Membrane potential oscillations in CA1 hippocampal pyramidal neurons in vitro: intrinsic rhythms and fluctuations...

The mechanisms mediating intrinsic and entrained CA1 pyramidal neuron rhythmic membrane potential oscillations were investigated in rat hippocampal slices. Intrinsic oscillations (6-14 Hz, < 10 mV) were evoked by long duration (2 s), depolarizing current pulses in 42% of the cells. Oscillations were also evoked by imposing sinusoidal transmembrane currents at 2, 7, and 14 Hz, adjusted at 7 Hz to imitate the synaptically mediated in vivo "intracellular theta". Slow all-or-none events (40 mV, 55 ms)--reminiscent of the rhythmic, high threshold slow spikes observed in vivo--were evoked and entrained by the sine wave current cycles with large, imposed depolarization in 35% of the cells. Intrinsic oscillations were insensitive to Ca(2+)-free, Co2+ (2 mM) and Mn2+ (2 mM) solutions, but were blocked by tetrodotoxin (TTX; 5 microM), illustrating that they were Na(+)-mediated. Tetraethylammonium (TEA; 15 mM) unmasked slow all-or-none events (40-50 mV, 20-55 ms) and plateau potentials (40-60 mV, 100-700 ms). Plateaus were Co2+ and Mn2+ resistant and were abolished by TTX, hence suggesting that the underlying persistent conductance was Na(+)-mediated. Plateaus were entrained one-to-one at all sinusoidal current frequencies in Ca(2+)-free, TEA + Co2+, or TEA + Mn2+ solutions. However, the high threshold Ca2+ spikes uncovered in TEA + TTX could only follow sinusoidal currents of less than 7 Hz. In conclusion, the high threshold Ca2+ and persistent Na+ conductances coexist in CA1 pyramidal cells. The persistent Na+ conductance mediated the intrinsic oscillations, and fluctuated at all the sine wave current frequencies used. The more sluggish high-threshold Ca2+ conductance exclusively oscillated at frequencies of less than 7 Hz and did not support the intrinsic rhythm.(ABSTRACT TRUNCATED AT 250 WORDS)