INCREASING AGE REDUCES EXPRESSION OF LONG-TERM DEPRESSION AND DYNAMIC-RANGE OF TRANSMISSION PLASTICITY IN CA1 FIELD OF THE RAT HIPPOCAMPUS

Long-term depression, depotentiation and long-term potentiation of fie ld excitatory postsynaptic potentials in the CA1 field of the hippocam pus were studied in slices from two-, 12-, 24- and 36-week-old rats. L ong-term potentiation was induced by stimulating afferent fibres for 1 s at 100 Hz. Long-term depression was induced either by stimulating t he afferent pathways twice for 15 min at 1 Hz (protocol 1), giving in total 1800 pulses, or by stimulating the fibres at 5 min intervals twi ce at 1 Hz for 5 min followed by 5 min stimulation at 5 Hz (protocol 2 ), giving in total 2100 pulses. We found significant long-term depress ion in slices of all groups stimulated with protocol 1; however. the m agnitude of long-term depression in slices from 24-and 36-week-old rat s was significantly lower than that in slices from two-and 12-week old rats, although there was no such difference in the magnitude of long- term potentiation between slices. Stimulation protocol 2 induced long- term depression only in slices from two- and 12-week-old rats. Compari son of the dynamic range of transmission plasticity in slices from two -and 36-week-old rats, calculated as the difference between the nearly saturated long-term potentiation and nearly saturated depotentiation, revealed a significantly smaller dynamic range in slices from 36-week -old rats in comparison with slices from two-week-old animals. The dec rease in the dynamic range in slices from 36-week-old rats was due to a diminished capacity to depotentiate the nearly saturated long-term p otentiation and not due to a decreased long-term potentiation expressi on in these slices. In contrast to long-term depression, in which the slope of the field excitatory postsynaptic potentials consistently and significantly decreased below the baseline level, the nearly saturate d depotentiation did not decrease below the original, pre-long potenti ation baseline level. The results demonstrate that increasing age redu ces expression of long-term depression and the dynamic range of transm ission plasticity. (C) 1998 IBRO. Published by Elsevier Science Ltd.