Learning and Behavioral-Long-Term Controlling for Motor Activity Potentiation: Importance of

A series of brief, high-frequency trains of electrical stimu- lation delivered to the perforant-path results in long-term potentiation (LTP) of the dentate gyrus as measured by av- erage evoked potentials (EPs). Similar increases in dentate evoked potentials have been reported after natural learning. Previous studies of this behavioral LTP have not adequately controlled for ongoing behavior at the time of recording, even though motor activity also influences the amplitude of EPs. Chronically implanted rats were trained in both a radial-arm maze and an avoidance task using a crossover design. EPs in the dentate gyrus following perforant-path stimulation were recorded daily under 3 different behavioral conditions: im- mobility, movement, and freely behaving. After completion of both tasks, animals were given tetanizing stimulation of the perforant path. Results indicated strong improvements in the performance of both tasks. Tetanization induced sig- nificant LTP, which was still present at the end of 5 d. Sig- nificant differences were found between EPs collected dur- ing immobility and movement throughout the experiment. No evidence of behavioral LTP was observed, and the EPs re- mained consistent with baseline measures. These data show the necessity of controlling for ongoing behavior at the time of recording in electrophysiological studies of learning. The data also indicate that the phenomenon of behavioral LTP, as assessed by hippocampal EPs, is not universal to all learning experiences. Long-term potentiation (LTP) is a term applied to an increase in synaptic efficacy, measured by evoked potentials (EPs), which follows the application of brief, high-frequency trains of elec- trical stimulation. The functional similarity of LTP to the hy- pothesized operation of the modifiable "Hebbian" synapse has made it one of the more popular phenomena with which to model and study the neural basis of memory (Teyler and Dis- cenna, 1984; Racine and Kairiss, 1987). However, the exact nature of the relationship of LTP to normal memory processes has yet to be determined (Teyler and Discenna, 1987). Support linking LTP to learning has come from a variety of sources, one of which is behavioral LTP. Behavioral LTP is a related electrophysiological phenomenon, in which LTP-like changes are reflected in hippocampal EPs as a result of natural