Central effects of tachykinin peptide on tracheal secretion

Tachykinin peptides acting on structures located on the ventral surface of the medulla can increase cholinergic outflow to the tracheal smooth muscles and augment respiratory motor output. In the experiments reported here (performed in anesthetized, paralyzed and artificially ventilated dogs), we examined the effects of tachykinin peptides substance P on secretion from submucosal glands. Changes in secretion were measured in an exposed section of tantalum-coated tracheal epithelium. Substances P was administered intracisternally or applied topically on the intermediate area of the ventral surface of medulla (VMS). Intracisternal infusion and the local medullary administration of tachykinin peptide caused a significant increase in tracheal submucosal gland secretion. Atropine given intravenously prevented the secretory changes induced by central action of tachykinins. In addition, prior application of 2% lidocaine to the medullary surface blocked the responses caused by substance P locally applied on the VMS. These findings suggest that substances P acting centrally can tracheal fluid secretion mainly via cholinergic mechanisms, and that the ventral surface of the medulla is one of the site of these action.

Modulation of respiratory reflexes by an excitatory amino acid mechanism in the ventrolateral medulla

Results from several studies suggest that the ventrolateral medulla (VLM) is involved in modulating the respiratory response to central and/or peripheral chemoreceptor stimulation. Furthermore, the excitatory amino acid (EAA) glutamate has been shown to have marked effects on respiration when administered to VLM sites. The purpose of this study was to determine if an excitatory amino acid mechanism in the VLM modulates the respiratory responses to hypoxia or hypercapnia in anesthetized rats. Exposure to hypoxic or hypercapnic gas under control conditions elicited increases in respiratory activity (diaphragmatic EMG activity and breathing frequency). Bilateral injection of kynurenic acid (KYN), an EAA antagonist, into rostral VLM sites evoked significant increases in breathing frequency; injections more caudal in the VLM typically caused apnea. Significantly larger increases in respiratory output were elicited by both hypoxia and hypercapnia after rostral VLM microinjections of KYN. The accentuated responses returned to control levels after a recovery of approximately 100 min. Microinjection of xanthurenic acid (XAN), an inactive analog of kynurenic acid, into the VLM prior to KYN had only slight effects on resting respiratory activity and no effects on the responses to hypoxia or hypercapnia. These results suggest two separate VLM sites which modulate respiration by EAA mechanisms. A more rostral site tonically inhibits respiratory activity and the respiratory responses to chemoreceptor stimulation and more caudal VLM sites may be required for the maintenance of respiratory activity.

Naloxone application to the ventrolateral medulla enhances the respiratory response to inspired carbon dioxide

Previous studies have shown that systemic administration of the opiate antagonist naloxone potentiates the ventilatory response to inspired carbon dioxide. The present study was designed to localize the site of action of naloxone for increasing the respiratory chemosensitivity to inhaled carbon dioxide (CO2) in cats. Naloxone applied topically to the caudal chemosensitive area on the ventral medullary surface (VMS) during hypercapnic breathing produced a 75% greater increase in minute ventilation than hypercapnic breathing alone. Furthermore, hypercapnic breathing produced a 200% increase in neuronal activity of VMS chemosensitive cells; this was further increased 120% by naloxone. It is concluded that naloxone increases the sensitivity of neurons in the caudal respiratory chemosensitive area of cats to hypercapnia, and that endogenous opiates may act as modulators at VMS chemosensitive sites during hypercapnic breathing.

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.

Is central chemoreceptor sensitive to intracellular rather than extracellular pH?

The chemosensitive area on the ventral surface of the brain stem responds to local acidosis by eliciting hyperventilation and to local alkalosis by hypoventilation. The stimulus is conventionally thought to be the hydrogen ion concentration in the area's extracellular fluid. It is pointed out, however, that the elegant studies by Loeschcke & Ahmad have demonstrated that [pH]e and [pH]i are normally tightly and rapidly coupled (Loeschcke & Ahmad, 1980). For this reason, the stimulus might just as well be the intracellular hydrogen ion concentration in the chemoreceptor area. The administration of acetazolamide allows the dissociation of [pH]e from [pH]i. With acetazolamide a sharp acid shift of CSF pH [( pH]c) is measured and in two consonance with this shift a marked increase in CBF is seen. Comparing these two reactions to that obtained with CO2 breathing, it is apparent that 7% CO2 causes about the same decrease in [pH]e and the same increase in CBF. In other words CBF acidosis can quantitatively account for the CBF increase induced by acetazolamide. But CO2 and acetazolamide influence [pH]i quite differently, as CO2 drops [pH]i to almost the same extent as [pH]c, while two recent studies by MR spectroscopy have shown that acetazolamide does not drop [pH]i measurably, if tissue hypercapnia is prevented in artificially ventilated rabbits or by the mild spontaneous hyperventilation caused by acetazolamide in normal man.(ABSTRACT TRUNCATED AT 250 WORDS)

Rete mirabile medullae: vascular specialization of the avian brain stem

A circumscribed vascular plexus is located between the conjoint proximal ganglion of cranial nerves IX-X and the lateral surface of the avian brain stem. Gross, light, and electron microscopic analyses reveal that this structure is a rete mirabile. This rete is unique in that it resides in the subarachnoid space where it is bathed by cerebrospinal fluid. Microscopic and ultrastructural studies document that the rete is composed of arterioles with wide luminal diameters, marginal folds project from the endothelial cells into the lumen of the blood vessel. The endothelial cells contain numerous pinocytotic vesicles. The underlying basal lamina is penetrated by myoendothelial junctions. One to three layers of smooth muscle cells comprise the tunica media. Bundles of unmyelinated nerve fibers course through the loose connective tissue encompassing the blood vessels.

Candidate cell populations for respiratory chemosensitive fields in the human infant medulla

The histology and location of human respiratory chemosensitive fields are not known. In contrast, the physiology of respiratory chemosensory areas in the ventral medulla of cats has been studied extensively, and their anatomy has been partially described. Using basic principles of comparative cytoarchitecture and computer-aided reconstructions of serial-sectioned medullae, we describe the histology and three-dimensional distribution of putative respiratory chemosensors in the feline and human infant medulla. We found that ventrolateral neurons of the human nucleus conterminalis are homologous to neurons identified in the feline L chemosensitive field by Trouth and others, and that ventrolaterally situated neurons in the human arcuate nucleus correspond to neurons predominating in the feline S and M fields. In addition, there are foci of thickened marginal glia along the feline ventral medullary surface that colocalize with chemosensitive fields identified by physiologic studies reported by others; we identify similar foci in the infant medulla. Thickened marginal glia are intermixed with neuronal fibers, often adjacent to neurons of the feline chemosensitive fields and their human counterparts, suggesting that they constitute a chemosensory neuropil at the medullary surface. Computer-aided reconstructions provide insight into the three-dimensional topography of putative respiratory chemosensors and their relationships to other brainstem structures in ways not obvious in single or even multiple sections. This delineation of candidate human respiratory chemosensors is a first step toward their postmortem analysis in patients with central ventilatory control disorders where finding histological abnormalities in these sites would support their role in human ventilation.

Depolarization and stimulation of neurons in nucleus tractus solitarii by carbon dioxide does not require chemical synaptic input

The effects of elevated CO2 (i.e. hypercapnia) on neurons in the nucleus tractus solitarii were studied using extracellular (n = 82) and intracellular (n = 33) recording techniques in transverse brain slices prepared from rat. Synaptic connections from putative chemosensitive neurons in the ventrolateral medulla were removed by bisecting each transverse slice and discarding the ventral half. In addition, the response to hypercapnia in 20 neurons was studied during high magnesium-low calcium synaptic blockade. Sixty-five per cent of the neurons (n = 75) tested were either insensitive or inhibited by hypercapnia. However, 35% (n = 40) were depolarized and/or increased their firing rate during hypercapnia. Nine out of 10 CO2-excited neurons retained their chemosensitivity to CO2 in the presence of high magnesium-low calcium synaptic blockade medium. Our findings demonstrate that many neurons in the nucleus tractus solitarii were depolarized and/or increased their firing rate during hypercapnia. These neurons were not driven synaptically by putative chemosensitive neurons of the ventrolateral medulla since this region was removed from the slice. Furthermore, because chemosensitivity persisted in most neurons tested during synaptic blockade, we conclude that some neurons in the nucleus tractus solitarii are inherently CO2-chemosensitive. Although the function of dorsal medullary chemosensitive neurons cannot be determined in vitro, their location and their inherent chemosensitivity suggest a role in cardiorespiratory central chemoreception.

gamma-Aminobutyric acid receptors at the ventral surface of the medulla inhibit respiratory motor outflow to the laryngeal musculature

The effect of activating gamma-aminobutyric acid (GABA) receptors at the ventral surface of the medulla on the activity of the recurrent laryngeal nerve and phrenic nerve was assessed in the cat. Characteristics of the effects of GABA on the activity of the recurrent laryngeal nerve were compared with those on that of the phrenic nerve which has previously been shown to be inhibited by the application of GABA to the ventral surface of the medulla. Application of GABA (0.017-4.05 mg) to the intermediate area produced a dose-related inhibition of respiratory activity in the recurrent laryngeal nerve, as well as the phrenic nerve, that culminated in apnea. The inhibition in each nerve was seen as a decrease in amplitude of nerve activity with no change in respiratory rate. The onset time, peak time and recovery time from GABA-induced inhibition of activity in the recurrent laryngeal and phrenic nerves were not significantly different. The ED50 value for GABA and its 95% confidence interval for inhibition of the activities of the recurrent laryngeal and phrenic nerves were 0.26 mg (0.19-0.36 mg) and 0.27 mg (0.20-0.37 mg), respectively. Therefore, the potency of GABA for the inhibition of the activity of these nerves was not significantly different. The GABA receptor antagonist, bicuculline (10 micrograms), reversed the inhibition of the activities of both the recurrent laryngeal and the phrenic nerves. The time for return of phasic activity in each nerve after bicuculline was not significantly different.(ABSTRACT TRUNCATED AT 250 WORDS)

Substance P-containing medullary projections to the intermediolateral cell column: identification with retrogradely transported rhodamine-labeled latex microspheres and immunohistochemistry

Substance P (SP)-containing medullary neurons that project to the intermediolateral cell column (IML) of the rat were studied. Neurons were retrogradely labeled with rhodamine-labeled latex microspheres (RITC-M) injected into the T-3 IML, and SP-immunoreactive neurons were identified with immunocytochemistry. RITC-M labeled cells occurred in the nucleus reticularis paragigantocellular lateralis (RPgcl), adjacent and lateral to the pyramidal tract at the level of the rostral inferior olivary nucleus and extended to the mid-facial nucleus in the medulla. Cells were also labeled caudal to the RPgcl, in the nucleus reticularis ventralis, pars alpha (RVa), rostral to the RVa, in the nucleus reticularis gigantocellularis (RGc), and in the raphe nuclei. SP immunoreactivity (SP-IR) was seen in cells that were also retrogradely labeled. These double-labeled cells were observed in the RPgcl, RVa and the raphe pallidus. These data show that the IML receives SP-neuronal projections from multiple locations in the medulla. The SP-neuronal projections from the RPgcl of the ventral medulla to the IML likely represent one component of the ventral medullary region that influences cardiovascular functions.

Target sites for anticholinesterases on the ventral surface of the medulla oblongata: hypotension elicited by organophosphorus agents

Sensitivity of the ventral surface of the medulla oblongata to organophosphorus agents, oxime reactivators, and muscarinic antagonists was examined in order to delineate sites of cholinergic activity in the central nervous system. The exposed ventral surface of the medulla oblongata in anaesthetized cats was treated with the organophosphorus anticholinesterase agents soman and (7-nitro-2-oxa-1,3-diazole) aminopentyl methylphosphonofluoridate (NBD-AP-MFP), a fluorescent active centre-selective probe of acetylcholinesterase. Topical application of soman (1-5 micrograms) or NBD-AP-MPF (5-120 micrograms) elicited a profound (80-90 mm Hg), long-lasting (0.5-3 h), dose-dependent vasodepression with only minor changes in heart rate and respiration. The vasodepression was rapidly reversed (7-10 min) upon topical application of muscarinic antagonists (atropine methylnitrate, atropine sulphate) and the bisquaternary oxime HI-6; systemic administration was without effect. Reversal of the hypotension by HI-6 occurred irrespective of whether the organophosphorus agent was NBD-AP-MPF, which forms conjugates with acetylcholinesterase that undergo no aging, or soman, which forms conjugates that undergo extensive aging rendering the enzyme refractory to oxime reactivation. Hence, oxime efficacy for reversal of the physiologic hypotension was not dependent solely on the fraction reactivatable enzyme. By virtue of the fluorescence distribution of NBD-AP-MPF the chemosensitive sites were estimated to reside no deeper than 50 microns into the medulla oblongata, providing a direct indication for localization of the chemosensitive cells on the superficial surface.

Intracisternal diethylpyrocarbonate inhibits central chemosensitivity in conscious rabbits

As a direct chemical test of the alpha-imidazole hypothesis for the function of mammalian central chemoreceptors (CCR), diethylpyrocarbonate (DEPC) a relatively specific reactant with imidazole groups in vitro has been administered in vivo via intracisterna magna (ICM) infusion in conscious rabbits using each rabbit as its own control. DEPC, in a dose-dependent fashion, induced resting hypoventilation and inhibited (1) the ventilatory response to CO2 in peripherally chemodenervated animals, and (2) both the PaCO2 and minute ventilation responses to ICM infusion of an acidic mock cerebrospinal fluid (CSF). DEPC had no effect on the hypoxic ventilatory response and had small non-dose-dependent effects on body temperature. ICM administration of hydroxylamine (HDA), a substance that reverses the DEPC-imidazole binding in vitro, prevented DEPC induced inhibition of CCR function. These data support but do not prove the alpha-imidazole hypothesis for mammalian central chemoreceptor function and demonstrate a potentially useful chemical tool for the study of central chemoreception.

Location of neurones with cardiovascular and respiratory function, at the ventral surface of the cat's medulla

A study has been made of the ventral surface of the medulla, to identify neurones with cardiovascular and respiratory functions. Experiments were performed on chloralose-anaesthetized, artificially ventilated cats. Ventral medullary neurones were stimulated by microinjections of excitant amino acid (which selectively activates cell bodies), and responses measured in blood pressure, heart rate, renal sympathetic and phrenic nerve activity. A small region of ventral medulla was found, corresponding to the "glycine-sensitive area", from which large increases in blood pressure and renal nerve activity were evoked by amino acid injections. More caudally, another cell group was localized lateral to the hypoglossal nerve roots, and these neurones depressed blood pressure and renal nerve activity. Two distinct regions were found to increase phrenic nerve activity: rostral to the pressor neurones, encroaching on the trapezoid body (roughly corresponding to area "M"), and a caudal group, close to the depressor neurones (i.e. lateral to the hypoglossal roots). No respiratory response could be evoked from medial to the hypoglossal roots (area "L") and stimulation of neurones in area "S" generally depressed phrenic activity. Neurones with cardiovascular and respiratory actions could be distinguished anatomically. Their locations have been mapped and compared with previous studies.

Central respiratory stimulation produced by thyrotropin-releasing hormone in the cat

Thyrotropin-releasing hormone (TRH) was administered intracerebroventricularly and it's effects on respiration were evaluated in the alpha-chloralose anesthetized cat. Respiratory activity was measured using a Fleisch pneumotachograph to monitor tracheal airflow. TRH (0.28-28 nmol) caused an elevation in respiratory minute volume which was due to an increase in respiratory rate with no effect on tidal volume. The site of TRH-induced tachypnea was in the hindbrain as both injections into the cisterna magna and the fourth ventricle produced similar effects. No changes in respiratory activity were seen when TRH injection was restricted to the lateral and third ventricles (forebrain). Furthermore, systemic administration of TRH (28 nmol) produced no significant respiratory effects. The active analogue, [3-Me-His2]-TRH (2.7 nmol) produced the same respiratory effects as TRH. The inactive analogue, TRH free acid (28-280 nmol), caused no significant change in respiratory activity. The data suggest that TRH interacts with a specific receptor in the hindbrain of the cat to affect respiration.

Electrophysiological study of sympathoexcitatory structures of the bulbar ventrolateral surface as related to vasomotor regulation

The responses in T3-4, T10-11 and L2-3 white rami to stimulation of different zones of the bulbar ventrolateral surface were maximal when the region of about 4 mm laterally to the midline was stimulated. A weak surface stimulation of all these zones elicited only a long latency response consisting of three waves. A short latency response appeared when supramaximal stimuli were applied only to the intermediate zone--the region up to 6 mm rostrally to the hypoglossal nerve root level (zone S and caudal part of zone M). The data presented show that long and short latency responses are conducted from the intermediate zone to the spinal cord via dorsolateral funiculus fibres with a conduction velocity of about 5.6 +/- 0.6 m/s. In addition, a special descending sympathoexcitatory pathway oriented to T2 preganglionic neurons with a conduction velocity of about 12.3 +/- 3.2 m/s was demonstrated. Antidromic discharges of the output sympathoexcitatory neurons elicited by dorsolateral funiculus stimulation were found in the intermediate zone only at a depth of about 400-2000 microM. Stimuli applied to different regions of the ipsilateral bulbar ventrolateral surface activate at least two groups of surface fibres (conduction velocities 6.7-8.0 and 2-3.2 m/s) which, in turn, activate the output neurons with a rather constant delay of about 20 ms equal to a difference between the latencies of long and short latency white rami responses. The mechanism of delay formation seems to be concentrated in the intermediate zone and formed probably by a chain of interneurons. A possible scheme of neuronal organization of the bulbar ventrolateral sympathoexcitatory structures is presented and discussed. The descending tonic activation of spinal vasomotor neurons is formed by spontaneous discharges of antidromically identified output neurons with a mean firing rate of about 14.4 imp./s. Some neurons are reflexly activated within the time limits of the late somatosympathetic reflex response. Coagulation of the intermediate zone resulted in a profound fall of blood pressure, disappearance of pressor and late somatosympathetic reflexes, whereas the spinobulbospinal somatosomatic reflex remains unchanged. The baroreceptor inhibition is partly realized through the elements of sympathoexcitatory intermediate zone because the preferable inhibition of the long latency white rami response was demonstrated in the middle of R-R interval and during a sharp increase in the arterial pressure induced by vasoconstrictor drugs. Thus, the structures of the intermediate zone seem to play a key role in supporting of blood pressure level and organization of pressure reflexes.(ABSTRACT TRUNCATED AT 400 WORDS)

Projections from the nucleus tractus solitarii to the rostral ventrolateral medulla

Projections from the nucleus tractus solitarii (NTS) to autonomic control regions of the ventrolateral medulla, particularly the nucleus reticularis rostroventrolateralis (RVL), which serves as a tonic vasomotor center, were analyzed in rat by anterograde, retrograde, and combined axonal transport techniques. Autonomic portions of the NTS, including its commissural, dorsal, intermediate, interstitial, ventral, and ventrolateral subnuclei directly project to RVL as well as to other regions of the ventrolateral medulla. The projections are organized topographically. Rostrally, a small cluster of neurons in the intermediate third of NTS, the subnucleus centralis, and neurons in proximity to the solitary tract selectively innervate neurons in the retrofacial nucleus and nucleus ambiguus. Neurons generally located in more caudal and lateral sites in the NTS innervate the caudal ventrolateral medulla (CVL). The RVL, CVL, and nucleus retroambiguus are interconnected. A combined retrograde and anterograde transport technique was developed so as to prove that projections from the NTS to the ventrolateral medulla specifically innervate the region of RVL containing neurons projecting to the thoracic spinal cord or the region of the nucleus containing vagal preganglionic neurons. When the retrograde tracer, fast blue, was injected into the thoracic spinal cord, and wheat germ agglutinin-conjugate horseradish peroxidase (HRP) was injected into the NTS, anterogradely labeled terminals from the NTS surrounded the retrogradely labeled neurons in the RVL and in the nucleus retroambiguus in the caudal medulla. Among the bulbospinal neurons in the RVL innervated by the NTS were adrenaline-synthesizing neurons of the C1 group. When fast blue was applied to the cervical vagus, and HRP was injected into the NTS, anterogradely labeled terminals from the NTS surrounded retrogradely labeled neurons in the rostral dorsal motor nucleus of the vagus, the region of the nucleus ambiguus, the retrofacial nucleus, and the dorsal portion of the RVL, a region previously shown to contain cardiac vagal preganglionic neurons. This combined anterograde and retrograde transport technique provides a useful method for tracing disynaptic connections in the brain. These data suggest that the RVL is part of a complex of visceral output regions in the ventrolateral medulla, all of which receive afferent projections from autonomic portions of the NTS. Bulbospinal neurons in the RVL, in particular the C1 adrenaline neurons, may provide a portion of the anatomic substrate of the baroreceptor and other visceral reflexes.

Vasopressor neurons in the rostral ventrolateral medulla of the rabbit

Neurons within the rostral ventrolateral medulla oblongata project directly to the intermediolateral column in the thoracolumbar spinal cord. This paper reviews evidence obtained from experiments in the rabbit regarding the anatomical connections and physiological, pharmacological and histochemical properties of these cells. The following hypotheses are discussed: an increase in the firing rate of these neurons leads to a rise in arterial pressure due to sympathetic vasoconstriction, but does not affect respiratory or other somatomotor activity; the bulbospinal pathway originating from these neurons is an essential component of the central pathways mediating baroreceptor and other cardiovascular reflexes; these neurons receive tonic GABAergic inhibitory inputs, which are not all of baroreceptor origin; many of these bulbospinal neurons synthesize adrenalize. The possible role of adrenaline in the function of these neurons is considered.

Central chemosensitivity to H+ and CO2 in the rat respiratory center in vitro

The brainstem, cervical cord and attached phrenic nerve were excised from newborn rats and superfused in vitro. Respiratory output was measured by integration of phrenic nerve discharges. Respiratory output was enhanced by an increase in pCO2 at constant pH as well as by decreased pH with constant pCO2. It is concluded that the adequate stimulus to central chemoreceptors is not restricted only to H+.

Cardiorespiratory effects produced by application of L-glutamic and kainic acid to the ventral surface of the cat hindbrain

The purpose of our study was to determine the cardiorespiratory effects of exciting cell bodies at the rostral (area M), intermediate (area S), and caudal (area L) chemosensitive sites on the ventral surface of the medulla. To do this, L-glutamic and kainic acid were applied bilaterally to each chemosensitive site while monitoring tidal volume (VT), respiratory rate (f), mean arterial pressure (BP), and heart rate (HR) in chloralose-anesthetized cats. Application of solutions (5 microliter) of L-glutamic acid ranging from 62.5 to 2000 mM to the intermediate area produced concentration-dependent increases in VT (from 2.0 +/- 0.6 ml to 14 +/- 1.1 ml) and BP (from 2.0 +/- 1.7 mm Hg to 39 +/- 5.3 mm Hg). No significant changes in f and HR were noted. Similar effects were observed with application of kainic acid. Application of L-glutamic acid to the caudal area produced hypotension (-24 +/- 5.4 mm Hg) with no accompanying changes in VT and f. No responses were observed after application of L-glutamic acid to the rostral area. These data suggest that activation of cell bodies on the intermediate area produces simultaneous stimulatory effects on BP and VT, whereas activation of cell bodies at the caudal area produces selective depressant effects on BP.

Monoamine distribution on the ventral surface of the rat medulla oblongata

The distribution of monoamine transmitters in the area near the ventral surface of the rat medulla oblongata was studied using the Falck-Hillarp histofluorescence method. Histological examination and scanning electron microscopy of these regions were also performed. It was found that there is a wide area dense with catecholamine terminals in the external layer of the ventral medulla oblongata. 5-Hydroxytryptamine-containing terminals and nerve cell bodies on and near the surface were also found. Due to their superficial localization these monoamines may influence the content of cerebrospinal fluid and in this way have effects on cardiovascular and other physiological functions.

Evidence for a capsaicin-sensitive vasomotor mechanism in the ventral medullary chemosensitive area of the cat

The effects of capsaicin applied to the exposed ventral surface of the medulla were studied on the mean arterial blood pressure, heart rate, respiration and sympathetic efferent nerve activity in chloralose-urethane-anaesthetized cats. The application of capsaicin produced a marked increase in the mean arterial blood pressure and sympathetic nerve activity, but not in the heart rate. The "intermediate area" proved to be the most sensitive to capsaicin. Pressor responses could be elicited repeatedly; tachyphylaxis was not noted provided a time interval of 30 min elapsed between consecutive applications. Repeated applications of capsaicin at intervals of less than 30 min led to tachyphylaxis. However, pressor responses evoked by either topical application of glutamate or pentamethylene-tetrazole or bilateral carotid occlusion could invariably be demonstrated during this period of tachyphylaxis. Histological studies revealed the existence of a hitherto unrecognized termination of capsaicin-sensitive nerve endings within the ventral medullary chemosensitive area of the cat. The results provide both functional and morphological evidence for the presence of a capsaicin-sensitive vasomotor mechanism in the ventral medullary chemo-sensitive area of the cat. It is suggested that the pressor effects of capsaicin applied to the ventral medullary chemo-sensitive area may be mediated by an activation of capsaicin-sensitive primary sensory afferents terminating in this area. Accordingly, capsaicin-sensitive neuronal mechanisms located in the ventral medullary chemosensitive area may play an important role in the central nervous regulation of blood pressure.

A study of intrathecal, cerebrospinal fluid-to-brain exchange

A cerebrospinal fluid (CSF) to brain exchange has been postulated for lipid-soluble and small molecular substances and has led to nearly 100 attempts per year to produce central effects via intrathecal injections. With few exceptions, however, modern neurological practice has avoided this approach because of its demonstrated ineffectiveness and dangers. The practicability of an intrathecal CSF to brain exchange was tested by cisternal infusions of mock CSF at different infusion pressures that might counteract central nervous system intoxications of systemic origin. Those efforts failed in different test situations with each of three barbiturates. Steady state doses at a selected level of barbiturate anesthesia were the same, whether induced by cisternal infusion or intravenously, and this was true for barbiturates of widely different lipid solubility. The cerebral response to pentylenetetrazol was delayed well beyond its rate of response when introduced intravenously. These results suggested that the bulk clearance rate and venous resorption of CSF were sufficient to prevent significant diffusion of the barbiturate or even mock CSF into the brain following intrathecal injection. Because central effects that follow venous resorption may be confused with direct central effects, many previous clinical reports are questioned. Apparent exceptions to the ineffectiveness of intrathecal therapy, such as spinal anesthesia, were discussed in terms of their special local effects. The relative effectiveness of intrathecal agents should be evaluated by comparing maintenance doses for a given central effect, when produced by both intrathecal and i.v. routes. Previous reports on rates of intrathecal infusion, intracranial pressure relationships, and the relative safety of such infusions were confirmed and extended.

Gallamine and vagotomy enhance respiratory modulation of reticular units

Respiration related units ( RRU ) were recorded in the brainstem of cats with spinal transection at the C7-Th 1 level and breathing N2O. The proportion of RRU in several structures was compared in control and in 3 experimental groups: (1) paralyzed with gallamine triethiodide; (2) vagotomized; and (3) both vagotomized and paralyzed. After gallamine, RRU percentage was multiplied by 3 in the bulbo-pontine reticular formation (RF) and as much as 20 in the mesencephalic RF. Vagotomy multiplied RRU proportions by 2 in the bulbo-pontine RF, by 12 in the mesencephalic RF and by 3 in the pneumotaxic complex (nucleus parabrachialis medialis ( NPBM ) and K olliker -Fuse (KF) nucleus). The effects of gallamine and vagotomy were not additive. Gallamine still increased proportion of RRU in the RF after vagotomy showing that the gallamine effect is not vagally mediated; this was in contrast to the previously reported suppression by vagotomy of phrenic discharge facilitation induced by gallamine. The total number of firing units in the RF was not modified by gallamine or vagotomy. It is concluded that the respiratory modulation of reticular neurons is selectively enhanced by gallamine and vagotomy through two independent mechanisms.

Neonatal breathing control mediated via the central chemoreceptors

Respiratory changes elicited via the central chemoreceptor system have been studied in anesthetized newborn guinea pigs and newborn rabbits. Periodic breathing was induced by inhibition of the central chemoreceptors by superfusion with alkaline cerebrospinal fluid. The periodic breathing was promptly reversed to steady by increasing the oxygen or carbon dioxide concentration in the inspired air or by intravenous theophylline. Elicitation of periodic breathing simply by exposing the animals to hypoxia succeeded only when very low oxygen concentrations were given. Clearcut respiratory excitation was produced by small amounts of theophylline applied onto the ventral surface of the medulla. Not only theophylline intravenously but also theophylline topically applied on the ventral medullary surface normalized spontaneously developed periodic breathing. Application of meperidine onto the ventral medullary surface gave respiratory inhibition with dosages considerably lower than required when given intravenously. The results emphasize the importance of an adequate respiratory drive from the central chemoreceptors for the maintenance of a regular breathing pattern. The findings support a view that at least part of the respiratory effects seen in the newborn following administration of meperidine or theophylline is due to effect of the drugs on the central chemosensitive system.

Does low pH stimulate central chemoreceptors located near the ventral medullary surface?

An in vitro preparation of the medulla oblongata of the rat was used to examine the responses to reducing pH, at constant CO2, of neurons in several identified nuclei. Neurons in an area close to the ventral surface, which is thought to be the location of pH-sensitive central respiratory chemoreceptors, did not respond differently from neurons in other medullary nuclei.

Chemoreceptor sensitivity and maladaptation to high altitude in man

Studies were carried out to find out the role of chemoreceptor sensitivity in the causation of maladaptation syndromes on acute exposure to altitude. The experiments were done in two phases. In phase I, the responses in chemoreceptor sensitivity were studied in altitude acclimatized subjects and compared with those who suffered from either High Altitude Pulmonary Oedema (HAPO) or Acute Mountain Sickness (AMS). In Phase II, a similar comparison was done in two groups of subjects, one representing normal sojourners at 3,500 m and the other being subjects who had just recovered from HAPO. The first phase was done at Delhi; and the second at an altitude of 3,500 m. Parameters of assessment were hypoxic sensitivity, carbon dioxide sensitivity, ventilation (VE), respiratory frequency (Rf), forced vital capacity (FVC), forced expiratory volume at the first second (FEV1), heart rate (HR), blood pressure (BP), and oral temperature (Tor). The results showed significantly lower sensitivity to both hypoxia and carbon dioxide in maladapted subjects, as compared to those who were acclimatized in both the categories suggesting thereby that reduced chemoreceptor sensitivity might be an initiating factor in the causation of maladaptation syndromes at altitude.

Morphological observations on superficial medullary CO2--chemosensitive areas

Physiological investigations have indicated that the ventrolateral surface of the medulla oblongata is involved in the chemical drive to respiration. In this investigation, light and electron microscopic investigations of the 3 chemosensitive regions reveal the following. (1) Evaginations of the ventral surface abut the overlying pia mater thereby delimiting discrete compartments; invaginations of the surface delimit wide cisternae lined with basement membrane. Neuronal elements with numerous synapses, were found scattered among astrocytic processes of the marginal glia in intermediate and caudal chemosensitive areas Microvasculature are conspicuously absent from the marginal glia. Intramedullary vessels are surrounded by perivascular spaces and the endothelium shows zonulae occludentes at cell junctions. (2) Horseradish peroxidase (HRP) applied to the ventral surface diffused throughout the interstitial and perivascular compartments, into synaptic clefts and neuronal soma. Diffusion of HRP into blood vessels was blocked at zonulae occludentes. Following intravenous injection of HRP, no reaction product was found outside cerebral vasculature in chemosensitive areas. (3) In spontaneously breathing cats, 2% procaine applied to the caudal chemosensitive area resulted in respiratory depression which began with the second breath. It is proposed, that substances which stimulate or depress respiration, when applied to the ventral medullary surface, produce their effects on superficial neurons located in the intermediate and caudal chemosensitive areas after diffusion through interstitial spaces.

Neurophysiological studies on superficial medullary chemosensitive area for respiration

In cats anesthetized with chloralose-urethane (40 mg/kg chloralose; 200 mg/kg urethane) pH sensitivity of neurons in the caudal chemosensitive area on the ventrolateral surface of the medulla oblongata was examined while monitoring phrenic nerve activity simultaneously. pH was varied by superfusion of the ventral medullary surface with mock cerebrospinal fluid (CSF) of different pH (pH 7.4 control, 7.0 and 7.8). A total of 130 units from 21 cats changed their firing rate in response to CSF-pH changes. These were subdivided into 3 groups. In Group I, 31 respiratory pH sensitive units increased their firing rate in response to decreased mock CSF-pH, noxious pinch, joint movement in contralateral forelimb, and increased inspired CO2. These responses may have originated from respiratory center neurons. Group II consisted of 59 non-respiratory pH sensitive units whose firing rate changed in an inverse manner with CSF-pH changes. Of these, 30 responded to contralateral distal forelimb movement, 15 to hair manipulation, 9 to heavy pressure and 5 to noxious pinch. Increased inspired CO2 (rebreathing) did not modify activity. The response to pH is believed to be from non-specific neurons. Group III consisted of 40 non-respiratory pH sensitive units responding to CSF-pH changes and to increased inspired CO2. The firing rate was irregular, the interval distribution approaching an exponential function. It may be postulated that the impulse frequency of chemosensitive impulses may be irregular at the site of impulse generation, the irregularity decreasing by convergence during transmission to the respiratory centers. The time course of Group III chemosensitive units was similar to phrenic nerve responses.

Morphology of the medullary chemosensitive fields. 1. Mapping of the neuronal matrix by a horseradish peroxidase technique

A modified horseradish peroxidase labelling technique was used to study the distribution pattern of neurons in the central chemosensitive fields of the medulla oblongata of cats. In several cryosectioned medullae a mapping of superficially located HRP-labelled neurons was achieved. The distribution and configuration of the labelled neurons indicate that most of them belong to the nucleus paragigantocellularis lateralis. However, by varying the time of incubation it was possible to identify different types of neurons. On the basis of certain aspects of the HRP incorporation mode and neuronal topography a specific type of small-sized neurons has been identified. Some functional implications of these small neurons with respect to their possible chemosensitive activity are discussed.

Ventilatory responses to hypocapnic vertebral artery perfusion in intact and carotid body denervated dogs

The ventilatory responses to step changes in vertebral artery PCO2 were investigated in intact and carotid body denervated dogs. The steady-state ventilatory responses of the denervated dogs were less than those of intact dogs. However, when expressed as a ratio to the control ventilation there was no difference between the two groups. While the arterial PCO2 was held at 56 mm Hg by adding CO2 to the inspired air the perfusion of the vertebral arteries was switched from the dog's own arterial supply to hypocapnic blood. The ventilation of the denervated dogs decreased at a faster rate (half time = 130 +/- 9 sec) to a level less than the room air control ventilation. The ventilation in the intact dogs decreased at a slower rate (half time = 184 +/- 23 sec) and was maintained above the room air control level after ten minutes of hypocapnic perfusion. Increasing the medullary blood flow, as measured with radiolabeled microspheres, augmented the rate of decline of ventilation in intact dogs. We conclude, (1) the influence of the peripheral chemoreceptors appears to increase as central drive is decreasing, and (2) the remaining time course of the decrease in ventilation is related to the rate of brain stem perfusion.

Serotonergic projections from the ventral medulla to the intermediolateral cell column in the rat

Serotonergic neurons of the ventral medulla (B1 and B3 cell groups are retrogradely labeled after HRP injection in the T1-T2 levels of the spinal cord. Following stereotaxic injections of 3H-labeled amino acids in this region, a direct projection to th intermediolateral cell column and ventral horn can be demonstrated.

The nucleus paragigantocellularis lateralis in the rat. Demonstration of afferents by the retrograde transport of horseradish peroxidase

Injections of horseradish peroxidase (HRP) were placed in the middle or caudal portion of the nucleus paragigantocellularis lateralis (PGCL) and 24 h later the entire spinal cord and brain were processed and examined for labeled neurons. Spinal afferents arise from all levels of the cord. Rexed's lamination scheme was adapted to the spinal cord of the rat and labelled neurons were localized to laminae IV, V, VII, VIII and X mainly on the side contralateral to the injection. At cervical levels, labeled neurons were consistently found bilaterally. The medial reticular nuclei of the medulla and pons contained HRP-labelled perikarya, which were concentrated most heavily in the nuclei reticularis medullae oblongatae ventralis, gigantocellularis, and pontis caudalis predominantly ipsilateral to the injection. The medial vestibular nucleus was consistently labeled. HRP-labeled perikarya were found bilaterally within the commissural portion and in the medial part of the nucleus of the solitary tract on the side of the injection. The rostral portion of the PGCL receives afferents from some secondary auditory nuclei: the ipsilateral inferior colliculus and the posterior ventral cochlear nucleus bilaterally. Thus, the rostral PGCL may be involved in auditory feedback loops. The caudal raphe nuclei are a major source of afferents to the caudal PGCL. The lateral hypothalamic area, paraventricular nucleus, and zona incerta also contain labeled neurons when injections are centered in the caudal portion of the nucleus.

Cardiovascular control by medullary surface chemoreceptors

The cardiovascular and respiratory effects of superfusion of the ventral surface of the medulla with acid hypercapnic or alkaline hypocapnic solutions have been studied in anaesthetized, paralyzed, artificially ventilated cats. Peripheral chemoreceptor and baroreceptor denervation was achieved by section of carotid sinus, aortic and cervical vagus nerves. Systemic arterial and central venous pressure, hindquarters blood flow, heart rate and phrenic nerve activity were recorded. Acid hypercapnic (pH 6.8, pCO2 85 mm Hg) superfusion caused increases in systemic arterial pressure, phrenic nerve activity and heart rate, and a decrease in hindquarters blood flow. Alkaline hypocapnic (pH u.i, pCO2 less than 10 mmHg) superfusion caused opposite effects. These experiments indicate a significant role of the chemoreceptors of the ventral surface of the medulla in cardiovascular control.

Projections to the spinal cord from neurons close to the ventral surface of the hindbrain in the rat

Neurons located within 100 micron of the ventral surface of the hindbrain were labeled following injections of horseradish peroxidase into the thoracic and lumbar spinal cord in the rat. Labeled neurons were distributed into two groups, one ventrolateral to the inferior olive and ventral to the facial nucleus, and one ventral to the raphe pallidus. The location of the ventrolateral subpial group is very similar to that of the brain stem chemosensitive zones, and suggests that the neurons in this group may be important in cardiovascular regulation.

Respiratory effects of beta-endorphin, D-Ala2-met-enkephalinamide, and Met-enkephalin injected into the lateral ventricle and the pontomedullary subarachnoid space

The respiratory effects of Met-enkephalin (900 microgram), D-Ala2-Met-enkephalinamide (10 microgram), and beta-endorphin (10 microgram) were studied and compared in lightly anesthetized cats, after injection into the lateral ventricle and into the pontomedullary subarachnoid space. The 3 peptides injected into the lateral ventricle induced equidepressant effects on respiration, but the duration of action and the involvement of either frequency or tidal volume varied considerably. Met-enkephalin was shorter-acting (45 min) than both D-Ala2-Met-enkephalinamide and beta-endorphin (over 5 h). The depression induced by beta-endorphin was preceded by a long-lasting stimulation of frequency. The effects were antagonized by i.v. naloxone, but the antagonism was not complete in one third of the animals. In the pontomedullary subarachnoid space, beta-endorphin failed to depress respiration significantly whereas Met-enkephalin induced an immediate and short-acting depression (15 min), and D-Ala2-Met-enkephalinamide depressed respiration for 2-4 h in a biphasic pattern. It is concluded that: (1) respiration is depressed by the 3 opiate peptides; (2) the effects of beta-endorphin on respiration, at the dose used in this study, are secondary to other actions on higher brain structures; and (3) Met-enkephalin and D-Ala2-Met-enkephalinamide seem to affect pontomedullary areas located near the ventral surface, although they may also interact with respiratory structures located more deeply in the brain stem.

Cardiovascular effects elicited from the ventral surface of medulla oblongata in the cat

Cardiovascular adjustments induced by topical application of drugs on a restricted area on the ventral surface of the medulla oblongata, corresponding to the caudal part of the rostral chemoreceptor area and the intermediate area, have been studied in chloralose-anesthetized cats. Topical application of GABA or glycine on these structures resulted in blood pressure fall, bradycardia, vasodilatation in the kidney and the skeletal muscles and also depression of respiration. Similar responses except for a slight tachycardia occurred with application of physostigmine. Application of GABA resulted in a marked attenuation of the reflex vasoconstrictor responses to removal of arterial baroreceptor restraint (carotid occlusion), particularly in the kidney, and to disappearance of the reflex renal vasodilatation to baroreceptor stimulation. The findings suggest that GABA application leads to a general diminution of the tonic vasomotor neuron activity, and with regard to renal vasomotor neurons a virtual cessation. Atropine methylnitrate application induced blood pressure rise, increased peripheral resistance in both skeletal muscle and kidney and a strongly potentiated renal vasoconstrictor response to carotid occlusion. The results indicate that the studied superficial medullary structures play an important role for the maintenance of tonic vasomotor neuron activity, especially renal.

Dickinson W. Richards Lecture: Circulatory adjustments to hypoxia

Circulatory adjustments during hypoxia act to redistribute blood flow and maintain arterial pressure. Redistribution of blood flow is accomplished by a local effect of hypoxia, which produces dilatation in coronary and cerebral vessels, and the chemoreceptor reflex, which produces vasoconstriction in skeletal muscle and the splanchnic bed and dilatation in coronary vessels. Arterial pressure is maintained primarily by the chemoreceptor reflex. If the chemoreceptor reflex fails to maintain arterial pressure, hypoxia and hypotension together activate the central pressor response. Compensatory mechanisms usually are sufficient to maintain homeostasis during hypoxia. However, when a hypotensive stress is superimposed during hypoxia, compensatory mechanisms may fail to maintain arterial pressure. Thus, systemic hypoxia interferes with autonomic cardiovascular adjustments.

Stimulation of phrenic nerve activity by an acetylcholine releasing drug: 4-aminopyridine

The effect of the acetylcholine releaser 4-aminopyridine on ventilation was studied by recording and quantifying the efferent phrenic nerve activity in 40 paralysed and vagotomized cats; with arterial Po2, PCO2 and pH kept constant. 4-Aminopyridine, given intravenously or in the vertebral artery, stimulates the phrenic nerve activity in a dose dependent manner. The stimulatory effects of 4-aminopyridine on the phrenic nerve activity could be abolished completely by administration of high doses of atropine. We conclude that 4-aminopyridine, which is used clinically for the reversal of a neuromuscular block, stimulates the phrenic nerve activity. Since the role of cholinergic mechanisms in the central chemoreception has been well established, the effect on the phrenic nerve activity is most probably by an increased release of acetylcholine at the site of the central chemoreceptors.

Topography of the respiratory and circulatory responses to acetylcholine and nicotine on the ventral surface of the medulla oblongata

1. Acetylcholine and nicotine were superfused on the ventral medullary surface between the ponto-medullary border and C1 in anaesthetized cats in order to determine the topical distribution of their actions on respiration and circulation. 2. Acetylcholine (10(-4) g . ml-1 = 5.5 . 10(-4) mMol . ml-1) produced an increase in respiration and a lowering of blood pressure. The magnitude and the time course of the responses varied according to the points of superfusion on the surface. 3. Nicotine (10(-4) g . ml-1 = 6.2 . 10(-4) mMol . ml-1) elicited hyperventilation and more often an increase in arterial pressure on unilateral superfusion of the surface. In some cases, however, a drop in blood pressure was also observed. 4. The responsive regions of the surface on which nicotine acted and elicited hyperventilation, bear a close resemblance to the regions responsive to acetylcholine. 5. The topographical distribution of the respiratory effects elicited by the above-mentioned drugs were similar to the distribution of the responses to changes in pH on the ventral medullary surface or to electrical stimulation. 6. Procaine (2 . 10(-2) g . ml-1 = 7.3 . 10(-2) mMol . ml-1) applied bilaterally in the intermediate zone (S) caused profound inhibition of respiration and of arterial pressure. Procaine at this concentration also inhibited respiratory hyperventilation caused by nicotine (10(-4) g . ml-1 = 6.2 . 10(-4) mMol . ml-1) applied to the caudal and rostral areas.

Influence of the CSF bicarbonate concentration on the ventilatory response to CO2 in relation to the location of the central chemoreceptors

In anaesthetized cats, in which the cerebrospinal fluid bicarbonate concentration was varied by a ventriculocisternal perfusion technique, the ventilatory response to CO2 during hyperoxia could be satisfactorily described by VE = S(PCSFCO2 -B). Both the slope S and the intercept B were positively and linearly related to the CSF bicarbonate concentration. Assuming that the PCSFCO2 is equal to the PCO2 in extracellular fluid, it can be shown that VE is a linear, but not a unique function of the [H+] at the site of the chemoreceptors; the slope of this relation varies with the bicarbonate concentration at that site, possibly due to chemical complex formation between HCO-3 and Ca2+ or Mg2+. Changes in the B-value were related to the location of the central chemoreceptors with the models of Pappenheimer and Berndt aand their coworkers. It was found that changes in the CSF bicarbonate concentration are reflected for 60 per cent at the site of the central chemoreceptors, and that this was independent of the cerebral perfusion. Using Berndt's model a distance between CSF and central chemoreceptors of approximately 100 micron was found; this calculated distance is relatively insensitive to relationship (logarithmic or not) between ventilation and H+ concentration and to changes in cerebral perfusion, owing to the approximate nature of the diffusion model.

Neurones within the "chemosensitive area" on the ventral surface of the brainstem which project to the intermediolateral column

With the method of retrograde transport of horseradish peroxidase it has been demonstrated that neurones within the "chemosensitive area" of the brainstem project to the thoracic intermediolateral column. The function of these neurones is discussed in regard to the regulation of blood pressure.