Physiological and pharmacological characteristics of quisqualic acid-induced K+-current response in the ganglion cells of Aplysia

The extracellular application of either quisqualic acid (QA) or Phe-Met-Arg-Phe-NH2 (FMRFamide) induces an outward current in identified neurons of Aplysia ganglion under voltage clamp. The time course of the QA-induced response is significantly slower than that induced by FMRFamide. The reversal potential for both responses was -92 mV and was shifted 17 mV in a positive direction for a twofold increase in the extracellular K+ concentration. The QA-induced response was markedly depressed in the presence of Ba2+, a blocker of inward rectifier K+-channel, whereas TEA, a Ca2+-activated K+-channel (BKCa) blocker, or 4-AP, a transient K+ (A)-channel blocker, had no effect on the response. The QA-induced K+-current was significantly suppressed by CNQX and GYKI52466, antagonists of non-NMDA receptors. However, the application of either kainate or AMPA, agonists for non-NMDA receptors, produced no type of response in the same neurons. The QA-induced K+-current response was not depressed at all by an intracellular injection of either guanosine 5 ' -O-(2-thiodiphosphate) (GDP-betaS) or guanosine 5 ' -O-(3-thiotriphosphate) (GTP-gammaS), but the FMRFamide-induced response was markedly blocked by both GDP-betaS and GTP-gammaS in the same cell. Furthermore, the QA- and FMR-Famide-induced K+-current responses were both decreased markedly when the temperature was lowered to 15 degreesC, from 23 degreesC. These results suggested that the QA-induced K+-current response is produced by an activation of a novel type of QA-receptor and that this response is not produced by an activation of the G protein.