Bulbospinal Neurons Research Articles

HYPERTENSION AND COMPONENTS OF VASCULAR RESISTANCE

1. In renal wrap hypertension in the rabbit and in essential hypertension in humans, non-autonomic components of resistance are the major factors contributing to elevated blood pressure. 2. The central nervous system plays an important role in moment to moment blood pressure control but is not responsible for maintenance of elevated pressure in essential hypertension. 3. Specific regions of the brain-stem maintain vascular tone through the effects of bulbospinal neurones which are organized both topographically and by chemical content. 4. Structural wall thickening in resistance vessels in hypertension is a manifestation of an abnormality of a growth controlling process. 5. Although several structural muscle growth factors have been identified, it is likely that only a few regulatory processes are critical. 6. Future studies in hypertension should examine associations between genetic markers of medial thickening and environmental influences.

Ventral respiratory group projections to phrenic motoneurons: Electron microscopic evidence for monosynaptic connections

The hypothesis that excitatory drive is transmitted monosynaptically from bulbospinal medullary respiratory neurons to spinal respiratory motoneurons was tested by an ultrastructural analysis of the phrenic motoneuronal pool in the rat. Combined anterograde labeling of the principal inspiratory bulbospinal neuron population (ventral respiratory group) and retrograde labeling of the phrenic motoneuron pool demonstrated the presence of labeled synaptic profiles, indicating that at least some bulbospinal inspiratory neurons make monosynaptic contacts with phrenic motoneurons. The synaptic boutons of ventral respiratory group neurons that were labeled in the phrenic nucleus had asymmetrical membrane densities at sites of synaptic contact with labeled phrenic somal or dendritic profiles, supporting the notion that this bulbospinal pathway has excitatory contacts with phrenic motoneurons. The morphological types of labeled boutons included three of the eight previously identified bouton types in the phrenic nucleus (Goshgarian and Rafols: Journal of Neurocytology 13:85-109, 1984), including the "S"-terminal, the "NFs"-terminal, and the "F"-terminal. There was no conclusive evidence of labeled double synapses, indicating that this type of synaptic contact is not common in the intact bulbospinal pathway.

GABA-immunoreactive boutons make synapses with inspiratory neurons of the dorsal respiratory group

Intracellular labelling with horseradish peroxidase (HRP) combined with γ-aminobutyric (GABA) immunocytochemistry was used to assess the GABAergic input to inspiratory bulbospinal neurons of the dorsal respiratory group in the cat. The relationship between GABA-immunoreactive (GABA-IR) boutons and intracellularly labelled neurons was examined at the light microscopic and ultrastructural levels. At the light microscopic level, GABA-IR boutons were frequently found in close apposition to dendrites and cell bodies of labelled neurons. The presence of synapses was confirmed with electron microscopy. In addition, synaptic specializations were observed between immunoreactive boutons and unlabelled terminals which in turn formed synaptic contacts with HRP-labelled dendrites, a finding consistent with presynaptic inhibition. These results demonstrate a direct GABAergic input to a functionally defined population of medullary respiratory neurons, and suggest involvement of this neurotransmitter in the control of this neurotransmitter in the control of these neurons.

Extensive monosynaptic inhibition of ventral respiratory group neurons by augmenting neurons in the Bötzinger complex in the cat

Axonal projections and synaptic connectivity of expiratory Bötzinger neurons with an augmenting firing pattern (Bot-Aug neurons) to neurons in the ipsilateral ventral respiratory group (VRG) were studied in anaesthetized cats. Antidromic mapping revealed extensive axonal arborizations of Bot-Aug neurons (24 of 45) to the rostral or caudal VRG, with some having arbors in both regions. Of 234 pairs of neurons studied with intracellular recording and spike-triggered averaging, monosynaptic inhibitory postsynaptic potentials (IPSPs) were evoked in 49/221 VRG neurons by 38/98 Bot-Aug neurons. The highest incidence of monosynaptic inhibition was found in inspiratory bulbospinal neurons (10 of 23 tested). Evidence was also found for monosynaptic inhibition, by a separate group of Bot-Aug neurons, of expiratory bulbospinal neurons (12/58), while excitatory postsynaptic potentials (EPSPs) were identified in another two of these neurons. In addition, monosynaptic IPSPs were recorded from 13 of 53 identified laryngeal motoneurons, and from 14 of 100 respiratory propriobulbar neurons. Presumptive disynaptic IPSPs were recorded from 11 of the 221 VRG neurons. We conclude that Bot-Aug neurons exert widespread inhibition on all major neuron categories in the ipsilateral VRG, and should be regarded as an important element in shaping the spatiotemporal output pattern of both respiratory motoneurons and premotor neurons.

Chemosensitivity of sympathoexcitatory neurones in the rostroventrolateral medulla of the cat

The hypothesis that sympathoexcitatory neurones within the rostroventrolateral medulla (RVLM) may be chemosensitive was tested in chloralose-anaesthetized cats by artificial perfusion of the RVLM via the left vertebral artery. The baroreceptors and peripheral chemoreceptors were denervated by bilaterally dissecting the carotid sinus and vagus nerves. Either white ramus T3 (WR-T3) or the renal nerve was recorded to monitor sympathetic activity. Perfusion with saline or Ringer solution bubbled with CO2 (10%-100%) produced a rapid and pronounced increase in sympathetic activity and blood pressure. Solutions adjusted to the same pH (pH 5.2 for 100% CO2) with HCl resulted in a much weaker excitation. A linear relationship between PCO2 and sympathetic activity was demonstrated. During prolonged perfusion (90 s) sympathetic activity returned to the control level after initial excitation and fell below control levels when perfusion ceased. The sympathetic activity response to CO2-bubbled solutions was unaffected by blockade of synaptic input by microinjection of CoCl2 into the RVLM, whereas spontaneous sympathetic activity and the supraspinal somato-sympathetic reflex from intercostal nerve T4 to WR-T3 were markedly reduced. It is therefore concluded that sympathoexcitatory bulbospinal neurones in the RVLM are directly chemosensitive to changes in arterial PCO2 and pH.

Mechanisms of abdominal muscle activation during vomiting

The possible contribution of spinal reflexes to abdominal muscle activation during vomiting was assessed in decerebrate cats. The activity of these muscles is partly controlled by bulbospinal expiratory neurons in the caudal ventral respiratory group (VRG). In a previous study it was found that the abdominal muscles are still active during vomiting after midsagittal lesion of the axons of these neurons between C1 and the obex (A.D. Miller, L.K. Tan, and I. Suzuki. J. Neurophysiol. 57: 1854-1866, 1987). The present experiments indicate that this postlesion activity was due to spinal stretch reflexes because 1) such midsagittal lesions eliminate abdominal muscle nerve activity during fictive vomiting in paralyzed cats in which there are no abdominal stretch reflexes, 2) the abdominal muscles are activated during vomiting by spinal reflexes after upper thoracic cord transections, and 3) the normal 100-ms delay between diaphragmatic and abdominal activation during vomiting is reduced to approximately 20-25 ms after both types of lesions, which is consistent with postlesion abdominal reflex activation. Our results also suggest that, during normal vomiting, abdominal stretch and tension reflexes have only a minor role if any and abdominal muscle activation is probably mediated primarily or exclusively by expiratory neurons in the caudal ventral respiratory group. However, our finding that phrenic activity is reduced both during vomiting after thoracic transections and during fictive vomiting after paralysis is consistent with a contribution of reflex activity from abdominal and/or intercostal muscles to phrenic discharge during normal vomiting.

Evidence that glutamate is the transmitter mediating respiratory drive from medullary premotor neurons to phrenic motoneurons: A double labeling study in the rat

Glutamate immunoreactivity was found in 81-84% of the bulbospinal neurons projecting to the phrenic motoneuron pool, which were located in the para-ambiguous nucleus and the ventral vicinity of the retrofacial nucleus. Their glutamate contents were identified by a double labeling technique in combination with the retrograde transport of horseradish peroxidase and immunocytochemistry of the glutamate. The remaining 16-19% of the bulbospinal neurons projecting to the phrenic motoneuron pool did not contain glutamate immunoreactivity. Thus it appears that glutamate may be a major transmitter of the inspiratory drive from the medullary respiratory neurons to the phrenic motoneurons.

Bo¨tzinger expiratory neurons may inhibit phrenic motoneurons and medullary inspiratory neurons during vomiting

During vomiting, phrenic motoneurons exhibit bursts of vigorous discharge while bulbospinal inspiratory neurons are mostly silent. To determine how this activity pattern correlates with the behavior of Bötzinger (BOT) expiratory (E) neurons, which inhibit both phrenic motoneurons and medullary inspiratory neurons, we recorded from 20 augmenting BOT E neurons in decerebrate, paralyzed cats. During fictive vomiting, all BOT E neurons fired between bursts of phrenic discharge and thus could contribute to the concurrent lack of firing of phrenic motoneurons.

Subnuclear organization of the lateral tegmental field of the rat. II: Catecholamine neurons and ventral respiratory group

Bulbospinal and propriobulbar respiratory neurons of the ventral respiratory group and catecholamine neurons of the A1 and C1 cell groups were simultaneously labelled in the rat medulla by a combination of retrograde tracing and immunohistochemical identification. The ventral respiratory group and catecholamine cell groups form adjacent, parallel cell columns in the lateral tegmental field of the ventrolateral medulla. The ventral respiratory group is located immediately dorsal to the A1 and C1 groups, although some A1 neurons are intermingled with neurons of the rostral ventral respiratory group, and some C1 neurons are intermingled with those of the Bötzinger complex. The proximate populations of respiratory, catecholamine, and (presumptive) cardiovascular neurons identified in this study provide further support to the hypothesis that this region of the lateral tegmental field of the ventrolateral medulla is a site of cardiorespiratory coordination.

Origins and terminations of bulbospinal axons that contain serotonin and either enkephalin or substance‐P in the North American opossum

We have shown previously that some enkephalin, substance-P, and serotoninergic neurons in the medullary raphe and adjacent reticular formation project to the spinal cord in the opossum. In the present study we have combined the retrograde transport of True Blue and immunofluorescence histochemistry to determine whether methionine enkephalin or substance-P containing bulbospinal neurons are serotoninergic. Furthermore, we have used the same immunofluorescence protocol to determine whether spinal axons contain the same substances. Neurons that immunostained for both enkephalin and serotonin were observed in many brainstem nuclei. However, those that projected to the spinal cord were limited to the nuclei raphe magnus and obscurus, and the ventral part of nucleus reticularis gigantocellularis, pars ventralis. Neurons that immunostained for both substance P and serotonin were fewer in number, but some of the ones in the above nuclei and within the nucleus raphe pallidus, projected to the spinal cord. Spinal axons exhibiting both enkephalin- and serotonin-like immunoreactivity were observed in the superficial laminae of the dorsal horn, lamina X, and the intermediolateral cell column, whereas those showing both substance-P and serotonin-like immunoreactivity were seen primarily in lamina X, the intermediolateral cell column, and the ventral horn. Some of the axons in the ventral horn were in close apposition to presumed motoneurons. Comparison of the above results with those obtained from previous studies of bulbospinal projections has allowed us to infer the origins of axons that innervate different spinal targets.

Brainstem connections of the rostral ventral respiratory group of the rat

The pontomedullary connections of the rostral division of the ventral respiratory group (rVRG), the largest medullary population of inspiratory bulbospinal and propriobulbar neurons, were identified in the rat by retrograde and anterograde tracing techniques. These experiments revealed that: (i) the Kölliker-Fuse nucleus, portions of the medial and lateral parabrachial nuclei, and all levels of the ipsilateral and contralateral VRG complex have dense reciprocal connections with the rVRG; (ii) the lateral paragigantocellular nucleus has reciprocal but less dense connections with rVRG; (iii) portions of the nucleus of the solitary tract have prominent projections to, but weaker inputs from rVRG; (iv) the raphe, magnocellular tegmental field and spinal trigeminal nuclei have minor projections to rVRG and receive only sparse inputs from rVRG, and; (v) the retrotrapezoid nucleus, pontine lateral tegmental field and area postrema each have only efferent projections to rVRG. These findings are consistent with previous studies of pontomedullary connections of rVRG in the cat, and further document the extensive reciprocal connections between principal respiratory groups. These connections are likely to be important for generation of the respiratory pattern, for the coordination of effector responses of the cranial and spinal respiratory motor neurons to afferent stimuli, and coordination of the central respiratory and cardiovascular control systems.

Respiratory muscle control during vomiting

The changes in thoracic and abdominal pressure that generate vomiting are produced by coordinated action of the major respiratory muscles. During vomiting, the diaphragm and external intercostal (inspiratory) muscles co-contract with abdominal (expiratory) muscles in a series of bursts of activity that culminates in expulsion. Internal intercostal (expiratory) muscles contract out of phase with these muscles during retching and are inactive during expulsion. The periesophageal portion of the diaphragm relaxes during expulsion, presumably facilitating rostral movement of gastric contents. Recent studies have begun to examine to what extent medullary respiratory neurons are involved in the control of these muscles during vomiting. Bulbospinal expiratory neurons in the ventral respiratory group caudal to the obex discharge at the appropriate time during (fictive) vomiting to activate either abdominal or internal intercostal motoneurons. The pathways that drive phrenic and external intercostal motoneurons during vomiting have yet to be identified. Most bulbospinal inspiratory neurons in the dorsal and ventral respiratory groups do not have the appropriate response pattern to initiate activation of these motoneurons during (fictive) vomiting. Relaxation of the periesophageal diaphragm during vomiting could be brought about, at least in part, by reduced firing of bulbospinal inspiratory neurons.

Activation and block of NMDA receptors by fast application of agonist and antagonists : Mark L. Mayer, Morris Benveniste and Doris K. Patneau, Unit of Neurophysiology & Biophysics, LDN, NICHD, NIH, Bethesda, MD USA

Most of the neurons of the caudal ventral respiratory group (cVRG) are bulbospinal expiratory neurons that receive their main excitatory drive from more rostral, but not yet defined regions. This study was devoted to investigate the functional role of ionotropic excitatory amino acid (EAA) receptors in the excitatory drive transmission to cVRG expiratory neurons during eupnoeic breathing and some respiratory reflexes including cough induced by mechanical stimulation of the tracheobronchial tree. The experiments were performed on spontaneously breathing rabbits under pentobarbitone anesthesia making use of microinjections (30–50 nl) of EAA receptor antagonists into the cVRG. Phrenic nerve and abdominal muscle activities were recorded. Bilateral microinjections of 50 mM kynurenic acid (KYN), a broad-spectrum EAA antagonist, and 10 mM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a non-NMDA antagonist, or 5 mM 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2,3-dione (NBQX), a more specific non-NMDA antagonist, completely suppressed spontaneous rhythmic abdominal activity and reflex expiratory responses either to tracheal occlusion at end-inspiration (Breuer–Hering inflation reflex) or to expiratory threshold loading (5 cm H2O); they also suppressed both the inspiratory and expiratory components of the cough reflex. Spontaneous rhythmic abdominal activity and the reflex respiratory responses were strongly reduced, but not completely abolished by microinjections of 10 mM d(−)-2-amino-5-phosphonopentanoic acid (D-AP5), an NMDA antagonist. The results provide evidence that the excitatory drive to cVRG bulbospinal expiratory neurons during eupnoeic breathing and the investigated respiratory reflexes is mediated by EAA receptors. They also support the view that neurons located in the cVRG are not merely elements of the expiratory output system.

Substance-P and somatostatin spinal fluid content in patients with cancer pain. Influence of 1spinal opiatle therapy

Intrathecal injection of the analogue of TRH, CG 3509, into conscious rats produced dose-related wet-dog shakes and forepaw licking, which showed a bell-shaped relationship of intensity to dose. Pretreatment with α-MPT intraperitoneally, markedly reduced levels of noradrenaline and dopamine in the spinal cord and brainstem and attenuated both CG 3509-induced responses, while intrathecal treatment with DSP4 selectively reduced noradrenaline in the spinal cord without affecting either behaviour. Since denervation supersensitivity may develop following treatment with DSP4, these results are not inconsistent with a proposal that bulbospinal noradrenergic neurones modulate the behaviour induced by CG 3509. Wet-dog shakes and forepaw licking induced by CG 3509 were reduced by pretreatment with phenoxybenzamine or prazosin, suggesting that a tonic noradrenergic pathway may facilitate both behavioural responses through α1-adrenoceptors. Methoxamine, combined with CG 3509 partially attenuated the wet dog shake behaviour, but methoxamine produced marked hindlimb jerking which might physiologically antagonise wet-dog shakes. Concomitant administration of clonidine and CG 3509 potently reduced wet-dog shakes in a dose-related manner but did not significantly affect forepaw licking, while idazoxan did not significantly affect either response. The latter findings imply that α2-adrenoceptors play different roles in modulating the two behavioural responses and the possible synaptic location of the receptors is discussed. Taken together these results suggest that CG 3509 may release noradrenaline from bulbospinal neurones regulating motor function.

Differences in the Central Hypotensive Actions of α-Methyldopa and Clonidine in the Spontaneously Hypertensive Rat

The rostral ventrolateral medulla (RVLM) is the proposed site of origin of bulbospinal excitatory vasomotor neurons, and this brainstem area gives rise to chemically distinct populations of neurons, including serotonin-containing neurons of the B3 group and epinephrine-containing neurons of the C1 group, which independently serve sympathoexcitatory functions. In the present study, we sought to establish (a) whether distinct and chemically specific pathways originating in the C1 or B3 regions are involved in the antihypertensive effects of alpha-methyldopa (methyldopa) and clonidine and (b) if so, whether these effects are related to an activation of alpha-adrenoceptors in these areas. Microinjections of methyldopa (6 nmol) or clonidine (5 nmol) were made in the C1 or B3 area in intact spontaneously hypertensive rats (SHR), pretreated with 5,7-dihydroxytryptamine (5,7-DHT) or with phentolamine. The microinjection of clonidine into both the B3 and the C1 area caused a rapid decrease in arterial pressure, whereas microinjection of methyldopa lowered the arterial pressure only after injection into the B3 area. Pretreatment with intracerebroventricular (i.c.v.) 5,7-DHT attenuated the hypotension produced by microinjection of clonidine into the B3 area, suggesting that this effect is mediated by serotonin-containing neurons. Central pretreatment with phentolamine reduced the hypotensive effects produced by injection of clonidine into either area and of methyldopa into the B3 region, consistent with previous suggestions that these central effects are mediated through alpha-adrenoceptors. These results suggest that both serotonin-containing and epinephrine-containing neurons contribute to the central action of clonidine, whereas the effects of methyldopa injection in RVLM appear to be mediated by serotonin-containing but not by epinephrine-containing neurons.

Diaphragmatic and external intercostal muscle control during vomiting: behavior of inspiratory bulbospinal neurons

1. The role of dorsal and ventral respiratory group (DRG and VRG) bulbospinal inspiratory (I) neurons in the control of diaphragmatic and external intercostal (inspiratory) muscle activity during vomiting was examined by recording from these neurons during fictive vomiting in decerebrate, paralyzed cats. Fictive vomiting was defined by a characteristic series of bursts of coactivation of phrenic and abdominal muscle nerves, elicited either by electrical stimulation of abdominal vagal afferents or by emetic drugs, which would be expected to produce vomiting if the animals were not paralyzed. 2. Data were recorded from 22 DRG and 29 VRG I neurons that were antidromically activated from the fourth cervical spinal segment (C4). Only 10% (5/51) of these neurons started to fire near the beginning of phrenic discharge during fictive vomiting and thus had the appropriate discharge pattern to contribute to the initial activation of the diaphragm and coactive external intercostal muscles during vomiting. The frequency of occurrence of these Active neurons was not significantly different in the DRG (3/22) and VRG (2/29) (chi 2 test). Most remaining neurons were either totally silent (n = 7) or had only sporadic, infrequent firing (n = 16) (Silent neurons, 23/51 = 45%), or else fired near the end of phrenic discharge during fictive vomiting (End neurons, 21/51 = 41%). Two neurons were categorized as having miscellaneous (Misc) behavior. 3. No differences were found among neurons having different response patterns during fictive vomiting in regard to the following: the manner in which fictive vomiting was elicited: cell location: conduction velocity; and neuronal firing onset, rate, and pattern during respiration.(ABSTRACT TRUNCATED AT 250 WORDS)

Bulbospinal Serotonergic Pathways in the Control of Blood Pressure

The anatomy, physiology, and pharmacology of bulbospinal 5-HT neurons innervating spinal sympathetic cell groups are reviewed. On the basis of the location of 5-HT cell bodies, size of axons, type of synapse, and diversity of chemical coding, it is argued that the group of bulbospinal neurons is anatomically heterogenous. The evidence for both sympathoexcitatory and sympathoinhibitory actions of these neurons is discussed, and, although unequivocal evidence is still lacking, the idea of a functional heterogeneity is attractive. This is supported by observations that 5-HT also has both sympathoexcitatory and sympathoinhibitory actions in the spinal cord. These actions are mediated via two receptors, 5-HT1-like and 5-HT2, respectively. The question of whether or not these receptors are tonically activated is discussed, and it is shown that most evidence indicates they are not.

Responses of inspiratory neurons of the dorsal respiratory group to stimulation of expiratory muscle and vagal afferents

In decerebrate, paralyzed and ventilated cats, we monitored the intracellular responses of 30 inspiratory neurons of the dorsal respiratory group (DRG) to stimulation of vagal and expiratory muscle (internal intercostal and abdominal) afferents. We hypothesized that the inhibitory effects of stimulation of expiratory muscle afferents, previously reported, would block the excitatory responses of inspiratory neurons of the DRG to vagal stimulation. Although prolonged stimulus trains to expiratory muscle afferents caused respiratory phase-switching, single shocks or short trains elicited no responses in 17 bulbospinal neurons, excitatory responses in 6, and inhibitory responses in 2. Of the 4 propriobulbar neurons tested, 2 had inhibitory responses and 2 did not respond. In only 2 neurons, both bulbospinal, did conditioning stimuli to expiratory muscle afferents block or reduce the excitatory effects of vagal stimulation. These results suggest that interaction of vagal and expiratory muscle afferents, which might account for the absence of a change in inspiratory duration despite increased vagal afferent feedback at elevated end-expiratory lung volumes, does not occur within the DRG.

Bulbospinal Serotonergic Pathways in the Control of Blood Pressure

The anatomy, physiology, and pharmacology of bulbospinal 5-HT neurons innervating spinal sympathetic cell groups are reviewed. On the basis of the location of 5-HT cell bodies, size of axons, type of synapse, and diversity of chemical coding, it is argued that the group of bulbospinal neurons is anatomically heterogenous. The evidence for both sympathoexcitatory and sympathoinhibitory actions of these neurons is discussed, and, although unequivocal evidence is still lacking, the idea of a functional heterogeneity is attractive. This is supported by observations that 5-HT also has both sympathoexcitatory and sympathoinhibitory actions in the spinal cord. These actions are mediated via two receptors, 5-HT1-like and 5-HT2, respectively. The question of whether or not these receptors are tonically activated is discussed, and it is shown that most evidence indicates they are not.

Evidence for respiratory interneurones in the C3-C5 cervical spinal cord in the decorticate rabbit.

In mammals, it has long been considered that the bulbo-spinal inspiratory drive provided a direct (monosynaptic) excitation of phrenic motoneurones (Phr Mns). Although such connections have been demonstrated, recent indirect data strongly suggested that the main inspiratory drive is polysynaptic. We tried to directly demonstrate relay respiratory interneurones at the C3-C6 spinal cord level where the Phr Mn pool is located. The experiments were performed on decorticate, unanaesthetized, bilaterally vagotomized and curarized rabbits and the firing pattern of spinal interneurones was compared to the phrenic bursting. Dorsally and dorso-medially to the Phr Mn pool, different classes of inspiratory (54%) and expiratory (46%) interneurones could be identified in the ventral horn. Three classes of inspiratory interneurones were characterized and classified as "I all" (26%), "I late" (43%) and "I tonic" (29%) according to the terminology used by other authors for the bulbospinal inspiratory neurones which drive the spinal respiratory motoneurones. The expiratory interneurones could also be divided into 3 classes: "E all" (48%), "E late" (10%) and "E tonic" (41%). This first direct evidence of inspiratory interneurones at the C3-C6 spinal cord levels can account for the major polysynaptic excitation of the Phr Mns while the presence of numerous expiratory interneurones at this level suggests a polysynaptic bulbo-spinal inhibitory action onto the Phr Mns. These classes of inspiratory and expiratory interneurones did not always coincide with the bulbo-spinal classes of neurones described elsewhere.(ABSTRACT TRUNCATED AT 250 WORDS)