Membrane currents in spinal motoneurons associated with the action potential and synaptic activity

Motor Neurons

Motor neurons translate synaptic input from widely distributed premotor networks into patterns of action potentials that orchestrate motor unit force and motor behavior. Intercalated between the CNS and muscles, motor neurons add to and adjust the final motor command. The identity and functional properties of this facility in the path from synaptic sites to the motor axon is reviewed with emphasis on voltage sensitive ion channels and regulatory metabotropic transmitter pathways. The catalog of the intrinsic response properties, their underlying mechanisms, and regulation obtained from motoneurons in in vitro preparations is far from complete. Nevertheless, a foundation has been provided for pursuing functional significance of intrinsic response properties in motoneurons in vivo during motor behavior at levels from molecules to systems. © 2017 American Physiological Society. Compr Physiol 7:463-484, 2017.

Axon-somatic back-propagation in detailed models of spinal alpha motoneurons

Antidromic action potentials following distal stimulation of motor axons occasionally fail to invade the soma of alpha motoneurons in spinal cord, due to their passing through regions of high non-uniformity. Morphologically detailed conductance-based models of cat spinal alpha motoneurons have been developed, with the aim to reproduce and clarify some aspects of the electrophysiological behavior of the antidromic axon-somatic spike propagation. Fourteen 3D morphologically detailed somata and dendrites of cat spinal alpha motoneurons have been imported from an open-access web-based database of neuronal morphologies, NeuroMorpho.org, and instantiated in neurocomputational models. An axon hillock, an axonal initial segment and a myelinated axon are added to each model. By sweeping the diameter of the axonal initial segment (AIS) and the axon hillock, as well as the maximal conductances of sodium channels at the AIS and at the soma, the developed models are able to show the relationships between different geometric and electrophysiological configurations and the voltage attenuation of the antidromically traveling wave. In particular, a greater than usually admitted sodium conductance at AIS is necessary and sufficient to overcome the dramatic voltage attenuation occurring during antidromic spike propagation both at the myelinated axon-AIS and at the AIS-soma transitions.

A computational model of motor neuron degeneration

To explore the link between bioenergetics and motor neuron degeneration, we used a computational model in which detailed morphology and ion conductance are paired with intracellular ATP production and consumption. We found that reduced ATP availability increases the metabolic cost of a single action potential and disrupts K+/Na+ homeostasis, resulting in a chronic depolarization. The magnitude of the ATP shortage at which this ionic instability occurs depends on the morphology and intrinsic conductance characteristic of the neuron. If ATP shortage is confined to the distal part of the axon, the ensuing local ionic instability eventually spreads to the whole neuron and involves fasciculation-like spiking events. A shortage of ATP also causes a rise in intracellular calcium. Our modeling work supports the notion that mitochondrial dysfunction can account for salient features of the paralytic disorder amyotrophic lateral sclerosis, including motor neuron hyperexcitability, fasciculation, and differential vulnerability of motor neuron subpopulations.

Electrophysiological correlates of sleep homeostasis in freely behaving rats

The electrical activity of the brain does not only reflect the current level of arousal, ongoing behavior, or involvement in a specific task but is also influenced by what kind of activity, and how much sleep and waking occurred before. The best marker of sleep-wake history is the electroencephalogram (EEG) spectral power in slow frequencies (slow-wave activity, 0.5-4 Hz, SWA) during sleep, which is high after extended wakefulness and low after consolidated sleep. While sleep homeostasis has been well characterized in various species and experimental paradigms, the specific mechanisms underlying homeostatic changes in brain activity or their functional significance remain poorly understood. However, several recent studies in humans, rats, and computer simulations shed light on the cortical mechanisms underlying sleep regulation. First, it was found that the homeostatic changes in SWA can be fully accounted for by the variations in amplitude and slope of EEG slow waves, which are in turn determined by the efficacy of corticocortical connectivity. Specifically, the slopes of sleep slow waves were steeper in early sleep compared to late sleep. Second, the slope of cortical evoked potentials, which is an established marker of synaptic strength, was steeper after waking, and decreased after sleep. Further, cortical long-term potentiation (LTP) was partially occluded if it was induced after a period of waking, but it could again be fully expressed after sleep. Finally, multiunit activity recordings during sleep revealed that cortical neurons fired more synchronously after waking, and less so after a period of consolidated sleep. The decline of all these electrophysiological measures-the slopes of slow waves and evoked potentials and neuronal synchrony-during sleep correlated with the decline of the traditional marker of sleep homeostasis, EEG SWA. Taken together, these data suggest that homeostatic changes in sleep EEG are the result of altered neuronal firing and synchrony, which in turn arise from changes in functional neuronal connectivity.

Cortical long-axoned cells and putative interneurons during the sleep-waking cycle

Knowledge of the input-output characteristics of various neuronal types is a necessary first step toward an understanding of cellular events related to waking and sleep. In spite of the oversimplification involved, the dichotomy in terms of type I (long-axoned, output) neurons and type II (short-axoned, local) interneurons is helpful in functionally delineating the neuronal circuits involved in the genesis and epiphenomena of waking and sleep states. The possibility is envisaged that cortical interneurons, which are particularly related to higher neuronal activity and have been found in previous experiments to be more active during sleep than during wakefulness, might be involved in complex integrative processes occurring during certain sleep stages. Electrophysiological criteria for the identification of output cells and interneurons are developed, with emphasis on various possibilities and difficulties involved in recognizing interneurons of the mammalian brain. The high-frequency repetitive activity of interneurons is discussed, together with various possibilities of error to be avoided when interpreting data from bursting cells. Data first show opposite changes in spontaneous and evoked discharges of identified output cells versus putative interneurons recorded from motor and parietal association cortical areas in behaving monkeys and cats during wakefulness (W) compared to sleep with synchronized EEG activity (S): significantly increased rates of spontaneous firing, enhanced antidromic or synaptic responsiveness, associated with shorter periods of inhibition in type I (pyramidal tract, cortico-thalamic and cortico-pontine) cells during W versus significantly decreased frequencies of spontaneous discharge and depression of synaptically elicited reponses of type II cells during W compared to S. These findings are partly explained on the basis of recent iontophoretic studies showing that acetylcholine, viewed as a synaptic transmitter of the arousal system, excites output-type neurons and inhibits high-frequency bursting cells. Comparing W and S to the deepest stage of sleep with desynchronized EEG activity (D) in type I and type II cells revealed that: (a) the increased firing rates of output cells in D, over those in W and S, is substantially due to a tonic excitation during this state, and rapid eye movements (REMs) only contribute to the further increase of discharge frequencies; (b) in contrast, the increased rates of discharge in interneurons during D is entirely ascribable to REM-related firing. On the basis of experiments reporting that increased duration of D has beneficial effects upon retention of information acquired during W, the suggestion is made that increased firing rates of association cortical interneurons during REM epochs of D sleep are an important factor in maintaining the soundness of a memory trace.

Transmission efficacy and plasticity in glutamatergic synapses formed by excitatory interneurons of the substantia gelatinosa in the rat spinal cord

Background

Substantia gelatinosa (SG, lamina II) is a spinal cord region where most unmyelinated primary afferents terminate and the central nociceptive processing begins. The glutamatergic excitatory interneurons (EINs) form the majority of the SG neuron population, but little is known about the mechanisms of signal processing in their synapses.

Methodology

To describe the functional organization and properties of excitatory synapses formed by SG EINs, we did non-invasive recordings from 183 pairs of monosynaptically connected neurons. An intact presynaptic SG EIN was specifically stimulated through the cell-attached pipette while the evoked EPSCs/EPSPs were recorded through perforated-patch from a postsynaptic neuron (laminae I-III).

Principal Findings

We found that the axon of an SG EIN forms multiple functional synapses on the dendrites of a postsynaptic neuron. In many cases, EPSPs evoked by stimulating an SG EIN were sufficient to elicit spikes in a postsynaptic neuron. EPSCs were carried through both Ca²⁺-permeable (CP) and Ca²⁺-impermeable (CI) AMPA receptors (AMPARs) and showed diverse forms of functional plasticity. The synaptic efficacy could be enhanced through both activation of silent synapses and strengthening of already active synapses. We have also found that a high input resistance (R_IN, >0.5 GΩ) of the postsynaptic neuron is necessary for resolving distal dendritic EPSCs/EPSPs and correct estimation of their efficacy.

Conclusions/Significance

We conclude that the multiple synapses formed by an SG EIN on a postsynaptic neuron increase synaptic excitation and provide basis for diverse forms of plasticity. This functional organization can be important for sensory, i.e. nociceptive, processing in the spinal cord.

Involvement of persistent Na+ current in spike initiation in primary sensory neurons of the rat mesencephalic trigeminal nucleus

It was recently shown that the persistent Na(+) current (I(NaP)) is generated in the proximal axon in response to somatic depolarization in neocortical pyramidal neurons, although the involvement of I(NaP) in spike initiation is still unclear. Here we show a potential role of I(NaP) in spike initiation of primary sensory neurons in the mesencephalic trigeminal nucleus (MTN) that display a backpropagation of the spike initiated in the stem axon toward the soma in response to soma depolarization. Riluzole (10 muM) and tetrodotoxin (TTX, 10 nM) caused an activation delay or a stepwise increase in the threshold for evoking soma spikes (S-spikes) without affecting the spike itself. Simultaneous patch-clamp recordings from the soma and axon hillock (AH) revealed that bath application of 50 nM TTX increased the delay in spike activation in response to soma depolarization, leaving the spike-backpropagation time from the AH to soma unchanged. This indicates that the increase in activation delay occurred in the stem axon. Furthermore, under a decreasing intracellular concentration gradient of QX-314 from the soma to AH created by QX-314-containing and QX-314-free patch pipettes, the amplitude and maximum rate of rise (MRR) of AH-spikes decreased with an increase in the activation delay following repetition of current-pulse injections, whereas S-spikes displayed decreases of considerably lesser degree in amplitude and MRR. This suggests that compared to S-spikes, AH-spikes more accurately reflect the attenuation of axonal spike by QX-314, consistent with the nature of spike backpropagation. These observations strongly suggest that low-voltage-activated I(NaP) is involved in spike initiation in the stem axon of MTN neurons.

An ex vivo preparation of mature mice spinal cord to study synaptic transmission on motoneurons

Mammalian spinal cord motoneurons are highly susceptible to chemical and mechanical disturbances, which imposes substantial difficulties for electrophysiological investigation in acute in vitro preparations. The aim of the present study was to establish an isolated spinal cord preparation from adult mice and to examine the synaptic activities of motoneurons in vitro. We removed the lumbo-sacral cord from the vertebral canal by hydraulic extrusion and maintained the isolated cord in vitro for extracellular recordings. Population spikes of motoneurons were evoked by electrical stimulation of dorsal roots (orthodromic) or ventral roots (antidromic) and these evoked responses could be continuously monitored for 5-6 h. The orthodromic population spikes were reversibly suppressed by the AMPA/kainate receptor antagonist 2,3-dihyro-6-nitro-7-sulfamoylbenzo quinoxaline (NBQX, 10 microM) but they persisted in the presence of the NMDA receptor antagonist D(-)-2-amino-5-phosphonovaleric acid (AP5, 50 microM). The antidromic population spikes exhibited evident paired pulse inhibition when evoked at inter-stimulus intervals of pound 6 ms. Histological examination revealed that basic morphological features of the lumbo-sacral motoneurons were preserved after 3-4 h of in vitro maintenance. This in vitro preparation is ideally suited for the electrophysiological study of synaptic transmission on adult mouse spinal motoneurons.

A modelling study of locomotion-induced hyperpolarization of voltage threshold in cat lumbar motoneurones

During fictive locomotion the excitability of adult cat lumbar motoneurones is increased by a reduction (a mean hyperpolarization of approximately 6.0 mV) of voltage threshold (Vth) for action potential (AP) initiation that is accompanied by only small changes in AP height and width. Further examination of the experimental data in the present study confirms that Vth lowering is present to a similar degree in both the hyperpolarized and depolarized portions of the locomotor step cycle. This indicates that Vth reduction is a modulation of motoneurone membrane currents throughout the locomotor state rather than being related to the phasic synaptic input within the locomotor cycle. Potential ionic mechanisms of this locomotor-state-dependent increase in excitability were examined using three five-compartment models of the motoneurone innervating slow, fast fatigue resistant and fast fatigable muscle fibres. Passive and active membrane conductances were set to produce input resistance, rheobase, afterhyperpolarization (AHP) and membrane time constant values similar to those measured in adult cat motoneurones in non-locomoting conditions. The parameters of 10 membrane conductances were then individually altered in an attempt to replicate the hyperpolarization of Vth that occurs in decerebrate cats during fictive locomotion. The goal was to find conductance changes that could produce a greater than 3 mV hyperpolarization of Vth with only small changes in AP height (< 3 mV) and width (< 1.2 ms). Vth reduction without large changes in AP shape could be produced either by increasing fast sodium current or by reducing delayed rectifier potassium current. The most effective Vth reductions were achieved by either increasing the conductance of fast sodium channels or by hyperpolarizing the voltage dependency of their activation. These changes were particularly effective when localized to the initial segment. Reducing the conductance of delayed rectifier channels or depolarizing their activation produced similar but smaller changes in Vth. Changes in current underlying the AHP, the persistent Na(+) current, three Ca(2+) currents, the "h" mixed cation current, the "A" potassium current and the leak current were either ineffective in reducing Vth or also produced gross changes in the AP. It is suggested that the increased excitability of motoneurones during locomotion could be readily accomplished by hyperpolarizing the voltage dependency of fast sodium channels in the axon hillock by a hitherto unknown neuromodulatory action.

Input-output functions of mammalian motoneurons

Our intent in this review was to consider the relationship between the biophysical properties of motoneurons and the mechanisms by which they transduce the synaptic inputs they receive into changes in their firing rates. Our emphasis has been on experimental results obtained over the past twenty years, which have shown that motoneurons are just as complex and interesting as other central neurons. This work has shown that motoneurons are endowed with a rich complement of active dendritic conductances, and flexible control of both somatic and dendritic channels by endogenous neuromodulators. Although this new information requires some revision of the simple view of motoneuron input-output properties that was prevalent in the early 1980's (see sections 2.3 and 2.10), the basic aspects of synaptic transduction by motoneurons can still be captured by a relatively simple input-output model (see section 2.3, equations 1-3). It remains valid to describe motoneuron recruitment as a product of the total synaptic current delivered to the soma, the effective input resistance of the motoneuron and the somatic voltage threshold for spike initiation (equations 1 and 2). However, because of the presence of active channels activated in the subthreshold range, both the delivery of synaptic current and the effective input resistance depend upon membrane potential. In addition, activation of metabotropic receptors by achetylcholine, glutamate, noradrenaline, serotonin, substance P and thyrotropin releasing factor (TRH) can alter the properties of various voltage- and calcium-sensitive channels and thereby affect synaptic current delivery and input resistance. Once motoneurons are activated, their steady-state rate of repetitive discharge is linearly related to the amount of injected or synaptic current reaching the soma (equation 3). However, the slope of this relation, the minimum discharge rate and the threshold current for repetitive discharge are all subject to neuromodulatory control. There are still a number of unresolved issues concerning the control of motoneuron discharge by synaptic inputs. Under dynamic conditions, when synaptic input is rapidly changing, time- and activity-dependent changes in the state of ionic channels will alter both synaptic current delivery to the spike-generating conductances and the relation between synaptic current and discharge rate. There is at present no general quantitative expression for motoneuron input-output properties under dynamic conditions. Even under steady-state conditions, the biophysical mechanisms underlying the transfer of synaptic current from the dendrites to the soma are not well understood, due to the paucity of direct recordings from motoneuron dendrites. It seems likely that resolving these important issues will keep motoneuron afficiandoes well occupied during the next twenty years.

Characteristics of fast Na(+) current of hypoglossal motoneurons in a rat brainstem slice preparation

Whole-cell patch clamp recordings were performed on hypoglossal motoneurons in a brainstem slice preparation from the neonatal rat brain to study the characteristics of the fast Na(+) current (I(Na)) which has not been hitherto investigated in these cells. To aid voltage clamping of I(Na), cells were bathed in low Na(+) solution, loaded intracellularly with Na(+) (to reverse the Na(+) gradient) or treated with a small dose (20 nM) of tetrodotoxin. In low extracellular Na(+) solution (Na(+) was replaced by choline or N-methyl-D-glucamine) I(Na) activated at membrane potentials positive to -45 mV and was half-maximally activated at -30 mV. Similar data were obtained when the Na(+) gradient was reversed or tetrodotoxin was applied. I(Na) rapidly activated (1--3.5 ms time constant) and inactivated (1.6 ms time constant at 0 mV) during membrane depolarization. Inactivation was strongly voltage-dependent (half inactivation at -44 mV) and developed mono-exponentially. Recovery from inactivation was bi-exponential with fast and slow time constants of 14 and 160 ms, respectively, at -58 mV. The rapid turning on of I(Na) was presumably responsible for the upstroke of the fast action potential generated by these cells while the slow phase of recovery from inactivation might modulate the ability to fire repetitively at high rate.

Differentiation of electrical excitability in motoneurons

Investigation of the differentiation of electrical properties of motoneurons has been stimulated by the importance of these neurons for embryonic behavior and facilitated by their experimental accessibility. In this review, we examine the development of different patterns of excitability and their functions, and discuss the emergence of repetitive firing and localization of ion channels in axons and dendrites. Finally, we summarize studies of the role of extrinsic factors in differentiation. These changes associated with differentiation of young motoneurons may presage those occurring later in the context of plasticity in the mature nervous system.

Excitability of the soma in central nervous system neurons

The ability of the soma of a spinal dorsal horn neuron, a spinal ventral horn neuron (presumably a motoneuron), and a hippocampal pyramidal neuron to generate action potentials was studied using patch-clamp recordings from rat spinal cord slices, the "entire soma isolation" method, and computer simulations. By comparing original recordings from an isolated soma of a dorsal horn neuron with simulated responses, it was shown that computer models can be adequate for the study of somatic excitability. The modeled somata of both spinal neurons were unable to generate action potentials, showing only passive and local responses to current injections. A four- to eightfold increase in the original density of Na(+) channels was necessary to make the modeled somata of both spinal neurons excitable. In contrast to spinal neurons, the modeled soma of the hippocampal pyramidal neuron generated spikes with an overshoot of +9 mV. It is concluded that the somata of spinal neurons cannot generate action potentials and seem to resist their propagation from the axon to dendrites. In contrast, the soma of the hippocampal pyramidal neuron is able to generate spikes. It cannot initiate action potentials in the intact neurons, but it can support their back-propagation from the axon initial segment to dendrites.

Synaptic control of motoneuronal excitability

Movement, the fundamental component of behavior and the principal extrinsic action of the brain, is produced when skeletal muscles contract and relax in response to patterns of action potentials generated by motoneurons. The processes that determine the firing behavior of motoneurons are therefore important in understanding the transformation of neural activity to motor behavior. Here, we review recent studies on the control of motoneuronal excitability, focusing on synaptic and cellular properties. We first present a background description of motoneurons: their development, anatomical organization, and membrane properties, both passive and active. We then describe the general anatomical organization of synaptic input to motoneurons, followed by a description of the major transmitter systems that affect motoneuronal excitability, including ligands, receptor distribution, pre- and postsynaptic actions, signal transduction, and functional role. Glutamate is the main excitatory, and GABA and glycine are the main inhibitory transmitters acting through ionotropic receptors. These amino acids signal the principal motor commands from peripheral, spinal, and supraspinal structures. Amines, such as serotonin and norepinephrine, and neuropeptides, as well as the glutamate and GABA acting at metabotropic receptors, modulate motoneuronal excitability through pre- and postsynaptic actions. Acting principally via second messenger systems, their actions converge on common effectors, e.g., leak K(+) current, cationic inward current, hyperpolarization-activated inward current, Ca(2+) channels, or presynaptic release processes. Together, these numerous inputs mediate and modify incoming motor commands, ultimately generating the coordinated firing patterns that underlie muscle contractions during motor behavior.

Integration of excitatory postsynaptic potentials in dendrites of motoneurons of rat spinal cord slice cultures

We examined the attenuation and integration of spontaneous excitatory postsynaptic potentials (sEPSPs) in the dendrites of presumed motoneurons (MNs) of organotypic rat spinal cord cultures. Simultaneous whole cell recordings in current-clamp mode were made from either the soma and a dendrite or from two dendrites. Direct comparison of the two voltage recordings revealed that the membrane potentials at the two recording sites followed each other very closely except for the fast-rising phases of the EPSPs. The dendritic recording represented a low-pass filtered version of the somatic recording and vice versa. A computer-assisted method was developed to fit the sEPSPs with a generalized alpha-function for measuring their amplitudes and rise times (10-90%). The mean EPSP peak attenuation between the two recording electrodes was determined by a maximum likelihood analysis that extracted populations of similar amplitude ratios from the fitted events at each electrode. For each pair of recordings, the amplitude attenuation ratio for EPSP traveling from dendrite to soma was larger than that traveling from soma to dendrite. The linear relation between mean ln attenuation and distance between recording electrodes was used to map 1/e attenuations into units of distance (micron). For EPSPs with typical time course traveling from the somatic to the dendritic recording electrode, the mean 1/e attenuation corresponded to 714 micron for EPSPs traveling in the opposite direction, the mean 1/e attenuation corresponded to 263 micron. As predicted from cable analysis, fast EPSPs attenuated more in both the somatofugal and somatopetal direction than did slow EPSPs. For EPSPs with rise times shorter than approximately 2.0 ms, the attenuation factor increased steeply. Compartmental computer modeling of the experiments with biocytin-filled and reconstructed MNs that used passive membrane properties revealed amplitude attenuation ratios of the EPSP traveling in both the somatofugal and somatopetal direction that were comparable to those observed in real experiments. The modeling of a barrage of sEPSPs further confirmed that the somato-dendritic compartments of a MN are virtually isopotential except for the fast-rising phase of EPSPs. Large, transient differences in membrane potential are locally confined to the site of EPSP generation. Comparing the modeling results with the experiments suggests that the observed attenuation ratios are adequately explained by passive membrane properties alone.

Modeling action potential initiation and back-propagation in dendrites of cultured rat motoneurons

Regardless of the site of current injection, action potentials usually originate at or near the soma and propagate decrementally back into the dendrites. This phenomenon has been observed in neocortical pyramidal cells as well as in cultured motoneurons. Here we show that action potentials in motoneurons can be initiated in the dendrite as well, resulting in a biphasic dendritic action potential. We present a model of spinal motoneurons that is consistent with observed physiological properties of spike initiation in the initial segment/axon hillock region and action potential back-propagation into the dendritic tree. It accurately reproduces the results presented by Larkum et al. on motoneurons in organotypic rat spinal cord slice cultures. A high Na+-channel density of Na = 700 mS/cm2 at the axon hillock/initial segment region was required to secure antidromic invasion of the somato-dendritic membrane, whereas for the orthodromic direction, a Na+-channel density of Na = 1,200 mS/cm2 was required. A "weakly" excitable (Na = 3 mS/cm2) dendritic membrane most accurately describes the experimentally observed attenuation of the back-propagated action potential. Careful analysis of the threshold conditions for action potential initiation at the initial segment or the dendrites revealed that, despite the lower voltage threshold for spike initiation in the initial segment, an action potential can be initiated in the dendrite before the initial segment fires a spike. Spike initiation in the dendrite depends on the passive cable properties of the dendritic membrane, its Na+-channel density, and local structural properties, mainly the diameter of the dendrites. Action potentials are initiated more easily in distal than in proximal dendrites. Whether or not such a dendritic action potential invades the soma with a subsequent initiation of a second action potential in the initial segment depends on the actual current source-load relation between the action potential approaching the soma and the electrical load of the soma together with the attached dendrites.

EXCITABILITY CHANGES OF THE MAUTHNER CELL DURING COLLATERAL INHIBITION

Excitability changes during collateral inhibition of the goldfish Mauthner cell (M cell) were measured directly by stimulating the cell with current pulses applied through an intracellular electrode. Excitability was suppressed during the extrinsic hyperpolarizing potential (EHP) as well as during the collateral IPSP. The inhibitory effect of the EHP was shown to be comparable in intensity to the effect of the IPSP. Excitability changes in the M cell during collateral IPSP depended on changes in the membrane conductance as well as in the membrane potential. Some simple equations are advanced which describe the excitability change during the IPSP in terms of changes in membrane potential and conductance. It was also found that invasion of antidromic impulses into the M cell was suppressed during the EHP, but not during the collateral IPSP. Conductance increase during the IPSP did not interfere with the invasion of antidromic impulses.

The minimal inhibitory synaptic currents evoked in neonatal rat motoneurones

1. Tight-seal whole-cell recordings were made from lumbar motoneurones visually identified in thin slices of neonatal rat spinal cord. The inhibitory postsynaptic currents (IPSCs) were evoked by extracellular stimulation of a neighbouring internuncial neurone in the presence of glutamate receptor antagonists. 2. Glycinergic IPSCs were recorded in the presence of bicuculline. The IPSCs appeared in an all-or-none manner as the graded stimulus intensity exceeded a certain threshold. Their latencies showed a unimodal distribution with a mean of 0.81 ms at 37 degrees C. Thus, the observed IPSCs are suggested to be monosynaptically evoked unitary IPSCs. The mean conductance of unitary IPSCs was 2.9 +/- 1.2 nS (+/- S.D.). 3. When the external Ca2+ concentration ([Ca2+]o) was reduced, the number of failures in response to stimulation increased, thereby reducing the mean amplitude of IPSCs. The mean amplitude of IPSCs was linearly related to the [Ca2+]o (0.35-1.4 mM) with a mean slope of 3.1 +/- 0.67 on double logarithmic co-ordinates. 4. The amplitude of individual IPSCs decreased with decrease in [Ca2+]o. However, below 0.7 mM [Ca2+]o, the mean amplitude of IPSCs (excluding failures) reached a steady minimum level. The mean conductance of these IPSCs measured in 0.5 mM [Ca2+]o was 657 +/- 281 pS. 5. The minimal IPSCs had a coefficient of variation of 0.50 +/- 0.13. No clear correlation was observed between the rise time and the amplitude of minimal IPSCs evoked in individual motoneurones, indicating that the amplitude variability is not due to the different synaptic locations. 6. Spontaneous miniature IPSCs were recorded from motoneurones in the presence of tetrodotoxin. The miniature IPSCs had a mean conductance of 739 +/- 278 pS, being comparable to the minimal evoked IPSCs. 7. Under various internal and external Cl- concentration, the reversal potential of the IPSCs (EIPSC) approximately coincided with the Cl- equilibrium potential. A 730-fold change in the potassium concentration gradient across the membrane did not affect the EIPSC. The permeability ratio of K+ to Cl- (Pk/PCl) was less than 0.05. 8. It is concluded that the IPSCs are carried almost exclusively by Cl- and that the minimal evoked IPSCs represent the quantal response of the transmitter.

The effect of omega-conotoxin GVIA on synaptic transmission within the nucleus accumbens and hippocampus of the rat in vitro

1. The actions of two calcium channel antagonists, the N-channel blocker omega-conotoxin GVIA (omega-CgTx) and the L-channel antagonist nisoldipine, on synaptic transmission were investigated in the hippocampus and nucleus accumbens of the rat in vitro. 2. omega-CgTx (100 nM for 10 min) produced a marked and irreversible reduction of focally evoked population spikes and intracellularly recorded excitatory postsynaptic potentials (e.p.s.ps) in the nucleus accumbens, which could not be overcome by increasing the stimulus strength. 3. Nisoldipine (10 microM for 10 min) had no effect on population spikes in the nucleus accumbens or the CA1 of the hippocampus. 4. In the hippocampus, population spikes were not irreversibly reduced by omega-CgTx (100 nM for 10 min) but rather, multiple population spikes were produced along with spontaneous synchronous discharges. This indicated that inhibitory synaptic transmission was being preferentially reduced. 5. Intracellular recordings demonstrated that omega-CgTx powerfully reduced inhibitory synaptic transmission in an irreversible manner and that excitatory transmission was also reduced but to a lesser extent. Unlike excitatory transmission in the nucleus accumbens and inhibitory transmission in the hippocampus, increasing the stimulus strength overcame the reduction of hippocampal excitatory transmission. 6. It is concluded that omega-CgTx-sensitive calcium channels are involved in the calcium entry that precedes the synaptic transmission in all these synapses. The apparent lower sensitivity of the hippocampal excitatory fibres to omega-CgTx may indicate that calcium entry that promotes transmitter release at central synapses may be mediated by pharmacologically distinct calcium channels.

Membrane currents in visually identified motoneurones of neonatal rat spinal cord

1. Ionic currents induced by depolarization of motoneurones were analysed by tight-seal, whole-cell recording in thin slices of neonatal rat lumbar spinal cord. Identification of motoneurones viewed under Nomarski optics was confirmed by retrograde labelling with the fluorescent dye, Evans Blue. 2. Under whole-cell voltage clamp, depolarizing command pulses from a holding potential of about -70 mV evoked a fast inward current followed by an outward current. The former was suppressed either by lowering external Na+ concentration or by application of tetrodotoxin (TTX). The apparent dissociation constant of TTX was about 13 nM. 3. The outward current remaining after TTX application was activated by depolarization above -50 mV, showing marked outward rectification in the current-voltage relation. Outward tail currents reversed in polarity near the K+ equilibrium potential calculated from the external and pipette K+ concentrations. 4. When external Ca2+ was replaced by Mg2+, the outward K+ current was suppressed markedly and reversibly. Subtraction of current recorded in Ca2+-free-Mg2+ solution from that in control solution revealed a Ca2(+)-dependent K+ current, IK(Ca) with both a transient, IC, and a sustained component IAHP; its tail current lasted for several hundred milliseconds. 5. The sustained outward current observed in Ca2(+)-free-Mg2+ solution was largely suppressed by external application of tetraethylammonium chloride (30 mM), suggesting that it was mostly the delayed rectifier current, IK. In Ca2(+)-free-Mg2+ solution containing TEA and TTX, another transient outward current was observed, which was inactivated by depolarizing pre-pulses in a time- and voltage-dependent manner. The steady-state inactivation curve indicated 50% inactivation at about -77 mV. 4-Aminopyridine (4-AP, 4 mM) largely and reversibly suppressed this current, whereas it did not affect IK observed in the absence of TEA. It is suggested that the transient outward current corresponds to the A-current (IA). 6. Action potentials were recorded in current-clamp mode. Replacement of external Ca2+ by Mg2+ markedly diminished the after-hyperpolarization. Concomitantly, the repolarizing phase of action potentials was slightly prolonged. In Ca2(+)-free-Mg2+ solution, application of 4-AP markedly prolonged action potential repolarization. In Ca2(+)-free-Mg2+ solution containing 4-AP, addition of TEA-Cl further prolonged the duration of the action potential. It is concluded that three different potassium currents, IC, IA and IK may all contribute to action potential repolarization in rat spinal motoneurones.

Voltage dependence of Ia reciprocal inhibitory currents in cat spinal motoneurones

1. Inhibitory postsynaptic currents (IPSCs) were recorded in voltage clamped posterior biceps or semitendinosus motoneurones of the cat during reciprocal inhibition. 2. Population IPSCs, recorded following stimulation of the whole quadriceps muscle nerve, had an average time-to-peak of 0.51 +/- 0.02 ms (+/- S.E.M., n = 22) and decayed exponentially, with an average time constant of 0.99 +/- 0.04 ms (at 37 degrees C) at resting membrane potentials. 3. Unitary IPSCs, recorded following spike-triggered averaging from an identified reciprocal inhibitory interneurone, had amplitudes of 120-220 pA with an average time-to-peak of 0.40 +/- 0.06 ms (n = 5). The decay of these unitary currents was exponential, with an average time constant of 0.82 +/- 0.07 ms (at 37 degrees C) at resting membrane potentials. 4. The time course of IPSCs was unaffected by either alpha-chloralose or pentobarbitone at concentrations necessary for deep anaesthesia. 5. The peak synaptic current varied linearly with the membrane potential over the range -90 to -30 mV, and had an average reversal potential of -80.7 +/- 1.5 mV (+/- S.E.M., n = 6) when measured using KCH3SO4-filled electrodes. 6. The reversal potential for the IPSC was used to calculate [Cl-]i. This was estimated to be 6.5 mM assuming that the inhibitory synaptic current was mediated purely by Cl- ions. 7. The rate at which synaptic currents decayed was exponentially dependent on the postsynaptic membrane potential, the decay time constant increasing e-fold for a 91 mV depolarization. This result was independent of [Cl-]i or of the magnitude of the synaptic conductance and was interpreted as a voltage dependence of the glycine channel open time. 8. The average unitary peak conductance was 9.1 +/- 1.7 nS (+/- S.E.M., n = 5), corresponding to the opening of approximately 200 glycine-activated postsynaptic channels following neurotransmitter release from a single Ia reciprocal interneurone.

Differential subcellular localization of the RI and RII Na+ channel subtypes in central neurons

Immunocytochemical localization of Na+ channel subtypes RI and RII showed that RI immunoreactivity is relatively low and homogeneous along the rostral-caudal extent of sagittal brain sections, whereas RII staining is heterogeneous and relatively dense in the forebrain, substantia nigra, hippocampus, and cerebellum. The somata of the dentate granule cells, hippocampal pyramidal cells, cerebellar Purkinje cells, and spinal motor neurons are immunoreactive for RI but not RII. In contrast, areas rich in unmyelinated nerve fibers, such as the mossy fibers of the dentate granule cells, the stratum radiatum and stratum oriens of the hippocampus, and the molecular layer of the cerebellum, are strongly immunoreactive for RII but not RI. Differential regulation of expression of RI and RII genes may allow differential modulation of Na+ channel density in somata and axons. The sites of RI localization correlate closely with sites where sustained Na+ currents have been recorded.

Synaptic potentials of chinchilla lateral superior olivary neurons

Neurons in the lateral superior olive (LSO) were characterized in vivo, by extracellular and intracellular recordings. Principal neurons of the LSO are excited by ipsilateral auditory stimuli and exhibit binaural inhibition, as observed in extracellular recordings. In subsequent intracellular recordings, ipsilateral acoustic stimuli evoked robust excitatory postsynaptic potentials (epsps), while contralateral stimuli evoked large inhibitory postsynaptic potentials (ipsps). The contralaterally evoked ipsps were reversed when the cell was polarized below resting membrane potential and when current was injected into neurons recorded with chloride-filled electrodes. The ipsp is probably a reflection of contralaterally evoked release of glycine acting through glycinergic receptors on the somata and proximal dendrites of these neurons. The properties of the epsps are consistent with data suggesting that ipsilaterally evoked excitation may be mediated by an excitatory amino acid-like substance acting through quisqualate or kainate receptors at dendritic locations.

An aminopyridine-sensitive, early outward current recorded in vivo in neurons of the precruciate cortex of cats using single-electrode voltage-clamp techniques

Studies were performed in cortical neurons to determine if voltage- and time-dependent membrane currents could be recognized and characterized in the dynamic, in vivo state. Intracellular measurements made in neurons of the precruciate cortex of awake cats with single-electrode voltage-clamp (SEVC) techniques disclosed an early outward current to depolarizing command steps in 124 of 137 cells studied. The voltage-dependent properties of the early outward current closely resembled those of A-currents studied in vitro in vertebrate and invertebrate neurons. The current was activated rapidly at onset latencies of less than two ms, fell to flat plateau levels within 60-120 ms during sustained depolarization, and was reduced or eliminated in 22 of 23 cells following intracellular administration of 3- or 4-aminopyridine. The magnitude of outward current in response to depolarizing commands was increased by preceding steady hyperpolarization and reduced by preceding steady depolarization. (The steady potentials were of 9.8 s duration and +/- 40 mV apart from the holding potentials.) Since return to the holding potentials occurred 80 ms before the onset of the command steps, the changes in membrane properties that were induced lasted beyond cessation of the steady polarizing stimuli themselves. Spiking did not prevent recognition of the early outward current as judged from its appearance before and after intracellular application of QX-314 to reduce spike activity. Apart from fast inward currents associated with spike potentials, the early outward current was the most conspicuous and characteristic membrane current noted in these recordings. An additional current component that was noted but not characterized in these studies was a slow, depolarization-induced inward current that could be reduced by intracellular injection of QX-314.

Suprarhinal cortex response to electrical stimulation of the lateral amygdala nucleus in the rat

Electrical stimulation of the lateral amygdala nucleus was found to evoke field potentials and influence unitary activity in the suprarhinal cortex of anesthetized rats. Laminar distributions of the field responses consisted of positive waves in superficial layers, that reversed to electronegatives from a depth of 0.4-0.5 mm. This response was followed by a shallow electropositive wave deeper than 0.7-0.8 mm. Extracellularly recorded units were studied in the posterior agranular insular area of the suprarhinal cortex. The data revealed that stimulation of the lateral amygdala produced a train of small amplitude spikes in association with a negative slow potential. Furthermore, such stimulation invariably elicited an inhibition of the spontaneous firing of large amplitude spikes, in association with a positive slow potential. The onset of this inhibitory response always occurred at longer latency than the excitatory one. The small amplitude spikes may well represent the firing of inhibitory interneurons after lateral amygdala stimulation. The study suggests that a feed-forward system of inhibition appears to be present in the connection between lateral amygdala and posterior agranular insular area of the suprarhinal cortex.

Binding experiment KD values and physiologically active GABA concentrations: an only apparent contradiction?

An apparent contradiction exists between GABAA receptor binding KD's (less than 10(-6) M) and GABA concentrations physiologically active in various systems (greater than 10(-6) M). Taking in account that synaptic cleft GABA action and removal should be rather quick (within 2 ms) we show that in principle there is no contradiction between low KD values (100-150 nM) and high physiologically active GABA concentrations (greater than 40 microM).

Localization of sodium channels in axon hillocks and initial segments of retinal ganglion cells

Affinity-purified antibodies against the sodium channel from rat brain were employed to localize sodium channels in the retina by immunocytochemical procedures. In rat retina, intense staining was observed in the ganglion cell axon layer and light staining was detected in fibers of the inner plexiform layer. In frog retina, only the ganglion cell axon layer was stained. Examination at higher magnification revealed that axon hillocks and initial segments of ganglion cells had a high density of immunoreactive sodium channels, whereas the cell bodies were devoid of stain. The sharply defined region of high sodium channel density at the axon hillock is likely to be responsible for the low threshold for action potential initiation in this region of vertebrate central neurons.

Synaptic currents at interpositorubral and corticorubral excitatory synapses measured by a new iterative single-electrode voltage-clamp method

A new iterative single-electrode voltage clamp method was applied to the measurement of synaptic currents in the red nucleus (RN) neuron of the cat. Voltage clamp was attained within 10 repetitions with great stability and the new algorithm was demonstrated to be superior to the original algorithm of iterative voltage clamp. With a conventional microelectrode, it was possible to measure the synaptic current with the time resolution of 50 microseconds. The synaptic currents evoked by stimulation of the contralateral interpositus nucleus (IP) had time-to-peak ranging from 200 to 540 microseconds and fitted well to alpha functions. Corticorubral (CR) synaptic current was also measured by making use of synaptic plasticity. The stimulation of the ipsilateral cerebral peduncle in cats with chronic lesion of the contralateral IP evoked fast rising EPSPs, as reported previously. The CR-EPSPs with times-to-peak less than 1 ms were subjected to voltage clamp. The CR synaptic currents had times-to-peak ranging from 350 to 880 microseconds. Since most of the interpositorubral (IR) synapses and a part of the CR synapses in IP-lesioned cats are situated on the somatic membrane of RN neurons and some of the CR synaptic currents were as rapid as the IR synaptic currents, the observed synaptic currents evoked by stimulation of the IP and those of the fast-rising CR-EPSPs were taken to originate from the synaptic membrane under space-clamp, i.e. soma. The present study provided additional evidence for the sprouting of the CR fibers as well as the time course of the synaptic current at the dendritic synapses remote from the soma, for the first time.

Inhibitory response in entorhinal and subicular cortices after electrical stimulation of the lateral and basolateral amygdala of the rat

The present study concerns an electrophysiological investigation of the responses of entorhinal and subicular neurons to electrical stimulation of the lateral and basolateral amygdaloid nuclei. All the neurons exhibited suppression of cell firing after the stimulation of these amygdaloid nuclei. Initial excitation following by the suppression characterized 91% of the units and 9% showed initial inhibition. The suppression effect was observed in association with an extracellularly recorded positive deflection. These data suggest that this inhibitory response is mediated by an interneuron.

gamma-Aminobutyric acid (GABA) removal from the synaptic cleft: a postsynaptic event?

In the present commentary we discuss the adequacy of Na+ transport-coupled presynaptic gamma-aminobutyric acid (GABA) uptake systems for the removal of GABA from the synaptic cleft. This discussion is based on the accepted stoichiometry for GABA presynaptic internalization, GABAout + 3Na+out + K+in in equilibrium GABAin + 3Na+in + K+out, on the parameters reported in the literature for typical synaptosomal preparations, and on the assumption that GABA removal must be a quick event (less than or equal to 2 msec), as derived from electrophysiological studies. On these bases, we have developed a calculation in order to evaluate the time course of synaptic cleft GABA removal by presynaptic systems and ended up with an overall value (t approximately 0.3 sec) which does not fit with the data derived from electrophysiological recordings. Moreover, we calculated that if such systems had the function of removing GABA within 2 msec, as it should be, a large depolarization would be brought about in GABAergic boutons, resulting ultimately in further GABA release. These considerations together with biochemical and pharmacological experimental results seem to exclude that presynaptic uptake systems have the function of removing GABA from the synaptic cleft. Our experimental data on the ability of a GABA-acceptive postsynaptic membrane (Deiters' neuron membrane) to transport GABA indicate that this system may have the correct characteristics for removing the neurotransmitter. This refers to both the kinetics and the electrophysiological consequences of the phenomenon.

Evaluation of the electrophysiological consequences of GABA removal from the synaptic cleft by Na+ ion transport-coupled neuronal uptake

The pre- and postsynaptic electrophysiological consequences of a carrier-mediated, Na+ ion transport-coupled removal of gamma-aminobutyric acid (GABA) from the relevant synaptic clefts are discussed. Assuming for the GABA internalization process a stoichiometry like GABAo + 3NA+o + K+i in equilibrium GABAi + 3Na+i + K+o and a synaptic cleft GABA maximal concentration of 100 microM we calculated the presynaptic depolarization associated with GABA removal between 11.5 and 38.2 mV. At the postsynaptic level the effect appears to be less marked.

Intracellular and extracellular electrophysiology of nigral dopaminergic neurons--2. Action potential generating mechanisms and morphological correlates

Intracellular recordings from identified nigral dopamine neurons in the rat revealed that their potentials are composed of four components: (1) a slow depolarization, (2) an initial segment spike, (3) a somatodendritic spike, and (4) an afterhyperpolarization. By combining intracellular and extracellular recording techniques with anatomical studies using intracellular injections of Lucifer yellow, an attempt was made to localize each of these potentials to various neuronal compartments. Lucifer yellow injections demonstrated that the dopamine neurons recorded have a pyramidal or polygonal shaped soma, 12-30 microns in diameter, with 3-6 thick major dendrites which extend 10-50 microns from the soma before bifurcating. The axon appears to rise from a major dendrite 15-30 microns from the soma. Based on this anatomical configuration, results from the electrophysiological studies suggest that: (1) the slow depolarization is a pacemaker-like conductance most likely localized to the somatic region, (2) the initial segment spike is a low-threshold spike probably located at the axon hillock, (3) the somatodendritic spikes are long duration spikes that rapidly inactivate with depolarization, have a high threshold, and are localized to the dendritic regions. The action potential is then terminated by a long duration afterhyperpolarization. Our data further suggest that spike generation may be initiated by a slow depolarization at the soma triggering a spike in the low-threshold axon hillock which then spreads across the already-depolarized soma to trigger the dendritic spike. Based on the above findings, dopamine neurons can be compartmentalized electrophysiologically and morphologically into subcomponents, each associated with spikes and specific ionic currents. The high threshold dendritic component of the action potential demonstrates rapid inactivation with depolarization, and thus occurs over a rather narrow range of membrane polarization. This limited range of action potential generation may be important in control of dendritic dopamine release and/or modulation of electrical coupling between dopaminergic neurons.

The synaptic current evoked in cat spinal motoneurones by impulses in single group 1a axons

Excitatory post-synaptic potentials (e.p.s.p.s) were evoked in motoneurones of anaesthetized cats by impulses in single group 1 a axons. E.p.s.p.s with a time course which indicated a somatic site of origin were voltage-clamped using a single micro-electrode clamp. Excitatory post-synaptic currents (e.p.s.c.s) were found to peak in less than 0.2 ms, and to decay with an exponential time course. The time constant of decay was usually in the range 0.3-0.4 ms (at 37 degrees C). At the resting membrane potential, an e.p.s.p. with a peak of 100 microV was generated by an average peak e.p.s.c. of 330 pA. This corresponded to an average peak conductance increase of 5 nS. The e.p.s.c. decreased with membrane depolarization, and reversed to become an outward current at a null potential of +4.6 +/- 2 mV (+/- S.E. of mean; n = 7). Membrane hyperpolarization caused the peak e.p.s.c. to increase and the time constant of decay of the e.p.s.c. to decrease. The total charge in the synaptic current did not increase with hyperpolarization. This observation can explain earlier observations which showed that the peak amplitude of the e.p.s.p. did not increase with hyperpolarization. The number of ion channels opened by transmitter release at a single somatic bouton was estimated to be in the range 40-240.

Analysis of glycine-activated inhibitory post-synaptic channels in brain-stem neurones of the lamprey

Voltage-clamp techniques were used to measure fluctuations in membrane current produced by the application of glycine to Müller cells in the brain stem of the lamprey. The power density spectrum of the glycine-induced current 'noise' was consistent with the hypothesis that glycine activated a single population of conductance channels with open times determined by first-order kinetics. In normal bathing solution the channel conductance was 73 +/- 12 pS (mean +/- S.D.) and the channel open time 34 +/- 6 msec at 5 degrees C. The reversal potential for the response was 66 +/- 5 mV. Neither channel conductance nor mean open time was voltage-dependent. Replacement of Cl- in the bathing solution by isethionate and sulphate reversibly abolished the response to glycine. Increasing intracellular Cl-, either by using Cl- -filled micropipettes or by raising extracellular K+, decreased channel conductance. This unexpected decrease was a direct effect of intracellular Cl- and was not related to coincident changes in reversal potential. Channel open time was unaffected by intracellular Cl- concentration. Reducing extracellular Cl- concentration from 126.5 to 31 mM reduced channel conductance at all levels of intracellular Cl- without affecting open time. Increasing the temperature of the preparation resulted in increase in channel conductance and a decrease in mean open time. Q10S for the effects were of the order of 1.3 and -2.3 respectively in the range 4-14 degrees C.

Inhibitory conductance changes at synapses in the lamprey brainstem

Although the conductance and kinetic behavior of inhibitory synaptic channels have been studied in a number of nerve and muscle cells, there has been little if any detailed study of such channels at synapses in the vertebrate central nervous system or of the relation of such channels to natural synaptic events. In the experiments reported here, current noise measurements were used to obtain such information at synapses on Müller cells in the lamprey brainstem. Application of glycine to the cells activated synaptic channels with large conductances and relaxation time constants (70 picosiemens and 33 milliseconds, respectively, at 3 degrees to 10 degrees C). Spontaneous inhibitory synaptic currents had a mean conductance of 107 nanosiemens and decayed with the same time constant. In addition, the glycine responses and the spontaneous currents had the same reversal potential and both were abolished by strychnine. These results support the idea that glycine is the natural inhibitory transmitter at these synapses and suggest that one quantum of transmitter activates about 1500 elementary conductance channels.

Synaptic mechanisms and circuitry involved in motoneuron control during sleep

Spontaneous amplitude fluctuations of the brainstem monosynaptic trigeminal jaw-closing reflex were examined in the freely moving chronic cat during wakefulness, quiet sleep, and active sleep. The largest amplitude responses occurred during active wakefulness; they decreased in size during quiet sleep. The lowest amplitude responses occurred during active sleep. A chronic cat preparation was developed in order to record intracellularly from identified trigeminal motoneurons for prolonged period of time throughout the states of sleep and wakefulness. The membrane potential of trigeminal motoneurons exhibited fluctuations that were correlated with changes in the animal's behavioral state. The fundamental pattern consisted of (a) slight hyperpolarization during quiet sleep, compared to arousal or alert wakefulness, (b) little if any hyperpolarization during quiet sleep compared to quiet wakefulness, and (c) dramatic hyperpolarization when active sleep was compared to quiet sleep. Sustained spike activity of trigeminal motoneurons, when present during wakefulness, decreased in frequency or tended to occur in bursts when the animal was in quiet sleep. During active sleep, activity ceased except for a few isolated spikes or short-duration bursts of action potentials. Based on an analysis of antidromically induced spike potentials and monosynaptically induced postsynaptic potentials, it was concluded that postsynaptic inhibition of trigeminal motoneurons during active sleep acts to suppress somatic reflex activity and produce muscular atonia. A companion study of the membrane potential of lumbar motoneurons in the chronic, unanesthetized, undrugged, normally respiring cat was performed during sleep and wakefulness. The antidromic field potential, antidromic and orthodromic spike, EPSP, membrane input resistance, and rheobasic current of lumbar motoneurons were studied during sleep and wakefulness. No change in motoneuron excitability occurred when quiet wakefulness was compared to quiet sleep. Postsynaptic inhibition resulted in decrease in excitability during active sleep. Further phasic decreases in excitability, also due to postsynaptic inhibition, occurred during active sleep in conjunction with clusters of rapid eye movements. The mesencephalon, pons, and medulla were explored in a conditioning-test paradigm in an attempt to find a site where electrical stimulation induced a pattern of somatomotor reflex and motoneuron membrane potential modulation comparable to that which occurs spontaneously during sleep and wakefulness. In unanesthetized, freely moving cats during wakefulness and quiet sleep, electrical stimulation within and in the vicinity of the nucleus pontis oralis produced facilitation of the masseteric reflex, whereas during active sleep the identical stimulus resulted in potent suppression of the reflex.(ABSTRACT TRUNCATED AT 400 WORDS)

An analysis of the epileptogenic potency of CO2+- its ability to induce acute convulsive activity in the isolated frog spinal cord

The action of Co2+ on the isolated frog spinal cord was studied by extracellular application of the ion in the superfusing solution. A complete and reversible blockade of chemical synaptic transmission by Co2+ (3 mmol/l) could be achieved after a superfusion period of 20-30 min. During continued Co2+ application (greater than 60 min) the following effects upon the motoneuron membrane, dorsal root and ventral root fibres were observed. Motoneurons and ventral root fibers: 1. prolongation of initial segment action potential to a maximum of 30 ms, 2. blockade of the long afterhyperpolarization, 3. abolition of adaptation, 4. increased duration of fibre action potential in the ventral root, 5. backfiring after ventral root stimulation. Dorsal root fibres: 1. prolongation of the extraspinal fibre action potential, 2. marked prolongation of the action potential of the terminal region, 3. backfiring of multiple action potentials after dorsal root stimulation. Even in the presence of Co2+, when synaptic transmission was completely blocked, strong convulsive reactions of the isolated spinal cord were observed. Intracellular injection of Co2+ into motoneurons did not affect the action potential, but led to a shift of the EIPSP towards the membrane potential. The results indicate that the induction of convulsive reactions by Co2+ is mainly due to a prolongation of action potentials. The plateau-like deformation of the action potential of the initial segment membrane and presumably of the terminal region of nerve endings results in retrograde propagation of action potentials and in some cases induces oscillatory discharge of single neurons.

Mechanism of inhibition by the amygdala in the lateral hypothalamic area of rats

The inhibition of neuronal activity in the lateral hypothalamus (LHA) of the rat by the basolateral nucleus of the amygdala (AL) was investigated by analyzing evoked potentials, single unit discharges and intracellular synaptic potentials. A single volley to the AL induced a negative-positive-wave in the LHA. The negative-wave threshold was lower than that of the positive wave. Analysis of depth profiles showed that the negative- and positive-waves appeared first at the dorsal margin of the LHA, peaked within the LHA, and were clearly different from each other. The effects of acute lesions showed the negative-wave to be conducted through the direct amygdalo-hypothalamic pathway. The positive-wave: through the stria terminalis. Stimulation of the stria terminalis produced positive evoked potentials with latencies shorter than those of the positive-waves. When conditioning and test stimuli were delivered to the AL, the negative-wave was inhibited for about 90 msec by the evoked positive-wave. Single AL stimuli evoked single unit discharges followed by inhibition of spontaneous firing for about 100 msec. Single stria terminalis stimuli inhibited spontaneous firing for the duration of the positive evoked potential. Intracellular LHA recording during single AL stimuli showed the presence of an EPSP followed by a 100 msec long lasting IPSP. The negative and positive extracellular potentials corresponded to these synaptic potentials. Inward current injection of 1 to 1.4 nA reversed the IPSP's indicating a -15 mV hyperpolarization difference between the IPSP reversal potential and the resting potential in LHA cells. The ionic mechanism of the IPSP is also discussed.

Amygdaloid or cortical facilitation of antidromic activity of trigeminal motoneurons in the rat

1. Electrical stimulation of the rat's contralateral central amygdaloid (CAm) nucleus or the contralateral frontal cortex markedly augmented the antidromic field potential evoked by stimulation of mylohyoid (Myl) nerve. 2. This facilitation was shown to be due to EPSPs of the mylohyoid-anterior digastric (Myl-Dig) motoneurons. 3. In a few motoneurons, cortical EPSPs had fixed short latencies following high-frequency double stimuli and this is believed to be due to a monosynaptic pathway. 4. The amygdaloid or cortically evoked EPSPs relieved IS-SD blockade in a few motoneurons and also facilitated antidromic discharge in others which did not show any IS or M spike response to the same subthreshold antidromic stimulation. The underlying mechanisms are discussed.