The effects of chronic lead exposure on long-term depression in area CA1 and dentate gyrus of rat hippocampus in vitro

Long-term potentiation (LTP) and long-term depression (LTD), two forms of s ynaptic plasticity, an believed to underlie the mechanisms of learning and memory. Previous studies have demonstrated that low-level lead exposure can impair the induction and maintenance of LTP in vivo and in vitro. The pres ent study was carried out to investigate whether the low-level lead exposur e affected the induction and maintenance of LTD. Neonatal Wistar rats were exposed to lead from parturition to weaning via milk of dams drinking 0.2% lead acetate solution. Field excitatory postsynaptic potentials (EPSPs) wer e recorded in hippocampal slices in adult rats (50-65 days) to study the al terations of LTD in area CA1 and dentate gyrus (DG) of hippocampus followin g chronic lead exposure. The input-output (I/O) curves before conditioning in both areas showed no evident alterations in basic synaptic transmission between the control and lead exposure groups. In area CA1, the mean amplitu de of EPSP slope in control rats (61+/-11%, n=15) decreased significantly g reater than that in lead-exposed rats (78 +/- 8%, n = 8, P < 0.05) followin g low frequency stimulation (LFS, 1 Hz, 15 min), which lasted at least 45 m in. In area DG, with application of the same LFS, the LTD was induced in co ntrol rats (72 +/- 22%, n = 8), while the LFS failed to induce LTD in lead- exposed rats (100 +/- 26%, n = 8). These results showed that chronic lead e xposure affected the induction of LTD in both area CA1 and DG. The effect o f lead on synaptic plasticity in area CA1 was also investigated. The altera tion of the amplitude of LTP in hippocampal slices caused by lead was reexa mined in order to compare with that on LTD (control: 189 +/- 23, n = 5; lea d-exposed: 122 +/- 12, n = 10). The result demonstrated that low-level lead exposure could reduce the range of synaptic plasticity, which might underl ie the dysfunction of learning and memory caused by chronic lead exposure. (C) 1999 Elsevier Science B.V. All rights reserved.