Frequency facilitation in the hippocampal slices of conditioned rats

Electrophysiological parameters in the hippocampal slices of two experimental (prepared immediately (n = 27) and one week (n = 9) after conditioning) and two control (passive (n = 10) and active (n = 8)) groups of rats have been compared. The experimental rats were trained in a two-way avoidance chamber. The active control rats received the same number of conditioned and unconditioned stimuli without pairing. The threshold and amplitude of population spikes recorded from the CA1 pyramidal cell body layer to the stimulation of stratum radiatum and its modification in a train of 10 pulses (0.2 Hz) were measured. The mean values of the threshold intensity were not significantly different between any pair of these groups. The direction of the changes in the population spike amplitudes following pseudo-conditioning or conditioning was the same. However, the population spike amplitudes decreased more significantly in the slices from the conditioned rats. The increase in the frequency facilitation was specific for the slices of conditioned rats. The modifications in the mechanism of frequency facilitation in the reinforced pathways may represent an important mechanism for behavioural learning.