Contribution of glia and neurons to the surface-negative potentials of the cerebral cortex during its electrical stimulation

In 20 cats anaesthetized with pentobarbital the suprasylvian gyrus was stimulated by single stimuli or by trains of 50 s stimuli and the potentials from the cortical surface and the intracellular potentials from glial and nerve cells were recorded. Glial cells were identified according to conventional electrophysiological criteria: the absence of action potentials and postsynaptic potentials; slow depolarization in response to electrical stimulation. The slow negativity of direct response to a single stimulus is similar in shape and time course to the depolarization of the cortical glial cells and is unlike the hyperpolarization of the cortical neurons. Quantitative analysis showed that the basic part of the slow negativity is the glial component, whereas the neuronal component—inhibitory postsynaptic potential—plays a much lesser role. The negative shift of the potential on the cortical surface evoked by its high-frequency stimulation is similar in shape and time course to the depolarization shift of the membrane potential of the cortical glial cells (the mean value and standard error of time to peak for glial depolarization were567.6 ± 26.8 ms and427 ± 24 ms for negative shift of potential). (The results are based on recordings from 37 cells.) The negative shift decays much quicker; it is not similar in shape and time course to the hyperpolarization shift of the neuronal membrane potentials (the mean value and standard error of time to peak for inhibitory postsynaptic potential was44.9 ± 4.5 ms). According to the quantitative analysis, the negative shift of the potential reflects mainly the depolarization of the cortical glial cells. The contribution of the hyperpolarization of neurons to the surface-negative shift can be distinctly observed during the first 0.2–0.3 s of stimulation.

It is supposed that accumulation of K⁺ ions in intercellular clefts results in depolarization of glial syncytium, which is reflected on the cortical surface as a slow negativity and a negative shift of the potential.     

Depolarization of cortical glial cells in response to electrical stimulation of the cortical surface

The responses of 155 neurones and 91 glial cells to the electrical stimulation of the cortex were recorded in the suprasylvian gyrus of 20 cats under pentobarbital anaesthesia. Glial cells were identified by electrophysiological criteria: absence of action potentials and postsynaptic potentials; high membrane potential; slow depolarization during the electrical stimulation of the cortex. 50 glial cells showed membrane potentials between 80 and 100 mV. Stimuli of low intensity which evoked only excitatory postsynaptic potentials of apical dendrites, the so-called dendritic potentials, failed to evoke glial depolarization. However, glial depolarization could be elicited at high-frequency stimulation. Stimuli, which evoked not only the dendritic potential but also subsequent slow negativity, could usually bring about glial depolarization too. The amplitude of glial depolarization in response to one stimulus did not exceed 2 mV, the latency being 3–5 ms. A phenomenon of decrementai summation of glial depolarization was observed. The stronger and more frequent the stimulation, the larger was glial depolarization. However, at frequencies over 50/s glial depolarization decay was observed already during the stimulation and in some cases, membrane potential was drastically reduced to zero. After cessation of stimulation, glial depolarization decayed exponentially in 3–4 s; in some cases the decay was prolonged up to 10s and slow irregular fluctuations of the membrane potential were recorded; at the same time, spikes of the neighbouring neurone could be recorded from the glial cell. With a decrease of the membrane potential glial depolarization was attenuated, but it could be elicited even at membrane potential below 20 mV.The results are interpreted in relation to the potassium ion hypothesis. It is suggested that glial depolarization is determined by release of K⁺, which is associated with excitation of non-myelinated fibres and with excitatory postsynaptic potentials generated in the cortical neuropile. Significant increases in the concentration of extracellular potassium ions could provoke actual movement of glial cells. It is supposed that glial depolarization of small magnitude which is recorded occasionally at the membrane potential below 30 mV is the result of electronic spread of glial depolarization from the neighbouring glial cells.     

Na(+) and K(+) concentrations, extra- and intracellular voltages, and the effect of TTX in hypoxic rat hippocampal slices

Severe hypoxia causes rapid depolarization of CA1 neurons and glial cells that resembles spreading depression (SD). In brain slices in vitro, the SD-like depolarization and the associated irreversible loss of function can be postponed, but not prevented, by blockade of Na(+) currents by tetrodotoxin (TTX). To investigate the role of Na(+) flux, we made recordings from the CA1 region in hippocampal slices in the presence and absence of TTX. We measured membrane changes in single CA1 pyramidal neurons simultaneously with extracellular DC potential (V(o)) and either extracellular [K(+)] or [Na(+)]; alternatively, we simultaneously recorded [Na(+)](o), [K(+)](o), and V(o). Confirming previous reports, early during hypoxia, before SD onset, [K(+)](o) began to rise, whereas [Na(+)](o) still remained normal and V(o) showed a slight, gradual, negative shift; neurons first hyperpolarized and then began to gradually depolarize. The SD-like abrupt negative DeltaV(o) corresponded to a near complete depolarization of pyramidal neurons and an 89% decrease in input resistance. [K(+)](o) increased by 47 mM and [Na(+)](o) dropped by 91 mM. Changes in intracellular Na(+) and K(+) concentrations, estimated on the basis of the measured extracellular ion levels and the relative volume fractions of the neuronal, glial, and extracellular compartment, were much more moderate. Because [Na(+)](o) dropped more than [K(+)](o) increased, simple exchange of Na(+) for K(+) cannot account for these ionic changes. The apparent imbalance of charge could be made up by Cl(-) influx into neurons paralleling Na(+) flux and release of Mg(2+) from cells. The hypoxia-induced changes in interneurons resembled those observed in pyramidal neurons. Astrocytes responded with an initial slow depolarization as [K(+)](o) rose. It was followed by a rapid but incomplete depolarization as soon as SD occurred, which could be accounted for by the reduced ratio, [K(+)](i)/[K(+)](o). TTX (1 microM) markedly postponed SD, but the SD-related changes in [K(+)](o) and [Na(+)](o) were only reduced by 23 and 12%, respectively. In TTX-treated pyramidal neurons, the delayed SD-like depolarization took off from a more positive level, but the final depolarized intracellular potential and input resistance were not different from control. We conclude that TTX-sensitive channels mediate only a fraction of the Na(+) influx, and that some of the K(+) is released in exchange for Na(+). Even though TTX-sensitive Na(+) currents are not essential for the self-regenerative membrane changes during hypoxic SD, in control solutions their activation may trigger the transition from gradual to rapid depolarization of neurons, thereby synchronizing the SD-like event.     

Patch-clamp study of postnatal development of CA1 neurons in rat hippocampal slices: membrane excitability and K+ currents.

1. The postnatal development of membrane properties and outward K+ currents in CA1 neurons in rat hippocampal slices was studied with the use of whole-cell patch-clamp techniques. 2. Neurons at all postnatal ages (2-30 days; P2-30) were capable of generating tetrodotoxin (TTX)-sensitive action potentials in response to intracellular injection of depolarizing current pulses. There was a gradual increase in the amplitude and a decrease in the duration of these action potentials with age. Stable values for spike duration were reached by P15, whereas spike amplitude increased until P20-25. In P2-5 neurons, the duration of action potentials was greatly prolonged by depolarization from the resting membrane potential, indicating a weak spike repolarizing mechanism at depolarized potentials. In contrast, the duration of spikes evoked in P20-30 neurons was not affected by similar changes in the membrane potential. 3. Application of tetraethylammonium (TEA, 10 mM) had no effect on the duration of spikes in P3-5 neurons, whereas application of 4-aminopyridine (4-AP, 2 mM) produced large increases in spike duration. In contrast, the duration of spikes in P26 neurons was greatly increased after TEA application, whereas 4-AP had smaller effects on spike duration in these neurons. 4. The input resistance and membrane time constant decreased with age from P2 to P15. The values for both parameters were considerably greater than those reported with conventional intracellular recording electrodes in the immature hippocampus. The resting membrane potential became more hyperpolarized with age. When the recording pipettes contained KCl (140 mM), the resting potential of P3-4 neurons was 34 mV depolarized compared with resting potentials observed with potassium gluconate-filled pipettes. Only a 13-mV change in resting potential was observed during similar comparisons in P27-28 neurons. 5. Outward currents activated by depolarization were examined with the use of voltage-clamp techniques in P2-30 neurons. In P2-5 cells, a small, slowly inactivating outward current was evoked with depolarizing commands from holding potentials near -50 mV. By preceding the depolarizing commands with a hyperpolarizing prepulse, an additional early transient outward current was evoked. The sustained and transient outward currents were separated by their kinetic properties and their sensitivity to cobalt (Co2+), TEA, and 4-AP.(ABSTRACT TRUNCATED AT 400 WORDS)     

A novel method for recording neuronal depolarization with recording at 125–825 Hz: implications for imaging fast neural activity in the brain with electrical impedance tomography

Electrical impedance tomography (EIT) is a recently developed medical imaging method which has the potential to produce images of fast neuronal depolarization in the brain. Previous modelling suggested that applied current needed to be below 100 Hz but the signal-to-noise ratio (SNR) recorded with scalp electrodes during evoked responses was too low to permit imaging. A novel method in which contemporaneous evoked potentials are subtracted is presented with current applied at 225 Hz to cerebral cortex during evoked activity; although the signal is smaller than at DC by about 10×, the principal noise from the EEG is reduced by about 1000×, resulting in an improved SNR. It was validated with recording of compound action potentials in crab walking leg nerve where peak changes of −0.2% at 125 and 175 Hz tallied with biophysical modelling. In recording from rat cerebral cortex during somatosensory evoked responses, peak impedance decreases of −0.07 ± 0.006% (mean ± SE) with a SNR of >50 could be recorded at 225 Hz. This method provides a reproducible and artefact free means for recording resistance changes during neuronal activity which could form the basis for imaging fast neural activity in the brain.     

Temperature effects on neuronal membrane potentials and inward currents in rat hypothalamic tissue slices

Preoptic–anterior hypothalamic (PO/AH) neurones sense and regulate body temperature. Although controversial, it has been postulated that warm-induced depolarization determines neuronal thermosensitivity. Supporting this hypothesis, recent studies suggest that temperature-sensitive cationic channels (e.g. vanilloid receptor TRP channels) constitute the underlying mechanism of neuronal thermosensitivity. Moreover, earlier studies indicated that PO/AH neuronal warm sensitivity is due to depolarizing sodium currents that are sensitive to tetrodotoxin (TTX). To test these possibilities, intracellular recordings were made in rat hypothalamic tissue slices. Thermal effects on membrane potentials and currents were compared in PO/AH warm-sensitive, temperature-insensitive and silent neurones. All three types of neurones displayed slight depolarization during warming and hyperpolarization during cooling. There were no significant differences in membrane potential thermosensitivity for the different neuronal types. Voltage clamp recordings (at −92 mV) measured the thermal effects on persistent inward cationic currents. In all neurones, resting holding currents decreased during cooling and increased during warming, and there was no correlation between firing rate thermosensitivity and current thermosensitivity. To determine the thermosensitive contribution of persistent, TTX-sensitive currents, voltage clamp recordings were conducted in the presence of 0.5 μ m TTX. TTX decreased the current thermosensitivity in most neurones, but there were no resulting differences between the different neuronal types. The present study found no evidence of a resting ionic current that is unique to warm-sensitive neurones. This supports studies suggesting that neuronal thermosensitivity is controlled, not by resting currents, but rather by currents that determine rapid changes in membrane potential between successive action potentials.     

Sodium influx blockade and hypoxic damage to CA1 pyramidal neurons in rat hippocampal slices.

We studied the effects of lidocaine and tetrodotoxin (TTX) on hypoxic changes in CA1 pyramidal neurons to examine the ionic basis of neuronal damage. Lidocaine (10 and 100 microM) and TTX (6 and 63 nM) delayed and attenuated the hypoxic depolarization and improved recovery of the resting and action potentials after 10 min of hypoxia. Lidocaine (10 and 100 microM) and TTX (63 nM) reduced the number of morphologically damaged CA1 cells and improved protein synthesis measured after 10 min hypoxia. Lidocaine (10 microM) attenuated the increase in intracellular sodium (181 vs. 218%) and the depolarization (-21 vs. -1 mV) during hypoxia but did not significantly attenuate the changes in ATP, potassium, or calcium measured at 10 min of hypoxia. Lidocaine (100 microM) attenuated the changes in membrane potential, sodium, potassium, ATP, and calcium during hypoxia. TTX (63 nM) attenuated the changes in membrane potential (-36 vs. -1 mV), sodium (179 vs. 226%), potassium (78 vs. 50%), and ATP (24 vs. 11%) but did not significantly attenuate the increase in calcium during hypoxia. These data indicate that the primary blockade of sodium channels can secondarily alter other cellular parameters. The hypoxic depolarization and the increase in intracellular sodium appear to be important triggers of hypoxic damage independent of their effect on cytosolic calcium; a treatment that selectively blocked sodium influx (lidocaine 10 microM) improved recovery. Our data indicate that selective blockade of sodium channels with a low concentration of lidocaine or TTX improves recovery after hypoxia by attenuating the rise in cellular sodium and the hypoxic depolarization. This blockade improves the resting and action potentials, histologic state, and protein synthesis of CA1 pyramidal neurons after 10 min of hypoxia to rat hippocampal slices. A higher concentration of lidocaine, which also improved ATP, potassium, and calcium concentrations during hypoxia was more potent. In conclusion, the depolarization and increased sodium concentration during hypoxia account for a portion of the neuronal damage after hypoxia independent of changes in calcium.     

Norepinephrine selectively reduces slow Ca2+- and Na+-mediated K+ currents in cat neocortical neurons

1. The effects of norepinephrine (NE) and related agonists and antagonists were examined on large neurons from layer V of cat sensorimotor cortex ("Betz cells") were examined in a brain slice preparation using intracellular recording, constant current stimulation and single microelectrode voltage clamp. 2. Application of NE (0.1-100 microM) usually caused a small depolarization from resting potential; hyperpolarizations were rare. Application of NE reversibly reduced rheobase and both the Ca2+- and Na+-dependent portions of the slow afterhyperpolarization (sAHP) that followed sustained firing evoked by constant current injection. The faster Ca2+-dependent medium afterhyperpolarization (mAHP), the fast afterhyperpolarization (fAHP), the action potential, and input resistance were unaffected. 3. The changes in excitability produced by NE application were most apparent during prolonged stimulation. The cells exhibited steady repetitive firing to currents that were formerly ineffective. The slow phase of spike frequency adaptation was reduced selectively and less habituation occurred during repeated long-lasting stimuli. The relation between firing rate and injected current became steeper if firing rate was averaged over several hundred milliseconds. 4. During voltage clamp in TTX, NE application selectively reduced the slow component of Ca2+-mediated K+ current. The faster Ca2+-mediated K+ current was unaffected, as were two voltage-dependent, transient K+ currents, the anomalous rectifier and leakage conductance measured at resting potential. Depolarizing voltage steps in the presence of Cd2+ revealed an apparent time- and voltage-dependent increase of the persistent Na+ current after NE application. The voltage-clamp results suggested ionic mechanisms for all effects seen during constant current stimulation except the depolarization from resting potential. The latter was insensitive to Cd2+ and TTX and occurred without a detectable change in membrane conductance. 5. NE application did not alter Ca2+ spikes evoked in the presence of TTX and 10 mM TEA. Inward Ca2+ currents examined during voltage clamp in TTX (with K+ currents reduced) became slightly larger after NE application. We conclude that NEs reduction of the slow Ca2+-mediated K+ current is not caused by reduction of Ca2+ influx. 6. Effects on membrane potential, rheobase, and the sAHP were mimicked by the beta-adrenergic agonist isoproterenol, but not by the alpha-adrenergic agonists clonidine or phenylephrine at higher concentrations.(ABSTRACT TRUNCATED AT 400 WORDS)     

Tetrodotoxin induced calcium spikes: in vitro and in vivo studies of normal and deafferented Purkinje cells

Summary Tetrodotoxin (TTX) is widely used to block the sodium dependent action potential in excitable cells to study their other ionic properties. TTX applied outside, selectively blocks voltage dependent sodium channels and is thought to have no other effects. We report here that TTX, applied to slices of rat cerebellum, suppressed sodium spikes of the Purkinje cells and induced firing in bursts of slower spikes. This activity was blocked by cobalt (2 mM) or cadmium (0.2 mM) in the medium as well as by hyperpolarizing currents showing that the slow spikes were due to voltage dependent calcium channels. The membrane potential was not significantly changed by TTX and the spikes during the bursts had the same threshold potentials and peak spike amplitudes as the voltage and Ca²⁺ dependent dendritic spikes evoked by injected current before adding TTX. This indicated that no marked changes in the membrane conductances were produced by the TTX. Unlike the burst firing induced by removing extracellular sodium, the TTX induced bursts were not followed by a large hyperpolarization. The same kind of results were obtained with extracellular recording in the in-vivo preparation with TTX applied topically or by pressure near the recording sites. TTX induced burst firing was not due to blocking afferent inhibitory input to the PC, since bicuculline (10^-6 M) applied without TTX, produced only increased firing of fast action potentials and no bursts. The bursts could be arrested within 1 to 2 min by intravenously administering 2 mg/kg sodium pentobarbital, the blockage lasted from 5 to 15 min. These effects of TTX were not due to a contaminant as TTX from two different suppliers produced the same effects. A possible mechanism based on a decrease of intracellular free sodium is discussed.     

Mechanical and electrical responses modulated by excitation of inhibitory nerves during stimulation with high-potassium solutions in circular smooth muscle of the rabbit rectum.

The properties of the mechanical responses produced by solutions containing high concentrations of potassium ion (high-K solution, [K(+)](o) = 9-27 mM) were investigated in circular smooth muscle preparations isolated from the rabbit rectum. Isometric recording of mechanical responses of the muscle revealed spontaneous contractions, which successively decreased and finally disappeared in most preparations. Stimulation of the smooth muscle with high-K solutions elicited an increase in both amplitude and frequency of twitch contractions (sustained component), with about a 2 min delay in the beginning (initial inhibition), and a transient large contraction shortly after the cessation of stimulation (after contraction). Transmural nerve stimulation (TNS) with electrical pulses for 1 min at 1 Hz frequency produced a sustained inhibition, but a transient contraction followed after termination of TNS. In the presence of tetrodotoxin (TTX), the TNS-induced responses were abolished, while a high-K solution elicited increased twitch contractions with a short delay and abolished the after contraction. Suramin produced effects similar to TTX on the responses produced by high-K solutions or TNS, but this was not the case for atropine, guanethidine or N(omega)-nitro-L-arginine (L-NA). Recording membrane potentials with microelectrodes revealed that TNS evoked an inhibitory junction potential (i.j.p.) which was non-adrenergic, non-cholinergic and non-nitrergic in nature. High-K solutions elicited a tri-phasic change in the membrane potential; an initial hyperpolarization, followed by a sustained depolarization and finally a transient depolarization on cessation of high-K stimulation. TTX or suramin inhibited the i.j.p.s and altered the tri-phasic change in the membrane potential produced by a high-K solution to a mono-phasic depolarization. No significant modulation of electrical responses of the membrane induced by TNS or high-K solution was elicited by atropine, guanethidine or L-NA. The results indicated that the circular smooth muscle of the rabbit rectum is innervated by inhibitory nerves, and that stimulation with high-K solutions caused inhibitory neuronal modulation of both electrical and mechanical responses of the smooth muscle, in a suramin-sensitive way.     

Longitudinal depolarization gradients along the somatodendritic axis of CA1 pyramidal cells: a novel feature of spreading depression

We studied the subcellular correlates of spreading depression (SD) in the CA1 rat hippocampus by combining intrasomatic and intradendritic recordings of pyramidal cells with extracellular DC and evoked field and unitary activity. The results demonstrate that during SD only specific parts of the dendritic membranes are deeply depolarized and electrically shunted. Somatic impalements yielded near-zero membrane potential (V(m)) and maximum decrease of input resistance (R(in)) whether the accompanying extracellular negative potential (V(o)) moved along the basal, the apical or both dendritic arbors. However, apical intradendritic recordings showed a different course of local V(m) that is hardly detected from the soma. A decreasing depolarization gradient was observed from the edge of SD-affected fully depolarized subcellular regions toward distal dendrites. Within apical dendrites, the depolarizing front moved toward and stopped at proximal dendrites during the time course of SD so that distal dendrites had repolarized in part or in full by the end of the wave. The drop of local R(in) was initially maximal at any somatodendritic loci and also recovered partially before the end of SD. This recovery was stronger and faster in far dendrites and is best explained by a wave-like somatopetal closure of membrane conductances. Cell subregions far from SD-affected membranes remain electrically excitable and show evoked unitary and field activity. We propose that neuronal depolarization during SD is caused by current flow through extended but discrete patches of shunted membranes driven by uneven longitudinal depolarization.     

Mechanisms of endothelin-induced augmentation of the electrical and mechanical activity in rat portal vein

Actions of porcine endothelin (ET) on the electrical and mechanical activity of the rat portal vein were investigated by means of the intracellular microelectrode and isometric tension recording techniques, ET (> 0.1 nM) enhanced the amplitude and frequency of the spontaneous contractions which ceased in the presence of 100 nM dihydropyridine derivatives (nifedipine or nicardipine). ET (0.15 nM) increased the frequency of the spontaneous action potentials, with no change in the basal membrane potential. Higher concentrations of ET (≧ 0.3 nM) further depolarized the membrane potential and increased the spike frequency. After blocking the spontaneous action potentials with nifedipine (100 nM), ET still depolarized the membrane. The depolarization was associated with a reduction in the electrotonic potential and was blocked in a Na-deficient solution (15.5 mM) but not in Ca-free, K-deficient or Cl-deficient solutions. In a Na-deficient solution, ET still evoked action potentials without depolarization. In Ca-free solution, ET depolarized the membrane potential with small oscillations, which were blocked by nifedipine (100 nM). The results indicate that in the rat portal vein, ET enhances electrical and mechanical responses through activation of the dihydropyridine-sensitive and voltage-dependent Ca channels. Acceleration of the Ca entry induced by ET can occur with or without depolarization of the membrane and can enhance the pacemaking mechanism.     

Synaptic and intrinsic responses of medical entorhinal cortical cells in normal and magnesium-free medium in vitro

1. Extracellular recordings were made from slices of hippocampus plus parahippocampal regions maintained in vitro. Field potentials, recorded in the entorhinal cortex after stimulation in the subiculum, resembled those observed in vivo. 2. Washout of magnesium from the slices resulted in paroxysmal events which resembled those occurring during sustained seizures in vivo. These events were greatest in amplitude and duration in layers IV/V of the medial entorhinal cortex and could occur both spontaneously and in response to subicular stimulation. Spontaneous seizure-like events were not prevented by severing the connections between the hippocampus and entorhinal cortex, but much smaller and shorter events occurring in the dentate gyrus were stopped by this manipulation. Both spontaneous and evoked paroxysmal events were blocked by perfusion with the N-methyl-D-aspartate (NMDA) receptor antagonist, DL-2-amino-5-phosphonovalerate (2-AP5). 3. Neurons in layers IV/V were characterized by intracellular recording. Injection of depolarizing current in most cells evoked a train of nondecrementing action potentials with only weak spike frequency accommodation and little or no posttrain after hyperpolarization. 4. A small number of cells displayed burst response when depolarized by positive current. The burst consisted of a slow depolarization with superimposed action potentials which decreased in amplitude and increased in duration during the discharge. The burst was terminated by a strong after hyperpolarization and thereafter, during prolonged current pulses a train of nondecrementing spikes occurred. The burst response remained if the cell was held at hyperpolarized levels but was inactivated by holding the cell at a depolarized level. 5. Depolarizing synaptic potentials could be evoked by stimulation in the subiculum. A delayed and prolonged depolarization clearly decremented with membrane hyperpolarization and, occasionally, increased with depolarization. 6. Washout of magnesium from the slices resulted in an enhancement of the late depolarization and a reversal of its voltage dependence. Eventually a single shock to the subiculum evoked a large all-or-none paroxysmal depolarization associated with a massive increase in membrane conductance. Similar events occurred spontaneously in all cells tested. The paroxysmal depolarizations, both spontaneous and evoked, were rapidly blocked by 2-AP5. 7. It is concluded that medial entorhinal cortical cells possess several intrinsic and synaptic properties which confer an extreme susceptibility to generation of sustained seizure activity.(ABSTRACT TRUNCATED AT 400 WORDS)     

Membrane capacitance of cortical neurons and glia during sleep oscillations and spike-wave seizures

Dual intracellular recordings in vivo were used to disclose relationships between cortical neurons and glia during spontaneous slow (<1 Hz) sleep oscillations and spike-wave (SW) seizures in cat. Glial cells displayed a slow membrane potential oscillation (<1 Hz), in close synchrony with cortical neurons. In glia, each cycle of this oscillation was made of a round depolarizing potential of 1.5-3 mV. The depolarizing slope corresponded to a steady depolarization and sustained synaptic activity in neurons (duration, 0.5-0.8 s). The repolarization of the glial membrane (duration, 0.5-0.8 s) coincided with neuronal hyperpolarization, associated with disfacilitation, and suppressed synaptic activity in cortical networks. SW seizures in glial cells displayed phasic events, synchronized with neuronal paroxysmal potentials, superimposed on a plateau of depolarization, that lasted for the duration of the seizure. Measurements of the neuronal membrane capacitance during slow oscillating patterns showed small fluctuations around the resting values in relation to the phases of the slow oscillation. In contrast, the glial capacitance displayed a small-amplitude oscillation of 1-2 Hz, independent of phasic sleep and seizure activity. Additionally, in both cell types, SW seizures were associated with a modulatory, slower oscillation ( approximately 0.2 Hz) and a persistent increase of capacitance, developing in parallel with the progression of the seizure. These capacitance variations were dependent on the severity of the seizure and the distance between the presumed seizure focus and the recording site. We suggest that the capacitance variations may reflect changes in the membrane surface area (swelling) and/or of the interglial communication via gap junctions, which may affect the synchronization and propagation of paroxysmal activities.     

Action potentials in the rat chromaffin cell and effects of acetylcholine.

1. Electrophysiological properties of the rat chromaffin cell were studied using intracellular recording techniques. 2. The resting potential in the chromaffin cell was -49 +/- 6 mV (mean +/- S.D., n = 14) in standard saline containing 10 mM-Ca whereas that in Na-free saline was -63 +/- 9 mV (n = 17). At rest, the membrane has a substantial Na permeability. 3. Action potentials were evoked by passing current through the recording electrode. In standard saline the major fraction of the action potential disappeared either upon omission of external Na ions from standard saline or addition of 1 muM tetrodotoxin (TTX). We conclude that action potentials in the chromaffin cell are due mainly to an increase in the permeability of the membrane to Na ions. 4. Small but significant regenerative action potentials were observed in Na-free saline, and when Ca in Na-free saline was replaced by Ba, prolonged action potentials occurred. We conclude that action potentials in the chromaffin cell also have a Ca component. 5. Iontophoretic application of acetylcholine (ACh) produced a transient membrane depolarization in standard saline. 6. Spontaneous action potentials were recorded extracellularly by microsuction electrodes. They occurred at a rate of 0-05-0-1/sec in almost all cells. 7. When the perfusion fluid contained 3 x 10(-7) M to 10(-4) M ACh the spike frequency increased up to about 2/sec. This stimulatory effect of ACh was blocked by 10(-7) M atropine but not by 10(-3) M hexamethonium nor by 10(-5) M-d-tubocurarine. 8. The importance of Ca entry during action potentials for catecholamine secretion is discussed     

Mechanisms regulating the activity of facial nucleus motoneurones--2. Synaptic activation from the caudal trigeminal nucleus

Field and postsynaptic potentials of facial motoneurones evoked by stimulation of the caudal trigeminal nucleus were studied in cats by means of extra- and intracellular recording. Mono- and polysynaptic input onto facial motoneurones from the caudal trigeminal nucleus were shown. Four types of responses were distinguished: excitatory postsynaptic potentials generating a single action potential; a gradual shift of depolarization inducing multiple discharges; a rhythmic discharge of action potentials appearing at a low level of depolarization; excitatory postsynaptic potentials or a sequence of excitatory and inhibitory postsynaptic potentials. Multiple discharge was shown to appear as a result of effective summation of high frequency excitatory influences from efferent neurones of the caudal trigeminal nucleus projecting into the facial nucleus. Factors facilitating the development of gradual depolarization are: dendritic localization of synaptic terminals, dendritic origin of after-depolarizing processes and the high input resistance of the facial motoneurone membrane. It is thought that specific features of facial motoneurones and properties of afferent inputs are supposed to provide high sensitivity of neuronal organization of the facial nucleus to afferent signals as well as wide diversity in controlling its activity.     

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.     

Hypoxia-activated Ca2+ currents in pacemaker neurones of rat rostral ventrolateral medulla in vitro.

We examined the effects of brief periods of hypoxia or application of cyanide on the discharge and membrane properties of medullary pacemaker neurones in slices of the rostral ventrolateral reticular nucleus (RVL) of the medulla oblongata of rats. Stable intracellular recordings were obtained from seventy-nine neurones within the RVL which exhibited spontaneous rhythmic discharge in the absence of excitatory postsynaptic potentials (EPSPs). The membrane potential cycles of these neurones could be reset with an evoked spike without eliciting EPSPs or inhibitory postsynaptic potentials and hence met criteria of RVL pacemaker neurones. Hypoxia, produced by reducing O2 from 95 to 20% for 40 s or exposure to cyanide (30-300 microM for 40 s), reversibly increased neuronal discharge 1.6-fold (20% O2) or 2.6-fold (300 microM cyanide), respectively, in association with membrane depolarization and a significant fall in membrane resistance. The membrane responses to hypoxia and cyanide were observed in the presence of tetrodotoxin (TTX) at a concentration (10 microM) which eliminated spontaneous spikes or spikes evoked by intracellular depolarization. When recorded at a holding potential of -70 mV by single-electrode voltage clamp, hypoxia or cyanide (300 microM) elicited inward currents of 0.44 +/- 0.06 and 0.58 +/- 0.08 nA, respectively, which are attenuated by reducing the concentration of extracellular Ca2+ ions, and abolished by 2 mM CoCl2 and 100 microM NiCl2, but not affected by 50 microM CdCl2, replacement of 83% extracellular Na+, or adenosine deaminase (2U ml-1). We conclude that hypoxia and cyanide directly excite RVL pacemaker neurones in vitro by a common mechanism: activation of Ca2+ channel conductance.     

Dendritic Ca2+ transients evoked by action potentials in rat dorsal cochlear nucleus pyramidal and cartwheel neurons

Simultaneous fluorescence imaging and electrophysiologic recordings were used to investigate the Ca(2+) influx initiated by action potentials (APs) into dorsal cochlear nucleus (DCN) pyramidal cell (PC) and cartwheel cell (CWC) dendrites. Local application of Cd(2+) blocked Ca(2+) transients in PC and CWC dendrites, demonstrating that the Ca(2+) influx was initiated by dendritic Ca(2+) channels. In PCs, TTX eliminated the dendritic Ca(2+) transients when APs were completely blocked. However, the Ca(2+) influx could be partially recovered during an incomplete block of APs or when a large depolarization was substituted for the blocked APs. In CWCs, dendritic Ca(2+) transients evoked by individual APs, or simple spikes, were blocked by TTX and could be recovered during an incomplete block of APs or by a large depolarization. In contrast, dendritic Ca(2+) transients evoked by complex spikes, a burst of APs superimposed on a slow depolarization, were not blocked by TTX, despite eliminating the APs superimposed on the slow depolarization. These results suggest two different mechanisms for the retrograde activation of dendritic Ca(2+) channels: the first requires fast Na(+) channel-mediated APs or a large somatic depolarization, whereas the second is independent of Na(+) channel activation, requiring only the slow depolarization underlying complex spikes.     

Synaptic and intrinsic control of membrane excitability of neostriatal neurons. I. An in vivo analysis

1. The relationship between membrane properties of neostriatal neurons and spontaneous and evoked synaptic potentials was studied with the use of intracellular recordings from anesthetized rats. Most of these neurons showed regular or irregular spontaneous depolarizing potentials that only in a few cases triggered action potentials at resting level. 2. The stimulation of the ipsilateral substantia nigra or of the sensorimotor cortex produced a relatively fast depolarizing post-synaptic potential (EPSP). In some cells this potential was followed by an inhibitory period that appeared as an hyperpolarization when the cell was depolarized from the resting level (inhibitory postsynaptic potential, IPSP). A late and long-lasting depolarization (LD) followed the EPSP or the EPSP-IPSP sequence. 3. Repetitive discharge with little adaptation was observed during direct depolarization. Most of the neurons tested for current-voltage (I-V) relationship showed nonlinearity of the input resistance in the hyperpolarizing direction. Spontaneous and evoked EPSPs were decreased in their amplitude and duration when the membrane potential was held at levels more hyperpolarized than -85 mV because of the strong rectification at these levels of hyperpolarization. 4. Local microiontophoretic application of bicuculline (BIC) or systemic administration of BIC and pentylenetetrazole (PTZ) produced a reduction of the IPSPs. The reduction of the inhibitory transmission caused a strong increase of the LD. The current-evoked firing pattern was not greatly altered. 5. The intracellular application of cesium increased the amplitude and the duration of the spontaneous depolarizations that triggered bursts of action potentials under this condition. Spikes were broadened and the rectification in the hyperpolarization direction was reduced. 6. Iontophoretically applied cadmium strongly depressed the amplitude of the spontaneous and evoked postsynaptic potentials. During cadmium application, nigral stimulation produced constant latency, all-or-none spikes in the absence of any synaptic potential. 7. Repetitive stimulation of the ipsilateral substantia nigra by electrical shocks (5 Hz, 25 s) produced a progressive and reversible decrease of the spontaneous depolarizing potentials (SDPs) and a decrease of the firing rate. In the same cells, when the train of stimulation was delivered in the ipsilateral cortex, a membrane depolarization coupled with an increase of the firing rate was observed. 8. We conclude that although synaptic circuits mediate a phasic inhibition in neostriatum, the low level of spontaneous firing of most neostriatal neurons is mainly because of the effects that membrane properties exert on the spontaneous and the evoked synaptic depolarizations in the striatum.(ABSTRACT TRUNCATED AT 400 WORDS)