Synthetic Ciguatoxin CTX 3C Induces a Rapid Imbalance in Neuronal Excitability Victor Martín, † Carmen Vale,* ,† Masahiro Hirama, ‡ Shuji Yamashita, ‡ Juan Andre ́ s Rubiolo, † Mercedes R. Vieytes, § and Luis M. Botana* ,† † Departamento de Farmacología, Facultad de Veterinaria, Universidad de Santiago de Compostela, 27002 Lugo, Spain ‡ Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan § Departamento de Fisiología, Facultad de Veterinaria, Universidad de Santiago de Compostela, 27002 Lugo, Spain * S Supporting Information ABSTRACT: Ciguatera is a human global disease caused by the consumption of contaminated fish that have accumulated ciguatoxins (CTXs), sodium channel activator toxins. Symptoms of ciguatera include neurological alterations such as paraesthesiae, dysaesthesiae, depression, and heightened nociperception, among others. An important issue to understand these long-term neurological alterations is to establish the role that changes in activity produced by CTX 3C represent to neurons. Here, the effects of synthetic ciguatoxin CTX 3C on membrane potential, spontaneous spiking, and properties of synaptic transmission in cultured cortical neurons of 11-18 days in vitro (DIV) were evaluated using electrophysiological approaches. CTX 3C induced a large depolarization that decreased neuronal firing and caused a rapid inward tonic current that was primarily GABAergic. Moreover, the toxin enhanced the amplitude of miniature postsynaptic inhibitory currents (mIPSCs), whereas it decreased the amplitude of miniature postsynaptic excitatory currents (mEPSCs). The frequency of mIPSCs increased, whereas the frequency of mEPSCs remained unaltered. We describe, for the first time, that a rapid membrane depolarization caused by CTX 3C in cortical neurons activates mechanisms that tend to suppress electrical activity by shifting the balance between excitatory and inhibitory synaptic transmission toward inhibition. Indeed, these results suggest that the acute effects of CTX on synaptic transmission could underlie some of the neurological symptoms caused by ciguatera in humans. ■ INTRODUCTION Neuronal homeostasis plays an essential role in the formation, maintenance, and modification of neuronal circuits and provides neurons with a reliable way to adapt to changes in levels of activity. 1,2 Neuronal changes in intracellular sodium concentration control action potential generation and mediate forms of synaptic plasticity that depend on neuronal firing. 3,4 In this sense, marine neurotoxins acting on voltage gated sodium channels (VGSC) have been very useful in the study of the role of excitability in various synaptic preparations. 5 The VGSC activator brevetoxin (PbTx) has been shown to increase N- methyl-D-aspartate receptor (NMDA) function and promote neurite growth in immature cortical neurons at concentrations that do not modify the intracellular calcium concentration. 4 Among the marine toxins that activate sodium channels, CTXs are the most potent activators of VGSC, causing cell membrane depolarization at rest, by increasing sodium influx, leading to persistent neurological changes in humans intoxicated by contaminated fish containing CTXs. 6-12 The physiological consequence of the binding of ciguatoxins to sodium channels is an initial increase in cellular excitability, which results in spontaneous and repetitive firing of action potentials, followed by a decrease in excitability as the membrane further depolarizes. 13 However, most of the studies to date have focused on the effects of ciguatoxin in the excitability of peripheral nervous system and not in central neurons. An important issue to understand the neurological consequences of ciguatera food poisoning in humans is to establish the role that changes in activity elicited by CTX could represent to central neurons. The neurological sequel of ciguatera fish poisoning in humans usually resolves within weeks of onset; however, some nervous symptoms may persist for months or even years. 14 In humans, symptoms of ciguatera fish poisoning manifest with paraesthesiae, dysaesthesiae, and heightened nociception as well as sensory abnormalities, which include subjective features of metallic taste, pruritus, arthralgia, myalgia, and dental pain. 14 Interestingly, alterations in inhibitory transmission are involved in altered body temper- ature and ataxia, 15 both of which are observed in ciguatera. 11,16 Received: December 9, 2014 Published: May 6, 2015 Article pubs.acs.org/crt © 2015 American Chemical Society 1095 DOI: 10.1021/tx500503d Chem. Res. Toxicol. 2015, 28, 1095-1108