Pertussis toxin treatment increases glutamate release and dihydropyridine binding sites in cultured rat cerebellar granule neurons

Abstract This study was designed to examine the ability of pertussis toxin to block various responses due to (−)-baclofen in cultured cerebellar granule neurons of the rat. Treatment with pertussis toxin for 3 h markedly reduced the ability of (−)-baclofen to stimulate GTPase in membranes, and its ability to inhibit forskolin-stimulated adenylyl cyclase in intact cells, whereas the ability of (−)-baclofen to inhibit glutamate release was not affected at 3 h, but was abolished after 16 and 48 h treatment with pertussis toxin. The amount of ADP-ribosylation of G i /G o due to pertussis toxin in intact cells correlated well with the former two effects, but not with the prevention of the ability of baclofen to inhibit glutamate release. Pertussis toxin treatment for up to 48 h did not significantly affect the levels of G s , G i and G o in membranes from granule neurons determined by immunoblotting. Pertussis toxin treatment for 16 or 48 h but not 3 h increased the total amount of stimulated release of glutamate by about 40% under normal conditions, and by 84% under depolarizing conditions. In parallel experiments it was observed that pertussis toxin treatment for 16 h increased the number of dihydropyridine binding sites by about 90% on intact granule neurons. Whole-cell calcium channel currents, recorded under several conditions in the cells, were not increased in amplitude by pertussis toxin treatment for up to 48 h, although the ability of baclofen to inhibit calcium channel currents was blocked by pertussis toxin. These results indicate that the pertussis toxin-induced increase in glutamate release may be due to an increase in dihydropyridine binding sites, possibly localized to the presynaptic terminals.