title: Differential effects of anesthetic and nonanesthetic cyclobutanes on neuronal voltage-gated sodium channels.
abstract: BACKGROUND
  Despite their key role in the generation and propagation of action potentials in excitable cells, voltage-gated sodium (Na+) channels have been considered to be insensitive to general anesthetics. The authors tested the sensitivity of neuronal Na+ channels to structurally similar anesthetic (1-chloro-1,2,2-trifluorocyclobutane; F3) and nonanesthetic (1,2-dichlorohexafluorocyclobutane; F6) polyhalogenated cyclobutanes by neurochemical and electrophysiologic methods.
  
  
  METHODS
  Synaptosomes (pinched-off nerve terminals) from adult rat cerebral cortex were used to determine the effects of F3 and F6 on 4-aminopyridine- or veratridine-evoked (Na+ channel-dependent) glutamate release (using an enzyme-coupled spectrofluorimetric assay) and increases in intracellular Ca2+ ([Ca2+]i) (using ion-specific spectrofluorimetry). Effects of F3 and F6 on Na+ currents were evaluated directly in rat lumbar dorsal root ganglion neurons by whole-cell patch-clamp recording.
  
  
  RESULTS
  F3 inhibited glutamate release evoked by 4-aminopyridine (inhibitory concentration of 50% [IC50] = 0.77 mM [approximately 0.8 minimum alveolar concentration (MAC)] or veratridine (IC50 = 0.42 mM [approximately 0.4 MAC]), and veratridine-evoked increases in [Ca2+]i (IC50 = 0.5 mM [approximately 0.5 MAC]) in synaptosomes; F6 had no significant effects up to 0.05 mM (approximately twice the predicted MAC). F3 caused reversible membrane potential-independent inhibition of peak Na+ currents (70+/-9% block at 0.6 mM [approximately 0.6 MAC]), and a hyperpolarizing shift in the voltage-dependence of steady state inactivation in dorsal root ganglion neurons (-21+/-9.3 mV at 0.6 mM). F6 inhibited peak Na+ currents to a lesser extent (16+/-2% block at 0.018 mM [predicted MAC]) and had minimal effects on steady state inactivation.
  
  
  CONCLUSIONS
  The anesthetic cyclobutane F3 significantly inhibited Na+ channel-mediated glutamate release and increases in [Ca2+]i. In contrast, the nonanesthetic cyclobutane F6 had no significant effects at predicted anesthetic concentrations. F3 inhibited dorsal root ganglion neuron Na+ channels with a potency and by mechanisms similar to those of conventional volatile anesthetics; F6 was less effective and did not produce voltage-dependent block. This concordance between anesthetic activity and Na+ channel inhibition supports a role for presynaptic Na+ channels as targets for general anesthetic effects and suggests that shifting the voltage-dependence of Na+ channel inactivation is an important property of volatile anesthetic compounds.
authors.affiliation: Department of Anesthesiology, Weill Medical College of Cornell University, New York, New York 10021, USA.
authors.forename: L
authors.initials: L
authors.lastname: Ratnakumari
authors.forename: T N
authors.initials: TN
authors.lastname: Vysotskaya
authors.forename: D S
authors.initials: DS
authors.lastname: Duch
authors.forename: H C
authors.initials: HC
authors.lastname: Hemmings
mesh_terms: D018685:Anesthetics, Inhalation / Q000494:pharmacology*; D000818:Animals; D020013:Calcium Signaling / Q000187:drug effects; D002540:Cerebral Cortex / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D003503:Cyclobutanes / Q000494:pharmacology*; D004594:Electrophysiology; D005727:Ganglia, Spinal / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D018698:Glutamic Acid / Q000378:metabolism; D066298:In Vitro Techniques; D015640:Ion Channel Gating / Q000187:drug effects*; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009474:Neurons / Q000187:drug effects*; D018408:Patch-Clamp Techniques; D011189:Potassium Chloride / Q000494:pharmacology; D017729:Presynaptic Terminals / Q000187:drug effects; D051381:Rats; D017207:Rats, Sprague-Dawley; D015222:Sodium Channels / Q000187:drug effects*; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism; D014701:Veratridine / Q000494:pharmacology
chemical_list: D018685:Anesthetics, Inhalation; D003503:Cyclobutanes; D015222:Sodium Channels; D018698:Glutamic Acid; D011189:Potassium Chloride; D014701:Veratridine