Glutamate and GABA activate different receptors and Cl- conductances in crab peptide-secretory neurons
Shumin Duan and Ian M. Cooke
J. Neurophysiol. 83: 31-37 (2000)
Summary
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 (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. 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 µmM) 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.
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