Characterization of neuropeptide Y receptor subtypes in the normal human brain, including the hypothalamus

The aim of the present study was to investigate the existence and distribution of neuropeptide Y receptor subtypes in various regions of the normal human brain using the peptide YY derivative receptor probes, [125I][Leu31,Pro34]polypeptide YY/Y1 and [125I]polypeptide YY(3-36)/Y2, in addition to the non-selective ligand [125I]polypeptide YY. Membrane binding assays performed with post mortem frontal cortex homogenates revealed that [125I]polypeptide YY and [125I]polypeptide YY(3-36) bound in a time- and protein concentration-dependent manner. Very low amounts of specific [125I][Leu31,Pro34]polypeptide YY binding could be detected even in the presence of high amounts of protein, contrasting with results obtained with [125I]polypeptide YY and [125I]polypeptide YY(3-36), a preferential Y2 receptor probe. Analysis of saturation isotherms revealed that [125I]polypeptide YY(3-36) bound to a single class of high-affinity sites (0.5-2 nM). Significantly higher binding capacities were evident for [125I]polypeptide YY(3-36) as compared to [125I][Leu31,Pro34]polypeptide YY, suggesting that the human frontal cortex, in contrast to the rat, is mostly enriched with Y2 receptors. Ligand selectivity profile confirmed the hypothesis that polypeptide YY(3-36), neuropeptide Y and polypeptide YY but not the [Leu31,Pro34] derivatives are potent competitors of [125I]polypeptide YY and [125I]polypeptide YY(3-36) binding sites. Autoradiographic studies demonstrated further that cortical areas, as well as most other regions of the human brain, are particularly enriched with Y2/[125I]polypeptide YY(3-36) sites, while only low to very low amounts of Y1 binding were detected except in the dentate gyrus of the hippocampal formation. In the human hypothalamus, a preponderance of Y2 binding sites was also noted. Taken together, these results clearly establish that the distribution of the Y1 and Y2 receptor subtypes in human is different from the rodent brain, the Y2 subtype being most abundant in the human brain.