Comparative autoradiographic distribution of neuropeptide Y Y1 receptors visualized with the Y1 receptor agonist [125I][Leu31,Pro34]PYY and the non-peptide antagonist [3H]BIBP3226.

BIBP3226, a non-peptide neuropeptide Y Y1 receptor antagonist, was used in its tritiated form to investigate the autoradiographic distribution of its binding sites in the rat brain. [125I][Leu31,Pro34]pPYY a recently developed Y1 agonist radioligand was used for comparison. 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 plexiform layer of the olfactory bulb, lateral septum nucleus tractus, solitarius and area postrema were only enriched with specific [125I][Leu31,Pro34]pPYY binding sites. These data suggest the possible heterogeneity of [125I][Leu31,Pro34]pPYY binding sites in the rat brain, a sub-population of which apparently fails to be recognized by the newly developed Y1 receptor antagonist, BIBP3226.