Calcium currents and calcium-dependent potassium currents in mammalian medullary respiratory neurones

1. Respiratory neurons of mammals are rhythmically active because their membrane potential fluctuates periodically over a voltage range of -70 to -55 mV. These respiratory drive potentials lead to periodic discharges of bursts of action potentials lasting for 1-2 s. The neuronal processes stabilizing this rhythmic activity involve excitatory and inhibitory synaptic processes that interact with specific membrane properties of the postsynaptic neurones. In the present experiments, performed on dorsal and ventral groups of respiratory neurones under in vivo and in vitro conditions, we verified the modulating feature of such intrinsic neuronal properties. 2. Intrinsic neuronal properties involve Ca2+ mechanisms that lead to intracellular Ca2+ accumulation, and consequently to activation of Ca(2+)-dependent K+ currents. 3. Blockade of intracellular Ca2+ accumulation significantly changed the amplitude and pattern of respiratory drive potentials, and blocked initial hyperpolarizing shifts of the membrane potential following each period of synaptic activation. 4. The data demonstrate that postsynaptic activities and action potential discharges activate low and high voltage-activated Ca2+ currents leading to intracellular Ca2+ accumulation and to activation of Ca(2+)-dependent K+ currents that significantly modulate the voltage response of medullary respiratory neurones to on-going synaptic activation. These intrinsic membrane properties also seem to be involved in the processes controlling termination of rhythmic burst discharges.

Second messenger-induced modulation of the excitability of respiratory neurones

The phospholipase inhibitor quinacrine and the protein kinase C activator phorbol-12,13-dibutyrate were injected intracellularly into expiratory neurones of the ventral respiratory group within the brain stem of anaesthetized cats. Neurones were identified by their on-going spontaneous respiratory activity and by antidromic excitation from the spinal cord at the C2-C3 level. Phorbol-12,13-dibutyrate and quinacrine, increased the amplitude of respiratory drive potentials reproducing an effect which is also obtained by potassium blockers. We conclude that modulation accounts for approximately a 40% reduction of the excitatory respiratory drive potentials provided by the respiratory rhythm generator. This modulation appears to be mediated by potassium currents that are controlled by intracellular messengers in brain stem respiratory neurones.

Somatostatin inhibition of hippocampal CA1 pyramidal neurons: mediation by arachidonic acid and its metabolites

We used electrophysiological methods in a slice preparation to study the mechanisms of somatostatin (SS) effects on hippocampal pyramidal neurons. SS hyperpolarizes hippocampal pyramidal neurons in part by augmenting the time- and voltage-dependent M-current (IM), which has been shown to be reduced by muscarinic agonists. The SS effects are abolished by the phospholipase A2 inhibitors 4-bromophenacyl bromide and quinacrine. Arachidonic acid (AA) mimics all the effects of SS on hippocampal pyramidal neurons. The effects of AA and SS on IM are blocked by the lipoxygenase inhibitor nordihydroguaiaretic acid but not by the cyclooxygenase inhibitor indomethacin. Prostaglandins E2, F2 alpha, and I2 do not increase IM. However, the specific 5-lipoxygenase inhibitors 5,6-methanoleukotriene A4 methylester and 5,6-dehydroarachidonic acid both blocked the IM-augmenting action of either SS or AA. Leukotriene C4 (but not leukotriene B4) increases IM to the same extent as AA. IM was not altered by the 12-lipoxygenase product 12-hydroperoxyeicosatetraenoic acid, and SS effects were not altered by the 12-lipoxygenase inhibitor baicalein. These data implicate 5-lipoxygenase metabolite(s) (probably leukotriene C4) as a mediator for the IM-augmenting effect of SS. In addition, when the IM effect is blocked by lipoxygenase inhibitors, both SS and AA elicit another outward current that is not blocked by either lipoxygenase or cyclooxygenase inhibitors, suggesting a direct role of AA itself distinct from the IM effect. SS did not alter significantly Ca(2+)-dependent action potentials or, in whole-cell recordings, inward currents likely to represent high-threshold Ca2+ currents. The combined results of these studies suggest that SS hyperpolarizes hippocampal neurons by two mechanisms, both mediated through the AA system. However, one mechanism (IM) involves a metabolite of AA and is most effective at slightly depolarized potentials, whereas the other may involve AA itself and be more effective at membrane potentials near rest.

Extracellular potassium, glial and neuronal potentials in the solitary complex of rat brainstem slices

Extracellular K+ activities (aKe) and neuronal and glial membrane potentials were recorded in the nucleus tractus solitarius (NTS) and in the dorsal vagal motor nucleus (DVMN) of rat brainstem slices after orthodromic stimulation of the tractus solitarius (TS). In glial cells, repetitive stimulation of the TS induced depolarizations of up to 30 mV followed by repolarizations which were fitted by exponential curves with a time constant of 1.6-5 s. Similar stimulations induced elevations of aKe of up to 8 mM, the recovery of which was fitted by single exponential curves with a time constant ranging between 1.6 and 4 s. These elevations in aKe were reduced by 75% during blockage of synaptic transmission in low Ca2+, high Mg2+ solution, and by 24% with application of 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX, 50 microM). Perfusion with a low Mg2+ solution increased the aKe response to stimulation of the TS, an effect that was reduced by the addition of 2-amino-5-phosphono-valeric acid (AP7, 50 microM) to the bath. No significant change in aKe and glial potential was seen when superfusing high concentrations of the C-terminal octapeptide of cholecystokinin (CCK8, 1-5 microM) and C-terminal tetrapeptide (CCK4, 50-100 microM). The effect of TS stimulations on solitary complex neurons suggests that extracellular K+ concentration is increased during synaptic activation of non-NMDA or NMDA ionotropic receptors. Conversely, slow depolarizations elicited by repetitive afferent activity or excitation by CCK agonists develop in neurons in the absence of measurable extracellular K+ fluctuations or glial depolarization.

Two distinct phases characterize maturation of neurons in the nucleus of the tractus solitarius during early development: morphological and electrophysiological evidence

We have used electrophysiology and light microscopy of intracellularly labeled neurons in the nucleus of the tractus solitarius (nTS) in brainstem slices of the newborn rat (P0 to P6) to examine the functional and morphological correlation of their development. Three-dimensional reconstruction of neurons injected intracellularly with biocytin, following electrophysiological recording, revealed a close correspondence between morphological immaturity (appearing as polarization of the dendritic tree) and the absence of a ramp-like voltage trajectory at the offset of hyperpolarizing current injections-IA negativity (8 of the 8 cells examined showed this correlation). These morphologically polarized IA negative neurons showed preferential dendritic sprouting in two diametrically opposite poles of the perikaryon. The orientation of the polarity differed according to the rostrocaudal location of the neuron. The appearance of a polarized dendritic tree during the first (immature) phase was transient and closely coincident with IA negativity. Following the development of adult-like electrophysiological characteristics, i.e., IA positivity, nucleus of the tractus solitarius neurons showed remarkably different morphological features (9 of 10 cells). These included a wide-spread branching of the dendritic tree in all directions, giving it a bushy appearance (cell body to dendrite ratio of 1:40). Numerous dendritic spines, growth cones on both dendrites and axons, and axon collateralization were present during both phases and indicate that nTS neurons during the two phases of early development demonstrate dynamic features of growth and maturation. The development of adult-like electrophysiological characteristics, i.e., IA positivity, progressively increased in the postnatal period. During the later part of the first postnatal week, twice as many neurons showed IA positivity in days P3 to P6 as compared with days P0 to P2. These results reveal the dynamic nature of neurons in the nTS during early development and illustrate the close link between morphology and functional characteristics in this region. We suggest that the establishment of adult-like morphology can be modified by appropriate environmental clues provided to nTS neurons during the initial (immature) phase of early postnatal development.

Responses to repetitive afferent activity of rat solitary complex neurons isolated in brainstem slices

The response of postsynaptic solitary complex neurons to repetitive stimulation (20-50 Hz) of the tractus solitarius were investigated by intracellular recordings in a brainstem slice preparation. Short duration stimuli (0.5 s) elicited increases in synaptic activity and short-term potentiation of synaptic potentials, both of which lasted approximately 1 min, plus a 10 s repolarization suppressed in the presence of glutamate ionotropic receptors antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) and 2-D-amino-7-phosphonoheptanoic acid (AP7, 50 microM). Longer (5 s) stimuli elicited 2-10 min depolarizations accompanied by membrane resistance increases and unaffected by glutamate ionotropic receptors antagonists. Our study reveals several mechanisms by which rhythmic visceral afferents may exert a tonic control of postsynaptic solitary complex neurons.

CCK modulates inhibitory synaptic transmission in the solitary complex through CCKB sites

The CCKB antagonists L-365, 260 and PD134308 and the CCKA antagonist L-364, 718 were applied to neurones of the rat solitary complex (SC) isolated in brainstem slices, while recording either intracellularly or by whole-cell patch-clamp. The CCKB antagonists increased the amplitude of spontaneous or solitary tract-evoked Cl(-)-dependent inhibitory synaptic events by 25 +/- 5% in 5/7 neurones. These events were identified as (1) reversed spontaneous potentials, (2) reversed multisynaptic potentials and (3) outward currents blocked by the GABAA antagonist bicuculline. The CCKB antagonists had no postsynaptic action and increased excitatory synaptic events by 16 +/- 5% in only 3/9 neurones. The CCKA antagonist depolarized neurones but had no effect on synaptic events. Results suggest that CCK, released from the SC tissue, modulates GABAergic interneurones through CCKB sites. This mechanism could contribute to panic attacks, probably mediated by CCKB receptors.

Rhythmic activities in the rat solitary complex in vitro

In adult rat brainstem slices, rhythmic discharge of action potentials occurred spontaneously in 10 out of 197 cells of the solitary complex. In 6 neurones, fast rhythms (2-6 per min) were characterized by volleys of synaptic activity presenting abrupt onset denoting synchronized discharge of presynaptic elements. Synchronizing signals may be generated by cells discharging bursts of high-frequency action potentials and presenting extensive axonal arborization, as observed in one cell. Slower rhythms (0.3-0.8 per min) monitored in three cells did not involve synchronizing processes and could be evoked in non-rhythmic cells by 15-30 min bath application of the cholecystokinin octapeptide (100 nM). These results suggest distinct operating mechanisms of fast and slow rhythms in the solitary complex in vitro.

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

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

CholecystokininA and cholecystokininB receptors in neurons of the brainstem solitary complex of the rat: pharmacological identification

The brainstem solitary complex, which receives projections from primary sensory afferents of the vagus nerve, appears to be a crucial site for the action of the cholecystokinin octapeptide (CCK8), because both peripheral-type (CCKA) and central-type (CCKB) binding sites are present in this structure. In the present study, we investigated the effects of recently developed receptor-specific pharmacological tools on neurons recorded in rat coronal brainstem slices, to ascertain whether CCK acts differently on each type of receptor. CCK8, which interacts with both binding sites, had three effects on neuronal discharge, brief excitation, prolonged excitation and delayed inhibition. BC 264, a novel CCK analog endowed with high affinity and selectivity for CCKB receptors, produced exclusively prolonged excitation. L-365,260, a novel nonpeptide antagonist of CCKB receptors, blocked the prolonged excitation induced by BC 264 or CCK8. L-364,718, a potent antagonist of CCKA receptors, blocked delayed inhibition and replaced brief CCK8-induced excitation by prolonged excitation. Altogether, these results show that CCK8 exerts, on neurons of the solitary complex, mixed effects due to simultaneous activation of CCKA inhibitory and CCKB excitatory binding sites. The hypothesis that exogenous CCK8 acts, in the solitary complex, through CCKA sites to slow down the motility of the digestive tract and through CCKB sites to modulate anxiety will be developed.

Burst generation in neocortical neurons after GABA withdrawal in the rat

1. gamma-Aminobutyric acid (GABA) withdrawal syndrome (GWS) represents a particular model of focal epilepsy consecutive to the interruption of a chronic intracortical GABA infusion and is characterized by the appearance of focal epileptic electroencephalographic (EEG) discharges and localized clinical signs on withdrawal of GABA. Effects of Ca2+ channel blockers and N-methyl-D-aspartate (NMDA) antagonists were evaluated in living rats presenting a GWS after interruption of a 5-day GABA infusion into the somatomotor cortex and in neocortical slices obtained from such rats. Bursting properties and morphology of neurons were also analyzed in slices. 2. In living rats, the noncompetitive NMDA antagonist phencyclidine [1-(1-phenylcyclohexyl)piperidine] and the Ca2+ antagonist flunarizine [E-1 (bis(4fluorophenyl)methyl)-4(3phenyl2-propenyl)-piperazine] were administered systemically to two groups of rats. Rats in the first group (n = 12) were injected with the drug 30-60 min before discontinuation of the GABA infusion. In this case, phencyclidine (10 mg/kg ip) prevented the development of GWS (n = 5), whereas flunarizine (40 mg/kg ip) had no consistent effect on the GWS appearance and characteristics (n = 7). Rats in the second group (n = 12) were injected 60-90 min after GABA discontinuation, i.e., during a fully developed GWS. In that case, neither drug suppressed GWS. 3. Neuronal activities in the epileptic focus were studied in slices with conventional intracellular recording and stimulation techniques. From the 65 neurons recorded, 29 responded with EPSPs and paroxysmal depolarization shifts (PDSs) to white matter stimulation (synaptic bursting or SB cells). Nineteen other neurons presented, in addition to synaptically induced PDSs, bursts of action potentials (APs) induced by intracellular depolarizing current injection (intrinsic bursting or IB cells). The remaining 17 neurons presented no bursting properties to either synaptic stimulation or depolarizing current injection (nonbursting or NB cells). 4. The recorded neurons were located 0.7-1.2 mm distant from the lesion because of the penetration of the GABA infusion cannula. Intracellular injection of neurons (n = 4) with biocytin or Lucifer yellow revealed that both SB and IB neurons were large, spiny pyramidal neurons localized in layer V of the sensorimotor cortex. 5. Bath application of the selective antagonist of NMDA receptors DL-2amino-5phosphonovalerate or DL-2amino-7phosphonoheptanoate (10-50 microM) reversibly reduced the amplitude (by 25-50%) and the duration (by 20-25%) of PDSs in all cases (n = 17).(ABSTRACT TRUNCATED AT 400 WORDS)

Control of breathing by endogenous opioid peptides: possible involvement in sudden infant death syndrome

Experiments have been performed in order to evaluate the respiratory consequences of a suppression or accumulation of endogenous opioid peptides, in the neuronal network which generates the motor respiratory activity. Iontophoretic application of naloxone onto respiratory neurons increases their firing activity and increases their respiratory modulation. On the other hand the local injection of kelatorphan (an enkephalinase inhibitor) decreases the firing of respiratory neurons and thus reduces the respiratory modulation. This effect of kelatorphan mimics the effect on respiratory neuron of an iontophoretic application of met-enkephalin. Furthermore the local injection of kelatorphan reduces the frequency of the respiratory output recorded from the phrenic nerve. This effect is reversed by systemic administration of naloxone. The results demonstrate the involvement of endogenous opioid peptides in the control of breathing suggesting that in Sudden Infant Death Syndrome a possible dysregulation in opioidergic system could occur.

In vitro study of newborn rat brain maturation: implication for sudden infant death syndrome

We have used slice preparation from newborn rats to study the development of the nucleus tractus solitarius neuronal network and brain intracellular phosphorus metabolites. As shown previously on adults, the newborn preparation retains local excitatory and inhibitory synaptic connections and enables study of intrinsic electrical properties in the nucleus tractus solitarius. Electrophysiological investigation of inhibitory synaptic transmission demonstrated a maturational step at days 4-6 after birth. Nuclear magnetic resonance spectroscopy of brain slices revealed a metabolic maturation between postnatal days 11 and 17. Results emphasize the differential maturation steps during the postnatal development of rat central nervous system. Possibly, Sudden Infant Death Syndrome may result from the abnormal timing in the occurrence of these steps.

Substance P and serotonin mutually reverse their excitatory effects in the rat nucleus tractus solitarius

Actions and interactions of serotonin and substance P are described in the nucleus tractus solitarius using coronal brainstem slices and intracellular recordings. Substance P (10-100 nM) and serotonin (10-100 microM) applied alone were excitatory, causing depolarization and increasing the input resistance. Reversing effect was obtained using a protocol of long (greater than 5 min) conditioning applications of substance P and shorter (20-60 s) test applications of serotonin: serotonin, which was excitatory by itself during controls, became inhibitory for the steady action potential discharges induced by conditioning substance P applications. In the reverse situation, inhibition was also obtained using prolonged conditioning exposures to serotonin and test applications of substance P. Prolonged conditioning applications (greater than 5 min) were required in these experiments since addition or potentiation, but not inhibition, was found when combining 20-60 s substance P and serotonin applications. In addition to their excitatory effects, substance P and serotonin, applied alone, had another mechanism of action. They reduced the duration of tetraethylammonium-prolonged action potentials. This mechanism was also reversed using conditioning applications of substance P or serotonin. Thus, reversing effects appeared simultaneously on multiple ionic mechanisms. Furthermore, the reversing effect was unaffected in tetrodotoxin-treated preparations, which indicates a postsynaptic phenomenon. Consequently, the control of two different postsynaptic ionic mechanisms during substance P and serotonin interaction suggests that the underlying mechanisms take place at a common level, possibly in relation to second messenger processes. From a functional point of view, these results support the idea that in the nucleus tractus solitarius the effects of either neurotransmitter, serotonin or substance P, can be completely reversed by a previous release of the other one.

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.

Relationship between tolerance to GABAA agonist and bursting properties in neocortical neurons during GABA-withdrawal syndrome

The interruption of intracortical, chronic GABA infusion is known to give rise to 'GABA withdrawal syndrome' (GWS) consisting of electroencephalographic paroxysmal focal activities, associated with behavioral epileptic signs. Neocortical slices were obtained from rats presenting the GWS (GWS slices), and intracellular recordings were performed in the vicinity of the gamma-aminobutyric acid (GABA)-infused site. Electrical stimulation of the underlying white matter induced paroxysmal depolarization shifts (PDSs) in virtually all neurons. Bath-applied GABA (1-10 microM) had no effect on these neurons, while the same dose range was found effective in blocking action potentials in saline-infused cortex slices obtained from control rats. In the GWS slices a population of neurons presented, in addition to synaptically induced PDSs, voltage-dependent and cobalt-sensitive PDSs and bursts of action potentials induced by depolarizing current injections. These intrinsic bursting neurons were unresponsive to high doses of GABA (100 microM). Dose-response curves of isoguvacine, a specific GABAA agonist, showed a shift to the right for the intrinsic bursting cells whatever the parameter measured (depolarization or conductance increase): the ED50 was 50-100 times higher for intrinsic bursting cells than for other non-intrinsic bursting cells, thus indicating that intrinsic bursting cells are tolerant to GABAA agonist. This tolerance may result from a decreased number of receptors or from a change in their properties as a consequence of the previous prolonged GABA infusion. The decrease in the GABA efficacy could lead to disinhibition and could thus give the appearance of epileptic events.

Metabolic action of N-methyl-D-aspartate in newborn rat brain ex vivo: 31p magnetic resonance spectroscopy

N-methyl-D-aspartate (NMDA) is an agonist used to identify neuronal receptive sites for dicarboxylic amino acid neurotransmitters; NMDA receptors are implicated in neuronal damage of ischemic or hypoglycemic origin in newborns although involved mechanisms remain to be identified. In the present study, 31P magnetic resonance spectroscopy with fast (6/min) data acquisition was used in newborn rat brain slices to measure changes of intracellular phosphocreatine and nucleotide triphosphate levels following extracellular NMDA applications. The rapid exhaustion of phosphocreatine stores in 50% of the total population of brain cells was induced in all cases by application of NMDA (30-45 s, 25-100 mM). It was not reproduced by other excitatory agents: potassium ions (24.6 mM, 4 min), isobutylxanthine (1mM), muscarine (10 mM), serotonin (0.1 mM) or substance P (10 microM). Such an effect of NMDA was not modified after tetrodotoxin (1 microM) and was reduced by extracellular 2-amino-5-phosphonovalerate (50 microM) or magnesium ions (2.2 mM). However it did develop during NMDA-induce neuronal excitations and was reversible within 10-30 min. This action of NMDA was followed by an irreversible decrease of phosphorus metabolites if mitochondrial creatine kinase and adenosine triphosphatase were decoupled by atractyloside (50 microM). Experiments revealed a link between selective NMDA action at neuronal plasma membranes, neurotoxicity and energy production by mitochondria.

1H COSY spectra of superfused brain slices of rat: ex vivo direct assignment of resonances

Realization of 1D 1H spectra of superfused brain slices of rats is described. Two-dimensional 1H-1H COSY spectra have been used for the direct assignment of resonances on living tissue (lactate, GABA, N-acetyl aspartate, aspartate, glutamine/glutamate, creatine/phosphocreatine, taurine, inositol, choline/ethanolamine). Utilization of the SUPERCOSY sequence permits simultaneously the enhancement of off-diagonal signals for small metabolites and the reduction of the signals for water and macromolecules.

Metabolic acidosis induced by N-methyl-D-aspartate in brain slices of the neonatal rat: 31P- and 1H-magnetic resonance spectroscopy

1H- and 31P-magnetic resonance spectroscopy was used to monitor intracellular lactate, phosphorus metabolites and pH in superfused brain slices from 2- to 9-day-old rats. N-Methyl-D-aspartate (NMDA) (100 microM, 0.5-3 min) was applied in the extracellular magnesium-free perfusion medium. NMDA induced intracellular metabolic acidosis, i.e., an increase of freely mobile lactate levels and an 0.3 pH unit acidification. This was abolished when the extracellular glucose supply was reduced. Experiments also indicate that acidosis is not responsible for the cell damage resulting from activation of NMDA receptors in hypoglycemic conditions.

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.

N-methyl-D-aspartate (NMDA) receptors control respiratory off-switch in cat

Functionally active N-methyl-D-aspartate (NMDA) receptors on cat medullary respiratory neurones were revealed by local iontophoretic application of DL-2-amino-7-phosphonoheptanoic acid (AP7). Blockade of NMDA receptors by systemic administration of NMDA antagonists (MK-801, phencyclidine, ketamine, AP7) in vagotomized cats increased the duration of inspiration (Ti) without increasing expiration and caused an apneustic breathing pattern. The increase in Ti which followed systemic MK-801, was accompanied by a shift and complete reversal of early expiratory neuronal discharge in relation to phrenic nerve discharge.

Somatostatin depresses excitability in neurons of the solitary tract complex through hyperpolarization and augmentation of IM, a non-inactivating voltage-dependent outward current blocked by muscarinic agonists

The synaptic function of somatostatin-containing fibers in the nervous system is controversial. Therefore, we used a slice preparation of the rat brain stem to test the electrophysiological effects of prosomatostatin-derived peptides on neurons of the solitary tract complex, which contains an abundance of somatostatin-containing fibers and cell bodies. Superfusion of both somatostatin-14 and somatostatin-28 (the precursor for somatostatin-14), but not somatostatin-28-(1-12) or -(1-10), predominantly inhibited spontaneous spike and subthreshold (probably synaptic) activity. In intracellular recordings, somatostatin-14 and -28 hyperpolarized most neurons in association with a slight (10-35%) but reproducible decrease in input resistance. These hyperpolarizing responses were augmented in depolarized cells and persisted in cells in which spontaneous inhibitory postsynaptic potentials became depolarizing after Cl- injection. These data suggest that somatostatin receptors regulate a K+ conductance. In voltage-clamp studies, somatostatin-28 and -14 induced a steady outward current and augmented the voltage-dependent, nonactivating outward K+ conductance (IM) shown to be blocked by activation of muscarinic cholinergic receptors. These results suggest (i) that somatostatin-containing elements in the solitary tract complex play an inhibitory role through the activation of postsynaptic permeability to potassium ions and (ii) that the same ion channel type may be coregulated by two neurotransmitter candidates, somatostatin and acetylcholine, through a reciprocal control mechanism.

Effects of pentobarbital and flurazepam on respiratory neurons in undrugged cats

Two hypnotic drugs known to enhance GABAergic transmission, a barbiturate (pentobarbital) and a benzodiazepine (flurazepam), were applied locally to respiratory-related neurons (RN) located in the ventral respiratory area in the medulla of non-anaesthetized cats which were either decerebrated or chronically implanted. Pentobarbital applied iontophoretically depressed the spontaneous discharge rate of most RN tested as well as the increase in firing of RN discharge induced by iontophoretic application of glutamate; pentobarbital also potentiated inhibition induced by iontophoresed GABA. Flurazepam applied by iontophoresis or pressure pulses depressed a minority of RN tested and did not enhance GABA-induced inhibition. These results suggest: that inhibition of RN activity through GABAergic mechanisms can be affected by drugs which act at the chloride ionophore but not those acting indirectly through the benzodiazepine binding site, and the effects of the agents on medullary neuronal activity are independent of an effect on the states of consciousness or on structures rostral to the medulla.

Excitability of 'silent' respiratory neurons during sleep-waking states: an iontophoretic study in undrugged chronic cats

An iontophoretic study of respiratory-related neurons (RN) was conducted in the medullary ventral respiratory area of chronically implanted, undrugged cats during states of sleep and wakefulness. Most RN recorded were unaffected by sleep-wake states but a few RN decreased their firing rate during sleep (sleep sensitive cells). The excitability of RN was assessed in the different states by local application of L-glutamate. Glutamate iontophoresis revealed the presence of 5 cells which were silent during sleep and completely or mostly silent during undisturbed wakefulness but always discharged with a respiratory-modulated pattern of the expiratory type in response to glutamate application. Arousing stimuli induced spontaneous firing of these cells and REM sleep reduced glutamate effectiveness. It was concluded that silent RN and RN which become inactive during sleep permanently receive subthreshold respiratory-modulated inputs which are amplified or depressed by state-dependent tonic inputs.

Organization of synaptic transmission in the mammalian solitary complex, studied in vitro

1. Synaptic transmission and neuronal morphology were studied in the nucleus tractus solitarius and in the dorsal vagal motor nucleus (solitary complex), in coronal brain-stem slices of rat or cat, superfused in vitro. 2. Electrical stimulation of afferent fibres of the solitary tract evoked two different types of post-synaptic response recorded intracellularly in different solitary complex neurones. Labelling with horseradish peroxidase showed that these two sorts of orthodromically evoked responses were correlated with different post-synaptic neuronal morphologies. 3. The majority of recorded neurones (n = 93) showed a prolonged reduction in excitability following the initial solitary-tract-evoked excitatory post-synaptic potential (e.p.s.p.). A smaller number of neurones (n = 53) showed a prolonged increase in excitability following solitary tract stimulation. In no case did the solitary tract stimulation induce a burst of action potentials at high frequency. 4. The time-to-peak and the half-width of the initial solitary-tract-evoked e.p.s.p. were shorter in neurones with prolonged increased excitability than in those with prolonged reduced excitability. In neurones with prolonged reduced excitability, this e.p.s.p. was followed by a hyperpolarization lasting 60-100 ms. The latency of this inhibitory post-synaptic potential (i.p.s.p.) was 3-5 ms longer than that of the initial e.p.s.p. and its reversal potential was 10 mV more negative than the reversal potential of the response measured following application of gamma-aminobutyric acid or glycine. In neurones with prolonged increased excitability, at a membrane potential of -40 to -50 mV, the initial solitary tract e.p.s.p. was followed by a prolonged depolarization lasting 100-400 ms. 5. Background synaptic activity was high in neurones with prolonged increased excitability, consisting of unitary e.p.s.p.s with an amplitude of more than 0.8 mV. This activity was increased for a period of 300-800 ms following solitary tract stimulation. Spontaneous excitatory potentials of more than 0.5 mV were not seen in neurones with prolonged reduced excitability. In these neurones, after intracellular injection of choride ions, reversed unitary i.p.s.p.s formed a background activity which was increased following stimulation of the solitary tract. 6. Neurones with prolonged reduced excitability were found in the medial, ventral and ventrolateral part of the nucleus tractus solitarius and in the dorsal vagal motor nucleus where they were identified by their antidromic response to stimulation ventral and lateral to the tractus solitarius.(ABSTRACT TRUNCATED AT 400 WORDS)

Voltage-dependent currents in neurones of the nuclei of the solitary tract of rat brainstem slices

Neurones within the ventral and ventrolateral nuclei of the solitary tract were analyzed under single-electrode current- and voltage-clamp conditions in rat brainstem slices. We present direct and indirect evidence for the existence of five different sorts of membrane currents: a tetrodotoxin-sensitive sodium current, a tetrodotoxin-resistant calcium current, a calcium-dependent potassium current, a non-inactivating potassium current which is inhibited by muscarine, an inactivating potassium current, which is inhibited by 4-aminopyridine. These membrane properties do not produce spontaneous bursting in these neurones. Assuming that neurones with such properties belong to the respiratory network, we discuss how conductances of this type may be involved in mechanisms regulating central respiratory activity.

Synaptic responses of neurons of the nucleus tractus solitarius in vitro

Postsynaptic responses of neurons in the nucleus tractus solitarius (NTS) have been studied in an in vitro slice preparation using extra- and intracellular recording. Single or paired pulse stimulations were delivered to afferent fibers within the tractus solitarius (TS) to activate orthodromic responses in these neurons. Most NTS neurons displayed an initial synaptic excitation followed by inhibition of spontaneous or evoked firing lasting up to 150-200 ms after stimulation. Excitatory postsynaptic potentials (EPSPs), recorded intracellularly, were increased in amplitude by membrane hyperpolarization. Large afterhyperpolarizations followed action potentials triggered by the EPSPs or evoked by intracellular current injections. Intracellular evidence for synaptic inhibition within the NTS included: (1) the presence, after Cl-injection, of flurries of spontaneous PSPs likely to be inverted inhibitory postsynaptic potentials; (2) reduction of the size of a test EPSP by a previous subthreshold TS conditioning volley; and (3) hyperpolarizing PSPs recorded in some neurons. Other NTS neurons exhibited prolonged excitatory responses to TS stimulation and could be local inhibitory interneurons. These results may help specify synaptic mechanisms in the NTS that could play an integrative role in the relay of visceral sensory inputs to higher order effectors.

Differentiation of two respiratory areas in the cat medulla using kainic acid

Kainic acid (KA) was used to destroy neuronal perikarya in different areas of the brainstem. Single KA microinjections were performed in 30 anaesthetized, vagotomized, artificially ventilated cats. Consequences were studied on the phrenic nerve activity (PNA) and blood pressure. We observed changes of the PNA unrelated to blood pressure alteration. Destruction of the dorsal respiratory area (DRA) including the nucleus tractus solitarius at the obex level produced a 40% decrease of the PNA frequency. Destruction restricted to the lateral part of the ventral respiratory area (VRA1) including the ambiguus nucleus induced a 60% decrease of the integrated PNA amplitude followed by a 40% increase of PNA frequency. These latter effects were also observed after destruction inside the infra solitary reticular formation (ISRF). No effect was observed after destruction in other brain structures. We concluded that ISRF and VRA1 form a single ventral bulbar respiratory area. This area controls respiration in a way different from that of the dorsal respiratory area (DRA).

Excitatory effects of iontophoretically applied substance P on neurons in the nucleus tractus solitarius of the cat: lack of interaction with opiates and opioids

The effects of iontophoretically applied substance P (SP), (D-Pro2, D-Trp7,9)-SP and (D-Pro4, D-Trp7,9,10)-SP were studied on neurons identified by their histological location in the nucleus tractus solitarius (NTS), their response to vagal or carotid sinus nerve stimulation and eventually their functional correlation with the central respiratory drive. Potent and consistent excitatory effects of SP were found supporting its role as a putative excitatory transmitter in the NTS. The effects of SP and L-glutamate (Glu) were differentiated by the relative insensitivity of SP-induced excitations to levorphanol and Met-enkephalin.

The effects of acetylcholine on bulbar respiratory related neurones. Consequences of anaesthesia by pentobarbital

The effects of cholinergic agonists and antagonists applied by microiontophoresis to bulbar respiratory neurones were determined in different preparations: cats anaesthetized by pentobarbital and immobilized by gallamine triethiodide, intercollicular decerebrate cats either not immobilized or immobilized by gallamine triethiodide. Respiratory neurones located at the bulbar level exhibited a muscarinic cholinergic sensitivity in all preparations. Muscarinic responses were either excitatory or inhibitory. The number of neurones showing excitatory responses was lower under pentobarbital anaesthesia than in decerebrate cats.

Pharmacological identification of delta and mu opiate receptors on bulbar respiratory neurons

The opiate receptors of central neurons related to the generation of the respiratory rhythm were identified using microiontophoresis of the synthetic opioid peptides DSLET (Tyr-D-Ser-Gly-Phe-Leu-Thr) and TRIMU-4 (Tyr-D-Ala-Gly-NH-CH (CH3)-CH2-CH(CH3)2), exhibiting a high selectivity for delta and mu receptors respectively. Both agonists induced depressions of spontaneous and L-glutamate-induced discharges; the effects were antagonized by naloxone and not mimicked by a related almost inactive peptide DSLLET (Tyr-D-Ser-Gly-Leu-Leu-Thr). The effect of DSLET had a faster time course than that of TRIMU-4 and persisted after prolonged applications of TRIMU-4. It is concluded that delta and mu receptors subtypes are distinct and are both present on central respiratory-related neurons.

Different effects of mu and delta opiate agonists on respiration

The involvement of different opiate receptor subtypes in opiate-induced respiratory depression was studied in the unanaesthetized rat. Synthetic opioid agonists, specific for mu or delta receptors, were administered intraperitoneally in freely moving rats while respiratory parameters were recorded by means of the whole body plethysmographic method. TRIMU-4 (Tyr-D-Ala-Gly-NH-CH(CH3)-CH2-CH(CH3)2), a specific agonist of the mu receptor, reduced the tidal volume and did not change the respiratory frequency. DSLET (Tyr-D-Ser-Gly-Phe-Leu-Thr), a relatively specific agonist of the delta receptor subtype, reduced respiratory frequency and was significantly less effective on tidal volume than was TRIMU-4. It is concluded that the respiratory depression occurring after the administration of opiates in clinical practice is a dual complementary effect involving mu and delta receptors.

Rhythmic neuronal activities in the nucleus of the tractus solitarius isolated in vitro

Coronal slices of the rat medulla were used for an electrophysiological study of the nucleus of the tractus solitarius isolated in vitro. Half of the spontaneously active neurons in the ventral NTS exhibited a rhythmic repetitive spike discharge. In contrast, bursting activity was virtually absent in the isolated NTS. Resetting of repetitive discharges could be obtained by electrical stimulation of the glossopharyngeal-vagal afferent terminals in the tractus solitarius. These results are discussed in relation to the proposed role of the NTS in the neurogenesis of the respiratory rhythm.

Involvement of amino acids in periodic inhibitions of bulbar respiratory neurones

As previously demonstrated, spontaneously firing bulbar inspiratory neurones are periodically inhibited either at the beginning of, or throughout expiration, while bulbar expiratory neurones are inhibited during inspiration. The aim of the present study was to test the hypothesis that amino acids act as transmitters of these periodic inhibitions. The study was performed using iontophoretic applications of drugs on bulbar respiratory neurones. On these neurones GABA and glycine-sensitive sites were identified and differentiated on the basis of the actions of agonist (muscimol) or antagonists (bicuculline, picrotoxin and strychnine). Using competitive antagonists (nipecotic acid, beta-alanine) mechanisms responsible for GABA uptake were found in the close vicinity of respiratory-related neurones. Some but not all types of periodic inhibition were found to be reduced following application of GABA or glycine antagonists. Strychnine was found to reduce periodic inhibitions occurring at the beginning of expiration in inspiratory neurones. GABA antagonists had an effect on those periodic depressions which were prolonged throughout expiration. A different and complementary role of glycine-like and GABA-like systems in central respiratory mechanisms is proposed.

Excitability of bulbar respiratory neurones: a study using microiontophoretic applications of depolarizing agents

The discharge of cat's bulbar respiratory neurones (RN) was shown to be modulated by periodic depressions which are characterized by their ability to reduce the effectiveness of microiontophoretically applied depolarizing agents: L-glutamate, acetylcholine and potassium. From the observation of cycle triggered time histograms (CTH), it appeared that these depressions have a determined and invariable phase relationship within the respiratory cycle. They were demonstrated in RN histologically located between and including the nucleus of the tractus solitarius and the nucleus ambiguus. Reproducibility and dose/response relationship of L-glutamate-induced depolarizations enabled an estimation of the functional effectiveness of these periodic depressions. In spontaneously phasic or 'silent' RN, depressions were demonstrated in the majority of cases (71%). Strongest depressions prevented spontaneous and L-glutamate-induced firing. Slighter depressions did not completely abolish L-glutamate effectiveness but reduced it by 20-90%. Conversely, in the majority of spontaneously tonic units (68%) depressions were not identified since the L-glutamate effect remained unchanged throughout the respiratory cycle. Four types of these respiration-related depressions were differentiated on the basis of their length, their phase relation to the respiratory cycle and their potentiation in barbiturate-anaesthetized preparations. A first type suppressed L-glutamate-evoked firing throughout inspiration; it was found in late-expiratory neurones. Two other types of depressions had a more restricted duration in the cycle: one was restricted to a portion of inspiration and was found in early-expiratory neurones; the other restricted to the beginning of expiration, was found in a special group of inspiratory neurones. A fourth type of inhibition was weaker and actively prolonged throughout expiration: it was found in another group of inspiratory neurones including the respiratory neurones located at the level of the nucleus of the tractus solitarius. These periodic depressions are interpreted in terms of synaptic inhibition; it is proposed that they play a major role in the functional organization of the respiratory centers at the bulbar level.

Pontine afferents to the medullary respiratory system: anatomofunctional correlation

A direct projection from the parabrachialis medialis, Kölliker-Fuse complex (PMKF) to the bulbar respiratory area is shown using autoradiography. Perikarya of PMKF project to the medulla, labelled fibers and terminals being located radially to the fourth ventricle from the nucleus hypoglossus to the ambiguus. The functional importance of this projection is demonstrated in pharmacological studies using iontophoresis of L-glutamate which enable identification of somatodendritic discharges. A good correlations exists between the localization in the medulla of labelled fibers and terminals and that of an inspiratory background activity related to inspiratory fibers or terminals.

Catecholaminergic depressant effects on bulbar respiratory mechanisms

On the basis of histochemical and pharmacological studies, catecholamines have been implicated in central mechanisms controlling respiration. This hypothesis was tested in iontophoretic studies on neurones located in bulbar respiratory centres. Adrenaline and noradrenaline had a predominantly depressant effect on respiratory as well as on closely situated non-respiratory units. These depressions were mimicked by the application of isoproterenol and clonidine; acetylcholine and serotonin had inconsistent effects on these neurones. In control experiments, microinjections, using a Hamilton syringe, were made in the area of bulbar respiratory centres: noradrenaline, but not serotonin, depressed the central respiratory activity reflected in the phrenic nerve discharge. These results suggest that specific adrenergic and noradrenergic depressant mechanisms could affect both respiratory and other physiological centres at the bulbar level.

Effects of opiates and methionine-enkephalin on pontine and bulbar respiratory neurones of the cat

The effects of microelectrophoretically applied opiate agonists and the antagonist naloxone have been investigated on extracellularly recorded neurones in pontine and bulbar respiratory centers of the cat. Morphine, levorphanol and methionine-enkephalin depressed the spontaneous discharge of respiration related units and the firing induced in these cells by phoretically applied L-glutamate. The rhythmic pattern of these neurones was modified, in such a way that basal activity was hardly affected while peak frequency was markedly reduced. These effects are mediated via stereospecific opiate receptors, since they were antagonized by naloxone and not mimicked by the D+ enantiomer of levorphanol, dextrophan. The few excitations observed following opiate agonist application were not antagonizable by naloxone. These data may provide a basis for the explanation of the depressant effect on central respiratory rhythm of systemically injected opiates which have been obtained in control experiments. The results are interpreted in terms of opiates lowering excitatory synaptic efficacy.