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

• Published in: Neuroscience Vol. 113; no. 1; pp. 115 - 123
• Main Authors: Bicalho, A.F.X, Guatimosim, C, Prado, M.A.M, Gomez, M.V, Romano-Silva, M.A
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.01.2002; Elsevier
• Subjects: Animals; Biological and medical sciences; calcium; Calcium - analysis; Calcium - metabolism; Calcium Channels - metabolism; Central nervous system; Central neurotransmission. Neuromudulation. Pathways and receptors; exocytosis; Exocytosis - drug effects; Fundamental and applied biological sciences. Psychology; Glutamic Acid - metabolism; Membrane Potentials - drug effects; Neurotoxins - toxicity; neurotransmitter; Rats; Rats, Wistar; Scorpion Venoms - toxicity; Sodium - analysis; Sodium - metabolism; Sodium Channels - drug effects; Sodium Channels - metabolism; Synaptosomes; tityustoxin; veratridine; Veratridine - toxicity; Vertebrates: nervous system and sense organs; [Na +] i, intrasynaptosomal free sodium concentration; ω-AgaIVA, ω-agatoxin IVA; PKA/C, protein kinase A/C; calcium; TsTX, tityustoxin; synaptosomes; EDTA, ethylenediaminetetra-acetate; ω-CgTX GVIA, ω-conotoxin GVIA; veratridine; HBSS, HEPES-buffered salt solution; EGTA, ethyleneglycol-bis(aminoethyl ether)tetra-acetate; ω-CgTX MVIIC, ω-conotoxin MVIIC; TTX, tetrodotoxin; tityustoxin; [Ca 2+] i, intrasynaptosomal free calcium concentration; HEPES, N-(α-hydroxyethyl) piperazine- N′-(α-ethanesulphonic acid; VSSC, voltage-sensitive sodium channel; exocytosis; neurotransmitter; VDCC, voltage-dependent calcium channel; Cerebral cortex; Rat; Rodentia; Central nervous system; Ionic channel; Glutamate; Toxin; Vertebrata; Synaptosome; Mammalia; Sodium; Neurotoxin; Excitatory aminoacid; Neurotransmitter; Release; Brain (vertebrata)
• ISSN: 0306-4522; 1873-7544
• DOI: 10.1016/S0306-4522(02)00139-2

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.