Sustained potential shifts and paroxysmal discharges in hippocampal formation.

Paroxysmal firing was provoked by electric stimulation of afferent pathways in hippocampal formation of intact, urethan-anesthetized rats, of freely moving unanesthetized rats, and in hippocampal tissue slices in vitro. The electric responses of fascia dentata and CA3 zone of the hippocampus of urethan-anesthetized rats were recorded with extracellular microelectrodes. Paroxysmal discharges were provoked by stimulating the ipsilateral angular bundle. During repetitive stimulation, intercurrent paroxysmal discharges (IPaD) took the form of compound action potentials (population spikes) of large amplitude, provoked by but not locked in time to the stimulus pulses. IPaD was often but not always followed by paroxysmal after-discharge (PaAD), usually consisting of bursts of population spikes, sometimes superimposed on a slow wave. Stimulus pulses that were not strong enough to evoke population spikes when applied singly could provoke the paroxysmal firing of large amplitude spikes when applied repetitively. The liminal frequency to provoke paroxysmal firing, with 10-s train duration and with pulses evoking 60 to 80% of maximal amplitude focal postsynaptic potential (PSP) waves, varied between 6 and 15 Hz in urethan-anesthetized rats. The outbreak of IPaD was always accompanied by a marked sustained potential (SP) shift. The polarity of the paroxysmal SP shift was the opposite of the polarity of the PSP waves. We conclude that the extracellular paroxysmal SP shifts in fascia dentata are probably generated mainly by current flowing from the dendritic trees toward the cell somata of granule cells. The amplitude of the population spikes fired during paroxysmal discharges could reach 30-40 mV, indicating the precise coincidence of the impulses fired by many neurons. These spikes often arose without a detectable preceding synaptic potential. We conclude that the synchronization of the action potentials fired by granule and pyramidal cells during paroxysmal discharge is probably due to electric interaction among the neurons. In unanesthetized freely moving rats IPaD and PaAD consisting of bursts of population spikes were provoked. These were similar to those observed in urethan-anesthetized rats. Motor seizures provoked in kindled rats were associated with intense and prolonged spike bursts followed by spikeless positive waves recorded in the granule cell layer of fascia dentata. In hippocampal tissue slices maintained in vitro, paroxysmal firing could be provoked in CA1 zone by repetitive stimulation of Schaffer collaterals. IPaD and PaAD could be provoked in some slices exposed to normal (3.5 mM) [K+] and in all slices exposed to elevated (5.5 or 7.0 mM) [K+].(ABSTRACT TRUNCATED AT 400 WORDS)