Inhibitory glutamate receptor channels in cultured lobster stomatogastric neurons

Inhibitory glutamate receptor channels (IGluRs) are ligand-gated ionotropic receptors related to ionotropic gamma-aminobutyric acid (GABA) and glycine receptors and expressed in neural and muscular tissues. In the crustacean stomatogastric ganglion (STG), IGluRs mediate recurrent synaptic inhibition central to the rhythmogenic capabilities of its embedded neural circuits. IGluRs expressed in cultured spiny lobster STG neurons exhibited an EC50 of 1.2 mM and a Hill coefficient of 1.4. They were neither cross-activated nor cross-desensitized by GABA, although a distinct GABA-gated chloride current was observed. Glycine did not evoke any current from STG neurons. The IGluR was weakly blocked by the chloride channel blocker furosemide and the excitatory glutamate receptor antagonist6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), but was not inhibited by bicuculline methiodide, strychnine, kynurenic acid, gamma--glutamylglycine, or aspartate. Outside-out patch-clamp recordings were analyzed using the mean-variance histogram technique. Under excised-patch conditions, the receptor exhibited only a single open state with an estimated unitary conductance of 80 +/- 8. 6 (SD) pS. The distinct GABA receptor also displayed a single open state with a conductance of 72 +/- 10 pS.