Increased persistent Na⁺current contributes to seizure in the slamdance bang-sensitiveDrosophilamutant

Abstract

There is clinical need to extend the understanding of epilepsy and to find novel approaches to treat this condition. Bang-sensitive (bs) Drosophila mutants, which exhibit reduced thresholds for seizure, offer an attractive possibility to combine tractable genetics, electrophysiology, and high-throughput screening. However, despite these advantages, the precise electrophysiological aberrations that contribute to seizure have not been identified in any bs mutant. Because of this, the applicability of Drosophila as a preclinical model has not yet been established. In this study, we show that electroshock of bs slamdance (sda) larvae was sufficient to induce extended seizure-like episodes. Whole cell voltage-clamp recordings from identified motoneurons (termed aCC and RP2) showed synaptic currents that were greatly increased in both amplitude and duration. Current-clamp recordings indicated that these inputs produced longer-lived plateau depolarizations and increased action potential firing in these cells. An analysis of voltage-gated currents in these motoneurons, in both first and third instar larvae, revealed a consistently increased persistent Na(+) current (I(Nap)) and a reduced Ca(2+) current in first instar larvae, which appeared normal in older third instar larvae. That increased I(Nap) may contribute to seizure-like activity is indicated by the observation that feeding sda larvae the antiepileptic drug phenytoin, which was sufficient to reduce I(Nap), rescued both seizure-like episode duration and synaptic excitation of motoneurons. In contrast, feeding of either anemone toxin, a drug that preferentially increases I(Nap), or phenytoin to wild-type larvae was sufficient to induce a bs behavioral phenotype. Finally, we show that feeding of phenytoin to gravid sda females was sufficient to both reduce I(Nap) and synaptic currents and rescue the bs phenotype in their larval progeny, indicating that a heightened predisposition to seizure may arise as a consequence of abnormal embryonic neural development.

Slamdance: seizing a fly model for epilepsy

“…To better understand seizures and how they may be prevented requires access to animal models where behavior and neurophysiology can be easily studied together, ideally in mutant strains relevant to human epilepsies. A recent study by Marley and Baines (2011) presents just such a model using the fruit fly Drosophila melanogaster, and in the process, this study provides evidence for correction mechanisms in two different neurons in response to seizure-inducing mutations or anti-epileptic drug (AED) therapy.…”

“…Limited access to brain electrophysiology in the fly has prevented a better understanding of how these mutations lead to epileptiform activity (Glasscock and Tanouye 2005;Pavlidis et al 1994). However, Marley and Baines (2011) provide a fresh insight by recording from larval neurons in the brains of slamdance (sda) b-s mutants. Voltage-gated sodium currents are compromised in sda mutants due to a defective aminopeptidase pathway (Zhang et al 2002), producing an epileptic phenotype in fly larvae following electric shock.…”

“…It has been reported that some children born to epileptic mothers who were administered AEDs throughout their pregnancies had impaired cognitive functions (Hanson et al 1976;Holmes 2009), and genetic predisposition to seizures was still evident in these children as well (Hanson et al 1976;Holmes 2009). Marley and Baines (2011) conducted an experiment in Drosophila to determine the effects on progeny of feeding AEDs to gravid seizure-prone sda females. Gravid sda females were fed phenytoin, which decreases I Nap and prevents seizures in those animals, as discussed above.…”

“…Therefore, why such a clear-cut result in Drosophila? Marley and Baines (2011) hypothesize that imbalances in excitation and inhibition in early stages of neural development could lead to seizures. Early pharmaceutical intervention by administering phenytoin may correct for this imbalance or prevent the imbalance from occurring.…”

“…The study by Marley and Baines (2011) demonstrates that the slamdance mutant at the larval developmental stage is an excellent model for studying the underlying pathophysiology of seizure susceptibility. Similar dysfunctions in sodium channels associated with seizure susceptibility are also seen in humans and rodents.…”