COMPARATIVE AUTORADIOGRAPHIC DISTRIBUTION OF NEUROPEPTIDE-Y Y-1 RECEPTORS VISUALIZED WITH THE Y-1 RECEPTOR AGONIST [I-125] [LEU(31),PRO(34)]PYY AND THE NONPEPTIDE ANTAGONIST [H-3] BIBP3226

BIBP3226, a non-peptide neuropeptide Y Y-1 receptor antagonist, was us ed in its tritiated form to investigate the autoradiographic distribut ion of its binding sites in the rat brain. [I-125][Leu(31), Pro(34)]pP YY a recently developed Y-1 agonist radioligand was used for compariso n. High levels of specific binding for both radioligands were seen in various areas of the rat brain, including superficial cortical laminae , anterior olfactory nucleus, claustrum, geniculate nuclei and medial mamilliary nucleus. However, a few regions, such as the external plexi form layer of the olfactory bulb, lateral septum nucleus tractus, soli tarius and area postrema were only enriched with specific [I-125][Leu( 31), Pro(34)]pPYY binding sites. These data suggest the possible heter ogeneity of [I-125][Leu(31), Pro(34)]pPYY binding sites in the rat bra in, a sub-population of which apparently fails to be recognized by the newly developed Y-1 receptor antagonist, BIBP3226.