Developmental onset of long‐term potentiation in area CA1 of the rat hippocampus.

Abstract

Long-term potentiation (l.t.p.) was studied in area CA1 of rat hippocampal slices during development at post-natal days 1-8, 15 and 60. Tetanic stimulation at 100 Hz for 1 s was delivered to the fibers in stratum radiatum and the time course of potentiation was recorded in stratum pyramidale for 20 min after tetanus. L.t.p. was measured at 20 min post-tetanus as an increase in the amplitude of the population spike. The time course and magnitude of post-tetanic potentiation (p.t.p.) differed with age. For 60-day-old animals p.t.p. was seen as a maximally potentiated response immediately post-tetanus that declined to a smaller potentiated response by 5 min post-tetanus. For animals younger than 15 days the response was also maximally potentiated immediately post-tetanus with subsequent decline. The duration of maximal potentiation was shorter and the magnitude was less. A different time course of p.t.p. was observed at 15 days. The maximal potentiation was approximately equal to that seen at 60 days, but instead of declining, the response remained maximally potentiated throughout the entire post-tetanus monitoring period. L.t.p. was 1st observed at post-natal day 5 and by post-natal days 7 and 8 substantial levels of l.t.p. were seen consistently. The greatest magnitude of l.t.p. was found at 15 days and was considerably more than that produced at 60 days. When the duration of l.t.p. was monitored for longer than 20 min the response declined back to pretetanus levels by 1-1.5 h for animals younger than 15 days. In 15-day-old rats the response remained maximally potentiated for the full 72 min that it was monitored, with no decline. In control experiments for low-frequency stimulation (l.f.s.) at 1/15 s for 100 stimuli, hippocampal slices from 60-day-old animals showed response elevation. L.f.s. resulted in response decrement over time for slices from 5- to 15-day-old animals. Three measures of pretetanus excitability in area CA1 suggested an increase with age. The stimulus intensity required for field excitatory post-synaptic potential (e.p.s.p.) threshold declined, the magnitude of the maximal population spike amplitude increased and the population spike latency decreased. Apparently, the magnitude of l.t.p. is not strictly related to the pretetanus excitability of CA1 cells. A 4th measure of excitability, the slope of the field e.p.s.p. at population spike threshold, showed a larger value at 15 days than at 1-8 or 60 days. A hypothesis is presented to show how a peak and decline in l.t.p. magnitude across ages might be mediated by the size of an available pool of plastic synapses, which diminishes as more synapses become consolidated by the production of l.t.p.