Synaptic mechanisms of inspiratory off-switching evoked by pontine pneumotaxic stimulation in cats

To elucidate synaptic mechanisms and the involvement of N-methyl-D-aspartate (NMDA) receptors in inspiratory off-switching (IOS) evoked by the stimulation of the nucleus parabrachialis medialis (NPBM), excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) were recorded from bulbar augmenting inspiratory (aug-I) and postinspiratory (PI) neurons in vagotomized cats. Stimulation of NPBM produced either transient inhibition or premature termination of inspiration (reversible or irreversible IOS), depending on the stimulus intensity. Each neuron displayed four-phasic postsynaptic responses during the reversible IOS, i.e. Phase 1 EPSPs, Phase 2 IPSPs, Phase 3 EPSPs and Phase 4 IPSPs in aug-I neurons, and Phase 1 plus 2 EPSPs, Phase 3 IPSPs and Phase 4 EPSPs in PI neurons. During the irreversible IOS, Phase 4 responses were replaced by sustained hyperpolarization in aug-I neurons and decrementing depolarization in PI neurons. Blockade of NMDA receptors by dizocilpine (0.3 mg kg(-1) i.v.) selectively increased Phase 4 potentials in both types of neurons and decreased the thresholds for evoking the irreversible IOS. The NPBM-induced responses had a pattern and time-course similar to those induced by vagal stimulation. The present results suggest that pneumotaxic and vagal inputs converge on the common IOS circuit, and the effectiveness of both inputs is modulated by NMDA receptors.     

Physiological properties of late inspiratory neurons and their possible involvement in inspiratory off-switching in cats

To assess the functional significance of late inspiratory (late-I) neurons in inspiratory off-switching (IOS), membrane potential and discharge properties were examined in vagotomized, decerebrate cats. During spontaneous IOS, late-I neurons displayed large membrane depolarization and associated discharge of action potentials that started in late inspiration, peaked at the end of inspiration, and ended during postinspiration. Depolarization was decreased by iontophoresis of dizocilpine and eliminated by tetrodotoxin. Stimulation of the vagus nerve or the nucleus parabrachialis medialis (NPBM) also evoked depolarization of late-I neurons and IOS. Waves of spontaneous chloride-dependent inhibitory postsynaptic potentials (IPSPs) preceded membrane depolarization during early inspiration and followed during postinspiration and stage 2 expiration of the respiratory cycle. Iontophoresed bicuculline depressed the IPSPs. Intravenous dizocilpine caused a greatly prolonged inspiratory discharge of the phrenic nerve (apneusis) and suppressed late-inspiratory depolarization as well as early-inspiratory IPSPs, resulting in a small constant depolarization throughout the apneusis. NPBM or vagal stimulation after dizocilpine produced small, stimulus-locked excitatory postsynaptic potentials (EPSPs) in late-I neurons. Neurobiotin-labeled late-I neurons revealed immunoreactivity for glutamic acid decarboxylase as well as N-methyl-D-aspartate (NMDA) receptors. These results suggest that late-I neurons are GABAergic inhibitory neurons, while the effects of bicuculline and dizocilpine indicate that they receive periodic waves of GABAergic IPSPs and glutamatergic EPSPs. The data lead to the conclusion that late-I neurons play an important inhibitory role in IOS. NMDA receptors are assumed to augment and/or synchronize late-inspiratory depolarization and discharge of late-I neurons, leading to GABA release and consequently off-switching of bulbar inspiratory neurons and phrenic motoneurons.     

Synaptic effects of intercostal tendon organs on membrane potentials of medullary respiratory neurons

1. Stimulation of intercostal muscle tendon organs or their afferent fibers reduces medullary inspiratory neuron activity, decreases motor output to inspiratory muscles, and increases the activity of expiratory laryngeal motoneurons. The present study examines the synaptic mechanisms underlying these changes to obtain information about medullary neurons that participate in the afferent limb of this reflex pathway. 2. Membrane potentials of medullary respiratory neurons were recorded in decerebrate paralyzed cats. Postsynaptic potentials (PSPs) elicited in these neurons by intercostal nerve stimulation (INS) were compared before and after intracellular iontophoresis of chloride ions. After chloride injection, the normal hyperpolarization caused by inhibitory (I) PSPs is "reversed" to depolarization. 3. In inspiratory neurons, reversal of IPSPs by chloride injection also reversed hyperpolarization produced by INS when applied during any portion of the respiratory cycle. This observation suggests that increased chloride conductance of the postsynaptic membrane mediated the inhibition. Further, it is very likely that the last-order interneuron in the afferent pathway must be excited by INS and alter inspiratory neuron activity via an inhibitory synapse. The linear relationship between the amplitude of the INS induced PSP and membrane potential of inspiratory neurons provided evidence that neurons in the afferent pathway are not respiratory modulated. 4. The membranes of expiratory vagal motoneurons and post-inspiratory neurons were depolarized by INS during all portions of the respiratory cycle before IPSP reversal. Reversal of IPSPs affected neither this depolarization of expiratory vagal motoneurons during stage I and II expiration nor that of post-inspiratory neurons during stage I expiration. Thus this depolarization probably resulted from synaptic excitation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of the ventrolateral region of the nucleus of the tractus solitarius in processing respiratory afferent input from vagus and superior laryngeal nerves

Summary The role of respiratory neurons located within and adjacent to the region of the ventrolateral nucleus of the tractus solitarius (vlNTS) in processing respiratory related afferent input from the vagus and superior laryngeal nerves was examined. Responses in phrenic neural discharge to electrical stimulation of the cervical vagus or superior laryngeal nerve afferents were determined before and after lesioning the vlNTS region. Studies were conducted on anesthetized, vagotomized, paralyzed and artificially ventilated cats. Arrays of 2 to 4 tungsten microelectrodes were used to record neuronal activity and for lesioning. Constant current lesions were made in the vlNTS region where respiratory neuronal discharges were recorded. The region of the vlNTS was probed with the microelectrodes and lesions made until no further respiratory related neuronal discharge could be recorded. The size and placement of lesions was determined in subsequent microscopic examination of 50 m thick sections. Prior to making lesions, electrical stimulation of the superior laryngeal nerve (4–100 A, 10 Hz, 0.1 ms pulse duration) elicited a short latency increase in discharge of phrenic motoneurons, primarily contralateral to the stimulated nerve. This was followed by a bilateral decrease in phrenic nerve discharge and, at higher currents, a longer latency increase in discharge. Stimulation of the vagus nerve at intensities chosen to selectively activate pulmonary stretch receptor afferent fibers produced a stimulus (current) dependent shortening of inspiratory duration. Responses were compared between measurements made immediately before and immediately after each lesion so that changes in response efficacy due to lesions per se could be distinguished from other factors, such as slight changes in the level of anesthesia over the several hours necessary in some cases to complete the lesions. Neither uni- nor bi-lateral lesions altered the efficacy with which stimulation of the vagus nerve shortened inspiratory duration. The short latency excitation of the phrenic motoneurons due to stimulation of the superior laryngeal nerve was severely attenuated by unilateral lesions of the vlNTS region ipsilateral to the stimulated nerve. Neither the bilateral inhibition nor the longer latency excitation due to superior laryngeal nerve stimulation was reduced by uni- or bi-lateral lesions of the vlNTS region. These results demonstrate that extensive destruction of the region of the vlNTS: a) does not markedly affect the inspiratory terminating reflex associated with electrical stimulation of the vagus nerve in a current range selective for activation of pulmonary stretch receptor afferents, and b) abolishes the short-latency increase, but not the bilateral decrease or longer latency increase in phrenic motoneuronal discharge which follows stimulation of the superior laryngeal nerve. We conclude that respiratory neurons in the region of the vlNTS do not play an obligatory role in the respiratory phase transitions in this experimental preparation. Neurons in the vlNTS region may participate in other reflexes, such as the generation of augmented phrenic motoneuronal discharge in response to activation of certain superior laryngeal or vagus nerve afferents.     

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

Summary 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 presynaptic 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 offswitch 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, postinspiratory and late-expiratory neurons. In apneusis there was a decoupling of the duration of inspiration and expiration. The variability of inspiratory duration increased five-fold whereas the variability of expiration was unchanged. We conclude that in the apneustic state, (1) inspiratory on-switching and the successive activation of the different inspiratory neuronal types are preserved; (2) near the end of the inspiratory ramp, the reversible phase of inspiratory off-switching is prolonged, producing the apneustic plateau, and (3) the irreversible phase of offswitching is impaired by a reduced activity of postinspiratory neurons. These results support the 3-phase model of respiratory rhythm generation, in which key roles are played by early-inspiratory and post-inspiratory neurons.     

NMDA and non-NMDA receptors may play distinct roles in timing mechanisms and transmission in the feline respiratory network.

1. Activation of N-methyl-D-aspartate (NMDA) glutamate receptors in the brainstem network of respiratory neurones is required to terminate inspiration in the absence of lung afferents, but it is not required in the inspiratory motor act of lung inflation. In the present study we examined the involvement of non-NMDA ionotropic glutamate receptors in these two mechanisms in the adult mammal. 2. Adult cats were either decerebrated or anaesthetized with sodium pentobarbitone, paralysed and ventilated. Inspiratory motor output was recorded from the phrenic nerve and central respiratory activity from neurones in the bulbar ventral respiratory group. 3. In decerebrate vagotomized cats, ionophoretic application of 2,3-dihydroxy-6-nitro-7-sulphamoylbenzo(F)quinoxaline (NBQX) onto single respiratory neurones decreased their spontaneous discharge rate and abolished the excitatory effect of exogenously applied (RS) alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) but not NMDA. 4. In these animals, intravenous infusion (12 mg kg-1) of the non-NMDA receptor blockers GYKI 52466 (1-(4-aminophenyl)-4-methyl-7,8-methylene-dioxy-5-H-2,3-benzodi aze pine) or NBQX: (1) decreased (in 10/15 cats) or abolished (in 5/15 cats) the inspiratory-related discharge of the phrenic nerve; (2) did not prolong the inspiratory phase; (3) reduced or abolished the spontaneous discharge of respiratory neurones; and (4) profoundly decreased the excitatory effects of AMPA but not NMDA ionophoresed onto these neurones. When both the phrenic nerve and the recorded respiratory neurone were silenced, neuronal excitation by ionophoretic application of NMDA first revealed a subthreshold respiratory modulation without lengthening of the inspiratory phase, then respiratory modulation became undetectable. 5. Additional blockade of NMDA receptors by a small dose (0.15 mg kg-1) of dizocilpine (MK-801), abolished the phrenic nerve activity which persisted after NBQX (apnoea), but the discharge or the subthreshold modulation of the bulbar respiratory neurones showed a lengthening of the inspiratory phase (apneusis). 6. Elevation of FA,CO2 increased or re-established phrenic nerve discharges after blockade of non-NMDA receptors or of both NMDA and non-NMDA receptors. 7. Small doses of NBQX or GYKI 52466 induced apnoea in five of five cats anaesthetized with sodium pentobarbitone. 8. In decerebrate animals with intact vagi, GYKI 52466 and NBQX depressed the Hering-Breuer expiratory-lengthening reflex. 9. The results suggest that: (1) there is a specialization of different classes of glutamate receptors participating in timing mechanisms and transmission within the mammalian respiratory network. Neural transmission predominantly involves activation of non-NMDA receptors, acting in synergy with NMDA receptors.(ABSTRACT TRUNCATED AT 400 WORDS)     

Inputs to intercostal motoneurons from ventrolateral medullary respiratory neurons in the cat

The investigation examined the synaptic input from medullary respiratory neurons in the nucleus retroambigualis (NRA) to external (EIM) and internal (IIM) intercostal motoneurons. Antidromic mapping revealed that 112/117 (96%) tested NRA units had axons descending into thoracic spinal cord with extensive arborizations at many thoracic segments, mainly contralaterally. The conduction velocities ranged from 10 to 105 m X s-1. The descending projections did not appear to be somatotopically arranged. Cross-correlation of the spike trains of NRA inspiratory units with the discharge of external intercostal nerves (performed usually with 4 contralateral nerves) showed significant narrow peaks only in 5 out of 40 averages. Of the 25 trigger units tested for the thoracic projection in this series of experiments, 24 were antidromically activated. Intracellular recordings were made from 52 IIMs [mean membrane potential 65.3 mV, central respiratory drive potentials (CRDPs) greater than 1 mV present in 23/52] and 53 EIM (mean membrane potential 54.3 mV, CRDPs in 31/53). During the depolarizing phase of the CRDPs, synaptic noise with frequent and apparently unitary EPSPs with amplitudes in excess of 1 mV was observed. Spike-triggered averages of synaptic noise were computed for 153 pairings between 137 NRA neurons and 105 contralateral intercostal motoneurons. Only four PSPs were revealed: two monosynaptic EPSPs between expiratory NRA units and IIMs and two probably disynaptic EPSPs between inspiratory NRA units and EIMs. When advancing the microelectrode down to the motoneuron pools, frequent recordings were made from interneurons with spontaneous respiratory discharge (inspiratory or expiratory) located dorsal and medial to the motor nuclei. The interneurons could be excited following stimulation of segmental afferents. It is concluded that monosynaptic connections between respiratory NRA neurons and intercostal motoneurons are rare (connectivity no more than approximately 4%). Segmental interneurons, interposed between the majority of descending respiratory axons and intercostal motoneurons, are likely to produce large unitary EPSPs and, thus, short-term synchronization in the discharge of intercostal motoneurons as observed by others.     

Dopamine1 receptor agonists reverse opioid respiratory network depression, increase CO2 reactivity

In adult pentobarbital-anesthetized and unanesthetized decerebrate cats, the D(1)R agonists (6-chloro-APB, SKF-38393, dihydrexidine) given intravenously restored phrenic nerve and vagus nerve respiratory discharges and firing of bulbar post-inspiratory neurons after the discharges were abolished by the micro-opioid receptor agonist fentanyl given intravenously. Reversal of opioid-mediated discharge depression was prevented by the D(1)R antagonist SCH23390. Iontophoresis of the micro-opioid receptor agonist DAMGO depressed firing of medullary bulbospinal inspiratory neurons. Co-iontophoresis of SKF-38393 did not restore firing and had no effect on bulbospinal inspiratory neuron discharges when applied alone. The D(1)R agonists given intravenously prolonged and intensified phrenic nerve and bulbospinal inspiratory neuron discharges. They also increased reactivity to CO(2) by lowering the phrenic nerve apnea threshold and shifting the phrenic nerve-CO(2) response curve to lower et(CO(2)) levels. Intravenous fentanyl on the other hand decreased CO(2) reactivity by shifting the phrenic nerve apnea threshold and the response curve to higher et(CO(2)) levels. Fentanyl effects on reactivity were partially reversed by D(1)R agonists.     

Connectivity of slowly adapting pulmonary stretch receptors with dorsal medullary respiratory neurons

1. Intracellular recordings were made from 50 dorsal respiratory group (DRG) neurons in the region of the ventrolateral nucleus of the solitary tract in anesthetized, paralyzed cats ventilated with a cycle-triggered pump whose inflation stroke was triggered by the onset of phrenic nerve inspiratory (I) discharge. Activity was recorded simultaneously in the ipsilateral nodose ganglion from sensory cell bodies of slowly adapting pulmonary stretch receptors (PSRs). 2. Respiratory cycle-related membrane potential changes of DRG neurons were recorded. Twenty-six neurons that did not exhibit spikes were classified as I alpha, I beta or pump (P)-cells by comparing their membrane potential trajectories during I in the presence of lung inflation with that observed during I, but with lung inflation withheld. The remaining 24 neurons were classified similarly, but the classification was based upon a comparison of their I-phase spike activity responses with and without lung inflation. I phase-related histograms of either membrane potential or spike activity were constructed to facilitate DRG neuronal classification. Additionally, steady lung inflation of varying magnitudes was applied during the expiratory phase. This prolonged expiration and produced different responses in the neurons. Generally, I beta and P-cells were depolarized, whereas I alpha cells were hyperpolarized. 3. Low-intensity electrical stimulation of the ipsilateral vagus nerve evoked excitatory postsynaptic potentials (EPSPs) in all three DRG neuronal types. P-cells and I beta cells exhibited EPSPs in response to the lowest intensity; generally this intensity was below threshold for the simultaneously recorded PSR. Overall, EPSPs in I alpha cells had the highest thresholds, but some EPSPs could be evoked at thresholds similar to those of the I beta cells. The distributions of the average onset latency of the evoked EPSP overlapped considerably. Thus vagal electrical stimulation cannot be used for unequivocal classification of DRG neurons into I alpha, I beta, and P-cell subpopulations. 4. Using intracellular spike-triggered averaging, single PSRs were shown to generate monosynaptic EPSPs in I beta neurons and P-cells but not I alpha cells. Divergence of single PSR afferents also was observed. Relationships between EPSP shape factors, amplitudes, and PSR afferent conduction velocity are similar to those previously observed for monosynaptic EPSPs in hindlimb motoneurons generated by spinal afferents.     

Effects of stimulation of phrenic afferent fibers on medullary respiratory neurons in cat

The effects of electrical stimulation of both cervical branches (C5 and C6) of the right phrenic nerve on medullary respiratory neuron activity were studied in anesthetized, spontaneously breathing cats. In 14 cats, the stimulation of the thin phrenic afferents had no effect on the inspiratory duration and evoked excitatory or inhibitory responses in only 3/86 inspiratory neurons tested. In 3 cats, the stimulation decreased the inspiratory duration and 26/26 inspiratory neurons showed a shortened discharge without modification of their discharge frequency. Although the effects of the stimulation were not analysed by averaging techniques, it is concluded that phrenic afferents do not exert an important control on the medullary respiratory neuron discharge.     

Intracellular recordings from different types of medullary respiratory neurons of the cat.

Respiratory neurons were recorded intracellularly within the lateral region of the lower brain stem of vagotomized and artificially ventilated cats. Bulbospinal, vagal, and antidromically nonresponsive types of neurons were distinguished by means of vagal and intraspinal stimulation. Almost all types of neurons discharged a burst of action potentials during one of the two phases of the central respiratory cycle, as indicated by phrenic nerve activity. The discharge pattern of the different types of neurons were described. The origin of the spntaneous changes of the membrane potential was investigated by measurements of the reversal potentials and membrane conductance changes. The results reveal that both inspiratory and expiratory types of neurons receive an excitatory input during their discharge period, and a reciprocal inhibitory input during their silent period. In addition, one type of neuron was described which receives inhibitory inputs during both inspiration and expiration. Recurrent inhibition, as indicated by hyperpolarizing postsynaptic potentials and membrane conductance changes following the antidromic action potential seems to exist only within the network of the vagal neurons. Suggestions are made about the functional organization of the neuronal network of the medullary respiratory system and the mechanism generating its rhythmic activity.     

Monosynaptic excitation of medullary inspiratory neurons by bulbospinal inspiratory neurons of the ventral respiratory group in the cat

Summary In Nembutal-anesthetized, immobilized, and artificially ventilated cats, we studied the connectivity of medullary collaterals of bulbospinal inspiratory (BS-I) neurons in the ventral respiratory group (VRG). BS-I neurons which projected to the contralateral spinal cord were isolated extracellularly and intracellular recordings were made from the respiratory neurons in the contralateral VRG. The intracellular membrane potentials were averaged using extracellular spikes of the BS-I neurons as triggers (spike-triggered averaging method). Fast-rising and short-lasting depolarizing potentials locked to the triggering spikes were obtained and shown to be unitary EPSPs induced monosynaptically by the medullary collaterals of BS-I neurons. A total of 137 pairs were analyzed and unitary EPSPs were found in 11 pairs. Four BS-I neurons and 7 inspiratory vagal motoneurons received EPSPs from the medullary collaterals of BS-I neurons. These findings suggest that 1) BS-I neurons in the VRG drive medullary motoneurons of accessory respiratory muscles and phrenic or intercostal motoneurons simultaneously, 2) BS-I neurons on both sides synchronize via the excitatory connections, and 3) the augmenting firing pattern of BS-I neurons might partly be produced by this reexcitatory connection within the population of BS-I neurons.     

Pneumotaxic mechanisms in the non-human primate: effect of the N-methyl-D-aspartate (NMDA) antagonist ketamine

We tested the possible involvement of N-methyl-D-aspartate (NMDA) receptors in the central inspiratory-termination mechanism in non-human primates. Inspiratory bursts were recorded from the phrenic nerve in Macaca fascicularis monkeys paralyzed and ventilated by means of a servoventilator driven by the inspiratory discharge of the phrenic nerve. The central inspiratory termination mechanism was tested by withholding lung inflation. This transiently suppressed the vagal feedback from the lungs which produces inspiratory off-switching independent from the central mechanism. Under anaesthesia with ketamine, a potent NMDA antagonist, non inflation increased inspiratory time to 4s (1s with lungs inflated) whereas no such effect was observed during halothane anaesthesia. We conclude that the termination of inspiration in primates is controlled via central mechanisms in which NMDA receptors are involved.     

Differential synaptic effects on physiological flexor hindlimb motoneurons from cutaneous nerve inputs in spinal cat.

1. We previously demonstrated in the spinal cat that superficial peroneal cutaneous nerve stimulation produced strong reflex contraction in tibialis anterior (TA) and semitendinosus (St) muscles but unexpectedly produced mixed effects in another physiological flexor muscle, extensor digitorum longus (EDL). The goal of the present study was to further characterize the organization of ipsilateral cutaneous reflexes by examining the postsynaptic potentials (PSPs) produced in St, TA, and EDL motoneurons by superficial peroneal and saphenous nerve stimulation in decerebrate, spinal cats. 2. In TA and St motoneurons, low-intensity cutaneous nerve stimulation that activated only large (A alpha) fibers [i.e., approximately 2-3 times threshold (T)], typically produced biphasic PSPs consisting of an initial excitatory phase and subsequent inhibitory phase (EPSP, IPSP). Increasing the stimulus intensity to activate both large (A alpha) and small (A delta) myelinated cutaneous fibers supramaximally (15-45 T) tended to enhance later excitatory components in TA and St motoneurons. 3. In EDL motoneurons, 2-3 T stimulation of the superficial peroneal nerve evoked initial inhibition (of variable magnitude) in 7/10 EDL motoneurons tested, with either excitation (n = 2) or mixed effects (n = 1) observed in the remaining EDL motoneurons. Saphenous nerve stimuli produced excitation either alone, or preceded by an inhibitory phase in EDL. Increasing the stimulus intensity enhanced later inhibitory influences from superficial peroneal and excitatory influences both from superficial peroneal and saphenous nerve inputs in EDL motoneurons. 4. Short-latency (less than 1.8 ms) EPSPs were observed in a few motoneurons in all reflex pathways examined, except for EPSPs in EDL motoneurons evoked by saphenous stimulation. IPSPs with central latencies less than 1.8 ms were also produced by both saphenous (TA, n = 1; EDL, n = 2) and superficial peroneal (EDL, n = 4) nerve stimulation. 5. The results, in comparison with other reports employing spinal and nonspinal preparations, suggest that removal of influences from higher centers reveals inhibitory circuits from the superficial peroneal and saphenous nerves to EDL motoneurons in the spinal preparation. The inhibitory inputs observed are thought to reflect the activation of "specialized" reflex pathways. Additionally, the demonstration of short-latency EPSPs and IPSPs suggest that the minimal linkage in both the excitatory and inhibitory cutaneous reflex pathways examined is disynaptic. The results are discussed in relation to previous studies on classically conditioned flexion reflex facilitation in spinal cat.     

Behavior of upper cervical inspiratory propriospinal neurons during fictive vomiting

1. The role of upper cervical inspiratory (UCI)-modulated neurons in respiratory muscle control during vomiting was examined by recording the impulse activity of these neurons during fictive vomiting in decerebrate, paralyzed cats. Fictive vomiting was identified by a characteristic series of bursts of coactivation of phrenic and abdominal muscle nerves, elicited either by electrical stimulation of supradiaphragmatic vagal nerve afferents or by emetic drugs, which would be expected to produce expulsion of gastric contents in nonparalyzed animals. 2. Data were recorded from 43 propriospinal UCI neurons, located in the C1-C3 spinal segments near the border of the intermediate gray matter and lateral funiculus, which were antidromically activated with floating pin electrodes placed in the ipsilateral lateral funiculus, usually at T1-T3. Some cells (9/21 tested) were also activated from the upper lumbar cord (L1). During respiration, most neurons (n = 40) had an augmenting discharge pattern during inspiration. In addition, more than one-half (55%) fired tonically during the remainder of the respiratory cycle. About 40% of UCI neurons showed variations in their firing pattern during the noninspiratory portion of respiration. These latter two properties of UCI neurons were not observed in dorsal and ventral respiratory group (DRG and VRG-, respectively) bulbospinal inspiratory (I) neurons previously recorded under similar conditions. 3. During fictive vomiting, the firing pattern of most UCI neurons fell into one of three main categories. More than one-half (53%) were active in phase with bursts of phrenic discharge and were thus classified as Active-type cells.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of large excitatory and inhibitory inputs on motoneuron discharge rate and probability.

We elicited repetitive discharge in hypoglossal motoneurons recorded in slices of rat brain stem using a combination of a suprathreshold injected current step with superimposed noise to mimic the synaptic drive likely to occur during physiological activation. The effects of repetitive en mass stimulation of afferent nerves were simulated by the further addition of trains of injected current transients of varying shapes and sizes. The effects of a given current transient on motoneuron discharge timing and discharge rate were measured by calculating a peristimulus time histogram (PSTH) and a peristimulus frequencygram (PSF). The amplitude and time course of the simulated postsynaptic potentials (PSPs) produced by the current transients were calculated by convolving the current transient with an estimate of the passive impulse response of the motoneuron. We then compared the shape of the injected current transient and the simulated PSP to the profiles of the PSTH and the PSF records. The PSTHs produced by excitatory PSPs (EPSPs) were characterized by a large, short-latency increase in firing probability that lasted slightly longer than the rising phase of the EPSP, followed by a reduced discharge probability during the falling phase of the EPSP. In contrast, the PSF analysis revealed a proportionate increase in discharge rate over the entire profile of the EPSP, even though relatively few spikes occurred during the falling phase. The PSTHs associated with inhibitory PSPs (IPSPs) indicated a reduction in discharge probability during the initial, hyperpolarizing phase of the IPSP, followed by an increase in the discharge probability during its subsequent repolarizing phase. Using the PSF analysis, the initial phase of the IPSP appeared as a large hole in the record where a very small number or no discharges occurred. The subsequent phase of the IPSP was associated with frequency values that were lower than the background values. The primary features of both PSTHs and PSFs can be used to estimate the relative amplitudes of the underlying EPSPs and IPSPs. However, PSTHs contain secondary peaks and troughs that are not directly related to the underlying PSP but instead reflect the regular recurrence of spikes following those affected by the PSP. The PSF analysis is more useful for indicating the total duration and the profile of the underlying PSP. The shape of the underlying PSP can be obtained directly from the PSF records because the discharge frequency of the spikes follow the PSPs very closely, especially for EPSPs.     

Augmentation of muscle nociceptive respiratory reflex facilitation by vagal afferents

Vagal influence on the facilitation of phrenic neural activity during respiratory phase-locked, gastrocnemius muscle nerve nociceptive electrical stimulation was examined in anesthetized, glomectomized, paralyzed, and artificially ventilated cats. (1) In the vagi-intact state, respiratory reflex facilitation was characterized by a sharp rise in peak amplitude, maximum rate of rise or slope, and mean rate of rise of integrated phrenic nerve activity. This was greater during inspiratory phase-locked (T1-locked) muscle nerve electrical stimulation than during expiratory phase-locked (TE-locked) muscle nerve electrical stimulation. "Evoked post-inspiratory phrenic activity" during the early expiratory phase was also observed during TE-locked muscle nerve electrical stimulation. (2) Bilateral vagotomy significantly attenuated the respiratory facilitation during both T1- and TE-locked muscle nerve electrical stimulation. In particular, the "evoked post-inspiratory phrenic activity" during TE-locked muscle nerve electrical stimulation was also attenuated or almost completely abolished. (3) Conditioning electrical stimulation of the vagus nerve revealed facilitatory reflexes which co-exist with inspiratory inhibitory reflexes. (4) The "evoked post-inspiratory phrenic activity" during TE-locked muscle nerve electrical stimulation, which was attenuated or abolished after vagotomy, was restored after vagal T1-locked conditioning stimuli combined with TE-locked muscle nerve electrical stimulation. The results suggest that vagal facilitatory reflexes augment the respiratory reflex facilitation during muscle nociceptive stimulation.     

Diaphragmatic and external intercostal muscle control during vomiting: behavior of inspiratory bulbospinal neurons

1. The role of dorsal and ventral respiratory group (DRG and VRG) bulbospinal inspiratory (I) neurons in the control of diaphragmatic and external intercostal (inspiratory) muscle activity during vomiting was examined by recording from these neurons during fictive vomiting in decerebrate, paralyzed cats. Fictive vomiting was defined by a characteristic series of bursts of coactivation of phrenic and abdominal muscle nerves, elicited either by electrical stimulation of abdominal vagal afferents or by emetic drugs, which would be expected to produce vomiting if the animals were not paralyzed. 2. Data were recorded from 22 DRG and 29 VRG I neurons that were antidromically activated from the fourth cervical spinal segment (C4). Only 10% (5/51) of these neurons started to fire near the beginning of phrenic discharge during fictive vomiting and thus had the appropriate discharge pattern to contribute to the initial activation of the diaphragm and coactive external intercostal muscles during vomiting. The frequency of occurrence of these Active neurons was not significantly different in the DRG (3/22) and VRG (2/29) (chi 2 test). Most remaining neurons were either totally silent (n = 7) or had only sporadic, infrequent firing (n = 16) (Silent neurons, 23/51 = 45%), or else fired near the end of phrenic discharge during fictive vomiting (End neurons, 21/51 = 41%). Two neurons were categorized as having miscellaneous (Misc) behavior. 3. No differences were found among neurons having different response patterns during fictive vomiting in regard to the following: the manner in which fictive vomiting was elicited: cell location: conduction velocity; and neuronal firing onset, rate, and pattern during respiration.(ABSTRACT TRUNCATED AT 250 WORDS)     

Whole cell recordings of intrinsic properties and sound-evoked responses from the inferior colliculus

Response features of inferior colliculus (IC) neurons to both current injections and tone bursts were studied with in vivo whole cell recordings in awake Mexican free-tailed bats. Of 160 cells recorded, 95% displayed one of three general types of discharge patterns in response to the injection of positive current: 1) sustained discharges; 2) adapting discharges; and 3) onset-bursting discharges. Sustained neurons were the most common type (N=78), followed by onset-bursting (N=57). The least common type was adapting (N=17). In 90 neurons the profiles of synaptic and discharge activity evoked by tones of different frequencies at 50 dB SPL were recorded. Three major tone-evoked response profiles were obtained; 1) neurons dominated by excitation (N=32) in which tones evoked excitatory post-synaptic potentials (EPSPs) or EPSPs with discharges over a range of frequencies with little or no evidence of inhibitory post-synaptic potentials (IPSPs) evoked by frequencies that flanked the excitation; 2) neurons that had an excitatory frequency region in which discharges were evoked that was flanked by frequencies that evoked predominantly IPSPs (N=26); 3) neurons in which all frequencies evoked IPSPs with little or no depolarizations (N=32). The question we asked is whether IC cells that express a particular profile of PSPs and discharges to acoustic stimulation also have the same current-evoked response profile. We show that, with one exception, the intrinsic features of an IC neuron are not correlated with the pattern of its synaptic innervation; the two features are unrelated in the majority of IC cells. The exception is a subtype of inhibitory dominated cell where most frequencies evoked IPSPs to both the onset and to the offset of the tone bursts. In those cells injected current steps always evoked an onset-bursting response.     

The role of dorsal respiratory group neurons studied with cross-correlation in the decerebrate rat

We examined the role of dorsal respiratory group (DRG) inspiratory neurons as transmitters of respiratory drive to phrenic and intercostal motoneurons and as relays of afferent information to ventral respiratory group (VRG) bulbospinal, inspiratory neurons. Attempts to antidromically activate 76 DRG neurons from the spinal cord at the C7 segment resulted in only 4 (5.3%) successes (3 contralateral, 1 ipsilateral). Cross-correlating DRG neuron discharge with that of the ipsilateral (56) and contralateral (20) phrenic nerve detected common activation peaks in 2 and 3 cases respectively, with no evidence for monosynaptic connections. Cross-correlating DRG neuron discharge with that of bulbospinal, inspiratory VRG neurons found some evidence for interaction. Peaks in 7 of 73 (10%) cross-correlation histograms were attributed to a monosynaptic excitation of DRG neurons by VRG neurons, although a common activation cannot be ruled out; troughs, some with an accompanying peak, in 9 (12.3%) histograms were interpreted as a combined excitation of the DRG neuron and delayed inhibition of the VRG neuron. In addition, 2 cross-correlation histograms showed peaks with latencies and half-amplitude widths consistent with a disynaptic excitation of a DRG neuron by a bulbospinal inspiratory VRG neuron. Cross-correlating the discharge of 57 pairs of DRG inspiratory neurons (6 contralateral) detected common activation peaks in 7 (12.3%) cases (none contralateral) and one case interpreted as evidence for a disynaptic excitation. These findings suggest that the role of the DRG inspiratory neurons in rats differs from that in cats, primarily because they do not act to transmit respiratory rhythmic drive directly to phrenic and intercostal motoneurons. The results offer some support for an excitation of DRG neurons by VRG inspiratory neurons, but no support for a role of DRG inspiratory neurons as mediators of afferent information transfer to VRG bulbospinal inspiratory neurons.