The Journal of Neurophysiology Vol. 83 No. 1 January 2000, pp. 31-37
Copyright ©2000 by the American Physiological Society

Glutamate and GABA Activate Different Receptors and Cl Conductances in Crab Peptide-Secretory Neurons

Békésy Laboratory of Neurobiology and Department of Zoology, University of Hawaii, Honolulu, Hawaii 96822

Duan, Shumin and Ian M. Cooke. Glutamate and GABA Activate Different Receptors and Cl Conductances in Crab Peptide-Secretory Neurons. J. Neurophysiol. 83: 31-37, 2000. Responses to rapid application of glutamic acid (Glu) and -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 EC₅₀ of 0.15 mM and a Hill coefficient of 1.05. Rapid (t_1/2 ~ 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, -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, quisqualate, and kainate (to 1 mM) were ineffective, nor were Glu responses influenced by glycine (1 µM) or Mg²⁺ (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 ~90 and 80% but GABA responses by ~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.