Calcium permeability and modulation of nicotinic acetylcholine receptor-channels in rat parasympathetic neurons.
Neuronal nicotinic acetylcholine (ACh)-activated currents in rat parasympathetic ganglion cells were examined using whole-cell and single-channel patch clamp recording techniques. The whole-cell current-voltage (I-V) relationship exhibited strong inward rectification and a reversal (zero current) potential of -3.9 mV in nearly symmetrical Na+ solutions (external 140 mM Na+/internal 160 mM Na+). Isosmotic replacement of extracellular Na+ with either Ca2+ or Mg2+ yielded the permeability (Px/PNa) sequence Mg2+ (1.1) > Na+ (1.0) > Ca2+ (0.65). Whole-cell ACh-induced current amplitude decreased as [Ca2+]0 was raised from 2.5 mM to 20 mM, and remained constant at higher [Ca2+]0. Unitary ACh-activated currents recorded in excised outside-out patches had conductances ranging from 15-35 pS with at least three distinct conductance levels (33 pS, 26 pS, 19 pS) observed in most patches. The neuronal nicotinic ACh receptor-channel had a slope conductance of 30 pS in Na+ external solution, which decreased to 20 pS in isotonic Ca2+ and was unchanged by isosmotic replacement of Na+ with Mg2+. ACh-activated single channel currents had an apparent mean open time (tau 0) of 1.15 +/- 0.16 ms and a mean burst length (tau b) of 6.83 +/- 1.76 ms at -60 mV in Na+ external solution. Ca(2+)-free external solutions, or raising [Ca2+]0 to 50-100 mM decreased both the tau 0 and tau b of the nAChR channel. Varying [Ca2+]0 produced a marked decrease in NP0, while substitution of Mg2+ for Na+ increased NP0. These data suggest that activation of the neuronal nAChR channel permits a substantial Ca2+ influx which may modulate Ca(2+)-dependent ion channels and second messenger pathways to affect neuronal excitability in parasympathetic ganglia.
Adams
D J
DJ
Department of Molecular and Cellular Pharmacology, University of Miami School of Medicine, FL 33101.
Nutter
T J
TJ
D000109:Acetylcholine / Q000494:pharmacology; D000595:Amino Acid Sequence; D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000502:physiology; D002413:Cations, Divalent / Q000493:pharmacokinetics; D004553:Electric Conductivity; D007473:Ion Channels / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D009474:Neurons / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D009538:Nicotine / Q000378:metabolism*; D010275:Parasympathetic Nervous System / Q000166:cytology / Q000378:metabolism* / Q000502:physiology; D010539:Permeability; D051381:Rats; D011978:Receptors, Nicotinic / Q000378:metabolism* / Q000502:physiology
D002413:Cations, Divalent; D007473:Ion Channels; D011978:Receptors, Nicotinic; D009538:Nicotine; D000109:Acetylcholine; D002118:Calcium