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Blocking effects of cobalt and related ions on the gamma‐aminobutyric acid‐induced current in turtle retinal cones.
Author(s) -
Kaneko A,
Tachibana M
Publication year - 1986
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.1986.sp016058
Subject(s) - biophysics , tetraethylammonium , chemistry , reversal potential , membrane potential , gamma aminobutyric acid , patch clamp , receptor , biology , biochemistry , potassium , organic chemistry
Red‐sensitive cone photoreceptors were isolated from the turtle retina, and GABA‐induced currents were recorded under voltage clamp. The effect of Co2+, widely used as a blocker of chemical synapses, on the GABA‐induced current was studied. Co2+ blocked the GABA‐induced current evoked by local application either at the synaptic region (cone pedicle) or at the extra‐synaptic region (cell body). 5 microM‐Co2+ suppressed the GABA‐induced current by 50%, and a few hundred microM‐Co2+ blocked it almost completely. Co2+ suppressed the GABA‐induced current non‐competitively: the saturating response amplitude decreased without a change in the threshold or saturating dose of GABA. The blocking was not voltage dependent in the physiological range of the membrane potential. Ni2+ and Cd2+ also blocked the GABA‐induced current non‐competitively, and were as effective as Co2+. Tetraethylammonium (25 mM) showed a similar but weaker blocking effect. On the other hand, Mg2+ (20 mM), Mn2+, Sr2+, Ba2+ (10‐100 microM each), D‐600 (10 microM) or Cs+ (10 mM) did not affect the GABA‐induced current. The Ca current in the turtle cones was blocked almost completely by 20 mM‐Mg2+ or 4 mM‐Co2+, or strongly suppressed by 10 microM‐D‐600. However, Cd2+ and Ni2+ (10 microM each) blocked the Ca current by ca. 50%, and Co2+ and Mn2+ (10 microM each) suppressed it only partially. The blocking of the GABA‐induced current by these agents was, therefore, not directly related to the blocking of the Ca current and/or Ca‐mediated currents. These observations present a warning on the use of some divalent cations, such as Co2+, Ni2+ or Cd2+, as a presynaptic blocker at the GABAergic synapse. High concentrations of Mg2+ are recommended as a more appropriate blocker.