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 Y4 and Y5 receptors, a series of selective molecules were used as radioligands and competitors in rat brain tissues. Binding data revealed that Y4 and Y5 receptor-related agonists and antagonists competed with high affinity for specific 125I-[Leu31,Pro34]human peptide YY (hPYY) binding in the presence of BIBO3304 [(R)-N-[[4-(aminocarbonylaminomethyl)-phenyl]-methyl]-N2-(diphenylacetyl)-argininamide trifluoroacetate] to mask Y1 sites as well as specific 125I-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 Y4 and Y5 subtypes. We were surprised to find that the visualization of these specific binding sites by receptor autoradiography clearly revealed the distinct distribution of specific 125I-[Leu31,Pro34]hPYY (in presence of Y1 and Y5 blockers) and 125I-hPP (in presence of Y5 blocker) binding sites. Moreover, significant amounts of specific 125I-hPP binding were observed in the medial preoptic area, paraventricular nucleus of the hypothalamus, interpeduncular nucleus, and various brainstem nuclei, even after masking Y4 and Y5 receptors. Similar results were obtained using 125I-hPYY(3-36) in presence of Y2 and Y5 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.