Serotonergic binding in the rat dorsal raphe nucleus: Critical role of MAO inhibition

The biochemical properties of the 5-HT(1A) receptor in dorsal raphe nucleus (DRN) were investigated using a micropunch procedure. Initially, the K(i) value for 5-hydroxytryptamine (5-HT) binding to a site labeled by the 5-HT(1A)-selective ligand [³H]8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) was 20-fold higher than the K(D) for [³H]5-HT. In addition, a number of putative 5-HT(1A) selective ligands displayed poor affinity for the [³H]8-OH-DPAT site. The possibility that these discrepant results were due to metabolism of the receptor ligands was investigated by increasing the concentration of the monoamine oxidase (MAO) inhibitor, 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-HT(1A)- selective compounds displayed high affinity for the DRN binding site consistent with [³H]8-OH-DPAT labeling a 5-HT(1A) receptor in this tissue. The present study describes a fast and easy method for measuring biochemical properties in small discrete brain areas. These studies also indicate that pargyline should be replaced in serotonergic binding assays with a more potent inhibitor of monoamine oxidase such as clorgyline.