Respiratory effects of the N-methyl-D-aspartate (NMDA) antagonist, MK-801, in intact and vagotomized chronic cats

NMDA receptor modulation and severe acute respiratory syndrome treatment

N-Methyl-D-aspartate (NMDA) subtype of glutamate receptors is expressed in the human lungs and central nervous system.  NMDA receptor potentiation could increase calcium ion influx and promote downstream signaling mechanisms associated with cellular contractions that are disrupted in severe acute respiratory syndrome. Pharmacological effects generated by triggering glutamate receptor function in the brain, coupled with concurrent stimulation of the respiratory tract, may produce a synergetic effect, improving the airway smooth muscle function. A novel multipronged intervention to simultaneously potentiate NMDA receptors expressed both in the central nervous system and airway muscles would be helpful for the treatment of severe acute respiratory syndrome that deteriorates peripheral and central nervous system function before causing death in humans.

Respiratory activity of the neonatal dorsolateral pons in vitro

The lateral and medial parabrachial and the Kölliker-Fuse nuclei (NPB/KF) are well known respiratory modulating centers in adulthood, but their role in neonates is largely unknown. We examined the role of the NPB/KF using hemi-sectioned pons-brainstem-spinal cord preparations in neonatal rats. Electrical stimulation applied at various intensities and delays in relation to the onset of spontaneous inspiratory C4 bursts, evoked transient depression or termination of C4 activity. This depression/termination was greatly attenuated either after perfusion of the NMDA-receptor antagonists (MK-801 or APV) or after microinjecting MK-801 into NPB/KF. Furthermore systemic application of the GABA-A receptor antagonist bicuculline reduced NPB/KF evoked inhibition of the C4 burst. Finally, we identified inspiratory, tonic inspiratory, expiratory, and inspiratory-expiratory (I-E) neurons which was major in the recorded neurons in the NPB/KF using the whole-cell patch-clamp method. MK-801 significantly decreased the driving potential and burst duration of I-E neurons. We conclude that neonatal NPB/KF mediated inspiratory off-switch operates on similar synaptic mechanisms as an adult.

Hypoxic ventilatory response in platelet-derived growth factor receptor-beta-knockout mice

The present study investigated whether the platelet-derived growth factor receptor (PDGFR)-beta-mediated mechanisms are involved in the hypoxic ventilatory response through modulating the N-methyl-D-aspartate (NMDA) function. The ventilatory changes during hypoxic challenge (10% O(2), 30 min) were measured plethysmographically in mice selectively lacking the PDGFR-beta in neurons (KO mice) and in control wild-type mice (WT mice) before and after blockade of NMDA receptors. In baseline breathing at rest, respiratory rate, tidal volume, and minute ventilation were similar between WT and KO mice. Hypoxia caused an increase of ventilation during the early period of exposure (an initial excitation), followed by a progressive decrease along with the exposure period (a late decline). The initial excitation occurred similarly in KO and WT mice, while the late decline was markedly attenuated in KO mice. Administration of an antagonist of NMDA receptors, dizocilpine (0.3 mg/kg, i.p.) decreased the initial excitation and hastened the late decline of hypoxic ventilatory response. Furthermore, the hypoxic ventilatory response in KO mice was indistinguishable from that in WT mice after blockade of NMDA receptors. The present study suggests that the PDGF-BB/PDGFR-beta signal axis contributes to the hypoxic ventilatory response by its inhibitory effect on the NMDA receptor-mediated function.

Spinal NMDA receptor activation is necessary for de novo, but not the maintenance of, A2a receptor-mediated phrenic motor facilitation

Adenosine 2a (A2a) receptor agonists elicit persistent increases in phrenic nerve activity by transactivating the neurotrophin receptor, TrkB, near phrenic motoneurons. Our working model proposes that A2a receptor-mediated TrkB receptor activation strengthens glutamatergic synapses onto phrenic motoneurons. Activation of glutamate N-methyl d-aspartate (NMDA) receptors has been implicated in other models of phrenic motor plasticity. Thus we hypothesized that NMDA receptor activation also would contribute to A2a receptor-mediated phrenic motor facilitation. Adult male Sprague-Dawley rats were anesthetized with urethane, mechanically ventilated, neuromuscularly paralyzed, and bilaterally vagotomized. The A2a receptor agonist CGS-21680 and the NMDA receptor-channel blocker MK-801 were administered intrathecally over the C4 spinal segment. Phrenic nerve activity was recorded before, during, and after drug administration. MK-801 (concentration range 0.1, 1.0, 10.0, and 100 microM) was administered 30 min before CGS-21680 (50 microM). MK-801 dose-dependently blocked A2a receptor-mediated phrenic motor facilitation. When administered at 60 min post-CGS-21680, MK-801 prevented further increases in phrenic nerve activity compared with the CGS-21680 alone (CGS-21680 alone at 120 min: 114 +/- 19%; CGS-21680 and MK-801 at 60 min post-CGS-21680: 61 +/- 11%, above baseline, P < 0.05) but did not return phrenic motor output to baseline values. Our data suggest that NMDA receptor activation is necessary for de novo A2a receptor-mediated phrenic motor facilitation and that the maintenance of preexisting phrenic motor facilitation does not involve NMDA receptor-dependent mechanisms.

Formation and maintenance of ventilatory long-term facilitation require NMDA but not non-NMDA receptors in awake rats

N-methyl-d-aspartate (NMDA) receptor antagonism in the phrenic motonucleus area eliminates phrenic long-term facilitation (pLTF; a persistent augmentation of phrenic nerve activity after episodic hypoxia) in anesthetized rats. However, whether NMDA antagonism can eliminate ventilatory LTF (vLTF) in awake rats is unclear. The role of non-NMDA receptors in LTF is also unknown. Serotonin receptor antagonism before, but not after, episodic hypoxia eliminates pLTF, suggesting that serotonin receptors are required for induction, but not maintenance, of pLTF. However, because NMDA and non-NMDA ionotropic glutamate receptors are directly involved in mediating the inspiratory drive to phrenic, hypoglossal, and intercostal motoneurons, we hypothesized that these receptors are required for both formation and maintenance of vLTF. vLTF, induced by five episodes of 5-min poikilocapnic hypoxia (10% O(2)) with 5-min normoxia intervals, was measured with plethysmography in conscious adult male Sprague-Dawley rats. Either (+/-)-2-amino-5-phosphonovaleric acid (APV; NMDA antagonist, 1.5 mg/kg) or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; non-NMDA antagonist, 10 mg/kg) was systemically (ip) injected approximately 30 min before hypoxia. APV was also injected immediately after or 20 min after episodic hypoxia in additional groups. As control, vehicle was similarly injected in each rat 1-2 days before. Regardless of being injected before or after episodic hypoxia, vehicle did not alter vLTF ( approximately 23%), whereas APV eliminated vLTF while having little effect on baseline ventilation or hypoxic ventilatory response. In contrast, CNQX enhanced vLTF ( approximately 34%) while decreasing baseline ventilation. Collectively, these results suggest that activation of NMDA but not non-NMDA receptors is necessary for formation and maintenance of vLTF in awake rats.

Role of NMDA receptors in development of respiratory control

The N-methyl-D-aspartate (NMDA) receptor has many functions throughout the central nervous system (CNS) including its role within the centers controlling respiration. Although NMDA receptors are important for normal breathing, they are specifically active under conditions of stress, such as hypoxia. Consistent with its role in other neurological functions, the NMDA receptor is also important to the prenatal development of normal neurological pathways for the control of ventilation. The importance of NMDA receptors to both normal breathing and stress responses is demonstrated by recent observations of antenatal effects of disturbances to the NMDA receptor which disrupts normal breathing as well as causing reduced ventilatory responses during stress in newborns. These characteristics fit with the known NMDA influences on neuronal development and plasticity. The methods used to evaluate these functions have mainly included pharmacological agents for activation (agonists) or depression (antagonists) of NMDA receptors. NMDA receptor expression has also been measured histologically, and more recently knockout animal models have been used to provide additional functional information.

Phrenic long-term facilitation requires NMDA receptors in the phrenic motonucleus in rats

Exposure to episodic hypoxia induces a persistent augmentation of respiratory activity, known as long-term facilitation (LTF). LTF of phrenic nerve activity has been reported to require serotonin receptor activation and protein syntheses. However, the underlying cellular mechanism still remains poorly understood. NMDA receptors play key roles in synaptic plasticity (e.g. some forms of hippocampal long-term potentiation). The present study was designed to examine the role of NMDA receptors in phrenic LTF and test if the relevant receptors are located in the phrenic motonucleus. Integrated phrenic nerve activity was measured in anaesthetized, vagotomized, neuromuscularly blocked and artificially ventilated rats before, during and after three episodes of 5 min isocapnic hypoxia (P(a,O2) = 30-45 mmHg), separated by 5 min hyperoxia (50% O2). Either saline (as control) or the NMDA receptor antagonist MK-801 (0.2 mg kg(-1), i.p.) was systemically injected approximately 1 h before hypoxia. Phrenic LTF was eliminated by the MK-801 injection (vehicle, 32.8 +/- 3.7% above baseline in phrenic amplitude at 60 min post-hypoxia; MK-801, -0.5 +/- 4.1%, means +/- S.E.M.), with little change in both the CO2-apnoeic threshold and the hypoxic phrenic response (HPR). Vehicle (saline, 5 x 100 nl) or MK-801 (10 microM; 5 x 100 nl) was also microinjected into the phrenic motonucleus region in other groups. Phrenic LTF was eliminated by the MK-801 microinjection (vehicle, 34.2 +/- 3.4%; MK-801, -2.5 +/- 2.8%), with minimal change in HPR. Collectively, these results suggest that the activation of NMDA receptors in the phrenic motonucleus is required for the episodic hypoxia-induced phrenic LTF.

Central histamine contributes to the inspiratory off-switch mechanism via H1 receptors in mice

Central histaminergic neurons are distributed in areas of the medulla and pons concerned with respiratory rhythm generation, but their effects on breathing pattern are unknown. We examined breathing pattern during hypercapnic responses in wild type (WT) and H1 receptor knockout (H1RKO) mice at 9-10 weeks of age before and after vagotomy. Minute ventilation increased with PaCO(2) increase equally in both genotypes; respiratory rate response was lower and tidal volume (V(T)) response higher in H1RKO mice than in WT mice. The V(T)-inspiratory time (T(I)) relation during hypercapnia was hyperbolic in both groups, with the curve in H1RKO mice shifted right-upward. After vagotomy, the V(T)-T(I) relation was a vertical line, which shifted right in H1RKO mice. We conclude that alterations of inspiratory off-switch and respiratory rhythm generation change breathing pattern without affecting central chemosensitivity in H1RKO. Histamine might affect breathing pattern centrally via H1 receptors.

Effects of NMDA receptor antagonists on opioid-induced respiratory depression and acute antinociception in rats

Although exogenous opioids alter the responses of animals to tissue-damaging stimuli and therefore are the cornerstone in the treatment of acute antinociception, they have profound side effects on ventilation. To diminish ventilatory effects, combination therapies have been advocated. Recent studies reported the effectiveness of the addition of N-methyl-D-aspartate (NMDA) receptor antagonists such as ketamine to morphine in the treatment of acute pain. However, NMDA receptors, together with non-NMDA receptors are known to be involved in the neurotransmission of inspiratory drive to phrenic motoneurons. Co-administration of NMDA and non-NMDA receptor antagonists has been shown to be deleterious to respiratory function. The present study investigated the hypothesis that the association of opioids and NMDA receptor antagonists may add to the impairment of respiratory parameters. In male Wistar rats, combinations of opioids (fentanyl or morphine) at antinociceptive doses and NMDA receptor antagonists (ketamine, 40 mg/kg, or dextromethorphan, 10 mg/kg) at subanesthetic doses were administered intraperitoneally. Antinociception was tested with the tail-withdrawal reaction (TWR) test, while the effect on respiratory parameters was investigated with blood-gas analysis. We found that, in rats, co-administration of NMDA receptor antagonists and opioids may result in an increased respiratory depression as compared to the opioids alone. The effect of the NMDA receptor antagonists on opioid-induced antinociception was limited.

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.

Respiratory network function in the isolated brainstem-spinal cord of newborn rats

The in vitro brainstem-spinal cord preparation of newborn rats is an established model for the analysis of respiratory network functions. Respiratory activity is generated by interneurons, bilaterally distributed in the ventrolateral medulla. In particular non-NMDA type glutamate receptors constitute excitatory synaptic connectivity between respiratory neurons. Respiratory activity is modulated by a diversity of neuroactive substances such as serotonin, adenosine or norepinephrine. Cl(-)-mediated IPSPs provide a characteristic pattern of membrane potential fluctuations and elevation of the interstitial concentration of (endogenous) GABA or glycine leads to hyperpolarisation-related suppression of respiratory activity. Respiratory rhythm is not blocked upon inhibition of IPSPs with bicuculline, strychnine and saclofen. This indicates that GABA- and glycine-mediated mutual synaptic inhibition is not crucial for in vitro respiratory activity. The primary oscillatory activity is generated by neurons of a respiratory rhythm generator. In these cells, a set of intrinsic conductances such as P-type Ca2+ channels, persistent Na+ channels and G(i/o) protein-coupled K+ conductances mediates conditional bursting. The respiratory rhythm generator shapes the activity of an inspiratory pattern generator that provides the motor output recorded from cranial and spinal nerve rootlets in the preparation. Burst activity appears to be maintained by an excitatory drive due to tonic synaptic activity in concert with chemostimulation by H+. Evoked anoxia leads to a sustained decrease of respiratory frequency, related to K+ channel-mediated hyperpolarisation, whereas opiates or prostaglandins cause longlasting apnea due to a fall of cellular cAMP. The latter observations show that this in vitro model is also suited for analysis of clinically relevant disturbances of respiratory network function.

Inspiration-promoting vagal reflex under NMDA receptor blockade in anaesthetized rabbits

1. This study describes a novel vagal respiratory reflex in anaesthetized rabbits. In contrast to the well-known inspiratory (I) off-switching by vagal afferent excitation, this vagal reflex initiates and maintains the central I activity of phrenic nerve discharges in rabbits pre-treated with antagonists of N-methyl-D-aspartate-type excitatory amino acid receptors (NMDA-Rs). 2. Under NMDA-R blockade with either dizocilpine (0.025-0.3 mg kg-1), D-2-amino-5-phosphonopentanoic acid (AP5, 0.5-1 mg, i.c.v.) or ketamine (10 mg kg-1), vagal stimulation at low frequencies (5-40 Hz) during the I phase prevented or markedly delayed the spontaneous I termination. In contrast, stimulation of the same vagal afferent at the same intensity but at a higher frequency (100-160 Hz) during the I phase immediately terminated the I phase. 3. In non-vagotomized rabbits, maintaining the tidal volume at end-expiratory levels during the I phase prevented spontaneous I termination and maintained apneusis after NMDA-R blockade with dizocilpine. 4. Brief stimulation of vagal afferents at low frequency (5-40 Hz) during the expiratory (E) phase constantly initiated phrenic I discharge after NMDA-R block. 5. We conclude that low-frequency discharge of vagal pulmonary stretch receptor afferents, as when lung volume is near functional residual capacity, promotes central I activity under NMDA-R blockade.

Modulation of hypoxic ventilatory response by systemic platelet-activating factor receptor antagonist in the rat

Platelet activating factor (PAF) has recently emerged as an important modulator of neuronal excitability by enhancing synaptic glutamate release. Since PAF receptors (PAFR) are ubiquitously distributed in the brain, we hypothesized that PAF may play a role in respiratory control. To examine this issue, hypoxic (10% O2 for 15 min, n = 14) and hypercapnic (5% CO2 for 30 min, n = 6) challenges were performed in chronically-instrumented, unrestrained adult rats following administration of the pre-synaptic PAFR antagonist BN52021 (i.p. 20 mg/kg in 0.5 ml) or vehicle (Veh). In normoxia, BN52021 elicited VT decreases and corresponding f increases such that minute ventilation (VE) was unaffected. During hypercapnia, peak VE increased similarly after both treatments (103+/-18% in BN52021 vs. 94+/-19% in Veh, p-NS). In contrast, significant reductions in the peak hypoxic VE response occurred after BN52021 (42+/-10% vs. 104+/-18% in Veh, P<0.002). BN52021 increased normoxic arterial blood pressure and decreased heart rate. However, hypoxia-induced chronotropic responses were attenuated and depressor responses were enhanced by BN52021. We further examined protein kinase C (PKC) translocation patterns during acute hypoxia after systemic BN52021 administration. Activation of PKC beta and delta was blocked by BN52021, PKC gamma was attenuated, with no effects on PKC alpha, epsilon, theta, iota, mu, and zeta. We conclude that systemic administration of a PAFR antagonist attenuates cardioventilatory recruitment to hypoxia and selectively attenuates activation of PKC in the rat brainstem. We speculate that enhanced regional PAF production and release during hypoxic conditions may contribute important excitatory inputs and signal transduction pathways within neuronal structures underlying cardiovascular and respiratory control.

Respiratory depression produced by intravenously administered NBQX

To determine whether blockade of non-N-methyl-D-aspartate (non-NMDA) excitatory amino acid receptors affects breathing, we administered the non-NMDA receptor antagonist, 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline (NBQX), to anesthetized cats while monitoring phrenic nerve discharge, blood pressure and heart rate. NBQX, 3 and 10 mg/kg, i.v., reduced phrenic amplitude 59 +/- 20% (n = 3) and 88 +/- 6% (n = 5), respectively, and decreased respiratory rate. Phrenic activity was completely silenced in 3 animals. These effects were accompanied by decreased blood pressure and heart rate. Our data indicate that NBQX, a competitive antagonist of non-NMDA receptors, is a powerful depressant of cardiorespiratory activity.

Respiratory effects of glutamate receptor antagonists in neonate and adult mammals

We determined the conditions (immaturity, species, anesthesia, receptor blockade selectivity) under which glutamate receptor blockade produces respiratory depression in mammals. In unrestrained 0- to 2-day-old neonate and adult mice and cats, ventilation was measured by the barometric method, before and after separate or sequential administration of a non-NMDA receptor antagonist, NBQX (2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(F)quinoxaline, 2-200 mg kg(-1) in mice, 10-40 mg kg(-1) in cats), and a NMDA receptor antagonist, dizocilpine (3 mg kg(-1) in mice, 0.15-1.0 mg kg(-1) in cats). NBQX or dizocilpine alone did not decrease ventilation in awake adults, but NBQX strongly depressed ventilation in neonate awake mice and in adult anesthetized animals. Given together, dizocilpine and NBQX always profoundly depressed ventilation by producing a lethal apnea in neonate mice, and an apneustic pattern of breathing in adults of both species and in neonate cats. We conclude that blockade of either NMDA or non-NMDA receptors is innocuous in awake adults. The factors which may potentiate respiratory depression are (1) anesthesia, (2) immaturity, and (3) combined blockade of both receptors types. The mechanism of depression is species-dependent and age-dependent.

NMDA receptors mediate peripheral chemoreceptor afferent input in the conscious rat

N-methyl-D-aspartate (NMDA) glutamate receptors mediate critical components of cardiorespiratory control in anesthetized animals. The role of NMDA receptors in the ventilatory responses to peripheral and central chemoreceptor stimulation was investigated in conscious, freely behaving rats. Minute ventilation (VE) responses to 10% O2, 5% CO2, and increasing intravenous doses of sodium cyanide were measured in intact rats before and after intravenous administration of the NMDA receptor antagonist MK-801 (3 mg/kg). After MK-801, eupcapnic tidal volume (VT) decreased while frequency increased, resulting in a modest reduction in VE. Inspiratory time (TI) decreased, whereas expiratory time remained unchanged. The VE responses to hypercapnia were qualitatively similar in control and MK-801 conditions, with slight reductions in respiratory drive (VT/TI) after MK-801. In contrast, responses to hypoxia were markedly attenuated after MK-801 and were primarily due to reduced frequency changes, whereas VT was unaffected. Sodium cyanide doses associated with significant VE increases were 5 and 50 microg/kg before and after MK-801, respectively. Thus 1-log shift to the right of individual dose-response curves occurred with MK-801. Selective carotid body denervation reduced VE during hypoxia by 70%, and residual hypoxic ventilatory responses were abolished after MK-801. These findings suggest that, in conscious rats, carotid and other peripheral chemoreceptor-mediated hypoxic ventilatory responses are critically dependent on NMDA receptor activation and that NMDA receptor mechanisms are only modestly involved during hypercapnia.

Effects of NMDA and non-NMDA receptor antagonists on the medullary inspiratory neuronal activity during spontaneous augmented breaths in anesthetized rats

To elucidate whether there is a difference between the effects of iontophoretically applied N-methyl-D-aspartate (NMDA) and non-NMDA receptor antagonists on the activity of inspiratory neurons during spontaneous augmented breaths, extracellular single unit recording of inspiratory neurons (I-augmenting, I-decrementing and I-other) was performed in pentobarbital anesthetized rats. The spontaneous augmented breath was divided into two different phases; the first phase (phase I) resembled a normal inspiration, but the second phase (phase II) consisted of a marked increase in diaphragm electromyogram activity. The mean firing frequency of I-aug type neurons was significantly decreased after 50 nA application of both D-2-amino-5-phosphonopentanoic acid (AP-5) (NMDA receptor antagonist) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (non-NMDA receptor antagonist). The mean firing frequency of both I-dec and I-other neurons was significantly decreased by both AP-5 and CNQX applications (70 nA). After AP-5 application, relative changes in the discharge rates during inspiratory phases I and II of spontaneous augmented breaths were significantly suppressed in all types of neurons, but CNQX application had no significant effect on the response changes during phase II. In all cell types of neurons, a significant difference between the iontophoretic AP-5 and CNQX applications in the relative mean firing rate was observed. These results suggested that activation of the NMDA receptor-induced neurotransmission can modify the discharge rate of medullary inspiratory neurons, irrespective of the cell types, during the inspiratory phase II of spontaneous augmented breaths, but that non-NMDA receptor blockade may not significantly influence their discharge rate.

Does dizocilpine (MK-801) inhibit the development of morphine-induced behavioural sensitization in rats?

Intermittent morphine pretreatment (10 mg/kg/day for 14 days) induced long-lasting (one month post-treatment) sensitization to the locomotor effects of morphine and amphetamine in rats. Co-administration of the non-competitive NMDA-receptor antagonist dizocilpine (MK-801) (0.1 mg/kg) with morphine did not prevent the development of long-term behavioural sensitization. However, this dose of MK-801 did cause long-term sensitization to its own locomotor effects. Co-administration of 0.25 mg/kg MK-801 with morphine caused death in 60% of the animals. In the animals that survived MK-801 plus morphine pretreatment, neither short-term (3 days) nor long-term morphine-induced sensitization was observed. MK-801 alone (0.25 mg/kg/day for 14 days) induced short-term cross-sensitization to morphine. Thus, the development of long-term morphine-induced locomotor sensitization could only be prevented by a dose of MK-801 that yields a lethal combination with morphine. In addition, MK-801 induced sensitization to its own locomotor effects and cross-sensitization to morphine. These findings seriously question whether MK-801 can be used to study the development of morphine-induced behavioural sensitization.

Membrane potentials of respiratory neurones during dizocilpine-induced apneusis in adult cats

1. In the vagotomized cat, blockade of NMDA receptors by dizocilpine (MK-801) produces an apneustic pattern of respiration characterized by a large increase in the duration of inspiration. 2. To identify dizocilpine-induced disfacilitations and disinhibitions in respiratory neurones generating the respiratory rhythm, membrane potential and input resistance of augmenting inspiratory (I; n = 11) and post-inspiratory (PI; n = 9) neurones were examined in the ventral respiratory group area, before and after administration of dizocilpine (0.1-0.3 mg kg-1 i.v.) in decerebrate, vagotomized, paralysed and artificially ventilated cats. 3. In I neurones, dizocilpine decreased the ramp depolarization and an 82% increase in input resistance was observed during inspiration. The inspiratory phase was prolonged, leading to a sustained level of depolarization during apneusis. The amplitude of stage 1 expiratory hyperpolarization decreased and its decay, which is normally slow, was faster. Throughout the remainder of expiration (stage 2) the membrane potential levelled off and the input resistance increased slightly (by 15%). 4. In PI neurones, dizocilpine depressed depolarization and suppressed firing in eight out of nine cells during the stage 1 expiratory phase. This was associated with a large (91%) increase of input resistance. The membrane potential switched quickly to stage 2 expiratory repolarization, during which a slight (19%) increase in input resistance occurred. 5. The hyperpolarization of PI neurones during early inspiration was reduced in amplitude by dizocilpine and input resistance was increased by 75% during inspiration, indicating that dizocilpine reduced the activity of the presynaptic inhibitory early-inspiratory (eI) neurones. 6. We conclude that NMDA receptor blockade in the respiratory network disfacilitates eI, I and PI neurones during their active phase. Decreased inhibitory processes during the inspiratory phase probably play a major role in the prolongation of inspiration.

Involvement of NMDA receptors in inspiratory termination in rodents: effects of wakefulness

We investigated the role of N-methyl-D-aspartate (NMDA) receptors in the off-switching of inspiration in rodents. Respiratory activity was measured by the plethysmographic method in Swiss and Balb c mice, Hartley guinea pigs, Wistar and Sprague-Dawley rats. The NMDA channel blocker dizocilpine (MK-801) administered systemically, had little effect on the timing of respiratory phases in intact animals. When dizocilpine was associated with a vagotomy performed under anesthesia, an apneustic respiratory pattern was obtained in all species and strains. As the anesthetic dissipated, the inspiratory pauses disappeared and the apneustic respiratory pattern was replaced by an eupneic respiratory pattern. Apneuses were re-instated by small doses of anesthetic (halothane, pentobarbital, alphaxolone-alphadolone or chloral hydrate) and suppressed by larger doses. We conclude that (i) the central NMDA-receptor dependent inspiratory off-switching mechanism previously described in cats and primates, also exists in rodents; (ii) wakefulness maintains a normal respiratory pattern after suppression of both the NMDA-receptor mediated and the vagally-mediated off-switching mechanisms; (iii) deep anesthesia suppresses inspiratory pauses in rodents.

NMDA receptors are involved at the ventrolateral nucleus tractus solitarii for termination of inspiration

The purpose of the present study was to determine whether blockade of excitatory amino acid receptors at the ventrolateral nucleus of the tractus solitarius would influence respiratory activity. This was done by microinjecting excitatory amino acid receptor antagonists into the ventrolateral nucleus of the tractus solitarius of alpha-chloralose-anesthetized animals while monitoring respiratory activity using a Fleisch pneumotachograph and arterial blood pressure and heart rate. Bilateral microinjection of the NMDA receptor antagonist, 3-[(R)-carboxypiperazin-4-yl]-propyl-1- phosphomic acid (CPP), 5.62 nmol per side, produced an increase in inspiratory duration (+4 +/- 1.6 s, n = 8) which progressed to an apneustic pattern of breathing. Similar results were obtained with CPP microinjected into the ventrolateral nucleus of the tractus solitarius of three vagotomized animals. Bilateral microinjection of a second NMDA receptor antagonist, 2-amino-7-phosphono-heptanoic acid (AP7), 562 nmol per side, produced qualitatively similar effects on respiration as seen with CPP. In contrast, blockade of non-NMDA receptors with 6-cyano-7-nitroquinoxaline-2,3-dione (CNXQ), 0.125 nmol per side, had very little effect on respiration. Activation of NMDA receptors at the ventrolateral nucleus of the tractus solitarius with bilateral microinjection of NMDA, 39 pmol, produced a large increase in expiratory duration (+11 +/- 3 s, n = 8), and apnea during the expiratory phase of the respiratory cycle in half of the animals studied. Similar results were obtained with D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazol-proprionate (AMPA). These results indicate that an endogenous excitatory amino acid released at the ventrolateral nucleus of the tractus solitarius and acting at the NMDA receptor, plays a significant role in respiratory timing.

Alteration by urethane of glutamatergic control of micturition

The i.v. administration of MK-801 (0.001-3 mg/kg), a non-competitive NMDA receptor antagonist, did not alter reflex bladder activity in unanesthetized decerebrate rat recorded during fast infusion (0.21 ml/min) cystometry or under isovolumetric conditions, but did depress reflex bladder contractions in doses between 0.1 and 3 mg/kg i.v. in the urethane-anesthetized (1.2 g/kg s.c.) intact rat during fast infusion cystometry. The ED50 and the dose to produce maximal inhibition in urethane-anesthetized intact rats were 0.25 mg/kg and 3 mg/kg i.v., respectively. During slow infusion (0.04 ml/min) cystometry, in unanesthetized decerebrate rats, MK-801 (0.1-1 mg/kg i.v. or 6-60 micrograms i.t.) decreased by 12-44% the micturition volume threshold (VT) but did not change the amplitude and duration of the bladder contractions. The administration of a larger i.t. dose (60 micrograms) of MK-801 produced no further decrease in VT but decreased the amplitude of bladder contractions by 24%. External urethral sphincter electromyogram activity was reduced or abolished by MK-801 (0.01-3 mg/kg i.v.) in both unanesthetized decerebrate and urethane-anesthetized intact rats with ED50 of 0.12 mg/kg and 0.05 mg/kg, respectively. These results indicate that NMDA receptors play an important role in both facilitatory and inhibitory central neural control of voiding function and that there is a significant interaction between urethane anesthesia and NMDA glutamatergic transmission. Thus, even though urethane anesthesia has been useful for studying the physiological characteristics of the micturition reflex, it seems inappropriate for analyzing the normal transmitter role of glutamic acid in reflex voiding.

Brainstem network controlling descending drive to phrenic motoneurons in rat

Contraction of the diaphragm is controlled by phrenic motoneurons that receive input from sources that are not fully established. Bulbospinal (second-order) neurons projecting to phrenic motoneurons and propriobulbar (third-order) neurons projecting to these bulbspinal neurons were investigated in rat by transsynaptic transport of the neuroinvasive pseudorabies virus. Bulbospinal neurons were located predominantly in the medullary lateral tegmental field in two functionally described regions, the ventral respiratory group and Bötzinger complex. An intervening region, the pre-Bötzinger complex, contained essentially no phrenic premotoneurons. Bulbospinal neurons were also located in ventral, interstitial, and ventrolateral subnuclei of the solitary tract, and gigantocellular, Kölliker-Fuse, parabrachial, and medullary raphe nuclei. A monosynaptic pathway to phrenic motoneurons from the nucleus of the solitary tract was confirmed; monosynaptic pathways from upper cervical spinal cord, spinal trigeminal nucleus, medical and lateral vestibular nuclei, and medial pontine tegmentum were not verified. Locations of third-order neurons were consistent with described projections to the ventral respiratory group, from contralateral ventral respiratory group, Bötzinger complex, A5 noradrenergic cell group, and the following nuclei; solitary, raphe, Kölliker-Fuse, parabrachial, retrotrapezoid, and paragigantocellular. Novel findings included a projection from locus coeruleus to respiratory premotoneurons and the lack of previously described pathways from area postrema and spinal trigeminal nucleus. These second- and third-order neurons from the output network for diphragm motor control which includes numerous behaviors (e.g., respiration, phonation, defecation). Of the premotoneurons, the rostral ventral respiratory group is the primary population controlling phrenic motoneurons.

The roles of K+ conductance in expiratory pattern generation in anaesthetized cats

1. The potassium current blockers caesium and tetraethylammonium were injected intracellularly by ionophoretic current into brainstem expiratory neurones of the ventral group. Neurones were identified by their spontaneous activity and by antidromic excitation from the spinal cord at the C2-C3 level. 2. The duration of action potentials increased and the early and late after-hyperpolarizations were completely suppressed. These effects on action potentials were reversible, recovered with an exponential time course within 3 min, and could be reproduced when blockers were applied repetitively into the same neurone. They were ascribed to blockade of potassium channels in the somatic membrane region. 3. Potassium channel blockers modified postsynaptic potentials: early-inspiratory hyperpolarizations were reversibly depressed while postinspiratory and expiratory depolarizations were irreversibly enhanced. The former effect was associated with a decrease of the neuronal input conductance. The latter effect was cumulative upon repetitive ionophoretic applications of potassium blockers. 4. The results demonstrate that potassium currents exert two different roles in expiratory pattern generation. Together with chloride currents, they contribute to the phasic early-inspiratory inhibition. They seem to be calcium-dependent and GABAB receptor-controlled currents which predominate near to the cell body. 5. Potassium currents also operate throughout the postinspiratory and late-expiratory periods. They seem to include persistent potassium currents which modulate the excitatory respiratory drive provided by the respiratory rhythm generator. We assume that these currents, widely distributed over the somatodendritic membrane area, are a target for neuromodulation by transmitters and intracellular second messengers.

Metabotropic glutamate receptors are involved in the control of breathing at the medulla oblongata level of anaesthetized rats

The goal of the present study was to identify sites in the medulla oblongata where metabotropic glutamate receptors are involved in regulating respiration. Unilateral microinjections (50 nl) of L-glutamate (L-glu) (10-25-50 mM) into the nucleus tractus solitarii (NTS) of anaesthetized rats elicited apnea (8.6 +/- 0.3 sec; 21.3 +/- 3.6 sec; 66.3 +/- 16.5 sec respectively; N = 6) and arterial hypotension (7.3 +/- 2.4 mmHg; 10.1 +/- 2.3 mmHg; 35.3 +/- 7.5 mmHg respectively; N = 6). Similarly, in other rats 1-aminocyclopentane-1, 3-dicarboxylic acid (ACPD) (1-5-10 mM), a selective agonist of metabotrophic glutamate receptors, also induced apnea (22.4 +/- 2.5 sec; 32.5 +/- sec; 92.5 +/- 1.4 sec respectively; N = 6) and arterial hypotension (12.7 +/- 2.2 mmHg; 19.6 +/- 4.3 mmHg; 26.5 +/- 1.5 mmHg respectively; N = 6). Paired experiments showed that unilateral microinjections of L-glu (50 mM) and ACPD (1 mM) into the nucleus retroambigualis (NRA) of anaesthetized rats elicited apnea (20.2 +/- 2.6 sec and 33.8 +/- 3.2 sec respectively; N = 6) and arterial hypotension (15.7 +/- 3.7 mmHg and 22.5 +/- 4.5 mmHg respectively; N = 6). The ACPD effects on apnea and hypotension in NTS and NRA were not prevented by a 3 min pretreatment with L-AP3 (30 mM), a putative antagonist of metabotropic glutamate receptors (19.5 +/- 1.4 sec; 12.3 +/- 3.2 mmHg and 30.6 +/- 2.9 sec; 23.4 +/- 3.8 mmHg respectively; N = 6). These data suggest that metabotropic glutamate receptors are involved in NTS and NRA regulation of cardiorespiratory functions.(ABSTRACT TRUNCATED AT 250 WORDS)

NMDA and non-NMDA receptors may play distinct roles in timing mechanisms and transmission in the feline respiratory network

1. Activation of N-methyl-D-aspartate (NMDA) glutamate receptors in the brainstem network of respiratory neurones is required to terminate inspiration in the absence of lung afferents, but it is not required in the inspiratory motor act of lung inflation. In the present study we examined the involvement of non-NMDA ionotropic glutamate receptors in these two mechanisms in the adult mammal. 2. Adult cats were either decerebrated or anaesthetized with sodium pentobarbitone, paralysed and ventilated. Inspiratory motor output was recorded from the phrenic nerve and central respiratory activity from neurones in the bulbar ventral respiratory group. 3. In decerebrate vagotomized cats, ionophoretic application of 2,3-dihydroxy-6-nitro-7-sulphamoylbenzo(F)quinoxaline (NBQX) onto single respiratory neurones decreased their spontaneous discharge rate and abolished the excitatory effect of exogenously applied (RS) alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) but not NMDA. 4. In these animals, intravenous infusion (12 mg kg-1) of the non-NMDA receptor blockers GYKI 52466 (1-(4-aminophenyl)-4-methyl-7,8-methylene-dioxy-5-H-2,3-benzodi aze pine) or NBQX: (1) decreased (in 10/15 cats) or abolished (in 5/15 cats) the inspiratory-related discharge of the phrenic nerve; (2) did not prolong the inspiratory phase; (3) reduced or abolished the spontaneous discharge of respiratory neurones; and (4) profoundly decreased the excitatory effects of AMPA but not NMDA ionophoresed onto these neurones. When both the phrenic nerve and the recorded respiratory neurone were silenced, neuronal excitation by ionophoretic application of NMDA first revealed a subthreshold respiratory modulation without lengthening of the inspiratory phase, then respiratory modulation became undetectable. 5. Additional blockade of NMDA receptors by a small dose (0.15 mg kg-1) of dizocilpine (MK-801), abolished the phrenic nerve activity which persisted after NBQX (apnoea), but the discharge or the subthreshold modulation of the bulbar respiratory neurones showed a lengthening of the inspiratory phase (apneusis). 6. Elevation of FA,CO2 increased or re-established phrenic nerve discharges after blockade of non-NMDA receptors or of both NMDA and non-NMDA receptors. 7. Small doses of NBQX or GYKI 52466 induced apnoea in five of five cats anaesthetized with sodium pentobarbitone. 8. In decerebrate animals with intact vagi, GYKI 52466 and NBQX depressed the Hering-Breuer expiratory-lengthening reflex. 9. The results suggest that: (1) there is a specialization of different classes of glutamate receptors participating in timing mechanisms and transmission within the mammalian respiratory network. Neural transmission predominantly involves activation of non-NMDA receptors, acting in synergy with NMDA receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Potentiation of MK-801-induced breathing impairment by 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline

The purpose of our study was to examine whether a significant interaction occurs between NMDA and non-NMDA receptor antagonists on respiratory function. For this purpose chloralose-anesthetized cats were used and respiratory minute volume (VE), tidal volume (Vt) respiratory rate (f), inspiratory and expiratory durations, and end tidal CO2 (FeCO2) were monitored. In some animals, phrenic nerve activity was also continuously recorded. In five spontaneously breathing animals, the NMDA receptor antagonist MK-801 was administered in a dose of 0.1 mg/kg i.v., and produced decreases in VE, Vt, f and increases in inspiratory duration and FeCO2. Using these five animals exhibiting respiratory effects from prior MK-801 dosing, we then administered the non-NMDA receptor antagonist NBQX (2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline) i.v. in a dose of 3 mg/kg. This dose is too low to produce a neuroprotective effect in animal models of brain ischemia. In each of the five animals NBQX administration produced an immediate impairment of respiration, culminating in apneusis within 55 s after i.v. injection. In terms of phrenic nerve discharge, inspiratory duration was increased approximately 4-fold by MK-801, and with the addition of NBQX, continuous discharge of the phrenic nerve occurred. Finally, NBQX given i.v. to animals not pretreated with MK-801 had only a slight depressant effect on respiratory activity. These results obtained with co-administration of low doses of two drugs that block NMDA and non-NMDA receptors raise the spector that combined use of these agents to ameliorate disorders in neurological function may be extremely deleterious to respiratory function.

Cardiorespiratory effects produced by blockade of excitatory amino acid receptors in cats

The aim of our study was to determine the role of excitatory amino acids in controlling cardiorespiratory activity. For this purpose we administered an antagonist of both N-methyl-D-aspartate (NMDA) and non-NMDA receptors (kynurenic acid), and an antagonist of the NMDA receptor complex (dizocilpine, more commonly known as MK-801) i.v. to chloralose-anesthetized cats while monitoring tracheal air flow, tidal volume, respiratory rate, inspiratory and expiratory durations, end tidal CO2, arterial blood pressure and heart rate. Administration of kynurenic acid in doses of 350 and 500 mg/kg produced respiratory depression as reflected by decreases in respiratory minute volume and increases in end tidal CO2. Inspiratory duration was increased with both doses and apnea (occurring during expiration) was observed with the high dose. Apnea was preceded by an apneustic pattern of breathing. Both doses resulted in an increase in blood pressure and, with the high dose, a later decrease in blood pressure was noted. Dizocilpine in doses ranging from 0.03 to 1 mg/kg produced dose-related decreases in respiratory minute volume, and increases in end tidal CO2. In addition, dizocilpine produced increases in inspiratory duration, an apneustic pattern of breathing and apnea (occurring during inspiration). Effects on blood pressure were similar to those observed with kynurenic acid. It is concluded that blockade of excitatory amino acid receptors results in pronounced effects on cardiorespiratory activity.

Effects of GABAB receptor agonists and antagonists on the bulbar respiratory network in cat

We examined the involvement of the GABAB receptor in central respiratory mechanisms. Respiratory neurons (RNs) from the ventral respiratory group in the medulla of the cat were subjected to iontophoretic applications of the GABAB receptor agonist baclofen and the antagonists saclofen and CGP 35348. In all types of RNs baclofen decreased the firing rate. This reduction was antagonized by CGP 35348. Application of either antagonist increased the spontaneous discharge in both inspiratory and expiratory RNs. CGP 35348 excited 57% of the neurons tested, on the average by 34% with ejection currents of 100 nA. Saclofen excited 6 of 9 neurons tested. Baclofen administered systemically (8-12 mg/kg i.v.) to either anesthetized, decerebrate or intact freely moving cats, induced a selective lengthening of the inspiratory phase, an effect comparable to the apneusis induced by the NMDA antagonist MK-801. Baclofen also produced either a pronounced decrease in the amplitude of phrenic nerve discharge or an apnea, both of which were reversed by increasing paCO2. The results suggest that endogenously released GABA acting on GABAB receptors may be involved in the control of respiratory neuronal discharge.

Effects of NMDA on respiratory neurons in newborn rat medulla in vitro

In 30 brain stem-spinal cord preparations isolated from newborn rats, we examined effects of 10 microM N-methyl-D-aspartate (NMDA) on 21 pre-inspiratory (Pre-I) and 26 inspiratory (I) neurons during perfusion in a synaptic blockade (low Ca2+, high Mg2+) bath, referred to here as low Ca. Application of NMDA to 9 Pre-I neurons that burst rhythmically by NMDA. Application of NMDA had no effect on 20 I neurons out of 26 tested. NMDA made one I neuron that was otherwise silent burst rhythmically in low Ca. The results suggest that, in this preparation, activation of NMDA receptors could contribute to induction or facilitation of burst activity by respiratory neurons, especially Pre-I neurons.

Blockade of NMDA receptor-channels by MK-801 alters breathing in adult rats

The role of N-methyl-D-aspartate (NMDA) receptor-channel activation in the production of respiratory pattern was studied by administration of the NMDA receptor-channel blocker (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801, 1-3 mg/kg, i.v.) to anesthetized adult rats. This dose of MK-801 blocked the excitatory effects of NMDA (applied iontophoretically) on brainstem respiratory neurons. The predominant respiratory response to systemic MK-801 administration was an increase in inspiratory duration and a decrease in amplitude of diaphragm electromyogram and phrenic nerve discharge. Effects on inspiratory timing and amplitude were most pronounced when the rats were vagotomized. Significant changes in arterial blood gases and pH after systemic MK-801 administration in spontaneously breathing rats (vagi intact or cut) indicated that ventilation was depressed by NMDA receptor-channel antagonism. Respiratory timing changes in response to systemic MK-801 administration differed between two rat strains studied. Breathing patterns resembling apneusis, i.e., with irregular inspiratory durations prolonged 2- to 30-fold, occurred in 60% of the vagotomized, spontaneously breathing Sprague-Dawley rats and none of the Wistar rats. Thus, the breathing pattern in Sprague-Dawley rats is more sensitive to interference with NMDA-mediated mechanisms. We propose that respiratory pattern generation and transmission of rhythmic respiratory drive are mediated by synergistic activation of NMDA and non-NMDA receptors at brainstem and spinal cord sites.

The bulbar network of respiratory neurons during apneusis induced by a blockade of NMDA receptors

Our aim was to study the mechanisms producing the transition from the inspiratory phase to the expiratory phase of the breathing cycle. For this purpose we observed the changes affecting the discharge patterns and excitabilities of the different types of respiratory neurons within the respiratory network in cat medulla, after inducing an apneustic respiration with the N-methyl-D-aspartate (NMDA) antagonist MK-801 given systemically. Respiratory neurons were recorded extracellularly through the central barrel of multibarrelled electrodes, in the ventral respiratory area of pentobarbital-anesthetized, vagotomized, paralyzed and ventilated cats. Inhibitions exerted on each neuron by the pre-synaptic pools of respiratory neurons were revealed when the neuron was depolarized by an iontophoretic application of the excitatory amino-acid analogue quisqualate. Cycle-triggered time histograms of the spontaneous and quisqualate-increased discharge of respiratory neurons were constructed in eupnea and in apneusis induced with MK-801. During apneustic breathing, the activity of the respiratory neuronal network changed throughout the entire respiratory cycle including the post-inspiratory phase, and the peak discharge rates of all types of respiratory neurons, except the late-expiratory type, decreased. During apneusis, the activity of the post-inspiratory neuronal pool, the post-inspiratory depression of other respiratory neurons, and the phrenic nerve after-discharge were reduced (but not totally suppressed), whereas the discharge of some post-inspiratory neurons shifted into the apneustic plateau. The shortened post-inspiration (stage 1 of expiration) altered the organization of the expiratory phase. Late-expiratory neurons (stage 2 of expiration) discharged earlier in expiration and their discharge rate increased. The inspiratory on-switching was functionally unaffected. Early inspiratory neurons of the decrementing type retained a decrementing pattern followed by a reduced discharge rate in the apneustic plateau, whereas early-inspiratory neurons of the constant type maintained a high discharge rate throughout the apneustic plateau. Inspiratory augmenting neurons, late-inspiratory and "off-switch" neurons also discharged throughout the apneustic plateau. During the apneustic plateau, the level of activity was constant in the phrenic nerve and in inspiratory neurons of the early-constant, augmenting, and late types. However, progressive changes in the activity of other neuronal types demonstrated the evolving state of the respiratory network in the plateau phase. There was a slowed but continued decrease of the activity of early-inspiratory decrementing neurons, accompanied by an increasing activity and/or excitability of "off-switch", post-inspiratory and late-expiratory neurons. In apneusis there was a decoupling of the duration of inspiration and expiration.(ABSTRACT TRUNCATED AT 400 WORDS)

Stimulant effect of thyrotropin-releasing hormone and its analog, RGH 2202, on the diaphragm respiratory activity, and their antagonism with morphine: possible involvement of the N-methyl-D-aspartate receptors

Thyrotropin-releasing hormone (TRH) was reported to stimulate respiration and abolish the respiratory depressant effect of morphine-like analgesics. Some TRH analogs which have a diminished hormonal activity may be of interest as potential non-specific opioid antagonists. The mechanism of this effect of TRH and its analogs is still unclear. Thus, in the present work the respiratory stimulant effect of TRH and its analog RGH 2202 was studied in the urethane-anesthetized vagotomized artificially-ventilated rats. The integrated diaphragmatic electromyogram was used to evaluate the effects of the drugs. TRH and RGH 2202 administered either i.v. or directly onto the dorsal medullary surface significantly increased the respiratory activity of the diaphragm. TRH and RGH 2202 also effectively antagonized the diaphragm activity depression caused by morphine. The latency, time course and activity of RGH 2202 turned out to be close to those of TRH. The possible involvement of N-methyl-D-aspartate (NMDA) receptors in the mechanism of action of TRH and RGH 2202 was also investigated. It was shown that the non-competitive NMDA antagonists ketamine and MK-801 and the competitive antagonist D-amino-5-phosphonovalerate after local or i.v. administration prevented or discontinued the diaphragm activity stimulation by TRH and RGH 2202. Moreover, they blocked the antagonistic action of TRH and RGH 2202 on the morphine-induced diaphragm activity depression. Thus, we conclude, that TRH and RGH 2202 cause similar stimulant effects on the respiratory activity of the diaphragm and effectively antagonize its depression by morphine. These effects are likely to be mediated by the NMDA receptors located in the central respiratory structures.

Role of excitatory amino acids in the generation and transmission of respiratory drive in neonatal rat

1. The involvement of excitatory amino acids in the generation and transmission of rhythmic respiratory drive was studied in an in vitro neonatal rat brain stem-spinal cord preparation. The subclasses of excitatory amino acid receptors studied included: (i) N-methyl-D-aspartate (NMDA) receptors, (ii) (R, S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid hydrobromide (AMPA) and kainate (non-NMDA) receptors and (iii) 2-amino-4-phosphonobutyric acid (AP-4)-sensitive receptors. Respiratory motoneurone population discharge was recorded from glossopharyngeal (IX), vagus (X), and hypoglossal (XII) cranial nerves, as well as cervical (C1-C5) and thoracic (T2-T5) spinal ventral roots. This activity is generated in the motoneurone pools that transmit respiratory drive to upper airway, accessory, diaphragm and intercostal muscles. Perturbations of motor nerve discharge were analysed after excitatory amino acid receptor antagonists or agonists were added to bathing solutions surrounding either the spinal cord or brain stem. The excitatory amino acid receptor antagonists included: (i) NMDA receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imin-H-maleate (MK-801) and (ii) non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). The agonists included: (i) NMDA, (ii) non-NMDA receptor agonists AMPA and kainic acid. The effects of perturbations of AP-4-sensitive receptors with AP-4, and of inhibiting excitatory amino acid uptake with dihydrokainic acid (DHK) were also studied. 2. Block of non-NMDA receptors in the medulla by CNQX resulted in an antagonist concentration-dependent decrease in the respiratory motoneuronal burst frequency. Non-NMDA receptor activation with kainic acid or AMPA caused a concentration-dependent increase in burst frequency, with competitive interactions with CNQX. 3. Inhibition of excitatory amino acid uptake in the medulla with DHK resulted in a reversible, dose-dependent increase in respiratory frequency. A similar increase in respiratory frequency was induced by DHK when medullary NMDA receptors were blocked with MK-801, confirming that endogenously released excitatory amino acids act at non-NMDA receptors to modulate rhythm. 4. Non-NMDA receptor block reduced and ultimately abolished the amplitude of integrated cranial and spinal respiratory motoneuronal discharge when added to the solution bathing the medulla and spinal cord, respectively. 5. NMDA receptor block in the medulla with MK-801 did not perturb the spontaneous respiratory burst frequency, although bath application of NMDA produced a dose-dependent increase in frequency, with non-competitive interactions with MK-801. MK-801 also did not perturb the amplitude of cranial or bulbospinal premotoneurone discharge.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of MK-801, an NMDA receptor antagonist, on the micturition reflex in the rat

The effects of MK-801 (Dizocilpine) on the micturition reflex were studied in rats anesthetized with urethane (1.2 g/kg, s.c.). MK-801, a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist (1-1000 micrograms/kg, i.v.) reduced, in a dose-dependent fashion the amplitude of rhythmic bladder contractions recorded isovolumetrically via a transurethral catheter. The largest doses (300-1000 micrograms/kg) completely abolished bladder activity. Bilateral section of the hypogastric nerves had no effect on MK-801 induced inhibition. MK-801 did not inhibit the bladder contractions induced by electrical stimulation of the pelvic nerve. These data suggest that MK-801 acts in the central nervous system to block glutaminergic excitatory transmission in the central micturition reflex pathway.

Endogenous activation of NMDA and non-NMDA glutamate receptors on respiratory neurones in cat medulla

The aim of this study was to evaluate the involvement of dicarboxylic amino acid neurotransmission in the periodic discharges of respiratory neurones. Respiratory neurones of the ventral and dorsal respiratory groups in the medulla of the cat were subjected to iontophoretic applications of (1) N-methyl-D-aspartate (NMDA) and a blocker of the NMDA subtype of glutamate receptor, D-2-amino-7-phosphonoheptanoic acid (AP7) and (2) an agonist and an antagonist of the non-NMDA subtypes of receptor: quisqualate and 6,7-dinitroquinoxaline-2,3-dione (DNQX), respectively. All five main types of respiratory neurones (all-, early- and late-inspiratory, transitional "off-switch", late expiratory) were excited by NMDA and quisqualate. Both agonists increased the peak firing rate but exerted different effects on the discharge pattern of respiratory neurones, within the respiratory cycle. Quisqualate induced discharges in the "silent" period of the neurone more readily than did NMDA which, in turn had a more pronounced effect during the burst period of the neurone. The effects of quisqualate and NMDA were suppressed by prior application of their selective antagonists, AP7 and DNQX. These antagonists decreased the spontaneous neuronal discharge of all cell types, throughout the entire firing phase, by a maximum of 24-63% with AP7 and by 30-50% with DNQX. The non-selective antagonist, gamma-D-glutamyl-glycine and the selective NMDA antagonists, CPP and MK-801, were also effective. It is concluded that respiratory neurones, of all types, within the medullary respiratory network are subjected to endogenous glutamate-like excitations, which may possibly shape the respiratory train of action potentials through the sequential activation of non-NMDA and NMDA subtypes of receptor.

Effects of N-methyl-D-aspartate (NMDA) receptor blockade on breathing pattern in newborn cat

We gave newborn kittens the N-methyl-D-aspartate (NMDA) receptor blocker MK-801 systemically while recording their breathing patterns by the barometric method. Unlike pentobarbital, MK-801 at an anaesthetic dose increased the relative length of inspiration within the respiratory cycle. The section of both vagus nerves under MK-801 produced apneustic breathing, whereas vagotomy under pentobarbital had no such effect. We conclude that the central inspiratory-termination mechanism mediated through NMDA receptors and the vagally-mediated mechanism that independently 'switches off' inspiration are both functional at birth.

Pneumotaxic mechanisms in the non-human primate: effect of the N-methyl-D-aspartate (NMDA) antagonist ketamine

We tested the possible involvement of N-methyl-D-aspartate (NMDA) receptors in the central inspiratory-termination mechanism in non-human primates. Inspiratory bursts were recorded from the phrenic nerve in Macaca fascicularis monkeys paralyzed and ventilated by means of a servoventilator driven by the inspiratory discharge of the phrenic nerve. The central inspiratory termination mechanism was tested by withholding lung inflation. This transiently suppressed the vagal feedback from the lungs which produces inspiratory off-switching independent from the central mechanism. Under anaesthesia with ketamine, a potent NMDA antagonist, non inflation increased inspiratory time to 4s (1s with lungs inflated) whereas no such effect was observed during halothane anaesthesia. We conclude that the termination of inspiration in primates is controlled via central mechanisms in which NMDA receptors are involved.

Effects of N-methyl-D-aspartate (NMDA) antagonist MK-801 on breathing pattern in rats

In anaesthetized, bivagotomized and artificially ventilated rats, the respiratory effects of systemic injection of MK-801, a non-competitive N-methyl-D-aspartate antagonist, were studied. In all the experiments (n = 11), the injection increased the inspiratory duration and decreased the expiratory duration. In 4 experiments, the inspiratory duration was drastically lengthened, resulting in an apneustic-like breathing pattern. These results demonstrate that apneusis is difficult but possible to induce in rats and suggest that termination of inspiration is controlled via central mechanisms in which NMDA-like receptors are involved.

Involvement of N-methyl-D-aspartate (NMDA) receptors in respiratory rhythmogenesis

The involvement of N-methyl-D-aspartate (NMDA) subtype of glutamate receptors in the control of inspiratory termination was studied in paralyzed decerebrated cats. Cats were either vagotomized, or had intact vagus nerves and were ventilated with a ventilator driven by the discharge of the phrenic nerve. The systemic administration of NMDA antagonists acting non-competitively (MK-801, ketamine, phencyclidine) or competitively (2-amino-7-phosphonoheptanoic acid: AP7), produced an apneusis in vagotomized animals or in animals transiently deprived of vagal pulmonary feedback by the 'no inflation test'. After NMDA receptor blockade, the inspiratory phase could be terminated by lung inflation or sensory stimulation. Thus pharmacologically distinct mechanisms control the termination of inspiration: vagal afferents which are NMDA-independent, and a central mechanism acting through the activation of NMDA receptors. The apneustic pattern induced by NMDA receptor blockade was characterized by a decrease of the amplitude of integrated phrenic nerve activity, the persistence of CO2 sensitivity and an enhancement of apneusis by anaesthesia. After injection of NMDA antagonists there was a decrease of the duration of expiration which thereafter remained constant and dissociated from inspiratory duration. The possible mechanisms by which NMDA receptors may contribute to respiratory rhythmogenesis are discussed.

Effects of N-methyl-D-aspartate antagonists on the cough reflex

The effects of antagonists of N-methyl-D-aspartate (NMDA) on the capsaicin-induced cough reflex in rats were studied. Intracisternal (i. cist.) injection of MK-801, a non-competitive antagonist of NMDA, significantly decreased the number of coughs in a dose-dependent manner. The competitive antagonists of NMDA, 2-DL-amino-5-phosphonovalerate and 2-DL-amino-7-phosphonoheptanoate, also decreased the number of coughs after i. cist. injection. The antitussive potencies of both the competitive and non-competitive antagonists were similar to that of dextromethorphan. Intraperitoneal injection of MK-801 also decreased the number of coughs in a dose-dependent manner. These data suggest that excitatory amino acid neurotransmitters and NMDA receptors may be involved in the regulation of the cough reflex.

Differentiation of phencyclidine and sigma receptor types affecting the central inspiratory termination mechanism in cat

The effects of 1) the phencyclidine receptor ligand TCP, 2) sigma receptor ligands (+)3-PPP and DTG, and 3) N-methyl-D-aspartate receptor blockers MK-801 and dextrorphan were determined on a brainstem mechanism which controls the termination of the inspiratory phase of the breathing cycle. Inspiratory bursts were recorded from the phrenic nerve in decerebrate paralyzed cats ventilated by means of a phrenic driven servoventilator. The central mechanism which terminates inspiration was tested by withholding lung inflation, thus suppressing the contribution of the vagal feedback from the lungs to inspiratory termination. TCP increased the duration of test inspiration (tTi) by 17% at 0.03 mg/kg and by 14-fold (from 1.6 to 23 s) at 1 mg/kg. With dextrorphan, tTi was significantly increased at 3 mg/kg. In contrast, (+)3-PPP and DTG did not increase tTi at doses up to 10 mg/kg, although MK-801 (0.03 mg/kg), given after the sigma ligands, increased tTi by 59-90%. It is concluded that phencyclidine but not sigma receptor ligands block the central mechanism which terminates inspiration and that the likely site of action is the NMDA receptor complex.