INTRACELLULAR RECORDS FROM HIPPOCAMPAL PYRAMIDAL CELLS IN RABBIT DURING THETA RHYTHM ACTIVITY

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Abstract
In the rabbit hippocampus, 71 pyramidal cells were successfully impaled with microelectrodes. Out of these, 61 cells showed a rhythmic change of the membrane potential in synchrony the eeg [electroencephalogram] theta rhythm. The rhythm was called the intracellular theta rhythm. The peak amplitude of the intracellular theta rhythm ranged from 5 to 36 mV (mean: 12mV). Burst of spikes were often seen on the depolarization-going phase of the intracellular theta rhythm. In 18 cells in which the intracellular theta rhythm was relatively large (21-36 mV, mean 28mV), spike inactivation was seen during the depolarization-going phase. Evidence was presented showing that EPSPs were largely responsible for the development of the intracellular theta rhythm. In a few cases IPSPs [inhibitory post-synaptic potential] followed the EPSPs [excitatory post-synaptlc potential]. Therefore, it could not be ruled out that they also contributed to the intracellular theta rhythm. The intracellular and EEG theta rhythm have similar properties. (1) They are synchronous. (2) Both rhythms increase in frequency with sciatic nerve stimulation. (3) Both diminish in amplitude for nearly 300 msec following stimulation of the Schaffer Col-laterals. Moreover, the positive- and negative- going phases of eeg theta rhythm recorded from the pyramidal cell body layer correspond to the hyper -polarization and depolarization -going phases of the intracellular theta rhythm respectively. It is suggested that the eeg theta rhythm is formed mainly by the extracellular field currents driven by the intracellular theta rhythm with some contribution from superposing bursts of spike.