Current-to-frequency transduction in CA1 hippocampal pyramidal cells: slow prepotentials dominate the primary range firing.

In order to study how hippocampal pyramidal cells transform a steady depolarization into discharges, CA1 pyramids (n = 32) were injected with 1.5 s long pulses of constant depolarizing current. The firing in response to weak currents was in most cells, characterized by low frequency (0.2-5 Hz), slowly increasing depolarizations preceding each action potential (slow prepotentials, SPPs), a long latency (0.2-5 s) to the initial spike and lack of adaptation. The SPPs, which lasted 30-2,000 ms, showed an increasing steepness with increasing current, and seemed to be a major regulating factor for the slow firing. In response to stronger currents the discharge had a high initial frequency (100-350 Hz), followed by adaptation to steady state firing (5-50 Hz). Thirty of 32 cells showed a dip in the frequency (n = 5), or a pause (n = 25) lasting 250-1,000 ms between the initial burst of firing and the steady state. The pause occurred only at intermediate current strengths. Additional spikes to the initial burst seemed to be recruited through the development of depolarizing waves. The initial slope of these waves resembled those of the SPPs. Similar waves occurred at the expected time of occasionally missing spikes during steady state firing. The variability (SD/mean) of the interspike intervals decreased with increasing frequency of firing. The frequency-current (f/I) relation for the steady state firing showed a simple linear or convex shape, and lacked a secondary range. In contrast, the f/I plots for the initial few interspike intervals had both primary, secondary and tertiary ranges, like motoneurones.