Potassium bromide, an anticonvulsant, is effective at alleviating seizures in the Drosophila bang-sensitive mutant bang senseless

Abstract

Human seizure disorders are a major health concern due to the large number of affected individuals, the potentially devastating consequences of untreated seizure occurrences, and the lack of an effective treatment for all patients. Although anticonvulsants have proven very helpful in treating seizures and remain the best option available for treatment, not all afflicted individuals respond to medication and many only do so in unique drug combinations or at the cost of adverse side-effects. Therefore, new and more effective anticonvulsants are continually sought after to combat this illness. In this study, we present results which offer the possibility of using Drosophila bang-sensitive (BS) mutants as a tool to screen anticonvulsants. By feeding the BS mutants a known anticonvulsant, potassium bromide, we have demonstrated that the drug dramatically reduces the seizures of bang senseless, the most severe of the BS mutants. This methodology suggests that the Drosophila system can potentially be a powerful instrument for assaying and testing new compounds with anticonvulsant properties.

Introduction

Epilepsy, a disorder characterized by recurrent seizures, is a prevalent neurological problem affecting more than 1% of the human population [16]. Seizures are brought about by insults to the brain such as head trauma, electroconvulsive shock, or illness and are discernible as disordered, synchronized firing of large neuronal populations in the brain [30], [22]. To date, anticonvulsants remain the most commonly used and effective remedy for treating epilepsy, although there are limitations associated with these medications. Firstly, more than one-third of all epileptic patients have intractable epilepsy; that is, these individuals still suffer spontaneous seizures despite drug treatment [12]. Furthermore, some epileptics respond only to a combination of drugs, an indication that more efficient anti-convulsants would be desirable. Finally, many anticonvulsants confer serious side-effects to the patients, such as ataxia, liver damage, and acneiform rashes. Consequently, there is a continual need to develop newer, more effective, and safer anticonvulsants to combat a disorder with such a diverse display of origins and treatment profiles.

Our laboratory uses a family of Drosophila melanogaster behavioral mutants, the bang-sensitives (BS), as a model with which to study seizures. The BS mutants possess an intriguing phenotype: when given a mechanical shock (such as a tap of the vial) or an electrical stimulus (wavetrains delivered to the central nervous system (CNS) of the fly), the BS mutants suffer from cycles of seizures and temporary paralysis (Fig. 1)[1], [5], [18]. There are many similarities between the seizures observed in these mutants and those seen in humans: (1) all individuals have a seizure threshold, (2) electrical shocks of a sufficient level can raise the seizure threshold, (3) seizure sensitivity can be modified by genetic backgrounds, (4) seizures can be segregated into specific regions of the CNS, and (5) seizures spread through the nervous system along particular pathways that are dependent on functional synaptic connections and recent electrical activity. Along with the powerful methodologies available for Drosophila, the BS mutants appear to be excellent models with which to study seizure susceptibility.

Because of the BS mutants' significantly lowered seizure sensitivity, our laboratory has explored the idea of using them as a means to evaluate compounds for anticonvulsant activity. We demonstrated the feasibility of this scheme in a previous study: by micro-puncturing the fly's head, infusing an anticonvulsant (valproate) solution into the brain, and then assaying the seizure sensitivity of the fly we were able to demonstrate that an anticonvulsant can suppress the BS phenotype [8]. Therefore, it appears that the BS phenotype can be used as a functional assay for anticonvulsant activity. However, although the method in the aforementioned study is conceptually simple, in practice it is technically difficult and invasive; therefore, it would be impractical to utilize this method in a mass-screen for anticonvulsants. Our hope is that we can develop a technique simple enough to test many flies simultaneously and at the same time sufficiently powerful to detect efficacy in a compound.

In this article we present the results of a new method we have developed towards this aim: direct feeding. This study focuses on examining whether the BS mutants would respond to one particular anticonvulsant, potassium bromide. Originally used in an anticonvulsant role in 1853, potassium bromide is still used occasionally on humans, particularly in patients with refractory seizures or tonic–clonic seizures [2], [24], [26], [28]. It has also gained popularity in recent years among veterinarians, especially in canine epilepsy where phenobarbital, the most popular remedy prescribed, has debilitating sideeffects [3], [20], [21], [25]. It is believed that potassium bromide functions by stabilizing excitable membrane through hyperpolarization of neurons, similar to the mechanism of ã-aminobutyric acid (GABA) [4], [15], [17], [31]. We chose the drug for this study because of its attractive combination of availability, cost, solubility, and ease of use. We demonstrate that potassium bromide has a dramatic effect on the seizures observed in bang senseless (bss), the most ‘severe’ mutant of the BS family whose gene product is currently unknown. Using both behavioral assays and electrophysiological analyses in various feeding experiments, we have shown that the seizures in bss mutants are reduced by about 50% and are noticeably less intense. This study, which demonstrates that a feeding regimen in Drosophila alone suffices to assay anticonvulsant activity, may provide a powerful method to use in the future for screening novel anticonvulsant compounds.