HIGH-AFFINITY DEXTROMETHORPHAN BINDING-SITES IN GUINEA-PIG BRAIN .1. INITIAL CHARACTERIZATION

  • 1 January 1983
    • journal article
    • research article
    • Vol. 23  (3) , 619-628
Abstract
Tritiated dextromethorphan [an antitussive] ([3H]DM) binds to 2 distinct sites in guinea-pig brain, a high-affinity site (Kd = 13-20 nM) and a low-affinity site (Kd > 200 nM). Binding of [3H] DM to the high-affinity site is rapid, reversible, saturable, proportional to tissue concentration and pH-dependent. The sites have a protein-like component, since preincubating brain homogenate in the presence of proteolytic enzymes and protein-modifying reagents significantly reduces binding. There is a progressive loss of binding when brain homogenate is heated to temperatures in excess of 37.degree.. Millimolar concentrations of Li, Ca, Mg and Mn decrease DM binding while Na, in concentrations as high as 100 mM, has little effect; Ca in .mu.mol concentrations slightly enhances binding. The pons-medulla and cerebellum contain the highest density of sites. Subcellular localization studies showed that high-affinity sites are confined almost exclusively to the microsomal fraction. Binding of DM to brain microsomes does not appear to be related to drug-metabolizing enzymes. The characteristics of DM binding suggest that DM sites are not a subclass of opiate receptors. Studies using tritiated dextrorphan as radioligand failed to reveal a high-affinity binding site for this compound in brain.