Inhibition of polyphosphoinositide turnover in rat cerebral cortex by clonidine

Abstract

In the rat brain, a number of receptors are linked to phospholipase C which catalyzes the hydrolysis of membrane inositol phospholipids; stimulation of α 1-adrenergic receptors, for example, increases polyphos-phoinositide turnover, but stimulation of α 2-receptors does not. The hydrolysis of inositol phospholipids in rat cortical slices was investigated using a direct assay involving prelabeling these lipids with 3H-inositol and then measuring the formation of 3H-inositol phosphates in the presence of lithium ions. As expected, clonidine, an α 2-agonist, did not stimulate the formation of 3H-inositol phosphates; however, clonidine antagonized the ability of noradrenaline to stimulate 3H-inositol phosphate formation. This effect was not blocked by antagonists of α 2, 5HT 2, H 2, or muscarinic receptors. Clonidine did not affect carbachol-stimulated 3H-inositol phosphate formation.