MATURATION OF LONG-TERM POTENTIATION IN THE HIPPOCAMPAL DENTATE GYRUSOF THE FREELY MOVING RATE

The ability of the perforant path/dentate granule cell synapse of the hippocampal formation to establish and maintain enhanced levels of syn aptic transmission in response to tetanization (longterm potentiation, LTP) was investigated in freely moving rats at 15, 30, and 90 days of age. Measures of 1) the slope of the population excitatory postsynapt ic potential (EPSP), and 2) the population spike amplitude (PSA) obtai ned before, and at several times following tetanization, were used to evaluate the magnitude and duration of LTP as a function of age. Signi ficant enhance ment of both EPSP slope and PSA measures was obtained f rom animals of all three ages in response to perforant path tetanizati on. The initial degree of enhancement was essentially the same across the age groups, ranging from +27% to +38% of pretetanization levels fo r EPSP slope measures and +60% to +75% of pretetanization levels for P SA measures, obtained 15 min after tetanization. The duration of this enhancement obtained from animals of the preweaning group was signific antly longer than that obtained from either 30- or 90-day-old animals. Enhanced measures of both EPSP dope and PSA decayed to baseline level s in these older animals 18 to 24 h after tetanization, while animals tetanized at 15 days of age maintained potentiated levels of both meas ures for a period of 5 days following tetanization. Tetanization of 15 -day-old animals resulted in a significant reduction in the latency to EPSP onset without affecting the time-based relationships among the o ther measured parameters, which included latency of the population spi ke onset, population spike minimum, and population spike offset. Tetan ization had no effect on the latency measure of any of these parameter s in either of the two older age groups. The primarily postnatal devel opment of the dentate granule cell population suggests that both funct ionally immature GABAergic modulation of granule cell activity and the differential development of components of the N-methyl-D-aspartate re ceptor complex may be involved in the age-related differences in the i nduction and expression of the LTP phenomenon. This study represents t he first developmental characterization of LTP in the perforant path/d entate granule cell synapse in freely moving rats during early develop ment. The results indicate that LTP can be reliably established and ma intained in behaving rats as young as 15 days of age. Whereas the degr ee of potentiation at this age is equivalent to that obtained from juv eniles and young adults, the duration of the effect significantly outl asts that obtained from older animals in which development of the dent ate gyrus is more functionally mature. (c) 1994 Wiley-Liss, Inc.