Accommodative reactions of medullary respiratory neurons of the cat.

In most of the bulbospinal respiratory neurons, threshold depolarization increased during the early period of their spontaneous burst discharge but decreased again at the end of a burst. In some vagal respiratory neurons, however, threshold depolarization increased steadily until the very end of their discharge period. These changes generally were accompanied by changes in the rate of depol1rization of the spikes, the amplitude of their overshoot, and their discharge frequency. For a given synaptic input, as indicated by the constancy of the interspike membrane potential trajectories, threshold depolarization of bulbospinal neurons remained constant or even decreased. Only in some vagal neurons was an increase in threshold deplarization observed under these conditions. With the exception of some vagal neurons, most of the respiratory neurons did not show a pronounced accommodative behavior when stimulated with linear rising currents. When stimulating with current pulses, all neurons discharged repetitively with only slight adaptation, which was already complete by the first few spike intervals. The current-frequency relationship was linear and revealed a primary and secondary range. The results support neither accommodation nor adaptation as important mechanisms in the genesis of the rhythmic activity of respiratory neurons.     

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 for the subthreshold currents ILeak and IH in the homeostatic control of excitability in neocortical somatostatin-positive inhibitory neurons

Cortical circuitry reconfigures in response to chronic (1-3 days) changes in activity levels. To understand this process, we must know the role played by inhibitory neurons because they crucially influence network properties by controlling action potential generation and synaptic integration. Using pharmacological blockade of activity in neocortical organotypic slice cultures, we examined the activity-dependent regulation of membrane excitability in a specific inhibitory neuron subtype: the somatostatin-positive (SOM+) neuron. Chronic action potential blockade (TTX, 2.5 days) resulted in increased excitability in SOM+ neurons. This result is consistent with a homeostatic process to maintain the average firing rate of SOM+ neurons at a particular level. Excitability changes were not ascribed to changing cell size or alterations in voltage-dependent sodium current. Instead, the excitability increase was largely the result of a decrease in the density of two subthreshold currents: a passive leak current (ILeak) and H-current (IH). The downregulation of these currents increased excitability mostly through a decrease in membrane input conductance. The coadaptation of ILeak and IH enabled a change in input conductance while helping to preserve membrane potential. Evidence indicated that ILeak was probably mainly mediated by K+. At earlier culture ages, this adaptation was superimposed on developmental changes, whereas at older ages, the same types of induced alterations occurred but with no developmental component. Together with other studies, these data indicate that both inhibitory and excitatory neurons increase membrane excitability with chronic reduction in activity, but through different mechanisms.     

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.     

Enhancement of visual responsiveness by spontaneous local network activity in vivo

Spontaneous activity within local circuits affects the integrative properties of neurons and networks. We have previously shown that neocortical network activity exhibits a balance between excitatory and inhibitory synaptic potentials, and such activity has significant effects on synaptic transmission, action potential generation, and spike timing. However, whether such activity facilitates or reduces sensory responses has yet to be clearly determined. We examined this hypothesis in the primary visual cortex in vivo during slow oscillations in ketamine-xylazine anesthetized cats. We measured network activity (Up states) with extracellular recording, while simultaneously recording postsynaptic potentials (PSPs) and action potentials in nearby cells. Stimulating the receptive field revealed that spiking responses of both simple and complex cells were significantly enhanced (>2-fold) during network activity, as were spiking responses to intracellular injection of varying amplitude artificial conductance stimuli. Visually evoked PSPs were not significantly different in amplitude during network activity or quiescence; instead, spontaneous depolarization caused by network activity brought these evoked PSPs closer to firing threshold. Further examination revealed that visual responsiveness was gradually enhanced by progressive membrane potential depolarization. These spontaneous depolarizations enhanced responsiveness to stimuli of varying contrasts, resulting in an upward (multiplicative) scaling of the contrast response function. Our results suggest that small increases in ongoing balanced network activity that result in depolarization may provide a rapid and generalized mechanism to control the responsiveness (gain) of cortical neurons, such as occurs during shifts in spatial attention.     

Anticonvulsant A(1) receptor-mediated adenosine action on neuronal networks in the brainstem-spinal cord of newborn rats

Membrane potential of ventral respiratory group neurons as well as inspiratory-related cranial (hypoglossal) and spinal (C(1)-Th(4)) nerve activities were analysed in brainstem-spinal cord preparations from neonatal rats. Block of Cl(-)-mediated inhibition with bicuculline (plus strychnine) affected neither rhythmic depolarizations nor spike discharge in 23 of 30 ventral respiratory group cells. In the other seven neurons, block of inhibitory postsynaptic potentials evoked pronounced depolarizations and spike discharge that was synchronous with seizure-like spinal nerve activity. Respiratory hypoglossal nerve activity persisted after transection at the spinomedullary junction, whereas spinal rhythm was blocked. After transection, the moderate bicuculline-evoked seizure-like perturbation of hypoglossal nerve activity was abolished and rhythmic ventral respiratory group neuron activity was not disturbed, whereas epileptiform discharge persisted in spinal nerves. The seizure-like nerve activity and depolarization of the minor subpopulation of perturbed ventral respiratory group neurons were reversed by either adenosine or the A(1) adenosine receptor agonist 2-chloro-N(6)-cyclopentyladenosine. The A(2) receptor agonist CGS 21860 had no effect. In control preparations, inspiratory nerve activity and membrane potential fluctuations (29 of 35 cells) were not changed by adenosine, 2-chloro-N(6)-cyclopentyladenosine or CGS 21860. In the other six cells, adenosine evoked a hyperpolarization (<10 mV) with no major change in input resistance. The anticonvulsant effects of adenosine and 2-chloro-N(6)-cyclopentyladenosine were antagonized by the A(1) adenosine receptor blocker 8-cyclopentyl-1,3-dipropylxanthine. After pre-incubation with 8-cyclopentyl-1,3-dipropylxanthine, bicuculline also evoked seizure-like discharge in the hypoglossal nerve.The results indicate that seizure-like spinal motor output of the respiratory network upon block of Cl(-)-mediated inhibition is caused by disinhibition of spinal neuronal networks with afferent connections to the ventral respiratory group. Presynaptic A(1) adenosine receptors exert an anticonvulsant action on the disinhibited spinal motor network, but have no depressing effect per se on the isolated medullary respiratory network.     

Chemical communication between vagal afferent somata in nodose Ganglia of the rat and the Guinea pig in vitro

The cell bodies of spinal afferents, dorsal root ganglion neurons, are depolarized several millivolts, and their probability of spiking increased when axons of neighboring somata in the same ganglion are electrically stimulated repetitively. This form of neural communication has been designated cross-depolarization (CD) and cross-excitation (CE). The existence of CD and CE between somata of vagal afferents (nodose ganglion neurons, NGNs) of rats and guinea pigs was investigated by electrically stimulating the vagus nerve while recording the electrical activity of NGNs in intact nodose ganglia with sharp intracellular microelectrodes. CD and CE in NGNs were manifested by a membrane depolarization (approximately 4 mV), the presence of spontaneous action potentials, and a decreased spike threshold. CD was dependent on the frequency and intensity of vagal nerve stimulation. Two distinct types of CD were observed: 1) in NGNs with large input resistances (R(in)), CD was dependent on [Ca2+]o, associated with increased membrane conductance, and had an extrapolated reversal potential (E(rev)) value of about -25 mV; and 2) in NGNs with low R(in), CD was independent of [Ca2+]o, not accompanied by a membrane conductance change, or a measurable E(rev) value. These data reveal the existence of a chemical communication pathway between vagal afferent somata and suggest the possibility that communication between different visceral organs may occur at the level of the primary vagal afferent neuron.     

Electrophysiological properties of neurons in the lateral habenula nucleus: an in vitro study

1. The electroresponsive characteristics of neurons in the lateral habenula were studied with intracellular recordings in a brain slice preparation of guinea pig diencephalon maintained in vitro. One hundred and two neurons met the criteria for recording stability, and of these, 18 were analyzed in detail. For these 18 neurons, the mean resting membrane potential was -61.9 mV, the mean input resistance was 124 M omega, and the mean spike amplitude of fast action potentials was 60.3 mV. 2. Lateral habenula neurons were found to have distinct patterns of activity dependent on membrane potential. At membrane potentials more positive than -65 mV, depolarization elicited trains of sodium-dependent fast action potentials. At membrane potentials more negative than -65 mV, slight depolarization elicited a tetrodotoxin-insensitive wave of depolarization, called a low-threshold spike (LTS), from which a burst of fast action potentials were triggered. The principal conductance underlying the LTS is a low-threshold calcium conductance, which is inactivated at membrane potential more positive than -65 mV and deinactivated when the membrane is hyperpolarized to potentials more negative than -65 V. 3. Upon termination of injected hyperpolarizing current, many neurons displayed oscillation in membrane potential at a frequency of 3-10 Hz, thereby generating repetitive bursts of fast spikes. 4. The pattern of neuronal activity in lateral habenula neurons was highly sensitive to slight alterations in membrane potential. The ability of these neurons to fire action potentials in two modes, tonically and in bursts, and the propensity of these neurons to dramatically alter their output in response to transient hyperpolarizing input, indicate that transmission through this relay in the dorsal diencephalic conduction system may be greatly augmented by relatively small hyperpolarizing influences on the individual neurons.     

Inhibitory processes in development of seizure activity in hippocampal slices

Summary The possibility that changes in inhibitory processes are necessary for development of seizure activity was examined using guinea pig hippocampal slices. After repeated tetanic stimulation to the stratum radiatum in region CA3, seizure discharges were evoked in the stratum pyramidale by test stimuli. The latency of the seizure discharges was shortened and the duration was prolonged progressively with the number of tetanic stimulations. The latency ranged from 30 to 100 ms and it was decreased successively as the distance between the recording site and the site of tetanic stimulation was decreased. This suggests that the foci of the seizure discharges existed near the site of tetanic stimulation. In neurons within the foci, inhibitory postsynaptic potentials and the suppressing action of alveus stimulation on glutamate-induced single cell discharges remained unchanged during development of seizure activity, although excitatory postsynaptic potentials were potentiated. In addition, no marked changes were detected in the input resistance, resting membrane potential and the amplitude, threshold and afterhyperpolarization of action potentials. These results suggest that suppression of inhibitory processes is not necessary for tetanus-induced seizure activity.     

Ionic currents and firing patterns of mammalian vagal motoneurons in vitro

The electrophysiological properties of guinea-pig dorsal vagal motoneurons were studied in an in vitro slice preparation. Antidromic, orthodromic and direct stimulation of the neurons demonstrated that the action potential is comprised of several distinct components: a fast initial spike followed by afterdepolarization and an early and a late afterhyperpolarizations. The fast initial spike and the early afterhyperpolarization were blocked by tetrodotoxin and tetraethylammonium ions, respectively. The afterdepolarization (present on the falling phase of the spike) and the late afterhyperpolarization were blocked by the addition of ions known to block calcium conductance (CdCl2, CoCl2 or MnCl2), indicating close association between these two potentials. Prolonged outward current injection through the recording electrode produced two different firing patterns, depending on the initial level of the membrane potential. From resting potential (usually -60 mV) the firing pattern was characterized by a short train of action potentials appearing shortly after the onset of the depolarization step. By contrast, when the depolarization was delivered from a hyperpolarized membrane potential level, a short train of repetitive firing appeared after an initial delay of 300-400 ms. The membrane current responsible for this initial reduction in excitability was studied by means of a single-electrode voltage-clamp technique. The magnitude, direction and kinetics of such current flow are consistent with the presence of early potassium current (IA), partly inactive at the resting potential. Synaptic activation of vagal motoneurons could be obtained by electrical stimulation of the tissue surrounding the vagal nucleus or by direct activation of the vagal nerve. Perivagal stimulation generated excitatory and inhibitory synaptic potentials which could be reversed by shifting the membrane potential. Vagal nerve stimulation, in addition to the antidromic activation of the cells, generated depolarizing responses which were unitary in nature and did not show much sensitivity to shifts in membrane potential. Perivagal and vagal nerve-evoked depolarizations could generate action potentials as well as partial dendritic spikes. We conclude that spike electroresponsiveness in vagal motoneurons is generated by voltage-dependent Na+ and Ca2+ conductances. In addition, the Ca2+-dependent current triggers a K+ conductance which is responsible for modulating the firing frequency obtained from the normal resting level.(ABSTRACT TRUNCATED AT 400 WORDS)     

Serotonin attenuates a slow inhibitory postsynaptic potential in rat hippocampal neurons

Activity of hippocampal neurons was recorded in an in vitro slice preparation. Topical application of serotonin produced hyperpolarization, blockade of a slow afterhyperpolarization which follows a burst discharge and blockade of a slow inhibitory postsynaptic potential. The slow inhibitory postsynaptic potential evoked by stimulation of the apical dendritic region of the hippocampus is more sensitive to serotonin than the membrane potential or conductance. The effects of serotonin on the inhibitory postsynaptic potentials are blocked by the 5-HT1a antagonist spiperone, and not by mianserin, a 5-HT2 antagonist. The attenuation of the inhibitory postsynaptic potentials is not accompanied by a change in postsynaptic reactivity to GABA or baclofen. Serotonin blocks repetitive large inhibitory postsynaptic potentials evoked in hippocampal neurons by topical application of 4-aminopyridine. Putative interneurons are more sensitive to topical application of serotonin than pyramidal neurons. Fenfluramine, a serotonin releaser mimics the effects of topical application of serotonin indicating that synaptically released serotonin can produce the changes in membrane potential and reactivity to afferent stimulation. It is suggested that serotonin attenuates slow inhibitory postsynaptic potentials by inhibiting feed forward inhibitory interneurons which impinge upon the recorded pyramidal neurons.     

Growth-inhibiting extracellular matrix proteins also inhibit electrical activity by reducing calcium and increasing potassium conductances

Inhibitionof neurite sprouting and electrical activity by extracellular matrix (ECM) glycoproteins was studied during neurite regeneration by using anterior pagoda (AP) neurons of the leech. Adult isolated neurons were plated in culture inside ganglion capsules, which among many ECM proteins, contain a group of inhibitory peanut lectin- (PNA) binding glycoproteins. These proteins inhibit neurite production and contribute to the formation of a bipolar outgrowth pattern by AP neurons. Addition of PNA lectin to the culture medium to block the inhibitory effects of ECM glycoproteins induced an increase of neurite sprouting, the loss of the bipolar pattern, and also an increase in the amplitude and duration of action potentials evoked by intracellular current injection. PNA lectin had independent effects on neurite sprouting and electrical activity, since there was no correlation between the total neurite length and the amplitude of the action potentials. Moreover, action potentials were increased by the presence of PNA lectin even in neurons that did not grow. The changes induced by PNA lectin on the active conductances underlying the action potentials were estimated by quantitative model simulations. We predict that the increases in the amplitude and duration of the action potential induced by PNA lectin were due to an increase in a calcium conductance and a reduction in the delayed rectifier potassium conductance. Our results suggest that inhibitory ECM glycoproteins may use independent signaling pathways to inhibit neurite sprouting and electrical activity. These proteins affect the action potential by changing the proportion of inward and outward active conductances.     

Suprachiasmatic nucleus communicates with anterior thalamic paraventricular nucleus neurons via rapid glutamatergic and gabaergic neurotransmission: state-dependent response patterns observed in vitro

The hypothalamic suprachiasmatic nucleus uniquely projects to the midline thalamic paraventricular nucleus. To characterize this projection, patch clamp techniques applied in acute rat brain slice preparations examined responses of anterior thalamic paraventricular nucleus neurons to focal suprachiasmatic nucleus stimulation. Whole cell recordings from slices obtained during daytime (n=40) revealed neurons with a mean membrane potential of -66+/-1.2 mV, input conductance of 1.5+/-0.1 nS and state-dependent tonic or burst firing patterns. Electrical stimulation (one or four pulses) in suprachiasmatic nucleus elicited monosynaptic excitatory postsynaptic potentials (mean latency of 12.6+/-0.6 ms; n=12), featuring both AMPA and N-methyl-D-aspartate-glutamate receptor-mediated components, and monosynaptic bicuculline-sensitive inhibitory postsynaptic potentials (mean latency of 16.6+/-0.6 ms; n=7) reversing polarity at -72+/-2.6 mV, close to the chloride equilibrium potential. Glutamate microstimulation of suprachiasmatic nucleus also elicited transient increases in spontaneous excitatory or inhibitory postsynaptic currents in anterior thalamic paraventricular neurons. Recordings from rats under reverse light/dark conditions (n=22) yielded essentially similar responses to electrical stimulation. At depolarized membrane potentials, suprachiasmatic nucleus-evoked excitatory postsynaptic potentials triggered single action potentials, while evoked inhibitory postsynaptic potentials elicited a silent period in ongoing tonic firing. By contrast, after manual adjustment of membrane potentials to hyperpolarized levels, neuronal response to the same "excitatory" stimulus was a low threshold spike and superimposed burst firing, while responses to "inhibitory" stimuli paradoxically elicited excitatory rebound low threshold spikes and burst firing. These data support the existence of glutamatergic and GABAergic efferents from the suprachiasmatic nucleus to its target neurons. Additionally, in thalamic paraventricular nucleus neurons, responses to activation of their suprachiasmatic afferents may vary in accordance with their membrane potential-dependent intrinsic properties, a characteristic typical of thalamocortical neurons.     

Orexin-induced modulation of state-dependent intrinsic properties in thalamic paraventricular nucleus neurons attenuates action potential patterning and frequency

The thalamic paraventricular nucleus (PVT) receives a dense innervation from orexin-synthesizing lateral hypothalamic neurons. Since PVT neurons display state-dependent tonic or low threshold spike-driven burst firing patterns, we examined how the response to exogenously applied orexins might modulate these features. Data were obtained with whole-cell patch clamp recording techniques in rat brain slices prepared during the subjective lights-on period. PVT neurons displayed a mean resting membrane potential of -61+/-6 mV and input conductance of 1.3+/-0.1 nS (n=60). The majority (90/107) of cells tested responded to orexin A and/or orexin B peptides (100-1000 nM), each inducing similar slowly rising and prolonged membrane depolarizations. We next evaluated associated changes in firing patterns and action potential frequency. Of 17 spontaneously silent neurons, 5 were induced into tonic firing and 4 into burst firing modes. Of nine spontaneously bursting neurons, three displayed an increase in burst frequency and in the number of action potentials within a burst. By contrast, another six cells were induced into tonic firing mode, with a marked decrease in instantaneous firing frequency and a shift in their excitatory postsynaptic potential-evoked responses from burst firing patterns to single action potentials. Under voltage clamp, orexins induced inward current (-21.8+/-2.4 pA at -60 mV) in 20/22 cells. In 13 cells, current-voltage (I-V) plots revealed a decrease in net conductance and reversal at -110+/-9 mV, while 3 cells displayed an increase in net conductance that reversed at -26+/-8 mV. These observations imply suppression of potassium and/or induction of nonselective cationic conductances in orexin-induced depolarization in PVT neurons, permitting these peptides to modulate intrinsic state-dependent properties. In vivo, such changes in firing patterns and frequency of action potential discharges could influence neurotransmission through PVT and activity-dependent synaptic plasticity at target sites of these neurons.     

Biphasic responses of thalamic neurons to GABA in isolated rat brain slices--II

In isolated thalamic slices, responses of relay neurons to electrophoretically applied GABA were recorded intracellularly and compared with inhibitory postsynaptic potentials evoked by electrical stimulation of the reticularis nucleus of the thalamus. Both reduced the excitability of thalamic neurons and were biphasic in the majority of neurons studied, consisting of an early, negative-going and a later, positive-going component, when recorded close to reversal potential (mean reversal potentials -66.6 and -57.7 mV). Bicuculline and picrotoxin applied electrophoretically reduced conductance increases evoked by GABA in all neurons. The later, positive-going component was more sensitive to these antagonists (applied with submaximal doses) than the early component. Current-voltage relations for responses to GABA, like those for inhibitory postsynaptic potentials, were non-linear in the majority of neurons. In particular, there was a region of reduced slope resistance close to the reversal potential. Holding the membrane at a conditioning potential was found to change the subsequent response and its reversal potential. Positive holding potentials shifted reversal potentials in the positive direction only when GABA was applied during the conditioning period. Negative holding potentials were effective whether GABA was applied during the conditioning period or not. Recovery from these effects followed a similar time course at all membrane potentials tested. Injection of Cl- produced a positive shift in the reversal potential for both components of the response to GABA and of the evoked inhibitory postsynaptic potential. Inhibitory postsynaptic potentials evoked in thalamic relay neurons by stimulation of the nucleus reticularis resembled responses to GABA in their biphasic nature, reversal potentials and sensitivity to antagonists and to changes in intracellular chloride.     

Whole cell recordings from respiratory neurons in the medulla of brainstem-spinal cord preparations isolated from newborn rats

In brainstem-spinal cord preparations isolated from newborn rats, a whole cell recording technique was applied to record membrane potentials of inspiratory (Insp) and pre-inspiratory (Pre-I) neurons in the ventrolateral medulla. Labelling of these respiratory neurons with Lucifer Yellow allowed analysis of their locations and morphology. Intracellular membrane potentials from 25 Insp neurons were recorded. Average resting membrane potential was –49 mV (n=25) and input resistance was 306 M. Insp neurons were classified into three types from the patterns of synaptic potentials. Type I neurons (n=11) had a high probability of excitatory postsynaptic potentials (EPSPs) in the pre- and post-inspiratory phases. Type II neurons (n=7) showed abrupt transition to the burst phase from the resting potential level without increased EPSPs in the preinspiratory phase. Type III neurons (n=7) were hyperpolarized by inhibitory postsynaptic potentials (IPSPs) in the pre- and post-inspiratory phases. These Insp neurons, located in the ventrolateral medulla 80–490m from the ventral surface, were 10–30 m in diameter, and had various soma shapes (pyramidal, spherical or fusiform). Intracellular membrane potentials from 24 Pre-I neurons were recorded. The average resting membrane potential was –45 mV (n=24), and the input resistance was 320 M. Typical Pre-I neurons showed fairly great depolarization accompanied by action potentials during their burst phase and repolarization during the inspiratory phase. Most Pre-I neurons appeared to have a high level of synaptic activity. These cells were located in the ventrolateral medulla 50–440 m below the ventral surface and had pyramidal or fusiform somas of 10–25 m in diameter. Stimulation of the ipsilateral IXth, Xth roots or the spinal cord (C3 level) induced orthodromic responses in most Insp or Pre-I neurons. An antidromic action potential was induced in only one Pre-I neuron by stimulation at the ipsilateral C3 level. Many Insp or Pre-I neurons had dendrites that terminated close to the ventral surface of the medulla. The present study revealed postsynaptic activity of respiratory neurons in the rostral ventrolateral medulla, which is consistent with the excitatory and inhibitory synaptic connections from Pre-I neurons to Insp neurons, and inhibitory synaptic connections for Insp neurons to Pre-I neurons.     

Selective actions of anesthetic agents on membrane potential trajectory in bulbar respiratory neurons of cats

The effects of two anesthetic agents, halothane and thiopental, on the membrane potential trajectory of respiratory-related neurons in the ventral respiratory group were investigated in decerebrate cats, of which the carotid sinus and vagal afferents were denervated. Infusion of halothane (2% for 90 s) depolarized the membrane in nearly half of the inspiratory (12/21), post-inspiratory (10/26) and expiratory (4/6) neurons and caused hyperpolarization in the rest of the population. Thiopental (2.5 mg/kg i.v.) produced depolarization in 11 inspiratory and 10 post-inspiratory neurons and hyperpolarization in 1 expiratory, 4 inspiratory and 7 postinspiratory neurons. In both hyperpolarized and depolarized neurons, reduction of the respiratory membrane potential fluctuations and an increase of input resistance were commonly observed. Both drugs depressed spontaneous firing in most of the neurons studied. An increase of firing was observed in 9 out of 47 depolarized cells. These two contrasting effects on the membrane potential trajectory occurred similarly in the known groups of respiratory neurons, but the response of a given cell was consistent for the two anesthetic agents. The present results demonstrate that the anesthetic drugs exert various influences on the ventral respiratory group neuron population in maintaining the membrane potential trajectory and discharge activity. This may reflect a functional heterogeneity in the bulbar respiratory network of neurons.     

Electrophysiological characteristics distinguish three classes of neuron in submucosal ganglia of the guinea-pig distal colon

Intracellular recordings were made from neurons in the submucosal ganglia of the guinea-pig distal colon. The recording electrode contained the intracellular marker biocytin, which was injected into neurons so that their electrophysiological characteristics could be correlated with their shape. Correlations of electrophysiology and shape have not been reported previously for neurons in this region. Three types of neuron were identified on electrophysiological grounds. Neurons of the first type (S neurons) had tetrodotoxin-sensitive soma action potentials, and received fast and slow excitatory synaptic inputs. They had uniaxonal morphologies and may function as secretomotor or possibly vasomotor neurons. The second type (AH neurons) received only slow synaptic input, while the soma action potential had tetrodotoxin-sensitive and -insensitive components with a shoulder on the falling phase and a long-lasting afterhyperpolarisation of the membrane potential following a single action potential. Neurons of this type had multipolar morphologies and provided dense innervation of adjacent submucosal ganglia. These neurons are similar to the submucosal intrinsic primary afferent neurons of the guinea-pig small intestine. The final type of neuron [the low-threshold (LT) neuron] had electrophysiological characteristics that set it apart from those described previously within enteric plexuses. They expressed tetrodotoxin-insensitive voltage-gated soma currents, did not have long-lasting afterhyperpolarisations and received only slow synaptic input. In addition, these neurons were very excitable: they had large input resistances and low thresholds for action potential discharge, and often fired action potentials in the absence of stimulation. Neurons with these characteristics were uniaxonal and thus are likely to be secretomotor or possibly vasomotor neurons. This study has shown that submucosal neurons of the distal colon fall into three distinct types, which can be distinguished by a combination of electrophysiological and morphological criteria.     

突触噪声对神经元兴奋性调节的模型研究

结合已有关于突触噪声输入下神经元放电形式现象的研究,建立一个类似生理条件下的运动神经元膜特性的数学模型。该模型再现了有无突触噪声两种情况下的神经元放电的基本形式,在含有噪声输入情况下再现了膜振荡的现象。在此基础上分别对上述两种情况神经元动作电位的个数、幅度以及阈值作了定量描述。与双斜坡刺激相比,兴奋性突触输入增加动作电位个数约为16个,但是兴奋阈值增加;抑制性突触输入减少动作电位个数约为19个,兴奋阈值减小。在抑制性噪声输入和混合突触输入的情况下,动作电位幅度增量明显比双斜坡刺激的要小,其平均值分别下降为0.68、0.33 mV。该数学模型能够很好地模拟电生理实验过程中神经元含有膜振荡的动作电位现象,这对后续电生理神经元的分析以及细胞放电的病理研究提供了手段与依据。     

The ionic mechanism of postsynaptic inhibition in motoneurones of the frog spinal cord

The ionic mechanism of postsynaptic inhibition in frog spinal motoneurones was studied with conventional and with ion-sensitive microelectrodes. In these neurones the inhibitory postsynaptic potential was depolarizing, its reversal potential being 15 mV less negative than the resting membrane potential. During the inhibitory postsynaptic potential the input resistance of the motoneurones was reduced to 20% of the resting value, indicating a strong increase of membrane conductance. The Cl- equilibrium potential calculated from intra- and extracellular Cl- activity measurements coincided with the reversal potential of the inhibitory postsynaptic potential to within a few millivolts. During repetitive inhibitory postsynaptic activity the intracellular Cl- activity decreased markedly, while the extracellular Cl- activity increased slightly. These changes of intra- and extracellular Cl- activities were no longer found after suppression of the inhibitory postsynaptic potential by strychnine. Blockade of an active, inward-going Cl- transport system in motoneurones by NH+4 led to a shift of the Cl- equilibrium potential and the reversal potential of the inhibitory postsynaptic potential towards the resting membrane potential. After prolonged action of NH+4, the Cl- equilibrium potential approached the membrane potential to within 5 mV, while the reversal potential of the inhibitory postsynaptic potential and resting membrane potential coincided. The difference between Cl- equilibrium potential and membrane potential after blockade of the Cl- pump is traced back to interfering intracellular ions, such as HCO-3 or SO42-, leading to an overestimation of intracellular Cl- activity and to the calculation of an erroneous Cl- equilibrium potential. Inhibitory amino acids like gamma-aminobutyrate or beta-alanine evoked depolarizations with reversal potentials similar to that of the inhibitory postsynaptic potential. These depolarizations were associated with a marked decrease of neuronal input resistance during inhibition. During the actions of these compounds a decrease of intracellular and a small increase of extracellular Cl- activity were found. The activities of other ions (K+, Ca2+ and Na+) did not change significantly, with the exception of extracellular K+ activity, which was slightly increased. Evidence is presented that the inhibitory postsynaptic potential, as well as the depolarizing action of inhibitory amino acids in motoneurones, is the result of an increase in membrane Cl- permeability and an efflux of Cl- from these cells, while other ions do not seem to be involved.