SEROTONERGIC BINDING IN THE RAT DORSAL RAPHE NUCLEUS - CRITICAL ROLE OF MAO INHIBITION

The biochemical properties of the 5-HT1A receptor in dorsal raphe nucl eus (DRN) were investigated using a micropunch procedure. Initially, t he K-i value for 5-hydroxytryptamine (5-HT) binding to a site labeled by the 5-HT1A-selective ligand [H-3]8-hydroxy-2-(di-n-propylamino)tetr alin (8-OH-DPAT) was 20-fold higher than the K-D for [H-3]5-HT. In add ition, a number of putative 5-HT1A selective ligands displayed poor af finity for the [H-3]8-OH-DPAT site. The possibility that these discrep ant results were due to metabolism of the receptor ligands was investi gated by increasing the concentration of the monoamine oxidase (MAO) i nhibitor, pargyline. Increasing the concentration of pargyline reduced , but did not abolish, the discrepancy between the K-i and K-D values for 5-HT. However, inclusion of clorgyline, which is a more potent MAO inhibitor, resulted in an excellent agreement between the K-i and K-D values for 5-HT. In addition, when clorgyline was used, 5-HT1A select ive compounds displayed high affinity for the . DRN binding site consi stent with [H-3]8-OH-DPAT labeling a 5-HT1A receptor in this tissue. T he present study describes a fast and easy method for measuring bioche mical properties in small discrete brain areas. These studies also ind icate that pargyline should be replaced in serotonergic binding assays with a more potent inhibitor of monoamine oxidase such as clorgyline. (C) 1996 Elsevier Science Inc.