Cross-species pharmacological characterization of the allylglycine seizure model in mice and larval zebrafish

Abstract

Treatment-resistant seizures affect about a third of patients suffering from epilepsy. To fulfill the need for new medications targeting treatment-resistant seizures, a number of rodent models offer the opportunity to assess a variety of potential treatment approaches. The use of such models, however, has proven to be timeconsuming and labor-intensive. In this study, we performed pharmacological characterization of the allylglycine (AG) seizure model, a simple in vivo model for which we demonstrated a high level of treatment resistance. (D,L)-Allylglycine inhibits glutamic acid decarboxylase (GAD) -the key enzyme in γ-aminobutyric acid (GABA) biosynthesis -leading to GABA depletion, seizures, and neuronal damage. We performed a side-by-side comparison of mouse and zebrafish acute AG treatments including biochemical, electrographic, and behavioral assessments. Interestingly, seizure progression rate and GABA depletion kinetics were comparable in both species. Five mechanistically diverse antiepileptic drugs (AEDs) were used. Three out of the five AEDs (levetiracetam, phenytoin, and topiramate) showed only a limited protective effect (mainly mortality delay) at doses close to the TD 50 (dose inducing motor impairment in 50% of animals) in mice. The two remaining AEDs (diazepam and sodium valproate) displayed protective activity against AG-induced seizures. Experiments performed in zebrafish larvae revealed behavioral AED activity profiles highly analogous to those obtained in mice. Having demonstrated crossspecies similarities and limited efficacy of tested AEDs, we propose the use of AG in zebrafish as a convenient and high-throughput model of treatment-resistant seizures.

Phenotypic Characterization of Larval Zebrafish (Danio rerio) with Partial Knockdown of the cacna1a Gene

“…In this study, we observed that spontaneous epileptiformlike events in the form of abrupt high-voltage spikes, spikewave complexes, and polyspike-wave discharges occurred in 92% of cacna1aa morphants. Previous studies indicated that chemicals (e.g., pentylentetrazole or allylglycine) [37,39,53] as well as mutations in different genes (e.g., scn1lab, aldh7a1) [22,24] increase locomotor activity in zebrafish larvae reminiscent of tonic-clonic-like seizures. Indeed, tonic-clonic-like seizures have been described both in genetic and pharmacological models of epilepsy in zebrafish [22,54].…”

Inhibition of glutamate decarboxylase (GAD) by ethyl ketopentenoate (EKP) induces treatment-resistant epileptic seizures in zebrafish

“…compared AG-induced acute seizures in zebrafish larvae and adult mice. The results showed clear cross-species similarities with regard to seizure behavior and demonstrated limited efficacy of ASDs 13 . A drawback of the AG zebrafish model is that seizures occurred asynchronously and with a long latency to onset.…”

“…Besides ease of handling and fast reproduction rate, zebrafish share high genetic, cellular and organ homologies to humans 10 . Consequently, over recent years several chemical and genetic zebrafish models of acute seizures or epilepsy have been generated either by immersion of larvae in chemical proconvulsants like PTZ 11 , 12 or D-AG 13 , or by knocking down or introducing mutations in epilepsy susceptible genes including lgi1 14 , scn1Lab 15 , 16 , and kcnq3 17 .…”

“…Consequently, a substantial number of ASDs target the GABAergic system to suppress seizures through enhancing GABA-mediated inhibition (benzodiazepines, and barbiturates), inhibiting GABA catabolism (valproate and vigabatrin) or preventing GABA reuptake (tiagabine). Conversely, chemicals like PTZ, bicuculline, penicillin and picrotoxin that antagonize the GABAergic pathway can cause seizures 13 , 19 .…”

Building a zebrafish toolkit for investigating the pathobiology of epilepsy and identifying new treatments for epileptic seizures

“…Within just a few minutes of treating zebrafish larvae or adults with PTZ, epileptiform discharges and immediate-early gene expression are detectable in the brain, and accompanied by vigorous locomotor convulsive behaviours (Baraban et al, 2005;Mussulini et al, 2013;Afrikanova et al, 2013). PTZ has become the most widely used and extensively validated pharmacological inducer of acute seizures in zebrafish, but it is only one of a wide range of chemicals that have been shown to induce seizures and convulsions in zebrafish (Winter et al, 2008), and further studies have extended this range to include other well-characterized experimental convulsants which are known to be effective in mammals, such as Kainic Acid (Alfaro et al, 2011), its marine neurotoxin analogue Domoic Acid (Tiedeken and Ramsdell, 2007), the plant natural product ginkgotoxin (Lee et al, 2012) and Allyl-Glycine (Leclercq et al, 2015). Thus, many compounds that provoke seizures in mammals, do so with comparable efficacy in zebrafish.…”