Inhibition of presynaptic sodium channels by halothane.
BACKGROUND
Recent electrophysiologic studies indicate that clinical concentrations of volatile general anesthetic agents inhibit central nervous system sodium (Na+) channels. In this study, the biochemical effects of halothane on Na+ channel function were determined using rat brain synaptosomes (pinched-off nerve terminals) to assess the role of presynaptic Na+ channels in anesthetic effects.


METHODS
Synaptosomes from adult rat cerebral cortex were used to determine the effects of halothane on veratridine-evoked Na+ channel-dependent Na+ influx (using 22Na+), changes in intrasynaptosomal [Na+] (using ion-specific spectrofluorometry), and neurotoxin interactions with specific receptor sites of the Na+ channel (by radioligand binding). The potential physiologic and functional significance of these effects was determined by measuring the effects of halothane on veratridine-evoked Na+ channel-dependent glutamate release (using enzyme-coupled spectrofluorometry).


RESULTS
Halothane inhibited veratridine-evoked 22Na+ influx (IC50 = 1.1 mM) and changes in intrasynaptosomal [Na+] (concentration for 50% inhibition [IC50] = 0.97 mM), and it specifically antagonized [3H]batrachotoxinin-A 20-alpha-benzoate binding to receptor site two of the Na+ channel (IC50 = 0.53 mM). Scatchard and kinetic analysis revealed an allosteric competitive mechanism for inhibition of toxin binding. Halothane inhibited veratridine-evoked glutamate release from synaptosomes with comparable potency (IC50 = 0.67 mM).


CONCLUSIONS
Halothane significantly inhibited Na+ channel-mediated Na influx, increases in intrasynaptosomal [Na+] and glutamate release, and competed with neurotoxin binding to site two of the Na+ channel in synaptosomes at concentrations within its clinical range (minimum alveolar concentration, 1-2). These findings support a role for presynaptic Na+ channels as a molecular target for general anesthetic effects.
Ratnakumari
L
L
Department of Anesthesiology, Cornell University Medical College, New York, New York 10021, USA.
Hemmings
H C
HC
D018685:Anesthetics, Inhalation / Q000494:pharmacology*; D000818:Animals; D001496:Batrachotoxins / Q000378:metabolism; D001921:Brain / Q000187:drug effects / Q000378:metabolism; D019706:Excitatory Postsynaptic Potentials / Q000187:drug effects; D005089:Exocytosis / Q000187:drug effects; D018698:Glutamic Acid / Q000378:metabolism; D006221:Halothane / Q000494:pharmacology*; D008297:Male; D009498:Neurotoxins / Q000378:metabolism; D017729:Presynaptic Terminals / Q000187:drug effects* / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012964:Sodium / Q000378:metabolism; D015222:Sodium Channels / Q000187:drug effects* / Q000378:metabolism; D013574:Synaptosomes / Q000187:drug effects* / Q000378:metabolism; D014701:Veratridine / Q000494:pharmacology
D018685:Anesthetics, Inhalation; D001496:Batrachotoxins; D009498:Neurotoxins; D015222:Sodium Channels; D018698:Glutamic Acid; D014701:Veratridine; C031775:batrachotoxinin A 20-alpha-benzoate; D012964:Sodium; D006221:Halothane