Injection of antisense oligos to nNOS into nucleus tractus solitarii increases blood pressure

Renin activates PI3K-Akt-eNOS signalling through the angiotensin AT₁ and Mas receptors to modulate central blood pressure control in the nucleus tractus solitarii

BACKGROUND AND PURPOSE

The renin-angiotensin system (RAS) is critical for the control of blood pressure by the CNS. Recently, direct renin inhibitors were approved as antihypertensive agents. However, the signalling mechanism of renin, which regulates blood pressure in the nucleus tractus solitarii (NTS) remains unclear. Here we have investigated the signalling pathways involved in renin-mediated blood pressure regulation, at the NTS.

EXPERIMENTAL APPROACH

Depressor responses to renin microinjected into the NTS of Wistar-Kyoto rats were elicited in the absence and presence of the endothelial nitric oxide synthase (eNOS)-specific inhibitor, N(5)-(-iminoethyl)-L-ornithine, Akt inhibitor IV and LY294002, a PI3K inhibitor and GP antagonist-2A [G(q) inhibitor]. Lisinopril (angiotensin converting enzyme inhibitor), losartan, valsartan (angiotensin AT(1) receptor antagonists), D-Ala7-Ang-(1-7) (angiotensin-(1-7) receptor antagonist) were used to study the involvement of RAS on renin-induced depressor effects.

KEY RESULTS

Microinjection of renin into the NTS produced a prominent depressor effect and increased NO production. Pretreatment with G(q) -PI3K-Akt-eNOS pathway-specific inhibitors significantly attenuated the depressor response evoked by renin. Immunoblotting and immunohistochemical studies further showed that inhibition of PI3K significantly blocked renin-induced eNOS-Ser ¹¹⁷ and Akt-Ser⁴⁷³ phosphorylation in situ. In addition, pre-treatment of the NTS with RAS inhibitors attenuated the vasodepressor effects evoked by renin. Microinjection of renin also increased Ras activation in the NTS.

CONCLUSIONS AND IMPLICATIONS

Taken together, these results suggest renin modulated blood pressure at the NTS by AT₁ and Mas receptor-mediated activation of G(q) and Ras to evoke PI3K-Akt-eNOS signalling.

Nitric oxide inhibits excitatory vagal afferent input to nucleus tractus solitarius neurons in anaesthetized rats

OBJECTIVE

Endogenous nitric oxide (NO) has been implicated in the regulation of neuronal activity which mediates cardiovascular reflexes. However, there is controversy concerning the role of NO in the nucleus tractus solitarius (NTS). The present study aims to elucidate the possible physiological role of endogenous NO in modulating the excitatory vagal afferent input to NTS neurons.

METHODS

All the experiments in the rat were conducted under anaesthetic conditions. Ionophoresis method was used for the application of NO donor or nitric oxide synthase (NOS) inhibitor, and single unit recording method was employed to detect the effects of these applications on vagal afferent- or cardio-pulmonary C-fibre reflex-evoked neuronal excitation in NTS.

RESULTS

Ionophoresis applications of L-arginine (L-Arg), a substrate of NOS, and sodium nitroprusside (SNP), a NO donor, both attenuated the vagal afferent-evoked discharge by (51.5+/-7.6)% (n = 17) and (68.3+/-7.1)% (n = 9), respectively. In contrast, application of D-Arg at the same current exerted no overall effect on this input. Also, both L-Arg and SNP inhibited spontaneous firing of most of the recorded neurons. In contrast, ionophoresis application of N(G)-nitro-L-arginine methyl ester (L-NAME) enhanced vagal afferent-evoked excitation by (66.3+/-11.4)% (n = 7). In addition, ionophoresis application of L-Arg and SNP significantly attenuated cardio-pulmonary C-fibre reflex-induced excitation in the tested NTS neurons.

CONCLUSION

Activation of local NO pathway in the NTS could suppress vagal afferent-evoked excitation, suggesting that NO is an important neuromodulator of visceral sensory input in the NTS.

IL-6 microinjected in the nucleus tractus solitarii attenuates cardiac baroreceptor reflex function in rats

Recent gene array and molecular studies have suggested that an abnormal gene expression profile of interleukin-6 (IL-6) in the nucleus tractus solitarii (NTS), a pivotal region for regulating arterial pressure, may be related to the development of neurogenic hypertension. However, the precise functional role of IL-6 in the NTS remains unknown. In the present study, we have tested whether IL-6 affects cardiovascular control at the level of the NTS. IL-6 (1, 10, and 100 fmol) was microinjected in the NTS of Wistar rats (280-350 g) under urethane anesthesia. Although the baseline levels of arterial pressure and heart rate did not change following IL-6 injections, the cardiac baroreflex in response to increased arterial pressure was dose-dependently attenuated. In addition, IL-6 (100 fmol) microinjections also attenuated l-glutamate-induced bradycardia at the level of the NTS. Immunohistochemical detection of IL-6 in naïve rats demonstrated that it was predominantly observed in neurons within the brain stem, including the NTS. These findings suggest that IL-6 within the NTS may play an important role for regulating cardiovascular control via modulation of input signals from baroreceptor afferents. Whether the abnormal gene expression of IL-6 in the NTS is associated in a causal way with hypertension remains to be resolved.

Microinjection of urocortin into the rat nucleus tractus solitarii decreases arterial blood pressure

Systemic administration of urocortin I (Ucn I), a member of the corticotrophin-releasing factor (CRF) peptide family, modulates cardiovascular system. In the central nervous system, Ucn I is found in the nucleus tractus solitarii (NTS), which plays an important role in regulating arterial blood pressure (ABP) and heart rate (HR) in response to activation of the baroreceptor afferents. In this study, we examined the effects of Ucn I, which has a high affinity for both type 1 and type 2 CRF receptors (i.e. CRF-R1 and -R2), on cardiovascular functions at the level of the NTS. A specific agonist of CRF-R1 (i.e. CRF) and a specific agonist of CRF-R2 (i.e. Urocortin II) were also tested to identify the specific cardiovascular effects induced by individual activation of either CRF-R1 or -R2. We found that Ucn I microinjected into the rat NTS produced a significant reduction in both ABP and HR. Both agonists for CRF-R1 and -R2 microinjected into the NTS also reduced ABP and HR. Our results suggest that Ucn I in the NTS may play an important role in cardiovascular regulation and the cardiovascular effects of Ucn I may be mediated by activation of both CRF-R1 and -R2, which are known to be present in the NTS.

Differential sensitivity of excitatory and inhibitory synaptic transmission to modulation by nitric oxide in rat nucleus tractus solitarii

The nucleus tractus solitarii (NTS) is a key central link in control of multiple homeostatic reflexes. A number of studies have demonstrated that exogenous and endogenous nitric oxide (NO) within NTS regulates visceral function, but further understanding of the role of NO in the NTS is hampered by the lack of information about its intracellular actions. We studied effects of NO in acute rat brainstem slices. Aqueous NO solution (NO(aq)) potentiated electrically evoked excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs, respectively) in different neuronal subpopulations and, in some neurones, caused a depolarization. Similar effects were observed using the NO donor diethylamine NONOate (DEA/NO). The threshold NO concentration as determined using an NO electrochemical sensor was estimated as approximately 0.4 nm (EC(50) approximately 0.9 nm) for potentiating glutamatergic EPSPs but approximately 3 nm for monosynaptic GABAergic IPSPs. Bath application of the soluble guanylate cyclase (sGC) inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) abolished NO(aq)- and DEA/NO-induced potentiation of evoked EPSPs, IPSPs and depolarization. All NO actions were mimicked by the non-NO-dependent guanylate cyclase activator Bay 41-2272. The effects of NO on EPSPs and IPSPs persisted in cells where postsynaptic sGC was blocked by ODQ and therefore were presynaptic, owing to a direct modulation of transmitter release combined with depolarization of presynaptic neurones. Therefore, while lower concentrations of NO may be important for fine tuning of glutamatergic transmission, higher concentrations are required to directly engage GABAergic inhibition. This differential sensitivity of excitatory and inhibitory connections to NO may be important for determining the specificity of the effects of this freely diffusible gaseous messenger.

Endothelial NO Synthase Activity in Nucleus Tractus Solitarii Contributes to Hypertension in Spontaneously Hypertensive Rats

NO is implicated as a major modulator of central nervous circuits regulating cardiovascular activity. Based on previous data, we hypothesized that overactivity of endothelial NO synthase (eNOS) within the nucleus tractus solitarii (NTS) could contribute to the hypertension in the spontaneously hypertensive rat (SHR). Using real-time PCR, we found that endogenous eNOS mRNA was greater in the NTS of mature, but not juvenile prehypertensive SHRs compared with aged-matched Wistar Kyoto (WKY) rats. To test the functional significance of this, we chronically blocked eNOS activity in the NTS in the adult SHR by in vivo adenoviral-mediated gene transfer of a dominant-negative form of eNOS; data were compared with WKY rats. This resulted in a fall in arterial pressure in the SHR but not WKY rats. In both rat strains, cardiac baroreceptor reflex gain and the high-frequency spectral component of heart rate variability increased. Thus, endogenous eNOS activity in the NTS plays a major role in determining the set point of arterial pressure in the SHR and contributes to maintaining high arterial blood pressure in this animal model of human hypertension.

Mechanism of nitric oxide action on inhibitory GABAergic signaling within the nucleus tractus solitarii

The cellular mechanisms mediating nitric oxide (NO) modulation of the inhibitory transmission in the nucleus tractus solitarii (NTS) remain unclear, even though this could be extremely important for various physiological and pathological processes. Specifically, in the NTS NO-evoked glutamate and gamma-aminobutyric acid (GABA) release might contribute to pathological hypertension. In cultured rat brainstem slices, NTS GABAergic neurons were targeted using an adenoviral vector to express enhanced green fluorescent protein and studied with a combination of patch clamp and confocal microscopy. Low nanomolar concentrations of NO increased intracellular Ca2+ concentration ([Ca2+]i) in somata, dendrites, and putative axons of GABAergic neurons, with axons being the most sensitive compartment. This effect was cGMP mediated and not related to depolarization or indirect presynaptic effects on glutamatergic transmission. Blockade of the cyclic adenosine diphosphate ribose (cADPR)/ryanodine-sensitive stores but not the inositol triphosphate-sensitive stores, inhibited NO effect. Since cADPR/ryanodine-sensitive stores are implicated in the Ca2+-induced Ca2+ release, NO can be expected to potentiate GABA release. In support of this notion, a cADPR antagonist abolished the NO-induced potentiation of GABAergic inhibitory postsynaptic potentials in the NTS. Thus, the NO-cGMP-cADPR-Ca2+ pathway, previously described in sea urchin eggs, also operates in mammalian GABAergic neurons. Potentiation of GABA release by NO may have implications for numerous brain functions.

Influence of the selective neuronal NO synthase inhibitor ARL 17477 on nitrergic neurotransmission in porcine stomach

Selective neuronal NOS (nNOS) inhibitors have been developed for possible application in cerebral ischemia and neurodegenerative disorders. To investigate the degree of interference with peripheral nNOS, the influence of the selective nNOS inhibitor ARL 17477 was studied on electrically induced nitrergic relaxations in pig gastric fundus strips and on gastric fundic compliance in conscious pig. Circular muscle strips of porcine gastric fundus were electrically stimulated (10 s trains at 4 Hz, 0.1 ms and 40 V). ARL 17477 inhibited the electrically induced relaxations in a concentration-dependent way (3x10(-6) M-10(-4) M). The inhibitory effect of ARL 17477 developed more progressively than that of N(G)-nitro-L-arginine methyl ester (L-NAME; 3x10(-4) M). In conscious pigs, instrumented with a fundic cannula, L-NAME (20 mg/kg i.v.) significantly increased mean arterial blood pressure and decreased fundic compliance in the fasted state (71+/-13 ml/mm Hg versus 185+/-37 ml/mm Hg after saline; P<0.05). ARL 17477 (3 mg/kg, i.v.) did not influence blood pressure but influenced gastric fundic volume-pressure curves in a similar way as L-NAME. Plasma concentration analysis of ARL 17477 indicated a half-life of less than 30 min in pig. ARL 17477 thus inhibits the effect of nitrergic neurons in the pig gastric fundus in vitro, leading to inhibited gastric compliance in the conscious pig. The study indicates that selective nNOS inhibitors, applied for cerebral disorders, might also interfere with neuronal nitrergic regulation of gastrointestinal motility.

Antisense oligos to neuronal nitric oxide synthase aggravate motoneuron death induced by spinal root avulsion in adult rat

The present study used nitric oxide synthase (nNOS) antisense oligos (nNOS AS-ODN) to assess the role of nNOS in motoneuron death induced by spinal root avulsion. A right seventh cervical (C7) spinal root avulsion was performed on adult male Sprague-Dawley rats. Two weeks later, FITC-labeled random oligos (FITC-R-ODN), nNOS AS-ODN, R-ODN or TE buffer was applied to the lesioned side of the C7 spinal segment and refreshed every 3 days. FITC-R-ODN was first detected inside the injured motoneurons at 10 h, accumulated to a maximum by 24 h and faded out from 72 h. Following avulsion, nNOS AS-ODN decreased the number of nNOS-positive motoneurons in the lesioned segment compared either with buffer (P < 0.001 at 15 days, 3 and 4 weeks post-injury) or with R-ODN control (P = 0.002 at 15 days, P < 0.001 at 3 and 4 weeks post-injury). Interestingly, nNOS AS-ODN also decreased the number of surviving motoneurons compared either with buffer (P = 0.005 at 15 days, P < 0.001 at 3 or 4 weeks) or with R-ODN control (P < 0.001 at 3 or 4 weeks). Meanwhile, there were no significant differences between R-ODN and buffer control either in the number of nNOS-positive motoneurons (P = 0.245 at 15 days, P = 0.089 at 3 weeks and P = 0.162 at 4 weeks) or in the number of surviving motoneurons (P = 0.426 at 15 days, P = 0.321 at 3 weeks or P = 0.344 at 4 weeks). These findings indicate that nNOS AS-ODN, applied from 2 weeks after avulsion, aggravates the motoneuron death due to root avulsion by specifically down-regulating nNOS gene expression and that the expression of nNOS in adult spinal motoneurons in response to root avulsion may play a beneficial role in the survival of injured neurons.

Nitric oxide modulation of glutamatergic, baroreflex, and cardiopulmonary transmission in the nucleus of the solitary tract

The neuromodulatory effect of NO on glutamatergic transmission has been studied in several brain areas. Our previous single-cell studies suggested that NO facilitates glutamatergic transmission in the nucleus of the solitary tract (NTS). In this study, we examined the effect of the nitric oxide synthase (NOS) inhibitor NG-nitro-l-arginine methyl ester (l-NAME) on glutamatergic and reflex transmission in the NTS. We measured mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA) from Inactin-anesthetized Sprague-Dawley rats. Bilateral microinjections of l-NAME (10 nmol/100 nl) into the NTS did not cause significant changes in basal MAP, HR, or RSNA. Unilateral microinjection of ( RS)-α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA, 1 pmol/100 nl) into the NTS decreased MAP and RSNA. Fifteen minutes after l-NAME microinjections, AMPA-evoked cardiovascular changes were significantly reduced. N-methyl-d-aspartate (NMDA, 0.5 pmol/100 nl) microinjection into the NTS decreased MAP, HR, and RSNA. NMDA-evoked falls in MAP, HR, and RSNA were significantly reduced 30 min after l-NAME. To examine baroreceptor and cardiopulmonary reflex function, l-NAME was microinjected at multiple sites within the rostro-caudal extent of the NTS. Baroreflex function was tested with phenylephrine (PE, 25 μg iv) before and after l-NAME. Five minutes after l-NAME the decrease in RSNA caused by PE was significantly reduced. To examine cardiopulmonary reflex function, phenylbiguanide (PBG, 8 μg/kg) was injected into the right atrium. PBG-evoked hypotension, bradycardia, and RSNA reduction were significantly attenuated 5 min after l-NAME. Our results indicate that inhibition of NOS within the NTS attenuates baro- and cardiopulmonary reflexes, suggesting that NO plays a physiologically significant neuromodulatory role in cardiovascular regulation.

Hypotensive effects of neuromedin U microinjected into the cardiovascular-related region of the rat nucleus tractus solitarius

Neuromedin U (NMU) is a brain-gut peptide with potent contractile effects on the uterus and smooth muscle. Intracerebroventricular injection of NMU reportedly decreased food intake and body-weight gain in the rat. We evaluated the effects of NMU delivered by microinjection into the rat nucleus tractus solitarius (NTS) on cardiovascular responses. At the concentrations used (5, 10 or 50 pmol), the intra-NTS injection of NMU in artificial cerebrospinal fluid produced a significant reduction in both the mean arterial pressure and heart rate. The hypotensive responses were dose dependent. Our findings suggest that NMU may act as a neurotransmitter or neuromodulative substance that causes excitation of neurons in the NTS and that it may play a role in cardiovascular regulation in vivo.

Subcellular localization of neuronal nitric oxide synthase in the rat nucleus of the solitary tract in relation to vagal afferent inputs

In the nucleus of the solitary tract (NTS), nitric oxide (NO) modulates neuronal circuits controlling autonomic functions. A proposed source of this NO is via nitric oxide synthase (NOS) present in vagal afferent fibre terminals, which convey visceral afferent information to the NTS. Here, we first determined with electron microscopy that neuronal NOS (nNOS) is present in both presynaptic and postsynaptic structures in the NTS. To examine the relationship of nNOS to vagal afferent fibres the anterograde tracer biotinylated dextran amine was injected into the nodose ganglion and detected in brainstem sections using peroxidase-based methods. nNOS was subsequently visualised using a pre-embedding immunogold procedure. Ultrastructural examination revealed nNOS immunoreactivity in dendrites receiving vagal afferent input. However, although nNOS-immunoreactive terminals were frequently evident in the NTS, none were vagal afferent in origin. Dual immunofluorescence also confirmed lack of co-localisation. Nevertheless, nNOS immunoreactivity was observed in vagal afferent neurone cell bodies of the nodose ganglion. To determine if these labelled cells in the nodose ganglion were indeed vagal afferent neurones nodose ganglion sections were immunostained following application of cholera toxin B subunit to the heart. Whilst some cardiac-innervating neurones were also nNOS immunoreactive, nNOS was never detected in the central terminals of these neurones. These data show that nNOS is present in the NTS in both pre- and postsynaptic structures. However, these presynaptic structures are unlikely to be of vagal afferent origin. The lack of nNOS in vagal afferent terminals in the NTS, yet the presence in some vagal afferent cell bodies, suggests it is selectively targeted to specific regions of the same neurones.

Nitric oxide in the gracile nucleus mediates depressor response to acupuncture (ST36)

The purpose of these studies was to determine the role of gracile nucleus and the effects of l-arginine-derived nitric oxide (NO) synthesis in the nucleus on the cardiovascular responses to electroacupuncture (EA) stimulation of "Zusanli" (ST36). Arterial blood pressure and heart rate were monitored during EA stimulation of ST36 following microinjections of agents into gracile nucleus. EA ST36 produced depressor and bradycardiac responses in anesthetized Sprague-Dawley rats. The cardiovascular responses to EA ST36 were blocked by bilateral microinjection of lidocaine into gracile nucleus. Microinjection of L-arginine into gracile nucleus facilitated the hypotensive and bradycardiac responses to EA ST36. The cardiovascular responses to EA ST36 were attenuated by bilateral microinjection of neuronal NO synthase (nNOS) antisense oligos into gracile nucleus. Microinjection of nNOS sense oligos into gracile nucleus did not alter the cardiovascular response to EA ST36. The results demonstrate that a blockade of neuronal conduction in the gracile nucleus inhibits the cardiovascular responses to EA ST36. The hypotensive and bradycardiac responses to EA ST36 are modified by influences of L-arginine-derived NO synthesis in the gracile nucleus. We conclude that NO plays an important role in mediating the cardiovascular responses to EA ST36 through gracile nucleus.

Effects of L-arginine-derived nitric oxide synthesis on cardiovascular responses to stimulus-evoked somatosympathetic reflexes in the gracile nucleus

The purpose of these studies was to determine the role of gracile nucleus (Gr) and the effects of L-arginine-derived nitric oxide (NO) synthesis in the nucleus on the cardiovascular responses to somatosympathetic reflexes (SSR). Electrical stimulation of sural and tibial nerve to evoke excitatory and inhibitory SSR was carried out in anesthetized Sprague-Dawley rats. Arterial blood pressure and heart rate were monitored during stimulus-evoked SSR following microinjections of the agents into Gr. Cardiovascular responses to electrical stimulation of the sural and tibial nerves were blocked by microinjection of lidocaine into Gr. The hypertensive and tachycardiac responses to stimulation of the sural nerve were attenuated by bilateral microinjection of L-arginine into Gr, but enhanced by the presence of nNOS antisense oligodeoxynucleotides (oligos) in the area. Microinjection of L-arginine into Gr facilitated the hypotensive and bradycardic responses to stimulation of the tibial nerve while pretreatment with nNOS antisense oligos into Gr attenuated the tibial stimulation evoked inhibitory SSR. The stimulus-evoked responses were not altered by microinjection of nNOS sense oligos into Gr. The results show that the cardiovascular responses to stimulus-evoked SSR were inhibited by the presence of a blockade of neuronal conduction in the Gr. L-Arginine-derived NO synthesis in the Gr attenuates the cardiovascular responses to stimulus-evoked excitatory SSR and facilitates the responses to inhibitory SSR. We conclude that NO in the Gr plays an inhibitory role in the central cardiovascular control through SSR regulation.

Functional NOS 1 in the rat mesenteric arterial bed

Previously we have demonstrated functional nitric oxide synthase (NOS) 1 in large arteries. Because resistance arteries largely determine blood pressure, this study examined whether functional NOS 1 also exists in resistance arteries. Phenylephrine (PE) contraction was measured in the absence and presence of the NOS 1 inhibitor N(5)-(1-imino-3-butenyl)-L-ornithine (VNIO) in isolated mesenteric resistance arteries (endothelium intact and denuded) from Sprague-Dawley rats. For NOS 1 activity and expression, the mesenteric arterial bed was separated into cytosolic and particulate fractions. NOS activity was assayed by measuring the conversion of [(3)H]arginine to [(3)H]citrulline inhibited by a nonselective NOS inhibitor or VNIO. VNIO increased PE sensitivity in endothelium-intact and -denuded arteries. In cytosolic and particulate fractions of the arterial bed, approximately 40% of NOS activity was inhibited by VNIO. Immunoprecipitation and Western blot analysis revealed two NOS 1 immunoreactive bands. One band corresponded to the rat brain isoform, whereas the second was of a slightly lower molecular mass. The cytosolic fraction contained both isoforms; however, the particulate fraction had only the lower molecular mass form. These studies demonstrate the existence of functional NOS 1 in resistance arteries.

Involvement of nitric oxide production via kynurenic acid-sensitive glutamate receptors in DOPA-induced depressor responses in the nucleus tractus solitarii of anesthetized rats

We have proposed the hypothesis that L-3,4-dihydroxyphenylalanine (DOPA) plays a role of neurotransmitter of the primary baroreceptor afferents terminating in the nucleus tractus solitarii (NTS). In the present study, we tried to clarify whether glutamate receptors and/or nitric oxide (NO), important modulators for central cardiovascular regulation, are involved in the DOPA-induced cardiovascular responses in the nucleus. Male Wistar rats were anesthetized with urethane and artificially ventilated. Compounds or antisense oligos (17-mer) for neuronal NO synthase were microinjected into depressor sites of the unilateral nucleus. DOPA 30-300 pmol microinjected into the nucleus dose-dependently induced depressor and bradycardic responses. Prior injection of kynurenic acid (600 pmol) suppressed DOPA (300 pmol)-induced responses by approximately 80%. Prior injection of N(G)-monomethyl-L-arginine 100 nmol, a potent NO synthase inhibitor, reversibly attenuated by approximately 90% DOPA-induced responses, while the D-isomer 100 nmol produced no effect. Furthermore, prior injection of neuronal NO synthase antisense oligos (20 pmol) reversibly reduced by approximately 70% responses to DOPA. Sense or scrambled oligos produced no effect. A NO precursor L-arginine (30 nmol) induced depressor and bradycardic responses, but these responses were not affected by kynurenic acid. These results suggest important roles for glutamate receptors and NO in DOPA induced-depressor and bradycardic responses in the NTS.

Vaginocervical stimulation-induced release of classical neurotransmitters and nitric oxide in the nucleus of the solitary tract varies as a function of the oestrus cycle

The effects of vaginocervical stimulation (VCS) on glutamate (GLU), aspartate (ASP), gamma-aminobutyric acid (GABA), noradrenaline (NA), arginine (ARG) and nitric oxide (NO) (citrulline) release in the nucleus of the solitary tract (nTS) were measured in anaesthetised female rats as a function of the oestrus cycle. During pro-oestrus/oestrus (P/E), but not during met-oestrus/di-oestrus (M/D), VCS significantly increased concentrations of NA, ASP, GLU, NO (citrulline) and GABA, but not ARG. Basal NA concentrations were also increased in P/E. These effects were prevented by bilateral section of either the vagus nerve or pelvic and hypogastric nerves. Vagotomy also significantly decreased basal NO concentrations in M/D and P/E while pelvic and hypogastric nerve section significantly increased GABA concentrations. Our results therefore confirm that the nTS is a relay structure for the visceral afferents sending information from the uterus into the central nervous system. The ability of VCS to trigger classical transmitter release and NO in the female is influenced by the stage of the oestrous cycle and is routed both via the vagus and pelvic/hypogastric nerves.

Adenoviral vector demonstrates that angiotensin II-induced depression of the cardiac baroreflex is mediated by endothelial nitric oxide synthase in the nucleus tractus solitarii of the rat

Angiotensin II (ANGII) acting on ANGII type 1 (AT1) receptors in the solitary tract nucleus (NTS) depresses the baroreflex. Since ANGII stimulates the release of nitric oxide (NO), we tested whether the ANGII-mediated depression of the baroreflex in the NTS depended on NO release. In a working heart-brainstem preparation (WHBP) of rat NTS microinjection of either ANGII (500 fmol) or a NO donor (diethylamine nonoate, 500 pmol) both depressed baroreflex gain by -56 and -67 %, respectively (P < 0.01). In contrast, whilst ANGII potentiated the peripheral chemoreflex, the NO donor was without effect. NTS microinjection of non-selective NO synthase (NOS) inhibitors (L-NAME; 50 pmol) or (L-NMMA; 200 pmol) prevented the ANGII-induced baroreflex attenuation (P > 0.1). In contrast, a neurone-specific NOS inhibitor, TRIM (50 pmol), was without effect. Using an adenoviral vector, a dominant negative mutant of endothelial NOS (TeNOS) was expressed bilaterally in the NTS. Expression of TeNOS affected neither baseline cardiovascular parameters nor baroreflex sensitivity. However, ANGII microinjected into the transfected region failed to affect the baroreflex.Immunostaining revealed that eNOS-positive neurones were more numerous than those labelled for AT1 receptors. Neurones double labelled for both AT1 receptors and eNOS comprised 23 +/- 5.4 % of the eNOS-positive cells and 57 +/- 9.2 % of the AT1 receptor-positive cells. Endothelial cells were also double labelled for eNOS and AT1 receptors. We suggest that ANGII activates eNOS located in either neurones and/or endothelial cells to release NO, which acts selectively to depress the baroreflex.

Central control mechanisms of circulation in the medulla oblongata by nitric oxide

Nitric oxide (NO) is involved in numerous physiological functions. Besides its role as an endothelium-dependent relaxing factor (EDRF), NO inhibits platelet aggregation, contributes to cytotoxicity against bacteria, is active in synaptic transmission within the brain, etc. NO synthase (NOS) is distributed in brain regions related to the regulation of cardiovascular functions. NO has been inferred not only to act directly on vascular vessels, but also to regulate circulation within the brain. In this review paper, we mainly consider the functions of NO in the cardiovascular center of the medulla oblongata. That is, we describe the anatomical distribution of NOS in the brain, effects of intravenous and intracerebroventricular administration of NOS inhibitors on the circulation, effects of microinjection of NO donors and NOS inhibitors into the nucleus tractus solitarius (NTS) and ventrolateral medulla (VLM), the results of electrophysiological studies on these areas, and finally, the data obtained by new molecular biological techniques.