The influence of developmental period of lead exposure on long-term potentiation in the adult rat dentate gyrus in vivo

Previous work has demonstrated an increase in the threshold for induction o f long-term potentiation (LTP) in the dentate gyrus of animals chronically exposed to lead (Pb) from birth (Gilbert et al., 1996). The present study s ought to extend these findings by evaluating the developmental periods crit ical for Pb-induced impairment oft TP. Rats were exposed to Pb through mate rnal milk and/or the drinking water over different developmental intervals: 1) beginning just prior to birth and continuing throughout life (PL); 2) b eginning just prior to birth and terminating at weaning (PW); or 3) contino usly from the early post-weaning period throughout life (WL). Pregnant darn s received 0.2% Pb-acetate in the drinking water on gestational day (GD)16, with male offspring switched to the same solution (PL group) or tap water (PW group) at weaning on postnatal day (PND)21. Postweaning exposure began on PND30 and continued throughout life. As adults (PND130-210), field poten tials evoked by perforant path stimulation were recorded in the dentate gyr us under urethane anesthesia, and an ascending series of stimulus trains wa s administered to induce LTP and to determine its threshold. The magnitute of population spike IFS) LTP was reduced relative to controls in animals ex posed throughout life (PL) and in animals exposed after weaning (WL). No im pairment in PS LTP was evident in animals removed from Pb at weaning and te sted as adults (PW). Similarly, thresholds for induction of PS LTP were ele vated relative to controls in the PL and WL groups, but were not affected b y Pb exposure limited to the lactational period (PW). Reductions in the mag nitude of LTP of the EPSP slope were evident in posttrain I/O functions in all Pb-exposed groups, including the PW group. An elevated LTP threshold wa s evident in the EPSP slope measure in the continuously exposed group (PL) only. Thus Pb exposure restricted to the lactational period appeared less d isruptive to adult LTP in the dentate gyrus than continuous exposure beginn ing around birth or weaning. However, EPSP slope LTP was impaired in animal s exposed to Pb for as little as 30 days in the early postnatal period. An attenuated ability to support neuroplastic change in synaptic function may contribute to cognitive deficits associated with Pb-induced toxicity. (C) 1 999 Intox Press, Inc.