Afferent connections and spinal projections of the pressor region in the rostral ventrolateral medulla of the cat

Vestibular influences on the autonomic nervous system

Considerable evidence exists to suggest that both sympathetic and respiratory outflow from the central nervous system are influenced by the vestibular system. Otolith organs that respond to pitch rotations seem to play a predominant role in producing vestibulo-sympathetic and vestibulo-respiratory responses in cats. Because postural changes involving nose-up pitch challenge the maintenance of stable blood pressure and blood oxygenation in this species, vestibular effects on the sympathetic and respiratory systems are appropriate to participate in maintaining homeostasis during movement. Vestibular influences on respiration and circulation are mediated by a relatively small portion of the vestibular nuclear complex comprising regions in the medial and inferior vestibular nuclei just caudal to Deiters' nucleus. Vestibular signals are transmitted to sympathetic preganglionic neurons in the spinal cord through pathways that typically regulate the cardiovascular system. In contrast, vestibular effects on respiratory motoneurons are mediated in part by neural circuits that are not typically involved in the generation of breathing.

The rostral ventrolateral medulla mediates suppression of the circulatory system by the ventromedial nucleus of the hypothalamus

We recently reported that a train of episodic neural discharges within the ventromedial nucleus of the hypothalamus (VMH) associated with suppression of the circulatory system had been determined by monitoring multiple unit activity (MUA). Abrupt increases in neural activity (MUA volleys; 1 to 4 min in duration) accompanied transient decreases in heart rate (HR) and blood pressure (BP), and showed circadian rhythm, occurring every 15 to 30 min in the light phase but seldom in the dark phase. The present study was aimed to determine if neurons in the vasomotor area of the rostral ventrolateral medulla (RVL) are involved in this VMH-induced cardiovascular suppression. MUAs of the VMH and RVL were monitored simultaneously with HR and BP in urethane-anesthetized rats. In synchrony with each MUA volley in the VMH, spontaneous activity of RVL neurons significantly decreased, as well as HR and BP. These RVL neurons are most likely vasomotor neurons because MUA of the RVL was attenuated by baroreceptor reflex activation, and electrical stimulation of these cells through the MUA recording electrodes produced pressor responses. These data suggest that VMH neurons that show a train of episodic discharges suppress the circulatory system at least in part by inhibiting the excitability of vasomotor neurons in the RVL.

Brainstem interneurons necessary for vestibular influences on sympathetic outflow

The objective of this study was to determine which general brainstem regions contain interneurons that are critical for mediating vestibulo-sympathetic responses in decerebrate cats, as a prelude for future cell recording studies. Large injections of kainic acid into the lateral reticular formation at levels caudal to the obex abolished sympathetic nerve responses elicited by electrical stimulation of the vestibular nerve. The same lesions also diminished excitatory and some inhibitory components of somato-sympathetic responses elicited by stimulation of the sciatic nerve, raising the possibility that a common pool of interneurons integrates signals from muscle, skin and the vestibular system that reflect body position in space. Large lesions of other brainstem regions containing interneurons involved in regulation of sympathetic activity, including nucleus tractus solitarius, the lateral tegmental field rostral to the obex, and the parabrachial nucleus, had no appreciable effect on the amplitude of either vestibulo-sympathetic or somato-sympathetic responses. Thus, interneurons critical for relaying vestibular and other somatic signals to descending cardiovascular-regulatory pathways appear to be located in the caudal and lateral parts of the medulla.

Depressor pathway involved in somatosympathetic reflex in cats

The present study aimed to identify direct vasodepressor pathways from the rostral ventrolateral medulla (RVLM) to the spinal cord and their role in mediation of somatosympathetic reflexes. Vasopressor and depressor areas were identified by stimulating various sites of RVLM electrically and/or chemically in anesthetized cats. Electrical lesions on the pressor areas abolished the pressor response evoked by peripheral C-fiber activation while the depressor response remained. Electrical lesions on the depressor areas decreased the depressor response evoked by A delta-fiber stimulation. To characterize the neurons involved, 17 medullospinal sympathetic neurons were identified electrophysiologically. While most of them were sympathoexcitatory, three medullospinal tract cells were found to be sympathoinhibitory neurons. From these results we concluded that a minor group of neurons in the RVLM is sympathoinhibitory and is involved in mediation of somatosympathetic depressor response.

Nucleus tractus solitarius efferent terminals synapse on neurons in the caudal ventrolateral medulla that project to the rostral ventrolateral medulla

The caudal ventrolateral medulla (CVL) contains neurons that are vasodepressor and are a critical component of the baroreceptor reflex pathway. While electrophysiological studies suggest that CVL neurons are intercalated in the baroreceptor pathway between the nucleus tractus solitarius (NTS) and the rostral ventrolateral medulla (RVL), there is no direct evidence for this projection. Therefore, we identified CVL neurons that project to RVL by retrogradely labelling them with wheat germ agglutinin-apo-horseradish peroxidase conjugated to colloidal gold (WAHG) injected into the RVL. Retrogradely labelled neurons were seen in previously identified vasodepressor areas of the rostral CVL that are critical for the baroreceptor reflex. Double labelling for WAHG and tyrosine hydroxylase (TH) immunocytochemistry indicated that CVL neurons that project to the RVL (CVL --> RVL neurons) are distinct from the noradrenergic neurons of the A1 cell group. To establish the presence of a direct projection from the NTS to CVL --> RVL neurons, the retrograde tracer WAHG was pressure injected into the RVL and the anterograde tracer biocytin was iontophoresed into the NTS of anesthetized rats. After 4-6 h, anesthetized rats were perfused transcardially with 3.75% acrolein in 2% paraformaldehyde and sections through the CVL were processed for both markers. By light microscopy, numerous biocytin-labelled varicose processes overlapped neurons containing WAHG in the CVL. By electron microscopy, biocytin was found in myelinated and unmyelinated axons and in axon terminals (0.9 + 0.02 microns) that contained primarily small clear vesicles. These terminals formed predominantly asymmetric synapses on large (1.5-6.0 microns in diameter) dendrites within the CVL. Some of the post-synaptic perikarya and large dendrites contained WAHG associated with lysosomes and multivesicular bodies, indicating that they belong to neurons which project to the RVL. We conclude that CVL --> RVL neurons are (a) distinct from A1 noradrenergic cells; (b) receive direct synaptic contacts from NTS efferent terminals; (c) are potently and monosynaptically excited (asymmetric synapses) by NTS efferent terminals. These data support the hypothesis that CVL neurons are intercalated between the NTS and the RVL in the baroreceptor reflex pathway.

Effects of nitric oxide on sympathetic baroreflex transmission in the nucleus tractus solitarii and caudal ventrolateral medulla in cats

We have previously shown that nitric oxide (NO) attenuates baseline sympathetic tone in the rostral ventrolateral medulla (RVLM), while having no effects on baroreflex transmission in this region in cats. In the present study, we tested the effects of microinjections (500 nl) of NG-nitro-L-arginine (L-NNA, 0.3 mM) or the NO donor S-nitroso-N-acetylpenicillamine (SNAP, 10 microM) in the nucleus tractus solitarii (NTS) and in the caudal ventrolateral medulla (CVLM) which are the two other relays of the sympathetic baroreflex within the brainstem. Neither L-NNA nor SNAP significantly changed the length of inhibition of renal sympathetic nerve activity (SNA) evoked by electrical stimulation of the ipsilateral carotid sinus nerve. In contrast, glutamate (1 mM) in the NTS markedly increased baroreflex inhibition of SNA and the glutamate receptor antagonist kynurenate (5 mM) in the CVLM significantly decreased baroreflex transmission in the same experiments. These results suggest that sympathetic baroreflex function is preserved during both impaired endogenous synthesis and excess exogenous supply of NO in the brainstem.

Fos expression in neurons projecting to the pressor region in the rostral ventrolateral medulla after sustained hypertension in conscious rabbits

Previous studies in anaesthetized animals have shown that the baroreflex control of sympathetic vasomotor activity is mediated to a large extent by inhibitory inputs to sympathoexcitatory pressor neurons in the rostral part of the ventrolateral medulla. The aim of this study was to determine, in conscious rabbits, the distribution of neurons within the brain that have two properties characteristic of interneurons conveying baroreceptor signals to the rostral ventrolateral medulla: (i) they are activated by an increase in arterial pressure; and (ii) they project specifically to the rostral ventrolateral medulla pressor region. In a preliminary operation, an injection of the retrogradely transported tracer, fluorescent-labelled microspheres, was made into the physiologically identified pressor region in the rostral ventrolateral medulla. After a waiting period of one to eight weeks, hypertension was produced in the conscious rabbit by continuous intravenous infusion of phenylephrine at a rate sufficient to increase arterial pressure by approximately 20 mmHg, maintained for a period of 60 min. A control group of animals was infused with the vehicle solution alone. In confirmation of our previous study, hypertension produced by phenylephrine resulted in the neuronal expression of Fos (a marker of neuronal activation) in the nucleus of the solitary tract, area postrema, the intermediate and caudal parts of the ventrolateral medulla parabrachial complex, and in the central nucleus of the amygdala. Approximately 50% of the Fos-immunoreactive neurons in both the caudal and intermediate parts of the ventrolateral medulla were also retrogradely labelled from the rostral ventrolateral medulla pressor region; such double-labelled neurons were confined to a discrete longitudinal column located just ventrolateral to the nucleus ambiguus. Significant numbers of double-labelled neurons were also found in the nucleus of the solitary tract and area postrema, although these represented a much lower proportion (13-16%) of the total number of Fos-immunoreactive neurons in these regions. In the parabrachial complex, Fos-immunoreactive and retrogradely labelled neurons were largely separate populations, while in the amygdala they were entirely separate populations. In the control group of rabbits, virtually no double-labelled neurons were found in any of these regions. The results indicate that putative baroreceptor interneurons that project to the pressor region of the rostral ventrolateral medulla are virtually confined to the lower brainstem. In particular, they support the results of previous studies in anaesthetized animals indicating that neurons in the intermediate and caudal ventrolateral medulla convey baroreceptor signals to the rostral ventrolateral medulla pressor region, and extend them by demonstrating the precise anatomical distribution of these neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Internal connections in the rostral ventromedial medulla of the rat

Physiological and pharmacological data suggest that the rostral ventromedial medulla (RVM) is an important site where integration between somatic and visceral functions might occur. The aim of the present study was to describe the interconnections between various nuclei of the rostral ventromedial medulla and thus reveal the possible anatomical basis for such functional interactions. The topography of anterogradely labelled internal projections was examined following iontophoretic microinjections of Phaseolus vulgaris leucoagglutinin (PHA-L). The results revealed that the nuclei of the rostral ventromedial medulla have strong interconnections and, to varying degrees, they also have bilateral projections into the rostral ventrolateral medulla. A particularly dense projection to widespread regions of the ventral medulla was traced from the raphe obscurus. Terminals, originating from the raphe pallidus were similarly dispersed but very low density in comparison. The focus of the projections of the gigantocellular nucleus pars ventralis and pars alpha shifted from the lateral paragigantocellular nucleus towards the RVM in rostral direction. Connections from the raphe magnus were altogether restricted to the RVM and the medial aspects of the lateral paragigantocellular nucleus. The diffuse and dense intramedullary connections of the raphe obscurus suggest that it might have an important role in coordinating the activity of rostral ventral medullary cells. The raphe pallidus and the ventral gigantocellular nuclei, areas that were innervated from widespread regions of the rostral ventral medulla but gave only limited projections there, are more likely to be involved in the direct descending control of spinal activities.

Abdominal vagal stimulation excites bulbospinal barosensitive neurons in the rostral ventrolateral medulla

We used extracellular recordings to examine the central pathway whereby electrical stimulation of abdominal vagal afferents elevates arterial pressure in the rabbit. Bulbospinal neurons in the rostral ventrolateral medulla were identified by antidromic activation from the dorsolateral funiculus of the thoracic spinal cord. Their barosensitivity was assessed by their response to intravenous phenylephrine and by their cardiac cycle-related rhythmicity. We used peristimulus histogram procedures to assess the effect of electrical stimulation of abdominal vagal afferents on the discharge rate of these neurons. Electrical stimulation (one to three pulses) activated 98 of 123 neurons tested (80%), had no effect on 22 neurons (18%) and inhibited the remaining three neurons. Latency to peak excitation was 228 +/- 3 ms, indicating that the conduction velocity of the vagal afferents was about 0.6 m/s, in the unmyelinated fibre range. Lower oesophageal distension with a balloon excited 22 of 48 neurons (46%), inhibited 12 neurons (25%) and had no effect on the remaining 14 cells (29%). Vagally induced excitation was reduced by aortic depressor nerve stimulation in nine of 13 neurons. Lightly touching the animal's back and legs had no effect on 56 of 60 neurons. Nociceptive stimuli failed to affect 47 of 60 neurons tested. No excitation was seen with electrical stimulation of the sciatic or central ear nerves. Our study identifies a robust excitatory input from the abdominal vagus to bulbospinal barosensitive neurons in the rostral ventrolateral medulla. Relevant physiological stimuli include lower oesophageal distension. The pathways may be relevant to cardiovascular changes which accompany upper gastrointestinal function.

Effects of preoptic warming on subretrofacial and cutaneous vasoconstrictor neurons in anaesthetized cats

1. Sympathetic and subretrofacial neuron responses to preoptic warming were studied in chloralose- or Saffan-anaesthetized, paralysed cats. 2. Warming a thermode in the preoptic region inhibited the activity of cutaneous vasoconstrictor fibres supplying hairy skin. Muscle vasoconstrictor fibre activity recorded at the same time was either unaffected or raised. 3. Small injections of sodium glutamate (5 nl, 0.1 M) were made into the region of the subretrofacial nucleus in the ventrolateral medulla. The part of that region where glutamate injections evoked brisk increases in cutaneous vasoconstrictor fibre activity was chosen for further study. 4. Extracellular single unit recordings were made in that area from seventy-seven subretrofacial neurons, which were identified by their barosensitivity (inhibition by carotid blind sac inflation). Forty-seven of them were antidromically activated by stimulation in the spinal cord. 5. The activity of twenty subretrofacial neurons (twelve proven bulbospinal) was significantly reduced by periods of preoptic warming. Cutaneous vasoconstrictor activity recorded at the same time also fell. Forty-nine subretrofacial neurons (thirty-five proven bulbospinal) were unaffected or excited by periods of preoptic warming that inhibited cutaneous vasoconstrictor fibres. The response of eight neurons was unclear. 6. No difference in either mean firing rate or axonal conduction velocity was found between neurons inhibited by preoptic warming and other subretrofacial neurons. 7. The subretrofacial neurons inhibited by warming were found intermingled with those unaffected or excited. Marked recording sites of warm-inhibited neurons were clustered around the ventromedial border of the subretrofacial nucleus. 8. In two cats, bilateral inhibition of subretrofacial neurons by surface application of 1 M glycine reduced cutaneous vasoconstrictor fibre activity to 32 and 44% of control levels. 9. The results suggest that specific cutaneous vasoconstrictor premotor neurons exist in the subretrofacial nucleus. These apparently provide most of the background excitatory drive to cutaneous vasomotor neurons. Central warming stimuli may act, at least in part, by withdrawing that drive.

Localization of barosensitive neurons in the caudal ventrolateral medulla which project to the rostral ventrolateral medulla

A population of depressor neurons in the caudal ventrolateral medulla that project to the rostral ventrolateral medulla may mediate the baroreceptor reflex. The aim of the present study was to determine the anatomical distribution of the population of neurons in the caudal ventrolateral medulla that mediate the baroreceptor reflex. Injection of the retrogradely transported tracer, rhodamine-labelled latex beads, into the pressor area of the rostral ventrolateral medulla of rats was used to identify neurons in the caudal ventrolateral medulla with projections to that area. Barosensitive neurons were identified by immunohistochemical detection of the protein Fos, a marker of neuronal activation, following infusion of the pressor agent phenylephrine (10 micrograms/kg/min, i.v. for 2 h n = 5). Isotonic saline was infused into control animals (n = 4). Neurons in the caudal ventrolateral medulla with projections to the rostral ventrolateral medulla were located at all rostrocaudal levels examined between 1 mm caudal and 0.4 mm rostral of the obex. Compared to saline infused rats, phenylephrine infusion induced a significant increase in the proportion of those neurons that expressed Fos (14% vs. 1% P < 0.000.1). These barosensitive neurons were found mainly at the level of the obex, between the lateral reticular nucleus and the nucleus ambiguus. In conclusion, this study is the first to show the distribution of the population of barosensitive neurons in the caudal ventrolateral medulla that project to the pressor region of the rostroventrolateral medulla. The results suggest there is a subpopulation of depressor neurons, confined to a small region of the rostral part of the caudal ventrolateral medulla, that are likely to be the interneurons that mediate the baroreceptor-reflex response.

Rostral ventrolateral medulla descending neurons excited by nucleus tractus solitarii inputs

Neurons in the rostral ventrolateral medulla (RVLM) were electrophysiologically characterized and anatomically identified using an intracellular recording technique in vivo. Of 49 neurons recorded, 7 were antidromically activated from the dorsolateral funiculus in the thoracic spinal cord, with axonal conduction velocities ranging from 16.6 to 55.0 m/s. The RVLM-spinal neurons were spontaneously active and non-bursting. Additionally, they demonstrated a flat post-R-wave histogram and a flat average of the neuronal membrane potential triggered by the pulsatile arterial pressure. Therefore their activity was not related to cardiac rhythm. Electrical stimulation of the nucleus tractus solitarii (NTS) at the level of the obex evoked monosynaptic excitatory postsynaptic potential (EPSP) on 3 RVLM-spinal neurons; median latency was 1.5 ms. The recorded neurons, intracellularly labeled with horseradish peroxidase (HRP) or biocytin, were located in the rostral pole of the RVLM, between 0.3 and 0.7 mm from the ventral medullary surface and in many cases close to the neurons containing phenylethanolamine-N-methyltransferase (PNMT). These findings are discussed in relation to the physiological role in cardiovascular and nociceptive functional regulation played by the neurons analyzed in this study.

Afferents to the nucleus reticularis parvicellularis of the cat medulla oblongata: a tract-tracing study with cholera toxin B subunit

The aim of this study was to examine anatomical evidence in cats of whether the nucleus reticularis parvicellularis (Pc) is part of the circuit responsible for the inhibition of brainstem motoneurons during paradoxical sleep. For this purpose, we made iontophoretic injections of the retrograde and anterograde tracer cholera toxin B subunit (CTb) in the Pc. After CTb injections in the Pc, a large number of retrogradely labeled neurons were seen in the central nucleus of the amygdala, the lateral part of the bed nucleus of the stria terminalis, the posterior hypothalamic areas, the mesencephalic reticular formation, the nucleus locus subcoeruleus, the nucleus pontis caudalis, other portions of the Pc, the nucleus reticularis dorsalis, the trigeminal sensory complex, and the nucleus of the solitary tract. We further found that the Pc receives 1) serotoninergic afferents from the raphe dorsalis, magnus, and obscurus nuclei; 2) noradrenergic inputs from the dorsolateral pontine tegmentum; 3) cholinergic afferents from the lateral medullary reticular formation; 4) substance P-like afferents from the central nucleus of the amygdala, bed nucleus of the stria terminalis, periaqueductal gray, and nucleus of the solitary tract; and 5) methionine-enkephalin-like projections from the periaqueductal gray, the nucleus of the solitary tract, the lateral pontine and medullary reticular formation, and the spinal trigeminal nucleus. We further found that the Pc do not receive afferents from brainstem structures responsible for muscle atonia, such as the ventromedial medulla and the dorsomedial pontine tegmentum, and therefore may not be part of the circuit inhibiting the brainstem motoneurons during paradoxical sleep.

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

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

Monosynaptic excitation of preganglionic vasomotor neurons by subretrofacial neurons of the rostral ventrolateral medulla

Extracellular single unit recordings were made from barosensitive neurons in the subretrofacial nucleus (SRF) of the rostral ventrolateral medulla in chloralose-anaesthetised cats. At the same time, single preganglionic neuron activity was recorded from filaments of the cervical sympathetic trunk (CST); barosensitive units were selected for study. Evidence for monosynaptic connections between the two neuron groups was sought by cross-correlation analysis of their ongoing activity. Cross-correlograms of 16/16 SRF/CST neuron pairs showed a broad peak (100-200ms wide), reflecting the synchronizing action of arterial baroreceptors on both neurons' activity. Two of the 16 cross-correlograms additionally showed a robust, statistically significant, narrow peak of a single 2 ms bin width, providing the first physiological demonstration that ventrolateral medullary neurons monosynaptically excite preganglionic sympathetic neurons. Deductions are made about the strength, convergence and divergence of the connection.

Periaqueductal gray area and cardiovascular function

The periaqueductal gray (PAG) area seems to play an important role in modulating several biological functions such as the triggering of stereotyped defence and reproductive behaviour, pain, anxiety and cardiovascular and respiratory activities. Anatomically this midbrain area is made up of symmetric neuronal columns arranged along the long axis of the aqueduct. In this paper we review the most important findings of the last 10-15 years about the interaction between the PAG area and the cardiovascular function. It is shown that these neuronal columns within the PAG area exhibit a viscerotropic organization which elicits both hypertensive and hypotensive responses. In particular, the stimulation of the ventral neuronal column evokes a hypotensive response associated with a regional decrease in the vascular resistance. On the contrary, the stimulation of the dorsal and lateral neuronal columns evokes arterial hypertension associated with specific changes of the vascular resistance. Recently the authors demonstrated that the glutamergic system in the PAG area (prevalently through NMDA subtype receptor) may also be involved in the control of cardiovascular system. Moreover, the involvement of the arginine vasopressin neuropeptide in the hypertension induced by administration of excitatory amino acids into the PAG area has been demonstrated.

Neurogenic hypertension related to basilar impression. Case report

The authors report the resolution of essential hypertension following transoral odontoidectomy and medullary decompression in a 39-year-old woman with basilar invagination. Current understanding of central regulation of the cardiovascular system is discussed and the pertinent neuroanatomy illustrated. Experimental and clinical evidence supporting the role of neurogenic mechanisms in the pathogenesis of hypertension is reviewed.

Projections from the nucleus tractus solitarii to the spinal cord

Projections from the nucleus tractus solitarii (NTS) to the spinal cord were demonstrated in the male Sprague-Dawley rat. In retrograde transport studies, a horseradish peroxidase conjugate or a fluorescent dye, FluoroGold, were injected into midcervical or upper thoracic spinal segments. Most solitariospinal neurons were multipolar or bipolar and located between the obex and spinomedullary junction. Solitariospinal neurons were concentrated in proximity to the ventral border of the solitary tract and extended dorsally into the intermediate division and ventrolaterally into the intermediate reticular zone (IRt) of the lateral tegmental field. This subgroup predominantly projects to midcervical spinal segments. A subset of small neurons was retrogradely labeled from cervical or thoracic spinal segments in the medial commissural nucleus and contiguous with a periventricular group surrounding the central canal. In anterograde transport studies, iontophoretic deposits of Phaseolus vulgaris leucoagglutinin were centered stereotaxically on sites in NTS identified by retrograde transport data. The lectin was incorporated by neurons of the solitary complex and transported bilaterally by axons that emerged from the nucleus and entered the reticular formation. The solitario-reticular (transtegmental) pathway irradiated diagonally across the IRt and extended caudally into the cervical lateral funiculus and spinal gray. A small periventricular-spinal pathway also descended longitudinally to the neuraxis. Solitariospinal neurons project to superficial lamina of the dorsal horn, laminae VII and X and ventral horn. The projections are predominantly contralateral to phrenic and intercostal motor nuclei and ipsilateral to the intermediolateral cell column. The solitariospinal projection represents the shortest route in the central nervous system, other than the local intraspinal reflex, through which first order visceral afferents signal cardiorespiratory and alimentary motor nuclei.

Ultrastructural localization and afferent sources of corticotropin-releasing factor in the rat rostral ventrolateral medulla: implications for central cardiovascular regulation

We investigated the ultrastructural localization, afferent sources, and arterial pressure effects of corticotropin-releasing factor (CRF) in the nucleus reticularis rostroventrolateralis (RVL), a region of the ventrolateral medulla containing C1 adrenergic neurons and sympatho-excitatory reticulospinal afferents to sympathetic preganglionic neurons. A polyclonal antibody to CRF was localized in acrolein-fixed sections through the rat RVL by the peroxidase-antiperoxidase (PAP) method. Light microscopy showed that 1-7 perikarya/30 micron section and numerous varicose processes contained CRF-like immunoreactivity (CRF-LI). By electron microscopy, CRF-LI was most intensely localized to large (80-100 nm) dense-core vesicles within numerous terminals and a few perikarya and large dendrites. Approximately half of the terminals containing CRF-LI were in direct contact with unlabeled perikarya or dendrites; the remainder were in apposition to either unlabeled terminals or astrocytes. Most synaptic specializations were asymmetric synapses on small, unlabeled dendrites. To examine potential extrinsic sources of CRF-containing terminals in the C1 area of the RVL, PAP immunocytochemical localization of CRF was combined with retrograde transport of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP). In all cases examined, a number of dually labeled neurons were found in the paraventricular nucleus (PVN) of the hypothalamus and a few dually labeled neurons were observed in the nuclei of the solitary tract; these labeled neurons were ipsilateral to the unilateral injection of WGA-HRP into the C1 area. Fewer dually labeled perikarya were detected in the lateral hypothalamic area and the lateral parabrachial nuclei, ipsilateral to the WGA-HRP injection. Additional physiological studies showed that bilateral microinjections of CRF into the C1 area of the RVL of urethane-anesthetized rats elicited a dose-related increase in arterial pressure. The results suggest that within the C1 area of the RVL, CRF released from terminals, arising predominantly from the PVN of the hypothalamus and probably from local neurons as well, may excite sympathoexcitatory reticulospinal neurons.

A monosynaptic pathway from an identified vasomotor centre in the medial prefrontal cortex to an autonomic area in the thoracic spinal cord

Chemical microstimulation (1 mM L-glutamate or 25 mM KCl) of the medial prefrontal cortex of anaesthetized rats produced falls in systolic and diastolic blood pressure of similar magnitude, without a change in heart rate. Application of the lectin Phaseolus vulgaris leucoagglutinin by iontophoresis from an adjacent barrel of the same micropipette revealed a direct projection to the central autonomic area of the thoracic spinal cord from this vasomotor area, which is equivalent to the region called prelimbic cortex by Krettek and Price [J. comp. Neurol. (1977) 171, 157-192] or Cg3 by Paxinos and Watson [The Rat Brain in Stereotaxic Coordinates (1986)]. Labelled axons descended in the dorsal corticospinal tract in the cervical spinal cord, where they displayed a few varicosities. In the thoracic spinal cord, labelled fibres occurred bilaterally in the gray matter, predominantly in the central autonomic area, where they displayed many varicosities. Electron microscope studies revealed that the anterogradely labelled varicosities in the central autonomic area were vesicle-filled boutons that formed asymmetric synaptic contacts. The synaptic targets were small dendrites or dendritic protrusions that were characterized by a high incidence of multivesicular bodies and coated vesicles. We conclude that a monosynaptic pathway that originates from a physiologically-defined vasomotor area in the medial prefrontal cortex terminates on a characteristic type of neuron in the central autonomic area of the thoracic spinal cord.

A comparison of hypotensive and non-hypotensive hemorrhage on Fos expression in spinally projecting neurons of the paraventricular nucleus and rostral ventrolateral medulla

The protein, Fos, detected immunohistochemically, was used to identify neurons in the brain that were activated after hemorrhage in the conscious rat. Spinally projecting neurons in the paraventricular nucleus (PVN) and rostral ventrolateral medulla (RVLM) were identified by the presence of rhodamine-labeled latex beads which had been previously injected into the upper thoracic spinal cord. On the experimental day, conscious rats underwent either (1) withdrawal of 4 ml of blood from a carotid cannula (n = 8) which reduced mean arterial pressure from 96.6 +/- 2.7 to 42.7 +/- 7.1 mmHg, (2) withdrawal of 2 ml of blood (n = 4) which did not affect mean arterial pressure. Animals that were not hemorrhaged were used as controls (n = 6). After the 4 ml hemorrhage, dense concentrations of Fos-positive cell nuclei were found in the lamina terminalis, supraoptic nuclei (SON), PVN and in the medulla. In contrast, the density of Fos-positive cells in 2 ml-hemorrhaged rats was not different from controls except in the SON and in the medial PVN in 2 of 4 rats. After the 4 ml hemorrhage 14.4 +/- 1.2% of the spinally projecting neurons in the PVN and 22.7 +/- 6.1% in the RVLM expressed Fos (P < 0.001 compared to control). After the 2 ml hemorrhage the proportion was 12.2 +/- 3.1% in the PVN (P < 0.001 compared control) but only 5.4 +/- 2.2% in the RVLM (P > 0.05 compared to control). The results suggest that spinally projecting neurons in the PVN and RVLM participate in the reflex responses to hemorrhage.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of NADPH diaphorase in neurons of the rostral ventral medulla: possible role of nitric oxide in central autonomic regulation and oxygen chemoreception

We studied whether neurons containing nitric oxide synthase (NOS) are localized to the rostral ventrolateral medulla (RVM) and, if so, whether they are distinct from the adrenergic neurons of the C1 group. NOS-containing neurons and/or C1 neurons were visualized using NADPH diaphorase histochemistry and phenylethanolamine N-methyltransferase (PNMT) immunohistochemistry, respectively. A column of NADPH diaphorase-positive neurons, extending 2 mm in the rostrocaudal plane, was observed lateral to the inferior olive and medial to the C1 neurons. Double labelling studies showed that NADPH diaphorase-positive neurons were not immunoreactive for PNMT, indicating that the two enzymes were localized in the different cells. Furthermore, only a small fraction of NADPH diaphorase neurons were retrogradely labelled after injections of fluorogold into the thoracic cord. We conclude that the RVM contains a well-defined group of neurons endowed with NOS that are distinct from the adrenergic neurons of the C1 group and have only limited monosynaptic projections to the spinal cord. Since the RVM is involved in the control of arterial pressure and in oxygen-conserving reflexes, the findings raise the possibility that nitric oxide participates in central autonomic regulation and oxygen chemoreception.

Localization of cardiac vagal preganglionic motoneurones in the rat: immunocytochemical evidence of synaptic inputs containing 5-hydroxytryptamine

The origin of cardiac vagal preganglionic motoneurones in the rat is still controversial and knowledge of the chemistry of synaptic inputs onto these neurones is limited. In this investigation vagal preganglionic motoneurones innervating the heart were identified by the retrograde transport of cholera toxin conjugated to horseradish peroxidase (CT-HRP) combined with the immunocytochemical localization of 5-hydroxytryptamine. Injection of CT-HRP into the myocardium resulted in the retrograde labelling of neurones primarily in the ventral regions of the nucleus ambiguus (75.1%). Labelled neurones were also distributed in a narrow band through the reticular formation extending between the dorsal motor nucleus of the vagus nerve and the nucleus ambiguus (17.3%) as well as in the dorsal motor nucleus itself (7.6%). A combination of retrograde labelling with immunocytochemistry for 5-hydroxytryptamine revealed that the neuronal perikarya and the dendrites of cardiac vagal motoneurones in the nucleus ambiguus were often ensheathed in 5-hydroxytryptamine-immunoreactive axonal boutons. Electron microscopic examination of this material confirmed that there were synaptic specializations between these boutons and the cardiac vagal motoneurones. The identification of 5-hydroxytryptamine-containing synaptic inputs to this population of vagal motoneurones provides further detail towards the understanding of the regulation of heart rate by the parasympathetic nervous system.

Comparison of the pressor effects of angiotensin II and III in the rostral ventrolateral medulla

Microinjection of angiotensin II and III into the rostral ventrolateral medulla of anesthetized barodenervated rabbits elicited in both cases pressor responses, which were of similar magnitude and time course. The responses to angiotensin II and III were either unchanged or increased in the presence of compounds which inhibit their degradation to shorter length peptides. The results indicate that both angiotensin peptides are independently capable of eliciting pressor responses in the rostral ventrolateral medulla.

Transneuronal labelling of neurons in rabbit brain after injection of herpes simplex virus type 1 into the renal nerve

We mapped the distribution of virus-labelled neurons in the brain after injection of Herpes simplex virus type 1 (HSV1) into the rabbit renal nerve. Seven days after injection, labelled neurons were observed in four brain regions, the rostral ventrolateral medulla (47 +/- 3% of neurons), the A5 area of the lower pons (38 +/- 4%), the caudal raphe nuclei and the parapyramidal area of the medulla (13 +/- 2%), and the paraventricular nucleus of the hypothalamus (1 +/- 1%). In the rostral ventrolateral medulla approximately one half of the HSV1-labelled neurons also contained tyrosine hydroxylase, characterizing them as C1 neurons. In the A5 area virtually all HSV1-labelled neurons also contained tyrosine hydroxylase. In the raphe nuclei and the parapyramidal area 47% of HSV1-positive neurons contained serotonin. The distribution of labelled neurons was similar to that observed after injection of HSV1 into the adrenal gland.

Actions of carotid chemoreceptors on subretrofacial bulbospinal neurons in the cat

Thirty-one barosensitive bulbospinal neurons were recorded from the subretrofacial (SRF) nucleus in eight chloralose-anaesthetised, paralysed cats. Close arterial injections of CO2-saturated saline were used to stimulate carotid body chemoreceptors. Seven neurons were abruptly excited and six neurons abruptly inhibited by chemoreceptor stimuli: these were primary responses, independent of changes in blood pressure or central respiratory drive (monitored from the phrenic nerve). A further eight neurons responded only indirectly to chemoreceptor stimuli, showing enhanced modulation of their activity coupled to the enhanced central respiratory drive. The distinction between primary and secondary responses was shown more clearly when central respiratory drive was inhibited by superior laryngeal nerve stimulation. The remaining ten neurons showed no clear response to chemoreceptor stimuli. SRF bulbospinal neurons thus show the same range of responses to chemoreceptor stimuli as the sympathetic neurons they are believed to drive.

Non-respiratory neurons in the Bötzinger complex exhibiting appropriate firing patterns to generate the emetic act in dogs

This work was performed to prove the hypothesis that the pattern generator for the emetic act exists in the Bötzinger complex (BOT) and is driven by vagal afferents via the subpostrema portion of the nucleus of the solitary tract (mNST). Non-respiratory neurons (78) intermingling with BOT respiratory neurons in decerebrate dogs responded to pulse train stimulation of vagal afferents with a mean latency of 387 ms. During retching induced by vagal stimulation, one-half of the non-respiratory neurons exhibited high frequency burst firings synchronous with each retch (SH-firing, SH-neurons) and one-third of these neurons showed similar firings synchronous with the periods between retches (BH-firing, BH-neurons). Two-thirds of the SH-neurons and one-half of the BH-neurons fired with gradually augmenting frequencies (augmenting firing) during the period prior to retching, which may correspond to the period of prodromal signs of vomiting. Three SH-neurons were observed at fictive expulsion: all 3 exhibited burst firings concomitant with expulsion. During cooling block of transmission in the mNST, stimulation of the vagus nerve ipsilateral to the cooling failed to induce not only retching but also augmenting firing and SH-firing in all 11 BOT SH-neurons observed. In contrast, contralateral vagal stimulation induced retching and neuronal firings which had been observed before the cooling. These results support the hypothesis mentioned above. Respiratory firings were changed during retching in all BOT respiratory neurons observed. Respiratory firings were depressed during retching in the majority (15/25) of inspiratory (I) neurons and in a few expiratory (E) neurons (6/45). SH-firing was exhibited by 3 I- and 13 E-neurons. A few (2) I- and half (23) E-neurons showed BH-firing. These results indicate that all BOT respiratory neurons participate in central patterning of the emetic act.

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

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

Rostrocaudal differences in morphology and neurotransmitter content of cells in the subretrofacial vasomotor nucleus

The rostral ventrolateral medulla (RVLM) contains sympathoexcitatory neurons that exert a powerful control over the sympathetic outflow to the cardiovascular system. In the cat there is a concentration of such neurons (but not neurons subserving other functions) within a narrow longitudinal column in the RVLM termed the subretrofacial (SRF) nucleus. Furthermore, it has been suggested that there are subgroups of cells, located at different rostrocaudal levels of the SRF nucleus, that preferentially or exclusively control different vascular beds (e.g. in the kidney and hindlimb). The aim of this study was to map quantitatively the rostrocaudal distribution within the nucleus of different cell types, defined according to morphological and/or chemical criteria, and to correlate this with the regional vasomotor effects (in hindlimb and kidney) evoked by stimulation of SRF cells at the corresponding rostrocaudal levels. SRF cells were highly heterogeneous with respect to both their morphology and chemical properties. They varied greatly in size (equivalent diameter ranging from 10-40 microns) as well as in shape and orientation. An immunohistochemical examination using the avidin-biotin procedure revealed that many SRF cells (estimated 57% of all SRF cells) were immunoreactive for tyrosine hydroxylase (TH, a marker of catecholamine cells). In addition, there were SRF cells immunoreactive for neuropeptide Y (NPY, 11% of total), enkephalin (ENK, 16% of total), and serotonin (5HT, 10% of total), but not for substance P, galanin or somatostatin. Different cell types, defined according to their morphology and/or chemical properties, were unevenly distributed throughout the nucleus. In the most caudal part of the SRF nucleus, virtually all cells were TH-positive, and the large majority (estimated 80%) were NPY-positive, suggesting that many cells at this level contained both TH and NPY. In contrast, in the most rostral part of the SRF nucleus, only 30% of cells were TH-positive, and no NPY-positive cells were observed. Both 5HT- and ENK-positive cells were found throughout the rostrocaudal extent of the nucleus, but predominantly within its rostral part. Furthermore, TH-positive cells in the rostral SRF nucleus were on average significantly larger (mean equivalent diameter 18-43% greater) than TH/NPY-positive cells in the caudal part of the nucleus, but smaller than 5HT- or ENK-positive cells at the same level. Overall, rostral cells (regardless of their chemical type) were larger than caudal cells within the SRF nucleus (mean equivalent diameter 13-28% greater).(ABSTRACT TRUNCATED AT 400 WORDS)

Hemorrhage induces c-fos immunoreactivity in spinally projecting neurons of cat subretrofacial nucleus

Neuronal expression of c-fos product in the cat rostral ventrolateral medulla was studied by immunohistochemistry. Spinally projecting neurons of the subretrofacial (SRF) nucleus were pre-labeled by retrograde transport of fluorescent latex microspheres from the lumbar cord. In 3 animals which were bled by approximately 25% blood volume from a carotid cannula, 48-62% of SRF-spinal neurons expressed c-fos immunoreactivity. In 2 control animals, the corresponding values were 2% and 4%. The data show that bulbospinal neurons of presumed vasomotor function express c-fos in response to hemorrhage.

Vestibular influences on the sympathetic nervous system

Studies using both electrical and natural stimulation have established that the vestibular system has prominent effects on sympathetic outflow and blood pressure. Preliminary evidence suggests that receptors in both otolith organs and semicircular canals are involved in producing these effects. Furthermore, vestibulosympathetic reflexes appear to be mediated by the medial vestibular nucleus and slowly conducting projections from the rostral ventrolateral medulla and caudal medullary raphe nuclei to preganglionic neurons in the thoracic spinal cord. However, many details are missing from our knowledge and understanding of the functional significance and neural substrate of vestibular influences on the sympathetic nervous system.

The Bötzinger complex as the pattern generator for retching and vomiting in the dog

To clarify the location of the pattern generator for the emetic act, the bulb was systematically stimulated and partially cut in decerebrate, paralyzed dogs. Stimulation of the following bulbar structures elicited the activities which could be recognized as retching and vomiting in the following muscle nerves. The bulbar structures were: the intra-bulbar bundle of the vagal afferents, the solitary tract and the medial subdivision of its nucleus (NTS), the area postrema, the commissural nucleus, the raphe area at the obex level, and the longitudinal reticular column which consists of 3 areas--the area between the caudal parts of the solitary complex (SC) and the nucleus ambiguus, the area ventromedial to the rostral part of the nucleus and the area dorsomedial to the retrofacial nucleus (RFN) which may correspond to the Bötzinger complex (BOT). The muscle nerves were: the phrenic branches to the dome and hiatal parts of the diaphragm, the abdominal muscle nerve, the pharyngo-esophageal branch of the vagus nerve, the mylohyoid muscle nerve, and the recurrent nerve branches to the adductors and abductor of the glottis. Emetic responses to stimulation of the vagal ventral trunk and the rostral SC still remained after cutting of the bilateral SCs at about 1 mm rostral to the obex, but disappeared after cutting at about 3.5 mm rostral to the obex. After the rostral cuts, stimulation of the SC part caudal to the cuts and the reticular column still induced the emetic act. Emetic responses to stimulation of the caudal SC remained after transection of the bulb at the rostral end of the RFN, but disappeared after transection at its caudal end or after partial cutting of the caudal BOT. The following hypothesis was proposed from these results. Emetic vagal afferents enter the rostral bulb, then descend through the SC to the area subpostrema. Subpostrema neurons project through the reticular column to the pattern generator of the emetic act in the BOT and activate it.

Cardiovascular regulation by cholinergic mechanisms in rostral ventrolateral medulla of spontaneously hypertensive rats

This study aimed to demonstrate the role of acetylcholine receptors in the rostral ventrolateral medulla (RVL) in the central regulation of the cardiovascular system in normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). The effects of cholinergic drugs, microinjected into the rostral ventrolateral medullary vasopressor area, on blood pressure and heart rate in anesthetized and artificially ventilated rats were investigated. Unilateral microinjection of carbachol (1 nmol/site), physostigmine (300 pmol/site) or 3,4-diaminopyridine (500 pmol/site) into the RVL elicited a pressor and tachycardiac response, of which only the pressor response was significantly greater in SHR than in WKY. Bilateral microinjection of atropine (1 nmol/site) caused a depressor and bradycardiac response. The depressor response produced by atropine injected in the RVL was also significantly greater in SHR than in WKY. These results suggest that there are tonic cholinergic mechanisms in the RVL of the rats, which exert an excitatory cardiovascular action, and that the enhanced responsiveness to acetylcholine receptor stimulation in the RVL may contribute to the sustained elevation of blood pressure in the SHR.

The ventrolateral medulla of the cat mediates vestibulosympathetic reflexes

Extracellular recordings were made from 94 neurons located in the ventrolateral medulla (VLM) whose firing rate was affected by vestibular nerve (VN) stimulation; 50 of these units were in the subretrofacial (SRF) nucleus, which contains cells that make direct excitatory connections with sympathetic preganglionic neurons. The sample included 12 SRF cells which were antidromically driven from the upper thoracic spinal cord and had conduction velocities of 10 m/s or less; the effect of VN stimulation on all but one of these units was inhibition. The onset latency of the response to VN stimulation was long [20.3 +/- 3.7 (S.E.M.) ms, n = 9, for the antidromically activated neurons and 12.1 +/- 1.2 ms, n = 73, for the others], suggesting that the effects were predominantly polysynaptic. In addition, most of the spontaneously active units tested (33/36) received convergent inputs from the carotid sinus nerve (CSN), as would be expected for neurons which influence sympathetic outflow. Vestibular-elicited inhibition of SRF neurons with projections to the intermediolateral cell column could account for late, long duration inhibition of sympathetic discharges produced by labyrinth stimulation.

Interaction between pressor and depressor areas in cat ventrolateral medulla

The purpose of the present study was to investigate whether stimulation of the caudal depressor area (CA) in the medulla lowered blood pressure (BP) by enhancing GABA release at the intermediate pressor area (IA) of the medulla. Application of the excitotoxin kainic acid (KA; 40 mM solution) to the caudal area lowered BP as has been previously described (Gatti, et al., Brain Research, 330 (1985) 21-29). Subsequent IA application of the GABA receptor antagonists picrotoxin (100 micrograms/side) or bicuculline (10 micrograms/side) consistently reversed this hypotensive effect. Picrotoxin raised BP by 92 +/- 10 mmHg (n = 5) after KA while by itself, picrotoxin only increased BP by 23 +/- 7.0 mmHg (n = 3) when applied to the IA. This effect was significantly greater following KA application to the caudal area (P less than 0.05). Likewise, bicuculline raised BP by 74 +/- 8.7 mmHg (n = 7) following KA while by itself, bicuculline only increased BP by 24 +/- 8.2 mmHg when applied to the IA (n = 4). This also was significantly different. This potentiation was not seen with the glycine receptor antagonist, strychnine. These data indicate that there is a GABAergic input from the caudal to the intermediate ventral surface areas of the cat which is involved in the central control of BP.

Viscerotopic organization of neurons subserving hypotensive reactions within the midbrain periaqueductal grey: a correlative functional and anatomical study

Microinjection of the excitatory amino acid D,L-homocysteic acid (40 nmol, in 200 nl) made into the ventrolateral part of the caudal half (A2.5-P1.5) of the midbrain periaqueductal gray (PAG) of the decerebrate cat evoked a hypotensive reaction associated with a slowing of the heart and a decrease in either external iliac or renal vascular resistance. The decrease in iliac vascular resistance was elicited from the pretentorial portion (A2.5-A0.6) of the PAG hypotensive area, whereas the decrease in renal vascular resistance was elicited from the subtentorial portion (A0.6-P1.5). Anatomical experiments using the method of retrograde transport of rhodamine-labelled microspheres or wheat germ agglutinin-horseradish peroxidase demonstrated topographically organized projections from the ventrolateral PAG to the subretrofacial (SRF) pressor nucleus in the rostral ventrolateral medulla. The pretentorial part of the ventrolateral PAG projected mainly to the caudal part of the SRF nucleus, which preferentially controls iliac vascular resistance. The subtentorial part of the ventrolateral PAG projected mainly to the rostral part of the SRF nucleus, which preferentially controls renal vascular resistance. Taken together, these findings suggest: (i) that neurons within the ventrolateral PAG are viscerotopically organized; and (ii) that their hypotensive function may be mediated by an inhibition of SRF pressor neurons. The results are discussed in relation to the recently described PAG hypertensive area which also is viscerotopically organized and projects to the SRF nucleus.

Tonic control of subretrofacial vasomotor neurons in the rostral ventrolateral medulla

1. Vasomotor pressor neurons in the subretrofacial nucleus of the rostral ventrolateral medulla receive afferent inputs from different sources that utilize different neurotransmitters. This paper briefly reviews recent studies on the role of inputs releasing: (i) GABA, and (ii) angiotensin II (AII) in the subretrofacial nucleus. 2. There are two types of tonic GABAergic inhibitory inputs: one arises from peripheral baroreceptors, while the second is independent of peripheral baroreceptors. 3. Blockade of receptors for AII elicits a decrease in blood pressure and sympathetic vasomotor activity, indicating that subretrofacial neurons are also tonically excited by AII. It is likely that the AII is released from nerve terminals in the subretrofacial nucleus, but the origin of the pathway is unknown.

A comparative analysis of neurons containing catecholamine-synthesizing enzymes and neuropeptide Y in the ventrolateral medulla of rats, guinea-pigs and cats

Neurons in the ventrolateral medulla oblongata of rats, guinea-pigs and cats that contain tyrosine hydroxylase, dopamine-beta-hydroxylase, phenylethanolamine-N-methyltransferase and neuropeptide Y have been demonstrated immunohistochemically in serial coronal sections of tissue taken from the level of the cervical spinal cord to the level of the facial nucleus. The anatomical distribution of these neurons has been described, quantified and reconstructed in three dimensions to compare the neuron populations between species. In all species, between 50 and 90% of immunoreactive neurons lay rostral to the level of the obex. There were no significant differences in the number and distribution of neurons containing catecholamine-synthesizing enzymes between control animals and those pretreated with colchicine, with two exceptions: all dopamine-beta-hydroxylase neurons were weakly immunoreactive without colchicine pretreatment in cats, and pretreatment with colchicine revealed a small rostral group of tyrosine hydroxylase-positive neurons in guinea-pigs. There were remarkable similarities in the rostrocaudal distributions of neurons containing tyrosine hydroxylase, dopamine-beta-hydroxylase and neuropeptide Y in relation to comparable anatomical landmarks across the species. However, the distributions of neurons containing tyrosine hydroxylase. Phenylethanolamine-N-methyltransferase-positive neurons, while densely stained in rats, were only faintly stained in cats and absent in guinea-pigs; the distribution of these neurons was similar to the distribution of neurons containing only tyrosine hydroxylase. The similarity of the distribution of neurons demonstrated using tyrosine hydroxylase, dopamine-beta-hydroxylase and neuropeptide Y immunohistochemistry implies that homologous catecholamine-containing neuron groups do exist in the ventrolateral medulla despite the variation in phenylethanolamine-N-methyltransferase between species. In contrast to the previous classification of neuron groups into A1 and C1 based on the presence or absence of this latter enzyme, the data suggest that a discrete group of tyrosine hydroxylase-immunoreactive neurons, which probably do not contain dopamine-beta-hydroxylase or neuropeptide Y, can be distinguished in the rostral ventrolateral medulla of all species. The absence of detectable dopamine-beta-hydroxylase in this group of neurons suggests that they may not synthesize either adrenaline or noradrenaline.

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.

Evidence that neurons of the central amygdaloid nucleus directly project to the site concerned with circulatory and respiratory regulation in the ventrolateral nucleus of the cat: a WGA-HRP study

Projections from the central amygdaloid nucleus (ACE) to the ventrolateral nucleus (VLN) of the medulla oblongata were studied using the wheat germ agglutinin-horseradish peroxidase (WGA-HRP) method. WGA-HRP was injected into the rostral VLN, where a pressor response associated with bradycardiac and apneustic responses was elicited by microinjection of glutamate, and into the caudal VLN, where a depressor response associated with bradycardiac and apneustic responses was elicited. In both experiments, HRP-labeled cells were found in the ACE. These results suggest that the ACE may send direct projections to the VLN and may play a role in cardiovascular and/or respiratory control mechanisms.

Anatomical substrates of cholinergic-autonomic regulation in the rat

Acetylcholine (ACh) plays a major role in central autonomic regulation, including the control of arterial blood pressure (AP). Previously unknown neuroanatomic substrates of cholinergic-autonomic control were mapped in this study. Cholinergic perikarya and bouton-like varicosities were localized by an immunocytochemical method employing a monoclonal antiserum against choline acetyltransferase (ChAT), the enzyme synthesizing ACh. In the forebrain, bouton-like varicosities and/or perikarya were detected in the septum, bed nucleus of the stria terminalis, amygdala (in particular, autonomic projection areas AP1 and AP2 bordering the central subnucleus), hypothalamus (rostrolateral/innominata transitional area, perifornical, dorsal, incertal, caudolateral, posterior [PHN], subparafascicular, supramammillary and mammillary nuclei). Few or no punctate varicosities were labeled in the paraventricular (PVN) or supraoptic (SON) hypothalamic nuclei. In the mid- and hindbrain, immunoreactive cells and processes were present in the nucleus of Edinger-Westphal, periaqueductal gray, parabrachial complex (PBC), a periceruleal zone avoiding the locus ceruleus (LC), pontine micturition field, pontomedullary raphe, paramedian reticular formation and periventricular gray, A5 area, lateral tegmental field, nucleus tractus solitarii (NTS), nucleus commissuralis, nucleus reticularis rostroventrolateralis (RVL), and the ventral medullary surface (VMS). In the PBC, immunoreactive varicosities identified areas previously unexplored for cholinergic autonomic responsivity (superior, internal, dorsal, and central divisions of the lateral subnucleus, nucleus of Koelliker-Fuse and the medial subnucleus). In the NTS, previously undescribed ChAT-immunolabeled cells and processes were concentrated at intermediate and subpostremal levels and distributed viscerotopically in areas receiving primary cardiopulmonary afferents. In the nucleus RVL, cholinergic perikarya were in proximity to the VMS and medial to adrenergic cell bodies of the C1 area. Punctate varicosities of unknown origin and dendrites extending ventrally from the nucleus ambiguus overlapped the C1 area and immediate surround of RVL.

IN CONCLUSION

1) Cholinergic perikarya and putative terminal fields, overlap structures that are rich in cholinoreceptors and express autonomic, neuroendocrine, or behavioral responsivity to central cholinergic stimulation (PHN, NTS, RVL). The role of ACh in most immunolabeled areas, however, has yet to be determined. Overall, these data support the concept that cholinergic agents act at multiple sites in the CNS and with topographic specificity.(ABSTRACT TRUNCATED AT 400 WORDS)

Lesions of rostral ventrolateral medulla abolish some cardio- and cerebrovascular components of the cerebellar fastigial pressor and depressor responses

We sought to establish whether the C1 area of the rostral ventrolateral reticular nucleus (RVL) of the medulla oblongata mediates: (1) the elevations in arterial pressure (AP), heart rate (HR) and regional cerebral blood flow (rCBF) elicited by electrical stimulation of the rostral cerebellar fastigial nucleus (FN), the fastigial pressor response (FPR); (2) the reductions in AP and HR elicited by chemical stimulation of intrinsic neurons of FN with excitatory amino acids, the fastigial depressor response (FDR). Studies were conducted on rats anesthetized (chloralose), paralyzed and artificially ventilated. The FN was stimulated electrically through microelectrodes and chemically by microinjection of D.L-homocysteic acid (100 nmol in 100 nl). rCBF was measured in homogenates of 11 brain regions by the 14C-iodoantipyrine technique. Bilateral electrolytic lesions restricted to the RVL abolished the elevations in AP, HR and rCBF elicited by electrical stimulation as well as the fall of AP and HR elicited by chemical stimulation of the FN. The disappearance of the responses was anatomically selective and could not be attributed to changes in resting AP, HR or rCBF, loss of reactivity of preganglionic sympathetic neurons, or variations in blood gases. Since the FN neither projects to nor receives afferents from the RVL the pathway to RVL is indirect. We conclude that: (1) the FPR results from excitation and the FDR inhibition of reticulospinal sympathoexcitatory axons of RVL; (2) the FPR is a consequence of excitation of axons arising from neurons in an as yet unidentified area of lower brainstem projecting to or through the FN and with collateral branches innervating RVL mono- or polysynaptically; (3) the FDR, in contrast, represents excitation of intrinsic FN neurons with a polysynaptic projection to RVL through unidentified regions of lower brainstem; (4) the RVL is a relay mediating the increase in rCBF associated with the FPR; and (5) the RVL plays a critical role in integrating actions on the systemic and cerebral circulation represented in cerebellum.