Long-lasting hippocampal potentiation and contextual memory consolidation

In order to ascertain whether there are hippocampal electrophysiological mo difications specifically related to memory, exploratory activity and emotio nal stress, extracellular electrical activity was recorded in hippocampal s lices prepared from the brains of male adult rats. Several groups of animal s were employed: (i) rats which had freely explored the experimental appara tus (8 min exposure); (ii) rats which had been subjected, in the same appar atus, to a fear conditioning paradigm training entailing the administration of aversive electrical footshocks (8 min exposure); (iii) rats to which th e same number of aversive shocks had been administered in the same apparatu s, but temporally compressed so as to make difficult the association betwee n painful stimuli and the apparatus (30 s exposure); (iv) naive rats never placed in the apparatus. Half of the rats from each treatment group were us ed for retrieval testing and the other half for hippocampal excitability te sting. The conditioned freezing response was exhibited for no less than 4 w eeks. Hippocampal excitability was measured by means of input-output curves (IOC) and paired-pulse facilitation curves (PPF). Retrieval testing or bra in slices preparation were performed at increasing delays after the trainin g sessions: immediately afterwards or after 1, 7 or 28 days. Only the rats subjected to the fear conditioning training exhibited freezing when placed again in the apparatus (retrieval testing). It was found that IOCs, with re spect to naive rats, increased in the conditioned animals up to the 7-day d elay. In free exploration animals the IOCs increased only immediately after the training session. In all other rats no modification of the curves was observed. IOC increases do not appear to imply presynaptic transmitter rele ase modifications, because they were not accompanied by PPF modifications. In conclusion, a clear-cut correlation was found between the increase in ex citability of the Schaffer collateral-CA1 dendrite synapses and freezing re sponse consolidation.