Effects of Pro-Leu-Gly-NH 2 and cyclo (Leu-Gly) on the binding of 3H-quinuclidinyl benzilate to striatal cholinergic muscarinic receptors

Abstract

The effects of Pro-Leu-Gly-NH 2 (melanotropin release inhibiting factor, MIF) and its analog, cyclo (Leu-Gly) on the mouse and rat striatal cholinergic muscarinic receptors labeled with 3H-quinuclidinyl benzilate (QNB) were investigated. 3H-QNB bound to the rat striatal muscarinic receptors at a single high affinity site with receptor density (B max value) of 1200 fmol per mg protein and an apparent dissociation constant (K d value) of 53.5 pM. At 140 pM concentration of 3H-QNB, the specific binding to the receptors was 724 fmol per mg protein. MIF in a concentration range of 10 −9 to 10 −4 M did not alter the binding of 3H-QNB but at 10 −3 M decreased the binding by 25%. Cyclo (Leu-Gly), on the other hand, in the concentration range of 10 −9 to 10 −3 M had no effect on the binding of 3H-QNB. A single injection of MIF (3 or 10 mg/kg IP) to rats did not alter the B max or the K d value of 3H-QNB to bind to the striatal membranes. 3H-QNB bound to the mouse striatal muscarinic receptors at a single high affinity site with a B max value of 991 fmol/per mg protein and a K d value of 21 pM. Neither acute administration of MIF (3 or 10 mg/kg IP) nor chronic treatment of the peptide (2, 8 or 32 mg/kg IP, daily for 5 days) to mice could influence the binding of 3H-QNB to the striatal muscarinic receptors. Although oxotremorine (0.5 mg/kg) given IP 1 hr prior to sacrifice decreased the B max value of 3H-QNB binding in mouse striatum, prior chronic treatment with MIF had no effect on the decreased receptor density (B max) during oxotremorine treatment. It is concluded that the previously reported anticholinergic activity of MIF may involve mechanisms other than its direct effect on the brain cholinergic muscarinic receptors.