Phorbol Esters Attenuate Glutamate‐Stimulated Inositol Phospholipid Hydrolysis in Neuronal Cultures

Abstract: The phorbol diesters 12‐O‐tetradecanoyl‐phorbol‐13‐acetate (TPA) and phorbol‐12,13‐dibutyrate, but not 4–α‐phorbol‐didecanoate, inhibited the stimulation of inositol phospholipid hydrolysis by excitatory amino acids and carbamylcholine in primary cultures of cerebellar neurons. This inhibition was mimicked by the synthetic diacylglycerol 1,2‐dioleoyl‐rac‐glycerol (DOG) and was selective for a specific glutamate‐phosphoinositide receptor subtype (GP₂ receptor) activated by glutamate and quis‐qualate. TPA was nearly inactive in inhibiting the stimulation of inositol phospholipid hydrolysis by N‐methyl‐d‐aspartate, a selective agonist of the GP₁ receptor. Phorbol diesters and DOG attenuated the stimulation of inositol phospholipid hydrolysis by glutamate and quisqualate also in cerebellar slices from 9–15‐day‐old rats; however, using this preparation, their action was weak and required high concentrations (> 1 μM). The inhibition of signal transduc‐tion by phorbol diesters was not consequent to a reduced binding of glutamate to its membrane recognition sites. In fact, TPA induced only a small increase in the K_D but no change in the B_max of [³H]glutamate binding in cerebellar membranes. Phorbol diesters may act to inhibit specific GTP‐binding proteins or particular molecular forms of phosphoinositidase C associated with GP₂ or muscarinic cholinergic receptors.