Seizure and epilepsy: studies of seizure disorders in Drosophila

Abstract

Despite the frequency of seizure disorders in the human population, the genetic and physiological basis for these defects has been difficult to resolve. Although many genetic contributions to seizure susceptibility have been identified, these involve disparate biological processes, many of which are not neural specific. The large number and heterogeneous nature of the genes involved makes it difficult to understand the complex factors underlying the etiology of seizure disorders. Examining the effect known genetic mutations have on seizure susceptibility is one approach that may prove fruitful. This approach may be helpful in both understanding how different physiological processes affect seizure susceptibility and identifying novel therapeutic treatments. We review here factors contributing to seizure susceptibility in Drosophila, a genetically tractable system that provides a model for human seizure disorders. Seizure-like neuronal activities and behaviors in the fruit fly are described, as well as a set of mutations that exhibit features resembling some human epilepsies and render the fly sensitive to seizures. Especially interesting are descriptions of a novel class of mutations that are second-site mutations that act as seizure suppressors. These mutations revert epilepsy phenotypes back to the wild-type range of seizure susceptibility. The genes responsible for seizure suppression are cloned with the goal of identifying targets for lead compounds that may be developed into new antiepileptic drugs.

Fig. 1. Drosophila para^bss1mutant behavior

(A) Cartoon depicting stereotypic behavioral phenotype of para^bss1 flies subjected to a mechanical shock (10 s vortex = BANG): initial seizure-like behavior, followed by complete paralysis, then a tonic–clonic-like period that is unique to para^bss1 and not evident in other BS mutant genotypes. In the figure, one clonus-like event is depicted, but the number can vary, as can the duration of the period. The tonic–clonic-like period is followed by a recovery seizure and the fly then recovers. Not depicted is a quiescent period of variable duration often observed between the recovery seizure and recovery, as well as the refractory period during which flies are resistant to further seizures that occurs immediately following recovery. (B) For para^bss1/Y hemizygous males, recovery time from behavioral paralysis is substantially longer than for para^bss1/+ heterozygous females or for another BS mutant, eas^PC80/Y.

Fig. 2. The BS mutantsdais more susceptible to seizures than wild type, and therefore has a lower seizure threshold

(A) seizure-like activity (initial seizure) is elicited in a para^bss1 fly by a high-frequency stimulus of low strength (4 V) and displayed at a high sweep speed. (B) A low-voltage high-frequency (HF) stimulus of 8 V fails to elicit a seizure in a wild-type Canton-Special (CS) fly because the stimulus is below the seizure threshold. Following the HF stimulus artifact, there is no seizure activity observed in this recording displayed at a high sweep speed. Note also that there is no period of synaptic failure and single-pulse stimulation of the giant fiber (GF) (0.5 Hz) continues to evoke dorsal longitudinal muscle (DLM) potentials. Two such effective single-pulse stimuli are depicted in this trace; each was effective in evoking a DLM potential. (C) Seizure-like activity is elicited in a wild-type CS fly by a high-voltage HF stimulus (32 V), which is above the threshold for seizure. The seizure in this recording begins within the large stimulus artifact and is displayed at a high sweep speed. Vertical calibration bar is 20 mV. Horizontal calibration bar is 300 ms. Adapted from Kuebler et al., 2001.

Fig. 3. Complex seizure-like electrophysiology phenotypes in para^bss1 mutants

(A) Electrical recording of seizure-like neuronal activity and synaptic failure in a para^bss1/Y animal subjected to a 4-V high-frequency electrical stimulus (HFS), as well as single-pulse stimuli to trigger the giant fiber circuit, allowing assessment of synaptic function (giant fiber response, GFR). Following an initial seizure (IS) is a period of synaptic failure within which GF stimulation fails to evoke a response (SF^1–5), unlike that seen prior to the seizure. As depicted, during the period of synaptic failure, spontaneous secondary seizures are observed (SS^1–4). Although in this trace four secondary seizures are observed, the number is variable. A final recovery seizure (RS) is observed, and shortly thereafter, GF system transmission is restored (response recovery, RR). (B) Initial seizure and secondary seizures at higher sweep speed. (C) Recovery seizure at higher sweep speed. Calibration bar is 20 mV, 10 s in (A), 20 mV, 1.5 s in (B) and (C).