The severe nervous system developmental risks of early life exposure to lead (Pb2+) are well known. It is now becoming increasingly clear that much lower concentrations of Pb2+ can produce significant detrimental effects in children, heightening the need to understand the properties and extent of Pb2+ actions on the brain. Our laboratory has recently discovered that in vitro exposure to very low levels of Pb2+ produces long-term impairments in presynaptic transmitter release in cultured hippocampal neurons, and that these actions mimic those observed in animals lacking the trophic factor brain-derived neurotrophic factor (BDNF). We propose studies in hippocampal slices from rats exposed to low levels of Pb2+ during development to utilize 1) state-of- the-art two-photon imaging methods to assess long-term effects on presynaptic Ca2+ influx and vesicular transmitter release in intact synapses, and 2) whole-cell patch-clamp recording from CA1 pyramidal neurons to characterize the long-term effects of low level Pb2+ exposure on postsynaptic N-methyl-D-aspartate receptor (NMDAR)-gated currents. Our working hypothesis is that early Pb2+ exposure produces impairments in NMDAR function that leads to reduced BDNF synthesis and release and subsequent impairments of presynaptic transmission that are critical to normal cognitive function. One manipulation known to increase BDNF levels and release is an enriched environment. To test our hypothesis and identify potential methods of protecting the brain from developmental damage from Pb2+, we propose to 1) characterize the effects of low [Pb2+] exposure on BDNF gene expression, promoter methylation and TrkB receptor activation, and 2) test the ability of an enriched environment, exogenous intraventricular BDNF infusion or administration of the TrkB agonist 7,8-dihydroxyflavone to prevent Pb2+-induced long-term damage to NMDAR function and transmitter release. The research program we propose addresses the critical question of whether early developmental exposure to low-levels of Pb2+ than previously thought have long-term detrimental effects on brain function. These studies will provide novel information about Pb2+ effects on both presynaptic and postsynaptic mechanisms critical to cognition and memory storage. Further, we will examine novel therapeutic manipulations that elevate BDNF release and TrKB receptor activation in order to protect against the long-term detrimental effects of Pb2+ exposure.
It is well recognized that exposure to lead (Pb2+) causes severe effects to children's long-term cognitive function. New data indicates the concerning possibility that early exposure to much lower concentrations of Pb2+ produces significant detrimental effects that can be long-term, perhaps even life-long. In order to set rational limits and develop therapeutic strategies to this environmental risk to young children, we need to know much more about the scope of these low-level effects of Pb2+. New evidence suggests that very low levels of Pb2+ can lead to long-term impairments in presynaptic transmitter release, and that these effects may be due to impairment of N-methyl-d-aspartate glutamate receptor-dependent release of brain-derived neurotrophic factor (BDNF). The studies proposed in this application will extend these findings to intact synapses in brain slices, directly test ths working hypothesis about the mechanism of action of Pb2+ on transmitter release, and evaluate the potential of therapeutic manipulations to help protect children from such effects. The novel finding that exposure to low levels of Pb2+ can reduced BDNF levels in the brain has important implications to brain volume changes throughout the lifespan. It is likely that large populations o children being exposed to very low concentrations of Pb2+ have subtle but life-long cognitive and structural brain damage that only now it is beginning to be recognized. Thus, the need to understand the severity of this danger and the key mechanisms involved is critical to dealing with a Public Health threat of significant proportion.
