A vasopressor cell group in the rostral dorsomedial medulla of the rabbit

2019 Ludwig Lecture: Rhythms in sympathetic nerve activity are a key to understanding neural control of the cardiovascular system

This review is based on the Carl Ludwig Distinguished Lecture, presented at the 2019 Experimental Biology Meeting in Orlando, FL, and provides a snapshot of >40 years of work done in collaboration with the late Gerard L. Gebber and colleagues to highlight the importance of considering the rhythmic properties of sympathetic nerve activity (SNA) and brain stem neurons when studying the neural control of autonomic regulation. After first providing some basic information about rhythms, I describe the patterns and potential functions of rhythmic activity recorded from sympathetic nerves under various physiological conditions. I review the evidence that these rhythms reflect the properties of central sympathetic neural networks that include neurons in the caudal medullary raphe, caudal ventrolateral medulla, caudal ventrolateral pons, medullary lateral tegmental field, rostral dorsolateral pons, and rostral ventrolateral medulla. The role of these brain stem areas in mediating steady-state and reflex-induced changes in SNA and blood pressure is discussed. Despite the common appearance of rhythms in SNA, these oscillatory characteristics are often ignored; instead, it is common to simply quantify changes in the amount of SNA to make conclusions about the function of the sympathetic nervous system in mediating responses to a variety of stimuli. This review summarizes work that highlights the need to include an assessment of the changes in the frequency components of SNA in evaluating the cardiovascular responses to various manipulations as well as in determining the role of different brain regions in the neural control of the cardiovascular system.

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.

Medullary lateral tegmental field: control of respiratory rate and vagal lung inflation afferent influences on sympathetic nerve discharge

We used spectral analysis and event-triggered averaging to determine the effects of chemical inactivation of the medullary lateral tegmental field (LTF) on 1) the relationship of intratracheal pressure (ITP, an index of vagal lung inflation afferent activity) to sympathetic nerve discharge (SND) and phrenic nerve activity (PNA) and 2) central respiratory rate in paralyzed, artificially ventilated dial-urethane-anesthetized cats. ITP-SND coherence value at the frequency of artificial ventilation was significantly ( P < 0.05; n = 18) reduced from 0.73 ± 0.04 (mean ± SE) to 0.24 ± 0.04 after bilateral microinjection of muscimol into the LTF. Central respiratory rate was unexpectedly increased in 12 of these experiments (0.28 ± 0.03 vs. 0.95 ± 0.25 Hz). The ITP-PNA coherence value was variably affected by chemical inactivation of the LTF. It was unchanged when central respiratory rate was also not altered, decreased when respiratory rate was increased above the rate of artificial ventilation, and increased when respiratory rate was raised from a value below the rate of artificial ventilation to the same frequency as the ventilator. Chemical inactivation of the LTF increased central respiratory rate in four of six vagotomized cats but did not significantly affect the PNA-SND coherence value. These data demonstrate that the LTF 1) plays a critical role in mediating the effects of vagal lung inflation afferents on SND but not PNA, 2) helps maintain central respiratory rate in the physiological range, but 3) is not involved in the coupling of central respiratory and sympathetic circuits.

Role of the medullary lateral tegmental field in reflex-mediated sympathoexcitation in cats

We tested the hypothesis that blockade of N-methyl-D-aspartate (NMDA) and non-NMDA receptors on medullary lateral tegmental field (LTF) neurons would reduce the sympathoexcitatory responses elicited by electrical stimulation of vagal, trigeminal, and sciatic afferents, posterior hypothalamus, and midbrain periaqueductal gray as well as by activation of arterial chemoreceptors with intravenous NaCN. Bilateral microinjection of a non-NMDA receptor antagonist into LTF of urethane-anesthetized cats significantly decreased vagal afferent-evoked excitatory responses in inferior cardiac and vertebral nerves to 29 +/- 8 and 24 +/- 6% of control (n = 7), respectively. Likewise, blockade of non-NMDA receptors significantly reduced chemoreceptor reflex-induced increases in inferior cardiac (from 210 +/- 22 to 129 +/- 13% of control; n = 4) and vertebral nerves (from 253 +/- 41 to 154 +/- 20% of control; n = 7) and mean arterial pressure (from 39 +/- 7 to 21 +/- 5 mmHg; n = 8). Microinjection of muscimol, but not an NMDA receptor antagonist, caused similar attenuation of these excitatory responses. Sympathoexcitatory responses to the other stimuli were not attenuated by microinjection of a non-NMDA receptor antagonist or muscimol into LTF. In fact, excitatory responses elicited by stimulation of trigeminal, and in some cases sciatic, afferents were enhanced. These data reveal two new roles for the LTF in control of sympathetic nerve activity in cats. One, LTF neurons are involved in mediating sympathoexcitation elicited by activation of vagal afferents and arterial chemoreceptors, primarily via activation of non-NMDA receptors. Two, non-NMDA receptor-mediated activation of other LTF neurons tonically suppresses transmission in trigeminal-sympathetic and sciatic-sympathetic reflex pathways.

Involvement of N-methyl-D-aspartate receptors in post-hypoxic depression of the dorsomedial medulla in cats

Pressor and sympathoexcitatory responses induced by microinjection of glutamate (Glu) into the dorsomedial medulla (DM) were depressed after hypoxia in anesthetized cats. This study was undertaken to investigate which Glu receptor subtypes would be involved in the post-hypoxic depression. Hypoxia was induced by inhalation of a 5% O(2) and 95% N(2) gas mixture. The pressor and sympathoexcitatory responses to microinjections of N-methyl-D-aspartic acid (NMDA) or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) into the DM were significantly depressed after hypoxia, suggesting the involvement of both NMDA and AMPA receptors. However, pretreatment with kynurenic acid or DL-2-amino-5-phosphonopentanoic acid, but not 6-cyano-7-nitroquinoxaline-2,3-dione, 5 min before hypoxia could effectively prevent the post-hypoxic depression of Glu-induced responses. These results further suggest that post-hypoxic depression of Glu-induced responses in the DM was predominately mediated by NMDA receptors.

Trigemino-reticulo-facial and trigemino-reticulo-hypoglossal pathways in the rat

This study was undertaken to identify premotor neurons in the pontomedullary reticular formation serving as relay neurons between the sensory trigeminal complex and the motor nuclei of the VIIth and XIIth nerves. Trigeminoreticular projections were first investigated after injections of anterogradely transported tracers (biotinylated dextran amine, biocytin) into single subdivisions of the sensory trigeminal complex. The results show that the trigeminoreticular projections were abundant from the pars interpolaris (5i) and caudalis (5c) and moderate from pars oralis (5o) of the spinal trigeminal nucleus. Injections into the 5i and 5c produce dense anterograde labeling (1) in the dorsal medullary reticular field; (2) in the parvocellular reticular field, medially adjacent to the 5i; and (3) more rostral in the region dorsal and lateral to the superior olivary nucleus. Some labeled terminals were also found in the intermediate reticular field, whereas only light anterograde labeling was observed in the gigantocellular and oral pontine reticular formation. The 5o sends fibers and terminals throughout the whole reticular formation, with no clear preferential projections within a particular field. Only light projections originated from the principal nucleus (5P). In a second series of experiments, we examined whether premotor neurons in the reticular formation are afferented by trigeminal fibers. Double labeling was performed by injection of an anterograde tracer in the 5i and 5c and retrograde tracer (gold-horseradish peroxidase complex) into the VII or the XII motor nucleus on the same side. Retrogradely labeled neurons in contact with anterogradely labeled boutons were found throughout the reticular formation with predominance in the parvocellular and intermediate reticular fields. These experiments demonstrate the existence of trigeminal disynaptic influences, via reticular neurons of the pontomedullary reticular formation, in the control of orofacial motor behaviors.

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.

Trigeminal-reticular connections: possible pathways for nociception-induced cardiovascular reflex responses in the rat

Cardiovascular regulatory neurons of the ventral medulla and pons are thought to have an important role in the mediation of trigeminal nociception-induced reflex cardiovascular responses. However, the neural pathways that link the spinal trigeminal nucleus with ventral medullary and pontine autonomic cell groups are poorly understood. The present study utilized injections of the highly sensitive anterograde tracer substance biotinylated dextran combined with immunocytochemistry for tyrosine hydroxylase, the synthesizing enzyme for catecholamines, to investigate the distribution and morphology of projections from the spinal trigeminal subnucleus caudalis to ventral medullary and pontine catecholaminergic cell groups. Injection of biotylinated dextran into the dorsal subnucleus caudalis produced dense anterograde labeling in dorsal regions of the medullary and pontine reticular formation including the dorsal medullary reticular field, the parvicellular reticular field, and the parvicellular reticular field pars anterior. In the ventral medullary and pontine reticular formation, light anterograde labeling tended to be distributed in close proximity to the distal dendrites of catecholaminergic neurons located in the C1, A1, and A5 regions. Injections of anterograde tracer into the dorsal medullary reticular field produced dense anterograde labeling in the ventral medullary and pontine reticular formation. Numerous terminal-like varicosities were observed in close proximity to catecholaminergic neurons located in the C1, A1, and A5 regions. These data suggest that trigeminal pain-induced reflex cardiovascular responses involve indirect projections that terminate in the dorsal medullary and pontine reticular formation before reaching ventral medullary and pontine catecholaminergic cell groups known to be involved in cardiovascular regulation.

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.

Attenuation of cardiovascular reactions of vocalized and non-vocalized defence areas of periaqueductal grey following lesions in dorsomedial or ventrolateral medulla of cats

In the pretentorial periaqueductal grey (PAG), the region producing pressor responses, vocalization and other somatic and visceral signs (VPR) of the defence reaction and another region producing pressor responses (PR) were localized by electrical stimulation in adult cats, anesthetized with intraperitoneal chloralose (40 mg/kg) and urethane (400 mg/kg). The pressor responses included increases of systemic arterial pressure (SAP) and heart rate, increases of blood flow in the common carotid and femoral arteries and a decrease of blood flow in the superior mesenteric artery. The VPR was found in a relatively restricted region of the dorsolateral PAG, while PR was found scattered within the dorsal and ventral portions of the lateral PAG. The increase of SAP and the changes of blood flow in the sampled arteries were slightly greater during VPR than PR stimulation. Mild vocalization with a slight increase of SAP but marked increases of carotid and femoral blood flow (vasodilation) could be induced by microinjection of N-methyl D-aspartate (0.2 M, 200 nl) into the VPR and the blood flow increase, particularly that of the femoral artery, was greatly attenuated by atropine (1 mg/kg, i.v.). In order to ascertain the contribution of the medullary pressor areas to the VPR- and PR-induced responses, extensive lesions were made in the dorsomedial (DM) or vetrolateral medulla (VLM) by microinjections of kainic acid (KA, 0.024 M) in 27 of the 42 cats. The resting SAP and blood flow of the three arteries were reduced by lesioning of the VLM more than that of the DM. Responses of SAP and blood flow from activation of the PR and VPR, particularly the latter, were affected more after DM compared to VLM lesioning. These data suggest that, while the pretentorial PAG constitutes the 'defence area,' vocalization is confined exclusively to its dorsolateral region and that both the VLM and DM contribute to the cardiovascular components of defence reactions. The DM appears to have a greater contribution compared to the VLM.

Evidence of a direct projection from the cardiovascular-reactive dorsal medulla to the intermediolateral cell column of the spinal cord in cats as revealed by light and electron microscopy

To ascertain whether the dorsal medulla, a well-established vasopressor structure, would project directly to the sympathetic intermediolateral cell column of the spinal cord, we have carried out both anterograde and retrograde tracing studies in cats. For anterograde tracing, biotin-dextran was iontophoretically delivered into the cardiovascular-reactive dorsal medulla following its functional identification by electrical stimulation. The anterogradely transported biotin-dextran was then visualized using the avidin-biotin-horseradish peroxidase complex method. By light microscopy, dextran-labelled varicose axons were observed bilaterally in the intermediolateral nucleus extending from segments T1 to L3, but concentrated in segments T1-T5, notably at levels T2-T4. Electron microscopic examination revealed the localization of biotin-dextran reaction product in some small myelinated axons and axon terminals in the intermediolateral cell column. The majority of tracer-labelled axonal boutons contained spherical synaptic vesicles and made asymmetric synaptic contacts primarily with small dendrites. A few boutons contained polymorphic synaptic vesicles and tended to form symmetric axodendritic synapses. Spinally projecting neurons of the dorsal medulla were identified using the retrograde transport of horseradish peroxidase injected into the electrically cardiovascular-reactive intermediolateral nucleus. The labelled neurons were localized in the medullary dorsomedial reticular formation ventromedial to the nucleus of the solitary tract, approximately 0.5-5 mm rostral to the obex. The projection was bilateral, but was relatively denser in the rostral portion of the contralateral dorsal medulla. The present findings support the hypothesis that the dorsal medulla, through its direct pathway innervating the intermediolateral nucleus, may serve as a sympathetic premotor structure for regulation of arterial pressure.

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

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

Vasodilatation produced by stimulation of parvocellular reticular formation in the medulla of anesthetized-decerebrate cats

In cats activation of the dorsal facial area (DFA) in the medulla produced an increase of blood flow in the common carotid artery (CCA). This involves flow increases in both intra- and extra-cranial vessels via cranial parasympathetic nerves. In this study, we attempted to explore transmitter mechanisms involved in vasodilatation in extracranial vascular beds due to DFA activation. Cats were anesthetized with intraperitoneal urethane (350 mg/kg) and chloralose (35 mg/kg). Electrical stimulation (100 microA, 20 Hz, 0.5 ms for 5 s) or microinjection of sodium glutamate (Glu, 0.25 M, 50 nl) in DFA increased the velocity of flow in CCA ipsilateral to the stimulation. After control values were obtained, the animals were subjected to decerebration with transection level just rostral to superior colliculi (precollicular decerebration). The increased CCA flow velocity induced by DFA activation was not altered before and after decerebration. Atropine (muscarinic blocker, 0.5-2.0 mg/kg, i.v.) alone only partially attenuated the increase, but the increase was totally blocked by additional N(omega)-L-arginine methyl ester (nitric oxide synthase inhibitor) in 7 out of 9 cats. These findings suggest that extracranial vasodilatation induced by DFA activation does not depend on the sympathetic nervous system, but involves the muscarinic- and nitric-oxide-mediated systems.

Convergence of multiple sensory inputs onto neurons in the dorsolateral medulla in cats

The dorsolateral medulla, including the nucleus reticularis parvicellularis, the cuneate nucleus, and the external cuneate nucleus, is an integrative region for a variety of sensory inputs. The purpose of this study was to determine whether individual neurons respond to a variety of different sensory modalities. To this end, responses of 40 neurons in the dorsolateral medulla to multiple sources of sensory input were assessed in cats anesthetized with alpha-chloralose. Neurons were located in the nucleus reticularis parvicellularis (24 cells, 60%), the cuneate nucleus (10 cells, 25%), and the external cuneate nucleus (6 cells, 15%). All neurons were tested for responses to: electrical stimulation of afferents coursing through the left stellate ganglion and afferents in the left cervical vagus nerve, and somatic, auditory, and visual stimulation. No neurons responded to all five stimuli. Three cells (7.5%) responded to four stimuli, 11 (27.5%) responded to three stimuli, 10 (25.0%) responded to two stimuli, and 15 (37.5%) responded to only a single stimulus. The remaining cell was unresponsive to any stimulus. As a group, neurons in the nucleus reticularis parvicellularis received input from the greatest number of sensory modalities, and cuneate nucleus neurons received input predominantly from somatosensory afferents. External cuneate nucleus neurons displayed response profiles intermediate between nucleus reticularis parvicellularis and cuneate nucleus. In addition, eight neurons (20% of the total) were sensitive to changes in blood pressure. Results of the present study support the hypothesis that neurons in the nucleus reticularis parvicellularis receive convergent inputs from different sensory modalities.2+ behaviors.

Role of the lateral tegmental field in the central sympatho-inhibitory effect of 8-hydroxy-2-(di-n-propylamino)tetralin in the cat

Cardiovascular neurones with sympathetic nerve and cardiac related activity have been detected in the lateral tegmental field and have been classified depending on their response to baroreceptor activation. Baroreflex activation decreased and increased the firing rate of sympatho-excitatory neurones and sympatho-inhibitory neurones respectively. Intravenous (i.v.) administration of 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin) (1 to 30 micrograms/kg) produced a dose-dependent decrease in the firing rate of sympatho-excitatory neurones in conjunction with a decrease in renal sympathetic nerve activity. 8-OH-DPAT had a variable effect (excitation or inhibition) on the firing rate of sympatho-inhibitory lateral tegmental field neurones. Both the excitatory and the inhibitory effects of 8-OH-DPAT were reversed by spiperone (1 mg/kg i.v.). Microinjection of 8-OH-DPAT (2 nmol/site) into the lateral tegmental field decreased blood pressure and renal sympathetic nerve activity. Compared with the effects elicited by microinjection of 8-OH-DPAT into the rostral ventrolateral medulla, the renal sympatho-inhibitory response elicited by 8-OH-DPAT injected into the lateral tegmental field appeared smaller. Chemical lesions of the lateral tegmental field neurones with kainic acid produced hypertension and sympatho-excitation. The correlation between renal sympathetic nerve activity and the R wave of the electrocardiogram disappeared after treatment with kainic acid. Subsequent i.v. 8-OH-DPAT (1 to 100 micrograms/kg) failed to alter blood pressure and renal sympathetic activity. In contrast clonidine (1 to 10 micrograms/kg) still decreased blood pressure, heart rate and renal sympathetic nerve activity in kainic acid-lesioned animals and these effects were reversed by the alpha 2-adrenoceptor antagonist, methoxy-idazoxan (10 micrograms/kg i.v.). The data described indicate that the lateral tegmental field plays a role in the central cardiovascular effects of 8-OH-DPAT and that the integrity of this area is necessary for the expression of the sympatho-inhibitory effects of the drug.

Cardiovascular effects of lesions of the rostral ventrolateral medulla and the nucleus reticularis parvocellularis in rats

This study explored the possibility that the nucleus reticularis parvocellularis (NRP) acts in concert with the rostral ventrolateral medulla (RVLM) in the maintenance of mean arterial pressure (MAP). Bilateral electrolytic or chemical lesions (kainic acid) were placed in three groups of rats anesthetized with sodium pentobarbital. In the different groups, lesions were placed only in the NRP or RVLM or in both the NRP and RVLM (NRPRVLM). Captopril (5 mg/kg, i.v.) and an arginine vasopressin antagonist (AVPX), d-pentamethylene methylated tyrosine (30 micrograms/kg, i.v.), were sequentially administered. A final procedure consisted of spinal cord transection. The RVLM lesions did not significantly alter MAP (before: 116 +/- 3 mmHg; after: 106 +/- 5 mmHg). Sequential administration of captopril and AVPX each reduced MAP to 87 +/- 5 mmHg and 62 +/- 4 mmHg, respectively. Spinal-cord transection lowered MAP to 38 +/- 2 mmHg. Lesions of the NRP also did not alter MAP (before: 113 +/- 4 mmHg; after: 118 +/- 5 mmHg). Captopril reduced MAP to 109 +/- 7 mmHg, AVPX had no effect, and spinal-cord transection decreased MAP to 31 +/- 3 mmHg. In contrast to the lack of effect of lesions of the RVLM or NRP on MAP, profound hypotension was observed after NRPRVLM lesions (before: 113 +/- 3 mmHg; after: 51 +/- 3 mmHg). Subsequent administration of captopril decreased MAP to 39 +/- 2 mmHg, and AVPX lowered MAP to 32 +/- 1 mmHg. Spinal-cord transection reduced MAP to 23 +/- 1 mmHg. Several conclusions can be drawn from this study. First, lesions of the RVLM do not decrease MAP because of compensation by the renin-angiotensin system and AVP secretion which is mediated by the NRP. Second, lesions of the NRP do not affect MAP because the intact RVLM can maintain sympathetic tone. Third, the profound hypotension observed after NRPRLVM lesions occurred because of the simultaneous impairment of sympathetic vasomotor activity and the neuroendocrine vasoconstrictor effects of the renin-angiotensin system and AVP secretion.

Inhibition of sympathetic hypertensive responses in the guinea-pig by prejunctional histamine H3-receptors

1. The effect of (R)-alpha-methylhistamine, a selective H3-histamine receptor agonist, was examined on the neurogenic hypertension and tachycardia that is induced by stimulation of areas in the medulla oblongata of guinea-pigs. Electrical medullary stimulation (32 Hz, 3-5 s trains, 0.5-1.0 ms square pulse, 25-400 microA) produced intensity-dependent increases in blood pressure and a more variable tachycardia. 2. (R)-alpha-methylhistamine inhibited the hypertension and tachycardia due to submaximal CNS stimulation. The inhibition of hypertension by (R)-alpha-methylhistamine was dose-dependent (10-300 micrograms kg-1, i.v.) and was not seen at high intensities of stimulation. 3. (R)-alpha-methylhistamine (300 micrograms kg-1, i.v.) did not attenuate the pressor response to adrenaline (1 and 3 micrograms kg-1, i.v.), indicating that the effect of (R)-alpha-methylhistamine was not mediated by a postjunctional action on smooth muscle. 4. The inhibition of CNS-induced hypertension by (R)-alpha-methylhistamine (300 micrograms kg-1, i.v.) was blocked by the H3 antagonists, thioperamide (ID50 = 0.39 mg kg-1, i.v.), impromidine (ID50 = 0.22 mg kg-1, i.v.) and burimamide (ID50 = 6 mg kg-1, i.v.). The rank order potency of these antagonists is consistent with activity at the H3B receptor subtype. Chlorpheniramine (30 micrograms kg-1, i.v.) and cimetidine (3 mg kg-1, i.v.) did not antagonize the inhibition of CNS-hypertension by (R)-alpha-methylhistamine. 5. These results suggest that (R)-alpha-methylhistamine inhibits sympathetic hypertensive responses in guinea-pigs by activation of prejunctional H3-receptors, possibly located on postganglionic nerve terminals. Furthermore, on the basis of the rank order potency to different H3-antagonists, it appears that the H3B-receptor subtype is involved with H3-receptor responses on vascular sympathetic nerves.

Coexistence of autonomic and somatic mechanisms in the pressor areas of medulla in cats

The effects of electrical stimulation and microinjection of sodium glutamate (0.5 M) in the sympathetic pressor areas of the dorsal medulla (DM), ventrolateral medulla (VLM), and parvocellular nucleus (PVC) on the knee jerk, crossed extension, and evoked potential of the L5 ventral root produced by intermittent electrical stimulation were studied in 98 adult cats anesthetized with chloralose and urethane. During electrical and glutamate stimulation of these pressor areas, in addition to the rise of systemic arterial blood pressure marked inhibition of the spinal reflex was produced, indicating presence of neuronal perikarya responsible for these actions. Mild to moderate augmentation of spinal reflexes was also observed during brain stimulation but only in a few cases. The magnitude of the somatic effects among the pressor areas of the VLM, DM, and PVC subsequent to glutamate activation was about the same. Induced spinal reflex inhibition, independent from the baroreceptor and vagal influence, remained essentially unaltered after acute midcollicular decerebration. The inhibition was also observed in cats decerebellated 8-10 days in advance. The inhibition was not affected after bilateral electrolytic- or kainic-acid-induced lesions in the paramedian reticular nucleus (PRN). On the contrary, PRN-induced spinal reflex inhibition was attenuated after bilateral lesions in the DM or VLM. Data suggest that there coexists neuronal subpopulations in the VLM, DM, and PVC that can affect both the sympathetic pressor systems and spinal reflexes.

Cardiovascular effects of microinjections of quipazine into nuclei of the medulla oblongata in anaesthetized cats: comparison with L-glutamate

Unilateral microinjections of quipazine (0.9 micrograms in 50 nl) into the subretrofacial nucleus produced hypertension and a slight tachycardia associated with an increase in renal sympathetic nerve activity. Microinjections of quipazine lateral, caudal or rostral to this nucleus failed to alter blood pressure and heart rate. Similarly, microinjections of l-glutamate (3 nmol in 15 nl) into the subretrofacial nucleus elicited hypertension, tachycardia and renal sympatho-excitation. The magnitude of the pressor response to quipazine was smaller than the response elicited by l-glutamate but its duration was longer. Microinjections of quipazine into the lateral tegmental field at l-glutamate hypertensive sites failed to alter arterial blood pressure and heart rate. In contrast, microinjections of quipazine into the caudal ventrolateral medulla or into the nucleus tractus solitarii produced hypotension and sympatho-inhibition. These effects were prevented by microinjections of the 5-HT2 receptor antagonists, LY 53857 or BW 501C. The present results indicate that stimulation of 5-HT2 receptors of the subretrofacial nucleus produces hypertension and sympatho-excitation whereas stimulation of 5-HT2 receptors in the caudal ventrolateral medulla and in the nucleus tractus solitarii produces hypotension and sympatho-inhibition.

Identification of cardiovascular neurons in the rostral ventromedial medulla in anesthetized rats

Recent studies have identified a region in the rostral ventromedial medulla (RVMM) of rats that appears to be involved in cardiovascular function. Since these studies used either microinjection of lidocaine or electrical stimulation, the exact contribution of intrinsic neurons as opposed to fibers of passage could not be determined. The present study was performed to map the location of neurons in RVMM from which changes in mean arterial pressure could be elicited by the microinjection of the excitatory amino acid analogue N-methyl-D-aspartic acid (NMDA) (20 ng/50 nl), which selectively activates cell bodies in barbiturate-anesthetized rats. Microinjection of NMDA into RVMM most often (53%) elicited pressor responses (31 +/- 7 mm Hg). On the basis of these responses, RVMM was determined to encompass a large portion of the nucleus gigantocellularis 0.5-1.5 mm lateral to the midline, 0.5-3.5 mm above the ventral surface, and extending from the rostral to the caudal pole of the facial nucleus. Depressor responses (-21 +/- 3 mm Hg) were found at all levels of RVMM but were most concentrated and of the largest magnitude in the rostral and caudal poles of RVMM. Microinjection of the inhibitory neurotransmitter glycine (500 mM) was used to determine whether neurons in RVMM were contributing to the maintenance of arterial pressure. Microinjection of glycine decreased arterial pressure (-15 +/- 2 mm Hg) throughout most of RVMM. Unexpectedly, increases in mean arterial pressure (24 +/- 3 mm Hg) were elicited by microinjection of glycine into the same region in RVMM in which NMDA most frequently elicited pressor responses.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiological evidence for hypothalamic defense area input to cells in the lateral tegmental field of the medulla of rabbits

Single cell recordings were made from neurons in the lateral tegmental field of the medulla (LTFM) during electrical stimulation of the hypothalamic defense area (HDA) of the rabbit. Fifty-four cells were inhibited by HDA stimulation; 23 of these cells received barosensory input. Twenty-two LTFM cells showed an increase in firing rate during HDA stimulation; 10 of these cells received barosensory input. The results of this study provide evidence that the hypothalamic defense area makes functional connections with cardiovascular-influenced neurons in the LTFM.

Sympathoadrenal excitation and inhibition by lower brainstem stimulation in cats

Effects of stimulation of brainstem sites on hemodynamics and plasma catecholamine levels were assessed in cats under chloralose-urethane anesthesia. Pressor areas of the dorsal medulla (DM) and ventrolateral medulla (VLM) and the depressor area of the paramedian reticular nucleus (PRN) were stimulated electrically using a monopolar electrode, or chemically using sodium glutamate microinjection. Plasma levels of norepinephrine (NE) and epinephrine (EPI) were measured in caval blood above the adrenal veins. Electrical stimulation of the DM and VLM produced increases in blood pressure and in plasma NE and EPI levels that were enhanced after acute vagotomies. The NE and EPI responses were attenuated after acute, bilateral adrenalectomies, confirming augmented adrenomedullary secretion, whereas the pressor responses were intact. Injection of sodium glutamate into the same pressor regions of the DM or VLM also produced pressor responses and elevated plasma catecholamine levels, indicating that the responses resulted from activation of neuronal perikarya. Stimulation of the PRN attenuated pressor and catecholamine responses during stimulation of the DM and VLM. The results indicate that pressor responses during stimulation of the DM and VLM are due at least partly to activation of perikarya in these regions, are associated with but not dependent on adrenomedullary activation, and are enhanced after vagotomy; and that neurons of the PRN exert inhibitory modulation of the pressor and adrenomedullary responses during stimulation of VLM and DM.

Cardiovascular and muscle tone changes produced by microinjection of cholinergic and glutamatergic agonists in dorsolateral pons and medial medulla

Cardiovascular and muscle responses to L-glutamic acid (Glut) and cholinergic agonists injected into the dorsolateral pontine tegmentum and medial medullary reticular formation (MMRF) were examined in unanesthetized, decerebrated cats. Glut, or cholinergic agonists acetylcholine (ACh) or carbachol (Carb), were injected into pons and MMRF at sites from which electrical stimulation produced bilateral suppression of muscle tone. Glut injection in MMRF produced hypotension without change in heart rate at doses as low as 1 mM. At higher doses (0.1-0.4 M), Glut induced hypotension with bradycardia in 23 out of 40 injections in both pons and MMRF. High concentrations of microinjected Glut decreased muscle tone or produced complete atonia in pons and rostral MMRF. Both N-methyl-D-aspartic acid (NMDA) and non-NMDA receptor blockers attenuated or completely blocked the cardiovascular response, while only non-NMDA antagonists blocked muscle inhibition to Glut injection. Microinjection of cholinergic agonists produced consistent hypotension in all of the injections in pons and MMRF, however, the heart rate response was variable with increase (27/42), decrease (2/42), or no change (13/42) in rate seen. Cholinergic injection produced muscle atonia in pons and caudal MMRF but not in rostral MMRF. Both muscle and cardiovascular responses were blocked by atropine but not by hexamethonium. The time course of muscle atonia and cardiovascular change differed in most of the experiments. We conclude that muscle tone suppression and cardiovascular response to Glut or cholinergic agonists use different receptor mechanisms and possibly different neurons. However, the co-localization of these mechanisms suggests that neuronal networks in the medial medulla and dorsolateral pons coordinate motor and cardiovascular responses.

Tonic cardiovascular effects of angiotensin II in the ventrolateral medulla

The rostral and caudal parts of the ventrolateral medulla play a major role in the control of blood pressure. Both regions contain a high density of receptor binding sites for angiotensin II, and it has been shown previously that microinjection of angiotensin II into the rostral ventrolateral medulla causes a rise in blood pressure. The aims of this study were to determine the cardiovascular effects of microinjection of angiotensin II and its specific antagonist [Sar1Thr8]angiotensin II into the caudal ventrolateral medulla and to characterize the regional vascular effects elicited by both compounds in the rostral ventrolateral medulla. Microinjections of angiotensin II (0.2-20 pmol) into histologically verified sites in the caudal ventrolateral medulla of anesthetized baroreceptor-denervated rabbits produced dose-dependent decreases in blood pressure and renal sympathetic nerve activity, whereas microinjection of [Sar1Thr8]angiotensin II (40 pmol) produced increases in these variables. In the rostral ventrolateral medulla, angiotensin II (0.02-20 pmol) elicited a dose-dependent increase in blood pressure, iliac vascular resistance, and renal sympathetic nerve activity, whereas [Sar1Thr8]angiotensin II (40 pmol) produced decreases in these variables. The effects on heart rate elicited by either compound in the rostral or caudal ventrolateral medulla were small but were in the same direction as the other cardiovascular variables. In contrast, angiotensin II had no detectable effect on sympathoexcitatory neurons within the rostral dorsomedial medulla, a region that lacks angiotensin II receptor binding sites. The results indicate that endogenous angiotensin II acts on specific receptors within the rostral and caudal parts of the ventrolateral medulla and has a tonic excitatory action on sympathoexcitatory and sympathoinhibitory neurons within these respective regions.

Evaluation of cardiovascular control by neurons in the dorsal medulla of rats

The contribution of sympathoexcitatory neurons in the dorsal medulla to the regulation of arterial pressure and the involvement of such neurons in integration of physiological responses or in the genesis of basal vasomotor tone are not well defined. In the present study discharge of neurons in the dorsal medulla of anesthetized rats was increased or decreased by microinjections of amino acids to examine effects on systemic arterial pressure, heart rate and blood flow and conductance of the renal and femoral vascular beds. Microinjections of excitatory D, L-homocysteic acid caused increases in arterial pressure of 18 +/- 2 mmHg, increases in heart rate ranging from 5-40 beats/min and renal vasoconstriction; the femoral bed constricted after some injections and dilated in response to others. Injections of the inhibitory amino acid glycine caused no consistent decreases in arterial pressure and heart rate and injections of the gamma-aminobutyric acid analog, muscimol were ineffective. These data demonstrate that neurons in the dorsal region of the rat medulla can contribute to regulation of arterial pressure and can integrate generalized differential changes in regional vascular resistance, but do not appear to be essential for the genesis of basal vasomotor tone.

Contribution between dorsal and ventrolateral regions of medulla oblongata in vasomotor function of cats

In cats anesthetized with alpha-chloralose and urethane, the medulla oblongata was explored with electrical and/or chemical stimulation for vasopressor actions. Chemical stimulation included semimicroinjection of sodium glutamate or microinjection of dl-homocysteic acid (DLH). The dorsal (DM), particularly its dorsomedial (DMM) portion, and the ventrolateral (VLM) regions were found to be most sensitive to both electrical and chemical stimulation. In between these two regions there was an area in the ventral portion of the gigantocellular and the parvocellular reticular nuclei that was sensitive to electrical and somewhat sensitive also to chemical stimulation. The responses to chemical stimulation indicate the existence of perikarya in these three areas for vasopressor actions. The most active response of VLM followed microinjection of DLH into the region of nu. paragigantocellularis lateralis (PGL) and subretrofacial nu. These responses were about 30% greater than those from DM where the most active area was found in the DMM region including the nu. and tractus solitarius. When an extensive area of VLM was lesioned bilaterally with kainic acid (KA, 1 micrograms/100 nl), the resting systemic arterial blood pressure (SAP) fell 42% and the pressor response on DM stimulation fell by more than 80%. In contrast, after similar KA bilateral destruction of DM the resting SAP fell 38% and the pressor responses from 67%. When only a limited area in the PGL/subretrofacial nu. of the rostral VLM (3-4 pressor sites, 1 mm apart) on one side was lesioned, the resting SAP remained unaltered while the pressor response on the ipsilateral DMM decreased 76%. On the other hand, when a similar smaller KA lesion (3-4 sites) was made in the DMM, although the resting SAP did not change, the pressor response on stimulation of the ipsilateral rostral VLM decreased 28%. Further lesioning of the contralateral DMM then decreased the rostral VLM pressor response 62% without much alternation in the resting SAP. Results of the present experiments are in accordance with our previous results that neuronal perikarya for vasopressor action exist not only in VLM but also in DM (18,34) with the function of VLM slightly predominant over DM. It seems highly possible that reciprocal innervation exists between DM and VLM, at least between DMM and the PGL/subretrofacial nu. region.

Homocysteic acid elicits pressor responses from ventrolateral medulla and dorsomedial medulla

In rats and cats anesthetized with alpha-chloralose and urethane, the pressor response and its relative reactivity were studied following microinjection of dl-homocysteic acid (DLH, 2 nmol in 40 nl of saline) into the dorsomedial medulla (DM) or the ventrolateral medulla (VLM). DLH, which excites only cell bodies of neurons but not fibers of passage, consistently produced pressor responses in DM and VLM in both cats and rats. The pressor effects elicited from VLM were more pronounced than those from DM. In cats, the most active areas for DLH were found in the rostral and midmedulla; the pons and caudal medulla were less active. The pressor responses from DM and VLM were augmented following bilateral vagotomy, persisted after sectioning of the carotid sinus and aortic depressor nerves, and after cauterization of the carotid bodies with phenol. The response induced by DLH was more apparent from VLM than that from DM. These pressor effects were evoked directly by activation of neurons in these two regions, and were not necessarily related to any homocysteate blockade of baroreceptor and/or chemoreceptor reflexes. These results suggest that in the medulla there reside at least two discrete pressor areas, DM and VLM containing neuronal perikarya.

Limitations of the technique of pressure microinjection of excitatory amino acids for evoking responses from localized regions of the CNS

The aim of this study, performed on anaesthetized cats and rabbits, was to test the assumption that pressure microinjections of excitatory amino acids cause long-lasting excitation of neurones located close to the injection site. Unitary action potentials or antidromic field potentials were recorded from respiratory or 'reticular' neurones in the medulla oblongata and from phrenic motoneurones at different distances from the injection site. Injection of 10-150 nl (5-150 nmol) of L-glutamate or DL-homocysteic acid into these areas resulted in complex and widespread neuronal events. Generally, more distant neurones (500-1300 microns) were excited for variable periods of time (3-15 min), while neurones in the vicinity of the injection site (0-500 microns) showed, after a brief period of excitation time, a long-lasting (up to 30 min) decrease in excitability or silencing of discharge, probably due to a depolarizing block and disturbances in the ionic composition of the extracellular space. These findings show that interpretation of physiological responses following such injections should not be based on an assumption of local neuronal excitation. Some recommendations regarding the use of this technique are made.

Origin of tonic GABAergic inputs to vasopressor neurons in the subretrofacial nucleus of the rabbit

The aims of this study were to determine (1) whether the vasomotor effects reflexly elicited by baroreceptor stimulation are dependent upon gamma-aminobutyric acid (GABA) receptors in the subretrofacial (SRF) nucleus in the rostral ventrolateral medulla; (2) the extent to which inputs other than those arising from peripheral baroreceptors, or transmitted via the nucleus tractus solitarius (NTS), contribute to the tonic GABAergic inhibition of SRF vasopressor cells. Following bilateral injection of a mixture of the GABA antagonist bicuculline methiodide (500 pmol) and GABA agonist muscimol (500 pmol) into the SRF nucleus, the sympathoinhibitory response normally evoked by a rise in arterial pressure (induced by inflating an aortic cuff) was abolished in 4 out of 8 rabbits and reduced in the remainder. For the whole group, the mean reduction in this response was 71%. In other experiments, the pressor response produced by injection of bicuculline methiodide into the SRF nucleus was still present after (1) destruction of the intermediate portion of the NTS, and (2) complete removal of the brain rostral to the pons. We conclude that (1) an inhibitory GABAergic input into the SRF nucleus is an important component of the central pathways mediating baroreceptor inhibition of sympathetic vasomotor tone; (2) the SRF nucleus also receives tonic GABAergic inputs that are intrinsic to the lower brainstem and are independent of baroreceptor or other cardiovascular inputs relayed by the NTS.

Pressor responses evoked by microinjections of L-glutamate into the caudal ventrolateral medulla of the rat

The caudal medulla of the rat was mapped for cardiovascular sensitive regions by recording changes in mean arterial pressure (MAP) and heart rate evoked by microinjections of L-glutamate (1 nmol/50 nl). Using this technique to selectively activate cell soma in the brainstem, a new pressor area in the caudal ventrolateral medulla has been identified. Several sites located approximately 1-1.5 mm posterior to the caudal medullary depressor zone were found where L-glutamate evoked pressor responses of 10-45 mm Hg. The most responsive area was located just dorsal to the lateral aspect of the lateral reticular nucleus at the level of the pyramidal decussation and the caudalmost pole of the inferior olives. Pressor responses at this site averaged 37 +/- 2 mm Hg. Changes in heart rate were inconsistent and both tachycardia and bradycardia were observed. Increases in arterial pressure elicited from the caudal pressor area (CPA) were abolished by ganglionic blockade. Pressor responses evoked from the CPA were also eliminated after functional inactivation of vasopressor neurons in the rostral ventrolateral medulla (RVM) was produced by microinjections of muscimol. Inhibition of CPA neurons by microinjections of GABA had no effect on MAP while GABA markedly reduced MAP when injected into the RVM. These studies demonstrate that a circumscribed region of the caudal ventrolateral medulla contains a population of 'vasopressor' neurons distinct from those located in the rostral medulla. No evidence was obtained to suggest that neural activity in the CPA contributes to the maintenance of arterial pressure. The precise functional role of the CPA in central cardiovascular regulation remains to be determined.

A role of insular cortex in cardiovascular function

We sought to determine whether the insular cortex contributes to the regulation of arterial blood pressure (AP). Responses to electrical and chemical stimulation of the cortex were studied in the anesthetized, paralyzed, and artificially ventilated Sprague-Dawley rat. The insular cortex was initially defined, anatomically, by the distributions of retrogradely labeled perikarya following injections of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) into the nucleus tractus solitarii (NTS). Injections of WGA-HRP into the insular cortex anterogradely labeled terminals in cardiopulmonary and other divisions of the NTS and confirmed projections revealed by retrograde tracing experiments. Electrical stimulation of the insular cortex elicited elevations of AP (less than or equal to 50 mm Hg) and cardioacceleration (less than or equal to 40 bpm). The locations of the most active pressor sites corresponded closely to the locations of retrogradely labeled cells in layer V of granular and posterior agranular areas of the insular cortex (areas 14 and 13) and the extreme capsule. Maximal pressor responses were obtained at a stimulus intensity of three to five times threshold current of 20-30 microA. Responses elicited mostly with higher-threshold currents were also mapped in areas 2a and 5lb and the claustrum and within the corpus callosum. Unilateral injections into the insular pressor area of the excitatory amino acid monosodium glutamate (L-Glu; 0.05 nmol to 10 nmol) or the rigid structural analogue of L-Glu, kainic acid (KA) (0.4 nmol) (which specifically excite perikarya), caused topographically specific elevations in AP and tachycardia. During the course of the anatomical transport studies, new findings were obtained on the organization and characteristics of the cortical innervation of the NTS and the nucleus reticularis parvocellularis. Topographic relationships between the cortex and the NTS were organized in a more complex manner than previously thought. Cells projecting to caudal cardiopulmonary segments of the NTS were fewer and generally located ventrally and caudally and in a more restricted area than cells projecting rostrally or to the parvicellular reticular formation. Anterograde transport data revealed new presumptive terminal fields in dorsolateral, ventral, periventricular, and commissural regions of the NTS, including an area overlapping the terminal field of the aortic baroreceptor nerve. We conclude that neurons within an area of the insular cortex projecting to multiple brainstem autonomic nuclei, including a region of the NTS innervated by baroreceptor afferents, increase arterial blood pressure and heart rate.(ABSTRACT TRUNCATED AT 400 WORDS)

Function of the ventrolateral medulla in the control of the circulation

The CNS control of the cardiovascular system involves the coordination of a series of complex neural mechanisms which integrate afferent information from a variety of peripheral receptors and produce control signals to effector organs for appropriate physiological responses. Although it is generally thought that these control signals are generated by a network of neural circuits that are widely distributed in the CNS, over the last two decades a considerable body of experimental evidence has accumulated suggesting that several of these circuits involve neurons found on or near the ventral surface of the medulla oblongata. Neurons in the VLM have been shown to be involved in the maintenance of vasomotor tone, in baroreceptor and chemoreceptor (central and peripheral) reflex mechanisms, in mediating the CIR and somatosympathetic reflexes and in the control of the secretion of vasopressin. These physiological functions of VLM neurons have been supported by neuroanatomical and electrophysiological studies demonstrating direct connections with a number of central structures previously implicated in the control of the circulation, including the IML, the site of origin of sympathetic preganglionic axons, and the SON and PVH, the site of origin of neurohypophyseal projecting axons containing AVP. Considerable suggestive evidence has also been obtained regarding the chemical messengers involved in transmitting information from VLM neurons to other central structures. There have been developments suggesting a role for monoamines and neuropeptides in mediating the neural and humoral control of SAP by neurons in the VLM. This review presents a synthesis of the literature suggesting a main role for VLM neurons in the control of the circulation.