Regionally-selective down-regulation of NPY receptor subtypes in the obese Zucker rat. Relationship to the Y5 ‘feeding’ receptor

Experiments were performed to examine whether there are regionally and subtype selective changes in the density of neuropeptide Y (NPY) receptors in the obese Zucker rat which has an increased synthesis and release of NPY confined to the hypothalamus. Competition binding assays were employed to examine the feasibility of using [¹²⁵I]peptide YY ([¹²⁵I]PYY) to measure neuropeptide Y (NPY) Y5 ‘feeding’ receptors in the hypothalamus, hippocampal and cerebral cortex following masking of Y1 and Y2 receptors. Y5 receptors could not be discriminated from the binding to Y1 and Y2 receptors in hypothalamic, hippocampal or cerebral cortex homogenates, possibly owing to the small population of Y5 receptors expressed in the brain and the lack of selective ligands for this receptor. Quantitative receptor autoradiography was used to examine for regional changes in NPY receptor subtypes in obese versus lean Zucker rats. The non-selective Y1, Y2, Y4 and Y5 receptor ligand [¹²⁵I]PYY and the more selective Y1, Y4 and Y5 ligand [¹²⁵I][Leu³¹,Pro³⁴]PYY were employed, in conjunction with masking compounds in an attempt to measure any regional changes in the recently cloned NPY Y5 ‘feeding’ receptor. Specific [¹²⁵I]PYY and [¹²⁵I][Leu³¹,Pro³⁴]PYY binding was significantly reduced in the hypothalamic dorsomedial and arcuate nuclei as well as in the dorsal and lateral (perifornical) areas of obese Zucker rats, as compared to lean rats. In addition there were significant reductions in binding to the thalamic reuniens and centromedial nucleus, and hippocampal dentate gyrus of obese rats as compared to lean rats. Masking [¹²⁵I]PYY binding to Y1 receptors using 1 μM BIBP3226 demonstrated that the reduced NPY receptor density was due to reductions in Y2 or Y5 receptor density. The binding which was sensitive to BIBP3226, i.e. Y1 receptor density, was not different between obese and lean rats. Attempts using [¹²⁵I]PYY and the relatively selective Y2 agonist, [13–36]NPY to mask Y2 receptors and reveal Y5 receptors failed to leave any specific binding suggesting that [13–36]NPY was not selective enough to separate binding to Y2 and Y5 receptors. However, using [¹²⁵I][Leu³¹,Pro³⁴]PYY, masking binding to Y1 receptors using 1 μM BIBP3226 and masking any binding to Y4 using 1 nM rat pancreatic polypeptide left a small amount of binding remaining in the thalamus and hypothalamus, presumably to Y5 receptors which was significantly reduced in obese versus lean rat brain. These data suggest that there is a selective down-regulation in Y5 ‘feeding’ receptors in the obese Zucker rat which is known to possess a hyperactive arcuate-paraventricular NPY system.