STIMULATION OF PHOSPHOINOSITIDE HYDROLYSIS BY IBOTENIC ACID IN CEREBELLAR NEURONS VIA 2 GLUTAMATE RECEPTOR SUBTYPES

In primary cultures of cerebellar granule cells, ibotenate produced a dose-dependent biphasic stimulation of phosphoinositide (PI) hydrolysis. The high-affinity component of ibotenate action (EC50 = 100 nM) was characteristic of the metabolotropic glutamate receptor activation. The action of ibotenate, like that of quisqualate, was inhibited by pretreatment of granule cells with pertussis toxin, confirming the participation of a guanosine triphosphate (GTP)-binding protein in ibotenate signal transduction. In contrast, the low-affinity component of ibotenate-stimulated PI hydrolysis (EC50 = 100-mu-M) exhibited the pharmacology of the N-methyl-D-aspartate (NMDA) receptor. This action was inhibited by NMDA receptor antagonists 3-(2-carboxy-piperazin-4-yl)propyl-1-phosphonic acid (CPP) and (+)5-methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine maleate (MK-801) and was potentiated by glycine. Moreover it was inhibited by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), acting as an antagonist at the positive modulatory site of the NMDA receptor. The low-affinity component of ibotenate-induced PI hydrolysis correlated well with the ability of ibotenate to induced Ca-45(2+) influx, confirming the action of ibotenate at the ionotropic NMDA receptor. Thus, in cerebellar granule cells, ibotenate appears to stimulate PI hydrolysis by acting at two distinct glutamate receptor subtypes.