Biological Plausibility

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The NMDA receptor activation by glutamate during development increases calcium influx, which acts as a secondary signal. Eventually, immediate early genes (IEG) activation is triggered by transcription factors and the proteins required for synapse formation are produced (reviewed in Ghiani et al., 2007).

Glutamate released from entorhinal cortex neurons has been shown to promote synaptogenesis in developing targeted hippocampal neurons (Mattson et al., 1988). Similarly, glutamate has been found to regulate synaptogenesis in the developing visual system of frogs (Cline and Constantine-Paton, 1990).

The ratio of synaptic NR2B over NR2A NMDAR subunits controls spine motility and synaptogenesis, and it has been suggested a structural role for the intracellular C terminus of NR2 in recruiting the signaling and scaffolding molecules necessary for proper synaptogenesis (Gambrill and Barria, 2011).

Empirical Evidence

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Include consideration of temporal concordance here

There is no direct evidence linking reduced presynaptic release of glutamate to decreased synaptogenesis as they have not been ever measured both in the same study after exposure to stressors. However, there are findings that strongly link reduced presynaptic release of glutamate to LTP.

Indeed, measures of presynaptic function at glutamatergic synapses in chronically exposed animals have produced results that can be related to the effects of Pb2+ on glutamate and LTP. Focal perfusion of high K+ is used to measure glutamate release and define the Ca+2-dependent and Ca+2-independent components by inclusion or removal of Ca+2 from the perfusion fluid. Animals exposed to 0.2% Pb2+ cause decrease in K+-evoked hippocampal glutamate release, which is an important factor in the elevated threshold and diminishes magnitude of hippocampal LTP (Gilbert et al., 1996, 1999; Lasley and Gilbert, 1996). Furthermore, the same research group showed that chronic exposure to 0.2% Pb2+ diminishes only the K+-stimulated increase in total extracellular glutamate compared to that in control but not in animals under Ca+2-free conditions, suggesting that the exposure-induced decrease in total glutamate release is due to Pb2+ -related decrements in the Ca+2-dependent component.

In animals exhibiting blood Pb2+ levels of 30-40 μg/100 ml, the perforant path stimulation to induce paired-pulse facilitation in dentate gyrus, which is a measure that is primarily mediated by enhanced glutamate release, is reduced (Lasley and Gilbert, 1996; Ruan et al., 1998). Microdialysis experiment in animals with the same Pb2+ values show diminished depolarization-induced hippocampal glutamate release (Lasley and Gilbert, 1996; Lasley et al., 1999).

In another study, rats continuously exposed to 0.1–0.5% Pb2+ in the drinking water beginning at gestational day 15-16 show decrease in total K+-stimulated hippocampal glutamate release (Lasley and Gilbert, 2002). Maximal effects have been seen in the 0.2% group (blood Pb = 40 μg/100 ml). However, these effects have been less evident in the 0.5% group and are no longer present in the 1.0% Pb2+ group (Lasley and Gilbert, 2002). The same finding was found in hippocampal cultures and brain slices acutely exposed to Pb2+ (Braga et al., 1999; Xiao et al., 2006).

More recently, Pb2+ has also been shown to decrease the levels of the vesicular proteins synaptophysin and synaptobrevin and inhibit vesicular release (Neal et al., 2010). Furthermore the same group has reported that chronic in vivo exposure to Pb2+ during development results in a marked inhibition of Schaffer-collateral-CA1 synaptic transmission by inhibiting vesicular release of glutamate, an effect that is not associated with a persistent change in presynaptic calcium entry (Zhang et al., 2015).

Uncertainties and Inconsistencies

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Response-response Relationship

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Time-scale

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Known modulating factors

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Known Feedforward/Feedback loops influencing this KER

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