L-DOPA appears to be the facilitation of release of DA rather than the conversion to DA.

Evidence for L-DOPA systems responsible for cardiovascular control in the nucleus tractus solitarii of the rat

Microinjections of L-DOPA (10-100 ng) into the medial area of the nucleus tractus solitarii (NTS) led to dose-dependent decreases in arterial blood pressure and heart rate in rats treated with i.p. 3-hydroxybenzylhydrazine, a central inhibitor of DOPA decarboxylase, or similarly with intraventricular 6-hydroxydopamine. D-DOPA, dopamine or noradrenaline (100 ng) produced no effect. L-DOPA methyl ester (1 microgram), a competitive antagonist for L-DOPA, microinjected into NTS, blocked the depressor and bradycardic responses to L-DOPA. High K+ (40 mM) released endogenous DOPA in a Ca(2+)-dependent manner from slices of the rat dorsomedial medulla including NTS. These results support the hypothesis that there exist systems of L-DOPA itself responsible for cardiovascular regulation in NTS of rats. This regulatory action of L-DOPA seems to be postsynaptic in nature.

Evidence for γ-aminobutyric acid receptor-mediated modulation of the aortic baroreceptor reflex in the nucleus tractus solitarii of the rat

Microinjections of the gamma-aminobutyric acid (GABA) receptor antagonist bicuculline into the medial area of the nucleus tractus solitarii of the rat enhanced the depressor and bradycardiac responses to aortic nerve stimulation whereas the glycine receptor antagonist strychnine did not affect them. The GABA receptor agonist muscimol and the GABA uptake inhibitor nipecotic acid reduced the responses to aortic nerve stimulation. These results provide evidence suggesting GABA receptor-mediated modulation of the aortic baroreceptor reflex in the nucleus tractus solitarii of the rat.

Blood pressure modulation by substance P in the rat nucleus tractus solitarius.

Substance P in a dose of 0.1-10 ng injected into the nucleus tractus solitarii (NTS) of the rat caused hypotension, bradycardia and apnea whereas a dose of 100 ng led to no response. A substance P antagonist injected into the NTS abolished the cardiovascular responses to substance P. The antagonist alone increased blood pressure and heart rate. The data suggest a role for substance P in the cardiovascular regulation by the NTS.

Unilateral blockade of excitatory amino acid receptors in the nucleus tractus solitarii produces an inhibiton of baroreflexes in rats

SummaryExcitatory amino acid receptors and l-glutamate in the nucleus tractus solitarii (NTS) may be involved in the regulation of baroreceptor reflexes. To evaluate this hypothesis, we microinjected amino acid antagonists unilaterally into the rat NTS, and examined their effects on cardiovascular responses to electrical stimulation of the aortic nerve and on depressor responses to excitatory amino acid agonists microinjected into the NTS. Male Wistar rats were anesthesized with urethane, paralyzed, and artifically ventilated. Kynurenate (227 ng), an excitatory amino acid antagonist, injected ipsilaterally but not contralaterally into the NTS, markedly inhibited the depressor response to aortic nerve stimulation. l-Glutamate diethylester (GDEE, 3 μg), another excitatory amino acid antagonist, injected ipsilaterally into the NTS, also markedly inhibited both reflex depressor and bradycardic responses. MK-801 (30 ng), an N-methyl-d-aspartate (NMDA) receptor channel blocker, slightly inhibited the baroreflex responses, while Joro spider toxin JSTX-3 (17 ng), a glutamate receptor antagonist, did not affect them. Kynurenate (227 ng) and GDEE (3 μg) markedly inhibited the depressor response to the NMDA receptor agonist NMDA (0.3 ng), the quisqualate receptor agonist quisqualate (0.1 ng), the kainate receptor agonist kainate (0.1 ng), and l-glutamate (10 ng), microinjected into the NTS, while MK-801 (30 ng) reduced only the depressor response to NMDA (0.3 ng), and JSTX-3 (17 ng) reduced only the depressor response to kainate (0.1 ng). These findings provide evidence for the presence of excitatory amino acid receptors involved in mediating the aortic baroreceptor reflex in the rat NTS. In addition, these observations are consistent with the hypothesis that l-glutamate or a related excitatory amino acid may be the neurotransmitter of baroreceptor information in the NTS. It appears that both NMDA and non-NMDA receptors in the rat NTS are responsible for the mediation of baroreflexes.