Fig. 3. Evoked field potential and interstitial DC potential changes associated with SD. Classical electrophysiological evidence for SD consists of a slowly propagating, large negative DC potential in the interstitial space associated with a transient loss of spontaneous and/or evoked neuronal activity. The association of the DC potential (thick record) with the change in evoked field potentials (thin traces) during SD in a HOTC is shown. This event was one of 40 such SD episodes induced every 10–15 min from the same culture over an 8 hr period. Field potentials were evoked at specific time points (0–8) shown on the DC record. A calibration pulse (10 mV, 2 msec) is shown toward the latter half of each field potential trace. The interstitial DC calibration time and voltage bars are shown. With NaAc exposure, the interstitial slow DC potential (large thick line) initially shifted more positive. Then with the evoked pulse from the hilus (1), interstitial DC potential briefly became more positive and then swung negative, reaching a peak (2) approximately −10 mV from baseline. Finally, SD occurred. This was evidenced by a massive shift in DC potential that reached −45 mV from baseline (0) in this example before returning to baseline. Further support that this DC change was SD came from alterations in the field potentials. Successive SDs (n = 12) could make pyramidal cells hyperexcitable. For example, two population spikes are seen (trace 0) before the associated slower, large fEPSP. Then, with exposure to NaAc–Ringer’s, evoked potentials became even more excitable (trace 1), showing multiple population spikes before becoming completely absent (traces 2–4) as the DC potential swung to its most negative extreme. Recovery frequently was associated with epileptiform activity, as in this example (points 5–7 along the slow DC record). In addition, pyramidal cells initially showed an exaggerated fEPSP (traces 5 and 6) and then multiple population spikes (trace 7). Shortly after SD, pyramidal cell fEPSPs were reduced slightly (compare trace 8 with 0).