An analysis of the epileptogenic potency of CO2+-its ability to induce acute convulsive activity in the isolated frog spinal cord

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Abstract
The action of Co2+ on the isolated frog spinal cord was studied by extracellular application of the ion in the superfusing solution. A complete and reversible blockade of chemical synaptic transmission by Co2+ (3 mmol/l) could be achieved after a superfusion period of 20–30 min. During continued Co2+ application (>60 min) the following effects upon the motoneuron membrane, dorsal root and ventral root fibres were observed. Motoneurons and ventral root fibres: prolongation of initial segment action potential to a maximum of 30 ms, blockade of the long afterhyperpolarization, abolition of adaptation, increased duration of fibre action potential in the ventral root, backfiring after ventral root stimulation. Dorsal root fibres: prolongation of the extraspinal fibre action potential, marked prolongation of the action potential of the terminal region, backfiring of multiple action potentials after dorsal root stimulation. Even in the presence of Co2+, when synaptic transmission was completely blocked, strong convulsive reactions of the isolated spinal cord were observed. Intracellular injection of Co2+ into motoneurons did not affect the action potential, but led to a shift of the EIPSP towards the membrane potential. The results indicate that the induction of convulsive reactions by Co2+ is mainly due to a prolongation of action potentials. The plateau-like deformation of the action potential of the initial segment membrane and presumably of the terminal region of nerve endings results in retrograde propagation of action potentials and in some cases induces oscillatory discharge of single neurons.