The kreisler mutation leads to the loss of intrinsically hypoxia-activated spots in the region of the retrotrapezoid nucleus/parafacial respiratory group

Acute hypoxia elicits a biphasic respiratory response characterized in the newborn by a transient hyperventilation followed by a severe decrease in respiratory drive known as hypoxic respiratory depression. Medullary O(2) chemosensitivity is known to contribute to respiratory depression induced by hypoxia, although precise involvement of cell populations remains to be determined. Having a thorough knowledge of these populations is of relevance because perturbations in the respiratory response to hypoxia may participate in respiratory diseases in newborns. We aimed to analyze the hypoxic response of ponto-medullary cell populations of kreisler mutant mice. These mice have defects in a gene expressed in two rhombomeres encompassing a part of the medulla oblongata implicated in hypoxic respiratory depression. Central responses to hypoxia were analyzed in newborn mice by measuring respiratory rhythm in ex vivo caudal pons-medullary-spinal cord preparations and c-fos expression in wild-type and kreisler mutants. The homozygous kreisler mutation, which eliminates most of rhombomere 5 and mis-specifies rhombomere 6, abolished (1) an early decrease in respiratory frequency within 10 min of hypoxia and (2) an intrinsic hypoxic activation, which is characterized by an increase in c-fos expression in the region of the ventral medullary surface encompassing the retrotrapezoid nucleus/parafacial respiratory group expressing Phox2b. This increase in c-fos expression persisted in wild-type Phox2b-negative and Phox2b-positive cells after blockade of synaptic transmission and rhythmogenesis by a low [Ca(2+)](0). Another central response was retained in homozygous kreisler mutant mice; it was distinguished by (1) a delayed (10-30 min) depression of respiratory frequency and (2) a downregulation of c-fos expression in the ventrolateral reticular nucleus of the medulla, the nucleus of the solitary tract, and the area of the A5 region. Thus, two types of ponto-medullary cell groups, with distinct anatomical locations, participate in central hypoxic respiratory depression in newborns.

Developmental basis of the rostro-caudal organization of the brainstem respiratory rhythm generator

The Hox genetic network plays a key role in the anteroposterior patterning of the rhombencephalon at pre- and early-segmental stages of development of the neural tube. In the mouse, it controls development of the entire brainstem respiratory neuronal network, including the pons, the parafacial respiratory group (pFRG) and the pre-Bötzinger complex (preBötC). Inactivation of Krox20/Egr2 eliminates the pFRG activity, thereby causing life-threatening neonatal apnoeas alternating with respiration at low frequency. Another respiratory abnormality, the complete absence of breathing, is induced when neuronal synchronization fails to develop in the preBötC. The present paper summarizes data on a third type of respiratory deficits induced by altering Hox function at pontine levels. Inactivation of Hoxa2, the most rostrally expressed Hox gene in the hindbrain, disturbs embryonic development of the pons and alters neonatal inspiratory shaping without affecting respiratory frequency and apnoeas. The same result is obtained by the Phox2a(+/-) mutation modifying the number of petrosal chemoafferent neurons, by eliminating acetylcholinesterase and by altering Hox-dependent development of the pons with retinoic acid administration at embryonic day 7.5. In addition, embryos treated with retinoic acid provide a mouse model for hyperpnoeic episodic breathing, widely reported in pre-term neonates, young girls with Rett's syndrome, patients with Joubert syndrome and adults with Cheyne-Stokes respiration. We conclude that specific respiratory deficits in vivo are assignable to anteroposterior segments of the brainstem, suggesting that the adult respiratory neuronal network is functionally organized according to the rhombomeric, Hox-dependent segmentation of the brainstem in embryos.

M(1)/M(3) and M(2)/M(4) muscarinic receptor double-knockout mice present distinct respiratory phenotypes

We investigated the role of muscarinic acetylcholine receptors in the control of breathing. Baseline breathing at rest and ventilatory responses to brief exposures to hypoxia (10% O(2)) and hypercapnia (3% and 5% CO(2)), measured by whole-body plethysmography in partially restrained animals, were compared in mice lacking either M(1) and M(3) or M(2) and M(4) muscarinic receptors, and in wild-type matched controls. M(1/3)R double-knockout mice showed at rest an elevated ventilation (V (E)) due to a large (57%) increase in tidal volume (V(T)). Chemosensory ventilatory responses were unaltered. M(2/4)R double-knockout mice were agitated and showed elevated V (E) and breathing frequency (f(R)) at rest when partially restrained, but unaltered V (E) and low f(R) when recorded unrestrained. Chemosensory ventilatory responses were unaltered. The results suggest that M(1) and M(3) receptors are involved in the control of tidal volume, while M(2) and M(4) receptors may be involved in the control of breathing frequency at rest and response to stress.

The pre-Bötzinger oscillator in the mouse embryo

Studies of the sites and mechanisms involved in mammalian respiratory rhythm generation point to two clusters of rhythmic neurons forming a coupled oscillator network within the brainstem. The location of these oscillators, the pre-Bötzinger complex (preBötC) at vagal level, and the para-facial respiratory group at facial level, probably result from regional patterning schemes specifying neural types in the hindbrain during embryogenesis. Here, we report evidence that the preBötC oscillator (i) is first active at embryonic stages, (ii) originates in the post-otic hindbrain neural tube and (iii) requires the glutamate vesicular transporter 2 for rhythm generation.

Abnormal inspiratory depth in Phox2a haploinsufficient mice

Mutations of genes encoding Phox2a or Phox2b transcription factors induce modifications of different brainstem neuronal networks. Such modifications are associated with defects in breathing behavior at birth. In particular, an abnormal breathing frequency is observed in Phox2a-/- mutant mice, resulting from abnormal development of the locus coeruleus (LC) nucleus. However, the role of Phox2a proteins in the establishment of respiratory neuronal pathways is unknown, largely because mutants die shortly after birth. In the present study, we examined the effects of a haploinsufficiency of the Phox2a gene. Phox2a heterozygotes survive and exhibit a significantly larger inspiratory volume both during normoxic breathing and in response to hypoxia and a delayed maturation of inspiratory duration compared to wild-type animals. This phenotype accompanied by an unaltered frequency is evident at birth and persists until at least postnatal day 10. Morphological analyses of Phox2a+/- animals revealed no anomaly in the LC region, but highlighted an increase in the number of cells expressing tyrosine hydroxylase enzyme, a marker of chemoafferent neurons, in the petrosal sensory ganglion. These data indicate that Phox2a plays a critical role in the ontogeny of the reflex control of inspiration.

Pre-/post-otic rhombomeric interactions control the emergence of a fetal-like respiratory rhythm in the mouse embryo

How regional patterning of the neural tube in vertebrate embryos may influence the emergence and the function of neural networks remains elusive. We have begun to address this issue in the embryonic mouse hindbrain by studying rhythmogenic properties of different neural tube segments. We have isolated pre- and post-otic hindbrain segments and spinal segments of the mouse neural tube, when they form at embryonic day (E) 9, and grafted them into the same positions in stage-matched chick hosts. Three days after grafting, in vitro recordings of the activity in the cranial nerves exiting the grafts indicate that a high frequency (HF) rhythm (order: 10 bursts/min) is generated in post-otic segments while more anterior pre-otic and more posterior spinal territories generate a low frequency (LF) rhythm (order: 1 burst/min). Comparison with homo-specific grafting of corresponding chick segments points to conservation in mouse and chick of the link between the patterning of activities and the axial origin of the hindbrain segment. This HF rhythm is reminiscent of the respiratory rhythm known to appear at E15 in mice. We also report on pre-/post-otic interactions. The pre-otic rhombomere 5 prevents the emergence of the HF rhythm at E12. Although the nature of the interaction with r5 remains obscure, we propose that ontogeny of fetal-like respiratory circuits relies on: (i) a selective developmental program enforcing HF rhythm generation, already set at E9 in post-otic segments, and (ii) trans-segmental interactions with pre-otic territories that may control the time when this rhythm appears.

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.

Ontogeny of central rhythm generation in chicks and rodents

Recent studies help in understanding how the basic organization of brainstem neuronal circuits along the anterior-posterior (AP) axis is set by the Hox-dependent segmentation of the neural tube in vertebrate embryos. Neonatal respiratory abnormalities in Krox20(-/-), Hoxa1(-/-) and kreisler mutant mice indicate the vital role of a para-facial (Krox20-dependent, rhombomere 4-derived) respiratory group, that is distinct from the more caudal rhythm generator called Pre-Bötzinger complex. Embryological studies in the chick suggest homology and conservation of this Krox20-dependent induction of parafacial rhythms in birds and mammals. Calcium imaging in embryo indicate that rhythm generators may derive from different cell lineages within rhombomeres. In mice, the Pre-Bötzinger complex is found to be distinct from oscillators producing the earliest neuronal activity, a primordial low-frequency rhythm. In contrast, in chicks, maturation of the parafacial generator is tightly linked to the evolution of this primordial rhythm. It seems therefore that ontogeny of brainstem rhythm generation involves conserved processes specifying distinct AP domains in the neural tube, followed by diverse, lineage-specific regulations allowing the emergence of organized rhythm generators at a given AP level.

Neural tube patterning by Krox20 and emergence of a respiratory control

Recent data begin to bridge the gap between developmental events controlling hindbrain neural tube regional patterning and the emergence of breathing behaviour in the fetus and its vital adaptive function after birth. In vertebrates, Hox paralogs and Hox-regulating genes orchestrate, in a conserved manner, the transient formation of developmental compartments in the hindbrain, the rhombomeres, in which rhythmic neuronal networks of the brainstem develop. Genetic inactivation of some of these genes in mice leads to pathological breathing at birth pointing to the vital importance of rhombomere 3 and 4 derived territories for maintenance of the breathing frequency. In chick embryo at E7, we investigated neuronal activities generated in neural tube islands deriving from combinations of rhombomeres isolated at embryonic day E1.5. Using a gain of function approach, we reveal a role of the transcription factor Krox20, specifying rhombomeres 3 and 5, in inducing a rhythm generator at the parafacial level of the hindbrain. The developmental genes selecting and regionally coordinating the fate of CNS progenitors may hold further clues to conserved aspects of neuronal network formation and function. However, the most immediate concern is to take advantage of early generated rhythmic activities in the hindbrain to pursue their downstream cellular and molecular targets, for it seems likely that it will be here that rhythmogenic properties will eventually take on a vital role at birth.

Increased ventilation and CO2 chemosensitivity in acetylcholinesterase knockout mice

To investigate the effects of a permanent excess of acetylcholine (AChE) on respiration, breathing and chemosensitivity were analyzed from birth to adulthood in mice lacking the AChE gene (AChE-/-), in heterozygotes, and in control wild-type (AChE+/+) littermates. Breathing at rest and ventilatory responses to brief exposures to hypoxia (10% O2) and hypercapnia (3-5% CO2) were measured by whole-body plethysmography. At rest AChE-/- mice show larger tidal volumes (VT, + 96% in adults), overall ventilation (VE, + 70%), and mean inspiratory flow (+270%) than wild-type mice, with no change in breathing frequency (fR). AChE-/- mice have a slightly blunted response to hypoxia, but increased VE and fR responses to hypercapnia. Heterozygous animals present no consistent alterations of breathing at rest and chemosensitivity is normal. Adult AChE-/- mice have an increased VE/VO2 and a marginally higher normalized VO2. The results suggest that the hyperventilation and altered chemosensitivity in AChE-/- mice largely reflect alterations of central respiratory control.

Generation of a novel functional neuronal circuit in Hoxa1 mutant mice

Early organization of the vertebrate brainstem is characterized by cellular segmentation into compartments, the rhombomeres, which follow a metameric pattern of neuronal development. Expression of the homeobox genes of the Hox family precedes rhombomere formation, and analysis of mouse Hox mutations revealed that they play an important role in the establishment of rhombomere-specific neuronal patterns. However, segmentation is a transient feature, and a dramatic reconfiguration of neurons and synapses takes place during fetal and postnatal stages. Thus, it is not clear whether the early rhombomeric pattern of Hox expression has any influence on the establishment of the neuronal circuitry of the mature brainstem. The Hoxa1 gene is the earliest Hox gene expressed in the developing hindbrain. Moreover, it is rapidly downregulated. Previous analysis of mouse Hoxa1(-/-) mutants has focused on early alterations of hindbrain segmentation and patterning. Here, we show that ectopic neuronal groups in the hindbrain of Hoxa1(-/-) mice establish a supernumerary neuronal circuit that escapes apoptosis and becomes functional postnatally. This system develops from mutant rhombomere 3 (r3)-r4 levels, includes an ectopic group of progenitors with r2 identity, and integrates the rhythm-generating network controlling respiration at birth. This is the first demonstration that changes in Hox expression patterns allow the selection of novel neuronal circuits regulating vital adaptive behaviors. The implications for the evolution of brainstem neural networks are discussed.

Respiratory function in adult mice lacking the mu-opioid receptor: role of delta-receptors

Mice lacking the mu-opioid receptor (MOR) provide a unique model to determine whether opioid receptors are functionally interactive. Recent results have shown that respiratory depression produced by delta-opioid receptor agonists is suppressed in mice lacking the mu-opioid receptor. Here we investigated the involvement of mu- and delta-opioid receptors in the control of ventilation and mu/delta receptor interactions in brainstem rhythm-generating structures. Unrestrained MOR-/- and wild-type mice showed similar ventilatory patterns at rest and similar chemosensory responses to hyperoxia (100% O2), hypoxia (10% O2) or hypercapnia (5%CO2-95%O2). Blockade of delta-opioid receptors with naltrindole affected neither the ventilatory patterns nor the ventilatory responses to hypoxia in MOR-/- and wild-type mice. In-vitro, respiratory neurons were recorded in the pre-Bötzinger complex of thick brainstem slices of MOR-/- and wild-type young adult mice. Respiratory frequency was not significantly different between these two groups. The delta2 receptor agonist deltorphin II (0.1-1.0 microM) decreased respiratory frequency in both groups whereas doses of the delta1 receptor agonist enkephalin[D-Pen2,5] (0.1-1.0 microM) which were ineffective in wild-type mice significantly decreased respiratory frequency in MOR-/- mice. We conclude that deletion of the mu-opioid receptor gene has no significant effect on ensuing respiratory rhythm generation, ventilatory pattern, or chemosensory control. In MOR-/- mice, the loss of respiratory-depressant effects of delta2-opioid receptor agonists previously observed in vivo does not result from a blunted response of delta receptors in brainstem rhythm-generating structures. These structures show an unaltered response to delta2-receptor agonists and an augmented response to delta1-receptor agonists.

Muscarinic Depression of Synaptic Transmission in the Epileptogenic GABA Withdrawal Syndrome Focus

The GABA withdrawal syndrome (GWS) is a model of local status epilepticus consecutive to the interruption of a prolonged GABA infusion into the rat somatomotor cortex. Bursting patterns in slices from GWS rats include intrinsic bursts of action potentials (APs) induced by intracellular depolarizing current injection and/or paroxysmal depolarization shifts (PDSs) induced by white matter stimulation. Possible changes in the effects of cholinergic drugs after in vivo induction of GWS were investigated on bursting cells ( n = 30) intracellularly recorded in neocortical slices. In GWS slices, acetylcholine (Ach, 200-1000 μM) or carbachol (Cch, 50 μM) applications increased the number of bursts induced by depolarizing current injection while synaptically induced PDSs were significantly diminished (by 50–60%) or even blocked independently of the cholinergic-induced depolarization. The intrinsic burst facilitation and PDS depression provoked by Ach or Cch were mimicked by methyl-acetylcholine (mAch, 100–400 μM, n = 11), were reversed by atropine application (1–50 μM, n = 3), and were not mimicked by nicotine (50–100 μM, n = 4), indicating the involvement of muscarinic receptors. In contrast, in nonbursting cells from the same epileptic area ( n = 42) or from equivalent area in control rats ( n = 24), a nonsignificant muscarinic depression of EPSPs was induced by Cch and Ach. The mAch depression of excitatory postsynaptic potential (EPSPs) was significantly lower than that seen for PDSs in GWS rats. None of the cholinergic agonists caused bursting appearance in these cells. Therefore the present study demonstrates a unique implication of muscarinic receptors in exerting opposite effects on intrinsic membrane properties and on synaptic transmission in epileptiform GWS. Muscarinic receptor mechanisms may therefore have a protective role against the development and spread of epileptiform activity from the otherwise-activated epileptic focus.

Genetic and developmental models for the neural control of breathing in vertebrates

The present paper reviews some of the possible mechanisms that may link gene function in the brainstem and breathing patterns in vertebrates. On one hand, adaptation and acclimatisation of mature breathing to environmental constraints such as hypoxia, involves complex regulation of the gene expression in precise cardiorespiratory sites of the brainstem. On the other hand, targeted inactivation of different genes suggests that postnatal respiratory variables at rest depend on genes controlling the prenatal development of the brainstem. During embryogenesis, neurotrophins (gdnf, bdnf) regulate the survival of specific cellular populations composing the respiratory neuronal network. The expression of developmental genes such as Hox and Krox-20 initiates hindbrain segmentation, the earliest sign of regionalisation in the brainstem. As shown in the chick embryo, segmental specifications allow the establishment of an active embryonic rhythmic network and later insertion of specific neuronal circuits increasing the primordial rhythm frequency to near mature values.

NMDA receptor activity in utero averts respiratory depression and anomalous long-term depression in newborn mice

Mutant mice lacking NMDA receptor 1 subunit (NR1) showed marked depression of respiratory and suckling activities in vivo and overexpression of synaptic long-term depression (LTD) in a brainstem cardiorespiratory-related region (nucleus tractus solitarius) in vitro. Pharmacological blockade of NMDA receptors in normal newborn mice mimicked the depression in suckling activity but not respiratory depression in vivo or brainstem LTD in vitro. Results at the behavioral and cellular levels demonstrate that NMDA receptor deficiency during prenatal development may unleash an anomalous form of NMDA receptor-independent LTD along with life-threatening respiratory depression consequences in the newborn. These findings raise the specter of cardiorespiratory dysregulation with increased risks of morbidity and mortality in the infant as a result of premature births or genetic or drug-induced NMDA receptor antagonism during pregnancy.

Branchiomotor activities in mouse embryo

Using a novel isolated hindbrain in vitro preparation, we demonstrate that, in the mouse, branchiomotor activities from trigeminal, facial, glossopharyngeal and vagal nerves start during segmentation, a crucial and conserved period of hindbrain embryogenesis. At embryonic day (E) 10.5, branchiomotor nerves are independently active in bursts, become coactive at a low frequency (about 0.5 min-1) at E12.5, before high frequency (about 15 min-1) fetal breathing starts at E14.5. Comparison with observations in chick reveals a transient episodic rhythmic pattern highly similar in mouse at E13.5 and chick at E7. This pattern is proposed as a marker identifying a phylotypic stage during the development of hindbrain neuronal networks in vertebrates.

The GABA-withdrawal syndrome: a model of local status epilepticus

The GABA-withdrawal syndrome (GWS) is a model of local status epilepticus following the interruption of a chronic GABA infusion into the rat somatomotor cortex. GWS is characterized by focal epileptic electroencephalographic discharges and associated contralateral myoclonus. In neocortical slices obtained from GWS rats, most neurons recorded in the GABA-infused area are pyramidal neurons presenting bursting properties. The bursts are induced by white-matter stimulation and/or intracellular depolarizing current injection and correlate with a decrease of cellular sensitivity to GABA, caused by its prolonged infusion. This effect is related to a calcium influx that may reduce the GABAA receptor-mediated inward current and is responsible for the bursting properties. Here we present evidence for the involvement of calcium- and NMDA-induced currents in burst genesis. We also report modulatory effects of noradrenaline appearing as changes on firing patterns of bursting and nonbursting cells. Complementary histochemical data reveal the existence of a local noradrenergic hyperinnervation and an ectopic expression of tyrosine hydroxylase mRNAs in the epileptic zone.

Segmental specification of GABAergic inhibition during development of hindbrain neural networks

A primordial rhythm-generating neural network emerges during the segmental period of vertebrate hindbrain development, suggesting a common genetic basis to both the structure and network activity of the region. We show here that segmentation influenced a postsegmental developmental step by which a GABAergic rhythm generator was incorporated into the primordial network and increased rhythm frequency to near mature values. This process depended on specifications in r3 and r5 that controlled, on the basis of a two-segment repeat, later maturation of GABAergic inhibition.

Pontine and medullary control of the respiratory activity in the trigeminal and facial nerves of the newborn mouse: an in vitro study

In vitro, the respiratory activity in rodents is characterized by: (i) the rapidly peaking, slowly decrementing pattern of spontaneous and rhythmic active phases recorded from the motor rootlets, and (ii) the specific location of their rhythmic generator in the ventrolateral medulla. The aim of the present study was to assess whether the trigeminal and facial motor rootlets still exhibit respiratory activity in the absence of peripheral and higher cerebral structures, and to compare the onset of their active phases with that of other respiratory rootlets, using the in vitro isolated brainstem--spinal cord preparation of the newborn mouse and rat. Spontaneous rhythmic activity was recorded from the trigeminal and facial rootlets. It was regular and synchronized bilaterally and ipsilaterally with the hypoglossal or cervical C1-C6 rootlets. Brainstem transection experiments demonstrated that for both the trigeminal and facial rootlets, the spontaneous rhythmic activity originates from the medulla, in a region consistent with the pre-Bötzinger complex and the rostral ventrolateral medulla. The pattern of the respiratory motor activity recorded from the trigeminal and facial rootlets was identical to the pattern recorded from the hypoglossal and cervical C1-C6 rootlets with rapidly peaking, slowly decrementing characteristics. The duration of the ascending part and the total duration of their active phases were similar. The onset of the active phases of the phrenic rootlets was delayed compared with that of the trigeminal, facial and hypoglossal rootlets. However, no difference in the onsets of the active phases of the cranial rootlets could be observed. Removal of the rostral pons suppressed the delay in onset of the active phases of the phrenic rootlets. Our findings show that: (i) rhythmic activities of the trigeminal and facial rootlets are preserved in absence of control by peripheral or high cerebral structures; (ii) the pattern and the location of the rhythmic generator for these activities are of the respiratory type; and (iii) the rostral pons is responsible for a delay in the onset of the active phases of the phrenic rootlets compared with that of the trigeminal, facial and hypoglossal rootlets.

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.

A ventral pontine pathway promotes rhythmic activity in the medulla of neonate mice

We have developed in new-born mice a ventral tilted-horizontal slice preparation for pontine stimulation and recording of spontaneous respiratory-like rhythmic trains of glutamatergic excitatory postsynaptic potentials (EPSPs) in medullary neurons. Electrical stimulations (10-50 Hz for 100-500 ms) of the caudal pontine reticular formation triggered a burst of EPSPs, recycling of the rhythmic activity and persistent increase of the rhythmic behaviour. These results identify a ventral pontine pathway that promotes rhythm generating mechanisms in the medulla and probably derives from a population of lateral reticular neurons identified in the embryonic hindbrain and eliminated after inactivation of the early developmental gene Krox-20.

K+ channels of expiratory neurones in the brain stem preparation of the newborn rat

Single channel activity of expiratory neurones was studied in outside-out recordings. Expiratory neurones were identified in the ventrolateral region of the in vitro isolated brain stem-spinal cord preparation of newborn rats in cell-attached and whole-cell configurations by their pattern of firing related to phrenic motor output. Three potassium (K+) channels of 10, 30 and 70 pS exhibited steady-state activity during long voltage commands (up to 5 min) and could be found associated together in the same patches. The 30pS channel showed voltage dependency, being most active at small depolarizations. The 70 pS channel showed little activity with < 1% of openings per sample time and 1 mM tetraethylammonium (TEA) sensitivity. At similar concentrations, the discharge of the phrenic nerve was also altered, as shown by the increase of the respiratory frequency and a tonic discharge. The association of these K+ channel types on the same patches may be specific of respiratory neurones and could contribute to their bursting activity.

Early ontogeny of rhythm generation and control of breathing

The ability of central networks to produce rhythmic motor behaviours linked to the respiratory function, is a remarkably conserved property of the brainstem reticular formation in vertebrates. Conserved cellular and molecular mechanisms also underlie the early embryonic development of the brainstem, leading to a segmented rhombencephalon in all vertebrates. We have proposed that the neural network that controls breathing after birth, derives from a primordial rhythmic network first active in the segmented hindbrain of the embryo. Observations on transgenic mice support this hypothesis: homozygous inactivation of Krox-20, a gene governing segmentation, leads to a lower-than-normal respiratory frequency (fR), despite fetal maturation of the respiratory network and functional compensatory control after birth.

Phasic and long-term depression in brainstem nucleus tractus solitarius neurons: differing roles of AMPA receptor desensitization

One important question concerning the homeostatic regulation of many physiological processes is whether the control mechanisms are purely reflexogenic or whether they may involve neural adaptation in the form of learning and memory in the brainstem. Using a brainstem slice preparation in the rat, we studied the modifiability of neural transmission in the first-order synapses of the medial and commissural nucleus tractus solitarius of the medulla. Sustained low-frequency stimulation (5 Hz) of primary afferent fibers in the tractus solitarius resulted in a phasic depression (accommodation) of synaptic strength as reflected by a concomitant decrease in the evoked excitatory postsynaptic potentials. In one group of neurons (type I), synaptic strength recovered rapidly after low-frequency stimulation, whereas in another group of neurons (type II), synaptic strength remained depressed for >30 min, i.e., manifesting long-term depression (LTD). The latter was switched into a short-term depression lasting 15-25 min after pharmacological blockade of NMDA receptor channels with D-APV or chelation of intracellular calcium ions with EGTA, whereas the accommodation phase was unaffected. Application of an AMPA receptor anti-desensitization agent cyclothiazide abolished the LTD, but not the accommodation response. These results suggest the presence of separate postsynaptic sites for the induction of LTD and accommodation, one being sensitive to cyclothiazide, whereas the other is not. Moreover, the maintenance of LTD is dependent on the level of intracellular Ca2+. These phasic and long-term synaptic plasticity in the nucleus tractus solitarius may play a role in the homeostatic regulation of cardiorespiratory functions.

Primordial respiratory-like rhythm generation in the vertebrate embryo

Respiration is a rhythmic motor behavior that appears in the fetus and acquires a vital importance at birth. It is generated centrally, within neuronal networks of the hindbrain. This region of the brain is of particular interest since it is the best understood with respect to the cellular and molecular mechanisms that underlie its development. Examination of hindbrain activities in the chick embryo has revealed that the central rhythm generator is active before fetal maturation and conforms to the rhombomeric organization of the embryonic hindbrain. Inactivation of genes required for the normal formation of rhombomeres in mice leads to perturbations of the reticular formation that affect respiration after birth and compromise the probability of survival. From studies of hindbrain development we might gain an understanding of how genes govern the early embryonic development of neuronal networks and how this might specify patterns of motor activities operating throughout life.

Reorganization of pontine rhythmogenic neuronal networks in Krox-20 knockout mice

We have shown previously that the inactivation of the zinc finger gene Krox-20 affects hindbrain segmentation, resulting in the elimination of rhombomeres 3 and 5. We demonstrate here that Krox-20 homozygous mutant mice exhibit abnormally slow respiratory and jaw opening rhythms, indicating that a modification of hindbrain segmentation influences the function of neuronal networks after birth. Central neuronal networks that control respiratory frequency are made predominantly depressant by the elimination of a previously undescribed rhythm-promoting system. Recordings of rhythmic activity from the isolated hindbrain following progressive tissue transections indicate that the reorganization takes place in the caudal pontine reticular formation. The newborn (PO) Krox-20-/- mice, in which apneas are ten times longer than in wild-type animals, may be a valuable model for the study of life-threatening apneas during early infancy.

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.

Mice lacking brain-derived neurotrophic factor exhibit visceral sensory neuron losses distinct from mice lacking NT4 and display a severe developmental deficit in control of breathing

The neurotrophins brain-derived neurotrophic factor (BDNF) and neurotrophin-4/5 (NT4) act via the TrkB receptor and support survival of primary somatic and visceral sensory neurons. The major visceral sensory population, the nodose-petrosal ganglion complex (NPG), requires BDNF and NT4 for survival of a full complement of neurons, providing a unique opportunity to compare gene dosage effects between the two TrkB ligands and to explore the possibility that one ligand can compensate for loss of the other. Analysis of newborn transgenic mice lacking BDNF or NT4, or BDNF and NT4, revealed that survival of many NPG afferents is proportional to the number of functional BDNF alleles, whereas only one functional NT4 allele is required to support survival of all NT4-dependent neurons. In addition, subpopulation analysis revealed that BDNF and NT4 can compensate for the loss of the other to support a subset of dopaminergic ganglion cells. Together, these data demonstrate that the pattern of neuronal dependencies on BDNF and NT4 in vivo is far more heterogeneous than predicted from previous studies in culture. Moreover, BDNF knockout animals lack a subset of afferents involved in ventilatory control and exhibit severe respiratory abnormalities characterized by depressed and irregular breathing and reduced chemosensory drive. BDNF is therefore required for expression of normal respiratory behavior in newborn animals.

Pharmacological properties of peripherally induced postsynaptic potentials in bulbar respiratory neurons of decerebrate cats

Intracellular recordings of bulbar inspiratory and post-inspiratory neurons, combined with extracellular iontophoresis of antagonists of putative neurotransmitters, were performed in decerebrate cats. Inhibitory postsynaptic potentials (IPSPs) evoked by stimulation of the superior laryngeal nerve or vagus nerve were depressed by bicuculline in all 22 neurons tested, but not modified by strychnine. The non-N-methyl-D-aspartate (NMDA) glutamate antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) decreased the neurally evoked excitatory postsynaptic potentials (EPSPs) in 23 out of 26 neurons tested, while the NMDA antagonist dizocilpine had no notable effect. The present results suggest that the peripherally induced IPSPs are mediated through gamma-aminobutyric acid (GABA)A receptors and the EPSPs through non-NMDA glutamate receptors in bulbar respiratory neurons.

A cholecystokinin-B receptor antagonist potentiates GABAergic and glycinergic inhibition in the nucleus of the solitary tract of the rat

In both rodent and primate in vivo models, cholecystokininB (CCKB) antagonists such as PD134,308 have anxiolytic effects that may involve the potentiation of GABAergic transmission. We have investigated this interaction using exogenous application of GABA and whole cell patch recording techniques in neurons of the nucleus of the solitary tract (NTS) in brainstem slice preparations. In the presence of PD143,308 the magnitude of the GABA-evoked decrease in membrane input resistance was enhanced by 41.2 +/- 3.1% and the duration of the response was prolonged by 34.8 +/- 2.2%. Also, PD134, 308 potentiated glycine-evoked decreases in membrane input resistance, increasing the amplitude of the response by 62.8 +/- 4. 85 and prolonging the duration of the response by 23.5 +/- 3.6%. The effect of PD134,308 persisted in the presence of tetrodotoxin, after reversal of the transmembrane gradient of chloride ions and under conditions of exaggerated GABAA receptor desensitization. Our results demonstrate that at least part of the functional link between PD134,308 and the GABAA response occurs postsynaptically.

Electroresponsive properties and membrane potential trajectories of three types of inspiratory neurons in the newborn mouse brain stem in vitro

1. The electrophysiological properties of inspiratory neurons were studied in a rhythmically active thick-slice preparation of the newborn mouse brain stem maintained in vitro. Whole cell patch recordings were performed from 60 inspiratory neurons within the rostral ventrolateral part of the slice with the aim of extending the classification of inspiratory neurons to include analysis of active membrane properties. 2. The slice generated a regular rhythmic motor output recorded as burst of action potentials on a XII nerve root with a peak to peak time of 11.5 +/- 3.4 s and a duration of 483 +/- 54 ms (means +/- SD, n = 50). Based on the electroresponsive properties and membrane potential trajectories throughout the respiratory cycle, three types of inspiratory neurons could be distinguished. 3. Type-1 neurons were spiking in the interval between the inspiratory potentials (n = 9) or silent with a resting membrane potential of -48.6 +/- 10.1 mV and an input resistance of 306 +/- 130 M omega (n = 15). The spike activity between the inspiratory potentials was burst-like with spikes riding on top of an underlying depolarization (n = 11) or regular with no evidence of bursting (n = 12). Hyperpolarization of the neurons below threshold for spike initiation did not reveal any underlying phasic synaptic activity, that could explain the bursting behavior. 4. Type-1 neurons showed delayed excitation after hyperpolarizing square current pulses or when the neurons were depolarized from a hyperpolarized level. This membrane behavior resembles the response seen in other CNS neurons expressing an IA. The response to 1-s long depolarizing pulses with a large current strength showed signs of activation of an active depolarizing membrane response leading to a transient reduction in the spike amplitude. The relationship between the membrane potential and the amplitude of square current pulses (Vm-I) showed a small upward rectification below -70 mV, and spike adaptation throughout a 1-s pulse had a largely linear time course. 5. Type-1 neurons depolarized and started to fire spikes 398 +/- 102 ms (n = 20) before the upstroke of the integrated XII nerve discharge. The inspiratory potential was followed by fast hyperpolarization, a short fast-repolarizing phase (1,040 +/- 102 ms, n = 5) and a longer slow-repolarizing phase (lasting until the next inspiratory discharge). 6. Type-2 neurons were spiking in the interval between the inspiratory potentials with no evidence of bursting behavior and had an input resistance of 296 +/- 212 M omega (n = 26). The response to hyperpolarizing pulses revealed an initial sag and postinhibitory rebound depolarization. This membrane behavior resembles the response seen in other CNS neurons expressing an Ih. The Vm-I relationship was linear at depolarized potentials and showed a marked upward rectification below -60 mV. Spike trains elicited by 1-s long pulses showed a pronounced early and late adaptation. 7. Type-2 neurons depolarized and started to fire spikes 171 +/- 87 ms (n = 23) before the upstroke of the integrated XII nerve discharge. The inspiratory potential had a variable amplitude from cell to cell and was followed by a short hyperpolarization in the cells displaying a large amplitude inspiratory potential.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyrotropin-releasing hormone (TRH) depolarizes a subset of inspiratory neurons in the newborn mouse brain stem in vitro

1. To extend the classification of respiratory neurons based on active membrane properties and discharge patterns to include responses to respiratory modulators, we have studied the effect of thyrotropin-releasing hormone (TRH, 1-5 microM) on the spontaneous respiratory-related neural activity in a thick brain stem slice preparation from the newborn mouse. The action of TRH on the respiratory output from the slice was investigated by recordings from the XII nerve. Cellular responses to TRH were investigated using whole cell recordings from hypoglossal motoneurons and three types of inspiratory neurons located in the rostral ventrolateral part of the slice. 2. Bath-applied TRH (1 microM) decreased the time between inspiratory discharges recorded on the XII nerve from 12.3 +/- 3.3 s to 4.9 +/- 1.1 s (n = 28; means +/- SD), i.e., caused an approximate threefold increase in the respiratory frequency. The coefficient of variation of the time between the inspiratory discharges decreased by one-half. Thus the respiratory output became more stable in response to TRH. The duration of the inspiratory discharges increased from 474 +/- 108 ms to 679 +/- 114 ms, and the amplitude decreased by 24%. An increase in the interdischarge noise on the XII nerve was recorded in the early phase of the TRH application. 3. Anatomically identified hypoglossal motoneurons (7 cells) responded to bath applied TRH with a depolarization eliciting spikes between the inspiratory potentials. The depolarization was accompanied by an increase in spontaneous excitatory synaptic activity that disappeared late during the TRH application. The duration of the inspiratory potentials was increased, indicating that the hypoglossal motoneurons received a longer duration synaptic input from the respiratory rhythm generator. 4. Type-1 inspiratory neurons showed a prolonged depolarization (3 cells), a transient depolarization (2 cells), or no change in membrane potential (2 cells) during 10 min of continued superfusion with a TRH-containing solution. The duration of the inspiratory potentials was increased during the TRH superfusion. With tetrodoxin (TTX, 1 microM) present in the superfusing solution TRH induced a prolonged depolarization (3 cells) or a transient depolarization (1 cell), demonstrating that type-1 inspiratory neurons are depolarized postsynaptically by TRH. The input resistance was not changed during the depolarizing response to TRH. 5. Type-2 inspiratory neurons showed a transient depolarization (7 cells) in response to bath-applied TRH. The duration of the inspiratory potentials was increased markedly during TRH. The transient depolarization was not the result of a postsynaptic action of TRH, because type-2 neurons (9 cells) showed no depolarization to TRH with TTX present in the superfusing solution. 6. Type-3 inspiratory neurons showed a transient depolarization (4 cells) with a partial recovery of the membrane potential late during the TRH application. The duration of the inspiratory potentials increased markedly during TRH. Four cells showed a transient depolarization with an increase in input resistance during TRH with TTX present in the superfusing solution. Thus type-3 neurons are depolarized postsynaptically by TRH. 7. We conclude that TRH increases the frequency of the respiratory rhythm in newborn mice through an action at the level of the brain stem.(ABSTRACT TRUNCATED AT 250 WORDS)

Brain stem energy metabolism response to acute hypoxia in anaesthetized rats: a 31P NMR study

Mammals react to acute hypoxia with an initial augmentation and a secondary depression of the respiratory rhythm generated by brain stem neuronal networks. To investigate the cytosolic level of energy rich phosphorus metabolites during these responses, we developed 31P nuclear magnetic resonance spectroscopy of the brain stem. Moderate hypoxia (paO2 = 40 mmHg, 2 min) caused a reversible 62 +/- 15% respiratory rhythm depression and decreased cytosolic phosphocreatine levels by 43 +/- 11% (p < 0.01, n = 7) without affecting adenosine triphosphate levels. Cellular metabolic depletion therefore contributes to the brain stem response to hypoxia, and appears to reflect adaptive mechanisms to limited oxygen availability in the brain stem.

Rhythm generation in the segmented hindbrain of chick embryos

1. The embryonic hindbrain of chick is segmented until stage 24, when it starts to generate rhythmic activities in cranial nerves. In order to recognize a possible influence of segmentation on the organization of neuronal systems generating motor rhythms, the activity of trigeminal, facial, glossopharyngeal, vagal and hypoglossal nerves was studied during embryonic stages 24-36, by simultaneous recording of different cranial nerves in an isolated, superfused chick hindbrain preparation. 2. Highly correlated recurrent episodes of cyclical burst discharges occurred in all nerves studied (correlation coefficients, 0.8 +/- 0.1) throughout stages 24-36. 3. Such coactivation is unlikely to be due to monosynaptic connections between widely divergent premotor neurons and motoneurons, or between motoneurons themselves, because no short-term correlation was apparent in the millisecond range between activities of different motor nerves. 4. Complete transverse or midsagittal sectioning of the hindbrain disrupted coactivation of nerves located at distinct rostrocaudal levels or occupying an ipsi- or contralateral position, respectively, while sparing the ability of individual nerves to generate rhythmic activity. Each hindbrain segment thus contains bilaterally the motor nuclei together with their own rhythm generator. Coactivation of motor patterns appears to result from intersegmental and cross-median connections between these rhythm generators. 5. The results are in keeping with the hypothesis of a segmental organization of the primordial hindbrain rhythm generator and give further support to the early determination of both the anatomical and the functional fate of neurons in this region of the vertebrate central nervous system.

A potassium current controls burst termination in rat neocortical neurons after GABA withdrawal

Bursting activities were investigated under conditions of reduced outward K+ currents in neocortical slices obtained from rats presenting the gamma-aminobutyric acid (GABA)-withdrawal syndrome (GWS), a focal epilepsy consecutive to the interruption of a chronic intracortical GABA infusion into the somatomotor cortex. These bursts were induced by intracellular depolarizing current injection and/or by white matter stimulation. Tetraethylammonium (TEA) at doses which did not change input resistance, spike duration or first interspike time interval abolished the burst terminating process and induced plateau-like potentials (up to 500 ms) which were tetrodotoxin-resistant and blocked by Ca2+ antagonists Cd2+ and Co2+. Therefore, it appears that bursts during GWS are generated by Ca(2+)-dependent plateau potentials which are terminated by a K+ current highly sensitive to TEA.

Calcium-dependent conductances control neurones involved in termination of inspiration in cats

Intracellular injection of the calcium chelator BAPTA into postinspiratory (PI) and late inspiratory neurones (late-I) of the ventral respiratory group of anaesthetised cat was performed to study the role of intracellular free calcium in patterning the activity of neurones controlling termination of inspiration. BAPTA injection into neurones resulted in an increase of input resistance and prolongation of action potential discharge with reduced adaptation. In addition, late-I neurones developed a secondary burst of action potentials during the postinspiratory phase of the cycle. We conclude that intracellular free calcium controls (1) the duration of activation and the degree of adaptation of PI neurones and (2) repolarisation of late-I neurones during postinspiration.

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.

Respiratory rhythm generation in chick hindbrain: effects of MK-801 and vagotomy

Hindbrain mechanisms generating the respiratory rhythm in chicks were analysed. In vivo, ventilation and intercostal muscle activity were recorded in chicks (1 and 2.5 weeks-old), vagotomized and treated with the NMDA receptor blocker MK-801 (dizocilpine). In vitro, synaptic transmission from vagal to second-order sensory neurones was studied in the nucleus of the solitary tract, using whole-cell recordings in slices. Vagal afferents were found to act through GABAergic synapses and control two hindbrain systems: a dizocilpine-sensitive control system and a rhythm generator. Although this organization is the same as in mammals, after vagotomy entirely different respiratory patterns emerge: (i) expiratory-inspiratory efforts triggered by the rhythm generator and (ii) periods of apnoea produced by the dizocilpine-sensitive system.

Onset and maturation of branchio-motor activities in the chick hindbrain

Branchio-motor activities from trigeminal, facial, glossopharyngeal and vagal nerves, in chick embryos have been recorded using suction electrodes on an isolated preparation of the hindbrain in vitro between developmental stages 20 and 36. They were composed of recurring episodes of cyclical burst discharges first identified at stage 24, therefore constituting one of the earliest organized activities generated in the chick central nervous system. Between stage 24 and 36, both the period between episodes and the number of bursts per episode were increased. This maturation sequence was preserved for several hours in vitro in the absence of supraspinal and sensory inputs. Results are in agreement with rhythmogenic properties constituting an early functional commitment of neuronal networks in this particular region of the neuroepithelium.

Noradrenaline mediates paradoxical effects on rat neocortical neurons after GABA withdrawal

1. The aim of the present study was to determine the role of noradrenergic neurotransmission in neuronal activities intracellularly recorded in neocortical slices obtained from rats presenting the gamma-aminobutyric acid (GABA) withdrawal syndrome (GWS), a focal epilepsy consecutive to the interruption of a chronic intracortical GABA infusion into the somatomotor cortex. Neurons recorded in the epileptic focus area (n = 52) were bursting or nonbursting cells. Intrinsic bursting (IB, n = 20) cells presented bursts of action potentials (APs) to an intracellular depolarizing current injection and paroxysmal depolarization shifts (PDSs) to white matter stimulation. Synaptic bursting (SB, n = 22) cells presented only PDSs. Nonbursting (NB, n = 10) cells presented no burst after either synaptic stimulation or depolarizing current injection. Results were compared with those obtained from NB neurons (n = 4) recorded in slices from saline-infused rats. 2. In all of the recorded neurons, bath application of norepinephrine (NE, 10 and 100 microM) provoked a depolarization (1-5 mV) associated with a decrease in input K+ conductance having a mean reversal potential at -90 to -102 mV, not significantly different for bursting and nonbursting cells. This reversal potential differed from that of Cl(-)-mediated inhibitory postsynaptic potentials (-70 mV) elicited in NB cells by electrical stimulation of the white matter. 3. In IB cells, the NE-induced depolarization replaced the intrinsic bursts by a sustained repetitive discharge of single APs and caused intrinsic bursts to appear during previously subthreshold depolarizing current pulses. These NE-increased activities were abolished by dihydropyridine nitrendipine (1 microM) and by Cd2+ (0.5 mM) or Co2+ (2 mM), thus confirming that Ca2+ currents contribute to burst generation in IB cells. 4. In both NB and SB cells recorded in slices from GWS rats, NE provoked the appearance of intrinsic bursts of APs during steps of depolarizing current injections. In addition, in NB cells, NE caused synaptic bursts to appear after white matter stimulation. These NE-induced bursts were dihydropyridine (nitrendipine, 1 microM)- and Cd2+ (0.5 mM)- or Co2+ (2 mM)-sensitive and were related to an increased AP-afterdepolarization. The fast AP-afterhyperpolarization was not affected by NE. In NB cells recorded in slices from saline-infused rats (n = 4) NE did not provoke the appearance of bursts even when stimulation intensity was increased up to three times.(ABSTRACT TRUNCATED AT 400 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)

Maturation of brain stem neurons involved in respiratory rhythmogenesis: biochemical, bioelectrical and morphological properties

Neonatal and adult respiratory-related functions of brain stem were compared using in vivo or in vitro approaches. The control of inspiratory off-switch by glutamate-like neurotransmitters was found active at birth. However, neurons from the nucleus tractus solitarius (NTS) are immature at birth because they present growth cones and the transient potassium current appears progressively during the first week of life in association with modification of the dendritic tree. These data support the hypothesis that the mechanisms of respiratory rhythmogenesis are different at birth and in the adult.

Spontaneous synaptic activities in rat nucleus tractus solitarius neurons in vitro: evidence for re-excitatory processing

The pattern of synaptic interactions between neurons of the nucleus tractus solitarius (NTS) has been analyzed using whole cell recording in rat brainstem slices. Following tractus solitarius (TS) stimulation 15/55 neurons presented a prolonged (up to 300 ms) increased excitability (PIE neurons) and 40/55 neurons presented a prolonged (up to 200 ms) reduced excitability (PRE neurons). In the absence of afferent sensory input all neurons showed spontaneous synaptic activity. Ongoing synaptic activity in PIE cells was glutamatergic and characterized by the absence of detectable inhibitory potentials while in PRE cells it was 90% GABAergic and 10% glutamatergic. Glutamatergic synaptic currents in PIE cells and GABAergic synaptic currents in PRE were studied using probability density and intensity functions. Distribution of time intervals between synaptic events indicated the latter were generated, in both PIE and PRE cells, by two simultaneous processes: (1) a close to Poisson process generating independent events; and (2) a subsidiary re-excitatory process generating synaptic events separated by intervals shorter than 20 ms. Blockade of glutamatergic transmission by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM) or blockade of action potentials by tetrodotoxin (TTX; 1 microM) suppressed the subsidiary process. In conclusion, we propose that PIE cells (1) form a re-excitatory network contributing to generation of excitatory activity in the NTS and (2) are located presynaptically with respect to PRE cells.

Cholecystokinin-gated currents in neurons of the rat solitary complex in vitro

1. Ionic conductances controlled by type A and type B cholecystokinin (CCK) receptors were studied in neurons of the rat nucleus tractus solitarius (NTS) and dorsal motor nucleus of the vagus (DMNV), using intracellular and whole-cell patch clamp recordings in current or voltage clamp configuration during bath application of agonists (CCK8, CCK4, BC 264) and antagonists. 2. CCKA receptor-related inhibition was associated with a membrane hyperpolarization and a decrease in input resistance that developed 2-6 min after the arrival of drug into the extracellular medium. These effects were induced by 5 nM CCK8 but not BC 264 and they were blocked by the CCKA antagonist, L-364,718, but not by the CCKB antagonist, L-365,260. 3. CCKA receptor-related inhibition was generated by a potassium current that reversed at a reversal potential E(rev) of -73 +/- 1 (mean +/- SE) mV with bathing potassium concentration [K+]o = 6 mM and at -88 +/- 1 with [K+]o = 3 mM, in agreement with the Nernst equation for potassium ions. 4. CCKB receptor-related excitation was associated with a membrane depolarization and an increase of the input resistance induced by the following agonists at threshold concentrations: CCK8 (0.2 nM) > or = BC 264 (0.4 nM) > CCK4 (10.9 nM). The increase of input resistance was abolished by L-365,260 and was maintained after blockade of the CCKA current by L-364,718. 5. CCKB receptor-related excitation, in the neurons (30% of cases) in which clear response reversal was observed, appeared to be generated by a decrease of a potassium conductance. Responses showed a reversal potential E(rev) of -68 +/- 4 mV with [K+]o = 6 mM and -89 +/- 1 mV with [K+]o = 3 mM, verifying predictions from the Nernst equation applied to potassium ions. However, in 70% of cases, clear reversal was not observed at membrane potentials negative to the theoretical potassium equilibrium potential EK. 6. In voltage clamp studies, CCK8 induced a 181 +/- 17 pA inward current associated with a 26 +/- 4% decrease in the instantaneous current (I(ins)) generated by hyperpolarizing voltage steps. This effect on I(ins) was demonstrated in the absence of effects on the outward noninactivating potassium current (IM) and on the inward noninactivating cationic current (IQ). 7. CCKB receptor-mediated excitation was not suppressed by cobalt, a blocker of calcium currents, and was not associated with a change of the calcium-dependent potassium current (IK(Ca)).(ABSTRACT TRUNCATED AT 400 WORDS)