Biomedical subjects

J F Paton

Publications and source records attributed to J F Paton.

At least 55 records · Page 3Linked to original sources

PubMed1997

An arterially-perfused trunk-hindquarters preparation of adult mouse in vitro.

We describe a preparation of arterially-perfused spinal cord with attached hindquarters, taken from adult mouse. This is the first preparation of adult mammalian spinal cord tissue to have the advantages of an in vitro approach whilst retaining intact intraspinal circuitry, sensory inputs, and somatic and sympathetic segmental outputs. The functional integrity of the preparation has been demonstrated by the motor and sympathetic reflexes that can readily be evoked by peripheral noxious thermal, mechanical and electrical stimuli, and also by bladder distension. The mechanical stability of the preparation allows intracellular recordings to be made from spinal dorsal or ventral horn neurones. The intact connectivity permits synaptic responses to be evoked by stimulation of functionally-defined peripheral sensory receptors. The preparation is relatively quick to set up and remains viable for more than 6 h. This model offers the opportunity to perform complex electrophysiological and pharmacological studies on functionally characterised synaptic responses of mature spinal neurones. The choice of the mouse will furthermore permit studies to be performed on genetically mutant strains.

Animals↗

PubMed1997

Rhythmic bursting of pre- and post-inspiratory neurones during central apnoea in mature mice.

1. Stimulation of pulmonary vagal C fibres (PCFs) inhibits inspiration but the response pattern of respiratory rhythm-generating neurones is unknown. This study provides the first description of the effects of PCF stimulation on six different types of respiratory neurones located in the ventrolateral medulla of the mature mouse. 2. Studies were performed in both urethane-anaesthetized (1.5 g kg-1 I.P.) mature mice and in an arterially perfused working heart-brainstem preparation (WHBP). In both preparations the respiratory motor pattern of phrenic and recurrent laryngeal nerves were comparable. Stimulation of PCFs, using phenylbiguanide (2-5 micrograms) injected into the right atrium, evoked a similar respiratory and cardiac response pattern in both anaesthetized and perfused mice, which included: (i) a significant prolongation of the inter-inspiratory interval; (ii) an increase in the duration and amplitude of post-inspiratory (PI) activity; and (iii) an atropine-sensitive bradycardia (50-260 beats min-1). 3. In the WHBP, PCF stimulation evoked a depolarization (11 +/- 1 mV) and high frequency tonic discharge (up to 64 Hz) in ten out of twenty-one PI neurones. During the PCF-induced prolongation of PI activity all other PI neurones (n = 11), as well as pre-inspiratory neurones (PreI; n = 11), displayed oscillations in membrane potential and/or rhythmic bursting at a similar frequency of 0.7-1.0 Hz. Other respiratory neurones recorded, including stage II expiratory neurones (n = 7), early- (n = 6), ramp- (n = 16) and late-inspiratory neurones (n = 4), ceased firing rhythmically during PCF stimulation. 4. The firing behaviour of PI and PreI neurones was assessed after switching to a low Ca2+ (0.2 mM)-high Mg2+ (5.25 mM) perfusate to block synaptic transmission in the WHBP. In the absence of synaptic transmission, PreI neurones (n = 7/8) continued to discharge rhythmically, whereas all other respiratory cell types (including PI neurones, n = 5) fired tonically. 5. In conclusion, stimulation of PCFs elicits a reflex-evoked prolongation of the PI phase of the respiratory cycle and excitation of PI neurones including rhythmic discharging. It is suggested that this rhythmic bursting depends on inhibitory connections from PreI neurones. The functional significance of these central 'apnoeic rhythms' are discussed.

Age Factors↗

PubMed1997

Changes in baroreceptor vagal reflex performance in the developing rat.

Ontogenesis of both vagal control of heart rate and the baroreceptor vagal reflex were evaluated in rats at postnatal ages (P) of 5/6, 10, 15, 20, 25 and >>42 days anaesthetised with urethane (1.5 g/kg). Between P5/6 and P25 heart rate rose from 372 +/- 12 to 448 +/- 20 beats per minute and mean arterial pressure increased from 33.9 +/- 3.1 to 74.59 +/- 3.25 mm Hg (mean +/- SEM, n = 7 and 11 respectively). Cardiac vagal tone was absent at P10 but significant at P20 (P < 0.05) as revealed with atropine (0.5-1 mg/kg i.v.). Baroreceptor cardiac reflex sensitivity, tested with phenylephrine (10-50 microg/kg i.v.), was attenuated significantly in P10-20 rats compared with P5/6, P25 and mature animals. In P14-17 rats stimulation of neurones in either the solitary tract or ambiguual nuclei, by microinjection of L-glutamate (100-200 pmol), evoked an atropine-sensitive bradycardia indicating a functional integrity of central and peripheral efferent pathways mediating the baroreceptor reflex. Thus, the baroreceptor vagal reflex is functional in P5/6 rats but becomes attenuated between P10-P20, which is coincident with the maturational rise in arterial pressure.

Aging↗

PubMed1997

Modeling neural mechanisms for genesis of respiratory rhythm and pattern. I. Models of respiratory neurons.

The general objectives of our research, presented in this series of papers, were to develop a computational model of the brain stem respiratory neural network and to explore possible neural mechanisms that provide the genesis of respiratory oscillations and the specific firing patterns of respiratory neurons. The present paper describes models of single respiratory neurons that have been used as the elements in our network models of the central respiratory pattern generator presented in subsequent papers. The models of respiratory neurons were developed in the Hodgkin-Huxley style employing both physiological and biophysical data obtained from brain stem neurons in mammals. Two single respiratory neuron models were developed to match the two distinct firing behaviors of respiratory neurons described in vivo: neuron type I shows an adapting firing pattern in response to synaptic excitation, and neuron type II shows a ramp firing pattern during membrane depolarization after a period of synaptic inhibition. We found that a frequency ramp firing pattern can result from intrinsic membrane properties, specifically from the combined influence of calcium-dependent K(AHP)(Ca), low-threshold Ca(T) and K(A) channels. The neuron models with these ionic channels (type II) demonstrated ramp firing patterns similar to those recorded from respiratory neurons in vivo. Our simulations show that K(AHP)(Ca) channels in combination with high-threshold Ca(L) channels produce spike frequency adaptation during synaptic excitation. However, in combination with low-threshold Ca(T) channels, they cause a frequency ramp firing response after release from inhibition. This promotes a testable hypothesis that the main difference between the respiratory neurons that adapt (for example, early inspiratory, postinspiratory, and decrementing expiratory) and those that show ramp firing patterns (for example, ramp inspiratory and augmenting expiratory) consists of a ratio between the two types of calcium channels: Ca(L) channels predominate in the former and Ca(T) channels in the latter respiratory neuron types. We have analyzed the dependence of adapting and ramp firing patterns on maximal conductances of different ionic channels and values of synaptic drive. The effect of adjusting specific membrane conductances and synaptic interactions revealed plausible neuronal mechanisms that may underlie modulatory effects on respiratory neuron firing patterns and network performances. The results of computer simulation provide useful insight into functional significance of specific intrinsic membrane properties and their interactions with phasic synaptic inputs for a better understanding of respiratory neuron firing behavior.

Adaptation, Physiological↗

PubMed1997

Modeling neural mechanisms for genesis of respiratory rhythm and pattern. II. Network models of the central respiratory pattern generator.

The present paper describes several models of the central respiratory pattern generator (CRPG) developed employing experimental data and current hypotheses for respiratory rhythmogenesis. Each CRPG model includes a network of respiratory neuron types (e.g., early inspiratory; ramp inspiratory; late inspiratory; decrementing expiratory; postinspiratory; stage II expiratory; stage II constant firing expiratory; preinspiratory) and simplified models of lung and pulmonary stretch receptors (PSR), which provide feedback to the respiratory network. The used models of single respiratory neurons were developed in the Hodgkin-Huxley style as described in the previous paper. The mechanism for termination of inspiration (the inspiratory off-switch) in all models operates via late-I neuron, which is considered to be the inspiratory off-switching neuron. Several two- and three-phase CRPG models have been developed using different accepted hypotheses of the mechanism for termination of expiration. The key elements in the two-phase models are the early-I and dec-E neurons. The expiratory off-switch mechanism in these models is based on the mutual inhibitory connections between early-I and dec-E and adaptive properties of the dec-E neuron. The difference between the two-phase models concerns the mechanism for ramp firing patterns of E2 neurons resulting either from the intrinsic neuronal properties of the E2 neuron or from disinhibition from the adapting dec-E neuron. The key element of the three-phase models is the pre-I neuron, which acts as the expiratory off-switching neuron. The three-phase models differ by the mechanisms used for termination of expiration and for the ramp firing patterns of E2 neurons. Additional CRPG models were developed employing a dual switching neuron that generates two bursts per respiratory cycle to terminate both inspiration and expiration. Although distinctly different each model generates a stable respiratory rhythm and shows physiologically plausible firing patterns of respiratory neurons with and without PSR feedback. Using our models, we analyze the roles of different respiratory neuron types and their interconnections for the respiratory rhythm and pattern generation. We also investigate the possible roles of intrinsic biophysical properties of different respiratory neurons in controlling the duration of respiratory phases and timing of switching between them. We show that intrinsic membrane properties of respiratory neurons are integrated with network properties of the CRPG at three hierarchical levels: at the cellular level to provide the specific firing patterns of respiratory neurons (e.g., ramp firing patterns); at the network level to provide switching between the respiratory phases; and at the systems level to control the duration of inspiration and expiration under different conditions (e.g., lack of PSR feedback).

Adaptation, Physiological↗

PubMed1997

Modeling neural mechanisms for genesis of respiratory rhythm and pattern. III. Comparison of model performances during afferent nerve stimulation.

The goal of the present study was to evaluate the relative plausibility of the models of the central respiratory pattern generator (CRPG) proposed in our previous paper. To test the models, we compared changes in generated patterns with the experimentally observed alterations of the respiratory pattern induced by various stimuli applied to superior laryngeal (SLN), vagus and carotid sinus (CS) nerves. In all models, short-duration SLN simulation caused phase-resetting behavior consistent with experimental data. Relatively weak sustained SLN stimulation elicited a two-phase rhythm comprising inspiration and postinspiration whereas a stronger stimulation stopped oscillations in the postinspiratory phase ("postinspiratory apnea"). In all models, sustained vagus nerve stimulation produced postinspiratory apnea. A short vagal stimulus delivered during inspiration terminated this phase. The threshold for inspiratory termination decreased during the course of the inspiratory phase. The effects of short-duration vagal stimulation applied during expiration were different in different models. In model 1, stimuli delivered in the postinspiratory phase prolonged expiration whereas the late expiratory phase was insensitive to vagal stimulation. No insensitive period was found in model 2 because vagal stimuli delivered at any time during expiration prolonged this phase. Model 3 demonstrated a short period insensitive to vagal stimulation at the very end of expiration. When phasic CS nerve stimulation was applied during inspiration or the first half of expiration, the performances of all models were similar and consistent with experimental data: stimuli delivered at the beginning inspiration shortened this phase whereas stimuli applied in the middle or at the end of inspiration prolonged it and stimuli delivered in the first half of expiration prolonged the expiratory interval. Behavior of the models were different when CS stimuli were delivered during the late expiratory phase. In model 1, these stimuli were ineffective or shortened expiration initiating the next inspiration. Alternatively, in models 2 and 3, they caused a prolongation of expiration. Although all CRPG models demonstrated a number of plausible alterations in the respiratory pattern elicited by afferent nerve stimulation, the behavior of model 1 was most consistent with experimental data. Taking into account differences in the model architectures and employed neural mechanisms, we suggest that the concept of respiratory rhythmogenesis based on the essential role of postinspiratory neurons is more plausible than the concept employing specific functional properties of decrementing expiratory (dec-E) neurons and that the ramp firing pattern of the late expiratory neuron is more likely to reflect intrinsic properties than disinhibition from the dec-E neurons.

Afferent Pathways↗

PubMed1996

The ventral medullary respiratory network of the mature mouse studied in a working heart-brainstem preparation.

1. This report provides the first description of respiratory network activity within the ventrolateral medulla of the mature mouse obtained from a unique working heart-brainstem preparation (WHBP). 2. In the WHBP three distinct respiratory phases were evident in recordings of both phrenic and vagal efferent nerves. These included a ramp inspiratory (I) discharge, post-inspiratory (PI) activity and a silent or expiratory interval (E2). 3. Extracellular recordings were made from different types of respiratory neurones located within, or in close proximity to, the nucleus ambiguus. Based on firing patterns and phase relative to phrenic nerve discharge, respiratory neurone types, including pre-inspiratory (PreI), early-inspiratory, throughout inspiratory (I), late-inspiratory, post-inspiratory (PI) and stage II expiratory or E2 neurones were characterized. 4. Intracellular recordings were made from four types of respiratory neurones (PreI, I, PI and E2 neurones). PreI neurones were depolarized maximally during the E2-inspiratory transition. I neurones exhibited a ramp depolarization which started either before or at the onset of phrenic discharge. Based on the kinetics of the inspiratory-related hyperpolarizations and duration of discharge, two types of PI neurones were found (rapidly adapting and slowly adapting). E2 neurones were hyperpolarized during both the inspiratory and post-inspiratory phases. 5. Phase-dependent chloride-mediated inhibition was studied in PreI, PI and E2 neurones and included: late inspiratory inhibition of PreI neurones; inspiratory-related inhibition of PI and E2 neurones; and post-inspiratory inhibition of PreI and E2 neurones. In addition, pre-inspiratory inhibition of PI neurones was also demonstrated. 6. The WHBP appears to be viable for analysing reflex, synaptic and cellular mechanisms regulating respiratory activity in an in vitro milieu. The synaptic organization of the respiratory network of the mouse appears comparable to that of the rat and cat. The possibility of a mutual inhibitory interaction between PreI and PI neurones is discussed in terms of the functional organization of the respiratory network in the mouse.

Animals↗

PubMed1996

A working heart-brainstem preparation of the mouse.

An intra-arterially perfused working heart-brainstem preparation (WH-BP) was developed to allow studies into functionally identified cardiovascular and respiratory neurones in an in vitro milieu. This report provides that first description of this preparation. Evidence is presented indicating that the WH-BP: (1) spontaneously generates eupneic-like phrenic nerve activity, indicative of adequate oxygenation of the brainstem; (2) preserves the integrity of central coupling between the central respiratory rhythm generator and cardiac vagal motor neurones and, (3), allow an intracellular analysis of medullary cardio-respiratory neurones. The WH-BP may provide an advantaged environment for analysis of both synaptic and cellular mechanisms within the medulla that regulate cardio-respiratory activity.

Animals↗

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

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↗

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

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

Characteristic firing behavior of cell types in the cardiorespiratory region of the nucleus tractus solitarii of the rat.

The present in vitro study was performed to characterize neurons within dorsal regions of the nucleus tractus solitarii (NTS), principally at the level of area postrema, and known to receive inputs predominantly from cardiovascular and respiratory afferents (i.e. cardiorespiratory NTS). This report describes 4 classes of neurons (S1-S4) that were silent at their resting membrane potential and received relatively short (< 3.6 ms) and consistent latency synaptic inputs (+/- 0.4 ms) comprising either an EPSP or EPSP/IPSP sequence following low intensity electrical stimulation of the solitary tract (ts). Intracellular recording with sharp electrodes were used to characterize neuron types based on their different firing response patterns to injection of depolarizing current. S1 cells showed a single action potential; S2 fired repetitively; S3 produced a 2-5 spike burst coincident with the start of the current pulse and S4 neurons showed delayed excitation. Accommodation of firing frequency was seen in S2, S3 and some S4 cells. The voltage dependency of the different discharge patterns of the 4 cell groups was tested by current pulse stimulation at different holding potentials. However, in the majority of cells in any one cell class the firing pattern was qualitatively similar. Based on these findings it is suggested that the different firing characteristics reflect differences in intrinsic membrane properties between neuron classes. Representative examples from each of the defined cell classes were further studied in current and voltage clamp using the whole cell patch technique to define the presence and role of certain ionic currents in the firing response patterns of the 4 cell groups. In the current clamp configuration the firing behavior of S1 neurons (single spiking) was unaltered during exposure to 4-aminopyridine (4-AP; 2 mM), cobalt chloride (Co; 5 mM), norepinephrine (NE; 20 microM) and muscarine chloride (50 microM). It is suggested that the relatively low excitability of this neuron is due a persistent outward current which occurred at -40 mV during depolarizing voltage steps in the voltage clamp configuration. A common characteristic of S2 neurons (repetitively firing) was that they showed accommodation during current injection which was greatly attenuated in the presence of Co or NE. In addition, 4-AP slowed the firing frequency, reduced the afterhyperpolarization and broadened the spike width of S2 cells. Interestingly, the amount of accommodation observed in S2 cells was variable for cells of this class and was proportional to the magnitude of a Co-sensitive inward current present during depolarizing voltage steps between -45 to -5 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

4-Aminopyridine↗

PubMed1993

Computational modeling of neuronal dynamics for systems analysis: application to neurons of the cardiorespiratory NTS in the rat.

The study constructs computational models of neurons in order to examine the contribution that their response dynamics may make to functional properties at the system level. As described in the accompanying study, neurons in the cardiorespiratory nucleus tractus solitarii (NTS) of the rat were recorded in vitro. When these cells were intracellularly injected with a constant current pulse, spike discharge patterns and subthreshold voltage trajectories were observed that were time- and voltage-dependent. The accompanying manuscript describes these dynamic responses in 4 classes of putative second-order cells that appear to receive direct primary afferent input, and a previous paper described two populations of rhythmically firing interneurons, one of which is intrinsically auto-active. In the present manuscript experimental neuronal voltage response data was collected across a current injection series for the S3 neuron type described in the accompanying study and for the auto-active neuron described previously. Using this data, computational model neurons have been constructed for these two neurons by using membrane ion channels to produce and match the observed neuronal voltage behavior. The channels were those implicated in the dynamic responses observed in the companion study, and include gNafast, gKdr, gKA, gKCa, gKAHP, gKM, gCaT and gCaL. The description of channel kinetics follows the Hodgkin-Huxley form. Different neuronal sources from the literature of channel kinetics were investigated and assembled into a 'channel kinetics library' from which both neuron models were tuned, primarily by adjusting the maximum channel densities, g, and time-dependence of kinetics. Methods are described for tuning the channel kinetics library to match various physiological responses. This approach created neuron models that were able to closely replicate the observed complex voltage and spiking responses of the two very different cardiorespiratory NTS neurons. The interaction of voltage- and calcium-dependent conductances were analyzed for their functional contributions by tuning their kinetics. Specific parameters are given that account for the behavior of each model. Sensitivity analyses by perturbing KCa and KA are shown for both neurons, and I/F curves are presented for the auto-active neuron's stimulated and recorded responses. The potential systems-level functional implications resulting from the different kinetics is demonstrated by driving the S3 model neuron in simulation with the pattern of input produced by model primary baroreceptor afferents. The limitations and significance of this approach are discussed.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗

PubMed1993

NTS neuronal responses to arterial pressure and pressure changes in the rat.

Central representation of arterial pressure by baroreceptor target neurons in the nucleus of the solitary tract (NTS) has not been studied. The present experiments sought to characterize response patterns of NTS baroreceptive cells in anesthetized, paralyzed, and artificially ventilated rats to both resting pressures and induced blood pressure challenges. Single-unit extracellular recordings were made from 83 baroreceptive NTS cells that received a synaptic input after electrical stimulation of the aortic nerve [latency, 2.46 +/- 0.78 (+/- SD) ms] and were located at an anatomically defined region known to receive baroreceptor afferents. Seventy-one neurons were presumed second order, since they received a short (< 5 ms) and invariant (< +/- 0.5 ms) synaptic input from the ipsilateral aortic nerve. Thirty-five of these neurons were silent at resting blood pressures and produced few (1-4) spikes when presented with induced pressor responses. The remainder (n = 36) had ongoing activity that was not pulse rhythmic and that varied in rate nonuniformly with arterial blood pressure during an induced challenge. Ongoing activity was analyzed for active neurons, revealing both R wave-related and lung inflation-related phasic activity. The present data suggest that baroreceptive NTS neurons may be sensitive to many characteristics of the input signal, such as dP/dt, mean pressure, and cardiac frequency, as well as pulmonary afferent drive.

Animals↗

PubMed1992

Influence of the cerebellar posterior vermis on the acquisition of the classically conditioned bradycardic response in the rabbit.

The magnitude of classically conditioned bradycardia was studied in rabbits in which various cerebellar regions (lobule IX or the posterior vermis or the hemispheres) had been removed surgically. Lesions were shown histologically to be restricted to the cortex and the underlying white matter without any damage to the deep cerebellar nuclei. In the conditioning procedure, tones were employed as conditioned stimuli (CS) and ear shocks as unconditioned stimuli (US). Cerebellar lesions did not affect the characteristics of the bradycardic orienting response, baseline heart rate or the unconditioned tachycardic response to US. The conditioned bradycardia was significantly reduced in magnitude with respect to controls in rabbits submitted to removal of posterior vermis, while it was unaffected in lobule IX and hemispheric lesioned rabbits. The temporal pattern of development and habituation of the bradycardiac response through the conditioning session, as well as its topography, did not differ from controls in any of the lesioned rabbits. After the first conditioning session, some control rabbits were submitted to removal of the posterior vermis and then conditioned again, following an identical procedure. Their pre- and post-lesion conditioned responses did not exhibit any appreciable differences and were similar to the responses exhibited by a group of unoperated controls which were submitted to a reconditioning session. It is concluded that in the rabbit the cerebellar posterior vermis is involved in the initial acquisition of the classically conditioned bradycardia, but it is not the site of its memory trace.

Animals↗

PubMed1992

Effect of anesthetic on sympathetic responses evoked from cerebellar uvula in decerebrate cats.

The sympathetic mechanisms involved in the conversion of the tachycardia-pressor response evoked by electrical stimulation of the uvula (lobule IX of the posterior cerebellar cortex) in the unanesthetized decerebrate cat to a bradycardia-depressor response in the same, but anesthetized preparation, were investigated. Sympathoexcitation was produced in the inferior cardiac and renal sympathetic nerves in response to short train stimulation (2-5 pulses, 100-500 Hz) of the uvula in the unanesthetized decerebrate cat, and when paired stimuli (conditioning and test) were applied, the test-evoked potential in both nerves was similar to the response elicited by the conditioning stimulus. Anesthetic administered to these same animals caused the test response in both sympathetic nerves to be greatly decreased, yet the conditioning response was unchanged. The attenuation of the test response by the conditioning stimulus diminished during recovery from anesthesia. The recovery of the test response paralleled the time course of the return of the tachycardia-pressor effect evoked by long train stimulation of the uvula. It appears that anesthesia does not block the sympathoexcitatory response but acts to augment sympathoinhibitory processes associated with uvula stimulation; some possible mechanisms are discussed.

Anesthetics↗

PubMed1991

The ventrolateral medulla as a source of synaptic drive to rhythmically firing neurons in the cardiovascular nucleus tractus solitarius of the rat.

We sought to determine whether the caudal ventrolateral medulla (cVLM), at the level of area postrema, influences the rhythmically beating neurons found within the dorsomedial NTS in rat brainstem slices. Intra- or extracellular recordings of neurons firing rhythmically at around 5 Hz were characterized as either auto-active (i.e. pacemaker; AA) or synaptically driven (SD) by pharmacological interventions. The nature of inputs evoked from the ipsilateral cVLM were orthodromic and the majority were excitatory (latency 3-20 ms). Further, this excitatory influence was found to be tonically active in 25/47 cells studied since inactivating the ipsilateral cVLM by localized cooling reduced the firing rate by 0.5-3.0 Hz (23% on average). Neuronal characterization showed that the most consistent and pronounced effect occurred on SD rather than AA cells. Control experiments that cooled other areas of the slice closer to the recording site proved ineffective. Additional studies showed that most rhythmically firing cells in the NTS received an excitatory synaptic input from the solitary tract (ts; latency 3-30 ms). This input was reduced or blocked by inactivating the cVLM in neurons in which the ts latency of activation was greater than 8 ms in half of the neurons tested. Subsequent pharmacological tests revealed that these neurons were predominantly SD. Identified AA neurons received an input from the ts at a shorter latency, typically less than 8 ms, and this was unperturbed by cooling the cVLM in all cases. Further, there was no obvious difference in the baseline discharge rates between cells in the hemi-slice and those recorded in an intact slice. In a hemi-coronal slice cooling the cVLM also produced a 20% decrease in firing rate in identified SD neurons but no consistent change in AA cells. We conclude that (1) the ipsilateral cVLM contributes principally tonic excitatory drive to rhythmically active neurons in the dorsomedial NTS in vitro and this preferentially effects SD neurons; (2) other excitatory drives other than those from the ipsilateral cVLM impinge upon SD cells, the origin of which are relatively local and likely to be in the NTS; (3) in the slice the projection from the cVLM to the NTS appears to be present but the reciprocal connection is absent.

2-Amino-5-phosphonovalerate↗