Sympathoexcitatory CVLM neurons mediate responses to caudal pressor area stimulation

Neurons in the caudal pressor area (CPA) are a source of tonic sympathoexcitation that is dependent on activation of cardiovascular sympathetic premotor neurons in the rostral ventrolateral medulla (RVLM). In the present study, we sought to clarify the mechanism through which CPA neurons elicit increases in RVLM neuronal discharge, vasoconstrictor sympathetic tone, and arterial pressure. In urethan-chloralose-anesthetized, paralyzed, and artificially ventilated rats, bilateral disinhibition of CPA with bicuculline (Bic) after bilateral disinhibition of caudal ventrolateral medulla (CVLM) caused increases in splanchnic sympathetic nerve activity (+277% control) and arterial pressure (+54 mmHg). Inhibition of CVLM neurons with muscimol abolished the pressor response to activation of CPA neurons, suggesting that neurons within CVLM mediate the excitatory responses from CPA. Disinhibition of CVLM and CPA with Bic enhanced the sympathoexcitatory responses to stimulation of CPA with DL-homocysteic acid, which were blocked by microinjections of kynurenic acid into CVLM. We conclude that the pathway from CPA to RVLM involves an obligatory glutamatergic activation of sympathoexcitatory neurons in the vicinity of CVLM.     

脑内血管紧张素Ⅱ系统在穹窿下器升压反应中的作用

文献报道脑内存在血管紧张素Ⅱ系统。与此一致,本工作用氨基甲酸乙脂麻醉、箭毒制动、人工呼吸的大鼠观察到：（１）穹窿下器（ＳＦＯ）、室旁核（ＮＰＶ）或ＮＰＶ的投射区：延髓头端腹外侧区（ＲＶＬＭ）、导水管周围灰质（ＰＡＧ）、蓝斑（ＬＣ）内注入血管紧张素Ⅱ（ＡⅡ）均引起升压反应;（２）ＳＦＯ升压反应可被双侧ＮＰＶ或ＲＶＬＭ内预先注入［Ｓａｒ１,Ｔｈｒ８］ＡⅡ（ＳＴＡⅡ,ＡⅡ拮抗剂）明显衰减,ＮＰＶ升压反应也可被ＲＶＬＭ内注入ＳＴＡⅡ削弱;（３）双侧ＰＡＧ用ＳＴＡⅡ预处理后,ＡⅡ引起的ＮＰＶ或ＳＦＯ升压反应均明显减小;（４）ＮＰＶ升压反应还可被双侧ＬＣ内预先注射ＳＴＡⅡ衰减,但ＳＦＯ升压反应不受影响。结合我们以往工作曾显示兴奋ＰＡＧ或ＬＣ均可作用于ＲＶＬＭ引起升压反应,目前的结果表明：ＳＦＯ内的ＡⅡ能神经元通过ＮＰＶ内ＡⅡ能神经元,不仅可直接作用于ＲＶＬＭ引起升压反应,而且还可间接通过ＰＡＧ作用于ＲＶＬＭ起升压作用,但ＬＣ不参与ＳＦＯ升压反应。     

An enteric signal regulates putative gastrointestinal presympathetic vasomotor neurons in rats

Ingestion of a meal results in gastrointestinal (GI) hyperemia and is associated with systemic and paracrine release of a number of peptide hormones, including cholecystokinin (CCK) and 5-hydroxytryptamine (5-HT). Systemic administration of CCK octapeptide inhibits a subset of presympathetic neurons of the rostroventrolateral medulla (RVLM) that may be responsible for driving the sympathetic vasomotor tone to the GI viscera. The aim of this study was to determine whether endogenous release of CCK and/or 5-HT also inhibits CCK-sensitive RVLM neurons. The effects of intraduodenal administration of the secretagogues sodium oleate (SO) and soybean trypsin inhibitor (SBTI) on circulating levels of CCK and 5-HT were examined. In separate experiments, the discharge rates of barosensitive, medullospinal, CCK-sensitive RVLM presympathetic vasomotor neurons were recorded after rapid intraduodenal infusion of SO-SBTI or water. Alternatively, animals were pretreated with the CCK1 receptor antagonists devazepide and lorglumide or the 5-HT3 antagonist MDL-72222 before SO-SBTI administration. Secretagogue infusion significantly increased the level of circulating CCK, but not 5-HT. SO-SBTI significantly decreased (58%) the neuronal firing rate of CCK-sensitive RVLM neurons compared with water (5%). CCK1 receptor antagonists did not reverse SO-SBTI-induced neuronal inhibition (58%), whereas the 5-HT3 antagonist significantly attenuated the effect (22%). This study demonstrates a functional relation between a subset of RVLM presympathetic vasomotor neurons and meal-related signals arising from the GI tract. It is likely that endogenously released 5-HT acts in a paracrine fashion on GI 5-HT3 receptors to initiate reflex inhibition of these neurons, resulting in GI vasodilatation by withdrawal of sympathetic tone.     

Regulation of histamine release in rat hypothalamus and hippocampus by presynaptic galanin receptors.

The effect of galanin, a peptide present in a subpopulation of histaminergic neurons emanating from the rat posterior hypothalamus, was investigated on K(+)-evoked [3H]histamine release in slices and synaptosomes from rat cerebral cortex, striatum, hippocampus and hypothalamus. Porcine galanin (0.3 microM) significantly inhibited histamine release induced by 25 mM K+ in slices from hypothalamus and hippocampus, but not from cerebral cortex and striatum, i.e., only in regions in which a colocalization of histamine and galanin has been described. The inhibitory effect of galanin was concentration dependent, with an EC50 value of 5.8 +/- 1.9 nM. The maximal inhibition was of 30-40% in hypothalamic and hippocampal slices depolarized with 25 mM K+. The galanin-induced inhibition observed in hypothalamic slices was not prevented in the presence of 0.6 microM tetrodotoxin and also occurred in hippocampal and hypothalamic synaptosomes, strongly suggesting the activation by galanin of presynaptic receptors located upon histaminergic nerve endings. The maximal inhibitory effect of galanin in slices or synaptosomes was lower than that previously reported for histamine acting at H3-autoreceptors, possibly suggesting that not all histaminergic axon terminals, even in the hypothalamus and hippocampus, are endowed with galanin receptors. It increased progressively in hypothalamic and hippocampal synaptosomes as the strength of the depolarizing stimulus was reduced. It is concluded that galanin modulates histamine release via presynaptic receptors, presumably autoreceptors located upon nerve terminals of a subpopulation of cerebral histaminergic neurons.     

Decompensated hemorrhage activates serotonergic neurons in the subependymal parapyramidal region of the rat medulla

According to prior evidence opioid and serotonin release by lower brain stem neurons may contribute to hemorrhage-induced sympathoinhibition (HISI). Here we seek direct evidence for the activation of opioidergic, GABAergic, or serotonergic neurons by severe hemorrhage in the medulla oblongata. Blood was withdrawn from awake rats (40-50% total volume) causing hypotension and profound initial bradycardia. Other rats received the vasodilator hydralazine, causing tachycardia and hypotension. Neuronal activation was gauged by the presence of Fos-immunoreactive (ir) nuclei after 2 h. Serotonergic, enkephalinergic, and GABAergic neurons were identified by the presence of a diagnostic enzyme or mRNA. Hemorrhaged rats had 30% fewer non-GABAergic Fos-ir neurons in the rostral ventrolateral medulla (RVLM) than hydralazine-treated rats, but they had six times more Fos-ir neurons within the subependymal parapyramidal nucleus (SEPPN). Fos-labeled SEPPN neurons were serotonergic (40-60%), GABAergic (31%), enkephalinergic (15%), or had mixed phenotypes. The data suggest that a reduced sympathoexcitatory drive from RVLM may contribute to HISI. SEPPN neuronal activation may also contribute to HISI or could mediate defensive thermoregulatory mechanisms triggered by hemorrhage-induced hypothermia.     

血管紧张素—（1—7）在大鼠延髓头端腹外侧通过氨基酸类递质调节血压的作用

采用微量注射、微透析、高效液相色谱－荧光测定等技术和方法,观察和 血管紧张素－（1－7）[Ang-(1-7)]在延髓头端腹外侧（RVLM）与氨基酸类递质释放之间的关系,在麻醉大鼠RVLM注射Ang-(1-7)可引起血压升高,同时伴RVLM兴奋性氨基酸（EAA）释放增多;在RVLM注射Ang-(1-7)选择性受体拮抗剂Ang779可引起血压降低,同时伴RVLM EAA释放减少和抑制性氨基酸（IAA）释放增多,Ang-(1-7)的升压作用和Ang779的降压作用均可被相应的氨基酸受体拮抗剂部分阻娄。结果提示,Ang-(1-7)在RVLM的升压效庆可能部分是通过EAA释放增多所致;而Ang779在 RVLM的降压效应可能部分是通过EAA释放减少、IAA释放增多所致。     

Studies on the specificity of uptake and release of radiolabelled histamine in rat brain slices

Previously it has been shown that radiolabelled histamine is taken up by brain slices and may subsequently be released by depolarizing stimuli in a calcium-dependent manner, indicating the involvement of neurons in uptake and release of histamine.The present study demonstrates that after incubation of brain slices with low (nM) concentrations of [³H]histamine the amine may be taken up by (and released from) dopaminergic and serotonergic neurons (nerve terminals). Thus 6-hydroxydopamine- and 5,7-dihydroxytryptamine-induced lesions not only reduced the uptake of [³H]dopamine (in striatal slices) and [³H]serotonin (in hippocampal slices), but also, though to a lesser extent, that of [³H]histamine. Immunocytochemical findings revealed that the neurotoxins did not visibly affect histaminergic neurons. Lesioning of noradrenergic neurons appeared not to alter significantly the uptake of [³H]histamine. Further, various drugs acting on either catecholamine-, serotonin- or opioid-receptors and known to cause presynaptic inhibition of the release of [³H]dopamine or [³H]wrotonin from striatal or hippocampal slices also inhibited [³H]histamine release.It is concluded that incubation of brain slices with low concentrations of [³H]histamine does not result in a selective labelling of histaminergic neurons. The possibility that, unlike other monoamines, histamine is not subject to high-affinity uptake by the nerve terminals from which it was released, is discussed.     

延髓头端腹外侧区血管紧张素Ⅱ对脊髓投射神经元氨基酸递质释放的调节

用脊髓(T8)中间外侧柱(IML)微透析方法结合高效液相色谱(HPLC)技术,研究延髓头端腹外侧区(RVLM)微量注射血管紧张素Ⅱ(ANGⅡ,100pmol,n=11)后脊髓IML氨基酸递质释放的变化.在RVLM区微量注射ANGⅡ(100pmol,n=11),能显著增加(P<0.01)脊髓(T8)内天门冬氨酸(ASP,从4.75±1.01升至8.90±2.28pmol/20μl)和谷氨酸(GLU,从18.99±8.64升至73.88±29.26pmol/20μl)的释放.在同一RVLM部位注射losartan(10nmol,n=8)可以显著抑制注射ANGⅡ引起的GLU释放升高反应(P<0.05).免疫荧光双标记结合共聚焦显微镜观察到RVLM内62%～91%的谷氨酸能神经元呈AT1受体免疫阳性.此结果提示ANGⅡ诱发的脊髓内谷氨酸释放可能来源于RVLM内AT1受体免疫阳性的谷氨酸能脊髓投射神经元.     

Effects of (S)-alpha -fluoromethylhistidine and metoprine on locomotor activity and brain histamine content in mice.

We examined the effects of (S)-alpha -fluoromethylhistidine (FMH), an inhibitor of histidine decarboxylase, and metoprine, an inhibitor of histamine N-methyltransferase, on the locomotor activity and the brain histamine content of ICR mice. The brain histamine content was decreased by FMH (12.5 or 50 mg/kg, i.p.) and increased by metoprine (4 mg/kg, i.p.). Under these conditions, the locomotor activity and the number of rearing were significantly decreased and increased by FMH and metoprine, respectively. The higher the brain histamine content, the greater the locomotor activity and vice versa. In a previous paper [Sakai et al., Life Sciences, 48, 2397-2404 (1991)], we showed that thioperamide, a histamine H3 antagonist, which enhances the release of histamine from histaminergic neurons, in doses of 12.5 and 25 mg/kg, i.p. increases the locomotor activity, whereas it decreases the brain histamine content. Taken together, these results support the hypothesis that central histaminergic neurons may be involved in the control of state of locomotion and rearing.     

Influence of ventrolateral surface of medulla on reflex tracheal constriction

To assess the role of structures located superficially near the ventrolateral surface of the medulla on the reflex constriction of tracheal smooth muscle that occurs when airway and pulmonary receptors are stimulated mechanically or chemically, experiments were conducted in alpha-chloralose-anesthetized, paralyzed, and artificially ventilated cats. Pressure changes within a bypassed segment of the trachea were used as an index of alterations smooth muscle tone. The effects of focal cooling of the intermediate areas or topically applied lidocaine on the ventral surface of the medulla on the response of the trachea to mechanical and chemical stimulation of airway receptors were examined. Atropine abolished tracheal constriction induced by mechanical stimulation of the carina or aerosolized histamine, showing that the responses were mediated over vagal pathways. Moderate cooling of the intermediate area (20 degrees C) or local application of lidocaine significantly decreased the tracheal constrictive response to mechanical activation of airway receptors. Furthermore, when the trachea was constricted by histamine, cooling of the intermediate area significantly diminished the increased tracheal tone, whereas rewarming restored tracheal tone to the previous level. These findings suggest that under the conditions of the experiments the ventral surface of the medulla plays an important role in constriction of the trachea by inputs from intrapulmonary receptors and in the modulation of parasympathetic outflow to airway smooth muscle.     

Cell immunoblot assay study demonstrating the release of PACAP from individual anterior pituitary cells of rats and the effect of PACAP on LH release

Orexin A (or hypocretin 1)-immunoreactive neurons in the rat lateral hypothalamus project to several areas of the medulla oblongata that are closely associated with cardiovascular regulation. The present study was undertaken to further strengthen the hypothesis that orexin A accelerates cardiovascular response by activating sympathoexcitatory neurons in the rat rostral ventrolateral medulla (RVLM). First, immunohistochemical studies revealed the presence of orexin A-immunoreactive fibers in the RVLM. Double labeling the sections with orexin A- and tyrosine hydroxylase (TH)-antisera further showed that orexin A-immunoreactive fibers are in close proximity with TH-immunoreactive neurons, some of which may be barosensitive, bulbospinal neurons in the RVLM. Second, microinjection of orexin A (6.35, 12.7 and 38.1 microM) into the RVLM, which was verified later by histological examination, caused a significant increase of mean arterial pressure (MAP) and a moderate increase of heart rate (HR) in awake rats. L-glutamate (33.3 mM) injected into the same sites, caused a larger increase in MAP, but a decrease in HR; whereas, saline injection was without significant effect. Results from this study suggest that orexin A, which may be released from the nerve fibers originating from the neurons in the lateral hypothalamus, acting on RVLM neurons in the medulla, increases sympathetic outflow targeted to the heart and blood vessels in awake animals.     

Nitric oxide- and superoxide-dependent mitochondrial signaling in endotoxin-induced apoptosis in the rostral ventrolateral medulla of rats

This study evaluated the hypothesis that the repertoire of cellular events that underlie circulatory fatality during endotoxemia may entail mitochondrial respiratory enzyme dysfunction, followed by the release of cytochrome c to the cytosol that triggers the activation of caspase cascades, leading to apoptotic cell death in the rostral ventrolateral medulla (RVLM) where sympathetic premotor neurons responsible for maintaining vasomotor tone are located. In adult Sprague-Dawley rats maintained under propofol anesthesia, nucleosomal DNA fragmentation was detected in the RVLM in a temporal profile that coincided positively with the progression of cardiovascular depression during experimental endotoxemia induced by Escherichia coli lipopolysaccharide (LPS). LPS also induced nitric oxide (NO) and superoxide (O(2)(-)) production, depressed mitochondrial Complex I and IV activity, promoted the release of cytochrome c from mitochondria to cytosol, upregulated the cytosolic expression of activated caspase-9 and -3, or increased caspase-3 enzyme activity in the RVLM. Microinjection bilaterally into the RVLM of an inducible nitric oxide synthase (iNOS) blocker, S-methylisothiourea, or a superoxide dismutase mimetic, Tempol, significantly blunted these apoptotic cellular events and antagonized the cardiovascular depression during endotoxemia. We conclude that caspase-dependent apoptotic cell death that results from NO- and O(2)(-)-associated mitochondrial signaling in the RVLM may underlie fatal cardiovascular depression during endotoxemia.     

Histamine Receptor and its Regulation of Energy Metabolism

In a series of studies on brain functions of histamine, probes to manipulate activities of histaminergic neuronal systems were applied to assess histaminergic function in non-obese normal, and lean and obese Zucker rats. Food intake was suppressed by both activation of H₁-receptors and inhibition of H₃-receptors in the ventromedial hypothalamic nucleus (VMH) and the paraventricular nucleus, each of which is a satiety center. Feeding circadian rhythm was decreased in its amplitude through histaminergic modulation in the hypothalamus. Histamine neurons in the mesencephalic trigeminal nucleus (Me5) were involved in regulation of masticatory functions, particularly eating speed, while histamine-containing neurons in the VMH controlled intake volume of meals. Energy deficiency in the brain enhanced satiation through histaminergic activation of VMH neurons, which in turn produced glycogenolysis in the hypothalamus to maintain homeostatic control of glucose supply. A very-low-calorie conventional Japanese diet, which is a fiber rich and low energy food source, enhanced satiation by increased mastication and because of the low energy supply of the diet. Hypothalamic histamine neurons were activated by high ambient temperature and also by interleukin-1β, an endogenous pyrogen, to maintain homeostatic thermoregulation. Behavioral and metabolic abnormalities of Zucker obese rats were mediated by a deficit in hypothalamic neuronal histamine, and the Zucker rat was evaluated as an animal model of histamine deficiency. Transplantation of the lean fetal hypothalamus into the third cerebroventricle of host obese Zuckers attenuated the abnormalities.     

Sympathoinhibition from ventrolateral periaqueductal gray mediated by 5-HT(1A) receptors in the RVLM

The role of 5-hydroxytryptamine 1A (5-HT(1A)) receptors located in the rostral ventrolateral medulla (RVLM) in the mediation of a sympathoinhibitory and depressor response elicited from the ventrolateral periaqueductal gray (vlPAG) matter of the midbrain was examined in pentobarbital sodium-anesthetized rats. Activation of neurons in the vlPAG evoked a decrease in renal and lumbar sympathetic nerve activities and a decrease in arterial blood pressure. After microinjection of the specific 5-HT(1A)-receptor antagonist WAY-100635 into the pressor area of the RVLM, the vlPAG-evoked sympathoinhibition and hypotension was attenuated to control levels (7 of 15 animals) or converted into a sympathoexcitation and pressor response (8 of 15 animals). Baroreflex inhibition of sympathetic nerve activity was not impaired by microinjection of WAY into the sympathoexcitatory region of the RVLM. These data suggest that sympathoinhibition and hypotension elicited by activation of neurons in the vlPAG are mediated by 5-HT(1A) receptors in the RVLM.     

Effects of high fructose intake on the development of hypertension in the spontaneously hypertensive rats: the role of AT1R/gp91PHOX signaling in the rostral ventrolateral medulla

Both genetic and dietary factors determine the development of hypertension. Whether dietary factor impacts the development of hereditary hypertension is unknown. Here, we evaluated the effect of daily high-fructose diet (HFD) on the development of hypertension in adolescent spontaneously hypertensive rats (SHR). Six-week-old SHR were randomly divided into two groups to receive HFD or normal diet (ND) for 3 weeks. The temporal profile of systolic blood pressure, alongside the sympathetic vasomotor activity, in the SHR-HFD showed significantly greater increases at 9–12 weeks of age compared with the age-matched SHR-ND group. Immunofluorescence was used to identify the distribution of reactive oxygen species (ROS), oxidants and antioxidants in rostral ventrolateral medulla (RVLM) where sympathetic premotor neurons reside. In RVLM of SHR-HFD, the levels of ROS accumulation and lipid peroxidation were elevated. The changes in protein expression were measured by Western blot. NADPH oxidase subunit gp91^phox and angiotensin II type I receptor were up-regulated in RVLM neuron. On the other hand, the expression of extracellular superoxide dismutase was suppressed. Both molecular and hemodynamic changes in the SHR-HFD were rescued by oral pioglitazone treatment from weeks 7 to 9. Furthermore, central infusion with tempol, a ROS scavenger, effectively ameliorated ROS accumulation in RVLM and diminished the heightened pressor response and enhanced sympathetic activity in the SHR-HFD. Together, these results suggest that HFD intake at adolescent SHR may impact the development of hypertension via increasing oxidative stress in RVLM which could be effectively attenuated by pioglitazone treatment.     

Increased superoxide anion in rostral ventrolateral medulla contributes to hypertension in spontaneously hypertensive rats via interactions with nitric oxide

Oxidative stress because of an excessive production of superoxide anion (O2*-) is associated with hypertension. The present study evaluated the hypothesis that in the rostral ventrolateral medulla (RVLM), where the premotor neurons for the maintenance of vascular vasomotor activity are located, increased O2*- contributes to hypertension in spontaneously hypertensive rats (SHR) by modulating the cardiovascular depressive actions of nitric oxide (NO). Compared with normotensive Wistar-Kyoto (WKY) rats, SHR manifested significantly increased basal O2*- production, along with reduced manganese superoxide dismutase (MnSOD) expression and activity, in the RVLM. The magnitude of hypotension, bradycardia, or suppression of sympathetic neurogenic vasomotor tone elicited by microinjection bilaterally into the RVLM of a membrane-permeable SOD mimetic, Mn(III)-tetrakis-(4-benzoic acid) porphyrin (MnTBAP), was also significantly larger in SHR. Transfection bilaterally into the RVLM of adenoviral vectors encoding endothelial nitric oxide synthase resulted in suppression of arterial pressure, heart rate, and sympathetic neurogenic vasomotor tone in both WKY rats and SHR. Microinjection of MnTBAP into the RVLM of SHR further normalized those cardiovascular parameters to the levels of WKY rats. We conclude that an elevated level of O2*- in the RVLM is associated with hypertension in SHR. More importantly, this elevated O2*- may contribute to hypertension by reducing the NO-promoted cardiovascular depression.     

Reduction in molecular synthesis or enzyme activity of superoxide dismutases and catalase contributes to oxidative stress and neurogenic hypertension in spontaneously hypertensive rats

A balance between production and elimination of reactive oxygen species such as superoxide anion (O2*-) and hydrogen peroxide (H2O2) tightly regulates the homeostasis of cellular oxidative stress, which contributes to a variety of cardiovascular diseases, including hypertension. The present study assessed the hypothesis that O2*- or H2O2 levels augmented by the reduced molecular synthesis or enzyme activity of superoxide dismutase (SOD), catalase (CAT), or glutathione peroxidase (GPx) in the rostral ventrolateral medulla (RVLM), where sympathetic premotor neurons that generate tonic vasomotor tone are located, contribute to the pathogenesis of hypertension. We found that copper/zinc SOD (SOD1), manganese SOD (SOD2), or CAT, but not GPx, mRNA or protein expression and enzyme activity in the RVLM of spontaneously hypertensive rats (SHR) were significantly lower than those in normotensive Wistar-Kyoto (WKY) rats, along with a significantly higher level of O2*- or H2O2. A causative relationship between these biochemical correlates of oxidative stress and neurogenic hypertension was established when gene transfer by microinjection of adenovirus encoding SOD1, SOD2, or CAT into the bilateral RVLM promoted a long-lasting reduction in arterial pressure in SHR, but not WKY rats, accompanied by an enhanced SOD1, SOD2, or CAT protein expression or enzyme activity and reduced O2*- or H2O2 level in the RVLM. These results together suggest that downregulation of gene expression and enzyme activity of the antioxidant SOD1, SOD2, or CAT may underlie the augmented levels of O2*- and H2O2 in the RVLM, leading to oxidative stress and hypertension in SHR.     

Rostral Ventrolateral Medulla: An Integrative Site for Muscle Vasodilation During Defense-Alerting Reactions

1. Evidence gathered over the last 30 years has firmly established that the rostral ventrolateral medulla (RVLM) is a major vasomotor center in the brainstem, harboring sympathetic premotor neurons responsible for generating and maintaining basal vasomotor tone and resting levels of arterial blood pressure. Although the RVLM has been almost exclusively classified as a vasopressor area, in this report we review some evidence suggesting a prominent role of the RVLM in muscle vasodilation during defense-alerting responses.2. Defense-alerting reactions are a broad class of behavior including flexion of a limb, fight/flight responses, apologies, etc. They comprise species-distinctive motor and neurovegetative adjustments. Cardiovascular responses include hypertension, tachycardia, visceral vasoconstriction, and muscle vasodilation. Since defense-alerting reactions generally involve intense motor activation, muscle vasodilation is regarded as a key feature of these responses.3. In anesthetized or unanesthetized-decerebrate animals, natural or electrical stimulation of cutaneous and muscle afferents produced hypertension, tachycardia, and vasodilation restricted to the stimulated limb.4. Unilateral inactivation of the RVLM contralateral to the stimulated limb abolished cardiovascular adjustments to stimulation of cutaneous and muscle afferents. Within the RVLM glutamatergic synapses mediate pressor responses, whereas GABAergic synapses mediates muscle vasodilation.5. In urethane-anesthetized rats, electrical stimulation of the hypothalamus elicited hypertension, tachycardia, visceral vasoconstriction, and hindlimb vasodilation. The hindlimb vasodilation induced by hypothalamic stimulation is a complex response, involving reduction of sympathetic vasoconstrictor tone, release of catecholamines by the adrenal medulla, and a still unknown system that may use nitric oxide as a mediator.6. Blockade of glutamatergic transmission within the RVLM selectively blocks muscle vasodilation induced by hypothalamic stimulation.7. The results obtained suggest that, besides its role in the generation and maintenance of the sympathetic vasoconstrictor drive, the RVLM is also critical for vasodilatory responses during defense reactions. The RVLM may contain several, distinctive mechanisms for muscle vasodilation. Anatomical and functional characterization of these pathways may represent a breakthrough in our understanding of cardiovascular control in normal and/or pathological conditions.     

Contribution to sympathetic vasomotor tone of tonic glutamatergic inputs to neurons in the RVLM

The role of excitatory amino acid (EAA) receptors in the rostral ventrolateral medulla (RVLM) in maintaining resting sympathetic vasomotor tone remains unclear. It has been proposed that EAA receptors in the RVLM mediate excitatory inputs both to presympathetic neurons and to interneurons in the caudal ventrolateral medulla (CVLM), which then provide a counterbalancing inhibition of RVLM presympathetic neurons. In this study, we tested this hypothesis by determining the effect of blockade of EAA receptors in the RVLM on mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA), after inhibition of CVLM neurons. In anesthetized rats, bilateral injections of muscimol in the CVLM increased MAP, HR, and RSNA. Subsequent bilateral injections of kynurenic acid (Kyn, 2.7 nmol) in the RVLM caused a modest reduction of approximately 20 mmHg in the MAP but had no effect, when compared with the effect of vehicle injection alone, on HR or RSNA. By approximately 50 min after the injections of Kyn or vehicle in the RVLM, the MAP had stabilized at a level close to its original baseline level, but the HR and RSNA stabilized at levels above baseline. The results indicate that removal of tonic EAA drive to RVLM neurons has little effect on the tonic activity of RVLM presympathetic neurons, even when inputs from the CVLM are blocked. Thus the tonic activity of RVLM presympathetic neurons under these conditions is dependent on excitatory synaptic inputs mediated by non-EAA receptors and/or the autoactivity of these neurons.     

Modulation of respiratory responses to chemoreflex activation by L-glutamate and ATP in the rostral ventrolateral medulla of awake rats

Presympathetic neurons in the different anteroposterior aspects of rostral ventrolateral medulla (RVLM) are colocalized with expiratory [B?tzinger complex (B?tC)] and inspiratory [pre-B?tzinger complex (pre-B?tC)] neurons of ventral respiratory column (VRC), suggesting that this region integrates the cardiovascular and respiratory chemoreflex responses. In the present study, we evaluated in different anteroposterior aspects of RVLM of awake rats the role of ionotropic glutamate and purinergic receptors on cardiorespiratory responses to chemoreflex activation. The bilateral ionotropic glutamate receptors antagonism with kynurenic acid (KYN) (8 nmol/50 nl) in the rostral aspect of RVLM (RVLM/B?tC) enhanced the tachypneic (120 ± 9 vs. 180 ± 9 cpm; P < 0.01) and attenuated the pressor response (55 ± 2 vs. 15 ± 1 mmHg; P < 0.001) to chemoreflex activation (n = 7). On the other hand, bilateral microinjection of KYN into the caudal aspect of RVLM (RVLM/pre-B?tC) caused a respiratory arrest in four awake rats used in the present study. Bilateral P2X receptors antagonism with PPADS (0.25 nmol/50 nl) in the RVLM/B?tC reduced chemoreflex tachypneic response (127 ± 6 vs. 70 ± 5 cpm; P < 0.001; n = 6), but did not change the chemoreflex pressor response. In addition, PPADS into the RVLM/B?tC attenuated the enhancement of the tachypneic response to chemoreflex activation elicited by previous microinjections of KYN into the same subregion (188 ± 2 vs. 157 ± 3 cpm; P < 0.05; n = 5). Our findings indicate that: 1) L-glutamate, but not ATP, in the RVLM/B?tC is required for pressor response to peripheral chemoreflex and 2) both transmitters in the RVLM/B?tC are required for the processing of the ventilatory response to peripheral chemoreflex activation in awake rats.