Vol. 123, No. 2, 1984 BIOCHEMICAL AND BIOPHYSICALRESEARCH COMMUNICATIONS September 17, 1984 Pages 703-709 Phorbol Esters Inhibit Agonist-Induced [3H] Inositol-l-Phosphate Accumulation in Rat Hippocampal Slices R. Labarca, A. Janowsky*, J. Patel** and S.M. Paul. Clinical Neuroscience Branch, NIMH; **Biological Psychiatry Branch, NIMH; National Institute of Mental Health and *National Institute of General Medical Sciences Bethesda, Maryland 20205 Received August 6, 1984 In rat hippocampal slices, carbachol and norepinephrine induce an accumulation of [3HI-inositol-l-phosphate which is markedly amplified in the presence of lithium. 13-dibutyrate no effect on [ 5 PDB) The tumor-promoting agents phorbol 12, and 48 phorbol, 12B-myristate, 13 a-acetate (PMA) have H] inositol-1-phophate accumulation alone, but when preincubated with hippocampal slices significantly inhibit the accumulation of [3H]-inositol-l-phosphate induced by carbachol and norepinephrine. The IC50 values for PDB and PMA are 0.2pM and 25pM respectively. In contrast, the weak tumor promoting agents 4-0-methylphorbol 12 myristate 13 acetate (MPMA) and phorbol 13,20-diacetate (P 13,20 DA) only slightly attenuate the agonist-induced response at concentrations SlOOpM, whereas 4a-phorbol (4a-PHR), a biologically inactive phorbol, has no effect. These data suggest that phorbol ester receptor-mediated events may be negatively coupled to agonist-induced phosphatidylinositol hydrolysis. Specific phorbol ester binding sites have recently been characterized in brain and various peripheral tissues (1,2,3). Structure-activity studies support the notion that these binding sites mediate the various biological effects of phorbol esters such as the induction of mitogenic activity, interference with cellular differentiation, as well as stimulation of platelet aggregation and ACTH secretion (cf ref. 4,5). In mouse brain there is a marked regional distribution of phorbol ester receptors, with the highest densities found in the hippocampus, cerebral cortex, striatum and substantia nigra (6). Evidence from a number of laboratories indicate that phorbol esters activate partially purified protein kinase C by shifting the Abbreviations: PDB, phorbol 12, 13-dibutyrate; PMA, 48 MPMA, 4-0-methylphorbol, 12 myristate, phorbol, 12e-myristate, 13 a-acetate; 20-diacetate; 4a-PHR, 4a-phorbol 13 acetate; P13,20 DA, phorbol 13, 0006-291X/84 $1.50 703 Copyright 0 1984 by Academic Press, Inc. AN rights qf reproduction in any form reserved.