Phasic cardiovascular responses to mevinphos are mediated through differential activation of cGMP/PKG cascade and peroxynitrite via nitric oxide generated in the rat rostral ventrolateral medulla by NOS I and II isoforms

The organophosphate insecticide mevinphos (Mev) acts on the rostral ventrolateral medulla (RVLM), where sympathetic vasomotor tone originates, to elicit phasic cardiovascular responses via nitric oxide (NO) generated by NO synthase (NOS) I and II. We evaluated the contribution of soluble guanylyl cyclase (sGC)/cyclic guanosine monophosphate (cGMP)/protein kinase G (PKG) cascade and peroxynitrite in this process. PKG expression in ventrolateral medulla of Sprague-Dawley rats manifested an increase during the sympathoexcitatory phase (Phase I) of cardiovascular responses induced by microinjection of Mev bilaterally into the RVLM that was antagonized by co-administration of 7-nitroindazole or Nomega-propyl-L-arginine, two selective NOS I inhibitors or 1-H-[1,2,4]oxadiaolo[4,3-a]quinoxalin-1-one (ODQ), a selective sGC antagonist. Co-microinjection of ODQ or two PKG inhibitors, KT5823 or Rp-8-Br-cGMPS, also blunted the Mev-elicited sympathoexcitatory effects. However, the increase in nitrotyrosine, a marker for peroxynitrite, and the sympathoinhibitory circulatory actions during Phase II Mev intoxication were antagonized by co-administration of S-methylisothiourea, a selective NOS II inhibitor, Mn(III)-tetrakis-(4-benzoic acid) porphyrin, a superoxide dismutase mimetic, 5,10,15,20-tetrakis-N-methyl-4'-pyridyl)-porphyrinato iron (III), a peroxynitrite decomposition catalyst, or L-cysteine, a peroxynitrite scavenger. We conclude that sGC/cGMP/PKG cascade and peroxynitrite formation may participate in Mev-induced phasic cardiovascular responses as signals downstream to NO generated respectively by NOS I and II in the RVLM.     

Differential engagements of glutamate and GABA receptors in cardiovascular actions of endogenous nNOS or iNOS at rostral ventrolateral medulla of rats

1. We evaluated in Sprague-Dawley rats anaesthetized with propofol the engagement of soluble guanylyl cyclase (sGC)/cGMP cascade, glutamatergic and GABAergic neurotransmission in the cardiovascular actions of endogenous nitric oxide (NO) at the rostral ventrolateral medulla (RVLM). 2. Microinjection bilaterally into the RVLM of a selective iNOS inhibitor, S-methylisothiourea (SMT, 250 pmoles), or a selective nNOS inhibitor, 7-nitroindazole (7-NI, 5 pmoles), induced respectively an enhancement or a reduction in systemic arterial pressure, heart rate and power density of the vasomotor components in the spectrum of arterial blood pressure signals, our experimental index for sympathetic neurogenic vasomotor tone. 3. The cardiovascular actions of SMT or 7-NI in the RVLM were significantly antagonized by co-administration into the RVLM of the sGC inhibitor, 1H-[1,2,4]Oxadiazole[4,3-alpha]quinoxalin-1-one (ODQ, 250 or 500 pmoles). 4. The cardiovascular excitatory effects after blockade of endogenous iNOS activity were significantly attenuated when N-methyl-D-aspartate (NMDA) receptor antagonist, dizocilpine (20 or 50 pmoles), or non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (250 or 500 pmoles), was co-microinjected bilaterally into the RVLM. 5. On the other hand, the cardiovascular depressive responses to blockade of endogenous nNOS activity were significantly antagonized on co-administration of GABA(A) receptor antagonist, bicuculline methiodine (5 or 10 pmoles), but not GABA(B) receptor antagonist, 2-hydroxy saclofen (50 or 100 pmoles). 6. We conclude that the cardiovascular actions of endogenous NO in the RVLM engage the sGC/cGMP pathway. In addition, whereas NO derived from nNOS induced sympathoexcitation via both NMDA and non-NMDA receptors in the RVLM, NO generated by iNOS elicited sympathoinhibition via GABA(A) receptors.     

Differential contributions of NOS isoforms in the rostral ventrolateral medulla to cardiovascular responses associated with mevinphos intoxication in the rat

The organophosphate poison mevinphos (Mev) elicits cardiovascular responses via nitric oxide (NO) produced on activation of M2 muscarinic receptors (M2R) in the rostral ventrolateral medulla (RVLM), where sympathetic vasomotor tone originates. This study further evaluated the contribution of nitric oxide synthase (NOS) isoforms at the RVLM to this process, using adult Sprague-Dawley rats. Bilateral co-microinjection into the RVLM of the selective NOS I inhibitor (250 pmol), 7-nitroindazole or N(omega)-propyl-L-arginine antagonized the initial sympathoexcitatory cardiovascular responses to Mev (10 nmol). Co-administration of a selective NOS II inhibitor, N6-(1-iminoethyl)-L-lysine (250 or 500 pmol) further enhanced these cardiovascular responses and reversed the secondary sympathoinhibitory actions of Mev. A potent NOS III inhibitor, N5-(1-iminoethyl)-L-ornithine (46 or 92 nmol) was ineffective. We also found that M2R co-localized only with NOS I- or NOS II-immunoreactive RVLM neurons. Furthermore, only NOS I or II in the ventrolateral medulla exhibited an elevation in mRNA or protein levels during the sympathoexcitatory phase, with further up-regulated synthesis of NOS II during the sympathoinhibitory phase of Mev intoxication. We conclude that whereas NOS III is not engaged, NO produced by NOS I and II in the RVLM plays, respectively, a sympathoexcitatory and sympathoinhibitory role in the cardiovascular responses during Mev intoxication.     

Mitochondrial respiratory enzyme complexes in rostral ventrolateral medulla as cellular targets of nitric oxide and superoxide interaction in the antagonism of antihypertensive action of eNOS transgene

Overproduction of nitric oxide (NO) by gene transduction of endothelial NO synthase (eNOS) in rostral ventrolateral medulla (RVLM), which is responsible for maintenance of vasomotor tone, reduces arterial pressure in spontaneously hypertensive rats (SHR). This NO-induced vasodepression, however, is not sustained and is followed by rebound hypertension. Because superoxide anion (O(2)(*-)) level is increased and synthesis or activity of mitochondrial manganese superoxide dismutase (SOD2) is reduced in RVLM during hypertension, we hypothesized that an interaction between NO and O(2)(*-) in RVLM, using mitochondrial respiratory enzyme complexes (MRC) as the cellular target, contributes to those cardiovascular outcomes after eNOS gene transduction in SHR. The present study assessed this hypothesis using adenoviral vectors to overexpress eNOS (AdeNOS) and/or SOD2 (AdSOD2) in RVLM of SHR or normotensive Wistar-Kyoto (WKY) rats. Microinjection of AdeNOS bilaterally into RVLM elicited 35% depression of MRC-I enzyme activity and evoked 60% and 50% increase in O(2)(*-) and peroxynitrite level in RVLM of SHR, but not WKY rats, which was reversed by cotransduced AdSOD2 or treatment with peroxynitrite decomposition catalyst. Cotransduction of AdeNOS and AdSOD2 in RVLM of SHR elicited significantly greater decreases in arterial pressure and heart rate than those promoted by the individual transgene and prevented the AdeNOS-induced rebound hypertension. We conclude that an interactive action between NO and O(2)(*-) on MRC-I in RVLM via formation of peroxynitrite contributes to the unsustained hypotensive effects of NO after overexpression of eNOS in SHR. The mitochondria-derived O(2)(*-) also mediates the rebound hypertension induced by eNOS transgene in RVLM of SHR.     

Protein kinase C-dependent mitochondrial translocation of proapoptotic protein Bax on activation of inducible nitric-oxide synthase in rostral ventrolateral medulla mediates cardiovascular depression during experimental endotoxemia

Sympathetic premotor neurons for the maintenance of vasomotor tone are located in rostral ventrolateral medulla (RVLM). We demonstrated previously that overproduction of nitric oxide (NO) by inducible NO synthase (iNOS) in RVLM, leading to caspase 3-dependent apoptotic cell death, plays a pivotal role in cardiovascular depression during endotoxemia induced by intravenous administration of Escherichia coli lipopolysaccharide. The interposing intracellular events remain unknown. We evaluated the hypothesis that these events encompass protein kinase C (PKC) activation, which triggers activation and translocation of Bax that opens mitochondrial permeability transition pore by interacting with adenine nucleotide translocase (ANT) or voltage-dependent anion protein (VDAC), followed by cytosolic release of cytochrome c. In Sprague-Dawley rats, coimmunoprecipitation and Western blot analyses revealed sequential manifestations during endotoxemia of membrane-bound translocation of PKC, dissociation of cytosolic PKC/Bax complex, mitochondrial translocation of activated Bax, augmented Bax/ANT or Bax/VDAC association, elevated cytosolic cytochrome c and caspase 3, and DNA fragmentation in ventrolateral medulla. Microinjection of iNOS inhibitor into bilateral RVLM significantly retarded PKC and Bax activation. The induced association of translocated Bax with ANT or VDAC and the triggered mitochondrial apoptotic signaling cascade were blunted by blockade in RVLM of PKC, mitochondrial translocation of Bax, Bax channels, ANT, or caspase 3, alongside significant amelioration of cardiovascular depression. We conclude that formation of mitochondrial Bax/ANT or Bax/VDAC complex that initiates caspase 3-dependent apoptosis in the RVLM as a result of PKC-dependent mitochondrial translocation of activated Bax activated by iNOS-derived NO plays a pivotal role in the manifestation of endotoxin-induced cardiovascular depression.     

The baroreflex and beyond: control of sympathetic vasomotor tone by GABAergic neurons in the ventrolateral medulla

1. Barosensitive, bulbospinal neurons in the rostral ventrolateral medulla (RVLM), which provide the major tonic excitatory drive to sympathetic vasomotor neurons, are prominently inhibited by GABA. 2. A major source of the GABAergic inhibition to presympathetic RVLM neurons arises from an area immediately caudal to the RVLM, known as the caudal ventrolateral medulla (CVLM). 3. Arterial baroreceptor afferents projecting to the nucleus tractus solitarius (NTS) provide a major tonic excitatory input to GABAergic CVLM neurons. These CVLM cells are a critical component for baroreflex-mediated changes in presympathetic RVLM neuronal activity, sympathetic nerve activity (SNA) and arterial pressure (AP). 4. Some GABAergic CVLM neurons are tonically activated by inputs independent of arterial baroreceptors or the NTS, providing a GABAergic-mediated inhibition of the presympathetic RVLM neurons that is autonomous of baroreceptor inputs. 5. GABAergic CVLM neurons appear to play two distinct, yet important, roles in the regulation of sympathetic vasomotor tone and AP. They dampen immediate changes in AP via the baroreflex and tonically inhibit the activity of the presympathetic RVLM neurons by baroreceptor-independent mechanisms. This baroreceptor-independent, GABAergic inhibition of presympathetic RVLM neurons may play an important role in determining the long-term level of sympathetic vasomotor tone and AP.     

Nitric oxide modulates cardiovascular function in the rat by activating adenosine A2A receptors and inhibiting acetylcholine release in the rostral ventrolateral medulla

1. Nitric oxide (NO), a gas transmitter, modulates many physiological processes, including the central regulation of cardiovascular activity. However, the mechanisms underlying the regulation of cardiovascular activity remain relatively unexplored. In the present study, we hypothesized that central NO-dependent sympathetic inhibition is mediated by activation of adenosine A(2A) receptors (A(2A)R) and inhibition of acetylcholine (ACh) release in the rostral ventrolateral medulla (RVLM). 2. L-Arginine (L-Arg; an NO donor; 100 nmol/100 nL) was microinjected into the RVLM of male Sprague-Dawley rats and heart rate variability (HRV) was assessed as an index of cardiac sympathovagal balance. Following microinjection of L-Arg, decreases were seen in mean arterial pressure (MAP), heart rate (HR) and the ratio of the low- to high-frequency components (LF/HF) of HRV. Pretreatment of rats with SCH58261 (40 pmol/60 nL into the RVLM), a competitive antagonist of the A(2A) R, attenuated these effects. 3. Western blot analysis and ELISA revealed that adenosine and A(2A)R levels increased in the RVLM following L-Arg microinjection, whereas ACh and muscarinic M(1) receptor levels decreased significantly, in parallel with the cardiovascular responses to L-Arg microinjection. The decrease in ACh levels was abolished by SCH58261 pretreatment. 4. Microinjection of N(G)-nitro-L-arginine methyl ester (a non-selective inhibitor of NO synthase; 15 nmol/100 nL) into the RVLM significantly increased MAP, HR and sympathetic activity, as evidenced by HRV (LF, HF and the LF/HF ratio were all increased). 5. The results indicate that the central NO/NO synthase system in the RVLM may modulate cardiovascular activity by activating the A(2A)R, which subsequently inhibits activation of the muscarinic M(1) receptor.     

Sympathoinhibitory effects of clonidine are transmitted by the caudal ventrolateral medulla in cats

We examined mechanisms of the central sympathoinhibitory actions of systemically administered clonidine in anesthetized cats. To avoid influences of sympathetic chemo- and baroreflexes, the animals were deafferentated by cutting the carotid sinus and vagal nerves bilaterally. Intravenous (i.v.) injections of clonidine (25-250 nmol/kg) caused significant (50-90%) decreases in preganglionic sympathetic nerve activity (SNA) recorded from the white ramus of the third thoracic segment. Microinjections (500 nl) into the rostral ventrolateral medulla (RVLM) of clonidine at doses (50-500 pmol in 500 nl), which probably produced higher local concentrations than produced by systemic administration, caused only slight reductions of SNA and small decreases in arterial blood pressure (BP). Furthermore, sympathoinhibition and hypotension caused by intravenous clonidine was almost unaffected by prior microinjection of alpha2-receptor antagonist rauwolscine (500 pmol) into the RVLM. Microinjections of clonidine into the caudal ventrolateral medulla (CVLM), which provides important inhibitory input to the RVLM, had no significant effects. However, chemical lesions of the CVLM with kainate (5.0 nmol), effectively blocked the sympathoinhibitory effects of subsequently administered intravenous clonidine. The results suggest that the central sympathoinhibitory effects of therapeutically relevant doses of systemically administered clonidine may be primarily mediated by pathways that activate the CVLM rather than by direct actions within the RVLM.     

Differential control of cardiac and sympathetic vasomotor activity from the dorsomedial hypothalamus

1. The dorsomedial hypothalamus (DMH) plays a crucial role in mediating the cardiovascular responses to different stressors, including acute psychological stress and cold stress. Activation of neurons in the DMH evokes increases in arterial pressure and in the activity of sympathetic nerves innervating the heart, blood vessels and brown adipose tissue. The descending pathways from the DMH to the spinal sympathetic outflow include synapses with neurons in medullary nuclei and possibly other brain stem regions. 2. Recent studies from our and other laboratories have indicated that neurons in the rostral ventrolateral medulla (RVLM) and in the region of the raphe pallidus (RP) in the medulla are important components of the descending pathways that mediate the cardiovascular response to activation of the DMH. Neurons in the RP primarily mediate the sympathetic cardiac components of the DMH-evoked response, whereas the RVLM neurons primarily mediate the sympathetic vasomotor component. 3. Activation of DMH neurons not only increases heart rate and sympathetic vasomotor activity, but also resets the baroreceptor reflex such that it remains effective, without any decrease in sensitivity, over a higher operating range of arterial pressure. 4. Activation of 5-hydroxytryptamine 5-HT(1A) receptors in the medulla oblongata leads to a selective suppression of cardiac and sympathetic vasomotor components of the DMH-evoked response, but does not affect sympathetic reflex responses evoked from baroreceptors or chemoreceptors. Thus, central 5-HT(1A) receptors modulate cardiovascular responses evoked from the DMH in a highly potent but selective fashion.     

Role of AT1 receptors in the central control of sympathetic vasomotor function

1. In a number of species, high concentrations of angiotensin II (AngII) receptors have been found in the rostral ventrolateral medulla (RVLM) in the hindbrain, which is an important region involved in the modulation of sympathetic vasomotor tone. The present review describes studies in which the contribution of angiotensin receptors in the brainstem to cardiovascular regulation, in particular sympathetic vasomotor reflexes, has been examined in conscious and anaesthetized rabbits. 2. In conscious rabbits, fourth ventricular infusions of AngII produced dose-dependent pressor responses as doses 400 times less than equipressor intravenous doses. Chronic baroreceptor denervation increased the sensitivity to AngII by 1000-fold. Administration of prazosin i.v. blocked the pressor response, suggesting that the mechanism involved sympathetic vasoconstriction. 3. The pattern of haemodynamic changes in response to AngII injected into the fourth ventricle (4V) involved decreased total peripheral conductance and mesenteric conductance, but a rise in hindlimb conductance. Sinoaortic denervation changed the hindlimb fall in conductance to an increase, suggesting that muscle vasomotor pathways were particularly inhibited by baroreceptor feedback mechanisms. 4. In anaesthetized rabbits, infusion of AngII into the RVLM increased blood pressure and transiently increased resting renal sympathetic nerve activity. The renal sympathetic baroreflex curves were shifted to the right and the upper plateau of the sympathetic reflex increase was markedly increased. 5. The pressor actions of 4V AngII were blocked by administration of a peptide antagonist injected into the RVLM or by the angiotensin AT(1) antagonist losartan injected into the 4V. These results suggest that mainly AT(1) receptors are involved and that the RVLM is a likely candidate site for the modulation of the renal sympathetic baroreflex. 6. Losartan administration into the 4V in conscious rabbits increased resting renal sympathetic tone and enhanced renal sympathetic baroreflex and chemoreflexes. 7. Our studies suggest that there are sympathoexcitatory AT(1) receptors in the RVLM accessible to AngII from the cerebrospinal fluid. In addition, an AT(1) receptor pathway normally inhibits the sympathoexcitation produced by baroreceptor unloading or chemoreceptor activation. The effect of losartan suggests that there is greater tonic activity within the sympathoinhibitory pathways. These two actions suggest that angiotensin receptors in the brainstem modulate sympathetic responses to specific afferent inputs, thus forming part of a potentially important mechanism for the integration of characteristic autonomic response patterns.     

Role of angiotensin II receptors in the regulation of vasomotor neurons in the ventrolateral medulla

1. There is a high density of angiotensin type 1 (AT1) receptors in various brain regions involved in cardiovascular regulation. The present review will focus on the role of AT1 receptors in regulating the activity of sympathetic premotor neurons in the rostral part of the ventrolateral medulla (VLM), which are known to play a pivotal role in the tonic and phasic regulation of sympathetic vasomotor activity and arterial pressure. 2. Microinjection of angiotensin (Ang) II into the rostral VLM (RVLM) results in an increase in arterial pressure and sympathetic vasomotor activity. These effects are blocked by prior application of losartan, a selective AT1 receptor antagonist, indicating that they are mediated by AT1 receptors. However, microinjection of AngII into the RVLM has no detectable effect on respiratory activity, indicating that AT1 receptors are selectively or even exclusively associated with vasomotor neurons in this region. 3. Under normal conditions in anaesthetized animals, AT1 receptors do not appear to contribute significantly to the generation of resting tonic activity in RVLM sympathoexcitatory neurons. However, recent studies suggest that they contribute significantly to the tonic activity of these neurons under certain conditions, such as salt deprivation or heart failure, or in spontaneously hypertensive or genetically modified rats in which the endogenous levels of AngII are increased or in which AT1 receptors are upregulated. 4. Recent evidence also indicates that AT1 receptors play an important role in mediating phasic excitatory inputs to RVLM sympathoexcitatory neurons in response to activation of some neurons within the hypothalamic paraventricular nucleus. The physiological conditions that lead to activation of these AT1 receptor-mediated inputs are unknown. Further studies are also required to determine the cellular mechanisms of action of AngII in the RVLM and its interactions with other neurotransmitters in that region.     

CONTROL OF SYMPATHETIC OUTFLOWS BY THE HYPOTHALAMIC PARAVENTRICULAR NUCLEUS

1. The functional role of the paraventricular nucleus (PVN) has been examined by studying its connections with cardiovascular neurons in the medulla and spinal cord and its influence on activity in several sympathetic nerves. 2. Chemical stimulation of neurons within the PVN can elicit pressor responses and can excite reticulo-spinal vasomotor neurons in the rostral ventrolateral medulla (RVLM). 3. The PVN-RVLM excitation is blocked by kynurenic acid applied iontophoretically in the vicinity of RVLM-spinal neurons, suggesting this is a glutamate-dependent pathway. 4. Electrical stimulation of PVN neurons evoked action potentials in RVLM neurons after 27 ms with a small variability. 5. Anterograde and retrograde labelling of PVN and RVLM neurons revealed PVN terminals closely associated with RVLM-spinal neurons and showed that the PVN is connected to the spinal cord via three pathways. 6. Chemical activation of PVN neurons can produce a pattern of activation of cardiovascular neurons similar to that occurring in defence against plasma volume expansion. 7. It is concluded that the PVN connections with the RVLM and spinal cord are important to a role in defending against life-threatening disturbances.     

Contribution of cGMP but not peroxynitrite to negative feedback regulation of penile erection elicited by nitric oxide in the hippocampal formation of the rat

We established previously that nitric oxide (NO) in the hippocampal formation (HF) participates actively in negative feedback regulation of penile erection. This study further evaluated whether this process engaged soluble guanylyl cyclase (sGC)/cGMP cascade or peroxynitrite in the HF. Intracavernous pressure (ICP) recorded from the penis in adult, male Sprague-Dawley rats anesthetized with chloral hydrate was employed as our experimental index for penile erection. Microinjection bilaterally of a NO-independent sGC activator, YC-1 (0.1 or 1 nmol) or a cGMP analog, 8-Bromo-cGMP (0.1 or 1 nmol), into the HF elicited a significant reduction in baseline ICP. Bilateral application into the HF of equimolar doses (0.5 or 1 nmol) of a sGC inhibitor, LY83583 or a NO-sensitive sGC inhibitor, ODQ significantly antagonized the decrease in baseline ICP induced by co-administration of the NO precursor, L-arginine (5 nmol), along with significant enhancement of the magnitude of papaverine-induced elevation in ICP. In contrast, a peroxynitrite scavenger, L-cysteine (50 or 100 pmol), or an active peroxynitrite decomposition catalyst, 5,10,15,20-tetrakis-(N-methyl-4'-pyridyl)-porphyrinato iron (III) (10 or 50 pmol), was ineffective in both events. These results suggest that NO may participate in negative feedback regulation of penile erection by activating the sGC/cGMP cascade in the HF selectively.     

Role of nitric oxide on pressor mechanisms within the dorsomedial and rostral ventrolateral medulla in anaesthetized cats

1. The role of nitric oxide (NO) in central cardiovascular regulation and the correlation between NO and glutamate-induced mechanisms is not clear. Microinjection of glutamate (3 nmol/30 nL) into dorsomedial medulla (DM) and rostral ventrolateral medulla (RVLM) increased arterial blood pressure (BP) and sympathetic vertebral nerve activity (VNA). Thus, in the present study, we examined the modulation by NO of glutamate-induced pressor responses in the DM and RVLM of cats. 2. Histochemical methods using nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) as a marker to stain neurons containing NO synthase (NOS), showed positive findings of NOS in both the DM and RVLM. 3. Microinjection of N(G)-nitro-L-arginine methyl ester (L-NAME), a NOS inhibitor, into the DM or RVLM did not alter resting BP and VNA, but it did cause a dose-dependent attenuation of glutamate-induced pressor responses. Interestingly, the increase in NO levels that resulted from pretreatment with L-arginine (L-Arg) or sodium nitroprusside (SNP) did not alter resting BP and VNA, but still inhibited glutamate-induced pressor responses in the DM and RVLM in a dose-dependent manner. 4. We also examined whether NO modulated the pressor responses induced by activation of different excitatory amino acid receptors. N-Methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA) were used. Consistent with the results from the initial glutamate studies, we observed that not only L-NAME, but also L-Arg and SNP attenuated pressor responses induced by NMDA and AMPA. No difference was found between the effects of NO on NMDA- and AMPA-induced pressor responses. 5. To investigate the possibility of a loss of agonist selectivity, the effects of D-2-amino-5-phosphonovalerate (D-AP5) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) on AMPA and NMDA responses in the DM were examined. The results showed that CNQX did not alter NMDA-induced pressor responses, while D-AP5 failed to alter AMPA-induced responses. 6. Our results suggest that activation of the glutamate-induced pressor mechanism is regulated by changes in NO levels in the DM and RVLM. This implies that NO may play a permissive role to allow operation of the glutamate-activation mechanism.     

Proceedings of the Symposium ‘Angiotensin AT1 Receptors: From Molecular Physiology to Therapeutics’: ROLE OF AT1 RECEPTORS IN THE CENTRAL CONTROL OF SYMPATHETIC VASOMOTOR FUNCTION

• 1 In a number of species, high concentrations of angiotensin II (AngII) receptors have been found in the rostral ventrolateral medulla (RVLM) in the hindbrain, which is an important region involved in the modulation of sympathetic vasomotor tone. The present review describes studies in which the contribution of angiotensin receptors in the brainstem to cardiovascular regulation, in particular sympathetic vasomotor reflexes, has been examined in conscious and anaesthetized rabbits.
• 2 In conscious rabbits, fourth ventricular infusions of AngII produced dose-dependent pressor responses as doses 400 times less than equipressor intravenous doses. Chronic baroreceptor denervation increased the sensitivity to AngII by 1000-fold. Administration of prazosin i.v. blocked the pressor response, suggesting that the mechanism involved sympathetic vasoconstriction.
• 3 The pattern of haemodynamic changes in response to AngII injected into the fourth ventricle (4V) involved decreased total peripheral conductance and mesenteric conductance, but a rise in hindlimb conductance. Sinoaortic denervation changed the hindlimb fall in conductance to an increase, suggesting that muscle vasomotor pathways were particularly inhibited by baroreceptor feedback mechanisms.
• 4 In anaesthetized rabbits, infusion of AngII into the RVLM increased blood pressure and transiently increased resting renal sympathetic nerve activity. The renal sympathetic baroreflex curves were shifted to the right and the upper plateau of the sympathetic reflex increase was markedly increased.
• 5 The pressor actions of 4V AngII were blocked by administration of a peptide antagonist injected into the RVLM or by the angiotensin AT₁ antagonist losartan injected into the 4V. These results suggest that mainly AT₁ receptors are involved and that the RVLM is a likely candidate site for the modulation of the renal sympathetic baroreflex.
• 6 Losartan administration into the 4V in conscious rabbits increased resting renal sympathetic tone and enhanced renal sympathetic baroreflex and chemoreflexes.
• 7 Our studies suggest that there are sympathoexcitatory AT₁ receptors in the RVLM accessible to AngII from the cerebrospinal fluid. In addition, an AT₁ receptor pathway normally inhibits the sympathoexcitation produced by baroreceptor unloading or chemoreceptor activation. The effect of losartan suggests that there is greater tonic activity within the sympathoinhibitory pathways. These two actions suggest that angiotensin receptors in the brainstem modulate sympathetic responses to specific afferent inputs, thus forming part of a potentially important mechanism for the integration of characteristic autonomic response patterns.

     

RENAL SYMPATHETIC BAROREFLEX EFFECTS OF ANGIOTENSIN II INFUSIONS INTO THE ROSTRAL VENTROLATERAL MEDULLA OF THE RABBIT

1. The effects of local infusion of angiotensin II (AII) into the rostral ventrolateral medulla (RVLM) pressor area on the renal sympathetic baroreflex were compared with the excitatory amino acid glutamate in urethane anaesthetized rabbits with chronically implanted renal nerve electrodes. Baroreflex blood pressure-renal nerve activity curves were obtained by intravenous infusion of phenylephrine and nitroprusside before and after treatments. 2. Infusion of 4 pmol/min of All into the RVLM increased blood pressure by 12 ± 2 mmHg and transiently increased resting sympathetic nerve activity. The renal sympathetic baroreflex curves were shifted to the right. The upper plateau of the sympathetic reflex increased by 29 ± 8% (n= 6, P < 0.025). 3. Infusions of glutamate into the RVLM, at a dose which was equipressor to that of AII, also increased resting renal sympathetic nerve activity. In contrast to AII, this increase was maintained throughout the infusion. Glutamate shifted the reflex curve to the right and increased the upper plateau of the sympathetic reflex by 44 ± 5% without affecting the lower plateau. 4. These results support the suggestion that AII can act at the level of the RVLM pressor area to facilitate baroreflex control of renal sympathetic activity in a similar fashion to that produced by fourth ventricular administration. 5. Thus the RVLM is a likely candidate site for modulation of the renal sympathetic baroreflex. The similarity of the actions of AII to those of glutamate suggest that it may directly excite sympathetic vasomotor cells in this region.     

The cardiovascular actions of clonidine and neuropeptide-Y in the ventrolateral medulla of the rat.

1. The cardiovascular responses to neuropeptide-Y (NPY) (25 and 50 pmol) and clonidine (10 and 20 nmol) were examined following microinjection into the rostral ventrolateral medulla (RVLM) and the caudal ventrolateral medulla (CVLM). Mean arterial pressure (MAP) and heart rate (HR) were measured in anaesthetized rats, pre- and post-injection. 2. The alpha 2-adrenoceptor agonist clonidine (10 and 20 nmol) reduced MAP and HR significantly when microinjected into the CVLM and RVLM. 3. NPY (25 and 50 pmol) microinjected into the CVLM decreased MAP and HR. However, in the RVLM neither dose had a significant cardiovascular effect. 4. The possibility of a functional interaction between the adrenergic system and NPY was examined by co-administration of clonidine and NPY in doses that gave submaximal blood pressure responses. In the CVLM this produced hypotension and bradycardia which was similar in magnitude to the sum of their individual responses, indicating that in this area their actions appear to be independent. 5. In the RVLM, where NPY has no significant cardiovascular effects, co-administration with clonidine, did not alter the response to clonidine. 6. It appears that in the areas investigated, there is no functional interaction between NPY and clonidine.