EFFECTS OF APPLIED CURRENTS ON SPONTANEOUS EPILEPTIFORM ACTIVITY-INDUCED BY LOW-CALCIUM IN THE RAT HIPPOCAMPUS

It is known that both applied and endogenous electrical fields can mod ulate neuronal activity. In this study, we have demonstrated that anod ic current injections can inhibit spontaneous epileptiform events in t he absence of synaptic transmission. Activity was induced with low-Ca2 + (0.2 mM) artificial cerebrospinal fluid (ACSF) and detected with a v oltage threshold detector. At the onset of an event, a current was inj ected into the stratum pyramidale via a tungsten electrode positioned within 150 mu m of the recording site. Data was recorded with a glass pipette electrode. The results show that spontaneous epileptiform acti vity can be fully suppressed by subthreshold anodic currents with an a verage amplitude of 3.9 mu A and a minimum amplitude of 1 mu A. In add ition, we observed that some events could be blocked by current pulses with shorter durations than the duration of the event itself. The pos sibility that increased tissue resistance could contribute to the effi cacy of the currents was tested by measuring the step-potential increa se evoked by anodic current injections. The data show a significant in crease in the amplitude of the evoked potential after introduction of a low-Ca2+ medium, suggesting that tissue resistance is increasing. Th ese results indicate that low-amplitude, subthreshold current pulses a re sufficient to block epileptiform activity in a low-Ca2+ environment . The increased tissue resistance induced by sustained exposure to a l ow-Ca2+ medium could contribute to the low current amplitudes required to block the epileptiform events. (C) 1998 Elsevier Science B.V. All rights reserved.