SUMMARY1. Fifty to ninety per cent of the Na efflux from axons of Loligo forbesi is inhibited by ouabain. The properties of the ouabain-sensitive component of the Na efflux are different from those of the ouabain-insensitive component.2. In unpoisoned axons with an average Na content of 75 m-mole/kg axoplasm the bulk of the ouabain-sensitive Na efflux is dependent on external K.3. In the presence of 460 mm Na in the external medium, raising the external K concentration from 0 to 100 mm increases the ouabain-sensitive Na efflux along a sigmoid curve which shows signs of saturating at high K concentrations.4. The curve relating ouabain-sensitive K influx to external K concentration is similar in shape to that for the ouabain-sensitive Na efflux. At all K concentrations examined the ouabain-sensitive K influx was less than the ouabain-sensitive Na efflux.5. Potassium-free sea water acts rapidly in reducing the Na efflux. There is no appreciable difference between the rates of action of K-free sea water on the Na pump and Na-free sea water on the action potential.6. Caesium and Rb can replace external K in activating the ouabainsensitive Na efflux. Both the affinity and maximum rate of the Na efflux mechanism are lower when Cs replaces K as the activating cation. P. F. BAKER AND OTHERS 7. Isosmotic replacement of external Na by either choline or dextrose, but not Li, increases the affinity of the ouabain-sensitive Na efflux mechanism for external K without appreciably affecting the maximum rate of pumping. External Li behaves like external Na and exerts an inhibitory action on the Na efflux.8. There is a large ouabain-sensitive Na efflux into K-free choline or dextrose sea waters. Addition of either Na or Li to the external medium reduces this efflux along a section of a rectangular hyperbola. The properties of this efflux suggest that there is a residual K concentration of up to 2 mm immediately external to the pumping sites in the axolemma.9. Over the range of internal Na concentrations studied (16-140 mmole/kg axoplasm) the ouabain-sensitive Na efflux increased linearly with Na concentration.10. Tetrodotoxin (10-6 g/ml.) reduces the Na influx by about half, but does not affect the ouabain-sensitive Na efflux.11. Isobutanol (1 % v/v) reversibly decreases both the ouabain-sensitive and ouabain-insensitive components of the Na efflux.12. Application of 2 mm cyanide to axons immersed in K-free sea water produces a transient rise in the Na efflux. This rise is not seen if ouabain is included in the sea water. The rise in efflux occurs at a time when the axons are partially poisoned and contain adenosine triphosphate (ATP) but no arginine phosphate (ArgP). A similar, but maintained rise can be obtained after application of dinitrophenol (DNP) at pH 8-0. The increased Na efflux in these partially poisoned axons is also inhibited by ouabain.13. Under conditions of partial-poisoning by alkaline DNP, there is a ouabain-sensitive Na influx from K-free sea water. The ouabain-sensitive Na influx is of similar size to the ouab...

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1 Pilocarpine administration has been used as an animal model for temporal lobe epilepsy since it produces several morphological and synaptic features in common with human complex partial seizures. Little is known about changes in extracellular neurotransmitter concentrations during the seizures provoked by pilocarpine, a non-selective muscarinic agonist. 2 Focally evoked pilocarpine-induced seizures in freely moving rats were provoked by intrahippocampal pilocarpine (10 mM for 40 min at a¯ow rate of 2 ml min 71 ) administration via a microdialysis probe. Concomitant changes in extracellular hippocampal glutamate, g-aminobutyric acid (GABA) and dopamine levels were monitored and simultaneous electrocorticography was performed. The animal model was characterized by intrahippocampal perfusion with the muscarinic receptor antagonist atropine (20 mM), the sodium channel blocker tetrodotoxin (1 mM) and the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 (dizocilpine maleate, 100 mM). The e ectiveness of locally (600 mM) or systemically (10 mg kg 71 day 71 ) applied lamotrigine against the pilocarpine-induced convulsions was evaluated.3 Pilocarpine initially decreased extracellular hippocampal glutamate and GABA levels. During the subsequent pilocarpine-induced limbic convulsions extracellular glutamate, GABA and dopamine concentrations in hippocampus were signi®cantly increased. Atropine blocked all changes in extracellular transmitter levels during and after co-administration of pilocarpine. All pilocarpine-induced increases were completely prevented by simultaneous tetrodotoxin perfusion. Intrahippocampal administration of MK-801 and lamotrigine resulted in an elevation of hippocampal dopamine levels and protected the rats from the pilocarpine-induced seizures. Pilocarpine-induced convulsions developed in the rats which received lamotrigine perorally. 4 Pilocarpine-induced seizures are initiated via muscarinic receptors and further mediated via NMDA receptors. Sustained increases in extracellular glutamate levels after pilocarpine perfusion are related to the limbic seizures. These are arguments in favour of earlier described NMDA receptor-mediated excitotoxicity. Hippocampal dopamine release may be functionally important in epileptogenesis and may participate in the anticonvulsant e ects of MK-801 and lamotrigine. The pilocarpine-stimulated hippocampal GABA, glutamate and dopamine levels re¯ect neuronal vesicular release.

Neurophysiological contributions to cérébral maturation and to périnatal pathology in man have been mostly concernée! with the spontaneous brain activity (

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