Maitotoxin Induces Phosphoinositide Turnover and Modulates Glutamatergic and Muscarinic Cholinergic Receptor Function in Cultured Cerebellar Neurons

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Abstract
Maitotoxin (MTX) stimulated inositol phosphate (IP) formation in primary cultures of rat cerebellar granule cells. MTX‐induced IP production was dependent on extracellular Ca2+ but independent of extracellular Na+. The stimulation of IP formation elicited by MTX was unaffected by pretreatment of cells with phorbol dibutyrate. pertussis toxin, and a variety of Ca2+ entry blockers, such as nimodipine, nisoldipine, Co2+, and Mn2+. The presence of MTX markedly attenuated IP production induced by carbachol and glutamate, with no apparent effect on the responses to norepinephrine (NE), histamine, 5‐hydroxytryptamine (5‐HT), and endothelin‐1. The inhibition of the carbachol‐ and glutamate‐induced responses by MTX was dose dependent with IC50 values of 1.2 and 0.5 ng/ml, respectively. Pretreatment of cells with a lower concentration of MTX (0.3 ng/ml) also attenuated carbachol‐ and glutamate‐induced IP formation, in a time‐dependent manner, with a decrease observed after 30 min prestimulation, but failed to affect NE‐, histamine‐, 5‐HT‐, endothelin‐1, and sarafotoxin S6b‐induced responses. Thus, MTX elicited a marked Ca2+‐dependent phosphoinositide (PI) turnover in cerebellar granule cells and selectively inhibited carbachol‐ and glutamate‐induced PI hydrolysis. Possible mechanisms underlying these selective modulations are discussed.