Neonatal stress alters LTP in freely moving male and female adult rats

We previously reported that neonatal isolation stress significantly changes measures of hippocampal long-term potentiation (LTP) in male and female ju venile rats, i.e., at 30 days of age. The changes in dentate granule popula tion measures, i.e., excitatory postsynaptic potential (EPSP) and populatio n spike amplitude (PSA), evoked by tetanization of the medial perforant pat hway, indicated that juvenile rats exposed to neonatal isolation exhibit di fferent enhancement profiles with respect to both the magnitude and duratio n of LTP in a sex-specific manner. Isolated males showed a significantly gr eater enhancement of LTP, while female "isolates" showed significantly long er LTP duration when compared to all other groups. The present study was de signed to determine whether the effects of the neonatal isolation stress pa radigm endures into adulthood. Rats isolated from their mothers for 1 h per day during postnatal days 2-9 were surgically prepared at 70-90 days of ag e, with stimulating and recording electrodes placed in the medial perforant pathway and the hippocampal dentate gyrus, respectively Prior to tetanizat ion, no significant effect of sex or treatment was obtained for baseline me asures of EPSP slope or PSA. In order to rule out baseline differences in h ippocampal cell excitability in female adult rats, we measured the response of dentate granule cells for one estrus cycle and found no pretetanization enhancement in the evoked response in either controls or previously stress ed rats. Following tetanization, there was a significant treatment and sex effect. During the induction of LTP, PSA values were significantly enhanced in both isolated males and females and had significantly longer LTP durati on when compared to the unhandled control group. Additionally, we observed that females took longer to reach baseline levels than males. Taken togethe r, these results indicate that repeated infant isolation stress enhances LT P induction and duration in both males and females. These results indicate that infant stress alters hippocampal neuroplasticity in such a way that it s effect endures into adulthood. (C) 1999 Wiley-Liss, Inc.