Pretreatment of Cerebellar Granule Cells with Concanavalin A Potentiates Quisqualate‐Stimulated Phosphoinositide Hydrolysis

The hydrolysis of phosphoinositides (PI) elicited in cerebellar granule cell cultures by agonists of metabolotropic glutamate receptors, glutamate and quisqualate, was enhanced when the cells were pretreated with concanavalin A (Con‐A). A similar effect was produced by wheat germ agglutinin, but not by several other lectins tested. Con‐A produced a dose‐dependent effect (EC50= 3 μM) and increased the efficacy but not the potency of the agonists. In contrast, Con‐A failed to enhance PI hydrolysis evoked by N‐methyl‐D‐aspartate, kainate, carbachol, the calcium ionophore A23187, or 50 mM K+. The Con‐A stimulatory effect was prevented by simultaneous pretreatment with the agonists of ionotropic quisqualate receptors quisqualate, kainate, and α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid, but not by the antagonist 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX). CNQX, which did not inhibit quisqualate‐stimulated PI hydrolysis in untreated cells, abolished the component of quisqualate response enhanced by Con‐A pretreatment. The pretreatment with Con‐A also increased the influx of 45Ca2+ in granule cells stimulated by quisqualate. This increase was inhibited by CNQX. Moreover, the potentiation of PI hydrolysis by Con‐A, but not the response to quisqualate alone, was abolished in the absence of Ca2+ and Na+. Pretreatment of granule cells with pertussis toxin inhibited PI hydrolysis stimulated by the metabolotropic quisqualate receptor and the Con‐A‐potentiated response by the same percentage, but Ca2+ influx induced by quisqualate was not affected. Hence, Con‐A appears to enhance PI hydrolysis by acting at the ionotropic quisqualate receptor which results in an enhanced Ca2+ influx and a subsequent activation of phospholipase C through a pertussis toxin‐sensitive G protein. This may indicate the existence of a functional cooperativity between the metabolotropic and ionotropic quisqualate receptors.