Difference in sensitivity of cardiovascular and respiratory control neurons in the subretrofacial nucleus to glutamate receptor subtype agonists in SHR, WKY and cats

Roles of cocaine- and amphetamine-regulated transcript peptide in the rostral ventrolateral medulla in cardiovascular regulation in rats

Cocaine- and amphetamine-regulated transcript peptide (CARTp) is present in neurons and varicose fibers in the rostral ventrolateral medulla (RVLM) that is crucial in the control of cardiovascular function. Prior research indicated that intracisternal administration of CARTp evokes hypertension and accumulation of Fos in the RVLM. Despite the interaction among CARTp, cardiovascular effect, and the RVLM, no studies have directly examined whether CARTp participates in cardiovascular regulation in the RVLM. The current study directly examined the modulation of blood pressure and baroreflex sensitivity by CARTp in the RVLM in the different strain of rats. Immunohistochemical study showed that CARTp immunoreactive (CART-IR) cell bodies and varicose CART-IR fibers were observed throughout the RVLM in the SD, WKY, and SHRs. Varicose CART-IR nerve fibers were particularly abundant in the WKY and SHRs. Bilateral microinjection of CARTp (30 pmol) into the RVLM caused a significant increase in mean arterial pressure (MAP) in WKY and SHRs. Bilateral microinjection of CARTp antibody (1:5000) into the RVLM displayed a fall in the basal level of the MAP in SHRs but had no effects in WKY rats. In SD rats, bilateral microinjection of CARTp (6, 30 or 60 pmol) into the RVLM did not change the MAP but attenuated phenylephrine-induced bradycardia in a dose-dependent manner. We propose that CARTp acting in the RVLM may involvement in the cardiovascular regulation either by increases in the blood pressure or by decreases in the baroreflex sensitivity in rats. Moreover, endogenous CARTp in the RVLM is associated with the maintenance of basal blood pressure of SHRs.

Age-related loss of cardiac vagal preganglionic neurones in spontaneously hypertensive rats

Despite the findings that impaired vagal control of the heart rate occurs in human hypertension, leading to greater cardiovascular risk, the mechanism of this impairment is as yet unknown. Observations in humans and experiments in the spontaneously hypertensive rat (SHR) suggested that such impairment may be related to an anomaly in central vagal neurones. We therefore set out to determine whether the numbers and distribution of cardiac-projecting vagal preganglionic neurones in the medulla of adult (12 week) hypertensive SHR are different from those in young (4 week) prehypertensive SHR and in age-matched Wistar-Kyoto (WKY) rats of two age groups. The number of vagal neurones, identified by labelling with the fluorescent tracer DiI applied to the heart, was essentially similar in the three areas of the medulla analysed (dorsal vagal nucleus, nucleus ambiguus and intermediate reticular zone) in young SHR and young or adult WKY rats. In contrast, fewer vagal neurones were labelled in adult SHR compared with young SHR or WKY rats. This difference was due to highly significant reductions in vagal neurones in the dorsal vagal nucleus and nucleus ambiguus on the right side of the medulla. These observations suggest that a loss of parasympathetic preganglionic neurones supplying the heart with axons in the right vagus nerve, or a remodelling of their cardiac projections, may explain the known impairment of the baroreceptor reflex gain controlling heart rate in hypertension.

ROLE OF THE CENTRAL NUCLEUS OF THE AMYGDALA IN THE CONTROL OF BLOOD PRESSURE: DESCENDING PATHWAYS TO MEDULLARY CARDIOVASCULAR NUCLEI

1. One of the key areas that links psychologically induced stress with the blood pressure-regulatory system is the central nucleus of the amygdala (CeA). This is an integratory forebrain nucleus that receives input from higher centres in the forebrain and has extensive connections with the hypothalamus and the medulla oblongata, areas involved in the regulation of the cardiovascular reflexes. 2. Based on studies using electrical or chemical stimulation or electrolytic lesions of the CeA, it has become clear that the CeA plays an important role in the regulation of blood pressure in response to stressful or fearful stimuli. 3. Two important medullary areas known to receive projections from the CeA are the nucleus tractus solitarius (NTS) and the rostral ventrolateral medulla (RVLM). The NTS is the site of the first synapse for afferent fibres originating from baroreceptors, chemoreceptors and the heart, whereas the RVLM contains neurons that maintain resting blood pressure and sympathetic nerve activity via projections to sympathetic preganglionic neurons in the intermediolateral cell column of the thoracolumbar spinal cord. 4. Electron microscopic studies using combined anterograde tracing and pre- and post-embedding immunogold labelling have shown that the pathways originating from the CeA to the NTS are inhibitory and may use GABA as a neurotransmitter. The results of these studies suggest that blood pressure changes produced by activation of the CeA may be mediated by attenuation of baroreceptor reflexes through a GABAergic mechanism at the level of the NTS. 5. Neuronal tract tracing combined with neurofunctional studies using the Fos protein as a marker of activated neurons indicate that the CeA projects directly to baroreceptive neurons in the NTS and RVLM that are activated by changes in blood pressure. 6. In conclusion, studies that have examined the efferent pathways of the CeA suggest that CeA neurons with projections to medullary baroreceptive neurons may play a vital role in the reflex changes in sympathetic nerve activity that are involved in blood pressure regulation in response to stress or anxiety.

Acute tolerance to ethanol inhibition of NMDA-induced responses in rat rostral ventrolateral medulla neurons

The present study was performed to examine the effects of acute ethanol exposure on N-methyl-D-aspartate (NMDA)-induced responses and the development of acute tolerance in rat rostral ventrolateral medulla (RVLM) in vivo and in vitro. Repeated microinjections of NMDA (0.14 nmol) into the RVLM every 30 min caused reproducible increases in mean arterial pressure in urethane-anesthetized rats weighing 325-350 g. Intravenous injections of ethanol (0.16 or 0.32 g, 1 ml) inhibited NMDA-induced pressor effects in a blood-concentration-dependent and reversible manner. The inhibitory effect of ethanol was reduced over time during continuous infusion of ethanol or on the second injection 3.5 h after prior injection of a higher dose of ethanol (0.32 g). A high dose of ethanol (0.32 g) had no significant effects on alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, gamma-aminobutyric acid and glycine-induced changes in blood pressure. In vitro studies showed that ethanol (10- 100 mM) dose-dependently inhibited inward currents elicited by pressure ejection of NMDA (10 mM) in RVLM neurons of neonatal brainstem slice preparations. When the superfusion time of ethanol (100 mM) was increased to 50 min, its inhibitory effect decreased gradually after 30-40 min in 60% of RVLM neurons examined. These data suggested that ethanol inhibition and subsequent tolerance development is associated with changed sensitivity to NMDA in the RVLM, which may play important roles in the ethanol regulation of cardiovascular function.

Increased expression of AMPA receptor subunits in the nucleus of the solitary tract in the spontaneously hypertensive rat

The expression of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subunits GluR1-4 in the nucleus of the solitary tract (NTS) of adult Wistar rats was examined by polymerase chain reaction (PCR), and the neuronal localisation of these receptor subunits in the NTS were confirmed by immunohistochemistry using subunit-specific antibodies. Semi-quantitative PCR was used to investigate differences in AMPA receptor subunit expression between spontaneously hypertensive rats (SH) and age-matched normotensive Wistar Kyoto rats (WKY). All four receptor subunits were expressed in both strains, but compared to WKY, total AMPA receptor and the GluR3 mRNA expressions were significantly higher in SH. No differences were detected in cDNA form the cerebral cortex or cerebellum. Immunolabelling for GluRs 1, 2 and 2/3 in the neuropil relative to neuronal somata in the cardioregulatory areas of the NTS appeared to be increased in SH, with an overall increase in the density of GluR2/3 labelling in the medial and commissural NTS of SH. These results indicate a possible role for changes in AMPA receptor subunit expression in NTS neurones, involving an increase in GluR3 associated with development of hypertension in SH.

Expression of metabotropic glutamate receptor 8 in autonomic cell groups of the medulla oblongata of the rat

Metabotropic glutamate receptors (mGluRs) in the medulla oblongata have been suggested to have a functional role in the regulation of cardiovascular baroreflexes. The present study examines the localization of mGluR8 autonomic nuclei of the medulla of the rat. mGluR8 immunoreactivity was observed in the cell bodies and/or processes of the dorsolateral, interstitial, medial, intermediate, ventral, ventrolateral, subpostremal, commissural, parvicellular and gelatinosus subnuclei of the nucleus tractus solitarius (NTS). The intensity of mGluR8 staining was highest in the commissural and interstitial subnuclei at the level of the area postrema. Commissural NTS is involved in regulation of baro-, and chemo-reflexes whereas the interstitial nucleus mediates respiratory reflexes. In the area postrema, diffuse staining was observed in the cell bodies, dendrites and fibers of the dorsal and central regions. In vagal outflow nuclei, mGluR8 immunoreactivity was observed in: (1). the cell bodies and processes of the dorsal motor nucleus of the vagus (DMN) throughout the rostro-caudal extent; and (2). the cell bodies and fibers throughout the rostro-caudal extent of the dorsal and ventral division the nucleus ambiguus (NA). Staining in the ventrolateral medulla was restricted to regions ventral to the nucleus ambiguus and dorsal to the lateral reticulate nucleus. The present study is the first to provide a detailed mapping of mGluR8 within the autonomic nuclei of the medulla and suggests that this subtype may be involved in shaping synaptic transmission in these central nuclei.

Magnesium decreases arterial pressure and inhibits cardiovascular responses induced by N-methyl-D-aspartate and metabotropic glutamate receptors stimulation in rostral ventrolateral medulla

OBJECTIVE

Magnesium sulfate (MgSO4) is widely used for the treatment of eclampsia. However, effects of Mg2+ in central cardiovascular regulation remain unclear. In the present study, the role of Mg2+ on cardiovascular regulation in the rostral ventrolateral medulla (RVLM) of rats was examined.

METHODS

Adult male Wistar rats were anesthetized with urethane, and artificially ventilated. The ventral surface of the medulla was exposed, and the RVLM was identified by microinjection (50 nl) of l-glutamate (l-Glu; 2 nmol). Then, MgSO4 (1, 3, 10 nmol, n = 7 for each dose) and magnesium chloride (MgCl2; 10 nmol, n = 7) were microinjected into the RVLM. l-Glu (2 nmol), N-methyl-D-aspartate (NMDA; 20 pmol), alpha-amino-3-hydroxy-5-methyl isoxazole-4-propionic acid (AMPA; 5 pmol) and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD, metabotropic glutamate receptor agonist; 1 nmol] were also microinjected with or without pretreatment of MgSO4 (10 nmol; n = 7 for each drug).

RESULTS

MgSO4 dose-dependently decreased mean arterial pressure (MAP) and heart rate (HR). The high dose of MgSO4 (10 nmol) significantly decreased MAP and HR (-25 +/- 4 mmHg and -43 +/- 6 bpm). Similarly, MgCl2 decreased MAP and HR (-27 +/- 4 mmHg and -30 +/- 6 bpm). The pressor response evoked by NMDA or (1S,3R)-ACPD was significantly attenuated by the pretreatment with MgSO4. In contrast, pressor response caused by l-Glu or AMPA was not affected by pretreatment with MgSO4.

CONCLUSIONS

These results suggest that Mg2+ has an inhibitory role on the RVLM neurons, and inhibits cardiovascular responses induced by NMDA and metabotropic glutamate receptor agonists.

Spontaneously hypertensive rats exhibit altered cardiovascular and neuronal responses to muscle contraction

We examined the cardiovascular and ventrolateral medullary neuronal responses to muscle contraction in the spontaneously hypertensive rat (SHR) and normotensive Wistar-Kyoto rat (WKY) control. Cardiovascular, respiratory and ventrolateral medullary neuronal responses to muscle contraction evoked by tibial nerve stimulation were recorded. SHRs exhibited significantly larger drops in arterial pressure compared to WKYs in response to muscle contraction (P < 0.05). Basal ventrolateral medulla neuronal discharge rates were similar between the SHR and the WKY groups. A majority of neurons recorded responded to muscle contraction in both the WKY (77 %; n = 53) and the SHR groups (68 %; n = 62). There was no difference in the percentage of neurons that responded with an increase (approximately 60 %) or decrease (approximately 40 %) in firing rate between hypertensive and normotensive rats. Pulse wave-triggered averaging techniques showed that most neurons that responded to muscle contraction also possessed a basal firing rhythm temporally related to the cardiac cycle (85 % in WKYs, 83 % in SHRs). However, decreases in neuronal firing rates in response to muscle contraction were significantly greater in SHRs than WKYs. Therefore, we conclude that muscle contraction unmasks a hyperexcitability of neurons in the ventrolateral medulla of SHRs that parallels the heightened blood pressure responses.

Integration of aortic nerve inputs in hypertensive rats

The integration of arterial baroreceptor afferent inputs was studied in renal wrap hypertensive (HT) and normotensive (NT) rats. In anesthetized and paralyzed rats, aortic nerve (AN)-evoked depressor responses were reduced in HT compared with NT rats (P<0.05). We tested the hypothesis that the attenuated baroreflex was associated with altered integration of baroreceptor inputs within the nucleus of the solitary tract. Based on onset latency and the ability of monosynaptic neurons (MSNs) to respond to each of 2 AN stimuli separated by 5 ms, cells in HT and NT rats were divided into 3 groups: short-latency MSNs (SLMSNs), long-latency MSNs (LLMSNs), and polysynaptic neurons (PSNs). A higher percentage of PSNs (73% versus 61%) and a lower percentage of SLMSNs (20% versus 27%) or LLMSNs (7% versus 12%) were found in HT rats (P<0.05). In addition, in HT compared with NT rats, the AN onset latency was greater in PSNs (29. 9+/-1.1 versus 26.7+/-0.8 ms) but not in SLMSNs (5.0+/-0.5 versus 5. 0+/-0.3 ms) or LLMSNs (22.9+/-1.2 versus 24.1+/-0.7 ms) (P<0.05). Finally, in HT compared with NT rats, the number of PSNs responding to a single AN stimulus with multiple action potentials was increased (40% versus 19%) (P<0.05). This was not observed in SLMSNs (26% versus 13%) or LLMSNs (12% versus 18%). The results indicate that renal wrap hypertension is associated with reduced AN-evoked depressor responses. There also were alterations in the integration of AN afferent inputs within the nucleus of the solitary tract, and these alterations were most marked in the PSN population.

Difference in topology and numbers of barosensitive catecholaminergic and cholinergic neurons in the medulla between SHR and WKY rats

We hypothesized that there may be a significant difference in the neuronal composition of the baroreceptor reflex pathway between normotensive Wistar Kyoto (WKY) and spontaneously hypertensive SHR rats. Using the double-immunoreactive (IR) method, the topology and numbers of barosensitive neurons that contain glutamate (Glu), glutamic acid decarboxylase (GAD), tyrosine hydroxylase (TH), phenylethanolamine N-methyltransferase (PNMT) and choline acetyltransferase (ChAT) were compared between the two strains. The control rats were sham-operated only for cannulation of the trachea and femoral artery/vein. The test rats were injected with the pressor agent phenylephrine to raise blood pressure and stimulate arterial baroreceptors. In both the control and test experiments, the c-Fos/Glu-, GAD-, TH- and PNMT-IR neurons were found in the nucleus tractus solitarii (NTS) and ventrolateral medulla (VLM), while the FosB/ChAT-IR neurons were found in the NTS, dorsal motor nucleus of the vagus (DMX) and nucleus ambiguus (AMB). In the control experiment, no significant difference in numbers was recognized in any of the double-IR neurons between the two strains. In the test experiment, the numbers of FosB/ChAT-IR neurons in the NTS, DMX and AMB were significantly smaller in SHR than in WKY. The numbers of c-Fos/TH-IR neurons in the caudal VLM were significantly larger in SHR than in WKY. These results suggest that a smaller number of barosensitive cholinergic neurons in the DMX and AMB in SHR causes the weaker baroreceptor-cardiac vagal reflex in SHR, and that a larger number of barosensitive catecholaminergic neurons in the caudal VLM in SHR are involved in the stronger baroreceptor-vasopressin reflex in SHR.

Reduced kynurenine aminotransferase-I activity in SHR rats may be due to lack of KAT-Ib activity

Kynurenine aminotransferase I (KAT-I), which also shows glutamine transaminase K (GTK) activity, catalyses the conversion of kynurenine to kynurenic acid, an endogenous glutamate antagonist. Both the GTK and KAT enzyme activities were found to be significantly reduced in kidney, brain and medulla oblongata homogenates of spontaneously hypertensive (SHR) compared to Wistar-Kyoto (WKY) rats. Enzyme activity stains on native gel separations of partially purified kidney homogenates was associated with two major bands of GTK (KAT-I)-activity in WKY and Wistar rats, KAT-Ia and KAT-Ib. SHR rats however showed only KAT-Ia activity. These findings suggest that the absence of KAT-Ib activity may result in a reduced ability to synthesise kynurenic acid in SHR rats, this may help to explain the enhanced sensitivity to glutamate seen in this strain.

Whole cell patch-clamp study of putative vasomotor neurons isolated from the rostral ventrolateral medulla

A distinct subpopulation of neurons in the rostral and ventrolateral part of the medulla oblongata (RVL) plays a key role in controlling sympathetic vasomotor tone. To characterize these neurons under conditions in which all cell-to-cell interactions are eliminated, RVL neurons were acutely dissociated from 13- to 19-day old rats. Cells projecting to the upper thoracic segments were retrogradely labeled with fluorescent beads. Fifty-two percent (17/33) of examined spinally projecting neurons were catecholaminergic, as demonstrated by single-cell reverse transcription-polymerase chain reaction or immunocytochemistry. No spontaneous (capacitive) spikes were revealed in the tight seal cell-attached configuration. Whole cell recordings were made from 54 spinally projecting neurons using Cs(+)- or K(+)-containing pipettes. No spontaneous firing was observed in current-clamp mode with K(+)-based pipettes (membrane potential, -61.5 +/- 2.3 mV). Step depolarizations (300- or 400-ms pulses, up to 100 pA) evoked regular firing or one to four spikes. Several voltage-gated currents, resembling the transient and persistent Na+, delayed rectifier and low- and high-threshold Ca2+, were revealed in voltage-clamp mode. These results show that isolated spinally projecting RVL neurons display no pacemaker-like activity. Because data from the literature indicate that these neurons are capable of generating such activity under different experimental conditions, the factors responsible for different behavior need to be determined. Dissociated RVL neurons provide a useful new model for studying biophysical and other properties of neurons involved in blood pressure control.

Immuno-localization of kynurenine aminotransferase (KAT) in the rat medulla and spinal cord

In the mammalian brain, kynurenine aminotransferase (KAT) is pivotal to the synthesis of kynurenic acid, a preferential antagonist at the strychnine-insensitive NMDA-glycine site. As NMDA receptors are involved in autonomic function, we have examined the immunohistochemical localization of KAT in the medulla and spinal cord of the rat. KAT immunoreactivity (KAT-li) was found throughout these areas, in both glia and neurons. Unlike the mainly astrocytic localization in forebrain structures, KAT-li was predominantly neuronal, notably in areas important for blood pressure and heart rate regulation: ventral medulla, nucleus ambiguus, nucleus of the solitary tract and intramediolateral cell column of the spinal cord. The presence of KAT in these nuclei supports a neuromodulatory role for kynurenic acid in NMDA-mediated autonomic function.

Role of spinal GABA receptors in depressor responses to chemical stimulation of the A5 area in normal and hypertensive rats

Chemical stimulation of neurons in the pontine A5 area by microinjection of L-glutamate lowers arterial blood pressure. The mechanism of this 'A5 depressor response' is not well-established. Here, we examine the involvement of spinal cord gamma-aminobutyric acid (GABA) receptors in this depressor response in normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). Experiments were conducted in male WKY and age-matched SHR anaesthetised with sodium pentobarbitone and chloral hydrate. An intrathecal catheter was implanted with the tip located between T9 and L2. Three days later, rats were re-anaesthetised and 10 nl of 40 mM L-glutamate was injected into the A5 area before, during and after, blockade of spinal cord GABA-A receptors by intrathecal injection of bicuculline methiodide (1 mM in 10 microliters phosphate-buffered saline). Injection of L-glutamate (10, 20, 40, 80 mM in 10 nl) produced depressor responses that were similar in WKY (n = 6) and SHR (n = 6). Intrathecal injection of bicuculline elicited a pressor response that was greater in SHR (n = 7, 28.5 +/- 7.6% increase in mean arterial pressure) than WKY (n = 11, 11.6 +/- 3.6%, p < 0.05). After bicuculline, the depressor response to injection of L-glutamate into the A5 area was eliminated in both WKY (n = 7) and SHR (n = 6). Intrathecal injection of vehicle had no effect on either resting arterial blood pressure or the depressor response to A5 stimulation. Basal blood pressure and control responses to A5 stimulation were fully restored by around 90 min after bicuculline injection in each animal. In separate groups of rats, intrathecal injection of muscimol elicited depressor responses that were greater in SHR (n = 6, -32.0 +/- 6.2%) than WKY (n = 6, -17.3 +/- 1.5%, p < 0.05). Our results suggest that the A5 depressor response is due to a projection from the A5 area to the spinal cord. This projection acts directly, or through a spinal interneuron and uses GABA as a neurotransmitter. Furthermore, our results indicate a hyper-responsiveness to GABA-A receptor stimulation in SHR since intrathecal bicuculline elicited much greater increases and intrathecal muscimol elicited much greater decreases, in blood pressure in SHR than in WKY. Finally, it seems likely that the A5-spinal depressor pathway is less effective in SHR than WKY under physiological conditions since chemical stimulation of the A5 area with L-glutamate produced a comparable depressor response in both strains.

Differences in the density of barosensitive neurons in the medulla of spontaneously hypertensive and Wistar-Kyoto rats

1. The density of barosensitive neurons in the medulla was examined in spontaneously hypertensive rats (SHR) and in normotensive Wistar-Kyoto (WKY) rats. In control experiments, rats were sham-operated, while in test experiments arterial baroreceptors were stimulated by pressor responses to i.v. administration of phenylephrine and the density of c-Fos-labelled neurons was immunocytologically examined. 2. In both control and test experiments, c-Fos-labelled neurons were distributed in cardiovascular control sites: the nucleus tractus solitarii (NTS) and the caudal and rostral ventrolateral medullas (CVLM/RVLM). 3. In both WKY rats and in SHR, the total density of labelled neurons in test experiments was significantly higher than in control experiments. 4. In control experiments, no significant difference was found in the distribution and density of labelled neurons in the NTS and in the CVLM/RVLM between rats and SHR. 5. In test experiments, no significant difference was found in the distribution and density of labelled neurons in the NTS between WKY rats and SHR. 6. In test experiments in SHR, the density of labelled neurons in the CVLM just caudal to the obex level was significantly higher than that in WKY rats, whereas the density of labelled neurons in WKY rats in the RVLM just rostral to the obex level was significantly higher than that in SHR. 7. These results indicate that stimulation of the arterial baroreceptor induces strain-specific differences in the density of barosensitive neurons in the CVLM/RVLM near the obex level.

Pressor and sympathetic responses to excitatory amino acids are not augmented in the ventrolateral medulla of Dahl salt-sensitive rats

We assessed the pressor and sympathetic responses to microinjection of excitatory amino acids (EAA) into the rostral ventrolateral medulla (RVLM) to see whether the response would be augmented in salt-induced hypertension. Seven-week-old Dahl-Iwai salt-sensitive rats were fed either a high- (8%, n = 10) or a low- (0.3%, n = 12) salt diet for 3 weeks. Then, L-glutamate (2 nmol), N-methyl-D-aspartate (NMDA; ionotropic EAA receptor agonist, 20 pmol) or (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD; metabotropic EAA receptor agonist, 1 nmol] was microinjected into the RVLM of urethane-anesthetized, artifically ventilated rats. The rats fed a high-salt diet showed a significantly (P < 0.001) higher mean arterial pressure (123 +/- 3 mmHg) than those fed a low-salt diet (99 +/- 2 mmHg). We found similar increases in mean arterial pressure and splanchnic sympathetic nerve activity elicited by microinjection of L-glutamate into the RVLM in the high- (33 +/- 2 mmHg and 52 +/- 10%) and low- (35 +/- 3 mmHg and 46 +/- 8%) salt groups. Similarly, pressor and sympathoexcitatory responses to either NMDA or (1S,3R)-ACPD did not differ between the groups. Microinjections of the lower doses of L-glutamate, NMDA and (1S,3R)-ACPD also showed comparable pressor responses between the groups. These results indicate that salt-induced hypertension in Dahl salt-sensitive rats is not associated with enhanced responsiveness of the RVLM to EAA. This is in contrast with our previous findings that pressor and sympathetic responses to EAA are enhanced in spontaneously hypertensive rats.

Involvement of non-NMDA and NMDA receptors in glutamate-induced pressor or depressor responses of the pons and medulla

1. Fifty-five intact and six baroreceptor denervated and vagotomized cats of either sex were anaesthetized intraperitoneally with urethane (400 mg/kg) and alpha-chloralose (40 mg/kg). Responses of the systemic arterial pressure (SAP), mean SAP (MSAP) and sympathetic vertebral nerve (VNA) and renal nerve activities (RNA) were recorded. 2. In intact animals, monosodium L-glutamate (Glu, 0.1 mol/L, 50 nL) was microinjected into pressor areas of the locus coeruleus (LC), gigantocellular tegmental field (GTF), rostral ventrolateral medulla (RVLM) and dorsomedial medulla (DM), and the depressor areas of caudal ventrolateral medulla (CVLM). The induced actions were compared before and after microinjection of either glutamate antagonists, glutamate diethylester (GDEE, 0.5 mol/L, 50-100 nL), a competitive AMPA receptor blocker, or 2-amino-5-phosphonovaleric acid (D-AP5, 0.025 mol/L, 50-100 nL), a competitive N-methyl-D-aspartate (NMDA) receptor blocker. GDEE completely blocked the increases of SAP and VNA elicited from all pressor areas. D-AP5 only partially blocked the pressor but slightly blocked VNA and RNA responses from LC, GTF and DM, particularly those from RVLM. Neither GDEE nor D-AP5 blocked the depressor responses of SAP and two nerve activities elicited from CVLM. 3. In baroreceptor denervated animals, NMDA (2 mmol/L, 50-100 nL) and AMPA (0.2 mmol/L, 50-100 nL) were micro-injected into the same pressor areas of GTF, RVLM and DM and the depressor area of CVLM responsive to Glu activation (0.1 mol/L, 30 nL). In RVLM, DM and CVLM, the results of either NMDA or AMPA were similar to those induced by Glu. However, in GTF, microinjection of either NMDA or AMPA did not induce similar responses to Glu. This suggests that the nature of GTF may differ from RVLM and DM. 4. The above results suggest that the Glu-induced pressor responses from LC, GTF, DM and especially RVLM, are primarily mediated through AMPA receptors. The Glu-induced depressor responses from CVLM may not be predominantly mediated by either AMPA or NMDA receptors. 5. In both baroreceptor-intact and -denervated cats stimulation of the pressor areas often produced an increase of VNA and a decrease of RNA, while in the depressor CVLM decreased both VNA and RNA. The VNA, but not RNA were positively correlated with the pressor responses, while both VNA and RNA were positively correlated with the depressor responses. This may suggest that neurons of the sympathetic vertebral and renal nerves are topographically organized in the brain.

Effects of Angiotensin II on the Spontaneous Activity of Rostral Ventrolateral Medullary Cardiovascular Neurons and Blood Pressure in Spontaneously Hypertensive Rats

The interactive role of rostral ventrolateral medulla (RVL) cardiovascular neurons and brain angiotensin II (Ang II) in regulating the arterial blood pressure was examined by recording simultaneously the spontaneous activity of these spinal projecting neurons and the arterial blood pressure in the pentobarbital-anesthetized spontaneously hypertensive rat (SHR) and its normotensive control, the Wistar Kyoto rat (WKY). It was found that Ang II elicited dose-dependent excitatory responses in a subpopulation of RVL cardiovascular neurons, followed by a subsequent increase in blood pressure. These effects of Ang II were significantly greater in SHR than in WKY. The effects were attenuated or abolished by co-administration of Ang II antagonist, [Sar(1), Ile(8)]-Ang II to RVL using bilateral microinjection attenuated the blood pressure effects of intracerebroventricularly administered Ang II by as much as 70%. These results indicated that spinal projecting RVL cardiovascular neurons are important in mediating the pressor action of Ang II. The enhanced sensitivity and responsiveness of RVL cardiovascular neurons to Ang II may be pertinent to the genesis of hypertension in adult SHR. Copyright 1996 S. Karger AG, Basel

Parapyramidal rostroventromedial medulla as a respiratory rhythm modulator

After inhalation of 15% CO2, immunoreactions to glutamate and glutamic acid decarboxylase were found in some c-Fos or c-Jun-labeled neurons distributed in the reticular region just dorsal to the pyramidal tract in the rostroventromedial medulla (parapyramidal RVMM). This region forms vertically the narrow strip between the nucleus raphe pallidus and nucleus parapyramidalis superficialis, and extends rostrocaudally from the level just ahead of the inferior olivary complex to the level just behind the nucleus of the trapezoid body. When we placed lesions with kainate in the parapyramidal RVMM, hyperpneic and tachypneic responses to brief inhalation of 15% CO2 were completely abolished, and the eupneic rhythm changed into the gasping rhythm. This study suggests that the parapyramidal RVMM consists of neuronal substrates that subserve as the respiratory rhythm modulator.

Cardiovascular effects of NMDA in the RVLM of spontaneously hypertensive rats

In this study we found that cardiovascular effects were differentially regulated by N-methyl-D-aspartate (NMDA) in the rostral ventral lateral medulla (RVLM) of spontaneously hypertensive rats (SHRs) compared to their normotensive controls (Wistar-Kyoto rats, WKYs). Adult SHRs and WKYs were anesthetized with urethane, cervically vagotomized, and placed in a sterotaxic frame. We found that electrical stimulation or local application of N-methyl-D-aspartate into the RVLM produced hypertension in both strains. Microinjection (3.5-4.0 nmol) of AP5 (2-amino-5-phosphono-valerate), an NMDA receptor antagonist, to the RVLM did not affect resting blood pressure; however, this agent antagonized hypertensive responses evoked by low-frequency electrical stimulation (5-20 Hz) in both strains. The elevation in blood pressure evoked by stimulation at a higher frequency (60 Hz) was not affected by AP5. These results suggest that NMDA receptors are involved in the low frequency, electrically evoked hypertension in both strains. We also found that SHRs had a larger pressor response to microinjection of NMDA and electrical stimulation than did WKYs. AP5 abolished the differences in evoked hypertension between WKYs and SHRs during low-frequency (5-10 Hz) electrical stimulation. These data suggest that the hypersensitivity of RVLM to low-frequency electrical stimulation in SHRs involve NMDA receptors. We previously reported that AP7 antagonizes NMDA and carotid clamping-induced hypertension. In this study, we found that when locally applied to RVLM, AP5 antagonized hypertension evoked by clamping the carotid arteries in SHRs and WKYs. Thus, carotid clamping-induced hypertension may also involve NMDA receptors in the RVLM.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of metabotropic glutamate receptors in ventrolateral medulla of hypertensive rats

Evidence is accumulating for the role of metabotropic, as well as ionotropic, glutamate receptors in cardiovascular regulation. We sought to determine whether stimulation of metabotropic glutamate receptors in the rostral ventrolateral medulla would evoke enhanced cardiovascular responses in spontaneously hypertensive rats (SHR). Thus, we microinjected (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD], a selective agonist of metabotropic glutamate receptors, into the rostral ventrolateral medulla of urethane-anesthetized adult SHR and age-matched Wistar-Kyoto (WKY) rats. Microinjection of (1S,3R)-ACPD (1 nmol/50 nL) produced increases in mean arterial pressure and splanchnic sympathetic nerve activity in SHR (+41 +/- 6 mm Hg and +34 +/- 4%, respectively) that were significantly greater than those observed in WKY rats (+18 +/- 3 mm Hg and +22 +/- 3%, P < .005 and P < .05, respectively). The pressor responses evoked by microinjection of L-glutamate (2 nmol), N-methyl-D-aspartate (20 pmol), or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (5 pmol) were also significantly (P < .001) augmented in SHR (+55 +/- 3, +61 +/- 7, and +53 +/- 5 mm Hg, respectively, in SHR versus +31 +/- 1, +30 +/- 3, and +28 +/- 2 mm Hg in WKY rats). Results indicate that stimulation of metabotropic, as well as ionotropic, glutamate receptors in the rostral ventrolateral medulla evokes enhanced cardiovascular responses in SHR, which may contribute to hypertension in this model.

Kynurenic acid, an endogenous glutamate antagonist, in SHR and WKY rats: possible role in central blood pressure regulation

1. Kynurenine aminotransferase catalyzes the conversion of kynurenine to kynurenic acid, an endogenous antagonist of excitatory amino acid receptors. The kynurenic acid content and kynurenine aminotransferase activity was measured in micro-dissected regions of spontaneously hypertensive rats (SHR) and their normotensive controls (Wistar-Kyoto rats: WKY). 2. Of the brain regions examined the highest kynurenine aminotransferase activity was found in the medulla followed by the olfactory bulb and the cerebellum, with the spinal cord showing the lowest activity. 3. All samples from SHR showed greatly reduced kynurenine aminotransferase activity compared to WKY. These reductions were most pronounced in the medulla and spinal cord, approximately 45-55%, and lowest in the cerebellum and olfactory bulbs, approximately 25-30%. 4. The kynurenic acid content of the rostral and caudal medulla as well as the spinal cord was also significantly lower in SHR. 5. These results suggest that there may be a deficiency in the kynurenic acid content and kynurenine aminotransferase activity in the SHR. 6. Given the accumulating evidence of the importance of medullary glutamatergic pathways in the control of blood pressure, as well as the higher sensitivity of cardiovascular neurons of SHR to applied glutamate, it seems possible that endogenous kynurenic acid in the brain may play a role in the control of blood pressure and the pathogenesis of experimental hypertension in the SHR.

Influence of humoral control peptides on medullary vasomotor control neurons: microstimulation and double-labeling studies using SHR and WKY rats

To study the influence of humoral control peptides on medullary vasomotor control neurons, angiotensin II (AII), arginine vasopressin (AVP) and atrial natriuretic peptide (ANP) were microinjected into three vasomotor control areas, i.e., the area postrema (AP), the nucleus tractus solitarii (NTS) and the rostral ventrolateral medulla (RVLM), of spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY), and the evoked cardiovascular response was observed. Unlike the injection areas, the threshold dose of one peptide for the cardiovascular response was similar, but the threshold dose differed from peptide to peptide. The threshold dose was lower for AII (0.15-0.29 pmol), in-between for ANP (0.9-1.5 pmol) and higher for AVP (14-30 pmol). No significant difference in the threshold dose was observed between SHR and WKY, suggesting that hypertension in SHR may not be due to the abnormal sensitivity to the three peptides of the vasomotor control neurons in the AP, NTS, and RVLM. The structural basis of the results of the microstimulation experiment was supported by the double-labeling study. The NTS neurons were innervated by (1) the AII-immunoreactive (ir) neurons in both sides of the lateral hypothalamic area (LH), the RVLM and the caudal ventrolateral medulla, and (2) the ANP-ir neurons in both sides of the paraventricular nucleus (Pa) and the LH. The RVLM neurons were innervated by (1) the AII-ir neurons in both sides of the LH and ipsilateral side of the lateral parabrachial nucleus (Pbl) and (2) the ANP-ir neurons in the ipsilateral Pbl. There was no evidence that the AVP-ir neurons in the Pa and the supraoptic nucleus innervate the NTS and the RVLM neurons, or that the AII, ANP or AVP-ir neurons innervate the AP neurons. This study suggests that in common with SHR and WKY rats AII and ANP may influence both the NTS and RVLM not by the humoral pathway but by the neural pathway, and AVP may not influence the three vasomotor control areas by the neural pathway.

Distribution of glutamate- and GABA-immunoreactive neurons projecting to the cardioacceleratory center of the intermediolateral nucleus of the thoracic cord of SHR and WKY rats: a double-labeling study

We aimed at (1) determining the distribution of glutamate (Glu)- and gamma-aminobutyric acid (GABA)-containing neurons in the brainstem with projections to the cardioacceleratory sympathetic preganglionic neurons in the intermediolateral nucleus (IML) of the upper thoracic cord and (2) determining whether such afferent projections in spontaneously hypertensive rats (SHR) differ from those of control Wistar-Kyoto (WKY) rats. We used a combination of electrophysiological methods to determine the site of HRP injection in the spinal cord and double-labeling methods for plotting the distribution of Glu- and GABA-immunoreactive neurons with projections to this site. HRP/Glu-labeled neurons (possibly glutamatergic) and HRP/GABA-labeled neurons (possibly GABAergic) were detected in 27% and 7% of the total HRP-labeled neurons of the central autonomic nuclei of 3 SHR rats and 3 WKY rats. HRP/Glu-labeled neurons were distributed predominantly ipsilaterally in 20 nuclei of the medulla oblongata, pons and hypothalamus, while HRP/GABA-labeled neurons were distributed in 7 nuclei of the medulla oblongata. No significant differences were found between the average percentages of HRP/Glu-labeled and HRP/GABA-labeled neurons in SHR and WKY rats. These findings indicate that: (1) the Glu-containing neurons represent a greater proportion than the GABA-containing neurons, (2) the proportions of these neurons appear to be similar in WKY and SHR rats and (3) generation of inbred tachycardia and hypertension in SHR rats can not be attributed to the topological and quantitative differences in the distribution of the glutamatergic and GABAergic neurons in the central autonomic nuclei.

Enhanced gamma-aminobutyric acid-mediated responses in nucleus tractus solitarius of hypertensive rats

Previous studies demonstrated that stimulation of type B gamma-aminobutyric acid (GABAB) receptors but not type A (GABAA) receptors in the nucleus tractus solitarius of spontaneously hypertensive rats elicited a larger increase in arterial pressure compared with control rats of the Wistar-Kyoto strain. The present studies extended that observation by examining the cardiovascular response to injection into the nucleus tractus solitarius of a selective GABAB receptor antagonist, CGP 35348, in these strains as well as examining the cardiovascular responses to stimulation or blockade of GABAB receptors in the nucleus tractus solitarius in another model of hypertension, the rat treated with deoxycorticosterone acetate and salt. In both groups of hypertensive rats the pressor response to injection into the nucleus tractus solitarius of the GABA uptake blocking drug nipecotic acid was significantly greater compared with control rats (P < .01 in each model). Similarly, in both models of hypertension, the depressor response elicited by blockade of GABAB receptors in the nucleus tractus solitarius by injection of CGP 35348 was approximately 75% greater compared with control rats (P < .05 in each model). These results suggest that alterations in GABAB-mediated neural transmission in the nucleus tractus solitarius may contribute to the elevated arterial pressure observed in these models of hypertension.

Metabotropic glutamate receptors in the ventrolateral medulla of rats

We investigated the hypothesis that stimulation of metabotropic excitatory amino acid receptors in the ventrolateral medulla evokes cardiovascular responses. Thus, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD], a selective agonist of metabotropic excitatory amino acid receptors, was microinjected into the rostral or caudal ventrolateral medulla of halothane-anesthetized Sprague-Dawley rats. Microinjections of (1S,3R)-ACPD (100 pmol-1 nmol) into the rostral ventrolateral medulla produced dose-dependent increases in mean arterial pressure (+20 +/- 4 mm Hg by 100 pmol and +35 +/- 2 mm Hg by 1 nmol, p < 0.01 versus artificial cerebrospinal fluid) and integrated splanchnic sympathetic nerve activity (+17 +/- 3% and +46 +/- 4%, respectively, p < 0.01), whereas (1S,3+)-ACPD microinjected into the caudal ventrolateral medulla decreased mean arterial pressure (-28 +/- 2 mm Hg by 100 pmol and -48 +/- 6 mm Hg by 1 nmol, p < 0.01 versus artificial cerebrospinal fluid) and splanchnic sympathetic nerve activity (-24 +/- 4% and -49 +/- 5%, p < 0.01). The blockade of ionotropic excitatory amino acid receptors by the combined injection of 2-amino-7-phosphonoheptanoic acid (200 pmol) and 6,7-dinitroquinoxaline-2,3-dione (200 pmol), which effectively blocked the responses elicited by either N-methyl-D-aspartate (20 pmol) or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (5 pmol), failed to affect the responses evoked by either (1S,3R)-ACPD (100 pmol) or L-glutamate (2 nmol) microinjected in the rostral and caudal ventrolateral medulla. These results suggest that metabotropic receptors are present and mediate cardiovascular responses evoked by L-glutamate injections into the rostral and caudal ventrolateral medulla.

Respiratory responses to microinjection of excitatory amino acid agonists in ventrolateral regions of the lateral parabrachial nucleus in the cat

Sites of tachypneic and hyperpneic responses to localized pressure injection of glutamate agonist were located in the small restricted regions in the ventrolateral regions of the lateral parabrachial nucleus (vlPb1) in the rostral and intermediate levels of the Pb1. The respiration-related neurons in the vlPb1 showed different sensitivity to different types of glutamate agonists. They are most sensitive, in descending order, to quisqualate, kainate, N-methyl-D-aspartate (NMDA), and glutamate. The respiratory response to kainate and NMDA was characterized by being slow and long-lasting, while the response to quisqualate and glutamate was rapid and brief. An immunocytochemical study suggested that the respiration-related neurons in the vlPb1 receive glutamatergic projections from the solitary tract nucleus, nucleus retroambigualis, nucleus para-ambigualis, para-retrofacial nucleus, subretrofacial nucleus and subfacial cell group in the medulla, Pb1 in the contralateral pons, paraventricular nucleus in the hypothalamus and central amygdaloid nucleus in the limbic system. These findings indicate that the respiration-related neurons in the vlPb1 play an important role in the modulation of respiration.

Glutamate receptor antagonists for neuroleptic malignant syndrome and akinetic hyperthermic parkinsonian crisis

Schizophrenia and Parkinson's disease have been considered inversely related neuropsychiatric disorders since the former has been attributed to increased dopaminergic transmission while the latter is thought to result from loss of dopaminergic neurons. It is in line with this concept that the classical neuroleptic (anti-schizophrenic) drugs cause as a side effect a drug-induced type of Parkinsonism. Most etiopathogenetic models hold that the "neuroleptic malignant syndrome" may result from "over-therapy" of schizophrenia, causing too widespread a block of dopaminergic transmission. The same clinical condition can be triggered by rapid discontinuation of dopaminergic medication in Parkinson's disease. Further, neuroleptic malignant syndrome shares key clinical features such as extrapyramidal motor disturbances and hyperthermia with a severe form of clinical deterioration in Parkinson's disease patients, the akinetic Parkinsonian crisis. Both conditions, neuroleptic malignant syndrome and Parkinsonian crisis, are resistant to anticholinergic treatment but may well respond to drugs with N-methyl-D-aspartate (NMDA) antagonistic properties such as amantadine and memantine. We advocate the use of NMDA receptor antagonists in these medical emergencies and link their clinical efficacy to the common pathophysiological pathway of increased excitatory amino acid neurotransmitter activity in neuroleptic malignant syndrome, Parkinsonian crisis, and dopamine agonist withdrawal states.

Difference in distribution of glutamate-immunoreactive neurons projecting into the subretrofacial nucleus in the rostral ventrolateral medulla of SHR and WKY: a double-labeling study

Glutamate immunoreactivity was found in 19% and 21% of the neurons of the central autonomic nuclei projecting into the subretrofacial nucleus (SRF) in the rostral ventrolateral medulla of Wistar-Kyoto rat (WKY) and spontaneously hypertensive rat (SHR), respectively, using a double-labeling technique in combination with glutamate immunocytochemistry. Double-labeled neurons were distributed in 22 nuclei or subnuclei in the limbic system, hypothalamus, midbrain, pons and medulla. The average number of glutamate-immunoreactive neurons per thousand in SHR was significantly higher in the ipsilateral lateral parabrachial nucleus (P less than 0.05) and Koelliker-Fuse nucleus (P less than 0.01) than in WKY, while it was significantly lower in the ipsilateral medial subnucleus (P less than 0.05) and the commissure subnucleus (P less than 0.05) of the nucleus tractus solitarii in SHR than in WKY. The results indicate that: (1) glutamate-immunoreactive neurons (possibly glutamatergic) in many central autonomic nuclei project into the sympathetic vasomotor control neurons in the SRF; (2) the large population of glutamate-immunoreactive neurons in the lateral parabrachial nucleus and the Koelliker-Fuse nucleus of SHR is likely to increase excitatory inputs to the SRF vasomotor control neurons, while the smaller population of glutamate-immunoreactive neurons in the medial and commissure subnuclei of the nucleus tractus solitarii is likely to decrease excitatory inputs to the GABAergic neurons intrinsic to the SRF.