Glutamate and GABA activate different receptors and Cl(-) conductances in crab peptide-secretory neurons.
Responses to rapid application of glutamic acid (Glu) and gamma-aminobutyric acid (GABA), 0.01-3 mM, were recorded by whole-cell patch clamp of cultured crab (Cardisoma carnifex) X-organ neurons. Responses peaked within 200 ms. Both Glu and GABA currents had reversal potentials that followed the Nernst Cl(-) potential when [Cl(-)](i) was varied. A Boltzmann fit to the normalized, averaged dose-response curve for Glu indicated an EC(50) of 0.15 mM and a Hill coefficient of 1.05. Rapid (t(1/2) approximately 1 s) desensitization occurred during Glu but not GABA application that required >2 min for recovery. Desensitization was unaffected by concanavalin A or cyclothiazide. N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, quisqualate, and kainate (to 1 mM) were ineffective, nor were Glu responses influenced by glycine (1 microM) or Mg(2+) (0-26 mM). Glu effects were imitated by ibotenic acid (0.1 mM). The following support the conclusion that Glu and GABA act on different receptors: 1) responses sum; 2) desensitization to Glu or ibotenic acid did not diminish GABA responses; 3) the Cl(-)-channel blockers picrotoxin and niflumic acid (0.5 mM) inhibited Glu responses by approximately 90 and 80% but GABA responses by approximately 50 and 20%; and 4) polyvinylpyrrolydone-25 (2 mM in normal crab saline) eliminated Glu responses but left GABA responses unaltered. Thus crab secretory neurons have separate receptors responsive to Glu and to GABA, both probably ionotropic, and mediating Cl(-) conductance increases. In its responses and pharmacology, this crustacean Glu receptor resembles Cl(-)-permeable Glu receptors previously described in invertebrates and differs from cation-permeable Glu receptors of vertebrates and invertebrates.
Duan
S
S
Békésy Laboratory of Neurobiology, Department of Zoology, University of Hawaii, Honolulu, Hawaii 96822, USA.
Cooke
I M
IM
D000818:Animals; D001581:Benzothiadiazines / Q000494:pharmacology; D003386:Brachyura; D002478:Cells, Cultured; D018118:Chloride Channels / Q000187:drug effects / Q000502:physiology*; D002712:Chlorides / Q000378:metabolism*; D003208:Concanavalin A / Q000494:pharmacology; D018698:Glutamic Acid / Q000494:pharmacology*; D005998:Glycine / Q000494:pharmacology; D007608:Kainic Acid / Q000494:pharmacology; D016202:N-Methylaspartate / Q000494:pharmacology; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D018408:Patch-Clamp Techniques; D016318:Quisqualic Acid / Q000494:pharmacology; D018091:Receptors, AMPA / Q000502:physiology; D018079:Receptors, GABA / Q000187:drug effects / Q000502:physiology*; D017470:Receptors, Glutamate / Q000187:drug effects / Q000502:physiology*; D018350:alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid / Q000494:pharmacology; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology*
D001581:Benzothiadiazines; D018118:Chloride Channels; D002712:Chlorides; D018091:Receptors, AMPA; D018079:Receptors, GABA; D017470:Receptors, Glutamate; D003208:Concanavalin A; D018698:Glutamic Acid; D005680:gamma-Aminobutyric Acid; D016202:N-Methylaspartate; D018350:alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; D016318:Quisqualic Acid; C004639:cyclothiazide; D007608:Kainic Acid; D005998:Glycine