Presynaptic and long-lasting postsynaptic inhibition during penicillin-induced spinal seizures.

In experiments on seven cats we tested the hypothesis that the epileptogenic effect of penicillin (PCN) on the spinal cord is mediated by a reduction of presynaptic inhibition. PCN-induced spinal hyperactivity was not associated with changes in either the presynaptic inhibition of extensor monosynaptic reflexes by conditioning volleys in flexor muscle nerves, or in evoked dorsal root potentials. Long-lasting inhibition of monosynaptic reflexes by repetitive cutaneous nerve volleys, shown by intracellular recording to be associated with prolonged inhibitory postsynaptic potentials (IPSPs), was also not changed by PCN. Antagonism of either pre- or postsynaptic spinal inhibition is not a necessary condition for induction of spinal seizures by PCN.

Effects of intracellular calcium chelation on voltage-dependent and calcium-dependent currents in cat neocortical neurons.

Large neurons from layer V in a slice preparation of cat sensorimotor cortex were impaled with microelectrodes containing KCl plus different concentrations of the Ca2+ chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid (BAPTA) or two of its derivatives. Impalement with electrodes containing high BAPTA (200 mM) quickly abolished Ca(2+)-dependent afterhyperpolarizations. Spike parameters were normal, but the usual time- and voltage-dependent rectification of subthreshold membrane potential was absent. Normally, this rectification results from activation of two voltage-gated currents, the persistent sodium current (INaP) and the hyperpolarizing inward rectifier current (Ih). Both of these currents were absent during voltage clamp with high BAPTA microelectrodes. Impalement with electrodes containing low BAPTA (2 mM) or derivatives caused a different effect. Injection of a 1-s current pulse evoked phasic firing instead of the tonic firing seen normally. Both the amplitude and the duration of the Ca(2+)-dependent afterhypolarization that followed repetitive firing were much greater than normal. The effectiveness of BAPTA derivatives in altering afterhyperpolarizations and firing properties were similar to their effectiveness in chelating Ca2+. It is assumed that the BAPTA effects result from reduction of intracellular Ca2+ concentration. Results with high BAPTA suggest that (i) both INaP and Ih require a minimal intracellular calcium concentration for normal expression, and that (ii) these voltage-gated currents may be modulated by changes in intracellular calcium concentration. Results with low BAPTA suggest that a small reduction of intracellular calcium concentration preferentially enhances a slow, Ca(2+)-dependent K+ current which then dominates the firing properties of the cell. The transformed firing properties resemble those of hippocampal pyramidal neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

A baseline stabilizer for electrophysiology.

We describe a circuit that compensates for a slowly drifting baseline without distorting the signal of interest. Implementation of this non-linear filter is quite simple; total cost of components is less than $20.00. The baseline stabilizer should be especially useful for intracellular recording and for recording evoked surface potentials.

Voltage clamp study of cat spinal motoneurons during strychnine-induced seizures.

Cat spinal motoneurons were examined by the technique of somatic voltage clamp during strychnine-induced spinal seizures. No clear alteration of voltage-dependent ionic currents was required in order for typical strychnine-induced paroxysmal depolarization shifts (PDSs) to develop in contrast to results previously obtained during penicillin-induced spinal seizures. Voltage clamp of evoked and spontaneous PDSs indicates these are generated by a synchronized mixture of excitatory and inhibitory synaptic currents wih excitation predominating.

Negative slope conductance at large depolarizations in cat spinal motoneurons.

The current-voltage relation of cat spinal motoneurons obtained by somatic voltage clamp exhibits a region of negative slope conductance at large depolarizations in addition to the previously reported N-shaped region at small depolarizations. The N shape at large depolarizations is caused by one outward current component which grows larger and then smaller with depolarization.

Electrical properties of facial motoneurons in brainstem slices from guinea pig.

Electrical properties of guinea pig facial motoneurons (FMNs) were studied in a brainstem slice preparation. FMNs were identified histologically and by antidromic activation. They displayed time-varying responses and inward rectification during both subthreshold depolarization and hyperpolarization. The depolarizing rectification was caused by a persistent Na+ current (INaP); the Cs+-sensitive hyperpolarizing response had a different mechanism. Hyperpolarizing prepulses caused a 4-aminopyridine-sensitive delay of spike initiation. An evoked spike was followed by a fast- and a medium-duration hyperpolarization (the fAHP and mAHP, respectively). Blockade of Ca2+ influx abolished the mAHP without affecting spike duration, whereas spikes were prolonged and the fAHP was abolished by TEA or 4-AP. Adequate depolarization evoked tonic repetitive firing characterized by a steep F-I slope and fast adaptation. Abolition of the mAHP was associated with reduced fast adaptation and increased F-I slope, whereas the mAHP was enhanced and firing rate was slowed after TEA application. Three outward ionic currents were identified during voltage clamp: a rapidly inactivating current, a slowly inactivating current and a slow persistent Ca2+-mediated current (IK(Ca]. We conclude that spike repolarization and the fAHP are governed mainly by fast voltage-dependent currents, whereas progressive activation of IK(Ca) causes fast adaptation and, together with INaP, regulates firing rate.

Epileptogenic action of tungstic acid gel on cat lumbar motoneurons.

Injection of tungstic acid gel (but not pH adjusted saline) into the cat lumbar ventral horn results in spontaneous, epileptiform activity consisting of waves of repetitive, high frequency, action potentials in motoneurons surrounding the injection site. In most motoneurons the action potentials are grouped in high frequency bursts composed of action potentials triggered from the delayed depolarization of the preceding action potential. The same kind of bursting can be triggered by intracellular current pulses, indicating that altered neuronal membrane properties are associated with the bursting activity. This type of bursting differs markedly from that seen in motoneurons during penicillin or strychnine-induced spinal seizures.

Negative slope conductance due to a persistent subthreshold sodium current in cat neocortical neurons in vitro.

The voltage dependent ionic currents of large layer V neurons of cat sensory/motor cortex were examined in an in vitro slice preparation using a single-microelectrode voltage clamp. These cells exhibit a persistent inward current in a voltage range below spike threshold. This inward current is responsible for the increase of input resistance upon depolarization seen in these cells in response to a constant current pulse and is activated at the same voltages traversed by the membrane potential between spikes during rhythmic firing. The inward current appears to be a persistent sodium current, since it is unaffected by extracellular Ba2+ or Co2+ but is blocked by extracellular TTX or intracellular QX314.

The induction and modification of voltage-sensitive responses in cat neocortical neurons by N-methyl-D-aspartate.

The actions of the excitatory amino acid, N-methyl-D-aspartate (NMDA), on layer V neurons of cat sensorimotor cortex were examined in an in vitro slice preparation using current clamp, single electrode voltage clamp (SEVC), and ionic substitution techniques. Low doses of NMDA evoked a slow depolarization with a net decrease of input conductance. Larger doses additionally evoked repetitive firing, rhythmic depolarization shifts (DSs), low-threshold calcium spikes (in the presence of TEA+) and bistable membrane potential behavior. Ionic substitution experiments suggested that entry of both Ca2+ and Na+ ions contributed to the NMDA responses. Attention was focused on the NMDA response with Ca2+ entry blocked. Examination by SEVC revealed that, in both normal cells and in the presence of several blocking agents, NMDA induced a highly voltage-dependent inward ionic current which could result in a region of negative slope conductance on the cell's current-voltage relation. The development of this current seems capable of accounting for all aspects of the observed response, including the DSs and low-threshold Ca2+ spikes. Substitution of TEA+ for most external Na+ (with Ca2+ entry blocked) largely eliminated the NMDA responses and corresponding ionic current. Our results in neocortical neurons are compared to those recently obtained in cultured murine neurons.

Multiple actions of N-methyl-D-aspartate on cat neocortical neurons in vitro.

The potent excitatory amino acid receptor agonist, N-methyl-D-aspartate (NMDA), was applied to cat neocortical neurons in an in vitro slice preparation. NMDA evokes a slow depolarization with a net input conductance decrease, repetitive firing, rhythmic depolarization shifts and bi-stable membrane potential behavior. Use of blocking agents, ion substitution and voltage clamp indicates that NMDA induces a highly voltage-dependent TTX-resistant inward sodium current which accounts for much of the NMDA response.

High- and low-threshold calcium currents in neurons acutely isolated from rat sensorimotor cortex.

Neurons were isolated by papain treatment and trituration of the frontoparietal cortex of 14 to 28-day-old rats. Whole cell voltage clamp revealed a slowly inactivating high-threshold Ca2+ current, activated positive to -45 mV, and a transient low-threshold Ca2+ current, activated positive to -65 mV. The high-threshold current was more sensitive to block by Cd2+ and the low-threshold current was more sensitive to block by Ni2+. Replacement of Ca2+ by Ba2+ increased the high-threshold current and reduced the low-threshold current. The high-threshold current was enhanced by Bay K 8644 and reduced by nimodipine and omega-conotoxin. The low-threshold current was also reduced by nimodipine but was insensitive to Bay K 8644 and omega-conotoxin. The properties of the currents were consistent with different underlying Ca2+ channel types.

Role of persistent inward and outward membrane currents in epileptiform bursting in mammalian neurons.

During the last five years, work in many laboratories has extended our knowledge of the ionic mechanisms of action potential generation in normal cells of the mammalian central nervous system (CNS). Here we review some of the important voltage-dependent currents present in mammalian CNS neurons. We discuss their possible role in epileptogenesis and normal behavior. We also emphasize the importance of these recent findings in relation to general models of neuronal behavior.

Amplification of synaptic current by persistent sodium conductance in apical dendrite of neocortical neurons.

1. Evidence for amplification of synaptic current by voltage-gated channels in dendrites of neocortical pyramidal neurons was demonstrated by examining the effect of specific channel blocking agents on the current arriving at the soma during iontophoresis of glutamate at a distal site on the apical dendrite. 2. Dendritic noninactivating Na+ channels were implicated in this voltage-dependent amplification of the transmitted current because it was maintained for > 1 s and because tetrodotoxin (TTX) eliminated much of this amplification. 3. Specific blockers of N-methyl-D-aspartate (NMDA) glutamate receptors reduced the amplitude of the glutamate-evoked current at all potentials and also reduced the non-TTX-sensitive component of voltage-dependent augmentation. The effects of TTX were identical whether or not NMDA channels were blocked. 4. We conclude that a persistent Na+ conductance exists in the apical dendrite of neocortical neurons. Together with the NMDA conductance at the synaptic site it provides a mechanism for the graded, voltage-dependent amplification of tonic, excitatory synaptic input. This amplification results in much more effective transmission of tonic excitatory current to the soma than would occur in a passive dendrite.

Modification of current transmitted from apical dendrite to soma by blockade of voltage- and Ca2+-dependent conductances in rat neocortical pyramidal neurons.

The axial current transmitted to the soma during the long-lasting iontophoresis of glutamate at a distal site on the apical dendrite was measured by somatic voltage clamp of rat neocortical pyramidal neurons. Evidence for voltage- and Ca2+-gated channels in the apical dendrite was sought by examining the modification of this transmitted current resulting from the alteration of membrane potential and the application of channel-blocking agents. After N-methyl-D-aspartate receptor blockade, iontophoresis of glutamate on the soma evoked a current whose amplitude decreased linearly with depolarization to an extrapolated reversal potential near 0 mV. Under the same conditions, glutamate iontophoresis on the apical dendrite 241-537 microm from the soma resulted in a transmitted axial current that increased with depolarization over the same range of membrane potential (about -90 to -40 mV). Current transmitted from dendrite to soma was thus amplified during depolarization from resting potential (about -70 mV) and attenuated during hyperpolarization. After Ca2+ influx was blocked to eliminate Ca2+-dependent K+ currents, application of 10 mM tetraethylammonium chloride (TEA) altered the amplitude and voltage dependence of the transmitted current in a manner consistent with the reduction of dendritic voltage-gated K+ current. We conclude that dendritic, TEA-sensitive, voltage-gated K+ channels can be activated by tonic dendritic depolarization. The most prominent effects of blocking Ca2+ influx resembled those elicited by TEA application, suggesting that these effects were caused predominantly by blockade of a dendritic Ca2+-dependent K+ current. When cells were impaled with microelectrodes containing ethylene glycol-bis(beta-amino-ethyl ether)-N,N',N'-tetraacetic acid to prevent a rise in intracellular Ca2+ concentration, blockade of Ca2+ influx altered the tonic transmitted current in different manner consistent with the blockade of an inward dendritic current carried by high-threshold-activated Ca2+ channels. We conclude that the primary effect of Ca2+ influx during tonic dendritic depolarization is the activation of a dendritic Ca2+-dependent K+ current. The hyperpolarizing attenuation of transmitted current was unaffected by blocking all known voltage-gated inward currents except the hyperpolarization-activated cation current (Ih). Extracellular Cs+ (3 mM) reversibly abolished both the hyperpolarizing attenuation of transmitted current and Ih measured at the soma. We conclude that activation of Ih by hyperpolarization of the proximal apical dendrite would cause less axial current to arrive at the soma from a distal site than in a passive dendrite. Several functional implications of dendritic K+ and Ih channels are discussed.