Alpha 1-adrenoceptor-mediated inositol phospholipid hydrolysis in rat cerebral cortex: relationship between receptor occupancy and response and effects of denervation

Abstract

The characteristics of catecholamine-mediated breakdown of inositol phospholipids in rat cerebral cortex slices have been examined using a direct assay involving prelabeling with [3H]inositol and examining the production of labelled inositol phosphates in the presence of lithium. Noradrenaline produced a marked stimulation of inositol phosphate accumulation and this response could be potently and competitively antagonised by the alpha 1-adrenoceptor antagonist prazosin. The alpha 2-antagonist yohimbine was almost 1000-fold less potent at antagonising noradrenaline inositol phospholipid response. Noradrenaline and adrenaline were full agonists at alpha 1-adrenoceptors but phenylephrine and methoxamine were only partial agonists in their ability to stimulate inositol phospholipid metabolism. There was a significant correlation between the ability of a variety of agonists and antagonists to activate or inhibit [3H]inositol phosphate accumulation and their ability to displace the alpha 1-adrenoceptor selective ligand [3H]prazosin from specific binding sites when assays were performed on rat cerebral cortical slices under identical conditions. The similarity of EC50 values of agonists stimulating inositol phosphate accumulation and their IC50 values in [3H]prazosin binding experiments suggested a close relationship between receptor occupancy and alpha 1-mediated inositol phosphate accumulation. Further experiments were performed to examine this directly by inactivating alpha 1-adrenoceptors with the alkylating antagonist phenoxybenzamine. After washing out unbound antagonist, [3H]prazosin binding was reduced to a very similar proportion to that observed on the maximal noradrenaline-stimulated accumulation of [3H]inositol phosphates in the slices. The EC50 values for noradrenaline-stimulated inositol phosphate accumulation was unaltered and the affinity of [3H]prazosin for the remaining sites was equally unaffected. In rats treated 14 days previously with i.c.v. 6-hydroxydopamine (2 X 250 micrograms) there was a small increase in alpha 1-adrenoceptor binding sites but a parallel shift to the left in the noradrenaline [3H]inositol phosphate accumulation dose-response curve. On the other hand, the partial agonist phenylephrine induced a larger maximal response in denervated animals without a change in the EC50 values. When slices from 6-hydroxydopamine treated animals were preincubated with phenoxybenzamine, the loss in alpha 1-adrenoceptor binding sites was greater than the reduction in the maximal response to noradrenaline. This may indicate the development of a small receptor reserve after denervation.(ABSTRACT TRUNCATED AT 400 WORDS)