Glutamate and GABA Activate Different Receptors and Cl- Conductances in Crab Peptide-Secretory Neurons. Shumin Duan and Ian M. Cooke. Békésy Laboratory of Neurobiology and Department of Zoology, University of Hawaii, Honolulu, Hawaii 96822.
APStracts 6:0484N, 1999.
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 (t1/2~1 s) desensitization occurred during Glu, but not GABA application that required >2 min for recovery. Desensitization was unaffected by concanavalin A or cyclothiazide. NMDA, AMPA, 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%; 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, 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.
Received 6 July 1999; accepted in final form 3 September 1999.
APS Manuscript Number J546-9.
Article publication pending Journal of Neurophysiology.
ISSN 1080-4757 Copyright 1999 The American Physiological Society.
Published in APStracts on 21 December 1999