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.