Potentiation of [³H]inositol phosphate formation by receptor activation and membrane depolarization in brain cortical slices (I)

Potentiation of [³H]inositol phosphate ([³H]IP) accumulation by receptor agonists combined with depolarizing agents was studied in rat brain cortical slices, prelabeled with [³H]inositol. Muscarinic agonists, α₁-adrenergic, histaminergic and serotonergic agonists remarkably enhanced (2-7-fold) the accumulation of [³H]IP in the presence of KCl (30 mM). The potentiated levels of [³H]IP were strongly dependent on K⁺ concentration and displayed a dose-response relationship with the agonist. Other depolarizing agents such as veratridine and ouabain induce potentiation of [³H]IP formation similarly to that observed by KCl, but to a lesser extent. The production of elevated levels of [³H]IP is Ca²⁺-dependent with maximal effect at 0.6 mM which is similar to the Ca²⁺ dependency observed for the agonist and the depolarizing agent alone. Enhanced [³H]IP levels induced by agonists in the presence of depolarizing agents affect V_max values only, since the apparent half maximal effective concentration of carbachol (CCh)-induced-IP-formation (1.2 × 10^-4 M and of the phenylephrine-induced IP-formation (8 × 10^-6 M), were not affected in the presence of either K⁺ or vertridine. In addition the efficacy of various muscarinic agonists as inducers of IP-accumulation was conserved under depolarizing conditions as compared to IP accumulation under normal conditions. In the presence of KCl (30 mM) the maximal degree of potentiation was at a range of 5-7-fold, with order efficacy of ACh > CCh > Oxo M > arecoline ≫ pilocarpine. Veratridine (1 μM) induced only 2-3-fold potentiation of the CCh-induced IP formation and less than two-fold potentiation of CCh-induced-[³H]IP accumulation was observed by Ca²⁺-ionophore A-23187 (10 μM), indicating that cytosolic Ca²⁺ is a sensitive measure for amplified phosphatidylinositol turnover. Enhanced phospholipase C (PLC) activity under depolarizing conditions may also provide a mechanism for enhanced diacylglycerol (DAG) formation and hence elevated protein kinase activity. These possibilities of PLC modulation which trigger 1,4,5-inositoltrisphosphate and DAG are discussed in the context of long-term potentiation and synaptic transmission.