Phorbol 12,13-dibutyrate regulates muscarinic receptors in rat cerebral cortical slices by activating protein kinase C.

Stimulation of muscarinic acetylcholine receptors (mAChR) elicits phosphatidylinositol turnover, which yields inositol phosphates (InsP) and diacylglycerol (DG) the latter activating protein kinase C (PKC). Activating PKC with phorbol esters inhibits mAChR agonist-stimulated phosphoinositide hydrolysis and InsP production. A possible mechanism of this inhibition may be down-regulation of mAChR by PKC. In the present work, rat cortical slices were preincubated with phorbol 12,13-dibutyrate (PDBu) followed by binding assays for [3H]quinuclidinyl benzilate [( 3H]QNB), [N-methyl-3H]scopolamine [( 3H]NMS) or [3H]pirenzepine [( 3H]PZ). Our data demonstrate that activation of PKC by phorbol esters causes a rapid down-regulation of muscarinic cholinergic receptors. This down-regulation is also rapidly reversible. Receptors on the cell surface appear to be more sensitive to the effect of PKC than do internal ones. This down-regulation occurs by a decrease in the number of receptors, rather than by changes in receptor affinity. The results suggest that PKC may exert negative feedback on its own activation by down-regulating the receptors that normally elicit phosphatidylinositol turnover.