Different changes in spontaneous field potential oscillations precede epileptiform bursting in hippocampal slices perfused with penicillin or reduced magnesium

Abstract

Power spectra were used to analyse spontaneous field potentials (SFPs) recorded in the CA3 distal apical dendritic region of guinea pig hippocampal slices perfused with either penicillin or reduced Mg 2+. High concentrations of penicillin (2000 IU/ml) progressively converted the low amplitudem irregular oscillations observed in control medium to higher amplitude, low frequency, rhythmic oscillations at apporximately 2–3 Hz just prior to the onset of spontaneous, synchronized bursting. Low concentrations (50–300 IU/ml) increased the power of frequencies below 10 Hz and suppressed higher in a dose-dependent fashion. Although Mg 2+-free medium also increased the magnitude of the SFPs prior to the onset of synchronous bursting, the changes were smaller than with penicillin and the frequency distribution was completely different. Low concentrations of Mg 2+ (0.0–0.5 mM) increased the power across all frequencies, however, the maximal effect was on frequencies between 5 and 25 Hz. The transition from normal to epileptiform activity may proceed through at least 2 distinct intermediate states. When recurrent inhibition is blocked (penicillin), synchronous synaptic activity precedes the onset of bursting, whereas non-specific increases in excitability and activation of NMDA receptors (reduced Mg 2+) produce an asynchronous transition state.