Lead (Pb) toxicity remains a significant public health problem among the population in this country with no threshold yet identified below which adverse effects are absent. Exposure that leads to blood Pb levels as low as 10 microgram/100 ml has been shown to cause significant impairment of cognitive functioning and significant delays in behavioral development in children. Of particular concern is that these alterations have persisted long after blood Pb levels have decreased following cessation of exposure. However, the magnitude and duration of exposure necessary to produce these deficits and the mechanisms underlying their manifestation have remained elusive. The use of neurochemical and neurophysiological approaches to investigate CNS dysfunction in rats has been fruitful in discriminating Pb toxicity in the presence of consistently applied exposure protocols. This research proposal consists of an investigation of the glutamate-mediated regulation of intracellular Ca+2 concentrations and induction of synaptic plasticity in hippocampal neurons of Pb-exposed rats. It is hypothesized that chronic developmental exposure to Pb impairs neuronal processes underlying synaptic plasticity via Pb-induced alterations at glutamatergic synapses that directly modulate intracellular Ca+2 levels. Experimentation will focus on the magnitude of the neurotransmitter signal generated at these synapses and the functional integrity of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor as well as definition of hippocampal physiology, particularly the ability to induce long-term potentiation (LTP), a model of synaptic plasticity. The NMDA receptor is associated with a ligand-operated Ca+2 channel and is primarily responsible for modulation of the Ca+2 entry into the neuron associated with synaptic plasticity. Studies will examine the effects of Pb exposure on hippocampal glutamate release employing intracerebral dialysis and paired-pulse facilitation of synaptic responses. NMDA-stimulated Ca+2 influx will be examined by the use of microdialysis and synaptoneurosomes, respectively, and by isolation of the NMDA component of the train-evoked synaptic response. Glutamate-stimulated binding of a radiolabelled probe of the NMDA receptor-operated ion channel will be conducted to identify a specific site of Pb action at these synapses. Other experiments will demonstrate the direct relationship of Pb-induced changes in Ca+2 influx with NMDA-induced potentiation in hippocampal slices by quantifying alterations in field potentials independent of Pb's effects on voltage-sensitive Ca+2 channels. These studies will determine the synaptic processes impaired by Pb exposure using functional neurophysiological measures in vivo to validate neurochemical findings so that subsequent studies can define precise toxic mechanisms. An animal model of asymptomatic Pb exposure will be utilized with hematologic and nutritional characteristics identical to those in the pediatric population at risk. Observed reductions in hippocampal glutamatergic synaptic function following exposure to Pb will provide a strong basis for the deficits in development and learning found clinically in children exposed to untoward levels of Pb. As a result of information obtained on toxic mechanisms, more sensitive and appropriate measures of toxicity can be developed and improved strategies for prevention, treatment and reversal of toxicity can be established.
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