Glutamate-gated inhibitory currents of central pattern generator neurons in the lobster stomatogastric ganglion

Abstract

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.