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

The role of neuronal excitation and inhibition in the pre-Bötzinger complex on the cough reflex in the cat

Brainstem respiratory neuronal network significantly contributes to cough motor pattern generation. Neuronal populations in the pre-Bötzinger complex (PreBötC) represent a substantial component for respiratory rhythmogenesis. We studied the role of PreBötC neuronal excitation and inhibition on mechanically induced tracheobronchial cough in 15 spontaneously breathing, pentobarbital anesthetized adult cats (35 mg/kg, iv initially). Neuronal excitation by unilateral microinjection of glutamate analog d,l-homocysteic acid resulted in mild reduction of cough abdominal electromyogram (EMG) amplitudes and very limited temporal changes of cough compared with effects on breathing (very high respiratory rate, high amplitude inspiratory bursts with a short inspiratory phase, and tonic inspiratory motor component). Mean arterial blood pressure temporarily decreased. Blocking glutamate-related neuronal excitation by bilateral microinjections of nonspecific glutamate receptor antagonist kynurenic acid reduced cough inspiratory and expiratory EMG amplitude and shortened most cough temporal characteristics similarly to breathing temporal characteristics. Respiratory rate decreased and blood pressure temporarily increased. Limiting active neuronal inhibition by unilateral and bilateral microinjections of GABA_A receptor antagonist gabazine resulted in lower cough number, reduced expiratory cough efforts, and prolongation of cough temporal features and breathing phases (with lower respiratory rate). The PreBötC is important for cough motor pattern generation. Excitatory glutamatergic neurotransmission in the PreBötC is involved in control of cough intensity and patterning. GABA_A receptor-related inhibition in the PreBötC strongly affects breathing and coughing phase durations in the same manner, as well as cough expiratory efforts. In conclusion, differences in effects on cough and breathing are consistent with separate control of these behaviors.

NEW & NOTEWORTHY This study is the first to explore the role of the inspiratory rhythm and pattern generator, the pre-Bötzinger complex (PreBötC), in cough motor pattern formation. In the PreBötC, excitatory glutamatergic neurotransmission affects cough intensity and patterning but not rhythm, and GABA_A receptor-related inhibition affects coughing and breathing phase durations similarly to each other. Our data show that the PreBötC is important for cough motor pattern generation, but cough rhythmogenesis appears to be controlled elsewhere.

Neurochemical and electrical modulation of the locus coeruleus: contribution to CO2drive to breathe

The locus coeruleus (LC) is a dorsal pontine region, situated bilaterally on the floor of the fourth ventricle. It is considered to be the major source of noradrenergic innervation in the brain. These neurons are highly sensitive to CO₂/pH, and chemical lesions of LC neurons largely attenuate the hypercapnic ventilatory response in unanesthetized adult rats. Developmental dysfunctions in these neurons are linked to pathological conditions such as Rett and sudden infant death syndromes, which can impair the control of the cardio-respiratory system. LC is densely innervated by fibers that contain glutamate, serotonin, and adenosine triphosphate, and these neurotransmitters strongly affect LC activity, including central chemoreflexes. Aside from neurochemical modulation, LC neurons are also strongly electrically coupled, specifically through gap junctions, which play a role in the CO₂ ventilatory response. This article reviews the available data on the role of chemical and electrical neuromodulation of the LC in the control of ventilation.

Ionotropic but not metabotropic glutamatergic receptors in the locus coeruleus modulate the hypercapnic ventilatory response in unanaesthetized rats

AIM

Central chemoreceptors are important to detect changes of CO2/H(+), and the Locus coeruleus (LC) is one of the many putative central chemoreceptor sites. Here, we studied the contribution of LC glutamatergic receptors on ventilatory, cardiovascular and thermal responses to hypercapnia.

METHODS

To this end, we determined pulmonary ventilation (V(E)), body temperatures (T(b)), mean arterial pressure (MAP) and heart rate (HR) of male Wistar rats before and after unilateral microinjection of kynurenic acid (KY, an ionotropic glutamate receptor antagonist, 10 nmol/0.1 μL) or α-methyl-4-carboxyphenylglycine (MCPG, a metabotropic glutamate receptor antagonist, 10 nmol/0.1 μL) into the LC, followed by 60 min of air breathing or hypercapnia exposure (7% CO2).

RESULTS

Ventilatory response to hypercapnia was higher in animals treated with KY intra-LC (1918.7 ± 275.4) compared with the control group (1057.8 ± 213.9, P < 0.01). However, the MCPG treatment within the LC had no effect on the hypercapnia-induced hyperpnea. The cardiovascular and thermal controls were not affected by hypercapnia or by the injection of KY and MCPG in the LC.

CONCLUSION

These data suggest that glutamate acting on ionotropic, but not metabotropic, receptors in the LC exerts an inhibitory modulation of hypercapnia-induced hyperpnea.

Hypoxia-sensing properties of the newborn rat ventral medullary surface in vitro

The ventral medullary surface (VMS) is a region known to exert a respiratory stimulant effect during hypercapnia. Several studies have suggested its involvement in the central inhibition of respiratory rhythm caused by hypoxia. We studied brainstem-spinal cord preparations isolated from newborn rats transiently superfused with a very low O(2) medium, causing reversible respiratory depression, to characterize the participation of the VMS in hypoxic respiratory adaptation. In the presence of 0.8 mM Ca(2+), very low O(2) medium induced an increase in c-fos expression throughout the VMS. The reduction of synaptic transmission and blockade of the respiratory drive by 0.2 mM Ca(2+)-1.6 mM Mg(2+) abolished c-fos expression in the medial VMS (at the lateral edge of the pyramidal tract) but not in the perifacial retrotrapezoid nucleus/parafacial respiratory group (RTN/pFRG) VMS, suggesting the existence of perifacial RTN/pFRG hypoxia-sensing neurons. In the presence of Ca(2+) (0.8 mM), lesioning experiments suggested a physiological difference in perifacial RTN/pFRG VMS between the lateral VMS (beneath the ventrolateral part of the facial nucleus) and the middle VMS (beneath the ventromedial part of the facial nucleus), at least in newborn rats. The lateral VMS lesion, corresponding principally to the most rostral part of the pFRG, produced hypoxia-induced stimulation, whereas the middle VMS lesion, corresponding to the main part of the RTN, abolished hypoxic excitation. This may involve relay via the medial VMS, which is thought to be the parapyramidal group.

Diencephalic and mesencephalic influences on ponto-medullary respiratory control in normoxic and hypoxic conditions: an in vitro study on central nervous system preparations from newborn rat

We investigated the effects of the diencephalon and mesencephalon on the central respiratory drive originating from ponto-medullary regions in normoxic and hypoxic conditions, using central nervous system preparations from newborn rats. We used two approaches: 1) electrophysiological analysis of respiratory frequency and the amplitude of inspiratory C4 activity and 2) immunohistochemical detection of Fos protein, an activity-dependent neuronal marker. We found that, in normoxic conditions, the mesencephalon moderated respiratory frequency, probably by means of an inhibitory effect on ventral medullary respiratory neurons. Diencephalic inputs restored respiratory frequency. Moreover, O(2)-sensing areas in the diencephalon (caudal lateral and posterior hypothalamic areas) and mesencephalon (ventrolateral and dorsolateral periaqueductal gray) seem to increase the amplitude of respiratory bursts during adaptation of the central respiratory drive to hypoxia. In contrast, decrease in respiratory frequency during hypoxia is thought to be mediated by a cluster of ventral hypothalamic neurons.

Nucleus isthmi and control of breathing in amphibians

Despite recent advances, the mechanisms of neurorespiratory control in amphibians are far from understood. One of the brainstem structures believed to play a key role in the ventilatory control of anuran amphibians is the nucleus isthmi (NI). This nucleus is a mesencephalic structure located between the roof of the midbrain and the cerebellum, which differentiates during metamorphosis; the period when pulmonary ventilation develops in bullfrogs. It has been recently suggested that the NI acts to inhibit hypoxic and hypercarbic drives in breathing by restricting increases in tidal volume. This data is similar to the influence of two pontine structures of mammals, the locus coeruleus and the nucleus raphe magnus. The putative mediators for this response are glutamate and nitric oxide. Microinjection of kynurenic acid (an ionotropic receptor antagonist of excitatory amino acids) and l-NAME (a non-selective NO synthase inhibitor) elicited increases in the ventilatory response to hypoxia and hypercarbia. This article reviews the available data on the role of the NI in the control of ventilation in amphibians.

Role of l-glutamate in the locus coeruleus of rats in hypoxia-induced hyperventilation and anapyrexia

Locus coeruleus (LC) is a noradrenergic nucleus in the pons which has been reported to play an inhibitory role in the ventilatory response to hypoxia. Since LC contains glutamatergic receptors and L-glutamate is known to participate in the ventilatory and thermoregulatory responses to hypoxia, the effects of kynurenic acid (KYN, a glutamatergic receptor antagonist) microinjected into the LC in the hypoxic hyperventilation and anapyrexia (a regulated drop in body temperature [Tb]) were examined. Ventilation (V) and Tb were measured before and after a microinjection of KYN (10 nmol/0.1 microl) into the LC, followed by hypoxia. Control rats received a saline injection. Under normoxia, KYN treatment did not affect V or Tb. Typical hypoxia-induced hyperventilation and anapyrexia were observed after saline injection. KYN injection caused an increase in the ventilatory response, acting on tidal volume (Vt), but did not affect the anapyrexic response to hypoxia. These data suggest that L-glutamate in the LC is an excitatory neurotransmitter that activates an inhibitory pathway to reduce the hypoxic ventilatory response, similarly to the data reported for rostral ventrolateral medulla (VLM). The role of L-glutamate into the LC and VLM opposes its effect on other nuclei such as the nucleus of the solitary tract and ventromedullary surface, where the neurotransmitter participates in an excitatory pathway of the ventilatory response.

Role of glutamate in the nucleus isthmi on the hypoxia- and hypercarbia-induced hyperventilation of toads

The nucleus isthmi (NI) is a mesencephalic structure of the amphibian brain that has been reported to participate in CO(2) chemoreception and in the ventilatory response to hypoxia. In the present study, we assessed the role of glutamatergic transmission inside the NI on the hypoxic and hypercarbic drive to breathing. We compared the respiratory responses to 7 and 5% inspired O(2) and 3% inspired CO(2) after microinjecting 10 nmol/100 nl of kynurenic acid (an antagonist of L-glutamate receptors) into the NI of toads (Bufo paracnemis). Kynurenic acid had no effect under resting conditions. Both hypoxia and hypercarbia elicited an increase in ventilation in all groups, with hypoxia acting on tidal volume (V(T)) and hypercarbia on frequency (f). The microinjection of kynurenic acid into the NI caused an increased ventilatory response to hypoxia and hypercarbia due to a higher V(T). We conclude that glutamatergic transmission in the NI has an inhibitory effect when the respiratory drive is high, acting on V(T).

Brainstem and hypothalamic areas activated by tissue hypoxia: Fos-like immunoreactivity induced by carbon monoxide inhalation in the rat

Acute ambient hypoxia interacts with the ventilatory and cardiocirculatory control systems, via the concomitant activation of arterial chemoreceptors and tissue oxygen-sensing mechanisms. Whether these latter mechanisms may trigger a specific pathway had not yet been elucidated. We addressed this issue, mapping Fos expression in adult conscious rats subjected to tissue hypoxia elicited by carbon monoxide inhalation, under conditions of minimal activation of arterial chemoreceptors. Brief stimuli have been delivered (1% carbon monoxide inhaled during 5, 10 or 20 min) to produce steady tissue hypoxia. Compared to normoxia, even the briefest stimuli led to marked neuronal activation within areas involved in ventilatory and cardiocirculatory control. In the brainstem, stimulated rats exhibited enhanced Fos expression in the nucleus of the solitary tract, the area postrema, the dorsal motor nucleus of the vagus nerve, the ventrolateral medulla, the parapyramidal group, the nucleus raphe pallidus, the lateral paragigantocellular nucleus, the locus coeruleus, the dorsal raphe nucleus, the lateral parabrachial area, and the ventrolateral central gray. In the hypothalamus, activated neurons were identified at the ventral border and in the supramamillary, posterior, and dorsomedial nuclei. Fos expression appeared with increasing the severity of tissue hypoxia in the retrotrapezoid nucleus, the ventral tegmental area and the arcuate and paraventricular hypothalamic nuclei. The present data support the idea that inputs related to tissue hypoxia might play a crucial role in patterning the physiological response to hypoxia.

Spontaneously hypertensive rats exhibit altered cardiovascular and neuronal responses to muscle contraction

We examined the cardiovascular and ventrolateral medullary neuronal responses to muscle contraction in the spontaneously hypertensive rat (SHR) and normotensive Wistar-Kyoto rat (WKY) control. Cardiovascular, respiratory and ventrolateral medullary neuronal responses to muscle contraction evoked by tibial nerve stimulation were recorded. SHRs exhibited significantly larger drops in arterial pressure compared to WKYs in response to muscle contraction (P < 0.05). Basal ventrolateral medulla neuronal discharge rates were similar between the SHR and the WKY groups. A majority of neurons recorded responded to muscle contraction in both the WKY (77 %; n = 53) and the SHR groups (68 %; n = 62). There was no difference in the percentage of neurons that responded with an increase (approximately 60 %) or decrease (approximately 40 %) in firing rate between hypertensive and normotensive rats. Pulse wave-triggered averaging techniques showed that most neurons that responded to muscle contraction also possessed a basal firing rhythm temporally related to the cardiac cycle (85 % in WKYs, 83 % in SHRs). However, decreases in neuronal firing rates in response to muscle contraction were significantly greater in SHRs than WKYs. Therefore, we conclude that muscle contraction unmasks a hyperexcitability of neurons in the ventrolateral medulla of SHRs that parallels the heightened blood pressure responses.

5-HT acting on 5-HT(1/2) receptors does not participate in the in vitro hypoxic respiratory depression

The involvement of serotoninergic mechanisms in the central respiratory depression produced by hypoxia was studied in the newborn rat brainstem-spinal cord preparation. The respiratory frequency measured by the C4 ventral root activity was recorded. 5-HT (30 microM) superfusion elicited a rapid increase in respiratory frequency, prevented by a treatment with methysergide (a 5-HT(1/2) receptor antagonist) (40 microM). To investigate the possible participation of 5-HT in hypoxic respiratory depression, this concentration of methysergide was added to the bathing medium during hypoxia. Methysergide did not modify the decrease in respiratory frequency produced by hypoxia. In order to ensure that other 5-HT subtype receptors were not involved in hypoxic respiratory depression, 5-HT was added to the bath during hypoxic-methysergide tests; no effect on respiratory frequency was observed. These results suggest that in the newborn rat brainstem-spinal cord preparation, serotoninergic mechanisms are not involved in the elaboration of the in vitro respiratory response to hypoxia.

The influence of NMDA receptor-mediated processes on breathing pattern in ground squirrels

The effects of blockade of N-methyl-D-aspartate (NMDA) type glutamate receptors by a non-competitive antagonist (MK-801) on cortical arousal, breathing pattern and ventilatory responses to hypoxia (10% O2 in N2) and hypercapnia (5% CO2 in air) were assessed in anesthetized (urethane) and unanesthetized golden-mantled ground squirrels (Spermophilus lateralis). Intra-cerebroventricular administration of MK-801 did not alter ventilation during wakefulness, although it did alter the pattern (breathing frequency and tidal volume components) of the hypercapnic ventilatory response, and suppressed the ventilatory response to hypoxia. Animals did not sleep following treatment with MK-801, and intravenous administration of MK-801 prevented expression of the sleep-like state normally observed in anesthetized animals. In anesthetized animals MK-801 elevated breathing frequency to levels observed without anesthesia, and suppressed the hypoxic ventilatory response. These data suggest that NMDA-type glutamatergic receptor-mediated processes influence cortical arousal and facilitate depression of breathing frequency during anesthesia and the hypoxic ventilatory response. Such processes are not essential for the hypercapnic ventilatory response.

Neurogenesis of patterns of automatic ventilatory activity

Normal respiration, termed eupnea, is characterized by periodic filling and emptying of the lungs. Eupnea can occur 'automatically' without conscious effort. Such automatic ventilation is controlled by the brainstem respiratory centers of pons and medulla. Following removal of the pons, eupnea is replaced by gasping, marked by brief but maximal inspiratory efforts. The mechanisms by which the respiratory rhythms are generated have been examined intensively. Evidence is discussed that ventilatory activity can be generated in multiple regions of pons and medulla. Eupnea and gasping represent fundamentally different ventilatory patterns. Only for gasping has a critical region for neurogenesis been identified, in the rostral medulla. Gasping may be generated by the discharge of 'pacemaker' neurons. In eupnea, this pacemaker activity is suppressed and incorporated into the pontile and medullary neuronal circuit responsible for the neurogenesis of eupnea. Evidence for ventilatory neurogenesis which has been obtained from a number of in vitro preparations is discussed. A much-used preparation is that of a 'superfused' brainstem of the neonatal rat. However, activities of this preparation differ greatly from those of eupnea, as recorded in vitro or in arterially perfused in vitro preparations. Activities of this 'superfused' preparation are identical with gasping and, hence, results must be reinterpreted accordingly. The possibility is present that mechanisms responsible for generating respiratory rhythms may differ from those responsible for shaping respiratory-modulated discharge patterns of cranial and spinal nerves. The importance of pontile mechanisms in the neurogenesis and control of eupnea is reemphasized.

Breathing of awake goats during prolonged dysfunction of caudal M ventrolateral medullary neurons

Cooling the caudal M ventrolateral medullary (VLM) surface for 30 s results in a sustained apnea in anesthetized goats but only a 30% decrease in breathing in awake goats. The purpose of the present study was to determine, in the awake state, the effect of prolonged (minutes, hours) caudal M neuronal dysfunction on eupneic breathing and CO2 sensitivity. Dysfunction was created by ejecting excitatory amino acid receptor antagonists or a neurotoxin on the VLM surface through guide tubes chronically implanted bilaterally on a 10- to 12-mm2 portion of the caudal M VLM surface of 12 goats. Unilateral and bilateral ejections (1 microliter) of selective antagonists for N-methyl-D-aspartic acid or non-N-methyl-D-aspartic acid receptors had no significant effect on eupneic breathing or CO2 sensitivity. Unilateral ejection of a nonselective excitatory amino acid receptor antagonist generally had no effect on eupneic breathing or CO2 sensitivity. However, bilateral ejection of this antagonist resulted in a significant 2-Torr hypoventilation during eupnea and a significant reduction in CO2 sensitivity to 60 +/- 9% of control. Unilateral ejection of the neurotoxin kainic acid initially stimulated breathing; however, breathing then returned to near control with no incidence of apnea. After the kainic acid ejection, CO2 sensitivity was reduced significantly to 60 +/- 7% of control. We conclude that in the awake state a prolonged dysfunction of caudal M VLM neurons results in compensation by other mechanisms (e.g., carotid chemoreceptors, wakefulness) to maintain near-normal eupneic breathing, but compensation is more limited for maintaining CO2 sensitivity.

Non-NMDA receptors modulate respiratory drive in fetal sheep

1. Experiments were carried out in unanaesthetized fetal sheep to evaluate the significance of non-N-methyl-D-aspartate (non-NMDA) receptor neurotransmission in the expression of fetal breathing movements. Catheters placed in the trachea and amniotic fluid and electrodes beneath the parietal bones and in the nuchal muscle were used to monitor breath amplitude and frequency and fetal behavioural state. 2. Experiments were carried out by instillation of neurotransmitter agonists, antagonists or receptor modulators into the cerebrospinal fluid (CSF) of the fourth ventricle by means of a chronic catheter introduced through the foramen magnum. 3. The non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) decreased respiratory rate in a dose dependent manner by lengthening both inspiratory time (T1) and expiratory time (T0). 4. Kainate and (R,S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) increased breath amplitude. Instillation of the antagonist 2,3-dihydro-6-nitro-7-sulphamoyl-benzo(f) quinoxaline (NBQX) prior to administering AMPA resulted in apnoea, which was not overcome by the agonist. 5. Cyclothiazide, which has been shown to prevent desensitization of AMPA receptors, caused an increase in both breath amplitude (152 +/- 73%; mean +/- S.D.; P = 0.004) and frequency (46 +/- 37%; P = 0.049). 6. These data suggest that glutamate acting at non-NMDA receptors is an essential component for the expression of fetal breathing movements, and that under resting conditions these non-NMDA receptors are desensitized following glutamate synaptic release.

Excitatory amino acid-mediated chemoreflex excitation of respiratory neurones in rostral ventrolateral medulla in rats

1. In anaesthetized rats, extracellular and intracellular recordings were made from 119 respiratory neurones in the rostroventrolateral reticular nucleus (RVL) of the medulla oblongata. 2. Two types of active respiratory neurones were detected in RVL: expiratory (E) and pre-inspiratory (Pre-I), based on the relationship between their discharge and that of the phrenic nerve. Some Pre-I but none of the E neurones could be antidromically excited from the C(3)-C(4) level of the spinal cord. 3. E and Pre-I neurones of RVL were excited by stimulation of the arterial chemoreceptors by a close arterial injection of sodium cyanide. The reflex excitation of RVL E neurones was preceded by increased phrenic nerve activity, while the excitation of RVL Pre-I neurones preceded the increases in phrenic nerve activity. 4. The chemoreflex excitation of the two types of RVL respiratory neurones as well as their resting discharge was abolished or significantly depressed by microionophoresis of kynurenate, a wide-spectrum antagonist of excitatory amino acid receptors, while xanthurenate, an inactive analogue of kynurenate, was without effect. 5. In ventilated rats, bilateral microinjection into RVL of kynurenate, but not xanthurenate, abolished resting activity and chemoreflex excitation of phrenic nerve activity, whilst in spontaneously breathing rats, kynurenate microinjection into RVL produced apnea and silenced phrenic nerves. 6. We conclude: (a) chemoreflex excitation of the phrenic nerves is mediated by stimulating Pre-I neurones of RVL by excitatory amino acidergic inputs and (b) RVL Pre-I neurones may directly and/or indirectly excite spinal phrenic motor neurones and hence are involved in inspiratory rhythmogenesis.

Modulation of the respiratory responses to hypoxia and hypercapnia by synaptic input onto caudal hypothalamic neurons

Prior results from this laboratory have demonstrated that the respiratory response to hypercapnia is enhanced by microinjection of GABA antagonists or GABA synthesis inhibitors into the caudal hypothalamus of both cats and rats. However, no evidence was found for modulation of the respiratory response to hypoxia by a hypothalamic GABAergic mechanism. The purpose of the present study was to determine if synaptic input other than GABAergic onto caudal hypothalamic neurons affects the respiratory responses to hypoxia. The respiratory (diaphragmatic EMG) responses to hypoxia (10% O2) and hypercapnia (5% CO2) were recorded in anesthetized rats before and after bilateral microinjection of a blocker of synaptic transmission (CoCl2, 100 mM) or an excitatory amino acid receptor antagonist (kynurenic acid, 50 mM) into the caudal hypothalamus. Both hypoxia and hypercapnia elicited increases in tidal diaphragmatic activity and respiratory frequency prior to the microinjections. The respiratory response to hypercapnia was increased (+10.5%) after CoCl2 microinjections, which is consistent with prior results obtained with blockade of GABAergic input. Kynurenic acid did not alter the respiratory response to hypercapnia. A new finding was that the respiratory response to hypoxia was diminished after both CoCl2 (-13.0%) and kynurenic acid (-25.0%) microinjections. The results of this study support our prior findings that neurons in the caudal hypothalamus modulate the respiratory response to hypercapnia. In addition, our findings suggest that an excitatory input acting through excitatory amino acid receptors in the caudal hypothalamus modulates the respiratory responses to hypoxia.

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)

In vivo and in vitro responses of neurons in the ventrolateral medulla to hypoxia

Neurons in the ventrolateral medulla (VLM) are known to be involved in several cardiorespiratory reflexes and to provide tonic drive to sympathetic preganglionic neurons. Recent studies have suggested that VLM neurons modulate the respiratory responses to hypoxia and to hypercapnia. The purpose of the present study was to determine with electrophysiological techniques if the discharge of these neurons is altered by hypoxia and/or by hypercapnia both in vivo and in vitro. Extracellular single-unit activity of VLM neurons (n = 39) was recorded during inhalation of a hypoxic gas (10% O2) and during inhalation of a hypercapnic gas (5% CO2) in anesthetized, spontaneously breathing rats (n = 16). Hypoxia elicited an increase in the discharge frequency in 64% of the VLM neurons studied; hypercapnia stimulated 42% of the neurons. Fifty-two percent of the neurons were stimulated by both hypoxia and hypercapnia. Signal averaging revealed that 76% of the hypoxia-stimulated neurons had a resting discharge related to the cardiac and/or respiratory cycle. Similar percentages of VLM neurons (35/54) were stimulated by hypoxia in a second group of animals (n = 14) that were studied after sinoaortic denervation. A rat brain slice preparation was then used to determine if hypoxia exerts a direct effect upon neurons in the VLM. Perfusing a hypoxic gas over the surface of medullary slices evoked an increase in the discharge frequency in the majority (39/49) of VLM neurons studied; responses were graded in relation to the magnitude of the hypoxic stimulus. Similar responses to hypoxia were observed in VLM neurons studied during perfusion with a synaptic blockade medium. Retrograde labeling of VLM neurons with rhodamine tagged microspheres injected into the thoracic intermediolateral cell column demonstrated that the hypoxia sensitive neurons were located in a region of the VLM that projects to the thoracic spinal cord. These results demonstrate that neurons in the ventrolateral medulla are excited by a direct effect of hypoxia; these neurons may play a critical role in the cardiorespiratory responses to hypoxia.

Excitatory and depressant respiratory responses to chemical stimulation of the rostral ventrolateral medulla in the cat

The rostral ventrolateral medulla (rVLM) is known to play an important role in cardiorespiratory control. In the rVLM an 'apnoea region', in which unilateral focal blocks induce strong depressant effects on inspiratory activity up to complete apnoea, has been described. This study was designed to systematically investigate the effects provoked by unilateral micro-injections (10-30 nl) of D,L-homocysteic acid 160 mM into this region on respiratory activity and arterial blood pressure in pentobarbitone anaesthetized, vagotomized, paralyzed and artificially ventilated cats. Micro-injections into the rostral portion of this area caused depressant respiratory responses up to complete apnoea, while micro-injections into more caudally located sites induced excitatory respiratory responses. Similar effects were observed in the activity of phrenic nerves and inspiration-related medullary neurons of both the dorsal and ventral respiratory group. The respiratory responses could be accompanied by marked increases in blood pressure (> or = 30 mmHg), especially at locations ventral to the retrofacial and facial nucleus; however, they could also occur in the absence of appreciable changes or even in association with slight decreases in blood pressure. Similar respiratory and pressor effects were observed in carotid sinus denervated cats. The results indicate that two distinct rVLM neuronal populations, one located more rostrally and the other more caudally, may have an important role in the genesis and/or maintenance of respiratory rhythm by exerting respectively inhibitory and excitatory influences on inspiratory activity. Furthermore, they support the hypothesis that different neural substrates of the rVLM are involved in the regulation of respiratory and cardiovascular functions.

Effect of substance P on respiratory rhythm and pre-inspiratory neurons in the ventrolateral structure of rostral medulla oblongata: an in vitro study

The pre-inspiratory (Pre-I) neurons which fire in the pre- and usually also during the post-inspiratory phase are located in the ventrolateral structures of the rostral medulla. They are suggested as primary rhythm generating neurons for respiration. These have been studied in isolated brainstem-spinal cord preparations from newborn 0-5-day-old rats. We have found that application of substance P (SP) enhanced the respiratory rhythm as measured by C4 ventral root and pre-I neuronal activities. Furthermore, the effect of SP was dependent on basal respiratory rate. An increase of the Pre-I and C4 burst rate by SP was clearer when the basal respiratory rhythm was somewhat lower. Moreover, long lasting depression of respiratory rate after the application of the alpha 2-agonist clonidine was reversed by SP. On the other hand, an inhibitory effect appeared in preparations with a higher basal respiratory rate, while the Pre-I burst rate tended to increase during SP perfusion. During chemical synaptic transmission blockade by perfusion with low Ca2+, high Mg2+ solution, a pre-I burst retained or completely blocked was found to be enhanced or reactivated by SP perfusion. The results suggest a direct postsynaptic action of SP, which could strongly stimulate burst generating properties of Pre-I neurons.