P. C. Schwindt

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.

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.

Synaptic depression in the localization of sound

Short-term synaptic plasticity, which is common in the central nervous system, may contribute to the signal processing functions of both temporal integration and coincidence detection. For temporal integrators, whose output firng rate depends on a running average of recent synaptic inputs, plasticity modulates input synaptic strength and thus may directly control signalling gain and the function of neural networks. But the firing probability of an ideal coincidence detector would depend on the temporal coincidence of events rather than on the average frequency of synaptic events. Here we have examined a specific case of how synaptic plasticity can affect temporal coincidence detection, by experimentally characterizing synaptic depression at the synapse between neurons in the nucleus magnocellularis and coincidence detection neurons in the nucleus laminaris in the chick auditory brainstem. We combine an empirical description of this depression with a biophysical model of signalling in the nucleus laminaris. The resulting model predicts that synaptic depression provides an adaptive mechanism for preserving interaural time-delay information (a proxy for the location of sound in space) despite the confounding effects of sound-intensity-related information. This mechanism may help nucleus laminaris neurons to pass specific sound localization information to higher processing centres.

The induction and modification of voltage-sensitive responses in cat neocortical nuerons 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 cells 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.

Two transient potassium currents in layer V pyramidal neurones from cat sensorimotor cortex.

1. Two transient outward currents were identified in large pyramidal neurones from layer V of cat sensorimotor cortex (‘Betz cells’) using an in vitro brain slice preparation and single‐microelectrode voltage clamp. Properties of the currents deduced from voltage‐clamp measurements were reflected in neuronal responses during constant current stimulation. 2. Both transient outward currents rose rapidly after a step depolarization, but their subsequent time course differed greatly. The fast‐transient current decayed within 20 ms, while the slow‐transient current took greater than 10 s to decay. Raised extracellular potassium reduced current amplitude. Both currents were present in cadmium‐containing or calcium‐free perfusate. 3. Tetraethylammonium had little effect on the slow‐transient current at a concentration of 1 mM, but the fast‐transient current was reduced by 60%. 4‐Aminopyridine had little effect on the fast‐transient current over the range 20 microM‐2 mM, but these concentrations reduced the slow‐transient current and altered its time course. 4. Both transient currents were evoked by depolarizations below action potential threshold. The fast‐transient current was evoked by a 7 mV smaller depolarization than the slow‐transient current, but its chord conductance increased less steeply with depolarization. 5. Voltage‐dependent inactivation of the fast‐transient was steeper than that of the slow‐transient current (4 vs. 7 mV per e‐fold change), and half‐inactivation occurred at a less negative potential (‐59 vs. ‐65 mV). The activation and inactivation characteristics of each current overlapped, however, implying the existence of a steady ‘window current’ extending over a range of approximately 14 mV beginning negative to action potential threshold. 6. The fast‐transient current displayed a clear voltage dependence of both its activation and inactivation kinetics, whereas the slow‐transient current did not. Recovery of either current from inactivation took about 1 s near ‐70 mV. The recovery of the slow‐transient current became faster with hyperpolarization. 7. The contribution of each transient current to repolarization of the action potential was assessed from pharmacological responses. Blockade of calcium influx had little or no effect on the rate of action potential repolarization, whereas the selective reduction of either transient current caused significant slowing of repolarization. 8. We conclude that Betz cells possess at least two transient potassium currents, each a member of the rapidly expanding family of voltage‐gated potassium currents that have been identified in various cell types.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Active currents in mammalian central neurons

Abstract The development of in-vitro techniques and the application of the voltage clamp have allowed the identification of voltage-dependent ionic currents in neurons of the mammalian CNS. Although action potential generation is firmly based on the ionic model of Hodgkin and Huxley, the differences in input—output responses of mammalian neurons reside in additional ionic conductance channels first activated in the near-threshold range of membrane potential. Several putative neurotransmitters affect the ionic current flowing through some of these newly described conductance channels thereby altering the input—output characteristics of mammalian central neurons. These changes in the response properties are likely to be important in both the normal and abnormal functions of neurons.

P-type calcium channels in rat neocortical neurones.

1. The high threshold, voltage‐activated (HVA) calcium current was recorded from acutely isolated rat neocortical pyramidal neurones using the whole‐cell patch technique to examine the effect of agents that block P‐type calcium channels and to compare their effects to those of omega‐conotoxin GVIA (omega‐CgTX) and nifedipine. 2. When applied at a saturating concentration (100 nM) the peptide toxins omega‐Aga‐IVA and synthetic omega‐Aga‐IVA blocked 31.5 and 33.0% of the HVA current respectively. 3. A saturating concentration of nifedipine (10 microM) inhibited 48.2% of the omega‐Aga‐IVA‐sensitive current, whereas saturating concentrations of both omega‐Aga‐IVA (100 nM) and omega‐CgTX (10 microM) blocked separate specific components of the HVA current. 4. Partially purified funnel web spider toxin (FTX) at a dilution of 1:1000 blocked 81.4% of the HVA current and occluded the inhibitory effect of omega‐Aga‐IVA. Synthetic FTX 3.3 arginine polyamine (sFTX) at a concentration of 1 mM blocked 61.2% of the HVA current rapidly and reversibly. The effects of sFTX were partially occluded by pre‐application of omega‐Aga‐IVA. We conclude that neither FTX nor sFTX blocked a specific component of the HVA current in these cells. 5. In view of the specificity of omega‐Aga‐IVA for P‐type calcium channels in other preparations and for a specific component of the HVA current in dissociated neocortical neurones we conclude that about 30% of the HVA current in these neurones flow through P‐channels.

Post-inhibitory excitation and inhibition in layer V pyramidal neurones from cat sensorimotor cortex

1. The effect of conditioning pre‐pulses on repetitive firing evoked by intracellular current injection was studied in layer V pyramidal neurones in a brain slice preparation of cat sensorimotor cortex. Most cells displayed spike frequency adaptation (monotonic decline of firing rate to a tonic value) for several hundred milliseconds when depolarized from resting potential, but the cells differed in their response when pre‐pulses to other potentials were employed. In one group of cells, the initial firing rate increased as the pre‐pulse potential was made more negative (post‐hyperpolarization excitation). Adaptation was abolished by depolarizing prepulses. In a second group, the initial firing rate decreased as the pre‐pulse potential was made more negative (post‐hyperpolarization inhibition). Hyperpolarizing pre‐pulses caused the initial firing to fall below and accelerate to the tonic rate over a period of several seconds. A third group displayed a mixture of these two responses: the first three to seven interspike intervals became progressively shorter and subsequent intervals became progressively longer as the conditioning pre‐pulse was made more negative (post‐hyperpolarization mixed response). 2. Cells were filled with horseradish peroxidase or biocytin after the effect of pre‐pulses was determined. All cells whose firing patterns were altered by pre‐pulses were large layer V pyramidal neurones. Cells showing post‐hyperpolarization excitation or a mixed response had tap root dendrites, fewer spines on the apical dendrite and larger soma diameters than cells showing post‐hyperpolarization inhibition. 3. Other electrophysiological parameters varied systematically with the response to conditioning pre‐pulses. Both the mean action potential duration and the input resistance of cells showing post‐hyperpolarization excitation were about half the values measured in cells showing post‐hyperpolarization inhibition. Values were intermediate in cells showing a post‐hyperpolarization mixed response. The after‐hyperpolarization following a single evoked action potential was 20% briefer in cells showing post‐hyperpolarization excitation compared to those showing inhibition. 4. Membrane current measured during voltage clamp suggested that two ionic mechanisms accounted for the three response patterns. Post‐hyperpolarization excitation was caused by deactivation of the inward rectifier current (Ih). Selective reduction of Ih with extracellular caesium diminished post‐hyperpolarization excitation, whereas blockade of calcium influx had no effect. Post‐hyperpolarization inhibition was caused by enhanced activation of a slowly inactivating potassium current. Selective reduction of this current with 4‐aminopyridine diminished the post‐hyperpolarization inhibition. 5. Chord conductances underlying both Ih and the slow‐transient potassium current were measured and divided by leakage conductance to control for differences in cell size.(ABSTRACT TRUNCATED AT 400 WORDS)

Epileptogenic doses of penicillin do not reduce a monosynaptic GABA-mediated postsynaptic inhibition in the intact anesthetized cat

Abstract The hypothesis that penicillin (PCN) causes seizures by interfering with γ-aminobutyric acid (GABA)-mediated inhibition was investigated by observing the effect of epileptogenic doses of systemic PCN on the monosynaptic inhibition of Deiters neurons by cerebellar stimuli. After intravenous PCN, typical epileptic spikes in cerebral cortical surface recordings occurred and were correlated with various degrees of hyperactivity and Deiters nucleus. Intracellular recordings in five Deiters neurons with stable penetrations before and after PCN-induced cortical spikes showed no reduction of the evoked monosynaptic inhibitory postsynaptic potential (IPSP). These few long-duration recordings were supported by statistical analysis of IPSP amplitude in small populations of neurons recorded either before or after PCN in 11 cats; no significant trend was seen after PCN. Extracellular unit and antidromic field potential recordings in Deiters nucleus did show alterations of net inhibition after PCN but do not refute the intracellular data. The epileptic effects of PCN on the intact mammalian nervous system may involve mechanisms other than antagonism of GABA-mediated inhibition.