Sleep-related apneic and apneustic breathing following pneumotaxic lesion and vagotomy

Role of the Kölliker-Fuse nucleus in cardiovascular responses to hypoxia and baroreceptor activation in anesthetized rats

Introduction: Parabrachial Kölliker-Fuse (KF) complex, located in dorsolateral part of the pons, is involved in the respiratory control, however, its role in the baroreflex and chemoreflex responses has not been established yet. This study was performed to test the contribution of the KF to chemoreflex and baroreflex and the effect of microinjection of a reversible synaptic blocker (Cocl₂) into the KF in urethane anesthetized rats. Methods: Activation of chemoreflex was induced by systemic hypoxia caused by N2 breathing for 30 seconds "hypoxic- hypoxia methods" and baroreflex was evoked by intravenous injection (i.v.) of phenylephrine (Phe, 20 µg /kg/0.05-0.1 mL). N2 induced generalized vasodilatation followed by tachycardia reflex and Phe evoked vasoconstriction followed by bradycardia. Results: Microinjection of Cocl₂ (5 mM/100 nL/side) produced no significant changes in the Phe-induced hypertension and bradycardia, whereas the cardiovascular effect of N2 was significantly attenuated by the injection of CoCl₂ to the KF. Conclusion: The KF played no significant role in the baroreflex, but could account for cardiovascular chemoreflex in urethane anesthetized rats.

Pontine mechanisms of respiratory control

Pontine respiratory nuclei provide synaptic input to medullary rhythmogenic circuits to shape and adapt the breathing pattern. An understanding of this statement depends on appreciating breathing as a behavior, rather than a stereotypic rhythm. In this review, we focus on the pontine-mediated inspiratory off-switch (IOS) associated with postinspiratory glottal constriction. Further, IOS is examined in the context of pontine regulation of glottal resistance in response to multimodal sensory inputs and higher commands, which in turn rules timing, duration, and patterning of respiratory airflow. In addition, network plasticity in respiratory control emerges during the development of the pons. Synaptic plasticity is required for dynamic and efficient modulation of the expiratory breathing pattern to cope with rapid changes from eupneic to adaptive breathing linked to exploratory (foraging and sniffing) and expulsive (vocalizing, coughing, sneezing, and retching) behaviors, as well as conveyance of basic emotions. The speed and complexity of changes in the breathing pattern of behaving animals implies that "learning to breathe" is necessary to adjust to changing internal and external states to maintain homeostasis and survival.

The emerging role of the parabrachial complex in the generation of wakefulness drive and its implication for respiratory control

The parabrachial complex is classically seen as a major neural knot that transmits viscero- and somatosensory information toward the limbic and thalamic forebrain. In the present review we summarize recent findings that imply an emerging role of the parabrachial complex as an integral part of the ascending reticular arousal system, which promotes wakefulness and cortical activation. The ascending parabrachial projections that target wake-promoting hypothalamic areas and the basal forebrain are largely glutamatergic. Such fast synaptic transmission could be even more significant in promoting wakefulness and its characteristic pattern of cortical activation than the cholinergic or mono-aminergic ascending pathways that have been emphasized extensively in the past. A similar role of the parabrachial complex could also apply for its more established function in control of breathing. Here the parabrachial respiratory neurons may modulate and adapt breathing via the control of respiratory phase transition and upper airway patency, particularly during respiratory and non-respiratory behavior associated with wakefulness.

Effects of intertrigeminal region NMDA and non-NMDA receptors on respiratory responses in rats

Respiratory disturbance, including apnea, can be induced by microinjection of glutamate into the intertrigeminal region (ITR) of the lateral pons, a region that is anatomically coupled to both the dorsal and ventral respiratory groups of the medulla. We showed that the ITR plays a functional role in regulating both vagal reflex apnea and spontaneous sleep-related apnea in rats, but the mechanisms have not been determined. This study shows that functional NMDA receptors are expressed in the ITR since the blockade of these receptors by AP5, a specific NMDA receptor antagonist, was fully effective in blocking apnea induced by glutamate injection within this region. Selective blockade of ITR NMDA receptors had no effect on the immediate apnea evoked by an intravenous 5-HT bolus, whereas the nonspecific glutamate receptor antagonist kynurenic acid significantly increased the duration of this vagal reflex apnea. These findings are of interest because pontine NMDA receptors participate in inspiratory off-switch mechanisms and have been implicated in various short- and long-term potentiation and depression phenomena. These data support the involvement of ITR non-NMDA receptors in modulation of reflex apnea per se, whereas NMDA receptors play a role in damping respiratory responses to transient disturbances.

The Kölliker-Fuse nucleus gates the postinspiratory phase of the respiratory cycle to control inspiratory off-switch and upper airway resistance in rat

Lesion or pharmacological manipulation of the dorsolateral pons can transform the breathing pattern to apneusis (pathological prolonged inspiration). Apneusis reflects a disturbed inspiratory off-switch mechanism (IOS) leading to a delayed phase transition from inspiration to expiration. Under intact conditions the IOS is irreversibly mediated via activation of postinspiratory (PI) neurons within the respiratory network. In parallel, populations of laryngeal premotoneurons manifest the IOS by a brief glottal constriction during the PI phase. We investigated effects of pontine excitation (glutamate injection) or temporary lesion after injection of a GABA-receptor agonist (isoguvacine) on the strength of PI-pool activity determined from respiratory motor outputs or kinesiological measurements of laryngeal resistance in a perfused brainstem preparation. Glutamate microinjections into distinct parts of the pontine Kölliker-Fuse nucleus (KF) evoked a tonic excitation of PI-motor activity or sustained laryngeal constriction accompanied by prolongation of the expiratory phase. Subsequent isoguvacine microinjections at the same loci abolished PI-motor or laryngeal constrictor activity, triggered apneusis and established a variable and decreased breathing frequency. In summary, we revealed that excitation or inhibition of defined areas within the KF activated and blocked PI activity and, consequently, IOS. Therefore, we conclude, first, that descending KF inputs are essential to gate PI activity required for a proper pattern formation and phase control within the respiratory network, at least during absence of pulmonary stretch receptor activity and, secondly, that the KF contains large numbers of laryngeal PI premotor neurons that might have a key role in the regulation of upper airway resistance during reflex control and vocalization.

Intertrigeminal region attenuates reflex apnea and stabilizes respiratory pattern in rats

The goal of the present study was to determine whether the newly described anatomical pathway involving the pontine intertrigeminal region (ITR), (Chamberlin and Saper, 1998), has a physiological role in mediating or modulating vagally-induced reflex apnea. We explored the ITR impact on vagal reflex apnea elicited by intravenuously injected 5-HT in ten anesthetized rats. The animals had a catheter inserted into the femoral vein for administration of 5-HT (0.00375 mg) and respiration was recorded by piezo-electric crystal. Multibarrel pipettes were used to pressure inject glutamate (5-10 nl, 10 mM), kynurenic acid (10 nl, 50 mM, a glutamate receptor antagonist), and red dye into the ITR, unilaterally and bilaterally. Intravenous administration of 5-HT produced an immediate 3-s apnea. Microinjections of glutamate into the ITR produced apneas, while microinjections of kynurenic acid blocked the glutamate effect. Following glutamate antagonism, subsequent administration of 5-HT produced apneas of much longer duration (8 s). Acute ponto-medullary transection in two animals yielded even greater prolongation of 5-HT-induced apnea. We conclude that a physiological role for the ITR in respiration is to attenuate vagally-induced reflex apneas. This finding is in agreement with the general modulatory role of pontine structures in autonomic activities including respiration, heart rate and regulation of blood pressure. In addition, our data indicate that medullary circuits, independent of pontine structures, are sufficient to produce 5-HT induced reflex apnea.

Pontine cholinergic mechanisms enhance trigeminally evoked respiratory suppression in the anesthetized rat

In the present study, we investigated in anesthetized rats the influences of the pontine rapid-eye-movement (REM) sleep center on trigeminally induced respiratory responses. We evoked the nasotrigeminal reflex by electrical stimulation of the ethmoidal nerve (EN5) and analyzed the EN5-evoked respiratory suppression before and after injections into the pontine reticular nuclei of the cholinergic agonist carbachol. After injections of 80-100 nl of carbachol (20 mM), we observed a decrease in respiratory rate, respiratory minute volume, and blood pressure but an increase in tidal volume. In those cases in which carbachol injections alone caused these REM sleep-like autonomic responses, we also observed that the EN5-evoked respiratory suppression was significantly potentiated. Unfortunately, carbachol injections failed to depress genioglossus electromyogram (EMG) effectively, because the EMG activity was already strongly depressed by the anesthetic alpha-chloralose. We assume that pontine carbachol injections in our anesthetized rats cause autonomic effects that largely resemble REM sleep-like respiratory and vascular responses. We therefore conclude that the observed potentiation of EN5-evoked respiratory suppression after carbachol might be due to REM sleep-associated neuronal mechanisms. We speculate that activation of sensory trigeminal afferents during REM sleep might contribute to pathological REM sleep-associated respiratory failures.

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.

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.

Apneustic breathing. A characteristic feature of brainstem compression in achondroplasia?

We have observed apneustic breathing in five patients with achondroplasia. In contrast to experimental models of apneusis, these patients appeared to have intact vagal function and no evidence of pontine disease. However, all our patients displayed clinical, structural, and electrophysiologic features of cervicomedullary compression, a well-recognized complication of achondroplasia. The degree of apneustic breathing was reduced in the majority of our patients following decompressive surgery. Traditional theories on the pathogenesis of apneustic breathing cannot satisfactorily explain the presence of apneustic breathing in our patients. We suggest that cervicomedullary compression may be capable of producing apneustic breathing in the absence of vagal or pneumotaxic center lesions.

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.

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

The effects on respiration of MK-801, an N-methyl-D-aspartate (NMDA) non-competitive antagonist, were studied in awake chronic cats by means of the plethysmographic technique. MK-801 (0.01-3.0 mg/kg) was first given i.v. in cumulative doses. The protocol was repeated 10-15 days later in the same animals after bilateral vagotomy. MK-801 selectively increased the duration of inspiration, causing an apneustic respiration but had no effect on the duration of expiration. The maximal inspiratory duration brought about by MK-801 in the intact cat (4.3 s; control 0.9 s) increased 4-fold after bilateral vagotomy (16.4 s; control 1.7 s). Such results suggest that the termination of the inspiratory phase in normal awake cats results from an interaction of pulmonary vagal afferent inputs (inactive on NMDA receptors) with a central mechanism in which NMDA-type glutamate receptors are activated by dicarboxylic amino acid neurotransmission.

Respiratory modulation of neuronal discharge in the central nucleus of the amygdala during sleep and waking states

The relationship between neuronal discharge in the central nucleus of the amygdala (ACE) and timing of the respiratory cycle was assessed during quiet and active sleep and during the waking state. Of 169 neurons recorded from the ACE in intact, drug-free cats, 22% discharged phasically with the respiratory cycle during at least one sleep or waking state. The dependency between neuronal discharge and the respiratory cycle was typically strong in only one state. Forty-three percent of the respiratory-related neurons were most strongly correlated with the respiratory cycle during the waking state (AW). An additional 30% were most strongly related to the respiratory cycle during quiet sleep (QS), whereas only 11% showed the strongest dependency during rapid eye movement (REM) sleep. Half of the ACE neurons (49%) discharged at frequencies less than 10 spikes per second, and the most common trend in firing rate across states was one in which neurons fired more rapidly during AW and REM than during QS. No relationship between discharge rate of ACE neurons in the three states and propensity for phasic discharge with the respiratory cycle could be demonstrated.

State-dependent alteration of respiratory cycle timing by stimulation of the central nucleus of the amygdala

The effect of electrical stimulation of the amygdaloid central nucleus (ACE) on respiration was studied in unanesthetized, unrestrained cats during sleep-waking states. Single 0.5-ms 500-microA constant-current pulses delivered to the ACE at various points on the respiratory cycle, produced a transient inspiratory effort which summated with ongoing inspiratory activity and reduced inspiratory time. Stimulus pulses delivered during the expiratory phase resulted in an earlier shift to inspiration. Repetitive single pulse stimuli delivered to the ACE at a rate slightly faster than the spontaneous respiratory cycle during the alert state, were capable of 'entraining' respiration at the stimulus frequency. This entrainment disappeared in quiet sleep. Atropine, however, which produced synchronous high voltage slow waves and 12-14-Hz EEG spindle activity in the alert cat, did not impair this entrainment. Short (300-500 ms) 100-Hz trains of 0.5-ms pulses to the ACE produced rapid onset, sustained inspiration and a rise in blood pressure in the alert animal. During quiet sleep the response was attenuated but qualitatively similar, and also aroused the animal. Single pulse stimuli, however, were not associated with cardiovascular changes or generalized arousal. These results suggest that the ACE contributes to excitation of the inspiratory cycle, possibly through the large projection of this nucleus to the parabrachial pons.