Chronic lead exposure alters presynaptic calcium regulation and synaptic facilitation in Drosophila larvae

Prolonged exposure to inorganic lead (Pb²⁺) during development has been shown to influence activity-dependent synaptic plasticity in the mammalian brain, possibly by altering the regulation of intracellular Ca²⁺ concentration (Ca²⁺]_i). To explore this possibility, we studied the effect of Pb²⁺ exposure on Ca²⁺]_i regulation and synaptic facilitation at the neuromuscular junction of larval Drosophila. Wild-type Drosophila (CS) were raised from egg stages through the third larval instar in media containing either 0 μM, 100 μM or 250 μM Pb²⁺ and identified motor terminals were examined in late third-instar larvae. To compare resting Ca²⁺]_i and the changes in Ca²⁺]_i produced by impulse activity, the motor terminals were loaded with a Ca²⁺ indicator, either Oregon Green 488 BAPTA-1 (OGB-1) or fura-2 conjugated to a dextran. We found that rearing in Pb²⁺ did not significantly change the resting Ca²⁺]_i nor the Ca²⁺ transient produced in synaptic boutons by single action potentials (APs); however, the Ca²⁺ transients produced by 10 Hz and 20 Hz AP trains were larger in Pb²⁺-exposed boutons and decayed more slowly. For larvae raised in 250 μM Pb²⁺, the increase in Ca²⁺]_i during an AP train (20 Hz) was 29% greater than in control larvae and the Ca²⁺]_i decay τ was 69% greater. These differences appear to result from reduced activity of the plasma membrane Ca²⁺ ATPase (PMCA), which extrudes Ca²⁺ from these synaptic terminals. These findings are consistent with studies in mammals showing a Pb²⁺-dependent reduction in PMCA activity. We also observed a Pb²⁺-dependent enhancement of synaptic facilitation at these larval neuromuscular synapses. Facilitation of EPSP amplitude during AP trains (20 Hz) was 55% greater in Pb²⁺-reared larvae than in controls. These results showed that Pb²⁺ exposure produced changes in the regulation of Ca²⁺]_i during impulse activity, which could affect various aspects of nervous system development. At the mature synapse, this altered Ca²⁺]_i regulation produced changes in synaptic facilitation that are likely to influence the function of neural networks.