Journal of Neuroscience, Vol 15, 6631-6639, Copyright © 1995 by Society for Neuroscience
Glutamate-gated inhibitory currents of central pattern generator neurons in the lobster stomatogastric ganglion
TA Cleland and AI Selverston
Biology Department, University of California at San Diego, La Jolla 92093-0357, USA.
Inhibitory glutamatergic neurotransmission is an elemental "building block"
of the oscillatory networks within the crustacean stomatogastric ganglion
(STG). This study constitutes the initial characterization of glutamatergic
currents in isolated STG neurons in primary culture. Superfusion of 1 mM
L-glutamate evoked a current response in 45 of 65 neurons examined. The
evoked current incorporated two kinetically distinct components in variable
proportion: a fast desensitizing component and a slower component. The
current was mediated by an outwardly rectifying conductance increase and
reversed at -48.8 +/- 5.3 mV. Reducing the external chloride concentration
by 50% deflected the glutamate equilibrium potential (Eglu) by +14 mV,
while increasing external potassium threefold shifted Eglu by up to +6 mV.
Ibotenic acid fully activated both components of the glutamate response.
Saturating concentrations of glutamate completely occluded neuronal
responses to ibotenic acid, indicating that ibotenic acid was activating
the same receptor(s) as glutamate. Millimolar concentrations of quisqualic
acid, kainate, AMPA, and NMDA each failed to evoke any response. Picrotoxin
(10(-4)M) completely blocked the glutamate response. Niflumic acid (100
microM) blocked > 80% of the desensitizing component and congruent to
50% of the sustained component. Reduction or elimination of extracellular
calcium did not abolish the response. This study extends the ionic and
pharmacological analysis of glutamatergic conductances in STG neurons. The
currents described are consistent with glutamatergic inhibitory synaptic
and agonist-evoked responses previously described in situ. We discuss their
pharmacology, ionic mechanisms, and functional significance.
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