MULTIPLICITY OF [H-3] 1,3-DI-O-TOLYLGUANIDINE BINDING-SITES WITH LOW-AFFINITY FOR HALOPERIDOL IN RAT-BRAIN

Specific binding of [H-3]1,3-di-o-tolylguanidine (DTG) was found not o nly in synaptic membrane fractions but also in subcellular fractions e nriched of microsomes, nuclei and mitochondria/myelins, with different sensitivities to displacement by the antipsychotic haloperidol. The h ighest binding was detected in microsomal fractions followed by, in or der of decreasing binding, fractions enriched in nuclei, synaptic memb ranes, mitochondria/myelins and homogenates. [H-3]DTG binding was comp letely abolished by prior treatment of the synaptic membranes with a l ow concentration of Triton X-100. [H-3]DTG. binding reached a plateau within 30 min of the incubation at 2-degrees-C, whereas raising the in cubation temperature to 30-degrees-C resulted in marked shortening of the time required to attain equilibrium, without altering the binding at equilibrium. The binding was inhibited by haloperidol in a concentr ation-dependent manner over a concentration range of 1 nM to 0.1 mM bu t with a potency more than 100 times weaker than the value reported in the literature, irrespective of the termination method employed and t he external proton concentrations. [H-3]DTG binding was markedly displ aced by a variety of compounds including sigma ligands, benzomorphan o piates and noncompetitive antagonists at the N-methyl-D-aspartate (NMD A) receptor in synaptic membranes of the cortex, hippocampus and cereb ellum. However, sigma ligands such as haloperidol, DTG and (+)-3-(3-hy droxyphenyl)-N-(1-propyl)piperidine were more potent in displacing [H- 3]DTG binding in cortical membranes than in hippocampal and cerebellar membranes, while the potencies of the NMDA antagonists were not signi ficantly different from each other among these 3 different central str uctures. These results suggest that the rodent brain may contain multi ple sigma sites with different low affinities for haloperidol in the m icromolar range.