A fall in arterial blood pressure produced by inhibition of the caudalmost ventrolateral medulla: the caudal pressor area

The brainstem network controlling blood pressure: an important role for pressor sites in the caudal medulla and cervical spinal cord

: Although medullary control of blood pressure (BP) has been extensively studied, the contribution of critical regions, such as pressor sites in the caudal medulla and upper cervical spinal cord and the lateral tegmental field, remains controversial and underappreciated. A series of pressor sites caudal to the caudal ventrolateral medulla (CVLM), including the caudal pressor area (CPA) and medullocervical pressor area, play an important role in control of BP. Activation and inhibition of these sites elicits pressor and depressor responses, respectively. Basal sympathetic tone is provided principally by the medullary lateral tegmental field and rostral ventrolateral medulla (RVLM). RVLM presympathetic neurons, which project to and drive preganglionic sympathetic somata in the intermediolateral cell column, are powerfully regulated by neurons in CVLM via tonic and phasic inhibition. The current state of knowledge is summarized thus: rostrocaudally organized columns of pressor sites caudal to CVLM extend to the upper cervical spinal cord; CPA pressor responses are RVLM-dependent; CPA mediates pressor responses by (first) inhibiting RVLM-projecting inhibitory CVLM units and (second) activating RVLM-projecting excitatory CVLM units; the chemoreflex is CPA-dependent; the baroreflex is CPA-independent; pressor responses to raphe obscurus stimulation are CPA-dependent; and medullocervical pressor area pressor responses are RVLM-independent, likely mediated by direct projections to the intermediolateral cell column. In this review, we seek to underscore and characterize the critical role played by the caudal medulla and upper cervical spinal cord in BP regulation and highlight important gaps in knowledge in interactions between the caudal medulla and other regions controlling BP, which may prove critical in revealing central mechanisms underlying pathophysiology of, and pharmacotherapeutic targets for, hypertension.

Paraventricular hypothalamic nucleus: axonal projections to the brainstem

The paraventricular hypothalamic nucleus (PVH) contains many neurons that innervate the brainstem, but information regarding their target sites remains incomplete. Here we labeled neurons in the rat PVH with an anterograde axonal tracer, Phaseolus vulgaris leucoagglutinin (PHAL), and studied their descending projections in reference to specific neuronal subpopulations throughout the brainstem. While many of their target sites were identified previously, numerous new observations were made. Major findings include: 1) In the midbrain, the PVH projects lightly to the ventral tegmental area, Edinger-Westphal nucleus, ventrolateral periaqueductal gray matter, reticular formation, pedunculopontine tegmental nucleus, and dorsal raphe nucleus. 2) In the dorsal pons, the PVH projects heavily to the pre-locus coeruleus, yet very little to the catecholamine neurons in the locus coeruleus, and selectively targets the viscerosensory subregions of the parabrachial nucleus. 3) In the ventral medulla, the superior salivatory nucleus, retrotrapezoid nucleus, compact and external formations of the nucleus ambiguous, A1 and caudal C1 catecholamine neurons, and caudal pressor area receive dense axonal projections, generally exceeding the PVH projection to the rostral C1 region. 4) The medial nucleus of the solitary tract (including A2 noradrenergic and aldosterone-sensitive neurons) receives the most extensive projections of the PVH, substantially more than the dorsal vagal nucleus or area postrema. Our findings suggest that the PVH may modulate a range of homeostatic functions, including cerebral and ocular blood flow, corneal and nasal hydration, ingestive behavior, sodium intake, and glucose metabolism, as well as cardiovascular, gastrointestinal, and respiratory activities.

Role of the medulla oblongata in normal and high arterial blood pressure regulation: the contribution of Escola Paulista de Medicina - UNIFESP

Several forms of experimental evidence gathered in the last 37 years have unequivocally established that the medulla oblongata harbors the main neural circuits responsible for generating the vasomotor tone and regulating arterial blood pressure. Our current understanding of this circuitry derives mainly from the studies of Pedro Guertzenstein, a former student who became Professor of Physiology at UNIFESP later, and his colleagues. In this review, we have summarized the main findings as well as our collaboration to a further understanding of the ventrolateral medulla and the control of arterial blood pressure under normal and pathological conditions.

Maps of cardiovascular and respiratory regions of rat ventral medulla: focus on the caudal medulla

The ventral medulla oblongata is critical for cardiorespiratory regulation. Here we review previous literature relating to sites within the ventral medulla that have been identified as having a 'cardiovascular' or 'respiratory' function. Together with the maps generated here, of sites from which cardiovascular and respiratory responses were evoked by glutamate microinjection, specific 'cardiovascular' regions have been defined and delineated. Commonly investigated regions, including the vasopressor rostral ventrolateral medulla (RVLM) and vasodepressor caudal ventrolateral medulla (CVLM), or areas only described by others, such as the medullary cerebral vasodilator area, are included for completeness. Emphasis is given to the caudal medulla, where three pressor regions, the caudal pressor area (CPA), the intermediate pressor area (IPA) and the medullo-cervical pressor area (MCPA), caudal to the vasodepressor CVLM were defined in the original data provided. The IPA is most responsive under pentobarbitone rather than urethane anaesthesia clearly delineating it from both the rostrally located CPA and the caudally located MCPA. The description of these multiple pressor areas appears to clarify the confusion that surrounds the identification of the 'CPA'. Also noted is a vasopressor region adjacent to the vasodepressor CVLM. Apart from the well described ventral respiratory column, a region medial to the pre-Bötzinger is described, from which increases in both phrenic nerve frequency and amplitude were evoked. Limitations associated with the technique of glutamate microinjection to define functionally specific regions are discussed. Particular effort has been made to define and delineate the regions with respect to ventrally located anatomical landmarks rather than the commonly used ventral surface or dorsal landmarks such as the obex or calamus scriptorius that may vary with the brain orientation or histological processing. This should ensure that a region can easily be defined by all investigators. Study of defined regions will help expedite the identification of the role of the multiple cell groups with diverse neurotransmitter complements that exist even within each of the regions described, in coordinating the delivery of oxygenated blood to the tissues.

Pressor responses to nasal stimulation are unaltered after disrupting the CPA

Stimulation of either the caudal pressor area (CPA) in the most caudal ventrolateral medulla with glutamate, or the nasal mucosa with ammonia vapors, induces an increase in mean arterial blood pressure (MABP). In the present study, we determined if neurons in the CPA serve as a relay for the increase in MABP seen after nasal stimulation. Ammonia vapors stimulated the nasal mucosa of rats anesthetized with either urethane alone or ketamine/xylazine and urethane to induce an increase in MABP, a bradycardia, and an apnea. Bilateral injections (50 nl) of glycine (1 M) or muscimol (2 mM) were placed in the CPA and the nasal mucosa again stimulated. The increases in MABP, the bradycardia and the duration of apnea to nasal stimulation were unchanged after either injection. However, resting MABP and HR were decreased significantly after glycine injections and resting MABP and resting respiratory rate were decreased after muscimol injections. The increase in MABP seen with nasal stimulation also did not change after multiple bilateral injections (3x40 nl) of ibotenate (5 microg/microl) in the CPA, but the bradycardia was eliminated and the duration of apnea was significantly shorter. These results suggest that the increase in MABP induced by nasal stimulation is via routes that do not include neurons in the CPA.

Commissural nucleus of the solitary tract is important for cardiovascular responses to caudal pressor area activation

The nucleus of the solitary tract (NTS) is the site of the first synapse of cardiovascular afferent fibers in the central nervous system. Important mechanisms for cardiovascular regulation are also present in the caudal pressor area (CPA) localized at the caudal end of the ventrolateral medulla. In the present study we sought to investigate the role of the commissural subnucleus of the NTS (commNTS) on pressor and tachycardic responses induced by l-glutamate injected into the CPA. Male Holtzman rats (n=8 rats/group) anesthetized with urethane (1.2 g/kg of body weight, iv) received injections of the GABA(A) receptor agonist muscimol into the commNTS. Unilateral injection of L-glutamate (10 nmol/100 nL) into the CPA increased mean arterial pressure (MAP, 31+/-4 mm Hg, vs. saline: 3+/-2 mm Hg) and heart rate (HR, 44+/-8 bpm, vs. saline: 10 +/-7 bpm). Inhibition of commNTS neurons with muscimol (120 pmol/60 nL) abolished the increase in MAP (9+/-4 mm Hg) and HR (17+/-7 bpm) produced by l-glutamate into the CPA. The present results suggest that the pressor and tachycardic responses to CPA activation are dependent on commNTS mechanisms.

Novel axonal projection from the caudal end of the ventrolateral medulla to the intermediolateral cell column

We used an optical imaging technique to investigate whether axons of neurons in the caudal end of the ventrolateral medulla (CeVLM), as well as axons of neurons in the rostral ventrolateral medulla (RVLM), project to neurons in the intermediolateral cell column (IML) of the spinal cord. Brain stem-spinal cord preparations from neonatal normotensive Wistar-Kyoto and spontaneously hypertensive rats were stained with a voltage-sensitive dye, and responses to electrical stimulation of the IML at the Th2level were detected as changes in fluorescence intensity with an optical imaging apparatus (MiCAM-01). The results were as follows: 1) depolarizing responses to IML stimulation during low-Ca high-Mg superfusion were detected on the ventral surface of the medulla at the level of the CeVLM, as well as at the level of the RVLM, 2) depolarizing responses were also detected on cross sections at the level of the CeVLM, and they had a latency of 24.0 ± 5.5 (SD) ms, 3) antidromic action potentials in response to IML stimulation were demonstrated in the CeVLM neurons where optical images were detected, and 4) glutamate application to the CeVLM increased the frequency of excitatory postsynaptic potentials (EPSPs) and induced depolarization of the IML neurons. The optical imaging findings suggested a novel axonal and functional projection from neurons in the CeVLM to the IML. The increase in EPSPs of the IML neurons in response to glutamate application suggests that the CeVLM participates in the regulation of sympathetic nerve activity and blood pressure and may correspond to the caudal pressor area.

A novel pressor area at the medullo-cervical junction that is not dependent on the RVLM: efferent pathways and chemical mediators

Chemical stimulation of a region extending from the most caudal ventrolateral medulla into the upper cervical spinal cord evoked large sympathetically mediated pressor responses. These responses were not dependent on the integrity of the rostral ventrolateral medulla (RVLM) and may be mediated by glutamatergic neurons embedded in the white matter that project to the thoracic spinal cord. We term this new region the medullo-cervical pressor area (MCPA). This region is distinct from the caudal pressor area, because blockade of the RVLM with muscimol inhibited this pressor response but not that evoked from the MCPA. This is the first study to provide functional evidence for a cardiovascular role for neurons in the cervical spinal cord white matter that innervate sympathetic preganglionic neurons (Jansen and Loewy, 1997). Using retrograde tracing, in combination with immunohistochemistry and in situ hybridization, we identified two groups of spinally projecting neurons in the region. Approximately 50% of neurons in one group were excitatory because they contained vesicular glutamate transporter 1 (VGluT1)/VGluT2 mRNA, whereas the other contained a mixed population of neurons, some of which contained either VGluT1/VGluT2 or GAD67 (glutamic acid decarboxylase 67) mRNA. Despite the fact that activation of the MCPA causes potent sympathoexcitation, it does not act to restore arterial pressure after chemical lesion of the RVLM so that a role for this novel descending sympathoexcitatory region remains to be elucidated.

Differential baroreceptor modulation mediated by the ventrolateral medulla

Previous studies have shown that pharmacological stimulation of a region denominated caudal pressor area (CPA), located in the caudal end of the ventrolateral medulla, induces increases in arterial blood pressure (BP). The aim of this study was to compare the responses on renal sympathetic nerve activity (rSNA) and BP responses mediated by stimulation of CPA or rostral ventrolateral medulla (RVLM), in intact or sino-aortic barodenervated rats. Male Wistar rats (300-350 g, n=15) were anesthetized (urethane 1.2 to 1.4 g/kg, i.v.) and artificially ventilated. The mean arterial pressure (MAP) and rSNA were measured during bilateral glutamate microinjection (10 nmo/100 nl) into the CPA or into the RVLM. Glutamatergic stimulation of the RVLM increased MAP (46+/-7 mm Hg) and rSNA (82+/-21%); during CPA stimulation, MAP and rSNA increased 60+/-7 mm Hg and 93+/-9%, respectively. However, despite the similarity of responses mediated by both regions, the duration of rSNA and blood pressure responses mediated by the CPA were significantly longer than the duration of the responses mediated by the RVLM. After barodenervation, there was an increase in the time-course and magnitude of sympathetic response only in response to RVLM stimulation but not in response to CPA. The results suggest a differential baroreceptor modulation on rSNA mediated by the ventrolateral medulla neurons. Glutamatergic activation of CPA neurons can cause large increases in the rSNA and BP with a weaker baroreceptor modulation when compared to responses mediated by the RVLM neurons.

Lesion of the caudal ventrolateral medulla prevents the induction of hypertension by adenosine receptor blockade in rats

The continuous infusion for 7 days of the adenosine receptor antagonist 1,3-dipropyl-8-sulfophenylxanthine (DPSPX) causes a sustained hypertension in rats, with an enhancement of sympathetic neurotransmission and activation of the renin-angiotensin system. We studied the involvement of the caudal ventrolateral medulla in the establishment of this hypertensive model by evaluating the effect of local lesioning in blood pressure (BP). Male adult Wistar rats received stereotaxic injections of 0.3 mul of saline or quinolinic acid (QA; 180 mM) in the caudal ventrolateral medulla followed by abdominal implant of minipump for infusion of saline or DPSPX (90 microg(-1) kg(-1) h(-1)). BP was measured in conscious animals every 2 days for 12 days. The sustained increase of BP (22.1 mm Hg; P < 0.001) detected in rats infused with DPSPX was reverted (6.7 mm Hg; P > 0.05) from day six onwards in animals with lesion of the lateralmost part of caudal ventrolateral medulla (VLMlat). The present results suggest that the development of hypertension induced by adenosine receptor antagonist involves the participation of the VLMlat. They further add new data as to the functional complexity of this medullary area involved in a variety of functions such as cardiovascular, respiratory, motor and pain control.

Defining projections from the caudal pressor area of the caudal ventrolateral medulla

We previously defined a functional area in the caudal medulla oblongata that elicits an increase in arterial pressure when stimulated (Sun and Panneton [2002] Am. J. Physiol. 283:R768-R778). In the present study, anterograde and retrograde tracing techniques were used to investigate the projections of this caudal pressor area (CPA) to the medulla and pons. Injections of biotinylated dextran amine into the CPA resulted in numerous labeled fibers with varicosities in the ipsilateral subnucleus reticularis dorsalis, commissural subnucleus of the nucleus tractus solitarii, lateral medulla, medial facial nucleus, A5 area, lateral vestibular nucleus, and internal lateral subnucleus of the parabrachial complex. Sparser projections were found ipsilaterally in the pressor and depressor areas of the medulla and the spinal trigeminal nucleus and contralaterally in the CPA. Injections of the retrograde tracer Fluoro-Gold into these areas labeled neurons in the CPA as well as the nearby medullary dorsal horn and reticular formation. However, we conclude that the CPA projects preferentially to the subnucleus reticularis dorsalis, commissural nucleus tractus solitarii, lateral medulla, A5 area, and internal lateral parabrachial nucleus. Weaker projections were seen to the CVLM and RVLM and to the contralateral CPA. The projection to the facial nucleus arises from nearby reticular neurons, whereas projections to the vestibular nucleus arise from the lateral reticular nucleus. Labeled neurons in the CPA consisted mostly of small bipolar and some triangular neurons. The projection to the CVLM, or to A5 area, may provide for the increase in arterial pressure with CPA stimulation. However, most of the projections described herein are to nuclei implicated in the processing of noxious information. This implies a unique role for the CPA in somatoautonomic regulation.

Cardiovascular responses to microinjections of GABA or anesthetics into the rostral ventrolateral medulla of conscious and anesthetized rats

The rostral ventrolateral medulla (RVLM) contains neurons involved in tonic and reflex control of arterial pressure. We describe the effects of gamma-aminobutyric acid (GABA) and anesthetics injected into the RVLM of conscious and urethane (1.2 g/kg, iv) anesthetized Wistar rats (300-350 g). In conscious rats, bilateral microinjection of GABA (50 nmol/200 nl) induced a small but significant decrease in blood pressure (from 130 +/- 3.6 to 110 +/- 5.6 mmHg, N = 7). A similar response was observed with sodium pentobarbital microinjection (24 nmol/200 nl). However, in the same animals, the fall in blood pressure induced by GABA (from 121 +/- 8.9 to 76 +/- 8.8 mmHg, N = 7) or pentobarbital (from 118 +/- 4.5 to 57 +/- 11.3 mmHg, N = 6) was significantly increased after urethane anesthesia. In contrast, there was no difference between conscious (from 117 +/- 4.1 to 92 +/- 5.9 mmHg, N = 7) and anesthetized rats (from 123 +/- 6.9 to 87 +/- 8.7 mmHg, N = 7) when lidocaine (34 nmol/200 nl) was microinjected into the RVLM. The heart rate variations were not consistent and only eventually reached significance in conscious or anesthetized rats. The right position of pipettes was confirmed by histology and glutamate microinjection into the RVLM. These findings suggest that in conscious animals the RVLM, in association with the other sympathetic premotor neurons, is responsible for the maintenance of sympathetic vasomotor tone during bilateral RVLM inhibition. Activity of one or more of these premotor neurons outside the RVLM can compensate for the effects of RVLM inhibition. In addition, the effects of lidocaine suggest that fibers passing through the RVLM are involved in the maintenance of blood pressure in conscious animals during RVLM inhibition.

Medullary and supramedullary mechanisms regulating sympathetic vasomotor tone

AIM

Neurons in the rostral ventrolateral medulla (RVLM) that project directly to sympathetic preganglionic neurons in the spinal cord play a critical role in maintaining tonic activity in sympathetic vasomotor nerves. Intracellular recordings in vivo from putative RVLM presympathetic neurons have demonstrated that under resting conditions these neurons display an irregular tonic firing rate, and also receive both excitatory and inhibitory synaptic inputs. This paper will briefly review some recent findings on the role of glutamate, GABA and angiotensin II (Ang II) receptors in maintaining the tonic activity of RVLM presympathetic neurons.

RESULTS

Based on these findings, the following hypotheses will be discussed: (1) RVLM neurons receive tonic glutamatergic excitatory inputs, which originate from both medullary and supramedullary sources; (2) at least some neurons that project to and tonically inhibit RVLM presympathetic neurons are themselves tonically inhibited by GABAergic inputs originating from neurons in the caudalmost part of the ventrolateral medulla (caudal pressor area); (3) under normal conditions, Ang II receptors in the RVLM do not contribute significantly to the tonic activity of RVLM presympathetic neurons, but may do so in abnormal conditions such as heart failure or neurogenic hypertension; (4) RVLM presympathetic neurons maintain a significant level of tonic resting activity even when glutamate, GABA and Ang II receptors on the neurons are completely blocked. Under these conditions, the tonic activity is a consequence either of the intrinsic membrane properties of the neurons (autoactivity) or of synaptic inputs mediated by receptors other than glutamate, GABA or Ang II receptors.

CONCLUSION

The current evidence indicates that the resting activity of RVLM presympathetic neurons is determined by the balance of powerful tonic excitatory and inhibitory synaptic inputs. Ang II receptors also contribute to the raised resting activity of these neurons in some pathological conditions.

Effects of prolactin-releasing peptide microinjection into the ventrolateral medulla on arterial pressure and sympathetic activity in rats

Prolactin-releasing peptide (PrRP), originally isolated from the hypothalamus, is highly localized in the cardiovascular regions of the medulla, and intracerebroventricular administration of PrRP causes a pressor response. In the present study we investigated the cardiovascular effects of PrRP applied to functionally different areas of the ventrolateral medulla (VLM), and to the nucleus tractus solitarius (NTS) and the area postrema (AP). In urethane-anesthetized rats, microinjection of PrRP into the pressor area of the most caudal VLM, recognized as the caudal pressor area in the rat, elicited dose-dependent increases in mean arterial pressure, heart rate, and renal sympathetic nerve activity. In the same injection area, neither thyrotropin-releasing hormone, corticotropin-releasing hormone nor angiotensin II affected these baseline cardiovascular variables. On the other hand, microinjection of PrRP into more rostral parts of the VLM, i.e. the depressor area of the caudal VLM and the pressor area of the rostral VLM, as well as the NTS and the AP, had no effect on these cardiovascular variables. Immunohistochemical analysis in the medulla revealed that the cardiovascularly PrRP-responsive region contained PrRP-immunoreactive cell bodies and nerve fibers. These results suggest that the most caudal VLM is an action site of PrRP to induce a pressor response, which is mediated, at least partly, by the increase in sympathetic outflow.

The caudal pressor area of the rat: its precise location and projections to the ventrolateral medulla

Investigators have demonstrated pressor areas in the medullas of various species. The present study precisely localized the pressor area in the caudal medulla of the rat and determined its projections to the caudal and rostral ventrolateral medulla. The caudal medulla first was mapped grossly in rats with injections (30 nl) of glutamate (30-, 15-, and 7.5-nmol doses) placed 0.5, 1.0, and 1.5 mm caudal to the calamus scriptorius, 1.0, 1.5, and 2.0 mm lateral to the midline, and 1.8, 1.7, and 1.6 mm ventral to the dorsal medullary surface, respectively, and their arterial pressures were recorded. One of these nine injections showed significant increases in arterial pressure. We micromapped this area with a total of 27 injections of glutamate (10 nl; 5 nmol) placed 300 microm apart at 3 different dorsoventral levels. This micromapping study pinpointed the precise location of caudal pressor area (CPA) neurons in a restricted region lateral to the caudal end of the lateral reticular nucleus and ventromedial to the medullary dorsal horn near the level of the pyramidal decussation. Injections of glutamate into this spot, 1.0 mm caudal to the calamus scriptorius, 2.0 mm lateral to the midline, and 1.7 mm ventral from the dorsal surface of the medulla, induced significant increases in arterial pressure. The neuroanatomic connections of neurons in the CPA to the ventrolateral medulla were then investigated with iontophoretic injections of either the anterograde tracer biotinylated dextran amine (BDA) made into the CPA or the retrograde tracer FluoroGold (FG) injected into either the caudal or rostral ventrolateral medulla. BDA injections resulted in bouton-laden fibers throughout both caudal and rostral portions of the ventrolateral medulla. Either of the FG injections resulted in numerous spindle-shaped neurons interspersed between the longitudinal fiber bundles running through the CPA area. The proximity of the CPA neurons to the A1 catecholaminergic cell group is discussed.

Evidence for tonic disinhibition of RVLM sympathoexcitatory neurons from the caudal pressor area

Previous studies in the rat have shown that a significant proportion of the tonic activity of presympathetic neurons in the rostral ventrolateral medulla (RVLM) is dependent on the tonic activity of neurons within the caudal pressor area (CPA), located in the most caudal part of the caudal ventrolateral medulla (CVLM). In this study, we determined the extent to which tonically active neurons in the CPA contribute to sympathetic vasomotor tone, and we also investigated the pharmacological mechanisms by which these neurons affect the tonic activity of RVLM presympathetic neurons. In anaesthetised rabbits, bilateral injections of the neuroinhibitory compound muscimol into the CVLM at the level of the most caudal part of the lateral reticular nucleus, which corresponds to the anatomical location of the CPA as mapped in the rat, resulted in an immediate profound hypotension and almost complete abolition of renal sympathetic nerve activity (rSNA). In contrast, microinjections into surrounding regions had little or no effect or else evoked a delayed hypotensive response. The hypotensive and sympathoinhibitory response evoked by inhibition of the CPA was greatly delayed by prior injections of the GABA receptor antagonist bicuculline into the RVLM. In contrast, injections of the glutamate receptor antagonist kynurenic acid into the RVLM did not alter the hypotensive and sympathoinhibitory response. The results indicate that neurons within the CPA tonically inhibit other neurons, which, in turn, inhibit RVLM sympathoexcitatory neurons, via a GABAergic synapse. This disinhibition of RVLM neurons by CPA neurons is essential for maintaining resting sympathetic vasomotor tone.

Cardiovascular and respiratory responses to microinjection of L-glutamate into the caudal pressor area in conscious and anesthetized rats

The role of the caudal pressor area (CPA) in the maintenance of vasomotor tonus in anesthetized and decerebrate animals has been clearly established. In conscious animals, however, the participation of CPA in the cardiovascular control remains to be fully elucidated. In the present study, unilateral L-glutamate (L-Glu) (10 and/or 20 nmol/70 nl) microinjection into CPA, in conscious male Wistar rats (250-280 g) caused a significant increase in mean arterial blood pressure (MAP; control: 112 +/- 1.9 mmHg; after 20 nmol L-Glu: 139 +/- 4.5 mmHg, N = 12, P<0.05) and respiratory rate (control: 81 +/- 3.5 breaths/min; after 10 nmol L-Glu: 92 +/- 3 breaths/min, P<0.05; after 20 nmol L-Glu: 104 +/- 5 breaths/min, N = 6, P<0.05). The subsequent anesthesia with urethane caused a significant increase in basal respiratory frequency (conscious: 81 +/- 3.5 breaths/min; under urethane: 107 +/- 1.3 breaths/min, N = 6, P<0.05). Anesthesia also significantly attenuated L-Glu-evoked pressor (conscious: DeltaMAP = +27 mmHg; anesthetized: DeltaMAP = +18 mmHg, P<0.05) and respiratory responses. These results suggest that glutamatergic receptors in the CPA are involved in cardiovascular and respiratory modulation in conscious rats.

Excitatory inputs to the RVLM in the context of the baroreceptor reflex

The central neural circuit mediating baroreceptor control of sympathetic vasomotor outflow involves an excitatory projection from arterial baroreceptors to nucleus tractus solitarius, an excitatory projection from nucleus tractus solitarius to the caudal ventrolateral medulla, an inhibitory projection from the caudal ventrolateral medulla to the rostral ventrolateral medulla (RVLM), and an excitatory projection from the RVLM to sympathetic preganglionic neurons in the spinal cord. For this circuit to be operational, the relevant neurons in the RVLM must be tonically active. Indeed, numerous studies have demonstrated that RVLM vasomotor neurons are tonically active; however, little is known regarding the nature of the tonic excitatory drive to these neurons. We present a model in which RVLM vasomotor neurons are tonically excited by inputs to the RVLM that can be blocked by the excitatory amino acid receptor antagonist, kynurenic acid, as well as an input from the caudal ventrolateral medulla that is not sensitive to kynurenic acid.

Sympathoexcitatory CVLM neurons mediate responses to caudal pressor area stimulation

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

Tonic drive to sympathetic premotor neurons of rostral ventrolateral medulla from caudal pressor area neurons

The responses of sympathetic premotor neurons in the rostral ventrolateral medulla (RVLM) to activation or inactivation of neurons in the caudal pressor area (CPA) were studied in urethan-anesthetized rats. Extracellular recordings were made from 32 barosensitive single units in the RVLM, of which 26 were antidromically activated from the cervical cord. Unilateral microinjections of L-glutamate (0.5-5 nmol) into the CPA increased firing in 13 of 14 premotor neurons by 90 +/- 30% while raising blood pressure. Both ipsilateral and contralateral injections were effective. Unilateral or bilateral inhibition of CPA neuron activity by microinjecting glycine (5-200 nmol/side) lowered blood pressure, while it reduced firing in 9 of 10 and 16 of 17 premotor neurons, respectively, by 45 +/- 9 and 39 +/- 6%. A significant proportion of tonic activity in RVLM sympathetic premotor neurons is thus driven, directly or indirectly, by neurons in the CPA.

Respiratory effects of kynurenic acid microinjected into the ventromedullary surface of the rat

Several studies demonstrate that, within the ventral medullary surface (VMS), excitatory amino acids are necessary components of the neural circuits involved in the tonic and reflex control of respiration and circulation. In the present study we investigated the cardiorespiratory effects of unilateral microinjections of the broad spectrum glutamate antagonist kynurenic acid (2 nmol/200 nl) along the VMS of urethane-anesthetized rats. Within the VMS only one region was responsive to this drug. This area includes most of the intermediate respiratory area, partially overlapping the rostral ventrolateral medulla (IA/RVL). When microinjected into the IA/RVL, kynurenic acid produced a respiratory depression, without changes in mean arterial pressure or heart rate. The respiratory depression observed was characterized by a decrease in ventilation, tidal volume and mean inspiratory flow and an increase in respiratory frequency. Therefore, the observed respiratory depression was entirely due to a reduction in the inspiratory drive. Microinjections of vehicle (200 nl of saline) into this area produced no significant changes in breathing pattern, blood pressure or heart rate. Respiratory depression in response to the blockade of glutamatergic receptors inside the rostral VMS suggests that neurons at this site have an endogenous glutamatergic input controlling the respiratory cycle duration and the inspiratory drive transmission.

Pharmacological properties of the CO2/H+-sensitive area in the ventral medullary surface assessed by the effects of chemical stimulation on respiration

We recently discovered that CO2/H+-sensitive neurons in the ventral medullary surface (VMS) are immunoreactive to glutamate, glutamic acid decarboxylase (GAD), calcineurin and cAMP. We then tested the hypothesis that glutamate, GABA, calcineurin and cAMP affect the activity of CO2/H+-sensitive neurons in the VMS. Using male Wistar rats anesthetized with urethane and pentobarbital, we checked for changes in relative tidal volume (VT) and respiratory frequency (f) in response to injecting the VMS with a variety of test agents dissolved in mock CSF. Respiratory changes occurred immediately and were dose-dependent. (1) 200-1600 pmol Glutamate increased VT but decreased f. The glutamate effect was never abolished by concomitant injection of AP5, a NMDA receptor antagonist, but was abolished by CNQX, an AMPA receptor antagonist, indicating predominance of AMPA receptors in the CO2/H+-sensitive neurons in the VMS. (2) 200-1600 pmol GABA decreased both VT and f. The GABA effect was never abolished by concomitant injection of saclofen, a GABA(B) receptor antagonist, but was abolished by bicuculline, a GABA(A) receptor antagonist, indicating predominance of GABA(A) receptors in the CO2/H+-sensitive neurons in the VMS. (3) 4-32 microg Calcineurin, a Ca2+/calmodulin-dependent protein phosphatase 2B, and 200-1600 pmol FK506, selective inhibitor of calcineurin, had no effect on respiration when they were applied extracellularly, but 400-3200 pmol BAPTA-AM, an intracellular Ca2+-chelating agent, decreased both VT and f, indicating involvement of intracellular Ca2+ in the excitatory mechanisms of respiration. (4) 100-800 pmol IBMX, an enhancer of intracellular cAMP, decreased both VT and f, indicating involvement of cAMP in the inhibitory mechanisms of respiration. These results indicate that the CO2/H+-sensitive neurons in the VMS contain glutamate and/or GABA in cytoplasma, possess AMPA and/or GABA(A) receptors on surface of plasma membrane, and compose the internal circuit, and that their activities are regulated by Ca2+ and cAMP.

The depressor caudal ventrolateral medulla: its correlation with the pressor dorsomedial and ventrolateral medulla and the depressor paramedian reticular nucleus

The functional correlation of the depressor caudal ventrolateral medulla (CVLM) with the two pressor regions, i.e. rostral ventrolateral medulla (RVLM) and dorsomedial medulla (DM), and with another inhibitory region, i.e. the paramedian reticular nucleus (PRN), were studied in cats anesthetized intraperitoneally with chloralose (40 mg/kg) and urethane (400 mg/kg). Systemic arterial pressure (SAP), heart rate (HR) and the sympathetic vertebral nerve activity (VNA) were recorded. The correct location of CVLM, RVLM or DM was determined by their specific responses, i.e. decreases of SAP, HR and VNA, for CVLM increases of these parameters for RVLM and DM, elicited first by electrical stimulation (80 Hz, 0.5 ms, 50-100 microA) then followed by microinjection of glutamate (Glu, 0.25 M, 70 nl). The depressor action of PRN was produced by electrical stimulation only. It was found that the depressor responses caused by the CVLM stimulation were greatly reduced 2 h after lesioning either the RVLM or DM by microinjection of kainic acid (KA, 24 mM, 200 nl) ipsilateral to the side of CVLM stimulation. The CVLM responses were further reduced after the remaining side of RVLM or DM was lesioned. The reduction of the CVLM-depressor responses was more apparent after the RVLM than DM lesioning. Data suggest that the CVLM-depressor responses are mediated through inhibition of the sympathetic-pressor neurons in both RVLM and DM with predominance of the former. Lesioning the PRN by KA and/or combination with DC electrolytic lesion reduced the CVLM-induced depressor responses. In turn, lesioning the CVLM by KA reduced the PRN-induced depressor responses. The reduction in the later manipulation was more apparent in the PRN-depressor responses than the CVLM-depressor responses. Data suggest that part of the PRN depressor action is mediated through activation of the CVLM.

The ventrolateral periaqueductal gray projects to caudal brainstem depressor regions: a functional-anatomical and physiological study

The reaction of shock, a precipitous, life-threatening fall in arterial pressure and heart rate, is evoked often by the combination of deep pain and blood loss following traumatic injury. A similar "shock-like" pattern of response can be evoked by excitation of the ventrolateral midbrain periaqueductal gray. Further, ventrolateral periaqueductal gray neurons are selectively activated by deep somatic or visceral pain and haemorrhage. The pathways mediating ventrolateral periaqueductal gray evoked hypotension and bradycardia are not known. In this study, the projections from the ventrolateral periaqueductal gray to "cardiovascular" regions in the caudal medulla of the rat were examined. Injections of the anterograde tracer, biotinylated dextran amine at physiologically-defined, ventrolateral periaqueductal gray depressor sites, revealed strong projections to the caudal midline medulla and to the depressor region of the caudal ventrolateral medulla. Injections of excitatory amino acids established that substantial falls in arterial pressure could be evoked from the ventrolateral periaqueductal gray-recipient parts of the caudal midline medulla. Injections of the retrograde tracer, cholera toxin subunit B at physiologically-defined, depressor sites in the caudal midline medulla and the caudal ventrolateral medulla confirmed the existence of substantial projections from the ventrolateral periaqueductal gray. Although previous studies have emphasized the importance of projections from the ventrolateral periaqueductal gray to the pressor region of the rostral ventrolateral medulla, this study has revealed the existence of strong ventrolateral periaqueductal gray projections to depressor regions within the caudal medulla (caudal midline medulla and caudal ventrolateral medulla) which likely contribute to ventrolateral periaqueductal gray-mediated hypotension and bradycardia.

Excitatory effects of nitric oxide within the rostral ventrolateral medulla of freely moving rats

The aim of the present study was to examine the participation of NO in the rostral ventrolateral medulla (RVLM) of freely moving rats. We utilized NO donors and L-arginine, which were microinjected into the RVLM. Unilateral microinjection (100 nL) of 2.5 nmol sodium nitroprusside produced a biphasic response consisting of an initial, rapid increase in arterial pressure (AP) from 125+/-5 to 161+/-8 mm Hg (P<.01) and a second, long-lasting response with a progressive increase in AP (maximum delta peak, 34+/-9 mm Hg; P<.01). Another NO donor, S-nitroso-N-acetylpenicillamine (SNAP; 2.5 nmol), also produced immediate hypertension from 118+/-5 mm Hg to 168+/-7 mm Hg (P<.01) but without the second, long-lasting response. L-Arginine (5, 24, and 140 nmol) produced a gradual increase in AP. L-Glutamate (5 nmol) microinjected into the RVLM produced an increase in AP from 122+/-9 mm Hg to 171+/-8 mm Hg (P<.01) and bradycardia from 342+/-10 to 315+/-8 beats/min. This AP response was significantly attenuated, from 115+/-7 to 128+/-9 mm Hg (P<.05), after microinjection of methylene blue (3 nmol) without alterations in heart rate. These results indicate that NO may have an excitatory effect on the RVLM of freely moving rats, probably in association with glutamatergic synapses via cGMP mechanisms.

Cardiorespiratory effects of L-glutamate microinjected into the rat ventral medulla

We investigated the cardiorespiratory effects elicited by microinjections of L-glutamate (L-glu, 25 nmol, 200 nl) at various sites in the ventral medulla (VMS) of urethane-anesthetized rats. The results demonstrated that regions responsive to the drug are located along a column in the VMS extending from the VI cranial nerve to the first cervical nerve in the caudal medulla. Within this column three breathing patterns were elicited from four distinct areas. In the most rostral and caudal portion of this hypothetical column, the breathing patterns observed in response to L-glu were similar and characterized by increases in minute ventilation, tidal volume, inspiratory drive, respiratory frequency, mean arterial blood pressure (MAP) and heart rate (HR). In the regions located between the areas described above two different breathing patterns were obtained without significant changes in MAP or HR. These patterns were characterized by decreases and increases in the respiratory indices analyzed, with the exception of respiratory frequency, which decreased in both regions. These results suggest that within the VMS discrete areas may act as functional units modulating cardiorespiratory responses while in others these functions are spatially segregated.

Role of the rostral ventrolateral medulla in maintenance of blood pressure in rats with Goldblatt hypertension

The aim of the present study was to examine the participation of the rostral ventrolateral medulla (RVLM) in the maintenance of hypertension in rats submitted to the renovascular Goldblatt (two-kidney, one clip) procedure. We inhibited or stimulated this area with the use of drugs such as glycine, L-glutamate, or kynurenic acid. (1) Bilateral microinjection of glycine (100 nmol, 200 nL, n = 13) into the RVLM of hypertensive rats produced a decrease in mean arterial blood pressure (MAP) from 177.2 +/- 29.3 to 102.3 +/- 20.9 mm Hg (P < .05), which was similar to the decrease produced by intravenous administration of hexamethonium. The inhibition of RVLM with glycine in normotensive rats produced a decrease in MAP from 106 +/- 17.1 to 59.7 +/- 7.3 mm Hg (P < .05, n = 9). (2) An impressive increase in MAP from 153.3 +/- 16.3 to 228 +/- 34.9 mm Hg (P < .05) occurred in hypertensive rats after microinjection of L-glutamate (50 nmol, 200 nL, n = 6) into the RVLM. The same procedure caused a significant but less intense increase in MAP from 105 +/- 13.8 to 148.3 +/- 24.9 mm Hg in normotensive rats (P < .05, n = 6). (3) A decrease in MAP from 151.6 +/- 25.3 to 96.8 +/- 22.5 mm Hg occurred in hypertensive rats after microinjection of the broad-spectrum glutamate antagonist kynurenic acid (4 nmol, 200 nL, n = 6) into the RVLM, whereas the same procedure did not change MAP in normotensive animals (n = 6). Heart rate was not significantly affected in any group.(ABSTRACT TRUNCATED AT 250 WORDS)