The central respiratory effects of acetylcholine vary with CSF pH

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.

Inhibitory and modulatory inputs to the vocal central pattern generator of a teleost fish

Vocalization is a behavioral feature that is shared among multiple vertebrate lineages, including fish. The temporal patterning of vocal communication signals is set, in part, by central pattern generators (CPGs). Toadfishes are well-established models for CPG coding of vocalization at the hindbrain level. The vocal CPG comprises three topographically separate nuclei: pre-pacemaker, pacemaker, motor. While the connectivity between these nuclei is well understood, their neurochemical profile remains largely unexplored. The highly vocal Gulf toadfish, Opsanus beta, has been the subject of previous behavioral, neuroanatomical and neurophysiological studies. Combining transneuronal neurobiotin-labeling with immunohistochemistry, we map the distribution of inhibitory neurotransmitters and neuromodulators along with gap junctions in the vocal CPG of this species. Dense GABAergic and glycinergic label is found throughout the CPG, with labeled somata immediately adjacent to or within CPG nuclei, including a distinct subset of pacemaker neurons co-labeled with neurobiotin and glycine. Neurobiotin-labeled motor and pacemaker neurons are densely co-labeled with the gap junction protein connexin 35/36, supporting the hypothesis that transneuronal neurobiotin-labeling occurs, at least in part, via gap junction coupling. Serotonergic and catecholaminergic label is also robust within the entire vocal CPG, with additional cholinergic label in pacemaker and prepacemaker nuclei. Likely sources of these putative modulatory inputs are neurons within or immediately adjacent to vocal CPG neurons. Together with prior neurophysiological investigations, the results reveal potential mechanisms for generating multiple classes of social context-dependent vocalizations with widely divergent temporal and spectral properties.

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.

Cardio-respiratory coupling indicates suppression of vagal activity in acute schizophrenia

Altered amygdala activity in patients with schizophrenia can influence respiratory patterns and consequently cardiovascular parameters. Hence, we examined respiration and heart rate time series complexity as well as coupling of both signals. Electrocardiograms and respiratory signals were obtained from 25 unmedicated patients with schizophrenia and controls. Approximate entropy (ApEn) was calculated for heart and respiratory rates as well as cross-ApEn reflecting coupling of both signals. Coupling was decreased indicating diminished vagal modulation at the brain stem level. Regularity of breathing correlated with disease severity. These data might reflect a lack of inhibitory control over brainstem centers in schizophrenia.

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.

Respiratory dysfunctions induced by prenatal nicotine exposure

1. Maternal tobacco smoking is the principal risk factor associated with sudden infant death syndrome (SIDS), a leading cause of death of infants under 1 year of age. Victims of SIDS show a higher incidence of respiratory control abnormalities, including central apnoeas, delayed arousal responses and diminished ventilatory chemoreflexes. 2. Nicotine is likely the link between maternal tobacco smoking and SIDS. Prenatal nicotine exposure can alter the breathing pattern and can reduce hypoxia- and hypercarbia-induced ventilatory chemoreflexes. In vitro approaches have revealed that prenatal nicotine exposure impairs central chemosensitivity, switching the cholinergic contribution from a muscarinic to a nicotinic receptor-based drive. In addition, serotonergic, noradrenergic, GABAergic, glycinergic and glutamatergic, among others, are affected by prenatal nicotine. 3. Here we propose that prenatal nicotine affects the respiratory network through two main processes: (i) reorganization of neurotransmitter systems; and (ii) remodelling of neural circuits. These changes make breathing more vulnerable to fail in early postnatal life, which could be related to the pathogenesis of 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.

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.

Sudden neonatal death in PACAP-deficient mice is associated with reduced respiratory chemoresponse and susceptibility to apnoea

Pituitary adenylate cyclase-activating polypeptide (PACAP)-deficient mice are more prone to sudden death during postnatal weeks 1-3 than wild-type littermates. Given that PACAP is localized in brainstem regions associated with respiratory chemosensitivity, we examined whether PACAP-null neonates have reduced respiratory responses to hypoxia and hypercapnia. Using unrestrained, whole-body, flow-through plethysmography we found that, by postnatal day 4, the PACAP-null neonates had significantly reduced ventilation during baseline breathing, and blunted responses to both hypoxia (10% O2-90% N2) and hypercapnia (8% CO2-92% air). To determine whether the respiratory phenotype of the PACAP-null mice may contribute to their greater neonatal mortality, we used ECG to examine respiration and cardiovascular function of littermates. We demonstrate that, under conditions that exacerbate mortality of knockout but not wild-type animals, PACAP-deficient mice experience prolonged apnoeas that precede atrio-ventricular block. Both apnoeas and atrio-ventricular block were absent in wild-type littermates. These data suggest that PACAP-deficiency results in higher neonatal mortality primarily as a result of respiratory control defects and raise the possibility that mutations in genes encoding components of the PACAP signalling pathways may contribute to neonatal breathing disorders in humans.

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.

Neuropharmacology of control of respiratory rhythm and pattern in mature mammals

This review summarizes the current understanding of the neurotransmitters and neuromodulators that are involved, firstly, in respiratory rhythm and pattern generation, where glutamate plays an essential role in the excitatory mechanisms and glycine and gamma-aminobutyric acid mediate inhibitory postsynaptic effects, and secondly, in the transmission of input signals from the central and peripheral chemoreceptors and of motor outputs to respiratory motor neurons. Finally, neuronal mechanisms underlying respiratory modulations caused by respiratory depressants and excitants, such as general anesthetics, benzodiazepines, opioids, and cholinergic agents, are described.

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.