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

CO2-Induced ATP-Dependent Release of Acetylcholine on the Ventral Surface of the Medulla Oblongata

Complex mechanisms that detect changes in brainstem parenchymal PCO2/[H+] and trigger adaptive changes in lung ventilation are responsible for central respiratory CO2 chemosensitivity. Previous studies of chemosensory signalling pathways suggest that at the level of the ventral surface of the medulla oblongata (VMS), CO2-induced changes in ventilation are (at least in part) mediated by the release and actions of ATP and/or acetylcholine (ACh). Here we performed simultaneous real-time biosensor recordings of CO2-induced ATP and ACh release from the VMS in vivo and in vitro, to test the hypothesis that central respiratory CO2 chemosensory transduction involves simultaneous recruitment of purinergic and cholinergic signalling pathways. In anaesthetised and artificially ventilated rats, an increase in inspired CO2 triggered ACh release on the VMS with a peak amplitude of ~5 μM. Release of ACh was only detected after the onset of CO2-induced activation of the respiratory activity and was markedly reduced (by ~70%) by ATP receptor blockade. In horizontal slices of the VMS, CO2-induced release of ATP was reliably detected, whereas CO2 or bath application of ATP (100 μM) failed to trigger release of ACh. These results suggest that during hypercapnia locally produced ATP induces or potentiates the release of ACh (likely from the medullary projections of distal groups of cholinergic neurones), which may also contribute to the development and/or maintenance of the ventilatory response to CO2.

Role of Astrocytes in Central Respiratory Chemoreception

Astrocytes perform various homeostatic functions in the nervous system beyond that of a supportive or metabolic role for neurons. A growing body of evidence indicates that astrocytes are crucial for central respiratory chemoreception. This review presents a classical overview of respiratory central chemoreception and the new evidence for astrocytes as brainstem sensors in the respiratory response to hypercapnia. We review properties of astrocytes for chemosensory function and for modulation of the respiratory network. We propose that astrocytes not only mediate between CO₂/H⁺ levels and motor responses, but they also allow for two emergent functions: (1) Amplifying the responses of intrinsic chemosensitive neurons through feedforward signaling via gliotransmitters and; (2) Recruiting non-intrinsically chemosensitive cells thanks to volume spreading of signals (calcium waves and gliotransmitters) to regions distant from the CO₂/H⁺ sensitive domains. Thus, astrocytes may both increase the intensity of the neuron responses at the chemosensitive sites and recruit of a greater number of respiratory neurons to participate in the response to hypercapnia.

Cholinergic control of ventral surface chemoreceptors involves Gq/inositol 1,4,5-trisphosphate-mediated inhibition of KCNQ channels

KEY POINTS

ACh is an important modulator of breathing, including at the level of the retrotrapezoid nucleus (RTN), where evidence suggests that ACh is essential for the maintenance of breathing. Despite this potentially important physiological role, little is known about the mechanisms responsible for the cholinergic control of RTN function. In the present study, we show at the cellular level that ACh increases RTN chemoreceptor activity by a CO2/H(+) independent mechanism involving M1/M3 receptor-mediated inositol 1,4,5-trisphosphate/Ca(+2) signalling and downstream inhibition of KCNQ channels. These results dispel the theory that ACh is required for RTN chemoreception by showing that ACh, similar to serotonin and other modulators, controls the activity of RTN chemoreceptors without interfering with the mechanisms by which these cells sense H(+). By identifying the mechanisms by which wake-on neurotransmitters such as ACh modulate RTN chemoreception, the results of the present study provide a framework for understanding the molecular basis of the sleep-wake state-dependent control of breathing.

ABSTRACT

ACh has long been considered important for the CO2/H(+)-dependent drive to breathe produced by chemosensitive neurons in the retrotrapezoid nucleus (RTN). However, despite this potentially important physiological role, almost nothing is known about the mechanisms responsible for the cholinergic control of RTN function. In the present study, we used slice-patch electrophysiology and pharmacological tools to characterize the effects of ACh on baseline activity and CO2/H(+)-sensitivity of RTN chemoreceptors, as well as to dissect the signalling pathway by which ACh activates these neurons. We found that ACh activates RTN chemoreceptors in a dose-dependent manner (EC50 = 1.2 μm). The firing response of RTN chemoreceptors to ACh was mimicked by a muscarinic receptor agonist (oxotremorine; 1 μm), and blunted by M1- (pirezenpine; 2 μm) and M3- (diphenyl-acetoxy-N-methyl-piperidine; 100 nm) receptor blockers, but not by a nicotinic-receptor blocker (mecamylamine; 10 μm). Furthermore, pirenzepine, diphenyl-acetoxy-N-methyl-piperidine and mecamylamine had no measurable effect on the CO2/H(+)-sensitivity of RTN chemoreceptors. The effects of ACh on RTN chemoreceptor activity were also blunted by inhibition of inositol 1,4,5-trisphosphate receptors with 2-aminoethoxydiphenyl borate (100 μm), depletion of intracellular Ca(2+) stores with thapsigargin (10 μm), inhibition of casein kinase 2 (4,5,6,7-tetrabromobenzotriazole; 10 μm) and blockade of KCNQ channels (XE991; 10 μm). These results show that ACh activates RTN chemoreceptors by a CO2/H(+) independent mechanism involving M1/M3 receptor-mediated inositol 1,4,5-trisphosphate/Ca(+2) signalling and downstream inhibition of KCNQ channels. Identifying the components of the signalling pathway coupling muscarinic receptor activation to changes in chemoreceptor activity may provide new potential therapeutic targets for the treatment of respiratory control disorders.

Alterations in cholinergic sensitivity of respiratory neurons induced by pre-natal nicotine: a mechanism for respiratory dysfunction in neonatal mice

Nicotine may link cigarette smoking during pregnancy with sudden infant death syndrome (SIDS). Pre-natal nicotine leads to diminished ventilatory responses to hypercarbia and reduced central chemoreception in mice at post-natal days 0-3. We studied how pre-natal nicotine exposure changes the cholinergic contribution to central respiratory chemoreception in neonatal isolated brainstem-spinal cord and slice preparations. Osmotic minipumps, implanted subcutaneously into 5-7 days pregnant mice, delivered saline or nicotine ditartrate 60 mg kg(-1) d(-1) for up to 28 days. In control preparations, acidification of the superfusion medium from pH 7.4 to 7.3 increased the frequency and reduced the amplitude of fictive respiration. In nicotine-exposed neonatal mice, the reduction in amplitude induced by acidification was reduced. In control preparations, atropine suppressed respiratory responses to acidification, while hexamethonium did not. By contrast, in nicotine-exposed preparations, hexamethonium blocked chemosensory responses but atropine did not. Our results indicate that pre-natal nicotine exposure switches cholinergic mechanisms of central chemosensory responses from muscarinic receptors to nicotinic receptors. Modification of the cholinergic contribution to central chemoreception may produce respiratory dysfunctions, as suggested by receptor-binding studies in victims of SIDS.

Influence of differential expression of acetylcholinesterase in brain and muscle on respiration

A mouse strain with a deleted acetylcholinesterase (AChE) gene (AChE knockout) shows a decreased inspiration time and increased tidal volume and ventilation .To investigate the respective roles of AChE in brain and muscle, we recorded respiration by means of whole-body plethysmography in knockout mice with tissue selective deletions in AChE expression. A mouse strain with the anchoring domains of AChE deleted (del E5+6 knockout mice) has very low activity in the brain and neuromuscular junction, but increased monomeric AChE in serum. A mouse strain with deletion of the muscle specific region of AChE (del i1RR knockout mice) exhibits no expression in muscle, but unaltered expression in the central nervous system. Neither strain exhibits the pronounced phenotypic traits observed in the complete AChE knockout strain. A third strain lacking the anchor molecule PRiMA, has no functional AChE and butyrylcholinesterase (BChE) in brain and an unaltered respiratory function. BChE inhibition by bambuterol decreases tidal volume and body temperature in del E5+6 and i1RR knockout strains, but not in PRiMA deletion or wild-type controls. We find that: (1) deletion of the full AChE gene is required for a pronounced alteration in respiratory phenotype, (2) BChE is involved in respiratory muscles contraction and temperature control in del E5+6 and i1RR knockout mice, and (3) AChE expression requiring a gene product splice to either exons 5 and 6 or regulated by intron1 influences temperature control.

A computational analysis of central CO2 chemosensitivity in Helix aspersa

We created a single-compartment computer model of a CO(2) chemosensory neuron using differential equations adapted from the Hodgkin-Huxley model and measurements of currents in CO(2) chemosensory neurons from Helix aspersa. We incorporated into the model two inward currents, a sodium current and a calcium current, three outward potassium currents, an A-type current (I(KA)), a delayed rectifier current (I(KDR)), a calcium-activated potassium current (I(KCa)), and a proton conductance found in invertebrate cells. All of the potassium channels were inhibited by reduced pH. We also included the pH regulatory process to mimic the effect of the sodium-hydrogen exchanger (NHE) described in these cells during hypercapnic stimulation. The model displayed chemosensory behavior (increased spike frequency during acid stimulation), and all three potassium channels participated in the chemosensory response and shaped the temporal characteristics of the response to acid stimulation. pH-dependent inhibition of I(KA) initiated the response to CO(2), but hypercapnic inhibition of I(KDR) and I(KCa) affected the duration of the excitatory response to hypercapnia. The presence or absence of NHE activity altered the chemosensory response over time and demonstrated the inadvisability of effective intracellular pH (pH(i)) regulation in cells designed to act as chemostats for acid-base regulation. The results of the model indicate that multiple channels contribute to CO(2) chemosensitivity, but the primary sensor is probably I(KA). pH(i) may be a sufficient chemosensory stimulus, but it may not be a necessary stimulus: either pH(i) or extracellular pH can be an effective stimuli if chemosensory neurons express appropriate pH-sensitive channels. The lack of pH(i) regulation is a key feature determining the neuronal activity of chemosensory cells over time, and the balanced lack of pH(i) regulation during hypercapnia probably depends on intracellular activation of pH(i) regulation but extracellular inhibition of pH(i) regulation. These general principles are applicable to all CO(2) chemosensory cells in vertebrate and invertebrate neurons.

Cellular mechanisms involved in CO(2) and acid signaling in chemosensitive neurons

An increase in CO(2)/H(+) is a major stimulus for increased ventilation and is sensed by specialized brain stem neurons called central chemosensitive neurons. These neurons appear to be spread among numerous brain stem regions, and neurons from different regions have different levels of chemosensitivity. Early studies implicated changes of pH as playing a role in chemosensitive signaling, most likely by inhibiting a K(+) channel, depolarizing chemosensitive neurons, and thereby increasing their firing rate. Considerable progress has been made over the past decade in understanding the cellular mechanisms of chemosensitive signaling using reduced preparations. Recent evidence has pointed to an important role of changes of intracellular pH in the response of central chemosensitive neurons to increased CO(2)/H(+) levels. The signaling mechanisms for chemosensitivity may also involve changes of extracellular pH, intracellular Ca(2+), gap junctions, oxidative stress, glial cells, bicarbonate, CO(2), and neurotransmitters. The normal target for these signals is generally believed to be a K(+) channel, although it is likely that many K(+) channels as well as Ca(2+) channels are involved as targets of chemosensitive signals. The results of studies of cellular signaling in central chemosensitive neurons are compared with results in other CO(2)- and/or H(+)-sensitive cells, including peripheral chemoreceptors (carotid body glomus cells), invertebrate central chemoreceptors, avian intrapulmonary chemoreceptors, acid-sensitive taste receptor cells on the tongue, and pain-sensitive nociceptors. A multiple factors model is proposed for central chemosensitive neurons in which multiple signals that affect multiple ion channel targets result in the final neuronal response to changes in CO(2)/H(+).

Increased ventilation and CO2 chemosensitivity in acetylcholinesterase knockout mice

To investigate the effects of a permanent excess of acetylcholine (AChE) on respiration, breathing and chemosensitivity were analyzed from birth to adulthood in mice lacking the AChE gene (AChE-/-), in heterozygotes, and in control wild-type (AChE+/+) littermates. Breathing at rest and ventilatory responses to brief exposures to hypoxia (10% O2) and hypercapnia (3-5% CO2) were measured by whole-body plethysmography. At rest AChE-/- mice show larger tidal volumes (VT, + 96% in adults), overall ventilation (VE, + 70%), and mean inspiratory flow (+270%) than wild-type mice, with no change in breathing frequency (fR). AChE-/- mice have a slightly blunted response to hypoxia, but increased VE and fR responses to hypercapnia. Heterozygous animals present no consistent alterations of breathing at rest and chemosensitivity is normal. Adult AChE-/- mice have an increased VE/VO2 and a marginally higher normalized VO2. The results suggest that the hyperventilation and altered chemosensitivity in AChE-/- mice largely reflect alterations of central respiratory control.

Ventilatory pattern and chemosensitivity in M1 and M3 muscarinic receptor knockout mice

Acetylcholine (ACh) acting through muscarinic receptors is thought to be involved in the control of breathing, notably in central and peripheral chemosensory afferents and in regulations related to sleep-wake states. By using whole-body plethysmography, we compared baseline breathing at rest and ventilatory responses to acute exposure (5 min) to moderate hypoxia (10% O(2)) and hypercapnia (3 and 5% CO(2)) in mice lacking either the M(1) or the M(3) muscarinic receptor, and in wild-type matched controls. M(1) knockout mice showed normal minute ventilation (V(E)) but elevated tidal volume (V(T)) at rest, and normal chemosensory ventilatory responses to hypoxia and hypercapnia. M(3) knockout mice had elevated V(E) and V(T) at rest, a reduced V(T) response slope to hypercapnia, and blunted V(E) and frequency responses to hypoxia. The results suggest that M(1) and M(3) muscarinic receptors play significant roles in the regulation of tidal volume at rest and that the afferent pathway originating from peripheral chemoreceptors involves M(3) receptors.

Cytoarchitecture of central chemoreceptors in the mammalian ventral medulla

We reviewed the previous reports on the fine anatomy of the mammalian ventral medulla with special attention to the cytoarchitecture of the superficial chemosensitive regions to summarize what is known, what is not yet known, and what should be studied in the future. We also reviewed studies on anatomical relationship between neurons and vessels, and morphological studies on dendrites of respiratory or chemosensitive neurons. When we compared the morphological reports on the ventral and dorsal putative chemosensitive regions, similarities were found as follows. Chemosensitive cells were often found not only near the ventral surface but near the dorsal surface of the brainstem. Dendritic projection towards the surface was a common characteristic in the ventral and dorsal chemosensitive neurons. Morphological abnormality in the brainstem of sudden infant death syndrome victims was also summarized. On the basis of the previous reports we discussed the perspective on the future study on central chemoreception. Among various unanswered questions in central chemosensitivity studies, physiological significance of surface cells and surface extending dendrites is the most important topic, and must be thoroughly investigated.

Maturation of the mammalian respiratory system

In this review, the maturational changes occurring in the mammalian respiratory network from fetal to adult ages are analyzed. Most of the data presented were obtained on rodents using in vitro approaches. In gestational day 18 (E18) fetuses, this network functions but is not yet able to sustain a stable respiratory activity, and most of the neonatal modulatory processes are not yet efficient. Respiratory motoneurons undergo relatively little cell death, and even if not yet fully mature at E18, they are capable of firing sustained bursts of potentials. Endogenous serotonin exerts a potent facilitation on the network and appears to be necessary for the respiratory rhythm to be expressed. In E20 fetuses and neonates, the respiratory activity has become quite stable. Inhibitory processes are not yet necessary for respiratory rhythmogenesis, and the rostral ventrolateral medulla (RVLM) contains inspiratory bursting pacemaker neurons that seem to constitute the kernel of the network. The activity of the network depends on CO2 and pH levels, via cholinergic relays, as well as being modulated at both the RVLM and motoneuronal levels by endogenous serotonin, substance P, and catecholamine mechanisms. In adults, the inhibitory processes become more important, but the RVLM is still a crucial area. The neonatal modulatory processes are likely to continue during adulthood, but they are difficult to investigate in vivo. In conclusion, 1) serotonin, which greatly facilitates the activity of the respiratory network at all developmental ages, may at least partly define its maturation; 2) the RVLM bursting pacemaker neurons may be the kernel of the network from E20 to adulthood, but their existence and their role in vivo need to be further confirmed in both neonatal and adult mammals.

Ventrolateral medullary control of cardiovascular activity during muscle contraction

An overview of the role of ventrolateral medulla (VLM) in regulation of cardiovascular activity is presented. A summary of VLM anatomy and its functional relation to other areas in the central nervous system is described. Over the past few years, various studies have investigated the VLM and its involvement in cardiovascular regulation during static muscle contraction, a type of static exercise as seen, for example, during knee extension or hand-grip exercise. Understanding the neural mechanisms that are responsible for regulation of cardiovascular activity during static muscle contraction is of particular interest since it helps understand circulatory adjustments in response to an increase in physical activity. This review surveys the role of several receptors and neurotransmitters in the VLM that are associated with changes in mean arterial pressure and heart rate during static muscle contraction in anesthetized animals. Possible mechanisms in the VLM that modulate cardiovascular changes during static muscle contraction are summarized and discussed. Localized administration of an excitatory amino-acid antagonist into the rostral portion of the VLM (RVLM) attenuates increases in blood pressure and heart rate during static muscle contraction, whereas its administration into the caudal part of the VLM (CVLM) augments these responses. Opioid or 5-HT1A receptor stimulation in the RVLM, but not in the CVLM, attenuates cardiovascular responses to muscle contraction. Furthermore, intravenous, intracerebroventricular or intracisternal injection of an alpha 2-adrenoceptor agonist or a cholinesterase inhibitor attenuates increases in blood pressure and heart rate during static muscle contraction. Finally, the possible involvement of endogenous neurotransmitters in the RVLM and the CVLM associated with cardiovascular responses during static muscle contraction is discussed. An overview of the role of the VLM in the overall cardiovascular control network in the brain is presented and critically reviewed.

Chemosensory and cholinergic stimulation of fictive respiration in isolated CNS of neonatal opossum

1. The aim of the present experiments was to characterize the central chemical drive of fictive respiration in the isolated CNS of the newborn opossum, Monodelphis domestica. This opossum preparation, in contrast to those of neonatal rats and mice, produces respiratory rhythm of high frequency in vitro. 2. Fictive respiration was recorded from C3-C5 ventral roots of the isolated CNS of 4- to 14-day-old opossums using suction electrodes. At room temperature (21-23 degrees C) the frequency of respiration was 43 +/- 5.3 min-1 (mean +/- S.E.M., n = 50) in basal medium Eagle's medium (BMEM) equilibrated with 5% CO2-95% O2, pH 7.37-7.40. Respiratory discharges remained regular throughout 8 h experiments and continued for more than 20 h in culture. 3. Superfusion of the brainstem confirmed that solutions of pH 6.3-7.2 increased both the amplitude and frequency of respiration. High pH solutions (7.5-7.7) had the opposite effect and abolished the rhythm at pH 7.7. Addition of ACh (50-100 microM) or carbachol (0.01-10 microM) to the brainstem superfusion also increased the amplitude and frequency of respiratory activity, as did physostigmine (50-100 microM) or neostigmine (20-50 microM). Conversely, scopolamine (50-100 microM) reduced the amplitude and frequency of the basal respiratory rhythm by about 30%. 4. H(+)- and cholinergic-sensitive areas on the surface of the isolated CNS were explored with a small micropipette (outer tip diameter, 100 microns) filled with BMEM (pH 6.5) or 1 microM carbachol. Carbachol applied to H(+)- and cholinergic-sensitive areas in the ventral medulla mimicked the changes of respiratory pattern produced by low pH application. Responses to altered pH and carbachol were abolished by scopolamine (50 microM). Histochemistry demonstrated several medullary groups of neurons stained for acetylcholinesterase. The superficial location of one of these groups coincided with a functional and anatomically well-defined pH- and carbachol-sensitive area placed medial to the hypoglossal roots. 5. Exploration of chemosensitive areas revealed that application of drugs or solutions of different pH to a single well-defined spot could have selective and distinctive effects upon amplitude and frequency of respiratory activity. 6. These results show that fictive respiration in the isolated CNS of the newborn opossum is tonically driven by chemical- and cholinergic-sensitive areas located on the ventral medulla, the activity of which regulates frequency and amplitude of respiration. They suggest that a cholinergic relay, although not essential for rhythm generation, is involved in the central pH chemosensory mechanism, or that cholinergic and chemical inputs converge upon the same input pathway to the respiratory pattern generator.

Cardiovascular effects elicited by central administration of physostigmine via M2 muscarinic receptors in conscious cats

The cardiovascular effects of an intracerebroventricular (i.c.v.) injection of physostigmine were studied using conscious cats. Physostigmine (5-25 micrograms: 5 microliters) caused a dose-dependent increase in mean arterial pressure (MAP) and heart rate (HR). The highest dose (25 micrograms) increased MAP and HR by 32 +/- 3 mmHg and 45 +/- 5 beats/min, respectively (n = 5). Pre-administration of the muscarinic receptor antagonist, atropine (25 micrograms; i.c.v.) blocked the effects of physostigmine (25 micrograms; i.c.v.). Also, the pre-administration of the M2 muscarinic antagonist, methoctramine (25 micrograms; i.c.v.), antagonized the cardiovascular effects of physostigmine without altering the baseline variables. However, the M1 muscarinic antagonist, pirenzepine (100 micrograms; i.c.v.) did not alter baseline MAP or HR, and also failed to inhibit the cardiovascular responses to physostigmine. Similarly, the M3 muscarinic blocker, 4-diphenyl-acetoxy-N-methylpiperidine methiodide (50 micrograms; i.c.v.), neither changed baseline cardiovascular variables nor blocked the effects of physostigmine. When the same cats were anesthetized with intravenous injection of sodium pentobarbital (25-30 mg/kg), physostigmine (25 micrograms; i.c.v.) evoked a decrease in MAP and HR of 13 +/- 6 mmHg and 15 +/- 6 bpm, respectively (n = 5). These results demonstrate that the increases in MAP and HR to the i.c.v. administration of physostigmine in conscious cats are possibly mediated through stimulation of central M2 muscarinic receptors. In addition, anesthesia reverses the effects elicited by the central administration of physostigmine to a decrease in MAP and HR.

Effects of cholinomimetics on cocaine-induced hypotension and apneusis at a ventral brainstem cardiorespiratory control site

The current study was undertaken to evaluate the effects of cholinomimetic drugs on cocaine-induced central cardiorespiratory depression. Cats anesthetized by urethane (2.0 g/kg) were subjected to topical application at the caudal ventrolateral medullary surface (cVMS) of cocaine and two cholinomimetic pretreatment drugs. The following drug regimens were tested: 37 mM cocaine 1) given alone; 2) given 5 min after 2.7 mM carbachol pretreatment; and 3) given 5 min after 3.6 mM physostigmine pretreatment. In 7 of 11 cats, pretreatment with physostigmine decreased the incidence of cocaine-induced apneusis and hypoventilation significantly (p < 0.05); these animals showed no significant change in the mean arterial blood pressure during the 5-min pretreatment before administration of cocaine. In 4 of 11 cats, the physostigmine pretreatment produced a significant decrease in mean arterial blood pressure followed by lethal cardiorespiratory arrest when cocaine was administered. Pretreatment with carbachol resulted in cardiorespiratory responses which were not significantly different from those produced by cocaine alone. In anesthetized cats not exhibiting hypotensive responses to physostigmine, pretreatment may ameliorate cocaine-induced respiratory failure by ventral brainstem control mechanisms.

Intramedullary sodium cyanide injection on respiratory and vasomotor responses in cats

To examine the effect of hypoxia confined to the ventrolateral medulla we microinjected NaCN into the cat medulla (1.0 mm below the ventral surface) unilaterally and investigated cardio-respiratory changes. We studied anesthetized artificially ventilated animals and measured the electrical activity of phrenic and cervical sympathetic nerves and blood pressure. Histotoxic hypoxia depressed phrenic amplitude and elevated sympathetic tone and blood pressure. These responses were obtained predominantly from the region 5.0-8.0 mm caudal to the foramen caecum and 3.0-5.0 mm lateral to the midline (intermediate area). A study with 14C-cyanide showed that total and covalently bound cyanide was confined within a 1 mm diffusion sphere following microinjection. Isolated areas in both rostral and caudal medulla responded to cyanide with elevated sympathetic tone in the absence of phrenic nerve depression, suggesting dissociation of respiratory and vasomotor responses to hypoxia. Thus, the respiratory depression and vasomotor excitation produced by central hypoxia can be reproduced by hypoxia limited to discrete regions of the ventrolateral medulla.

Cardiovascular effects of central administration of cholinomimetics in anesthetized cats

The cardiovascular effects of central administration of cholinomimetics were investigated in anesthetized cats, to identify the site and mechanism of their action. Physostigmine, 10-100 micrograms, given by intracerebroventricular administration (i.c.v.) caused a dose-dependent reduction in blood pressure and renal sympathetic nerve discharges and no change in heart rate, which were antagonized by intravenous injection (i.v.) of atropine but not by methscopolamine or pirenzepine, given intravenously. Carbachol 3-30 micrograms (i.c.v.) reduced blood pressure and renal sympathetic nerve discharges and caused no change in heart rate. The M1 muscarinic agonist, McN-A-343, 100-1000 micrograms (i.c.v.) did not affect blood pressure, heart rate or renal sympathetic nerve discharges. Bilateral application of physostigmine, 10-100 micrograms/site on the ventral medullary surface, decreased blood pressure and renal sympathetic nerve discharges but not heart rate. Carbachol, 3-30 micrograms/site, caused reductions in blood pressure and renal sympathetic nerve discharges and no change in heart rate. Atropine, but not pirenzepine or methscopolamine, reversed the effects of physostigmine or carbachol. Treatment with McN-A-343, 100-1000 micrograms/site, did not alter blood pressure, heart rate or renal sympathetic nerve discharges. Under pretreatment with atropine into the ventral medulla but not pirenzepine, physostigmine, given intravenously, did not influence blood pressure. It is concluded that a cholinergic mechanism, concerned with a depressor response, is located on the ventral medulla. Muscarinic receptors of the non-M1 subtype, possibly M2, are related to this mechanism.

Central cocaine neurotoxicity at brainstem cardiorespiratory control sites

Cocaine hydrochloride was applied topically to the ventrolateral medullary surface (VMS) where chemosensitive respiratory and vasomotor control sites are colocalized. Cats (n = 16) were anesthetized with urethane (2.0 g/kg, 80 percent of dose titrated over 60 min). The trachea of each animal was cannulated and the VMS was surgically exposed. Tidal volume (VT), frequency of breathing (f), systolic and diastolic blood pressure (SBP and DBP, respectively), and heart rate (HR) were measured. Cocaine (62.5 micrograms per site) administered at the VMS control sites decreased f, SBP, and DBP significantly (p < 0.05), without changing HR or VT values. This cocaine-induced hypoventilation was associated with brief intervals of inspiratory cramp (apneusis). Central cocaine neurotoxicity may result from interaction of cocaine with VMS sites, producing increased inspiratory drive and decreased vasomotor tone.

Nicotinic activity in the interpeduncular nucleus of the midbrain prolongs recovery from halothane anesthesia

The influence of nicotinic transmission in the interpeduncular nucleus of the ventral midbrain on recovery from general anesthesia (3% halothane in oxygen) was assessed in rats. Immediately upon withdrawal of the anesthetic, nicotine (2 microliters, 10(-5) to 10(-1) M) was injected into the interpeduncular nucleus. Larger doses of nicotine (10(-2) and 10(-1) M) significantly (P < 0.05) prolonged the recovery of righting reflexes (to 371 +/- 55 and 362 +/- 67 sec, respectively, mean +/- SE), compared with injection of saline (187 +/- 19 sec). Prior intramuscular administration of the nicotinic antagonist, mecamylamine (2 mg/kg) significantly reduced the effect of 10(-2) M nicotine (to 211 +/- 43 sec). Injection of the nicotinic antagonist, hexamethonium (10(-1) M) led to a low mean recovery time (181 +/- 21 sec), not significantly different from control. Prolongation of recovery by 10(-2) M nicotine was not found to be significant when sites more dorsal to the interpeduncular nucleus were injected. An observed tendency for injection of nicotine to slow the post-anesthesia rate of breathing was not statistically significant and not correlated anatomically with the injection site in the midbrain. Increased release of acetylcholine has been shown previously to occur in the interpeduncular nucleus during anesthesia. The present results suggest that nicotinic activation of the interpeduncular nucleus facilitates or sums with the mechanisms in the brain that produce anesthesia under halothane.

Dopamine and nicotine, but not serotonin, modulate the crustacean ventilatory pattern generator

Dopamine (DA) causes a dose-dependent increase in the frequency of motor neuron bursts [virtual ventilation (fR)] produced by deafferented crab ventilatory pattern generators (CPGv). Domperidone, a D2-specific DA antagonist, by itself reversibly depresses fR and also blocks the stimulatory effects of DA. Serotonin (5HT) has no direct effects on this CPGv. Nicotine also causes dramatic dose-dependent increases in the frequency of motor bursts from the CPGv. The action is triphasic, beginning with an initial reversal of burst pattern typical of reversed-mode ventilation, followed by a 2- to 3-min period of depression and then a long period of elevated burst rate. Acetylcholine chloride (ACh) alone is ineffective, but in the presence of eserine is moderately stimulatory. The inhibitory effects of nicotine are only partially blocked by curare. The excitatory action of nicotine is blocked by prior perfusion of domperidone, but not by SKF-83566.HCl, a D1-specific DA antagonist. SKF-83566 had no effects on the ongoing pattern of firing. These observations support the hypothesis that dopaminergic pathways are involved in the maintenance of the CPGv rhythm and that the acceleratory effects of nicotine may involve release of DA either directly or via stimulation of atypical ACh receptors at intraganglionic sites.

Cardiorespiratory changes induced by vertebral artery injection of sodium cyanide in cats

Brain stem hypoxia caused by vertebral artery injection of sodium cyanide (NaCN) (1-20 micrograms) in artificially ventilated cats depressed phrenic and stimulated sympathetic nerve activity with a simultaneous increase in arterial blood pressure. Larger doses of NaCN caused greater effects. Hypercapnia produced by inhalation of 7% CO2 in O2 tended to reduce NaCN-induced responses on phrenic activity but not on blood pressure or sympathetic activity. Infusion into the vertebral artery with hypoxic saline (3% CO2 in N2) altered blood pressure, also affecting phrenic and sympathetic nerves similarly to NaCN administration. However, washout of CO2 by infusion of 100% O2 bubbled saline at high flow rates (3.6 ml/min) depressed phrenic as well as sympathetic activity and blood pressure. Spinal transection at the first cervical level eliminated sympathetic excitatory response to intravertebral cyanide injection. However, a large dose of NaCN (600 micrograms) given intravenously in spinal animal excited sympathetic activity. We conclude that intravertebral injection of NaCN can be used to study the effects of local hypoxia of the brain stem on cardiorespiratory responses and that hypoxia acts at both these sites (brain stem and spinal cord) to stimulate sympathetic excitation.

Endothelin-sensitive areas in the ventral surface of the rat medulla

In urethane-anesthetized rats, subregions of the ventral surface of the medulla (VSM) in which endothelin (ET) caused cardiorespiratory effects were mapped by topically applying 1 pmol of ET-1. Two distinct subregions, termed the rostral and caudal ET-sensitive areas, were identified. The rostral area was also sensitive to L-glutamate and glycine. It extended between the caudal end of the trapezoid body and the rootlet of the XIIth nerve partly overlying the pyramidal tract. In this position ET-1 caused the type I response consisting of an initial increase (excitatory component) in arterial pressure (AP), renal sympathetic nerve activity (RSNA), heart rate (HR), phrenic nerve activity (PNA) and the number of bursts of PNA (burst rate) followed by a sustained decrease (inhibitory component) in them. The caudal ET-sensitive area was located near the rootlet of the XIIth nerve. In this position ET-1 caused the type II response consisting of a decrease in PNA and an increase in burst rate. Part of this area responded to nicotine but not to glutamate or glycine. ET-3 (10 pmol) applied to the two ET-sensitive areas produced responses similar to those elicited by ET-1. The dose-response relationship was investigated by delivering ETs to the rostral area. The excitatory component of most of the variables was elicited at a dose of 1 fmol of ET-1 or 1 pmol of ET-3, whereas the inhibitory component was produced at 10 fmol of ET-1 or 10 pmol of ET-3. These results suggest that subregions of the rat's VSM may participate in the central cardiorespiratory control by ET.

Stimulation of serotonin2 receptors in the ventrolateral medulla of the cat results in nonuniform increases in sympathetic outflow

Topical application of the serotonin2 agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane or DOI, in a dose of 30 micrograms/side to the intermediate area of the ventrolateral surface of the medulla produced a significant increase in mean arterial pressure with no significant change in heart rate both in intact animals (n = 8) and in cervically vagotomized animals (n = 3). The pressor response of DOI was blocked by pretreatment of the intermediate area with ketanserin, a serotonin2 antagonist (n = 7). Pretreatment with intravenous phentolamine did not block the pressor response of DOI (n = 3). However, this pressor response could be counteracted by intravenous propranolol (n = 5) or by bilateral stellate ganglionectomy (n = 3). These data suggest that sympathoexcitation by centrally applied DOI selectively increased cardiac inotropy but not chronotropy. Further studies indicate that DOI increased contractile force without increasing heart rate and that the positive inotropic effect of DOI could be counteracted by bilateral stellate ganglionectomy. Bilateral microinjections of DOI into the subretrofacial nucleus in a dose of 100 ng (n = 3) and a dose of 300 ng (n = 3) increased mean arterial blood pressure by 23 +/- 2 and 44 +/- 6 mm Hg, respectively, without producing any changes in heart rate. These data suggest that DOI has a central site of action in the ventrolateral medulla, presumably at the subretrofacial nucleus, which leads to selective sympathoexcitation of the cardiac ventricles.

Acetylcholine and central chemosensitivity: in vitro study in the newborn rat

In vitro experiments were performed in the superfused brainstem-spinal cord preparation of newborn rats in order to analyse the central respiratory effects of acetylcholine. The central motor output was assessed from recording electrical activity in nerves supplying respiratory muscles. Acetylcholine added to the bathing medium induced dose-dependent increases in respiratory frequency which were blocked by muscarinic (but not nicotinic) antagonists and enhanced by physostigmine. These effects originated from the medullary ventral surface where chemosensitive structures have been previously located. The respiratory central chemosensitivity of the isolated brainstem was analysed using a CO2 free, pH 7.9 medium instead of the normal medium (bubbled with 5% CO2, pH 7.3). Decreases at the H+ and CO2 stimuli led to decreased inspiratory activity, resulting mainly from a decrease in the amplitude of the motor output. These responses were enhanced by atropine and diminished by physostigmine. These results obtained in vitro on the newborn rat suggest that cholinergic synapses are not directly involved in the genesis of respiratory rhythmicity but confirm previous results obtained in vivo in adult animal revealing that acetylcholine is implicated in the central respiratory chemosensitivity.

Tolerance to nicotine-induced sympathoadrenal stimulation and cross-tolerance to stress: differential central and peripheral mechanisms in rats

Nicotine stimulates the secretion of catecholamines from sympathetic nerve endings and adrenal medulla by acting on peripheral nicotinic cholinergic receptors. Nicotine is also a potent stimulant in the central nervous system but the significance of nicotinic receptors in brain in mediating cardiovascular and sympathoadrenal responses to nicotine is unclear. The responses of resting plasma catecholamines, blood pressure and heart rate were compared in rats receiving nicotine, administered either systemically or intracerebroventricularly (i.c.v.). Sympathoadrenal stress responses were also studied in rats rendered tolerant to nicotine from repeated systemic or intraventricular injections. Nicotine, given either intraventricularly or systemically, produced dose-related increases in the concentration of epinephrine in plasma. Little effect on norepinephrine in plasma was observed with nicotine given intraventricularly, indicating predominant stimulation of adrenomedullary pathways. In contrast, nicotine, given systemically, produced comparable increases in both epinephrine and norepinephrine. Blood pressure increased and heart rate fell in response to either intraventricular or systemic administration of nicotine. Rats exhibited tolerance to nicotine 24 hr after a single intraventricular injection; however, tolerance was not detected with systemically injected nicotine unless the injections were given at least every 30 min. Whereas rats rendered tolerant to systemic administration of nicotine were cross-tolerant to stress, with respect to sympathoadrenal stimulation, cross-tolerance with stress was not detected in rats treated with nicotine repeatedly by the intraventricular route. These results indicate that nicotinic receptors in brain modulate the central sympathetic outflow and adapt readily to nicotine stimulation with prolonged tolerance, but are probably not involved in sympathoadrenal stress responses. Peripheral nicotinic receptors, regulating sympathoadrenal secretion of catecholamines, displayed much shorter-lasting tolerance.

Synthesis, release and receptor binding of acetylcholine in the C1 area of the rostral ventrolateral medulla: contributions in regulating arterial pressure

This study sought to determine whether release of acetylcholine (ACh) within the C1 area of nucleus reticularis rostroventrolateralis (RVL) contributes to the tonic maintenance of arterial pressure (AP) in the rat. The activity of choline acetyltransferase (ChAT), the biosynthetic enzyme for ACh, varied 5.5-fold in micropunches of the 6 medullary regions examined. ChAT activity in the C1 area (179 +/- 35 nmol [14C]ACh formed/mg protein/60 min; n = 4) was intermediate between that of the hypoglossal nucleus (249 +/- 38; highest) and the pyramids (45 +/- 11; lowest) and equivalent to that found in the parietal cortex (147 +/- 15). Release of [3H]ACh from C1 area micropunches was increased by raising extracellular K+ concentrations (5-55 mM) and was entirely Ca2(+)-dependent. Muscarinic receptor binding density was assessed using [3H]quinuclidinyl benzylate ([3H]QNB) as ligand and a recently developed 'electronic micropunch' technique which allows measurement of quench-corrected [3H]QNB binding within corresponding cylinders of tissue obtained by the mechanical micropunch cannula. [3H]QNB binding density varied 2.6-fold: lateral reticular nucleus pars lateralis greater than C1 area greater than nucleus ambiguus = hypoglossal nucleus = pyramid = oral spinal trigeminal nucleus. In urethane-anesthetized rats, inhibition of ACh synthesis by hemicholinium-3 (HC-3, 3 nmol/50 nl), or blockade of muscarinic receptors by scopolamine (SCOP, 3 nmol/50 nl), reduced resting mean AP by 18-24 mm Hg following bilateral microinjection into the C1 area. These concentrations of HC-3 and SCOP were sufficient to attenuate by 70-80% the increase in local cholinergic neurotransmission elicited by the cholinesterase inhibitor physostigmine given systemically. High concentrations of SCOP (30-150 nmol/50 nl) lowered AP by 46-60 mm Hg. Similarly, bilateral microinjections of GABA (10 nmol/50 nl) into the C1 area markedly reduced mean AP by 51 +/- 6 mm Hg to levels normally found after transection of the spinal cord. Thus, a substantial portion of tonic sympathetic activity may be driven by activation of muscarinic receptors in the C1 area. In the spontaneously hypertensive rat (SHR), a genetic model of hypertension, neither spontaneous nor K(+)-evoked release of [3H]ACh from the C1 area differed from that of normotensive Wistar-Kyoto rats (WKY).(ABSTRACT TRUNCATED AT 400 WORDS)

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)

Neurophysiological concomitants of soman-induced respiratory depression in awake, behaving guinea pigs

Soman-induced respiratory failure was investigated in awake, behaving guinea pigs chronically instrumented to allow concurrent recordings of medullary respiratory-related unit (RRU) activity, diaphragm electromyogram (DEMG), and electrocorticogram. Responses to soman typically began with hyperpnea. Loss of consciousness, as indicated by the development of seizure activities, took place shortly after the onset of hyperpnea. This was followed by dyspnea, hypopnea, and finally, respiratory failure. The most profound respiratory dysfunctions were seen during the development of dyspnea characterized by a progressively degenerative RRU-DEMG phase relationship (phase anomalies) and mixed patterns of ataxic breathing. Electrophysiographic records indicated that the anomalous RRU-DEMG phase phenomenon is attributable to a state of functional dissociation in some brainstem mechanisms that are normally involved in the orchestration of a synchronous respiratory drive. The failure of bulbar rhythmogenic mechanisms to maintain an orderly and synchronous recruitment of respiratory drive, which led to untimely and chaotic activations of respiratory muscles, was apparently the underlying cause of various ataxic breathing patterns and a reduced ventilatory efficiency. Spectral analyses of DEMG activities showed that, despite episodic muscle fasciculations and signs of fatigue, the functional integrity of the diaphragm was not significantly compromised by soman at a dose sufficient to produce respiratory failure. These findings not only support the notion of a relatively more important involvement of central respiratory mechanisms in soman-induced respiratory failure, but also identify a state of functional dissociation of central respiratory timing mechanisms as being a significant component in soman intoxication.

High activity neurons in the reticular formation of the medulla oblongata: a high-resolution autoradiographic 2-deoxyglucose study

Accumulation of radioactivity in the medulla oblongata of the rat was assessed with high resolution autoradiography after injection of [3H]2-deoxyglucose or [14C]2-deoxyglucose. The autoradiographic approach employed allowed analysis at the cellular level. A salient feature of autoradiograms was the presence of foci of very high silver grain density in the reticular formation. These heavily labeled foci were shown to be associated with neurons by combined Acridine Orange staining and autoradiography. The high activity neurons in the ventral lateral medulla were predominantly located in the caudal portion of the paragigantocellular reticular nucleus. In addition, a group of high activity neurons was present in the dorsal reticular formation. The potential involvement of the high activity neurons in cardiovascular and respiratory regulation is discussed. In dry-mount autoradiograms produced after injection of [3H]2-deoxyglucose, certain small cells, presumably glial cells, were observed throughout the brainstem to accumulate radioactivity to a greater extent than the surrounding tissue. High activity neurons and high activity glial cells were observed for 5 min and 45 min survival intervals after intravenous injection of [3H]2-deoxyglucose. The similar appearance of autoradiograms at 5 min and 45 min after injection of [3H]2-deoxyglucose indicates that the utility of a 5-min survival period deserves further evaluation for assessment of functional activity patterns in brain.

Mechanisms involved in the respiratory depressant actions of nicotine in anesthetized rats

In the urethane-pentobarbital anesthetized rat, the respiratory depressant and lethal effects of intravenously infused (-)-nicotine (120 micrograms/kg/min) or (+)-nicotine (600 micrograms/kg/min) were effectively prevented by pretreatment with the opioid antagonist, naltrexone, whereas the lethal effect of (-)-nicotine (120 micrograms/kg/min) was not altered by bilateral adrenalectomy. Further, pretreatment with either the nicotinic ganglion-blocker, mecamylamine, a secondary amine, or the quarternary nicotinic ganglion-blocker, hexamethonium, completely prevented the lethal effects of (-)-nicotine (120 micrograms/kg/min). These data suggest that central opioidergic and nicotinic processes are involved in nicotine's respiratory depressant and lethal effects.

Effect of hypercapnia on ventilatory response to intravenous nicotine administration in anesthetized dogs

We studied the effects of hypercapnia on the ventilatory response to nicotine in thirty anesthetized mongrel dogs. Ventilatory (VE) and occlusion pressure (P0.2) changes were assessed before and after intravenous injection of nicotine at concentrations of 1, 4, 16 and 64 micrograms/kg in four different groups of five dogs each. An end-tidal CO2 (PETCO2) was set at 40 mm Hg or 60 mm Hg by inspiration of 7% CO2 in oxygen through a non-rebreathing valve. With PETCO2 maintained at 40 mm Hg, P0.2 had increased 1 min after nicotine injection from 1 to 16 micrograms/kg in a dose-dependent manner, and a subsequent decrease in P0.2 below the initial value was observed at around 4 min. Injection of 64 micrograms/kg of nicotine produced a marked increase in P0.2 and subsequent apnea. With PETCO2 at 60 mm Hg, the time course of P0.2 was qualitatively similar to that observed with PETCO2 at 40 mm Hg, except that the change in P0.2 was larger in the former case than in the latter, for a given nicotine dose. The ratio of the difference in maximal P0.2 observed with PETCO2 of 40 mm Hg and that at 60 mm Hg to the difference between PETCO2 values (delta PO2/delta PETCO2) increased with nicotine dose from 1 to 4 micrograms/kg and, with a further increase in nicotine dose, the maximal delta P0.2/delta PETCO2 plateaued, while delta P0.2/delta PETCO2 obtained from the minimal PO2 values decreased in a nicotine dose-dependent fashion. These results suggest that hypercapnia enhances both stimulative and subsequent depressive ventilatory responses to nicotine.

Tonic and baroreflex effects on arterial pressure and ventilation of pentobarbital and nicotine on the rat ventral medullary surface

Unilateral aortic depressor nerve stimulation caused depression of arterial pressure (baropressure reflex) and ventilation (baro-ventilatory reflex) in urethane-anesthetized, spontaneously breathing Sprague-Dawley rats. Application of sodium pentobarbital to the ventral medullary surface (VMS) depressed baseline arterial pressure and ventilation, and attenuated the baro-pressure reflex, but not the baro-ventilatory reflex. Application of nicotine on the VMS decreased baseline arterial pressure and increased ventilation, but left both baro-pressure and baro-ventilatory reflexes unaltered. The results suggest that some of the structures that affect vasomotor tone may not be involved in the baroreflex inhibition of arterial pressure. Additionally, neither the neural structures near the VMS which modulate CO2 control of ventilation, nor those that affect tonic control of vasomotor tone are likely to be significantly involved in the baro-ventilatory reflex.

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)

Function of the ventrolateral medulla in the control of the circulation

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

Modulation of the centrally-evoked visceral alerting/defence response by changes in CSF pH at the ventral surface of the medulla oblongata and by systemic hypercapnia

In the present study on nine cats, repeated tests were made of the effects of superfusion of the ventral surface of the medulla oblongata with acid or alkaline CSF. Only two animals showed slight hyperventilation, tachycardia, mesenteric vasoconstriction and variable changes in hindlimb vascular conductance when the ventral surface was superfused with acid CSF; alkaline CSF produced opposite effects. These changes are qualitatively similar to, but much smaller than, published results which support the idea that the central chemoreceptor areas for CO2 are near the surface of the ventral medulla. But, in accord with those who have disputed this idea, the remaining 7 animals showed no response to superfusion with acid or alkaline CSF. Yet, all 9 animals showed marked hyperventilation in response to inhalation of 5% or 8% CO2. These findings accord with the view that chemosensitive structures on the ventral medulla represent part, but not all of the central chemosensitive mechanism for CO2. Inhalation of CO2 also induced bradycardia, mesenteric vasodilatation and either vasodilatation or vasoconstriction in hindlimb, attributable to a predominance of the direct myocardial depressant and local vasodilator effects of CO2, over the increase in sympathetic activity produced by central hypercapnia. But, despite the different effects of acid CSF and inhaled CO2 on baselines, they produced comparable effects on the visceral altering/defence response evoked by electrical stimulation in the ventral amygdalo-hypothalamic pathway viz, the magnitude of the characteristic hindlimb dilatation was reduced while that of the mesenteric constriction was increased.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Inhibition of feline spinal cord dorsal horn neurons following electrical stimulation of nucleus paragigantocellularis lateralis. A comparison with nucleus raphe magnus

The effect of electrical stimulation applied to the nucleus paragigantocellularis lateralis (PGL) was assessed on the somatosensory responses of functionally identified spinal cord dorsal horn neurons in the cat. Neurons were classified as low threshold mechanoreceptive, wide dynamic range or nociceptive specific. The responses of over 95% of all neurons tested were inhibited by a conditioning stimulus to the PGL. For each cell the threshold current intensity necessary to produce inhibition from the PGL (inhibitory threshold) was determined. Analysis of the incidence of inhibition and the inhibitory thresholds showed that the PGL-induced inhibition was not selective for a particular class of neuron. Due to the many similarities between the PGL and the nucleus raphe magnus (NRM), a comparison was made between each region's potency in inhibiting the responses of spinal cord neurons. Based on an analysis of inhibitory thresholds, the PGL was found to be significantly more potent than the NRM. These results indicate the PGL to be an important site from which descending modulation of spinal cord somesthetic information emanates.

Mapping of cholinoceptive(nicotinoceptive)neurons in the lower brainstem: with special reference to the ventral surface of the medulla

The distribution of cholinoceptive neurons in the lower brainstem of the rat was investigated by means of a histochemical method for specific acetylcholinesterase. Nicotinoceptive neurons were characterized using an alpha-bungarotoxin-horseradish peroxidase conjugate for the detection of nicotinic acetylcholine receptors. For the first time a nearly complete mapping of the location of cholinoceptive (nicotinoceptive) neurons of the lower brainstem was achieved. Special attention was focused on the organization of the cholinoceptive neuronal matrix of the ventral surface of the medulla, where regulative centers for vasomotor and respiratory control are located.

Substance P-immunoreactive neurons in the brainstem of the cat related to cardiovascular centers

The distribution of substance P-immunoreactivity (SP-IR) in the brainstem and spinal cord of normal and colchicine-pretreated cats was analysed using the peroxidase-antiperoxidase (PAP) technique. Numerous SP-IR fibers are present in the nucleus solitarius, nucleus dorsalis nervi vagi and nucleus spinalis nervi trigemini, various parts of the formatio reticularis, substantia grisea centralis mesencephali, locus coeruleus and nucleus parabrachialis. SP-IR perikarya occur in the substantiae gelatinosa and intermedia of the spinal cord, the nucleus spinalis nervi trigemini-pars caudalis, the nucleus dorsalis nervi vagi, and the nucleus solitarius, as well as in the adjacent formatio reticularis and the medullary nuclei of the raphe. In addition, SP-IR cell bodies are located in the nuclei raphe magnus and incertus, ventral and dorsal to the nucleus tegmentalis dorsalis (Gudden), nucleus raphe dorsalis, substantia grisea centralis mensencephali, locus coeruleus, nucleus parabrachialis and colliculus superior. The results indicate that SP-IR neurons may be involved in the regulation of cardiovascular functions both at the central and peripheral level. A peripheral afferent portion seems to terminate in the nucleus solitarius and an efferent part is postulated to originate from the nucleus dorsalis nervi vagi and from the area of the nuclei retroambiguus, ambiguus and retrofacialis.

Effect of low-molecular-weight proteins on protein (lysozyme) binding to isolated brush-border membranes of rat kidney

Filtered proteins including the low-molecular-weight protein lysozyme are reabsorbed by the proximal tubule via adsorptive endocytosis. This process starts with binding of the protein to the brush-border membrane. The binding of 125I-labelled egg-white lysozyme (EC 3.2.1.17) to isolated brush-border membranes of rat kidney and the effect of several low-molecular weight proteins on that binding was determined. The Scatchard plot revealed a one-component binding type with a dissociation constant of 5.3 microM and 53.0 nmol/mg membrane protein for the number of binding sites. The binding of the cationic lysozyme was inhibited competitively by the addition of cationic cytochrome c to the incubation medium, while the neutral myoglobin had no effect. The anionic beta-lactoglobulin A inhibited the lysozyme binding in a noncompetitive manner. These data suggest that the binding takes place between positively charged groups of the protein molecule and negative sites on the brush-border membrane, and, the competition between the cationic cytochrome c and the cationic lysozyme for the binding sites may be responsible for the inhibitory effect of cytochrome c on renal lysozyme reabsorption. The binding step at the brush-border membrane appears to be cation-selective.

Distribution of NT-IR perikarya in the brain of the guinea pig with special reference to cardiovascular centers in the medulla oblongata

The occurrence and distribution of neurotensin-immunoreactive (NT-IR) perikarya was studied in the central nervous system of the guinea pig using a newly raised antibody (KN 1). Numerous NT-IR perikarya were found in the nuclei amygdaloidei, nuclei septi interventriculare, hypothalamus, nucleus parafascicularis thalami, substantia grisea centralis mesencephali, ventral medulla oblongata, nucleus solitarius and spinal cord. The distribution of NT-IR perikarya was similar to that previously described in the rat and monkey. In the gyrus cinguli, hippocampus and nucleus olfactorius, though, no NT-IR neurons were detected in this investigation. Additional immunoreactive perikarya, however, were observed in areas of the ventral medulla oblongata, namely in the nucleus paragigantocellularis, nucleus retrofacialis and nucleus raphe obscurus. The relevance of the NT-IR perikarya within the ventral medulla oblongata is discussed with respect to other neuropeptides, which are found in this area, and to cardiovascular regulation.

Cholinoceptive properties of respiratory neurones in the rat medulla

The effects of iontophoretically applied acetylcholine, the acetylcholine agonists nicotine and muscarine, and the antagonists atropine, dihydro-beta-erythroidine (DH beta E) and mecamylamine, together with the excitatory amino acids, glutamate and D,L-homocysteic acid (DLH) were examined on the activity of respiratory-related neurones in the rat medulla and were compared with effects on non-respiratory brain stem neurones. Most neurones were excited by acetylcholine and no inhibitory responses were seen. Glutamate and DLH also excited but there was a trend for the phasic activity of respiratory neurones to be converted to a tonic discharge. Nicotine also excited most neurones to which it was applied and these responses were blocked by DH beta E but not by atropine. Muscarine also caused excitation and these responses were blocked by atropine but not by DH beta E. Both antagonists blocked acetylcholine-induced excitation but had no effect on responses to glutamate or DLH. Mecamylamine was without effect. It is concluded that the proportion of cholinoceptive respiratory neurones in the rat brain stem is similar to that for non-respiratory neurones. It seems likely that both nicotinic and muscarinic receptors are present on the majority of respiratory neurones and that both contribute to the response produced by iontophoretically-applied acetylcholine.