Fast and slow depolarizing potentials induced by short pulses of kainic acid in hippocampal neurons.

Responses of hippocampal neurons to short pulses of alpha-kainic acid (KA) were studied intracellularly in thin brain slices of the guinea pig. A KA pulse induced a slow depolarizing potential either with or without a preceding much faster depolarization. Large fast responses were induced only at the center of glutamate-sensitive spots, where short pulses of L-glutamate (Glu) induced large depolarizations in the impaled neuron. The fast responses resembled Glu-induced depolarizations in time-course, in sensitivity to movement of the tips of amino acid-pipettes, in sensitivity to Glu-antagonists and in reversal potential. The slow response was much more resistant to movement of the amino acid-pipettes and to Glu-antagonists. Mn2+ was without effect on the fast as well as the slow responses. Glu-induced depolarizations super-imposed on the slow response were simply depressed. These results indicate that two different types of receptors are activated by administration of KA, and suggest that the slow response results from a direct action of KA and the fast response is produced as a consequence of either the direct action of KA on the Glu receptors or a calcium-independent release of Glu by KA.