DIFFERENTIAL SENSITIVITY OF [H-3] 7-OH-DPAT-LABELED BINDING-SITES IN RAT-BRAIN TO INACTIVATION BY N-ETHOXYCARBONYL-2-ETHOXY-1,2-DIHYDROQUINOLINE

The effects of alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) on the binding of [H-3]7-OH-DPAT, a ligand for the D-3, dopamine receptor, were assessed in ventral striatal (n. accumbens and elf. tubercle) membranes of adult, male Sprague-Dawley rats. [H-3]Spiperone binding to D-2-like receptors in striatal membranes was also assayed as a positive control. In vitro, EEDQ was equipotent in inactivating [H-3]7-OH-DPAT- and [H-3]spiperone-labeled binding sites. In vivo, [H-3]spiperone binding was rapidly eliminated in a dose-dependent manner following EEDQ administration. In contrast, [H-3]7-OH-DPAT binding was not significantly altered by any dose of the alkylating agent at any time point examined. Depletion of endogenous catecholamines with alpha-methyltyrosine and reserpine revealed a second, higher affinity binding site for [H-3]7-OH-DPAT. Administration of EEDQ in catecholamine-depleted animals reduced [H-3]7-OH-DPAT binding by one-third relative to controls and resulted in the detection of a single population of sites. These findings suggest that [H-3]7-OH-DPAT labels two distinct populations of binding sites in rat brain membranes, only one of which is susceptible to inactivation by EEDQ. These sites may represent high and low affinity states of the D-3 receptor. In addition, this discovery may provide a useful method for examining the function of some D-3 receptors in brain independent of other monoaminergic systems.