STEREOTYPICAL CHANGES IN THE PATTERN AND DURATION OF LONG-TERM POTENTIATION EXPRESSED AT POSTNATAL DAYS 11 AND 15 IN THE RAT HIPPOCAMPUS

1. Extracellular recordings from hippocampal area CA1 lasting 2-8 h po sttetanus were used to evaluate the duration of long-term potentiation (LTP) at two key developmental ages. 2. At day 11 LTP consistently en dured for approximately 1 h before declining to baseline by 2.5 h post tetanus. The response could then be repotentiated, and in some cases, the repotentiation lasted longer than the original potentiation. 3. At day 15 two patterns of potentiation were observed. The first pattern was similar to that observed at day 11 in that the potentiation did no t persist; however, it did endure for approximately 2-2.5 h before dec lining to baseline by 4 h posttetanus. In the second pattern the poten tiation persisted indefinitely; these responses were monitored for 6-8 h posttetanus. 4. These patterns are similar to the temporal phases o f LTP that have been revealed in adult rat hippocampus through pharmac ological manipulations. They may reflect developmental changes during which the different cellular mechanisms underlying LTP become sequenti ally activated, 5. These findings are important for several reasons. F irst, because the different temporal phases of LTP seem to be added st epwise during development, animals of different ages could be used exp licitly to elucidate the underlying cellular mechanisms of these phase s in LTP. Second, because LTP is a candidate mechanism for some forms of learning and memory, these results have implications for sequential steps in the ontogeny of learning and memory. Finally, because studie s of LTP have used animals of widely varying ages, including these two ages, it is important to consider whether differences in the developm ental properties of LTP could influence experimental observations.