C. E. Stafstrom

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.

Hyperpolarizing Potentials in Guinea Pig Hippocampal CA3 Neurons

Summary1.There is a bewildering variety of hyperpolarizing potentials which control activity in hippocampal pyramidal cells. These include an inhibitory postsynaptic potential (IPSP) with early and late components, voltage- and calcium-dependent potassium conductances, a voltage-dependent potassium conductance modulated by muscarinic agents (the M-current), and a complex and poorly understood afterhyper-polarization following epileptiform bursts.2.In hippocampal CA3 pyramidal cells, mossy fiber stimulation elicits an IPSP which is made up of two readily separable components. Using thein vitro slice preparation, we investigated the underlying ionic basis of these IPSP components and compared them to other hyperpolarizing potentials characteristic of the CA3 neurons.3.Intracellular recordings were obtained and then tissue was exposed to bathing medium low in chloride concentration or high in potassium concentration; the ion “blockers” EGTA (intracellular); tetraethylammonium (TEA) (intra- and extracellular), and barium and cobalt (extracellular); and the γ-aminobutyric acid (GABA)/ chloride antagonists penicillin, bicuculline and picrotoxin.4.As has been reported by others, the early part of the CA3 IPSP is a GABAergic, chloride-dependent potential generated at proximal (probably somatic) membrane. The later component of the IPSP is a potassium-dependent synaptic potential, which is picrotoxin and bicuculline insensitive, not dependent on increases in intracellular calcium, and apparently produced on distal (dendritic) membranes.5.The later component of the IPSP is clearly different from the calcium-dependent potassium conductance [gK(Ca)] responsible for hyperpolarizations following normal spontaneous and current-induced burst discharge in hippocampal pyramidal cells. The late component of the IPSP may have some similarities to the afterhyperpolarization seen following penicillin-induced burst discharges.

Inward Currents in Cat Neocortical Neurons Studied In Vitro

Two distinct types of clinical recurrent seizures or epilepsy have been identified: those that begin focally in cortex (partial seizures) and those that appear to begin synchronously in both hemispheres (generalized seizures). Because most experimentalists use focal physical or chemical techniques to initiate seizures, our concepts about epileptic mechanisms are, therefore, more applicable to clinical partial epilepsy.