Evidence for the existence of an additional class of neuropeptide Y receptor sites in rat brain

Five distinct neuropeptide Y (NPY) receptors have been cloned thus far. Selective agonists and antagonists have recently been developed allowing for detailed functional studies as to the pathophysiological role of a given subtype as well as receptor binding characteristics and distribution. To precisely investigate the discrete localization and ligand selectivity profile of Y-4 and Y-5 receptors, a series of selective molecules were used as radioligands and competitors in rat brain tissues. Binding data revealed that Y-4 and Y-5 receptor-related agonists and antagonists competed with high affinity for specific I-125-[Leu(31), Pro(34)]human peptide YY (hPYY) binding in the presence of BIBO3304 [(R)-N-[[4-(aminocarbonylaminomethyl)-phenyl]methyl]-N-2-(diphenylacetyl)-argininamide trifluoroacetate] to mask Y-1 sites as well as specific I-125-labeled human pancreatic polypeptide (hPP) binding. Competition binding profiles were best fitted to a two-site model for both radioligands, suggesting the likely recognition of the Y-4 and Y-5 subtypes. We were surprised to find that the visualization of these specific binding sites by receptor autoradiography clearly revealed the distinct distribution of specific I-125-[Leu(31), Pro(34)]hPYY (in presence of Y-1 and Y-5 blockers) and I-125-hPP (in presence of Y-5 blocker) binding sites. Moreover, significant amounts of specific I-125-hPP binding were observed in the medial preoptic area, paraventricular nucleus of the hypothalamus, interpeduncular nucleus, and various brainstem nuclei, even after masking Y-4 and Y-5 receptors. Similar results were obtained using I-125-hPYY(3-36) in presence of Y-2 and Y-5 blockers. These results suggest the possible existence of at least one additional subtype of NPY receptor sites in the rat brain, with enrichment seen in midbrain and brainstem areas involved in the regulation of food intake and cardiorespiratory parameters.