CHARACTERIZATION OF NONADRENERGIC [H-3] CLONIDINE BINDING-SITES IN RAT STOMACH - HIGH-AFFINITY OF IMIDAZOLINES, GUANIDINES AND SIGMA-LIGANDS

We have identified and characterized non-adrenergic [H-3]clonidine bin ding sites in rat stomach. The binding of [H-3]clonidine was rapid, re versible, partly specific (as defined by cirazoline 0.1 mmol/l; 68% sp ecific binding at [H-3]clonidine 10 nmol/l), saturable and of high aff inity. The specific binding of [H-3]clonidine to rat stomach membranes was concentration-dependently inhibited by various imidazolines and g uanidines including the sigma site ligand 1,2-di-(2-tolyl)guanidine (D TG), by the butyrophenone derivative haloperidol and by the piperidine derivative 3-PPP[(R)-3-(3-hydroxyphenyl)-N-propylpiperidine]; the lat ter two compounds are also known to exhibit affinity for sigma sites. In contrast, rauwolscine, histamine, ranitidine and the non-hydrolysab le GTP-analogue Gpp(NH)p (5' guanylylimidodiphosphate) did not, or wit h negligible affinity, inhibit [H-3]clonidine binding. In most cases, the competition curves were best fitted to a two-site model. The rank order of affinity for the high affinity site (in a few cases for a sin gle detectable site) was as follows: cirazoline>idazoxan greater than or equal to DTG> (+)-3-PPP> clonidine>guanabenz>haloperidol. This rank order is not compatible with the pharmacological properties of either I-1- or I-2-imidazoline binding sites. However, the ability of halope ridol, (+)-3-PPP and DTG to displace [H-3]clonidine (the latter two wi th high affinity) suggests that the [H-3] clonidine binding sites in r at stomach may be related to sigma-like sites.