Time course and role of extracellular Ca2+ in veratridine-induced glutamate release

Sodium channel toxins and neurotransmitter release

Voltage-dependent sodium channels (VDSC) are an important class of ion channels in excitable cells, where they are responsible for the generation and conduction of action potential. In addition, the release of neurotransmitters from nerve terminals is influenced by sodium channel activity. The function of VDSC is subject to modulation by various neurotoxins, such as scorpion toxins, which have long been used as tools in the investigation of neurotransmitter release. This opens an interesting perspective concerning modulation of neurotransmission via pharmacological manipulation of sodium channel properties, which can lead to a better understanding of their physiological and pathological roles. Here we briefly review the studies of neurotoxins acting on sodium channels, focusing primarily on the view of the mechanisms of neurotransmitter release.

Investigation of the modulation of glutamate release by sodium channels using neurotoxins

The modulation of neurotransmitter release by calcium channels is well established, yet, sodium channels were regarded mainly as charge carriers. Many lines of evidence suggest a more fine-tuning role played by sodium channels. Using rat cerebrocortical isolated nerve endings (synaptosomes) and two toxins that have separate sites of action on sodium channels and provoke distinct changes in channel kinetics, we were able to show that depending on the rate of increase in channel conductance, the outcome in terms of neurotransmitter release and calcium channel types coupled to that event are different. Mainly, our study focused on veratridine, an alkaloid from lilaceous plants that binds to sodium channel toxin site 2, and tityustoxin, a toxin purified from the venom of the Brazilian yellow scorpion Tityus serrulatus that binds to site 3. Veratridine induces a slower increase in intrasynaptosomal sodium and calcium concentrations, slower depolarization, delayed exocytosis and a slower and predominantly calcium-independent glutamate release, when compared to tityustoxin.Thus, we have used these two toxins to investigate the events that start with sodium entry and culminate with the release of glutamate in isolated nerve endings (synaptosomes) from rat cerebral cortex. With that in mind we measured intrasynaptosomal free sodium concentration [Na(+)](i), intrasynaptosomal free calcium concentration [Ca(2+)](i), membrane potential, exocytosis and glutamate release using fluorescent probes.

Effect of antidepressant drugs on veratridine-evoked glutamate and aspartate release in rat prefrontal cortex

In vivo microdialysis in conscious rats was used to evaluate the effect of local application, through a microdialysis probe, of desipramine (DMI), imipramine and citalopram (CIT), on veratridine-evoked glutamate and aspartate release in rat prefrontal cortex (PFCx). All antidepressant drugs (ADs), given at a concentration of 0.1 mM, significantly inhibited glutamate release, while aspartate release was affected only by DMI and CIT. In contrast, local administration of ADs markedly potentiated veratridine-evoked dopamine and noradrenaline release. Perfusion of clonidine, quinpirole and 1-[3-(trifluoro-methyl)phenyl]-piperazine (TFMPP) at 0.1 mM concentration also diminished, evoked release of glutamate and aspartate. The regulation of amino acid release in rat PFCx may be achieved by direct effect of ADs on Na+ channels or indirectly, by involvement of D2/D3, alpha 2 or 5-HT1B heteroceptors activated by the increased level of monoamines in response to the blockade of respective transporters.

In vitro induction of microcyst-like structures in the superior olivary complex

To investigate the etiology of hole formation in the gerbil and rat central auditory system, organotypic cultures were grown in control and veratridine-containing media. The latter condition is known to increase neuronal activity. Tissue was obtained at postnatal day 6 and grown for 6-9 days in vitro, a period prior to the formation of holes in vivo. In both rats and gerbils, veratridine led to the appearance of large numbers of holes, and these were phenotypically similar to those found in vivo. These results support the idea that hole formation is an activity-dependent phenomenon, and suggest that it is not restricted to the mature gerbil auditory system.