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

Abstract

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

A Drosophila model for toxicogenomics: Genetic variation in susceptibility to heavy metal exposure

“…Lead exposure is especially detrimental during early development and even low doses can result in intellectual disability [ 16 – 20 ] as well as behavioral disorders [ 21 – 24 ]. The neurotoxic effects of lead on the nervous system may be mediated through its effects on the function of hippocampal NMDA receptors and inhibition of presynaptic calcium channels [ 25 , 26 ]. Lead compounds have been used in paints, and lead paint exposure is a major route for lead ingestion in children [ 27 , 28 ].…”

Accumulation, elimination, sequestration, and genetic variation of lead (Pb2+) loads within and between generations of Drosophila melanogaster

“…Together these mechanisms may serve to minimize Pb exposure in the brain and to offspring, especially during the first d of adult life when experience plays an important role in the development of complex behaviors (Hirsch et al, 2003). The Drosophila model facilitates extending behavioral observations to include genetic (Hirsch et al, 2009; Ruden et al, 2009) and neural mechanisms (He et al, 2009; Morley et al, 2003) mediating these effects, as well as transgenerational effects of anthropogenic Pb.…”

“…Several measurable endpoints have been suggested by others for the use of Drosophila as a toxicology model (Rand, 2010; Rand et al, 2014); these include, but are not limited to, lethality, morphology, behavioral traits, embryonic development, and targeted mechanistic studies. We have established D. melanogaster as a model system to investigate the behavioral, physiological, and genetic effects of chronic, developmental Pb exposure (He et al, 2009; Hirsch et al, 2003; Hirsch et al, 2009; Morley et al, 2003; Ruden et al, 2009). …”

Adverse Outcome Pathway on chronic binding of antagonist to N-methyl-D-aspartate receptors (NMDARs) during brain development induces impairment of learning and memory abilities

“…Besides the above studies described in rodents, intracellular Ca2+ regulation has been studied at the neuromuscular junction of larval Drosophila exposed to 0, 100 μM or 250 μM Pb2+ (He et al, 2009 …”