Biomedical subjects

D W Richter

Publications and source records attributed to D W Richter.

At least 19 recordsLinked to original sources

PubMed1995

Role of fast inhibitory synaptic mechanisms in respiratory rhythm generation in the maturing mouse.

1. The importance of glycinergic and GABAAergic synaptic mechanisms for respiratory rhythm generation in the maturing mouse were investigated in vivo and in an in vitro slice preparation generating respiratory rhythmic activity spontaneously at all postnatal ages. 2. The effect on respiration of topical application of strychnine or bicuculline to the surface of the ventrolateral medulla was assessed in spontaneously breathing anaesthetized mice of different ages (postnatal (P) days 0 to > 56). Glycine receptor antagonization with concentrations of strychnine up to 25 microM was ineffective in altering the breathing pattern in neonates (P1-P8). However, in mature mice (P > 15), low doses of strychnine (0.2-2 microM) abolished regular rhythmic discharge in the phrenic nerve. Bicuculline (0.5-50 microM) produced dose-dependent increases in inspiratory time, amplitude and cycle length of phrenic nerve discharge in anaesthetized neonatal mice whereas both cycle length and duration of inspiratory activity were reduced in mature animals. In addition, in both neonates and mature mice low concentrations of bicuculline (0.5-5 microM) abolished phrenic nerve discharge intermittently. 3. The response of respiratory-modulated hypoglossal (XII) neurones recorded in tilted sagittal slices from newborn and mature mice during blockade of glycine and GABAA receptors was similar to the phrenic nerve changes observed in vivo: in slices from neonates, the rhythmic activity of XII neurones was resistant to concentrations of strychnine up to 50 microM whereas low doses of strychnine (0.2-2 microM) abolished rhythmic activity in preparations from mature mice. Bicuculline (1-50 microM) produced a dose-dependent prolongation of burst duration and a slowing of rhythmic discharge in slices from neonatal mice whereas in mature mice rhythmic XII bursts were shortened and their frequency increased. At all maturational stages, bicuculline (1-50 microM) induced severe disruption of the regular rhythm of XII neurone activity causing maintained depolarizations and oscillations in membrane potential. 4. On-going inhibitory postsynaptic potentials of neurones located in the ventral respiratory group region of tilted sagittal slices from both immature and mature mice were sensitive to low concentrations of either bicuculline or strychnine (1-5 microM) indicating an absence of a maturational change in the sensitivity of GABAA and glycine receptors to their respective antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Age Factors↗

PubMed1995

Adenosinergic modulation of respiratory neurones and hypoxic responses in the anaesthetized cat.

1. The modulatory effects of intracellularly injected adenosine on membrane potential, input resistance and spontaneous or evoked synaptic activity were determined in respiratory neurones of the ventral respiratory group. 2. The membrane potential hyperpolarized and sometimes reached values which were beyond the equilibrium potential of Cl(-)-dependent IPSPs. At the same time, neuronal input resistance decreased. 3. Spontaneous and stimulus-evoked postsynaptic activities were decreased, as were mean respiratory drive potentials. 4. Systemic injection of the A1 adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 0.01-0.05 mg kg-1) resulted in an increase in mean peak phrenic nerve activity when arterial chemoreceptors were denervated. In contrast, phrenic nerve activity decreased when arterial chemoreceptors were left intact. 5. The depressant effect of adenosine on synaptic activity was abolished after systemic DPCPX administration. DPCPX caused an increase in respiratory drive potentials, increased the amplitude of stimulus-evoked IPSPs, and hyperpolarized membrane potential. 6. Administration of DPCPX blocked the early hypoxic depression of stimulus-evoked IPSPs, doubled the delay of onset of hypoxic apnoea and shortened the time necessary for recovery of the respiratory rhythm. 7. The data indicate that adenosine acts on pre- and postsynaptic A1 receptors resulting in postsynaptic membrane hyperpolarization and depression of synaptic transmission. Blockade of A1 receptors increases respiratory activity, indicating that adenosine A1 receptors are tonically activated under control conditions. Further activation contributes to the hypoxic depression of synaptic transmission in the respiratory network.

Adenosine↗

PubMed1995

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.

Action Potentials↗

PubMed1995

Anoxic disturbance of the isolated respiratory network of neonatal rats.

Tissue oxygen (PO2), K+ (aKe), pH (pHe) and Ca2+ ([Ca2+]e) were measured in the region of the ventral respiratory group (VRG) in the in vitro brainstem-spinal cord preparation of neonatal rats. During tissue anoxia, elicited by superfusion of N2-gassed solutions, an initial increase in the frequency of respiratory activity, lasting between 2 and 12 min, turned into a frequency depression. During anoxia periods of up to 60 min, respiratory activity persisted in solutions containing CO2/bicarbonate, whereas a complete blockade was observed after 15-25 min in N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid- (Hepes)-buffered salines. After such anoxic apnea, respiratory rhythmicity could be reactivated by superfusion of hypoxic, CO2/bicarbonate-buffered solutions. In both types of hypoxic solutions, aKe increased by maximally 1.5 mM, whereas an initial increase of pHe by up to 0.05 pH units turned, after 2-4 min, into an acidification which could exceed 0.5 pH units. In contrast, [Ca2+]e remained unaffected by anoxia. Addition of 2-5 mM cyanide (CN-) to oxygenated Hepes-buffered saline evoked an increase in PO2 in the VRG from 100 to more than 300 mmHg. The effects of CN- on respiratory activity, aKe and pHe were almost identical to those during anoxia. In oxygenated, CO2/bicarbonate-free solutions of different pH, however, an increase in pHe in the VRG led to a decrease in respiratory frequency, whereas a fall of pHe produced a frequency acceleration. A rise of aKe in the VRG by more than 2 mM as induced by superfusion of a 7 mM K+ solution led to a sustained increase of respiratory frequency. The results indicate that blockade of aerobic metabolism does not severely perturb K+ and Ca2+ homeostasis and that the biphasic response to anoxia is not directly related to the observed changes in PO2, aKe, pHe, or [Ca2+]e. In the respiratory network of neonatal mammals, CO2 might provide a stimulus for long-term maintenance of respiratory activity under oxygen depletion.

Animals↗

PubMed1995

Maturational changes in the respiratory rhythm generator of the mouse.

The changes in motor activity of the respiratory rhythm generator were quantitatively analysed in mice (from birth to at least 56 days old) in both awake and anaesthetized preparations, as well as in vitro to define the age at which the respiratory network is mature. In awake and anaesthetized spontaneously breathing mice respiratory-related thoracic movements were recorded and revealed an age-dependent increase in both inspiratory time (45%) and cycle length (22%) over the first 15 days of life. Similarly, the pattern of phrenic nerve activity recorded from anesthetized animals also changed from a short, rapid onset and offset burst, without a post-inspiratory phase (0-10 days old), to a discharge of longer duration which included both ramp and post-inspiratory components (> 15 days). This pattern was comparable to that seen in adult mice (> 56 days old). A recently developed tilted-sagittal brainstem slice preparation containing an isolated, but functionally intact, medullary respiratory network was employed in our in vitro studies. Since this preparation generates respiratory rhythmic activity spontaneously in both neonatal and mature mice (> 56 days old) it has permitted a direct comparison of the respiratory motor output pattern, recorded from the hypoglossal (XII) motor nucleus, during post-natal development in similar preparations. Consistent with our in vivo findings there was an age-dependent change in the motor pattern. The rhythmic burst of XII neurones recorded from slices of neonates (0-10 days old) was short in duration and decremented whereas a longer discharge (increase of 625% compared to neonate) containing a plateu component was seen in animals more than 15 days old. In addition, the cycle length of rhythmic XII neurones increased (143%) and, together with the changes in burst duration, reached a steady-state value over a similar time course to the maturational changes in phrenic nerve activity recorded in vivo.

Afferent Pathways↗

PubMed1995

Pre-Bötzinger complex in the cat.

1. Patterns of respiratory neuronal activity were examined in pentobarbitone anesthetized adult cats in a circumscribed area of the ventrolateral medulla, which has previously been defined as the pre-Bötzinger complex (pre-BOTC) from electrophysiological and morphological criteria in the brain stem-spinal cord preparation of the neonatal rat. The pre-BOTC has been proposed to play a critical role in respiratory rhythm generation in mammals, but electrophysiological properties of the region have not been thoroughly characterized in the adult brain stem in vivo. 2. From intra- and extracellular recordings, we verified the existence of a well-defined zone with a distinct profile of neuronal activity between the rostral Bötzinger complex containing expiratory neurons and the more caudal medullary pool of inspiratory neurons of the ventral respiratory group (VRG) in the para-ambigual region. This zone corresponds to the pre-BOTC. It was characterized by a concentration of the various types of respiratory neurons, particularly those proposed to be involved in respiratory phase transitions, including neurons discharging immediately before the onset of inspiratory phase activity (pre-inspiratory neurons), early-inspiratory, and postinspiratory neurons. The majority of these neurons were presumed interneurons because they were not antidromically activated by spinal cord or cranial nerve stimulation. 3. The locus of the pre-BOTC corresponded histologically to the rostral part of the nucleus ambiguus and ventrolateral reticular formation. It was located caudal to the retrofacial nucleus and rostral to the lateral reticular nucleus, extending 3.0-3.5 mm rostral to the obex, and 3.2-4.0 mm lateral from the midline. This location was homologous to that established in the neonatal rat. 4. Pre-inspiratory neurons (pre-I neurons) were specifically found in the pre-BOTC. Intracellular recordings from these neurons revealed two types of activity patterns. Type 1 of pre-I neurons exhibited a steady membrane depolarization during expiration and a steep membrane depolarization with a high-frequency burst of action-potential discharge during the phase transition from expiration to inspiration. This was followed by a decline of depolarization and spike discharge during the remainder of the inspiratory phase. A second type of pre-I neurons exhibited a secondary graded membrane depolarization and burst discharge during the late-inspiratory period. 5. Synaptic events were examined in other respiratory neurons during the 40-160 ms preceding the onset of phrenic nerve activity when pre-I neurons exhibited peak spike discharge. Early-inspiratory, throughout-respiratory, and postinspiratory neurons were disinhibited during this period, whereas stage-2 expiratory neurons exhibited a decrease in spike activity and repolarization.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗

PubMed1994

Activity-related pH changes in respiratory neurones and glial cells of cats.

Intracellular pH (pHi) and membrane potential (Em) were measured in vivo in expiratory neurones and glial cells in the medulla of anaesthetized cats using double-barrelled H(+)-sensitive microelectrodes. In glial cells, stimulation of spinal pathways evoked a depolarization of up to 12 mV amplitude and an increase of pHi (7.25 +/- 0.15) by maximally 0.1 pH unit. IN expiratory neurones, pHi (7.15 +/- 0.18) fell by up to 0.2 pH unit during inspiratory inhibition. In axons of expiratory neurones, pHi remained unaffected during rhythmic action potential discharges. We suggest that the glial alkalinization is due to activation of Na+/HCO3- cotransport, whereas the neuronal acidification is caused by efflux of HCO3- via receptor-coupled anion channels.

Animals↗

PubMed1994

Anoxia induced functional inactivation of neonatal respiratory neurones in vitro.

In the isolated brain stem-spinal cord preparation of neonatal rats, respiratory (C4) activity persisted at reduced frequency during tissue anoxia. In seven biphasic expiratory (Bi-E) and in 12 of 22 inspiratory (Insp) neurones, recorded with the 'blind' patch-clamp technique, anoxia evoked a persistent hyperpolarization by up to 16 mV whereas membrane potential of the remaining Insp neurones depolarized slightly or remained unaffected. Respiration-related synaptic potentials and action potential discharges were blocked in all Bi-E and in about 50% of the Insp neurones, whereas rhythmic activity persisted in the other cells. The results indicate that a major population of neonatal respiratory neurones is functionally inactivated during anoxia. This mechanism might contribute to the anoxia tolerance of the immature brain.

Animals↗

PubMed1994

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.

Action Potentials↗

PubMed1994

Spontaneous activation of KATP current in rat dorsal vagal neurones.

Membrane currents were measured in dorsal vagal motoneurones (DVMN) of rat brain stem slices. One to eight minutes after establishing the whole cell configuration, a spontaneous outward current with an amplitude of 137 +/- 54 pA and a reversal potential of -79 +/- 5 mV developed in 20% of DVMN. Tolbutamide (100-200 microM) or glibenclamide (10-50 microM) reversibly abolished the spontaneous outward current whereas only a partial blockade was detected upon administration of 20 mM tetraethylammonium. In four of 12 DVMN which did not show a progressive outward current, diazoxide evoked a tolbutamide-sensitive outward current. The results indicate that DVMN have ATP-dependent K+ channels which are activated by changes in the intracellular milieu induced by diffusion via the patch pipette.

Adenosine Triphosphate↗

PubMed1994

Serotonin 1A-receptor activation suppresses respiratory apneusis in the cat.

Malfunction of inhibitory synaptic processes in the brainstem result in abnormal prolonged inspiration (apneusis). Since we previously found that the serotonin (5-hydroxytryptamine; 5-HT) 5-HT1A receptor agonist 8-hydroxy-dipropylaminotetralin (8-OH-DPAT) shortens inspiratory discharges, we tested its ability to suppress apneusis. We recorded phrenic nerve activity and the membrane potential of medullary expiratory (E-2) and postinspiratory (PI) neurons in 14 anaesthetized, paralyzed, artificially ventilated cats. Systemic hypoxia or i.v. injection of pentobarbital sodium or the N-methyl-D-aspartate (NMDA) receptor blocker ketamine induced apneustic phrenic nerve discharges, delayed depolarization to threshold of E-2 neurons and prolonged hyperpolarization in PI neurons. 8-OH-DPAT (10-40 micrograms/kg i.v.) produced partial to complete restoration of normal phrenic nerve discharges and membrane potential.

8-Hydroxy-2-(di-n-propylamino)tetralin↗

PubMed1994

5-HT-1A receptor-mediated modulation of medullary expiratory neurones in the cat.

The involvement of the 5-HT-1A receptor in serotoninergic responses of stage 2 expiratory (E-2) neurones was investigated in pentobarbitone-anaesthetized, mechanically ventilated cats. The specific agonist of the 5-HT-1A receptor, 8-hydroxy-diproplaminotetralin (8-OH-DPAT), administered systemically or by ionophoresis directly on to the neurones, had a clear depressant effect. Administration of 8-OH-DPAT at doses of 10-50 micrograms kg-1 (I.V.) increased the membrane hyperpolarizations of E-2 neurones during the inspiratory and postinspiratory phases, and shortened their duration of activity in association with shortening of phrenic nerve activity. Discharges of E-2 neurones were also less intense. At doses of 50-90 micrograms kg-1, 8-OH-DPAT reduced or abolished inspiratory hyperpolarizations, and reduced expiratory depolarizations of membrane potential and discharge in parallel with inhibition of phrenic nerve discharges. The effects of the larger doses were reversed by I.V. injection of NAN-190, an antagonist at the 5-HT-1A receptor. Dose-dependent effects on the membrane potential and discharge of E-2 neurones, but not on phrenic nerve activity, were also seen by ionophoretic administration of 8-OH-DPAT on to E-2 neurones. At low currents, ejection of 8-OH-DPAT hyperpolarized the neurones without affecting the duration of inspiratory hyperpolarization and expiratory depolarization. This hyperpolarization depressed the intensity and the duration of expiratory discharges. Ejection with larger currents hyperpolarized the E-2 neurones further, and depressed expiratory depolarization leading to blockade of expiratory discharges. The effects on membrane potential were accompanied by decreased neuronal input resistance. This depressed the excitability of E-2 neurones as tested by discharge evoked by intracellular current injection. The amplitudes of action potentials decreased in parallel with the changes in input resistance. The effects were attributed to a postsynaptic effect of 8-OH-DPAT leading to a gradually developing inhibition by activation of 5-HT-1A receptors. Hyperventilatory apnoea depressed on-going synaptic activity and unmasked the effect of ionophoretically applied 8-OH-DPAT. The responses of the E-2 neurone were enhanced, as evidenced by increased membrane hyperpolarization and greater reduction of input resistance. Both responses faded appreciably, indicating receptor desensitization. The degree and rate of apparent desensitization depended on the dose/ejecting current. The greater sensitivity and faster desensitization to 8-OH-DPAT were attributed to the hyperventilatory alkalinization of the extracellular fluid, which might influence agonist binding to 5HT-1A receptors and/or receptor properties.

8-Hydroxy-2-(di-n-propylamino)tetralin↗

PubMed1994

Mechanisms of respiratory rhythm generation change profoundly during early life in mice and rats.

To study the ontogenesis of central respiratory rhythm generation, a novel brainstem slice preparation was developed that generates respiratory rhythmic activity spontaneously in mice and rats at all post-natal ages. The slice was made by tilting the brainstem to include both the ventrolateral and dorsomedial medulla. This 'tilted-sagittal' slice contained the nucleus ambiguus, the hypoglossal motor nucleus (XII) and the nucleus of the solitary tract which were preserved intact throughout their rostro-caudal extent. Using this rhythmic preparation it has been possible for the first time to directly compare the significance of glycinergic mechanisms for respiratory rhythmogenesis between newborns and mature rodents in vitro. Our findings demonstrate that during the first two weeks of life there are profound changes in both the motor pattern of rhythmic XII neurons and sensitivity of the respiratory rhythm to strychnine blockade of glycine receptors. Thus, developmental changes in strychnine-sensitive receptors are vital for the maturation of the respiratory network and it is suggested that any disturbance in their development may be lethal.

Aging↗

PubMed1994

Pontine respiratory neurons in anesthetized cats.

The pontine respiratory neurons (PRG) in the 'pneumotaxic centre' have been hypothesized to contribute to phase-switching of neural respiratory activity, especially in terminating inspiration. To define the neural elements involved in phase-switching, we recorded respiratory neurons extra- and intracellularly in anesthetized cats with an intact central nervous system. In total, 54 neurons were recorded: 49 neurons with activity modulated by central respiratory rhythm (20 inspiratory, 17 postinspiratory and 12 expiratory) and 5 neurons with activity correlated to tracheal pressure. The recorded neurons were clustered in dorsolateral pontine tegmentum within the Kölliker-Fuse (KF) subnucleus of the parabrachial nuclei. Stable intracellular membrane potential was recorded in 11 of the 49 respiratory neurons (8 postinspiratory, 1 early inspiratory and 2 inspiratory). During continuous injection of chloride ions (n = 6), synaptic noise increased and IPSPs reversed, including a wave of IPSPs during stage-2 expiration in postinspiratory neurons. Further, relative input resistance varied through the respiratory cycle such that the least input resistance occurred during the neuron's (n = 5) quiescent period. No IPSPs nor EPSPs were evoked in pontine respiratory neurons by vagal stimulation. In conclusion, various types of respiratory neurons were recorded in the KF nucleus. Prominent excitatory and inhibitory postsynaptic activities were similar to those described for medullary neurons. These pontine respiratory neurons do not appear to receive a strong afferent input from the vagus. Rather, vagal afferent inputs seem to be directed towards non-respiratory neurons that are located more medially in the dorsal pons.

Anesthesia↗

PubMed1994

Functionally intact in vitro preparation generating respiratory activity in neonatal and mature mammals.

The present report describes a novel rhythmically active brainstem slice preparation that generates respiratory activity spontaneously in both mice and rats of varying maturational states. The brainstems of neonatal (0-4 days) and mature (3-8 weeks) mice and rats were isolated and a 600- to 750-microns thick slice cut to include the dorsomedial and the ventrolateral regions of the complete rostro-caudal extent of the medulla. This plane of section we have termed "tilted-sagittal". Rhythmically discharging neurones were recorded extracellularly from both the dorsal and ventral regions of the slice. The recording sites of these neurones were found in the hypoglossal motonucleus (XII) and in areas of the ventrolateral medulla that includes the ventral respiratory group (VRG) region. Histological examination revealed the preservation of neuronal structures important for cardiorespiratory regulation and reflex control including the nucleus of the solitary tract as well as the nucleus ambiguus. In addition, pontine structures including the A5 region were also preserved. Rhythmic activity was found only in slices where the ambiguual column was preserved in its entirety. The mean frequency of discharge of XII neurones was 20 and 10 bursts per minute in neonates and mature rodents respectively. In preparations of mature animals we demonstrate that this frequency increased significantly (P < 0.05) by either raising temperature from 29 degrees C to 38 degrees C (54%), elevating extracellular potassium concentration from 4 to 7.5 mM (52%), blocking potassium channels (20%) or decreasing pH from 7.4 to 7.0 (18%). The burst duration to frequency ratio of XII and VRG rhythmic neurones was similar and therefore indicative of a common brainstem oscillator. Consistent with this finding was that rhythmic activity in the VRG persisted despite removal of the dorsomedial region of the slice. In contrast, rhythmic XII neurones became tonic following mechanical disconnection of the VRG.

Aging↗

PubMed1993

Synapses between slowly adapting lung stretch receptor afferents and inspiratory beta-neurons in the nucleus of the solitary tract of cats: a light and electron microscopic analysis.

Previous neuroanatomic and physiologic studies indicated that afferent fibres from slowly adapting pulmonary stretch receptors (SAR) project to the nuclei of the solitary tract and terminate on inspiratory beta-neurons. In the present study we combined electrophysiologic and morphologic approaches to verify the presumed monosynaptic connections between SARs and beta-neurons. Single identified beta-neurons and single identified SAR afferent fibres were labelled intrasomally and intraaxonally, respectively, with horseradish peroxidase (HRP) in the same anesthetized cats. Under the light microscope, we analyzed the morphology of beta-neurons and their dendritic fields and of the terminal projection pattern of fibres from SARs and identified potential synaptic connections between boutons of SAR afferent fibres and the soma and dendrites of beta-neurons. The identified tissue was then processed further for electron microscopic analysis. On average, beta-neurons had 6 primary dendrites that bifurcated 3-8 times. The dendritic trees extended 1.5 mm both rostrocaudally in the ventrolateral nucleus of the solitary tract and medially into the intermediate subnucleus. Axons of beta-neurons curved toward the midline and no collateral branches were evident over its stained length (2.5-3.4 mm). Axodendritic synaptic contacts between SAR fibres and beta-neurons were identified electron microscopically in four of six tissue samples chosen by light microscopy. In addition, we located 2 axodendritic and 2 axosomatic synaptic contacts that were not observed under light microscopic screening. The boutons of SAR fibres contained clear, round vesicles and formed asymmetrical synapses with beta-neurons. Multiple synaptic connections were found between collaterals of a single SAR and single beta-neurons, indicating a dense terminal projection of single SAR afferent fibres onto beta-neurons. These morphologic data prove monosynaptic connections between electrophysiologically identified SAR afferent fibres and beta-neurons.

Animals↗

PubMed1993

Microenvironment of respiratory neurons in the in vitro brainstem-spinal cord of neonatal rats.

1. O2-, K(+)- and pH-sensitive microelectrodes were used to measure extracellular oxygen pressure (PO2), K+ activity (aKo) and pH (pHo) in ventral regions of the medulla oblongata containing respiratory neurons in the in vitro brainstem-spinal cord preparation from 0 to 4-day-old rats. 2. The location of respiratory neurons was mapped by extracellular recordings with conventional microelectrodes, or with the reference barrel of ion-sensitive microelectrodes. The major populations of respiratory neurons were distributed in the ventrolateral reticular formation near the nucleus ambiguus at depths of 300-600 microns. In this area, aKo baseline increased from 3.2 to 3.8 mM whereas steady-state values of PO2 and pHo fell from 120 to 7 mmHg and from 6.9 to 6.7, respectively. 3. During rhythmic inspiratory discharges recorded with suction electrodes from ventral roots of spinal (C3-C5) and cranial (IX, X, XII) nerves, aKo transiently increased by up to 100 microM, and PO2 fell maximally by 0.4 mmHg. During episodes of non-rhythmic neuronal discharge, aKo increased by as much as 0.4 mM and PO2 decreased by about 10 mmHg. In contrast, no variations in pHo could be detected during such activities. 4. Activation of medullary neurons by tetanic electrical stimulation of axonal tracts in the ventrolateral column of the spinal cord at the level of the phrenic motoneuron pool produced aKo elevations of up to 5 mM, decreases of PO2 by up to 50 mmHg, and pHo increases by a maximum of 0.07 pH units. These aKo and PO2 transients were reduced by more than 80% during blockade of synaptic transmission with 5 mM manganese (Mn2+) and completely blocked by 1 microM tetrodotoxin (TTX). 5. The tissue PO2 gradient as well as activity-related decreases of PO2 were completely abolished after block of oxidative cellular metabolism by addition of 2-10 mM cyanide (CN-) to the bathing solution. 6. Inhibition of the Na(+)-K+ pump by addition of 3-50 microM ouabain (3-10 min) caused a reversible increase of aKo by 0.8-3 mM, a delayed recovery of stimulus-induced aKo elevations, and produced a disturbance of the respiratory rhythm. 7. The sensitivity of the respiratory network to oxygen depletion was tested by superfusing the neuraxis with hypoxic solutions gassed with N2 instead of O2 (5-20 min).(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗

PubMed1993

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.

Action Potentials↗