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

Abstract

Responses to rapid application of glutamic acid (Glu) and γ-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, α-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.