Dorsal medullary inspiratory neurons: effects of superior laryngeal afferent stimulation

Functional Neuroplasticity in the Nucleus Tractus Solitarius and Increased Risk of Sudden Death in Mice with Acquired Temporal Lobe Epilepsy

Sudden unexpected death in epilepsy (SUDEP) is the leading cause of death in individuals with refractory acquired epilepsy. Cardiorespiratory failure is the most likely cause in most cases, and central autonomic dysfunction has been implicated as a contributing factor to SUDEP. Neurons of the nucleus tractus solitarius (NTS) in the brainstem vagal complex receive and integrate vagally mediated information regarding cardiorespiratory and other autonomic functions, and GABAergic inhibitory NTS neurons play an essential role in modulating autonomic output. We assessed the activity of GABAergic NTS neurons as a function of epilepsy development in the pilocarpine-induced status epilepticus (SE) model of temporal lobe epilepsy (TLE). Compared with age-matched controls, mice that survived SE had significantly lower survival rates by 150 d post-SE. GABAergic NTS neurons from mice that survived SE displayed a glutamate-dependent increase in spontaneous action potential firing rate by 12 wks post-SE. Increased spontaneous EPSC frequency was also detected, but vagal afferent synaptic release properties were unaltered, suggesting that an increase in glutamate release from central neurons developed in the NTS after SE. Our results indicate that long-term changes in glutamate release and activity of GABAergic neurons emerge in the NTS in association with epileptogenesis. These changes might contribute to increased risk of cardiorespiratory dysfunction and sudden death in this model of TLE.

Central pathways of pulmonary and lower airway vagal afferents

Lung sensory receptors with afferent fibers coursing in the vagus nerves are broadly divided into three groups: slowly (SAR) and rapidly (RAR) adapting stretch receptors and bronchopulmonary C fibers. Central terminations of each group are found in largely nonoverlapping regions of the caudal half of the nucleus of the solitary tract (NTS). Second order neurons in the pathways from these receptors innervate neurons located in respiratory-related regions of the medulla, pons, and spinal cord. The relative ease of selective activation of SARs, and to a lesser extent RARs, has allowed for more complete physiological and morphological characterization of the second and higher order neurons in these pathways than for C fibers. A subset of NTS neurons receiving afferent input from SARs (termed pump or P-cells) mediates the Breuer-Hering reflex and inhibits neurons receiving afferent input from RARs. P-cells and second order neurons in the RAR pathway also provide inputs to regions of the ventrolateral medulla involved in control of respiratory motor pattern, i.e., regions containing a predominance of bulbospinal premotor neurons, as well as regions containing respiratory rhythm-generating neurons. Axon collaterals from both P-cells and RAR interneurons, and likely from NTS interneurons in the C-fiber pathway, project to the parabrachial pontine region where they may contribute to plasticity in respiratory control and integration of respiratory control with other systems, including those that provide for voluntary control of breathing, sleep-wake behavior, and emotions.

Impaired cardiorespiratory recovery after laryngeal stimulation in nicotine-exposed young lambs

The hypothesis that postnatal nicotine exposure weakens cardiorespiratory recovery from reflex apnea and bradycardia was tested in eight lambs continuously infused with nicotine from the day of birth at a dose of 1 to 2 mg.kg(-1).d(-1). Eight age-matched lambs infused with saline served as controls. Apnea and bradycardia were elicited by laryngeal stimulation with 1 mL of water (laryngeal chemoreflex) both during air breathing [0.21 fraction of inspired oxygen (FiO(2))] and mild hypoxia (0.10 FiO(2)) at a mean postnatal age of 5 +/- 1, 14 +/- 1, and 28 +/- 1 d. Ventilation, heart rate, and blood pressure were similar in the two groups at rest. In response to laryngeal chemoreflex stimulation, nicotine-treated lambs had a more pronounced decrease in ventilation (p < 0.05), longer reflex apnea (p < 0.001 in 0.21 FiO(2); p < 0.01 in 0.10 FiO(2)), and greater reflex bradycardia (p < 0.01). During reflex apnea, sighs were less efficient in restoring heart rate to prestimulation level, and a greater decrease in heart rate was observed before sighs in nicotine-treated lambs. These effects were most apparent at 5 d of age, when nicotine-treated lambs also had lower ventilation during hypoxia (p < 0.05). The response to hyperoxia was comparable in the two groups at all ages. The ability to terminate laryngeal chemoreflex-induced apnea is attenuated in young lambs continuously exposed to nicotine. This attenuation is present both in normoxia and in hypoxia and is accompanied by reduced effects from sighing on cardiac autoresuscitation.

Reflex respiratory response to changes in upper airway pressure in the anaesthetized rat

1. We examined the upper airway (UA) motor response to upper airway negative pressure (UANP) in the rat. We hypothesized that this response is mediated by superior laryngeal nerve (SLN) afferents and is not confined to airway dilator muscles but also involves an increase in motor drive to tongue retractor and pharyngeal constrictor muscles, reflecting a role for these muscles in stabilizing the UA. 2. Experiments were performed in 49 chloralose-anaesthetized, tracheostomized rats. Subatmospheric pressure in the range 0 to -30 cmH(2)O was applied to the isolated UA. Motor activity was recorded in separate experiments from the main trunk of the hypoglossal nerve (XII, n = 8), the pharyngeal branch of the glossopharyngeal nerve (n = 8), the medial and lateral branches of the XII (n = 8) and the pharyngeal branch of the vagus (n = 8). Afferent activity was recorded from the whole SLN in six experiments. 3. All UA motor outflows exhibited phasic inspiratory activity and this was significantly (P < 0.05) increased by UANP. Tonic end-expiratory activity increased significantly in response to pressures more negative than -20 cmH(2)O. Bilateral section of the SLN also increased (P < 0.05) motor activity and abolished the responses to UANP. Electrical stimulation of the SLN inhibited inspiratory XII activity. SLN afferents were tonically active and inhibited by UANP. 4. We conclude that UANP causes a reflex increase in motor drive to pharyngeal dilator, tongue retractor and pharyngeal constrictor muscles via afferent fibres in the SLN. Tonic activity in SLN afferent fibres at zero transmural pressure exerts a marked inhibitory effect on UA motor outflow.

Respiratory neuronal activity during apnea and poststimulatory effects of laryngeal origin in the cat

We investigated the behavior of medullary respiratory neurons in cats under pentobarbitone anesthesia, vagotomized, paralysed, and artificially ventilated to elucidate neural mechanisms underlying apnea and poststimulatory respiratory depression induced by superior laryngeal nerve (SLN) stimulation. Inspiratory neurons were completely inhibited during SLN stimulation and poststimulatory apnea. During recovery of inspiratory activity, augmenting inspiratory neurons were depressed, decrementing inspiratory neurons were excited, and late inspiratory neurons displayed unchanged bursts closely locked to the end of the inspiratory phase. Augmenting expiratory neurons were either silenced or displayed different levels of tonic activity during SLN stimulation; some of them were clearly activated. These expiratory neurons displayed activity during poststimulatory apnea, before the onset of the first recovery phrenic burst. Postinspiratory or decrementing expiratory neurons were activated during SLN stimulation; their discharge continued with a decreasing trend during poststimulatory apnea. The results support the three-phase theory of rhythm generation and the view that SLN stimulation provokes a postinspiratory apnea that could represent the inhibitory component of respiratory reflexes of laryngeal origin, such as swallowing. In addition, because a subpopulation of augmenting expiratory neurons displays activation during SLN stimulation, the hypothesis can be advanced that not only postinspiratory, or decrementing expiratory neurons, but also augmenting expiratory neurons may be involved in the genesis of apnea and poststimulatory phenomena. Finally, the increase in the activity of decrementing inspiratory neurons after the end of SLN stimulation may contribute to the generation of poststimulatory respiratory depression by providing an inhibitory input to bulbospinal augmenting inspiratory neurons.

Central control of the cardiovascular and respiratory systems and their interactions in vertebrates

This review explores the fundamental neuranatomical and functional bases for integration of the respiratory and cardiovascular systems in vertebrates and traces their evolution through the vertebrate groups, from primarily water-breathing fish and larval amphibians to facultative air-breathers such as lungfish and some adult amphibians and finally obligate air-breathers among the reptiles, birds, and mammals. A comparative account of respiratory rhythm generation leads to consideration of the changing roles in cardiorespiratory integration for central and peripheral chemoreceptors and mechanoreceptors and their central projections. We review evidence of a developing role in the control of cardiorespiratory interactions for the partial relocation from the dorsal motor nucleus of the vagus into the nucleus ambiguus of vagal preganglionic neurons, and in particular those innervating the heart, and for the existence of a functional topography of specific groups of sympathetic preganglionic neurons in the spinal cord. Finally, we consider the mechanisms generating temporal modulation of heart rate, vasomotor tone, and control of the airways in mammals; cardiorespiratory synchrony in fish; and integration of the cardiorespiratory system during intermittent breathing in amphibians, reptiles, and diving birds. Concluding comments suggest areas for further productive research.

Synaptic potentials in respiratory neurones during evoked phase switching after NMDA receptor blockade in the cat

1. Blockade of NMDA receptors by dizocilpine impairs the inspiratory off-switch (IOS) of central origin but not the IOS evoked by stimulation of sensory afferents. To investigate whether this difference was due to the effects of different patterns of synaptic interactions on respiratory neurones, we stimulated electrically the superior laryngeal nerve (SLN) or vagus nerve in decerebrate cats before and after i.v. administration of dizocilpine, whilst recording intracellularly. 2. Phrenic nerve responses to ipsilateral SLN or vagal stimulation were: at mid-inspiration, a transient inhibition often followed by a brief burst of activity; at late inspiration, an IOS; and at mid-expiration, a late burst of activity. 3. In all neurones (n = 16), SLN stimulation at mid-inspiration evoked an early EPSP during phase 1 (latency to the arrest of phrenic nerve activity), followed by an IPSP in inspiratory (I) neurones (n = 8) and by a wave of EPSPs in post-inspiratory (PI) neurones (n = 8) during phase 2 (inhibition of phrenic activity). An EPSP in I neurones and an IPSP in PI neurones occurred during phase 3 (brief phrenic burst) following phase 2. 4. Evoked IOS was associated with a fast (phase 1) activation of PI neurones, whereas during spontaneous IOS, a progressive (30-50 ms) depolarization of PI neurones preceded the arrest of phrenic activity. 5. Phase 3 PSPs were similar to those occurring during the burst of activity seen at the start of spontaneous inspiration. 6. Dizocilpine did not suppress the evoked phrenic inhibition and the late burst of activity. The shapes and timing of the evoked PSPs and the changes in membrane potential in I and PI neurones during the phase transition were not altered. 7. We hypothesize that afferent sensory pathways not requiring NMDA receptors (1) terminate inspiration through a premature activation of PI neurones, and (2) evoke a late burst of phrenic activity which might be the first stage of the inspiratory on-switch.

Role of respiratory and non-respiratory neurones in the region of the NTS in the elaboration of the sneeze reflex in cat

Extracellular recordings were made in the dorsal respiratory group (DRG) and adjacent reticular formation following single-shock stimulation of the anterior ethmoidal nerve (AEN) and during sneeze evoked by repetitive stimulation of the AEN in nembutal-anaesthetized, curarized and ventilated cats. These neurones were characterised according to (i) their activity during the respiratory cycle (as inspiratory augmenting or decrementing (I Aug or I Dec), expiratory augmenting or decrementing (E Aug or E Dec), silent or tonic), and (ii) their axonal projection (bulbospinal or non-bulbospinal-non-vagal (BS or NBS-NV)). Following single-shock stimulation of the AEN, most of the inspiratory neurones were transiently inhibited, whereas E Aug neurones were activated and E Dec neurones were activated and then inhibited. Silent neurones responded with a multispike or a paucispike pattern. Following repetitive stimulation of the AEN and during the resulting sneeze reflex, I Aug neurones increased their activity in parallel with the phrenic activity, I Dec neurones fired at the onset and at the end of the inspiration, E Dec and some silent neurones fired either during the compressive phase or after the expulsive phase, whereas E Aug and some silent neurones fired during the expulsive phase. We conclude that sneeze involves a reconfiguration of the central respiratory drive which uses, at least partly, the respiratory network to trigger a non-ventilatory defensive motor act.

Synaptic inputs to medullary respiratory neurons from superior laryngeal afferents in the cat

Synaptic inputs from afferents in the superior laryngeal nerve (SLN) to medullary respiratory neurons (n = 154) in the dorsal respiratory group (DRG), ventral respiratory group (VRG) and the region of the Bötzinger complex (BOT) were studied in anesthetized cats. Single pulse stimulation of the SLN-evoked monosynaptic EPSPs in most inspiratory bulbospinal (I-BS) neurons in the DRG, and disynaptic or oligosynaptic chloride-dependent IPSPs in other I-BS neurons in the DRG and VRG. Stimulation of laryngeal afferents also inhibited oligosynaptically expiratory bulbospinal neurons in the VRG, and all types of respiratory neurons recorded in the BOT region. Oligosynaptic potentials (usually EPSPs) were recorded in inspiratory and expiratory laryngeal motoneurons. These results provide evidence of a processing of SLN-evoked synaptic responses by all tested groups of medullary respiratory neurons. The pathways mediating these synaptic responses are discussed.

Morphology and electrophysiology of superior laryngeal nerve afferents and postsynaptic neurons in the medulla oblongata of the cat

Intra-axonal recordings were made from 24 afferent fibres of the superior laryngeal nerve in and around the nucleus tractus solitarius, in 26 pentobarbitone-anaesthetized cats. Conduction velocity ranged from 15 to 38 m/s. Four afferents were injected with horseradish peroxidase. They showed dense terminal arborization in the region of the ventral and ventrolateral subnuclei of the nucleus tractus solitarius, both rostral and caudal to the obex. Six other intra-axonal recordings were thought to originate from axons of neurons postsynaptic to superior laryngeal afferents; one of these was injected with horseradish peroxidase and showed a similar arborization pattern to that of the afferent axons. In the same region, intracellular recordings were made from 124 neurons which responded to superior laryngeal nerve stimulation with excitatory postsynaptic potentials (mean latency 2.7 +/- 1.0 ms). Ninety-nine of these neurons were thought to receive a monosynaptic input. The stimulation threshold evoking these responses was similar to that which inhibited phrenic nerve discharge. Eleven of the monosynaptically excited neurons were injected with horseradish peroxidase. They had fusiform or stellate somata and simple dendritic trees, radiating mainly in the transverse plane. In one experiment, in which both a superior laryngeal nerve afferent fibre and a neuron were labelled, afferent terminal varicosities were found in close apposition with the postsynaptic membrane of the injected neuron. Four of 14 (29%) tested neurons could be antidromically activated from the C3 spinal segment. The stimulus thresholds and onset latencies of the responses of superior laryngeal nerve afferents and medullary neurons to stimulation of the superior laryngeal nerve are consistent with their involvement in the reflex inhibition of respiratory neurons evoked by superior laryngeal nerve stimulation.

Synaptic events in ventral respiratory neurones during apnoea induced by laryngeal nerve stimulation in neonatal pig

1. Postsynaptic potentials evoked by electrical stimulation of superior laryngeal nerve (SLN) were recorded during SLN-induced apnoea from the respiratory neurones of the ventral respiratory group (VRG) in pentobarbitone-anaesthetized, vagotomized and artificially ventilated newborn piglets (n = 14, 4-7 days old). All recorded inspiratory (n = 10), post-inspiratory (n = 10) and expiratory (n = 20) neurones had a triphasic pattern of membrane potential and were identified for their projections to the spinal cord or cervical vagus nerve. 2. During long-lasting apnoea, induced by SLN stimulation, the membrane potential trajectory of each type of recorded neurone was held at the level corresponding approximately to the membrane potential reached during stage I of expiration. Compound postsynaptic potentials evoked in most respiratory-related neurones had an early short-lasting and a late long-lasting component. 3. Postsynaptic potentials in four out of seven inspiratory neurones, in which postsynaptic potentials were well demonstrated, were characterized by an early depolarization followed by long-lasting hyperpolarization. In three other inspiratory neurones only late hyperpolarization was present. The reversal of the late hyperpolarization by intracellular chloride injection was achieved to a different degree in the early and late portions of late hyperpolarization. 4. Postsynaptic potentials evoked in expiratory neurones were studied in sixteen neurones and displayed two patterns: early hyperpolarization followed by long-lasting hyperpolarization (n = 7, six were not antidromically activated after spinal cord stimulation) or early hyperpolarization followed by late depolarization (n = 9, eight projected to the spinal cord). The early hyperpolarization was readily reversed by chloride injection. The late hyperpolarization was more difficult to reverse and usually the reversal was not completed. 5. Postsynaptic potentials evoked in post-inspiratory neurones showed a pattern of two consecutive phases of depolarization. 6. The present study revealed that during long-lasting apnoea evoked by SLN stimulation each category of VRG respiratory neurones received a temporally synchronized combination of an initial fast input derived reflexly from laryngeal afferents, and of late inputs representing involvement of the whole respiratory network in the response.

Effects of contralateral superior laryngeal nerve stimulation on dorsal medullary inspiratory neurons

The effects of superior laryngeal nerve (SLN) stimulation on phrenic (PHR) nerve activity and on activity of dorsal respiratory group (DRG) inspiratory (I) neurons contralateral to the stimulus were examined in decerebrate, paralyzed cats. Stimulation caused bilateral PHR suppression followed by recovery at ca. 30 ms. Most DRG neurons (70%) contralateral to the stimulus were inhibited, but average onset of inhibition lagged that of PHR suppression. This contrasts sharply with the observation in an earlier study that inhibition of ipsilateral I neurons on the average preceded PHR suppression. The remaining neurons (30%) were not inhibited. Only 22% of contralateral neurons were excited by SLN stimulation, in contrast to 52% of ipsilateral neurons. Thus, contralateral DRG I neurons do not mediate the onset of bilateral PHR suppression by SLN stimulation and are probably inhibited through a longer pathway than that for the ipsilateral unit responses.