Cumulative long-term potentiation in the rat dentate gyrus correlates with, but does not modify, performance in the water maze

The electrically induced increase in hippocampal synaptic strength known as long-term potentiation (LTP) is thought to involve some of the same mechanisms as those mediating information storage during spatial learning. Physiological saturation of synaptic weights might therefore be expected to occlude spatial learning. In support of this, Castro et al. (Castro CA, Silbert LH, McNaughton BL, Barnes CA, 1989, Nature 342:545-548) reported that repeated induction of LTP to asymptotic levels, over a 14-day period, blocked the ability of rats to learn the position of a hidden platform in a water maze shortly afterwards. The authors have attempted to replicate this finding in two experiments. In Experiment 1, rats were given either 400 Hz stimulation as 50 trains/day over a 5-day period, to induce LTP, or were given the same number of stimulus pulses at 1 Hz. Two hours after the last stimulation session they underwent eight spatial learning trials, occurring at 2-hour intervals. In Experiment 2, the procedure of Castro et al. was followed exactly, with LTP induced by 10 trains of 400 Hz stimulation daily for 14 days. Spatial learning trials began 10 minutes after the last stimulation session and consisted of 12 trials with alternating 30-second and 2-minute intervals. In neither experiment was a significant impairment of spatial learning observed. However, there was a strong positive correlation, in both experiments, between the final level of LTP and subsequent performance in the water maze.