N-methyl-D-aspartate receptors in the spinal cord mediate pressor responses to stimulation of the rostral ventrolateral medulla in the rat

Selective C1 Lesioning Slightly Decreases Angiotensin II Type I Receptor Expression in the Rat Rostral Ventrolateral Medulla (RVLM)

Cardiovascular homeostasis is regulated in large part by the rostral ventrolateral medulla (RVLM) in mammals. Projections from the RVLM to the intermediolateral column of the thoracolumbar spinal cord innervate preganglionic neurons of the sympathetic nervous system causing elevation of blood pressure and heart rate. A large proportion, but not all, of the neurons in the RVLM contain the enzymes necessary for the production of epinephrine and are identified as the C1 cell group. Angiotensin II (Ang II) activates the RVLM acting upon AT1 receptors. To assess the proportion of AT1 receptors that are located on C1 neurons in the rat RVLM this study employed an antibody to dopamine-beta-hydroxylase conjugated to saporin, to selectively destroy C1 neurons in the RVLM. Expression of tyrosine hydroxylase immunoreactive neurons in the RVLM was reduced by 57 % in the toxin injected RVLM compared to the contralateral RVLM. In contrast, densitometric analysis of autoradiographic images of (125)I-sarcosine(1), isoleucine(8) Ang II binding to AT1 receptors of the injected side RVLM revealed a small (10 %) reduction in AT1-receptor expression compared to the contralateral RVLM. These results suggest that the majority of AT1 receptors in the rat RVLM are located on non-C1 neurons or glia.

Mapping the cellular electrophysiology of rat sympathetic preganglionic neurones to their roles in cardiorespiratory reflex integration: a whole cell recording study in situ

Sympathetic preganglionic neurones (SPNs) convey sympathetic activity flowing from the CNS to the periphery to reach the target organs. Although previous in vivo and in vitro cell recording studies have explored their electrophysiological characteristics, it has not been possible to relate these characteristics to their roles in cardiorespiratory reflex integration. We used the working heart–brainstem preparation to make whole cell patch clamp recordings from T3–4 SPNs (n = 98). These SPNs were classified by their distinct responses to activation of the peripheral chemoreflex, diving response and arterial baroreflex, allowing the discrimination of muscle vasoconstrictor-like (MVC_like, 39%) from cutaneous vasoconstrictor-like (CVC_like, 28%) SPNs. The MVC_like SPNs have higher baseline firing frequencies (2.52 ± 0.33 Hz vs. CVC_like 1.34 ± 0.17 Hz, P = 0.007). The CVC_like have longer after-hyperpolarisations (314 ± 36 ms vs. MVC_like 191 ± 13 ms, P < 0.001) and lower input resistance (346 ± 49  MΩ vs. MVC_like 496 ± 41 MΩ, P < 0.05). MVC_like firing was respiratory-modulated with peak discharge in the late inspiratory/early expiratory phase and this activity was generated by both a tonic and respiratory-modulated barrage of synaptic events that were blocked by intrathecal kynurenate. In contrast, the activity of CVC_like SPNs was underpinned by rhythmical membrane potential oscillations suggestive of gap junctional coupling. Thus, we have related the intrinsic electrophysiological properties of two classes of SPNs in situ to their roles in cardiorespiratory reflex integration and have shown that they deploy different cellular mechanisms that are likely to influence how they integrate and shape the distinctive sympathetic outputs.

Effects of intrathecal kynurenate on arterial pressure during chronic osmotic stress in conscious rats

Increased plasma osmolality elevates mean arterial pressure (MAP) through activation of the sympathetic nervous system, but the neurotransmitters released in the spinal cord to regulate MAP during osmotic stress remain unresolved. Glutamatergic neurons of the rostral ventrolateral medulla project to sympathetic preganglionic neurons in the spinal cord and are likely activated during conditions of osmotic stress; however, this has not been examined in conscious rats. This study investigated whether increased MAP during chronic osmotic stress depends on activation of spinal glutamate receptors. Rats were chronically instrumented with an indwelling intrathecal (i.t.) catheter for antagonist delivery to the spinal cord and a radiotelemetry transmitter for continuous monitoring of MAP and heart rate. Osmotic stress induced by 48 h of water deprivation (WD) increased MAP by ~15 mmHg. Intrathecal kynurenic acid, a nonspecific antagonist of ionotropic glutamate receptors, decreased MAP significantly more after 48 h of WD compared with the water-replete state. Water-deprived rats also showed a greater fall in MAP in response to i.t. 2-amino-5-phosphonovalerate. Finally, i.t. kynurenic acid also decreased MAP more in an osmotically driven model of neurogenic hypertension, the DOCA-salt rat, compared with normotensive controls. Our results suggest that spinally released glutamate mediates increased MAP during 48-h WD and DOCA-salt hypertension.

Sympathoexcitation during chemoreflex active expiration is mediated by l-glutamate in the RVLM/Bötzinger complex of rats

The involvement of glutamatergic neurotransmission in the rostral ventrolateral medulla/Bötzinger/pre-Bötzinger complexes (RVLM/BötC/pre-BötC) on the respiratory modulation of sympathoexcitatory response to peripheral chemoreflex activation (chemoreflex) was evaluated in the working heart-brain stem preparation of juvenile rats. We identified different types of baro- and chemosensitive presympathetic and respiratory neurons intermingled within the RVLM/BötC/pre-BötC. Bilateral microinjections of kynurenic acid (KYN) into the rostral aspect of RVLM (RVLM/BötC) produced an additional increase in frequency of the phrenic nerve (PN: 0.38 ± 0.02 vs. 1 ± 0.08 Hz; P < 0.05; n = 18) and hypoglossal (HN) inspiratory response (41 ± 2 vs. 82 ± 2%; P < 0.05; n = 8), but decreased postinspiratory (35 ± 3 vs. 12 ± 2%; P < 0.05) and late-expiratory (24 ± 4 vs. 2 ±1%; P < 0.05; n = 5) abdominal (AbN) responses to chemoreflex. Likewise, expiratory vagal (cVN; 67 ± 6 vs. 40 ± 2%; P < 0.05; n = 5) and expiratory component of sympathoexcitatory (77 ± 8 vs. 26 ± 5%; P < 0.05; n = 18) responses to chemoreflex were reduced after KYN microinjections into RVLM/BötC. KYN microinjected into the caudal aspect of the RVLM (RVLM/pre-BötC; n = 16) abolished inspiratory responses [PN ( n = 16) and HN ( n = 6)], and no changes in magnitude of sympathoexcitatory ( n = 16) and expiratory (AbN and cVN; n = 10) responses to chemoreflex, producing similar and phase-locked vagal, abdominal, and sympathetic responses. We conclude that in relation to chemoreflex activation 1) ionotropic glutamate receptors in RVLM/BötC and RVLM/pre-BötC are pivotal to expiratory and inspiratory responses, respectively; and 2) activation of ionotropic glutamate receptors in RVLM/BötC is essential to the coupling of active expiration and sympathoexcitatory response.

Effects of an oral dose of l-glutamic acid on circulating neurotransmitters: Possible roles of the C1(Ad) and the A5(NA) pontomedullary nuclei

Objective

Investigation of the effects of an oral administration of a small dose of l-glutamic acid on the two peripheral sympathetic branches (neural and adrenal) of the autonomic nervous system.

Research design and methods

Circulating neurotransmitters and cardiovascular parameters were assessed in 28 healthy volunteers before and after the administration of 500 mg of l-glutamic acid or placebo.

Results

The drug triggered a significant and sustained enhancement of the noradrenaline and dopamine circulating levels which were paralleled and positively correlated with the diastolic blood pressure increases. Conversely, both platelet and plasma serotonin showed significant falls throughout the test. Significant positive correlations were registered between noradrenaline, dopamine, and noradrenaline/dopamine ratio versus diastolic blood pressure but not versus systolic blood pressure or heart rate.

Conclusion

The above results allowed us to postulate that the drug provoked a significant enhancement of peripheral neural sympathetic activity and the reduction of adrenal sympathetic and parasympathetic drives. Both sympathetic branches are positively correlated with the A₅ noradrenergic and the C₁ adrenergic pontomedullary nuclei, which interchange inhibitory axons that act at post-synaptic α₂ inhibitory autoreceptors. In addition, we discussed the mechanisms able to explain why the drug acted preferentially at the A₅ noradrenergic rather than the C₁ adrenergic nuclei.

Angiotensin modulation of rostral ventrolateral medulla (RVLM) in cardiovascular regulation

The rostral ventrolateral medulla (RVLM) and the presympathetic bulbospinal neurons in this region play a critical role in cardiovascular regulation. However, there is ambiguity regarding the precise anatomical coordinates of the RVLM and much still needs to be learned regarding the regulation and neurochemistry of this region. This brief review discusses some of these issues and focuses on the role of angiotensin-mediated signaling in the RVLM in blood pressure regulation.

Neurochemical phenotypes of cardiorespiratory neurons

Interactions between the cardiovascular and respiratory systems have been known for many years but the functional significance of the interactions is still widely debated. Here I discuss the possible role of metabotropic receptors in regulating cardiorespiratory neurons in the brainstem and spinal cord. It is clear that, although much has been discovered, cardiorespiratory regulation is certainly one area that still has a long way to go before its secrets are fully divulged and their function in controlling circulatory and respiratory function is revealed.

VGLUT1 and VGLUT2 innervation in autonomic regions of intact and transected rat spinal cord

Fast excitatory neurotransmission to sympathetic and parasympathetic preganglionic neurons (SPN and PPN) is glutamatergic. To characterize this innervation in spinal autonomic regions, we localized immunoreactivity for vesicular glutamate transporters (VGLUTs) 1 and 2 in intact cords and after upper thoracic complete transections. Preganglionic neurons were retrogradely labeled by intraperitoneal Fluoro-Gold or with cholera toxin B (CTB) from superior cervical, celiac, or major pelvic ganglia or adrenal medulla. Glutamatergic somata were localized with in situ hybridization for VGLUT mRNA. In intact cords, all autonomic areas contained abundant VGLUT2-immunoreactive axons and synapses. CTB-immunoreactive SPN and PPN received many close appositions from VGLUT2-immunoreactive axons. VGLUT2-immunoreactive synapses occurred on Fluoro-Gold-labeled SPN. Somata with VGLUT2 mRNA occurred throughout the spinal gray matter. VGLUT2 immunoreactivity was not noticeably affected caudal to a transection. In contrast, in intact cords, VGLUT1-immunoreactive axons were sparse in the intermediolateral cell column (IML) and lumbosacral parasympathetic nucleus but moderately dense above the central canal. VGLUT1-immunoreactive close appositions were rare on SPN in the IML and the central autonomic area and on PPN. Transection reduced the density of VGLUT1-immunoreactive axons in sympathetic subnuclei but increased their density in the parasympathetic nucleus. Neuronal cell bodies with VGLUT1 mRNA occurred only in Clarke's column. These data indicate that SPN and PPN are densely innervated by VGLUT2-immunoreactive axons, some of which arise from spinal neurons. In contrast, the VGLUT1-immunoreactive innervation of spinal preganglionic neurons is sparse, and some may arise from supraspinal sources. Increased VGLUT1 immunoreactivity after transection may correlate with increased glutamatergic transmission to PPN.

Characterization of baroreflex gain in the domestic pigeon (Columba livia)

Birds have a remarkable capacity to regulate circulation yet little is known about the avian baroreflex. Although both linear regression and curve-fitting techniques are frequently used to assess baroreflex function in mammals, only the former technique has been used in birds. We characterized baroreflex gain in domestic pigeons (Columba livia) and compared gain values derived from applying linear regression to ramp changes in mean arterial pressure (MAP) to values derived from fitting a four-parameter sigmoidal function to steady-state alterations in MAP. We found that, unlike mammals, pigeons do not display circadian patterns in MAP, HR or gain derived from bolus injections of vasoactive drugs. The pressor, but not depressor response, was attenuated by administration of the NMDA-antagonist ketamine, suggesting that central processing of the baroreflex may be similar in birds and mammals despite anatomical differences in arterial baroreceptive zones. Because graded infusions of vasoactive drugs could not consistently produce a plateau in the HR response, fitting data to a sigmoidal curve was difficult. Thus, we propose that variations of the Oxford method and linear regression analysis are superior method to assess baroreflex gain in pigeons than curve fitting.

Glutamate and GABA content of calbindin-immunoreactive nerve terminals in the rat intermediolateral cell column

Immunoreactivity for calbindin-D28K (calbindin) occurs in some bulbospinal vasopressor neurons in the rostral ventrolateral medulla and calbindin-immunoreactive terminals form synapses in the intermediolateral cell column (IML), where the cell bodies of sympathetic preganglionic neurons are located. In this study, we used post-embedding immunogold labelling to determine whether calbindin terminals in the IML contained the excitatory amino acid neurotransmitter glutamate. We also assessed GABA immunoreactivity in semi-serial sections through the same terminals since this inhibitory amino acid transmitter is present in the inputs to sympathetic preganglionic neurons that lack glutamate. Analysis of 42 calbindin-positive terminals whose postsynaptic targets were not identified revealed two major groups on the basis of amino acid content. One group was immunoreactive for glutamate; and the other, for GABA. In addition, about 20% of the calbindin terminals were positive for both glutamate and GABA. Our anatomical methods cannot differentiate whether this third group is a subset of the GABAergic terminals or a separate population capable of co-releasing the two amino acids.

GABA in the control of sympathetic preganglionic neurons

1. Amino acid neurotransmitters are critical for controlling the activity of most central neurons, including sympathetic preganglionic neurons (SPN), the spinal cord neurons involved in controlling blood pressure and other autonomic functions. 2. In studies reviewed here, SPN were identified either by retrograde tracing from a peripheral target (superior cervical ganglion or adrenal medulla) or by detection of immunoreactivity for choline acetyltransferase (ChAT), the acetylcholine-synthesizing enzyme that is a marker for all SPN, in intact or completely transected rat spinal cord. 3. Postembedding immunogold labelling on ultrathin sections was then used to detect GABA and sometimes glutamate in nerve terminals on SPN or near them in the neuropil of the lateral horn. 4. In some cases, the terminals were prelabelled to show an anterograde tracer or immunoreactivity for ChAT or neuropeptide Y. 5. This anatomical work has provided information that is helpful in understanding how SPN are influenced by their GABAergic innervation. 6. Immunogold studies showed that the proportion of input provided by GABAergic terminals varies between different groups of SPN. For some groups, this input may be preferentially targeted to cell bodies. 7. Anterograde tracing demonstrated that supraspinal as well as intraspinal GABAergic neurons innervate SPN and investigations on completely transected cord suggested that supraspinal neurons may provide a surprisingly large proportion of the GABAergic terminals that contact SPN. 8. The double-labelling studies in which other amino acids, ChAT or neuropeptide Y were localized along with GABA indicate that GABAergic terminals contain other neurochemicals that could modulate the actions of GABA, depending on the complement of receptors that are present pre- and post-synaptically. 9. Taken together, these data indicate that GABAergic transmission to SPN may be much more complicated than suggested by the currently available electrophysiological studies.

Role of spinal NMDA and non-NMDA receptors in the pressor reflex response to abdominal ischemia

Abdominal ischemia induces a pressor reflex caused mainly by C-fiber afferent stimulation. Because excitatory amino acids, such as glutamate, bind to N-methyl-D-aspartate (NMDA) and non-NMDA [dl-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)] receptors and serve as important spinal neurotransmitters, we hypothesized that both receptors play a role in the abdominal ischemia pressor reflex. In chloralose-anesthetized cats, NMDA receptor blockade with 25.0 mM dl-2-amino-5-phosphonopentanoate did not alter the pressor reflex (33 +/- 9 to 33 +/- 7 mmHg, P > 0.05, n = 4), whereas AMPA receptor blockade with 4.0 mM 6-nitro-7-sulfamylbenzo(f)quinoxaline-2,3-dione significantly attenuated the reflex (29 +/- 5 to 16 +/- 4 mmHg, P < 0.05, n = 6). Because several studies suggest that anesthesia masks the effects of glutamatergic receptors, this experiment was repeated on decerebrate cats, and in this group, NMDA receptor blockade with 25.0 mM dl-2-amino-5-phosphonopentanoate significantly altered the pressor reflex (36 +/- 3 to 25 +/- 4 mmHg, P < 0.05, n = 5). Our combined data suggest that spinal NMDA and AMPA receptors play a role in the abdominal ischemia pressor reflex.

Altered glutamate receptor function during recovery of bladder detrusor-external urethral sphincter coordination in a rat model of spinal cord injury

Coordination of the bladder detrusor and the external urethral sphincter is a supraspinally controlled reflex that is essential for efficient micturition. This coordination is permanently lost after spinal cord transection but can recover chronically after incomplete spinal cord injury (SCI). As glutamatergic transmission plays a key role in all levels of detrusor-external urethral sphincter coordination, we examined the role of potential alterations in glutamatergic control in its recovery after SCI. Rats were subjected to standardized incomplete contusion injury. Detrusor-external urethral sphincter coordination was evaluated urodynamically at 5 days (subacute) and 8 weeks (chronic) after SCI. Sensitivity of coordinated activation of the external urethral sphincter in response to bladder distension to the alpha -amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid/kainate antagonist 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo(f)quinoxaline-7-sulfonamide disodium (NBQX) and to the N-methyl-D-aspartate (NMDA) antagonist R(--3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonic acid (CPP) was determined by intrathecal application at the L6 spinal cord level during urodynamic recordings. We found that while detrusor contractions recovered at 5 days after SCI, coordinated activation of the external urethral sphincter was significantly impaired at 5 days and recovered only by 8 weeks. There was no difference in sensitivity of detrusor-external urethral sphincter coordination to NBQX at the subacute or chronic time points. However, external urethral sphincter response to bladder distension was sensitive to a 50% lower dose of CPP at 5 days compared with uninjured rats or chronic recovered SCI rats. Thus, alterations in NMDA receptor function appeared to be involved in recovery of detrusor-external urethral sphincter coordination after incomplete SCI.

Lesions of the C1 catecholaminergic neurons of the ventrolateral medulla in rats using anti-DbetaH-saporin

Phenylethanolamine-N-methyltransferase (PNMT)-containing neurons in the rostral ventrolateral medulla (RVLM) are believed to play a role in cardiovascular regulation. To determine whether injection of anti-dopamine beta-hydroxylase (DbetaH)-saporin directly into the RVLM in rats could selectively destroy these cells and thereby provide an approach for evaluating their role in cardiovascular regulation, we studied rats 2 wk after unilateral injection of 21 ng anti-DbetaH-saporin into the RVLM. There was an approximately 90% reduction in the number of PNMT-positive neurons in the RVLM, although the number of non-C1, spinally projecting barosensitive neurons of this area was not altered. The A5 cell group was the only other population of DbetaH-containing cells that was significantly depleted. The depressor response evoked by injection of tyramine into the RVLM was abolished by prior injection of toxin. The pressor response evoked by injection of glutamate into the RVLM was attenuated ipsilateral to the toxin injection but was potentiated contralateral to the toxin injection. Thus anti-DbetaH-saporin can be used to make selective lesions of PNMT-containing cells, allowing for the evaluation of their role in cardiovascular regulation.

Effect of propranolol on central neurotransmitter release in Wistar rats analysed by brain microdialysis

1. The effect of propranolol on amino acid neurotransmitter release in the rostral ventrolateral medulla (RVLM) was examined in urethane-anaesthetized male Wistar rats. 2. Amino acids released in the RVLM in response to intravenous administration of propranolol (0.4 mg/kg per min; n = 6) or nitroglycerin (0.02 mg/kg per min; n = 5) were determined by the brain microdialysis method. 3. Amino acids in dialysates were analysed by high-performance liquid chromatography and were quantified by ultraviolet absorbance. 4. Administration of both intravenous propranolol and nitroglycerin significantly decreased arterial blood pressure. Heart rate was decreased only by propranolol. 5. The reduction in arterial blood pressure produced by intravenous propranolol was accompanied by a decrease in the release of the excitatory amino acid glutamate in the RVLM. 6. The reduction in arterial blood pressure following intravenous nitroglycerin was not accompanied by a release of glutamate. 7. There were no significant changes in the levels of other amino acids (glycine, taurine, GABA) following either propranolol or nitroglycerin. 8. The decrease in glutamate release in the RVLM may account, in part, for the central depressor mechanism of propranolol.

Volhard Lecture. Brain, blood pressure and stroke

BRAIN AND BLOOD PRESSURE IN EXPERIMENTAL ANIMALS: Our experiments in models of experimental hypertension in the rabbit in the early 1970s demonstrated that increased activity of bulbospinal pressor neurons containing noradrenaline or serotonin mediated the elevated arterial blood pressure. Other workers had demonstrated decreased activity of noradrenergic neurons in the medulla. Accordingly, I proposed the hypothesis that the hypertension in these models arose from 'disinhibition', due to unrestrained activity of descending pressor pathways, released from the inhibitory influences present in normal animals. Over the next 15-20 years, experiments from our group and from other laboratories demonstrated that there were two distinct bulbospinal pressor pathways descending from the rostral ventral medulla, one containing adrenaline, neuropeptide Y and glutamate, and the other containing serotonin, substance P and glutamate. It has also been established that the key depressor area is in the caudal ventrolateral medulla and that the main inhibitory input, restraining the activity of the bulbospinal pressor pathways, is a short gamma-aminobutyric acid (GABA) projection ascending from the caudal ventrolateral medulla to the rostral ventral medulla. More recent experiments in the spontaneously hypertensive rat (SHR) using the immediate-early gene c-fos as a marker of neuronal activity, have demonstrated that impaired activity of this short inhibitory GABA pathway in the SHR disinhibits the bulbospinal pressor pathway, thus contributing to the hypertension in this model. BLOOD PRESSURE AND STROKE IN HUMANS: The risks of primary stroke and of secondary or recurrent stroke are both directly related to the level of blood pressure and clinical trials have clearly demonstrated that lowering blood pressure markedly reduces the incidence of primary stroke. The Perindopril Protection Against Recurrent Stroke Study (PROGRESS) was launched to test the hypothesis that lowering the blood pressure in subjects who have already had a stroke or a transient ischaemic attack will also reduce the risk of stroke. A major unresolved issue for practising clinicians is how to manage the raised blood pressure that is so common in the acute phase of stroke. Accordingly, the PROGRESS investigators are planning another major multinational trial to assess the benefits and risks of lowering blood pressure in the first 3 days after the onset of a stroke.

Effect of spinal cord transection on N-methyl-D-aspartate receptors in the cord

Spinal cord injury can lead to an exaggeration of transmission through spinal pathways, resulting in muscle spasticity, chronic pain, and abnormal control of blood pressure and bladder function. These conditions are mediated, in part, by N-methyl-D-aspartate (NMDA) receptors on spinal neurons, but the effects of cord injury on the expression or function of these receptors is unknown. Therefore, antibodies to the NMDA-R1 receptor subunit and binding of [3H]MK-801 were used to assess NMDA receptors in the spinal cord. Receptor density in rats with intact spinal cords was compared to that in rats 1 and 2 weeks after spinal cord transection (SCT) at the mid-thoracic level. At 1 and 2 weeks after SCT, [3H]MK-801 binding was reduced in most laminae in cord segments caudal to the injury, whereas no decrease in amount of R1 subunit immunoreactivity was observed. No significant changes in [3H]MK-801 binding and NMDA-R1 immunoreactivity could be seen rostral to the transection. Since [3H]MK-801 binding requires an open ion channel, the discrepancy between [3H]MK-801 binding and immunocytochemistry may indicate a loss of functional receptors without a consistent change in their total number. Therefore, the exaggerated reflexes that are well established in rats 2 weeks after cord injury must be mediated by a mechanism that withstands attenuation of NMDA receptor function.

GABA- and glutamate-immunoreactive synapses on sympathetic preganglionic neurons projecting to the superior cervical ganglion

Our previous work suggests that virtually all of the synapses on sympathetic preganglionic neurons projecting to the rat adrenal medulla are immunoreactive for either the inhibitory amino acid, gamma-aminobutyric acid (GABA) or the excitatory amino acid, L-glutamate. To investigate whether or not this is true for other groups of sympathetic preganglionic neurons, and to determine whether or not the proportion of inputs containing each type of amino acid neurotransmitter is the same for different groups of sympathetic preganglionic neurons, we retrogradely labelled rat and rabbit sympathetic preganglionic neurons projecting to the superior cervical ganglion and used post-embedding immunogold on ultrathin sections to localise GABA- and glutamate-immunoreactivity. The cell bodies and dendrites of both rat and rabbit sympathetic preganglionic neurons projecting to the superior cervical ganglion received synapses and direct contacts from nerve fibres immunoreactive for GABA and from nerve fibres immunoreactive for glutamate. In the rat, GABA was present in 48.9% of the inputs to sympathetic preganglionic neurons projecting to the superior cervical ganglion, and glutamate was present in 51.7% of inputs. Double immunogold labelling for glutamate and GABA on the same section, as well as labelling of consecutive serial sections for the two antigens, indicated that GABA and glutamate occur in separate populations of nerve fibres that provide input to rat sympathetic preganglionic neurons projecting to the superior cervical ganglion. We now have shown that GABA or glutamate is present in virtually all of the inputs to sympathetic preganglionic neurons projecting to the superior cervical ganglion and in essentially all of the inputs to sympathetic preganglionic neurons supplying the adrenal medulla. These findings are consistent with the hypothesis that all fast synaptic transmission in central autonomic pathways may be mediated by either excitatory or inhibitory amino acids. Furthermore, we showed a statistically significant difference in the proportion of glutamate-immunoreactive inputs between sympathetic preganglionic neurons projecting to the superior cervical ganglion and sympathoadrenal neurons (data from Llewellyn-Smith et al. [Llewellyn-Smith, I.J., Phend, K.D., Minson, J.B., Pilowsky, P.M., Chalmers, J.P., 1992. Glutamate immunoreactive synapses on retrogradely labelled sympathetic neurons in rat thoracic spinal cord. Brain Res. 581, 67-80]), with preganglionics supplying the adrenal medulla receiving more excitatory inputs than those supplying the superior cervical ganglion. This increased excitatory input to sympathoadrenal neurons may explain the predominant activation of these neurons following baroreceptor unloading.

Glutamate- and GABA-immunoreactive synapses on sympathetic preganglionic neurons caudal to a spinal cord transection in rats

Spinal cord injury destroys bulbospinal amino acid-containing pathways to sympathetic preganglionic neurons and severely disrupts blood pressure control, resulting in resting or postural hypotension and episodic hypertension. Almost all immunoreactivity for the excitatory amino acid L-glutamate has been reported to disappear from autonomic areas of the cord caudal to a transection, apparently depriving autonomic neurons of their major excitatory input. However, the magnitude of the neurogenic episodic hypertension after cord injury suggests that excitatory inputs to sympathetic preganglionic neurons must still be present. Moreover, the hypotension associated with high spinal injuries may reflect a enhanced role for inhibitory transmitters, such as GABA. This apparent contradiction regarding the presence of glutamate and lack of information about GABA prompted the present investigation. In rats seven days after spinal cord transection, we examined identified sympathetic preganglionic neurons caudal to the injury for the presence of synapses or direct contacts from varicosities that were immunoreactive for the amino acids, L-glutamate and GABA. Adrenal sympathetic preganglionic neurons were retrogradely labelled with cholera toxin B subunit and amino acid immunoreactivity was revealed with post-embedding immunogold labelling. In single ultrathin sections, 46% (98/212) of the synapses or direct contacts on adrenal sympathetic preganglionic neurons were immunoreactive for glutamate and 39% (83/214) were immunoreactive for GABA. Analysis of inputs with the physical disector yielded similar results for the two amino acids. The proportions of glutamatergic or GABAergic synapses on cell bodies and dendrites were similar. When alternate ultrathin sections were stained to reveal glutamate or GABA immunoreactivity, either one or the other amino acid occurred in 78.4% (116/148) of inputs; 4.1% (6/148) of inputs contained both amino acids and 17.5% (26/148) of inputs contained neither. These results demonstrate that nerve fibres immunoreactive for the neurotransmitter amino acids, glutamate and GABA, provide most of the input to sympathetic preganglionic neurons caudal to a spinal cord transection. Synapses containing glutamate and GABA could provide the anatomical substrate for the exaggerated sympathetic reflexes and the low sympathetic tone that result from spinal cord injury.

Central control mechanisms in hypertension

There is substantial evidence for an activation of the sympathetic nervous system in man as well as in genetic models of hypertension, such as the spontaneously hypertensive rat (SHR), but we are only beginning to understand the central mechanisms that generate changes in sympathetic activity and elevate blood pressure (BP). Significant recent advances have been made in defining the neural pathways involved in BP regulation and in identifying the neurotransmitters these neurones utilise. In this overview, we describe the neural pathways within the medulla oblongata and spinal cord that participate in BP control and examine the role of amino acid neurotransmitters within these pathways. We demonstrate how alterations in these pathways explain the sympathetic activation observed in the SHR and contribute to hypertension in this model. Lastly, we examine the application of modern molecular biological approaches to further our understanding of the neural regulation of the circulation. In these studies, we used the administration of antisense oligonucleotides to interrupt gene expression.

Spinal cord excitatory amino acid receptors and plasma catecholamine autonomic responses in the conscious rabbit

Intrathecal administration of the specific glutamate subtype receptor agonist NMDA (N-methyl-D-aspartate), or the non-NMDA receptor agonist AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid), at the level of the lower thoracic spinal cord in the conscious rabbit, produces increased levels of plasma norepinephrine and a rise in blood pressure. These responses are specifically inhibited with prior intrathecal administration of the NMDA receptor antagonist AP-5 (2-amino-5-phosphonovaleric acid) or non-NMDA receptor antagonist DNQX (6, 7-dinitroquinoxaline-2, 3-dione), respectively. In contrast, arterial baroreflex activation induced by transient hypotension following intravenous sodium nitroprusside increases levels of plasma norepinephrine which are inhibited with intrathecal AP-5, but not DNQX. The experiments are consistent with glutamate acting as a neurotransmitter in the spinal cord influencing control of sympathetic nerve function via activation of spinal cord NMDA and non-NMDA subtype receptors. The results support previous work suggesting that baroreflex function is predominantly mediated by spinal NMDA receptors and that spinal glutamate receptors are important in baroreflex control of the circulation.

The one hundred percent hypothesis: glutamate or GABA in synapses on sympathetic preganglionic neurons

The synaptic input from glutamate- or gamma-aminobutyric acid (GABA)-immunoreactive nerve fibers to sympathetic preganglionic neurons projecting to the superior cervical ganglion was assessed using retrograde tracing with cholera toxin B subunit and post-embedding immunogold labelling. Half the boutons that synapsed on or directly contacted this group of neurons were immunoreactive for glutamate and half were immunoreactive for GABA. In previous studies, about 2/3 of the synapses on sympathoadrenal neurons were found to be glutamate-immunoreactive and about 1/3, GABA immunoreactive and these two amino acids were not co-localized. Thus, 100% of the boutons that synapse on sympathetic preganglionic neurons supplying the superior cervical ganglion or the adrenal medulla are likely to contain either glutamate or GABA. Anterograde tracing combined with amino acid immunocytochemistry indicated that at least some glutamate-containing and some GABA-containing nerve fibers in the intermediolateral cell column could come from the rostral ventral medulla.

Excitatory amino acid receptors in central cardiovascular regulation

Excitatory amino acid neurotransmitters and their receptors are the principal mediators of fast synaptic transmission within the central nervous system. Accumulating evidence suggests that synaptic activation of excitatory amino acid receptors in the nucleus of the tractus solitarius, rostral and caudal ventrolateral medulla, and in the spinal cord play a key role in neural transmission of cardiovascular information in the central nervous system. Pharmacological blockade of excitatory amino acid receptors at these sites eliminates a variety of centrally-mediated cardiovascular responses. These include baroreceptor reflexes and increases in arterial pressure produced by stimulation of various brain regions as well as peripheral afferent nerves. These observations indicate that synaptic activation of EAA receptors at specific sites within the brainstem and in the spinal cord play an important role in central cardiovascular regulation.

Substance P and serotonergic inputs to sympathetic preganglionic neurons

Sympathetic preganglionic neurons are the final central links in the sympathetic pathways that control the heart and blood vessels. The neurotransmitters present in the supraspinal pathways that control the activity of sympathetic preganglionic neurons include amino acids, amines and peptides. In this paper we discuss evidence that suggests a role for serotonin and substance P in these pathways. Both of these neurotransmitters are present in bulbospinal neurons. Our results suggest that they have an important physiological role in the central regulation of blood pressure.

Central neurons and neurotransmitters in the control of blood pressure

1. In this paper we review recent work from our laboratory on two major pathways important in the central control of blood pressure. 2. We report experiments on the sympatho-excitatory bulbospinal pathway from the rostral ventral medulla. Here we focus particularly on the role of excitatory amino acids. 3. We review studies on the short inhibitory or depressor pathway ascending from the caudal to the rostral ventral medulla, which is thought to use gamma-aminobutyric acid (GABA) as its neurotransmitter. We report on experiments with the immediate early gene, c-fos, demonstrating that its expression in the bulbospinal pressor neurons is increased by stimuli that activate these nerves, and that this expression can be blocked in vivo by treatment with an antisense oligonucleotide. We also show that basal and stimulated expression of the c-fos gene is important in the central control of blood pressure.

Pharmacological effects of the non-competitive NMDA antagonist CNS 1102 in normal volunteers

1. Non-competitive antagonists at the glutamatergic N-methyl D-aspartate receptor significantly reduce the volume of ischaemic cerebral infarction in animals and are potential agents for the treatment of acute stroke in humans. 2. CNS 1102, a novel non-competitive NMDA antagonist, was administered as a 15 min intravenous infusion to healthy male volunteers in a double-blind, placebo-controlled, dose-ranging study. This was the first administration to man. 3. Clinically significant sedation, increased mean arterial pressure and pulse rate were seen at doses of 30 micrograms kg-1 and above. Symptoms of sedation and central nervous excitation became unacceptable for conscious individuals at doses of 45 micrograms kg-1 and above. 4. Rapid onset of central nervous system effects after administration is in keeping with rapid distribution of CNS 1102 to brain. Steady state volume of distribution was large (444 l) and terminal elimination half-life from plasma was approximately 4 h. 5. Pharmacokinetic properties are favourable for a potential neuroprotective therapy. The maximum tolerated dose for conscious individuals was 30 micrograms kg-1 given intravenously over 15 min. Further assessment of CNS 1102 should seek methods of drug administration which maximise administered dose with minimal side effects.

Disinhibition of the rostral ventral medulla increases blood pressure and Fos expression in bulbospinal neurons

The GABA agonist muscimol, injected into the depressor area of the caudal ventrolateral medulla, increased blood pressure and increased the expression of the immediate early gene c-fos in the rostral ventral medulla (RVM) of the rat. The number of Fos-immunoreactive (Fos-IR) neurons seen in the RVM was increased 3-fold after muscimol compared to Fos-IR after vehicle treatment. In the rostral aspect of the RVM approximately half of the Fos-IR neurons were identified as spinally projecting after the injection of the retrograde tracer cholera toxin B subunit into the upper thoracic spinal cord. These bulbospinal Fos-IR neurons were identified in the lateral aspects of the RVM, in the area where baroreceptor-sensitive neurons have been identified in electrophysiological studies, and also in more medial areas of the RVM. Fos-IR neurons were also identified in the intermediolateral cell column of the thoracic spinal cord after muscimol injection, but were rarely observed in this area after vehicle treatment. This study demonstrates the functional connectivity of the caudal and rostral areas of the medulla oblongata and the spinal cord, supporting the view that the caudal ventrolateral medulla contains neurons that provide a tonic inhibitory control over neurons in the RVM and that, in turn, the spinally projecting neurons in the RVM provide an excitatory input to the spinal cord sympathetic preganglionic neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Intrathecal kynurenate but not benextramine blocks hypoxic sympathoexcitation in chemodenervated anesthetized rats

In peripherally chemodenervated anesthetized and ventilated rats, inhalation of 100% N2 for 20 s rapidly excites reticulospinal vasomotor neurons of the rostroventrolateral reticular nucleus (RVL) and sympathetic nerves. After an initial fall, arterial pressure is also elevated. The sympathoexcitatory response and secondary increases in arterial pressure were abolished by microinjections of muscimol (250 pmol) into the RVL or by intrathecal administration of kynurenate (500 nmol), a broad-spectrum antagonist of excitatory amino acid receptors, but not by benextramine, an alpha-adrenoceptor antagonist. The results indicate that excitation of pre-ganglionic sympathetic neurons in the spinal cord by hypoxia results from excitation of RVL-spinal vasomotor neurons, probably the glutamatergic neurons.

Prevention by NMDA receptor antagonists of the centrally-evoked increases of cardiac inotropic responses in rabbits

1. The purpose of this study was to investigate further the role of the excitatory amino acid (EAA) system of neurotransmission, particularly of the NMDA receptor, in the central regulation of cardiac function. 2. Electrical stimulation of the paraventricular nucleus of the hypothalamus (PVN) in pentobarbitone anaesthetized rabbits induced a cardiovascular response mainly characterized by a positive inotropic effect, hypertension and a marked increase in the myocardial oxygen demand index. 3. The intracerebroventricular (i.c.v.) or intravenous (i.v.) injection of different EAA antagonists acting on different sites of the NMDA receptor/channel complex dose-dependently blunted the excitatory cardiovascular effects of PVN stimulation. 4. 5,7 Dichlorokynurenic acid was used as a specific glycine site antagonist and 2-amino-5-phosphonovaleric acid was used to block the agonist recognition site; ketamine was used as a channel blocker site antagonist and ifenprodil as a blocker of the polyamine binding site. 5. 5,7 Dichlorokynurenic acid (125 and 250 micrograms kg-1, i.c.v.) virtually abolished the cardiovascular responses, inducing only haemodynamic depression at the highest dose used. 2-Amino-5-phosphonovaleric acid (0.1 to 1.0 mg kg-1, i.c.v.) elicited a reduction of the peak values observed during PVN stimulation which was accompanied by a decrease of the basal cardiovascular parameters. Ketamine (2.5 and 10 mg kg-1) and ifenprodil (1 mg kg-1), injected intravenously, blocked the haemodynamic response induced by PVN stimulation without marked reduction of the basal haemodynamics. 6. It is concluded that glutamate neurotransmission is not only involved in vasomotor tone control but also in the central control of cardiac function and can therefore modulate the myocardial oxygen demand.

Contribution of spinal N-methyl-D-aspartic acid receptors to control of sympathetic outflow by the paraventricular nucleus

Spinal NMDA receptors contribute to control of the cardiovascular system by the ventrolateral medulla. However, little is known about the contribution of these receptors to suprabulbar regulation of hemodynamics and sympathetic outflow. Hence, the involvement of spinal NMDA receptors in regulation of the cardiovascular system by the paraventricular nucleus (PVN) of the hypothalamus was investigated. In urethane-anesthetized rats, the change in mean arterial pressure (MAP), heart rate (HR), and renal nerve activity (RNA) produced by electrical or chemical activation of the PVN was determined before and after intrathecal administration of the NMDA receptor antagonist, 2-amino-5-phosphonovaleric acid (AP5). Intrathecal AP5 decreased resting MAP, HR, and RNA, but had no effect on the increase in RNA produced by electrical or chemical stimulation of the PVN. The pressor and renal vasoconstrictor effects resulting from electrical, but not chemical, stimulation of the PVN were significantly reduced by intrathecal AP5. These data show that much of the cardiovascular control exerted by the PVN does not depend on a spinal NMDA receptor mechanism.

Chlormethiazole attenuates the derangement of sensory evoked potential (SEP) induced by ICV administration of NMDA

Administration to anaesthetized rats of N-methyl-D-aspartate (NMDA 30 nmol ICV) induced a profound derangement of stimulation evoked potential (f-SEP) and also autonomic excitation with an increase in arterial blood pressure and heart rate. These effects were antagonised by pretreatment with dizocilpine (0.5 mg/kg IV). Pretreatment with chlormethiazole (20 mg/kg IV 26 min before NMDA) also markedly diminished the derangement of f-SEP. At the end of the registration period 2 h after NMDA the SEP had recovered to 72.7 +/- 3.4 (% of control; mean +/- SEM) in saline-treated rats as compared to 96.1 +/- 5.6% in chlormethiazole treated animals (P < 0.01). In contrast to dizocilpine, chlormethiazole alone had no effect on heart rate or blood pressure and did not alter the autonomic effects of ICV NMDA. These results demonstrate that chlormethiazole can antagonise some NMDA-receptor mediated functions, even though there is no evidence that it is an NMDA antagonist.

An excitatory amino acid synapse in the thoracic spinal cord is involved in the pressor response to muscular contraction

The increase in arterial pressure and heart rate elicited during exercise are produced by descending central command and by feedback from contracting limb muscles. Previous studies from this laboratory have demonstrated that neurons in the ventrolateral medulla (VLM) that project to the intermediolateral (IML) columns of the thoracic spinal cord are involved in the mediation of the pressor response to contraction of hind limb muscles. This study determines if these VLM neurons utilize excitatory amino acids (EAA) as the neurotransmitter at the synapse on IML neurons. The arterial pressure and heart rate responses to static muscular contraction, elicited by stimulation of the L7 and S1 ventral roots, and to electrical stimulation in the caudal hypothalamus were examined in anesthetized cats. Both muscular contraction and hypothalamic stimulation elicited significant increases in arterial pressure and heart rate. Intrathecal administration of the broad spectrum, postsynaptic EAA antagonist, kynurenic acid, greatly reduced (-77%) the pressor response to muscular contraction. A smaller (-27%) decrease in the magnitude of pressor response elicited by muscular contraction was produced by intrathecal administration of 2-amino-4-phosphonobutyric acid which acts on a presynaptic EAA receptor. Neither antagonist affected the heart rate responses associated with muscular contraction or the cardiovascular responses to hypothalamic stimulation. These results indicate that the pressor response elicited by feedback from contracting hind limb muscles is mediated through an excitatory amino acid synapse in the spinal cord.

Sympathoexcitation from the rostral ventrolateral medulla is mediated by spinal NMDA receptors

These studies examined the role of spinal N-methyl-D-aspartic acid (NMDA) receptors in mediating sympathoexcitation evoked by stimulation of neurons in the rostral ventrolateral medulla (RVLM). In urethane-anesthetized rats, blood pressure, heart rate, and splanchnic sympathetic nerve activity (SNA) were recorded. The NMDA receptor antagonist D-2-amino-7-phosphonoheptanoic acid (D-AP7) was administered to the spinal cord via intrathecal (IT) catheter. Blockade of spinal NMDA receptors reduced arterial blood pressure, heart rate, and SNA. Spinal administration of D-AP7 markedly attenuated the pressor and sympathoexcitatory responses evoked by L-glutamate stimulation of the RVLM. The small increases in heart rate evoked by stimulation of the RVLM were not affected by IT administration of D-AP7. These results indicate that NMDA receptors in the spinal cord mediate the pressor and sympathoexcitatory responses evoked by activation of a bulbospinal pathway originating from the RVLM. Moreover, these data suggest that excitatory amino acid neurotransmitters and NMDA receptors in the spinal cord play an important role in the maintenance and regulation of SNA and cardiovascular function.

Glutamate, NMDA and NMDA receptor antagonists: cardiovascular effects of intrathecal administration in the rat

Selected excitatory amino acids and antagonists were tested for their effects on arterial pressure and heart rate when administered intrathecally at the second (T2) or ninth (T9) thoracic spinal levels in urethane-anesthetized Sprague-Dawley rats with spontaneous or artificial respiration. Intrathecal administration of glutamate (1 mumol) and N-methyl-D-aspartic acid (NMDA; 2 nmol) at T9 increased arterial pressure and heart rate. The response began within 1 min, peaked at 2-3 min and persisted for 8-15 min. The maximum changes were 20-25 mm Hg for arterial pressure and 40-50 beats/min for heart rate. These responses were prevented by systemic administration of hexamethonium (10 mg/kg). Responses to administration of NMDA at the two spinal levels were essentially the same. Effects elicited by NMDA but not by glutamate were blocked by pretreatment with the NMDA receptor antagonists, D,L-2-amino-5-phosphonovaleric acid (APV; 10 nmol, intrathecal administration) and ketamine (7 mg/kg, i.v.). Intrathecal administration of APV (10, 50 and 200 nmol) at T2 produced dose-dependent decreases in arterial pressure without changing heart rate. The results support the hypothesis that NMDA receptors are involved in regulation of sympathetic output at the spinal level. They also indicate that in this preparation there is a tonic activation of NMDA receptors in sympathetic pathways to the vessels but not to the heart. Finally, the persistence of the response to glutamate in the presence of NMDA receptor antagonists suggests the involvement of non-NMDA receptors in spinal control of sympathetic output.

NMDA receptor antagonists block cardiovascular responses to intrathecal administration of D-baclofen in the rat

In previous studies we found that D and L-baclofen have different effects on sympathetic output when administered intrathecally, yet the actions of both enantiomers are blocked by intrathecal administration of phaclofen. The present experiments were done to determine the mechanism by which D-baclofen expresses its effects. In urethane-anaesthetized Sprague-Dawley rats, when D-baclofen was given intrathecally at the T9 spinal level following pretreatment with 2 nmol of the NMDA receptor antagonist, DL-2-amino-5-phosphonovaleric acid (APV), it increased systolic and diastolic arterial pressures (n = 7), as in the previous studies. However, after intrathecal administration of 10 nmol of APV, administration of D-baclofen had no effect on these parameters (n = 7). Intravenous administration of ketamine (7.5 mg/kg), another NMDA receptor antagonist, also blocked the effect of D-baclofen (n = 6) but it had no effect on the pressor responses produced by intrathecal administration of carbachol (27.4 nmol; n = 6). In additional experiments, L-baclofen (70 nmol) had no effect on the increases in heart rate and arterial pressure produced by N-methyl-D-aspartic acid (NMDA) (2 nmol; n = 8). These results indicate that D-baclofen increases arterial pressure via an NMDA receptor-mediated mechanism, perhaps by provoking the release of an endogenous ligand which activates these receptors.

Glutamate-immunoreactive synapses on retrogradely-labelled sympathetic preganglionic neurons in rat thoracic spinal cord

Retrograde tracing with cholera toxin B subunit (CTB) combined with post-embedding immunogold labelling was used to demonstrate the presence of glutamate-immunoreactive synapses on sympathetic preganglionic neurons that project to the adrenal medulla or to the superior cervical ganglion in rat thoracic spinal cord. At the electron microscope level, glutamate-immunoreactive synapses were found on retrogradely labelled nerve cell bodies and on dendrites of all sizes. Two-thirds of the vesicle-containing axon profiles that were directly apposed to, or synapsed on, CTB-immunoreactive sympathoadrenal neurons were glutamate positive. The proportion of glutamate-immunoreactive contacts and synapses on sympathoadrenal neurons decreased to zero when the anti-glutamate antiserum was absorbed with increasing concentrations of glutamate from 0.1 mM to 10 mM. Double immunogold labelling for glutamate and gamma-aminobutyric acid (GABA) showed that glutamate-immunoreactive profiles did not contain GABA and that GABA-immunoreactive profiles did not contain glutamate. These results suggest that glutamate is the major excitatory neurotransmitter to sympathoadrenal neurons and possibly to other sympathetic preganglionic neurons in the intermediolateral cell column of the spinal cord.

Properties of ventrolateral medullary neurons that respond to muscular contraction

Previous results from this laboratory have suggested that neurons in the ventrolateral medulla (VLM) modulate the pressor response to muscular contraction. The purpose of the present study was to determine 1) if VLM neurons with a discharge pattern related to sympathetic discharge and/or the cardiac cycle are stimulated during muscular contraction, 2) if the neurons activated by muscular contraction project to the intermediolateral columns of the spinal cord and 3) the location of glutamate immunoreactive neurons in the medulla. Single-unit responses of ventrolateral medullary neurons to hindlimb muscular contraction evoked by ventral root (L7 and S1) stimulation were recorded in one group of anesthetized cats. Computer analyses were performed to determine if the resting discharge of VLM neurons correlated temporally with sympathetic nerve discharge and/or the cardiac cycle. The discharge rate of 21 of 27 neurons which had a discharge related to sympathetic nerve activity increased during muscular contraction. Neurons in some of the experiments were tested for axonal projections to the intermediolateral nucleus (T2 or T5) of the spinal cord with antidromic activation techniques. The discharge pattern of 78% of the VLM neurons which were activated antidromically was related to the cardiac cycle or sympathetic nerve discharge. Most (92%) reticulospinal VLM neurons with cardiovascular related discharge were excited by muscular contraction. In a second set of experiments, glutamate immunoreactivity was demonstrated in neurons within an area overlapping the location of VLM neurons which were excited by muscular contraction. These findings suggest that reticulospinal neurons in the ventrolateral medulla which have a discharge pattern related to cardiovascular activity contribute to the pressor reflex evoked by muscular contraction. These neurons may utilize glutamate as a neurotransmitter.

Characteristics of endothelin binding sites in the spinal cord of spontaneously hypertensive rats

The binding of [125I]sarafotoxin 6b (SRT 6b) and [125I]endothelin-1 (ET-1) to endothelin (ET) receptors of neuronal membranes prepared from cerebral cortex and spinal cord of 8-week-old spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) was determined. SHR had significantly higher blood pressure as compared to WKY. Heart rate was similar in SHR and WKY. [125I]SRT 6b and [125I] ET-1 bound to the membranes of cerebral cortex and spinal cord at a single, high affinity site. The binding of [125I]SRT 6b and [125I]ET-1 in the cerebral cortex and spinal cord membranes was found to be similar in SHR and WKY. Concentration-dependent inhibition of [125I]ET-1 binding in spinal cord membranes by unlabeled ET-1, ET-2 and ET-3 was performed. The Ki values were found to be 2.35 +/- 0.68, 0.29 +/- 0.08 and 24.10 +/- 5.90 nM for ET-1, ET-2 and ET-3, respectively in WKY. The Ki values of ET-1 and ET-2 were found to be similar in WKY and SHR. However, the Ki value of ET-3 were found to be significantly lower (P less than 0.004) in SHR as compared to WKY. Concentration-dependent inhibition of [125I]SRT 6b binding in spinal cord membranes by unlabeled ET-1, ET-2 and ET-3 were also performed. The Ki values were found to be 9.50 +/- 2.10 pM, 0.17 +/- 0.04 nM and 31.20 +/- 6.00 nM for ET-1, ET-2 and ET-3, respectively in WKY. The Ki values of ET-1 and ET-2 were found to be similar in WKY and SHR.(ABSTRACT TRUNCATED AT 250 WORDS)

Rostral ventrolateral medulla: a source of the glutamatergic innervation of the sympathetic intermediolateral nucleus

To determine whether the sympathoexcitatory projection from the rostral ventrolateral medulla (RVL) to the sympathetic intermediolateral nucleus (IML) of the spinal cord might use glutamate as an excitatory transmitter, we performed a dual-label, transport and immunocytochemical ultrastructural study. Axon terminals within the IML were examined to determine whether anterogradely transported Phaseolus vulgaris-leucoagglutinin (PHA-L) following injections into the RVL, was colocalized with glutamate immunoreactivity using an antibody to hemocyanin-conjugated L-glutamate (Hepler et al., J. Histochem. Cytochem., 36 (1988) 13-22). Transported PHA-L was visualized with the peroxidase-antiperoxidase technique while glutamate-like immunoreactivity was localized within the same section of the thoracic spinal cord with immunoautoradiography. By light microscopy, PHA-L immunoreactivity was found within a plexus of fine fibers and varicose processes localized to the IML. Silver grains indicative of glutamate immunoreactivity were concentrated over the IML and also over the superficial layers of the dorsal horn. Electron microscopic analysis revealed PHA-L immunoreactivity in axons and axon terminals within the IML. They ranged in diameter from 0.5 to 2.0 microns, contained numerous small clear and 0-3 large, dense-core vesicles, and formed primarily asymmetric synaptic contacts on small dendrites of IML neurons. Some of the PHA-L immunoreactive terminals making asymmetric (excitatory) synaptic contacts on the small dendrites of IML neurons also contained glutamate-like immunoreactivity. We conclude that at least a portion of the input to the IML from the RVL uses glutamate as its transmitter.(ABSTRACT TRUNCATED AT 250 WORDS)

Glutamate in spinally projecting neurons of the rostral ventral medulla

Phosphate activated glutaminase (PAG), an enzyme of glutamate synthesis, was localized by immunohistochemistry in all PNMT-immunoreactive and all serotonin-immunoreactive neurons in the rostral ventral medulla of the rat. Between 71 and 83% of bulbospinal neurons localised in the rostral ventral medulla projecting to the intermediolateral cell column in the upper thoracic spinal cord contained PAG immunoreactivity. Of these bulbospinal PAG-immunoreactive neurons 17-27% contained PNMT immunoreactivity and 9-16% contained serotonin immunoreactivity. Other bulbospinal PAG-immunoreactive neurons (60-70%) contained neither PNMT- nor serotonin immunoreactivity. The results provide anatomical evidence suggestive of a glutamatergic input to the sympathetic preganglionic neurons of the spinal cord arising from different populations of neurons located in the rostral ventral medulla.

Effect of blockade of spinal NMDA receptors on sympathoexcitation and cardiovascular responses produced by cerebral ischemia

Pharmacological blockade of N-methyl-D-aspartic acid (NMDA) receptors in the spinal cord was produced by intrathecal administration of the NMDA receptor antagonist D-2-amino-7-phosphonoheptanoic acid. Blockade of spinal NMDA receptors significantly reduced arterial pressure, heart rate and sympathetic nerve activity and reduced by approximately 50% the pressor and sympathoexcitatory responses evoked by cerebral ischemia. These results indicate that NMDA receptors in the spinal cord participate in the maintenance and regulation of the sympathetic nervous system and suggest that excitatory amino acid neurotransmitters may play a role in sympathoexcitation produced by physiological activation of descending bulbospinal pathways.

Midbrain central grey: regional haemodynamic control and excitatory amino acidergic mechanisms

The haemodynamic responses to electrical and chemical stimulation of the periaqueductal or central grey (CG) was investigated in urethane-anaesthetized rats. CG stimulation resulted in a characteristics pattern of mesenteric and renal vasoconstriction accompanied by modest hindquarter vasodilatation. This haemodynamic response was also accompanied by widening of the palpebral fissure, tachycardia and by twitching of the vibrissae. This constellation of physiological responses constitutes the 'defence reaction' and indicates that the CG area under investigation is involved in these phenomena. Both electrical and chemical (kainic acid) lesions of the pressor area of the rostral ventrolateral medulla (RVLM) attenuated the pressor responses to CG stimulation. Intrathecal administration of the excitatory amino acid receptor antagonist kynurenic acid (0.5 mumole/10 microliter) markedly reduced the pressor responses produced by stimulation of both the CG and the RVLM. These results provide additional evidence in support of the notion that neurons arising in the CG relay in the RVLM where they may, in turn, communicate with a descending excitatory amino-acidergic pathway involved in cardiovascular control.

Intrathecal kynurenate reduces arterial pressure, heart rate and baroreceptor-heart rate reflex in conscious rats

In the present study, an excitatory amino acid (EAA) pathway in the spinal cord which maintains sympathetic vasomotor tone in conscious rats has been investigated. To this end, the cardiovascular effects of an intrathecally administered EAA antagonist, kynurenate (KYN), were studied in conscious rats. KYN (0.5 mumol in 10 microliters) caused a dramatic reduction in mean arterial pressure (MAP) and heart rate (HR) that persisted for 2-3 h, and also resulted in extensor paralysis of the hindlimbs. The time courses of fall in MAP and HR and hindlimb paralysis were similar. Baroreceptor-HR reflex activity was also markedly impaired after KYN, suggesting functional diminution of sympathetic outflow at the level of the spinal cord after blockade of EAA receptors by KYN. Xanthurenate, a metabolite of KYN without EAA antagonistic properties, produced negligible effects at the same dose of KYN. While these findings do not identify the putative EAA pathway, they do provide the first demonstration that this system is tonically active in conscious rats.

Preferential release of neuroactive amino acids from the ventrolateral medulla of the rat in vivo as measured by microdialysis

The basal overflow of extracellular endogenous amino acids was measured from the ventrolateral medulla of urethane anaesthetized rats in vivo by microdialysis. Inclusion of a mercury salt, p-chloromercuriphenylsulphonic acid, in the dialysate (Krebs' solution), results in a preferential increase in the overflow of aspartate, glutamate, glycine and GABA. A smaller increase in the overflow of the glutamate precursor and metabolite, glutamine, was also found. There was no significant change in the basal extracellular levels of taurine, asparagine, alanine, serine, ornithine or lysine. Inclusion of a specific GABA uptake inhibitor, nipecotic acid, in the dialysate results in an immediate, dose dependent increase in the overflow of GABA, and to a lesser extent, taurine. Since it is likely that mercury salts increase neurotransmitter release by increasing free intracellular calcium ion concentrations, it is suggested that these results provide further evidence for a physiologically relevant neurotransmitter role for aspartate, glutamate, glycine and GABA in the ventrolateral medulla.

Central serotonergic mechanisms in cardiovascular regulation

This paper reviews the role of central serotonin-containing neurons in the control of blood pressure. Central serotonin nerves have their cell bodies in the brainstem in a number of discrete collections, from where they ascend to ramify throughout the brain, descend to terminate in the spinal cord, or send shorter projections terminating in medulla, pons, and midbrain. Activation of one important ascending serotonin pathway innervating the preoptic region of the hypothalamus causes an increase in blood pressure. Activation of a bulbospinal serotonin projection descending from the ventrolateral medulla (the B3 cell group) to terminate in the intermediolateral cell column (IML) also evokes a pressor response. This pressor response is independent of that elicited by stimulation of the ventrolateral medulla in the adjacent but separate area containing the C1 adrenaline cell group. The pressor action appears to depend on increased release of serotonin, as detected by microdialysis in the area of the IML, and to be mediated by serotonin receptors of the 5HT1 subclass, probably located on sympathetic preganglionic neurons. It is possible that neuroactive excitatory amino acids, such as glutamate or aspartate, and neuropeptides such as substance P, also play a part in the pressor response evoked by stimulation of the ventrolateral medulla in the area of the lateral B3 serotonin cells. This descending serotonin pathway also appears important in mediating the hypotensive action of the antihypertensive drugs methyldopa and clonidine.