A cholinergic mechanism involved in the neuronal excitation by H+ in the respiratory chemosensitive structures of the ventral medulla oblongata of rats in vitro

Effects of acetylcholine on hypoglossal and C4 nerve activity in brainstem-spinal cord preparations from newborn rat

Effects of acetylcholine (ACh) on respiratory activity have been an intriguing theme especially in relation to central chemoreception and the control of hypoglossal nerve activity. We studied the effects of ACh on hypoglossal and phrenic (C4) nerve activities and inspiratory and pre-inspiratory neurons in the rostral ventrolateral medulla in brainstem-spinal cord preparations from newborn rats. ACh application increased respiratory rhythm, decreased inspiratory hypoglossal and C4 nerve burst amplitude, and enhanced pre-inspiratory hypoglossal activity. ACh induced membrane depolarization of pre-inspiratory neurons that might be involved in facilitation of respiratory rhythm by ACh. Effects of ACh on hypoglossal and C4 nerve activity were partially reversed by a nicotinic receptor blocker, mecamylamine. Further application of a muscarinic receptor antagonist, oxybutynin, resulted in slight increase of hypoglossal (but not C4) burst amplitude. Thus, ACh induced different effects on hypoglossal and C4 nerve activity in the brainstem-spinal cord preparation.

The retrotrapezoid nucleus and the neuromodulation of breathing

Breathing is regulated by a host of arousal and sleep-wake state-dependent neuromodulators to maintain respiratory homeostasis. Modulators such as acetylcholine, norepinephrine, histamine, serotonin (5-HT), adenosine triphosphate (ATP), substance P, somatostatin, bombesin, orexin, and leptin can serve complementary or off-setting functions depending on the target cell type and signaling mechanisms engaged. Abnormalities in any of these modulatory mechanisms can destabilize breathing, suggesting that modulatory mechanisms are not overly redundant but rather work in concert to maintain stable respiratory output. The present review focuses on the modulation of a specific cluster of neurons located in the ventral medullary surface, named retrotrapezoid nucleus, that are activated by changes in tissue CO₂/H⁺ and regulate several aspects of breathing, including inspiration and active expiration.

M4-muscarinic acetylcholine receptor into the pedunculopontine tegmental nucleus mediates respiratory modulation of conscious rats

The pedunculopontine tegmental nucleus (PPTg) has been shown to have important functions relevant to the regulation of behavioral states and various motor control systems, including breathing control. The PPTg is considered an important nucleus in the mesopontine region with considerably cholinergic input to the ventral respiratory column. In addition, recent studies indicate that cholinergic innervation of the ventral respiratory column may play an important role in modulation of breathing. Here, we investigated the cholinergic stimulation of the PPTg and the changes in breathing output in conscious rats. Male Wistar rats (280-350 g, N = 5-12/group) with unilateral stainless steel cannula implanted into the PPTg were used. Respiratory parameters (tidal volume (V_T), respiratory frequency (f_R) and ventilation (V_E)) were analyzed by whole body plethysmography. In unrestrained awake rats, unilateral injection of the cholinergic muscarinic agonist carbachol (10 mM-100 nL) in the PPTg decreased f_R, and increase V_T, without changing V_E. The changes in f_R and V_T elicited by carbachol into the PPTg are abolished by previous blockade of the M4 muscarinic cholinergic receptors tropicamide into the PPTg. No significant changes in f_R and V_T elicited by carbachol were observed after blockade of the M1 and/or M3 muscarinic cholinergic receptors pirenzepine or 4-DAMP into the PPTg. Our data suggest that the changes in f_R and V_T produced by muscarinic cholinergic stimulation of PPTg is presumably mediated through a Gi-coupled M4 muscarinic receptors.

Cholinergic neurons in the pedunculopontine tegmental nucleus modulate breathing in rats by direct projections to the retrotrapezoid nucleus

KEY POINTS

Cholinergic projections from the pedunculopontine tegmental nucleus (PPTg) to the retrotrapezoid nucleus (RTN) are considered to be important for sleep-wake state-dependent control of breathing. The RTN also receives cholinergic input from the postinspiratory complex. Stimulation of the PPTg increases respiratory output under control conditions but not when muscarinic receptors in the RTN are blocked. The data obtained in the present study support the possibility that arousal-dependent modulation of breathing involves recruitment of cholinergic projections from the PPTg to the RTN.

ABSTRACT

The pedunculopontine tegmental nucleus (PPTg) in the mesopontine region has important physiological functions, including breathing control. The PPTg contains a variety of cell types, including cholinergic neurons that project to the rostral aspect of the ventrolateral medulla. In addition, cholinergic signalling in the retrotrapezoid nucleus (RTN), a region that contains neurons that regulate breathing in response to changes in CO₂ /H⁺ , has been shown to activate chemosensitive neurons and increase inspiratory activity. The present study aimed to identify the source of cholinergic input to the RTN and determine whether cholinergic signalling in this region influences baseline breathing or the ventilatory response to CO₂ in conscious male Wistar rats. Retrograde tracer Fluoro-Gold injected into the RTN labelled a subset of cholinergic PPTg neurons that presumably project directly to the chemosensitive region of the RTN. In unrestrained awake rats, unilateral injection of the glutamate (10 mm/100 nL) in the PPTg decreased tidal volume (V_T ) but otherwise increased respiratory rate (f_R ) and net respiratory output as indicated by an increase in ventilation (V_E ). All respiratory responses elicited by PPTg stimulation were blunted by prior injection of methyl-atropine (5 mm/50-75 nL) into the RTN. These results show that stimulation of the PPTg can increase respiratory activity in part by cholinergic activation of chemosensitive elements of the RTN. Based on previous evidence that cholinergic PPTg projections may simultaneously activate expiratory output from the pFRG, we speculate that cholinergic signalling at the level of RTN region could also be involved in breathing regulation.

Cholinergic modulation of the parafacial respiratory group

KEY POINTS

This study investigates the effects of cholinergic transmission on the expiratory oscillator, the parafacial respiratory group (pFRG) in urethane anaesthetized adult rats. Local inhibition of the acetyl cholinesterase enzyme induced activation of expiratory abdominal muscles and active expiration. Local application of the cholinomimetic carbachol elicited recruitment of late expiratory neurons, expiratory abdominal muscle activity and active expiration. This effect was antagonized by local application of the muscarinic antagonists scopolamine, J104129 and 4DAMP. We observed distinct physiological responses between the more medial chemosensitive region of the retrotrapezoid nucleus and the more lateral region of pFRG. These results support the hypothesis that pFRG is under cholinergic neuromodulation and the region surrounding the facial nucleus contains a group of neurons with distinct physiological roles.

ABSTRACT

Active inspiration and expiration are opposing respiratory phases generated by two separate oscillators in the brainstem: inspiration driven by a neuronal network located in the preBötzinger complex (preBötC) and expiration driven by a neuronal network located in the parafacial respiratory group (pFRG). While continuous activity of the preBötC is necessary for maintaining ventilation, the pFRG behaves as a conditional expiratory oscillator, being silent in resting conditions and becoming rhythmically active in the presence of increased respiratory drive (e.g. hypoxia, hypercapnia, exercise and through release of inhibition). Recent evidence from our laboratory suggests that expiratory activity in the principal expiratory pump muscles, the abdominals, is modulated in a state-dependent fashion, frequently occurring during periods of REM sleep. We hypothesized that acetylcholine, a neurotransmitter released in wakefulness and REM sleep by mesopontine structures, contributes to the activation of pFRG neurons and thus acts to promote the recruitment of expiratory abdominal muscle activity. We investigated the stimulatory effect of cholinergic neurotransmission on pFRG activity and recruitment of active expiration in vivo under anaesthesia. We demonstrate that local application of the acetylcholinesterase inhibitor physostigmine into the pFRG potentiated expiratory activity. Furthermore, local application of the cholinomimetic carbachol into the pFRG activated late expiratory neurons and induced long lasting rhythmic active expiration. This effect was completely abolished by pre-application of the muscarinic antagonist scopolamine, and more selective M3 antagonists 4DAMP and J104129. We conclude that cholinergic muscarinic transmission contributes to excitation of pFRG neurons and promotes both active recruitment of abdominal muscles and active expiratory flow.

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.

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.

The caudal solitary complex is a site of central CO(2) chemoreception and integration of multiple systems that regulate expired CO(2)

The solitary complex is comprised of the nucleus tractus solitarius (NTS, sensory) and dorsal motor nucleus of the vagus (DMV, motor), which functions as an integrative center for neural control of multiple systems including the respiratory, cardiovascular and gastroesophageal systems. The caudal NTS-DMV is one of the several sites of central CO(2) chemoreception in the brain stem. CO(2) chemosensitive neurons are fully responsive to CO(2) at birth and their responsiveness seems to depend on pH-sensitive K(+) channels. In addition, chemosensitive neurons are highly sensitive to conditions such as hypoxia (e.g., neural plasticity) and hyperoxia (e.g., stimulation), suggesting they employ redox and nitrosative signaling mechanisms. Here we review the cellular and systems physiological evidence supporting our hypothesis that the caudal NTS-DMV is a site for integration of respiratory, cardiovascular and gastroesophageal systems that work together to eliminate CO(2) during acute and chronic respiratory acidosis to restore pH homeostasis.

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.

Timing and duration of developmental nicotine exposure contribute to attenuation of the tadpole hypercapnic neuroventilatory response

The ability for air-breathing vertebrates to adjust ventilation in response to increased CO(2) (hypercapnia) is fundamental to maintaining pH homeostasis. Developmental nicotine exposure has been shown to impair tadpole neuroventilatory responses to hypercapnia following 8-12 weeks of exposure. It is not clear, however, to what extent the timing of exposure during development and/or the duration over which the exposure takes place contribute to this impairment. Here, tadpoles were exposed to 30 microg/L of nicotine for 3- or 10-week durations, either early or late in tadpole development. Correlates of tadpole lung neuroventilation were monitored during normocapnic (1.5% CO(2)) and hypercapnic (5% CO(2)) conditions of isolated brainstems. Preparations derived from early metamorphic tadpoles failed to increase lung neuroventilation in response to hypercapnia whether they had been exposed to nicotine for 3 or 10 weeks. Preparations derived from late metamorphic tadpoles failed to respond to hypercapnia after being exposed to nicotine for 10 weeks. These results suggest that both the developmental timing and duration of exposure are important when considering nicotine's effect on the hypercapnic neuroventilatory response.

Central cholinergic regulation of respiration: nicotinic receptors

Nicotinic acetylcholine receptors (nAChRs) are expressed in brainstem and spinal cord regions involved in the control of breathing. These receptors mediate central cholinergic regulation of respiration and effects of the exogenous ligand nicotine on respiratory pattern. Activation of alpha4* nAChRs in the preBötzinger Complex (preBötC), an essential site for normal respiratory rhythm generation in mammals, modulates excitatory glutamatergic neurotransmission and depolarizes preBötC inspiratory neurons, leading to increases in respiratory frequency. nAChRs are also present in motor nuclei innervating respiratory muscles. Activation of post- and/or extra-synaptic alpha4* nAChRs on hypoglossal (XII) motoneurons depolarizes these neurons, potentiating tonic and respiratory-related rhythmic activity. As perinatal nicotine exposure may contribute to the pathogenesis of sudden infant death syndrome (SIDS), we discuss the effects of perinatal nicotine exposure on development of the cholinergic and other neurotransmitter systems involved in control of breathing. Advances in understanding of the mechanisms underlying central cholinergic/nicotinic modulation of respiration provide a pharmacological basis for exploiting nAChRs as therapeutic targets for neurological disorders related to neural control of breathing such as sleep apnea and SIDS.

Inhalation of the nerve gas sarin impairs ventilatory responses to hypercapnia and hypoxia in rats

Sarin, a highly toxic nerve gas, is believed to cause bronchoconstriction and even death primarily through respiratory failure; however, the mechanism underlying the respiratory failure is not fully understood. The goals of this study were to ascertain whether sarin affects baseline ventilation (VE) and VE chemoreflexes as well as airway resistance and, if so, whether these changes are reversible. Four groups of F344 rats were exposed to vehicle (VEH) or sarin at 2.5, 3.5, and 4.0 mg h m(-3) (SL, SM, and SH, respectively). VE and VE responses to hypercapnia (7% CO2) or hypoxia (10% O2) were measured by plethysmography at 2 h and 1, 2, and 5 days after VEH or sarin exposure. Total pulmonary resistance (RL) also was measured in anesthetized VEH- and SH-exposed animals 2 h after exposure. Our results showed that within 2 h after exposure 11% of the SM- and 52% of the SH- exposed groups died. Although the SM and SH significantly decreased hypercapnic and hypoxic VE to similar levels (64 and 69%), SH induced greater respiratory impairment, characterized by lower baseline VE (30%; P<0.05), and total loss of the respiratory frequency response to hypercapnia and hypoxia. VE impairment recovered within 1-2 days after sarin exposure; interestingly, SH did not significantly affect baseline RL. Moreover, sarin induced body tremors that were unrelated to the changes in the VE responses. Thus, LC50 sarin causes a reversible impairment of VE that is not dependent on the sarin-induced body tremors and not associated with changes in RL.

Muscarinic receptors and alpha2-adrenoceptors interact to modulate the respiratory rhythm in mouse neonates

The respiratory rhythm generator (RRG) is modulated by several endogenous substances, including acetylcholine (ACh) and noradrenaline (NA) that interact in several modulatory processes. To know whether ACh and NA interacted to modulate the RRG activity, we used medullary "en bloc" and slice preparations from neonatal mice where the RRG has been shown to receive a facilitatory modulation from A1/C1 neurons, via a continuous release of endogenous NA and activation of alpha2 adrenoceptors. Applying ACh at 25 microM activated the RRG but ACh had no effects at 50 microM. Applying the ACh receptor agonists nicotine and muscarine facilitated and depressed the RRG, respectively. After yohimbine pre-treatment that blocked the alpha2 facilitation, the nicotinic facilitation was not altered, the muscarinic depression was reversed and ACh 50 microM significantly facilitated the RRG. After L-tyrosine pre-treatment that potentiated the alpha2 facilitation, the muscarinic depression was enhanced. Thus, ACh regulates the RRG activity via nicotinic and muscarinic receptors, the muscarinic receptors interacting with alpha2 adrenoceptors.

Molecular responses to acidosis of central chemosensitive neurons in brain

Significant advances have been made in understanding how neurons sense and respond to acidosis at the cellular level. Decrease in pH of the cerebrospinal fluid followed by hypercapnia (increased arterial CO2) is monitored by the chemosensory neurons of the medulla oblongata. Then the intracellular signalling pathways are activated to regulate specific gene expression, which leads to a hyperventilatory response. However, little is known about molecular details of such cellular responses. Recent studies have identified several transcription factors such as c-Jun, Fos and small Maf proteins that may play critical roles in the brain adaptation to hypercapnia. Hypercapnic stimulation also activates c-Jun NH2-terminal kinase (JNK) cascade via influx of extracellular Ca2+ through voltage-gated Ca2+ channels. In addition, several transmembrane proteins including Rhombex-29 (rhombencephalic expression protein-29 kDa) and Past-A (proton-associated sugar transporter-A) have been implicated in regulation of H+ sensitivity and brain acidosis-mediated energy metabolism, respectively. This review discusses current knowledge on the signalling mechanisms and molecular basis of neuronal adaptation during acidosis.

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(+).

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.

Involvement of cholinergic mechanisms in the central control of respiration in the cane toad, Bufo marinus

Chemical substrates, central sites and central mechanisms underlying the regulation of breathing in lower vertebrates have not been well characterized. The present study was undertaken to determine the effect of pH changes and cholinergic agents on the central control of respiration in the cane toad, Bufo marinus. Adult toads were anesthetized, catheterized and unidirectionally ventilated before exposing the brainstem. An airtight buccal cannula was also inserted through the tympanum to record buccal pressure. The animal was decerebrated, anesthetic removed and the responses to pH changes of solutions bathing the ventral surface of the medulla (VSM) were tested by superfusing the VSM with mock cerebrospinal fluid (mCSF) of pH 7.8-normal, 7.2-acidic and 8.4-basic. The acidic solution increased respiratory activity, the basic solution decreased activity and the normal solution had no effect. In addition, cholinergeric agents (acetylcholine-ACh, physostigmine-Phy, nicotine-Nic, and atropine-Atr) dissolved in mCSF were applied bilaterally onto the VSM using filter paper pledgets. ACh, Phy and Nic increased episodic breathing frequency by 14.3+/-9.7, 9.4+/-5.4 and 29.1+/-11.8 %, respectively, whereas, Atr caused a decrease (-26.6+/-16.6%). These agents had no effect on blood pressure. It is therefore, concluded that the VSM is pH sensitive and a cholinergic mechanism is involved in the central modulation of respiration in Bufo.

Neurotransmitters in central respiratory control

A diverse group of processes are involved in central control of ventilation. Both fast acting neurotransmitters and slower acting neuromodulators are involved in the central respiratory drive. This review deals with fast acting neurotransmitters that are essential centrally in the ventilatory response to H(+)/CO(2) and to acute hypoxia. Data are reviewed to show that the central response to H(+)/CO(2) is primarily at sites in the medulla, the most prominent being the ventral medullary surface (VMS), and that acetylcholine is the key neurotransmitter in this process. Genetic abnormalities in the cholinergic system lead to states of hypoventilation in man and that knock out mice for genes responsible for neural crest development have none or diminished CO(2) ventilatory response. In the acute ventilatory response to hypoxia the afferent impulses from the carotid body reach the nucleus tractus solitarius (NTS) releasing glutamate which stimulates ventilation. Glutamate release also occurs in the VMS. Hypoxia is also associated with release of GABA in the mid-brain and a biphasic change in concentration of another inhibitory amino acid, taurine. Collectively changes in these amino acids can account for the ventilatory output in response to acute hypoxia. Future studies should provide more data on molecular and genetic basis of central respiratory drive and the role of neurotransmitter in this essential function.

Sensing arterial CO(2) levels: a role for medullary P2X receptors

ATP has been shown to act as an excitatory neurotransmitter in the central nervous system. In this review, evidence is presented to indicate that when ATP is micro-injected into the ventrolateral medulla (VLM) of the rat, changes in respiratory activity are elicited. These effects, and accompanying changes in heart rate and blood pressure are mediated by P2X purinoreceptors. Immunocytochemistry indicates a prevalence of P2X(2) and P2X(6) purinoreceptors in this region of the medulla. The P2 purinoceptor antagonists, suramin and PPADS blunt the respiratory responses to changes in arterial CO(2) levels when micro-injected into the VLM. This effect is shown electrophysiologically to be mediated by purinoreceptors located primarily on respiratory neurones of the VLM including the Bötzinger complex. As the effects of agonist activation of P2X(2) purinoceptors expressed in HEK293 cells and Xenopus oocytes are potentiated by lowering pH, these data imply that the central respiratory response to CO(2) depends in part on the pH sensitivity of purinoreceptors located on inspiratory neurones. The implications for respiratory activity and control are discussed.

Acetylcholine modulates respiratory pattern: effects mediated by M3-like receptors in preBötzinger complex inspiratory neurons

Perturbations of cholinergic neurotransmission in the brain stem affect respiratory motor pattern both in vivo and in vitro; the underlying cellular mechanisms are unclear. Using a medullary slice preparation from neonatal rat that spontaneously generates respiratory rhythm, we patch-clamped inspiratory neurons in the preBötzinger complex (preBötC), the hypothesized site for respiratory rhythm generation, and simultaneously recorded respiratory-related motor output from the hypoglossal nerve (XIIn). Most (88%) of the inspiratory neurons tested responded to local application of acetylcholine (ACh) or carbachol (CCh) or bath application of muscarine. Bath application of 50 microM muscarine increased the frequency, amplitude, and duration of XIIn inspiratory bursts. At the cellular level, muscarine induced a tonic inward current, increased the duration, and decreased the amplitude of the phasic inspiratory inward currents in preBötC inspiratory neurons recorded under voltage clamp at -60 mV. Muscarine also induced seizure-like activity evident during expiratory periods in XIIn activity; these effects were blocked by atropine. In the presence of tetrodotoxin (TTX), local ejection of 2 mM CCh or ACh onto preBötC inspiratory neurons induced an inward current along with an increase in membrane conductance under voltage clamp and induced a depolarization under current clamp. This response was blocked by atropine in a concentration-dependent manner. Bath application of 1 microM pirenzepine, 10 microM gallamine, or 10 microM himbacine had little effect on the CCh-induced current, whereas 10 microM 4-diphenylacetoxy-N-methylpiperidine methiodide blocked the current. The current-voltage (I-V) relationship of the CCh-induced response was linear in the range of -110 to -20 mV and reversed at -11.4 mV. Similar responses were found in both pacemaker and nonpacemaker inspiratory neurons. The response to CCh was unaffected when patch electrodes contained a high concentration of EGTA (11 mM) or bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (10 mM). The response to CCh was reduced greatly by substitution of 128 mM Tris-Cl for NaCl in the bath solution; the I-V curve shifted to the left and the reversal potential shifted to -47 mV. Lowering extracellular Cl(-) concentration from 140 to 70 mM had no effect on the reversal potential. These results suggest that in preBötC inspiratory neurons, ACh acts on M3-like ACh receptors on the postsynaptic neurons to open a channel permeable to Na(+) and K(+) that is not Ca(2+) dependent. This inward cation current plays a major role in depolarizing preBötC inspiratory neurons, including pacemakers, that may account for the ACh-induced increase in the frequency of respiratory motor output observed at the systems/behavioral level.

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.

Hans Loeschcke, Robert Mitchell and the medullary CO2 chemoreceptors: a brief historical review

In the late 1950s, stimulated by reports from Leusen in Belgium and Winterstein in Germany on ventilatory responses to spinal fluid acid, Hans Loeschcke from Göttingen, and Robert Mitchell of the University of California in San Francisco were independently seeking the site of respiratory chemosensitivity to CO2 which they presumed to be mediated by cerebro-spinal fluid hydrogen ions. In 1960 Loeschcke came to San Francisco to join Mitchell for 3 months of intensive hunting for the site of action. This essay describes the events surrounding the localization of ventral medullary superficial (VMS) chemosensitivity to topical acidification, and some of their subsequent and largely independent work on the location, nature and function of this structure. The discovery led to a vast literature on all aspects of the regulation of respiration.

Pharmacological properties of the CO2/H+-sensitive area in the ventral medullary surface assessed by the effects of chemical stimulation on respiration

We recently discovered that CO2/H+-sensitive neurons in the ventral medullary surface (VMS) are immunoreactive to glutamate, glutamic acid decarboxylase (GAD), calcineurin and cAMP. We then tested the hypothesis that glutamate, GABA, calcineurin and cAMP affect the activity of CO2/H+-sensitive neurons in the VMS. Using male Wistar rats anesthetized with urethane and pentobarbital, we checked for changes in relative tidal volume (VT) and respiratory frequency (f) in response to injecting the VMS with a variety of test agents dissolved in mock CSF. Respiratory changes occurred immediately and were dose-dependent. (1) 200-1600 pmol Glutamate increased VT but decreased f. The glutamate effect was never abolished by concomitant injection of AP5, a NMDA receptor antagonist, but was abolished by CNQX, an AMPA receptor antagonist, indicating predominance of AMPA receptors in the CO2/H+-sensitive neurons in the VMS. (2) 200-1600 pmol GABA decreased both VT and f. The GABA effect was never abolished by concomitant injection of saclofen, a GABA(B) receptor antagonist, but was abolished by bicuculline, a GABA(A) receptor antagonist, indicating predominance of GABA(A) receptors in the CO2/H+-sensitive neurons in the VMS. (3) 4-32 microg Calcineurin, a Ca2+/calmodulin-dependent protein phosphatase 2B, and 200-1600 pmol FK506, selective inhibitor of calcineurin, had no effect on respiration when they were applied extracellularly, but 400-3200 pmol BAPTA-AM, an intracellular Ca2+-chelating agent, decreased both VT and f, indicating involvement of intracellular Ca2+ in the excitatory mechanisms of respiration. (4) 100-800 pmol IBMX, an enhancer of intracellular cAMP, decreased both VT and f, indicating involvement of cAMP in the inhibitory mechanisms of respiration. These results indicate that the CO2/H+-sensitive neurons in the VMS contain glutamate and/or GABA in cytoplasma, possess AMPA and/or GABA(A) receptors on surface of plasma membrane, and compose the internal circuit, and that their activities are regulated by Ca2+ and cAMP.

In vitro study of H+-sensitive neurons in the ventral medullary surface of neonate rats

We hypothesized that the direct stimulus of the central chemoreceptor neurons is the CO2/H+-induced change in intracellular pH (pHi). If it is true, pHi responses during hypercapnic stimulation should be exhibited in the central chemoreceptor neurons in the ventral medullary surface (VMS) and some neurons in the CO2/H+ sensitive regions such as the nucleus tractus solitarii of the medial dorsal medulla (MDM). To test this hypothesis, the cultured VMS and MDM neurons (control) derived from one day-old neonate rats were labeled with H+-sensitive fluorescent indicator 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF), and were exposed to perfusate of various pHs. The H+-sensitive neurons were determined by a rapid decrease in the intracellular BCECF fluorescence intensity. In almost all the MDM neurons (99.6%) and 94% of the VMS neurons, the intracellular BCECF fluorescence intensity remained unchanged when the extracellular pH (pHo) was decreased. In contrast, in 0.4% of the MDM neurons (8/1800) and in 6% of the VMS neurons (111/1800), the intracellular BCECF fluorescence intensity decreased when the pHo was decreased from 7.4 to 7.2. This subpopulation of MDM and VMS neurons were considered to be H+-sensitive neurons. The H+-sensitive neurons in the VMS showed positive immunoreactivity to glutamate (57%, 17/30) and glutamic acid decarboxylase (23%, 7/30), but no immunoreactivity to choline acetyltransferase, tyrosine hydroxylase, phenylethanolamine N-methyltransferase, somatostatin, serotonin and substance P. These results indicate that the H+-sensitive neurons are present specifically in the VMS, and are mainly glutamatergic and GABAergic.

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.

The central respiratory effects of acetylcholine vary with CSF pH

Hydrogen ion concentration [H+] centrally is a major determinant of ventilation. Its action involves central cholinergic mechanisms. The point(s) where increased [H+] induces its changes in the cholinergic system is unclear. If H+ acts presynaptically by increasing endogenous ACh synthesis and release, its effect should be absent when ACh is supplied exogenously. If H+ acts postsynaptically by changing ACh degradation or ACh receptor sensitivity, its effect should persist in the presence of exogenous ACh. We perfused the brain ventricular system in spontaneously breathing anesthetized dogs with progressively higher concentrations of ACh (0-52.8 mM) in cerebrospinal fluid (CSF) at pH 7.4 and CSF pH 7.1. Increasing concentrations of ACh increased ventilation > 4-fold in a linear manner in the presence of non-acidic and acidic CSF. With acidic CSF the ACh ventilatory response line was shifted to a higher y-intercept, resulting in a higher ventilation at any [ACh]. These findings are consistent with the hypothesis that central acidosis augments ventilation by postsynaptic cholinergic events.

Effects of M1-selective antimuscarinics on respiratory chemosensitivity in humans

We examined effects of selective M1 antagonists on hypercapnic and hypoxic ventilatory responses in 17 healthy human volunteers. Subjects were intravenously treated with placebo, pirenzepine (10 mg) and biperiden lactate (4 mg) on three separate days in a randomized double-blind design. Ventilatory responses to hyperoxic progressive hypercapnia and isocapnic progressive hypoxia were studied after the drug administration. There were no statistically significant differences in the mean delta VE/delta PET CO2 or delta VE/delta SaO2 among the three treatments. However, the delta VE/delta PET CO2 with placebo negatively correlated with the difference in delta VE/delta PET CO2 between the biperiden and placebo studies (r=-0.65, P < 0.01), but not with that between the pirenzepine and placebo studies. On the other hand, the delta VE/delta SaO2 with placebo negatively correlated with the difference in delta VE/delta SaO2 between the pirenzepine and placebo studies (r = -0.79, P < 0.001), but not with that between the biperiden and placebo studies. These data suggest the possible involvement of M1 cholinergic receptors in the central CO2 and peripheral O2 sensing mechanisms in humans, although the degree of its involvement is not consistent among subjects. These findings may explain the interindividual variation in the control of breathing in humans.

Rhythm generation in organotypic medullary cultures of newborn rats

Organotypic transverse medullary slices (obex level) from six-day-old rats, cultured for two to four weeks in chemically defined medium contained rhythmically discharging neurones which were activated by CO2 and H+. The mechanisms underlying this rhythmicity and the spread of excitation and synaptic transmission within this organotypic tissue were examined by modifying the composition of the external solution. Our findings showed that (1) Exposure to tetrodotoxin (0.2 microM) or to high magnesium (6 mM) and low calcium (0.2 mM) concentrations abolished periodic activity. (2) Neither the blockade of GABAergic potentials with bicuculline methiodide (200 microM) and/or hydroxysaclofen (200 microM) nor the blockade of glycinergic potentials with strychnine hydrochloride (100 microM) abolished rhythmicity. (3) While atropine sulphate (5 microM) was ineffective in modulating periodic discharges nicotine (100 microM) - like CO2-shortened the intervals between the periodic events; hexamethonium (50-100 microM) reduced both periodic and aperiodic activity. (4) Exposure to the NMDA antagonist 2-aminophosphonovaleric acid (50 microM) suppressed periodic events only transiently. In the presence of 2-aminophosphonovaleric acid rhythmicity recovered. However, the AMPA-antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (10-50 microM), abolished periodic activity reversibly within less than 5 min. When 6-cyano-7-nitroquinoxaline-2,3-dione and nicotine were administered simultaneously periodic events persisted for up to 10 min. These findings indicate that synaptic excitatory drive is a prerequisite for the generation of rhythmic discharges of medullary neurones in this preparation. This drive may activate voltage-dependent channels or it may facilitate endogenous cellular mechanisms which initiate oscillations of intracellular calcium concentration. To test the latter possibility (5) calcium antagonists were added to the bath saline. The organic calcium antagonists verapamil and flunarizine (50-100 microM each) and the inorganic calcium antagonists cobalt (2 mM) and magnesium (6 mM) suppressed periodic activity and abolished or weakened the chemosensitivity towards CO2/acidosis. (6) Dantrolene (10 microM). an inhibitor of intracellular calcium release decreased the periodicity, while thapsigargin (2 microM) which blocks endoplasmic Ca(2+)-ATPase, transiently accelerated the occurrence of periodic events. (7) Oscillations of intracellular free calcium concentrations in Fura-2 AM-loaded cells were weakened or abolished by cobalt (2 mM). The results of (5)-(7) indicate that transmembrane calcium fluxes as well as intracellular Ca(2+)-release and -clearance mechanisms are a prerequisite for intracellular free calcium oscillations which may be important in the generation of rhythmic discharges in medullary neurones.

Decreased muscarinic receptor binding in the arcuate nucleus in sudden infant death syndrome

Muscarinic cholinergic activity in the human arcuate nucleus at the ventral medullary surface is postulated to be involved in cardiopulmonary control. A significant decrease in [3H]quinuclidinyl benzilate binding to muscarinic receptors in the arcuate nucleus is now shown to occur in sudden infant death syndrome (SIDS) infants, compared to infants dying acutely of known causes. In infants with chronic oxygenation abnormalities, binding is low in other nuclei, as well as in the arcuate nucleus. The binding deficit in the arcuate nucleus of SIDS infants might contribute to a failure of responses to cardiopulmonary challenges during sleep.

Studies of the respiratory center using isolated brainstem-spinal cord preparations

It has been ten years since a brainstem-spinal cord preparation isolated from a newborn rat was introduced for study of the mammalian respiratory center. Here, I briefly summarize first, these studies, which include the tissue condition of in vitro preparations, respiratory reflexes, pharmacology, rhythm generation, respiratory chemoreception, phrenic motoneurons, regulation from pons, and development of a respiratory center. In the latter half of this paper, I focus on the neural mechanisms of respiratory rhythm generation. A current hypothesis for the central pattern generator of respiration proposed by the author's group is that the respiratory rhythm generator, composed of pre-inspiratory neurons in the rostral ventrolateral medulla, produces the primary rhythm of respiration and triggers an inspiratory pattern generator composed of inspiratory neurons in the rostral and the caudal ventrolateral medulla.

A cholinergic mechanism involved in fetal breathing during the high voltage ECoG state

The effects of muscarinic cholinergic neurotransmission on fetal breathing was determined by administration of carbachol or carbachol plus a muscarinic receptor antagonist into the cerebrospinal fluid of the fourth ventricle in unanesthetized fetal sheep. In the hour following the instillation of carbachol (1.0 microgram), the incidence of fetal breathing in high voltage electrocortical state (ECoG) increased to 63 +/- 11.7 (SEM) percent compared to 1.2 +/- 0.9 after instillation of vehicle (Ringer solution). The cholinergic agonist increased breath amplitude from 4.5 +/- 0.4 to 10.6 +/- 1.4 mmHg. These effects were eliminated when the M1 receptor antagonist pirenzepine (50-100 micrograms) was administered with carbachol but not by antagonists which are relatively selective for M2 or M3 receptors. When administered alone, muscarinic receptor antagonists did not effect the incidence or amplitude of fetal breathing in low voltage. These data indicate that the apnea which occurs during high voltage in the sheep fetus involves an inhibition of acetylcholine acting at M1 muscarinic receptor bearing neurons.

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.

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.

Synaptic response of bulbar respiratory neurons to hypercapnic stimulation in peripherally chemodenervated cats

Effects of hypercapnia on the membrane potential and synaptic activity of bulbar respiratory neurons were studied in decerebrate, vagotomized, glomectomized and artificially ventilated cats. Coaxial multibarrelled electrodes were used for intracellular recording and extracellular iontophoresis of drugs. Hypoventilation with oxygen-enriched air (hyperoxic hypercapnia) produced an increase of depolarization together with an increase of spiking during the active phase and an increase of hyperpolarization during the inactive phase of each respiratory cycle in the inspiratory, postinspiratory and expiratory neurons of the ventral respiratory group. Both depolarizing and hyperpolarizing effects were associated with a decrease in input resistance. Intracellular injection of Cl- reversed the polarity of the hyperpolarizing synaptic wave to depolarization during the inactive phase, and hypercapnia increased the depolarization at that phase. Iontophoresis of tetrodotoxin eliminated the CO2-induced changes in membrane potential and input resistance. In 20 out of 58 neurons examined, iontophoretically applied atropine partly or totally suppressed the depolarizing response to hypercapnia. For these neurons, iontophoresed acetylcholine produced a sustained depolarization that was antagonized by atropine, but not by hexamethonium. The present study shows that both depolarizing and hyperpolarizing responses of medullary respiratory neurons to hyperoxic hypercapnia are synaptically mediated. A muscarinic mechanism is involved in part of the respiratory neuronal excitation evoked by hypercapnia.

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.

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.

Neurons sensitive to pH in slices of the rat ventral medulla oblongata

The effects of extracellular pH changes on neurons in slices of the rat ventral medulla oblongata were investigated by extracellular recording. Changes in discharge rate were correlated with pH changes in the tissue next to the recorded cell, as measured by H(+)-selective microelectrodes. pH was altered by varying the bicarbonate concentration ([HCO3-]) in the superfusion solution. In 136 out of 316 neurons, the number of spontaneous or electrically evoked discharges per unit time increased with decreasing pH and decreased with increasing pH. Changes of only 0.01-0.04 pH unit were effective in these pH-sensitive neurons. The response was transient; the discharge rate returned to the control value within a few minutes. The pH sensitivity persisted in the presence of 0.5 microM atropine, 20 microM bicuculline and after replacing Ca2+ by Mg2+ in the superfusion solution to reduce synaptic transmission. The response to the same pH decrease was stronger when increasing PCO2 than when reducing [HCO3-]0. The pH-induced response significantly increased during hypoxia. The results show that in the ventral medulla oblongata neurons exist that transiently respond to small decreases and increases of pH. The pH sensitivity is an intrinsic property of these neurons; it is not due to a synaptic mechanism but is modulated by PCO2 and PO2.

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)

Cardiorespiratory effects induced by acetazolamide on the ventromedullary surface of the cat

1. Inhibition of carbonic anhydrase by acetazolamide in alpha-chloralose-anaesthetized cats, in a region of the brain stem co-extensive with the glycine-sensitive area, intermediate chemosensitive area, and probably C1 catecholaminergic neurones produces hypotension, bradycardia and depression of the central respiratory drive. 2. These responses are concentration dependent, and can still be observed when the enzyme substrate (CO2) is elevated. Therefore, in both the hypercapnic and the normocapnic condition, similar responses in arterial blood pressure, heart rate and respiratory rate are observed when acetazolamide is topically applied to the glycine-sensitive area. 3. To investigate further the contribution of peripheral baro-, chemo- and cardiopulmonary receptors to these responses, acetazolamide was topically applied to the glycine-sensitive area under three different conditions: intact gallamine-paralysed (5 mg kg-1 h-1) and artificially ventilated (A), sinoaortic denervated (B), and sinoaortic denervated plus bilaterally vagotomized cats (C). Under all conditions, similar responses were observed. The fall in arterial blood pressure was 75 +/- 11 (A), 90 +/- 13 (B), and 75 +/- 9 mmHg (C). Changes in heart rate during acetazolamide application were -23 +/- 6, -20 +/- 8, and -26 +/- 6 beats min-1, respectively. The decreases in respiratory rate were 9 +/- 2 (A), 11 +/- 2 (B), and 11 +/- 2 breaths min-1 (C). 4. The data indicate that the responses to topical application of acetazolamide are mainly due to its central action at the glycine-sensitive area and are not influenced by peripheral baroreceptor and chemoreceptor inputs.

Effects of pilocarpine on breathing movements in normal, chemodenervated and brain stem-transected fetal sheep

1. We examined the effects on fetal breathing movements (FBM) and electrocortical activity (ECoG) of the administration of pilocarpine to three groups of fetal sheep in utero: one group had peripheral chemodenervation; one group had peripheral chemodenervation and transection of the brain stem at the level of the colliculi; a third group of sham-operated animals acted as a control. 2. Pilocarpine induced a period of FBM characterized by their high amplitude (ca. 24 mmHg) and low frequency (ca. 0.65 Hz). It also produced low-voltage ECoG. These effects were seen both in normoxia and in hypoxia. They were independent of the ECoG state in which the drug was given. 3. In normoxia, pilocarpine-induced FBM were blocked by cholinergic antagonists which enter the central nervous system, atropine sulphate and scopolamine hydrobromide, but were either unaffected or were reduced in amplitude by antagonists with restricted access to the central nervous system, atropine methylnitrate and scopolamine methylbromide. In hypoxia, atropine sulphate and scopolamine hydrobromide again blocked FBM but now scopolamine methylbromide and atropine methylnitrate blocked the FBM in 42% of trials. In the remaining trials FBM remained virtually unchanged. 4. No differences were observed in the effects of pilocarpine on FBM or ECoG between chemodenervated fetuses and those which had been sham-operated. 5. In fetuses with transection of the brain stem, breathing was continuous irrespective of the ECoG. Pilocarpine increased the amplitude of FBM but did not alter the frequency of these FBM. These effects occurred both in normoxia and in hypoxia. 6. We conclude that pilocarpine produces effects on fetal behaviour via at least two sites in the central nervous system. One site lies in the pons or medulla and is responsible for the stimulation of FBM. The other site is above the level of the colliculi and is responsible for producing sustained low-voltage ECoG. These effects do not require the integrity of the peripheral chemoreceptors and we find no evidence to support the previous suggestion that the action of pilocarpine is via stimulation of these receptors.

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.

Decreased excitability of respiratory motoneurons during hypercapnia in the acute spinal cat

This study was undertaken to examine the effects of hypercapnia on the excitability of respiratory motoneurons. The action of CO2 on phrenic (inspiratory) and internal intercostal (expiratory) motoneurons was compared with that exerted on non-respiratory motoneurons of the musculocutaneous nerve. The experiments were performed on spinalized (C1 segment), partially deafferented cats that were exposed to different CO2/O2 mixtures (end-tidal CO2 3 +/- 0.3, 6 +/- 0.5 and 9 +/- 0.5%). Changes in neuronal excitability were assessed by: measuring the amplitudes of antidromic field potentials recorded from a population of motoneurons; analysis of the amplitude and latency of the orthodromic response recorded from a given nerve and evoked by microstimulation within the corresponding motor nucleus; monitoring the membrane potentials during intracellular recordings from phrenic motoneurons; and recording ongoing activity of the phrenic and internal intercostal nerves. Hypercapnia (end-tidal CO2 6 +/- 0.5 or 9 +/- 0.5%) decreased the excitability of phrenic and musculocutaneous motoneurons, the effect being larger at the higher CO2 level. Internal intercostal motoneurons were generally more resistant to the effects of CO2. A depression of their excitability was observed only at end-tidal CO2 9 +/- 0.5%. The decreased excitability of phrenic motoneurons was associated with membrane hyperpolarization. It is concluded that the depressant action of CO2 is present in both respiratory and non-respiratory spinal motoneurons. The action of hypercapnia on respiratory motoneurons may oppose the excitatory effects exerted through specific chemoreflexes.

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)