A comparison of the binding of [3H]proprionyl-neuropeptide Y to rat and human frontal cortical membranes

Non-competitive binding of the nonpeptide antagonist BIBP3226 to rat forebrain neuropeptide Y1 receptors

[3H]Neuropeptide Y labelled neuropeptide Y receptors in rat forebrain membranes as a homogenous class of high-affinity sites. Between 80 and 85% of these receptors showed high affinity for Y1-selective antagonists such as (R)-N2-(diphenylacetyl)-N-[(4-hydroxyphenyl)methyl]-D-arginine amide (BIBP3226). While competitive in functional studies, BIBP3226 produced parallel shifts of the Scatchard plots of [3H]neuropeptide Y saturation binding in rat forebrain membranes. Mechanisms which are routinely invoked to explain non-competitive binding do not apply to BIBP3226. Wash-out experiments, involving successive treatment of the membranes with BIBP3226, buffer (wash-out step) and [3H]neuropeptide Y, argue against irreversible or a pseudo-irreversible binding of the antagonist. Allosteric inhibition is also unlikely since BIBP3226 did not affect the rate of dissociation of [3H]neuropeptide Y in isotope dilution experiments. The non-hydrolyzable guanine nucleotide, 5'-guanylylimidodiphosphate (Gpp(NH)p), abolished the binding of [3H]neuropeptide Y and increased its rate of dissociation in isotope dilution experiments. This suggests that the initial [3H]neuropeptide Y-receptor association is a low affinity process and that the observed binding of [3H]neuropeptide Y is related to the formation of a ternary [3H]neuropeptide Y-receptor-G protein complex. Two- or even multistate models (in which BIBP3226 could potentially behave as an inverse agonist) could therefore be needed to explain the non-competitive antagonism of BIBP3226 in broken cell preparations.

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.

Neuropeptide Y receptors from calf brain: effect of crude Conus venom preparations on [3H]NPY binding

NPY receptors are identified in calf frontal cortex and hippocampus membrane preparations by binding of N-[propionyl-3H] neuropeptide Y. Saturation and competition binding data with PYY, NPY-(18-36) and NPY itself fit with a single class of sites: for the radioligand KD = 1.4 +/- 0.5 nM, Bmax = 434 +/- 180 fmol/mg protein in frontal cortex, KD = 0.7 +/- 0.2 nM, Bmax = 267 +/- 50 fmol/mg protein in hippocampus. Competition curves of the Y1-subtype selective agonist [Leu31, Pro34]NPY are biphasic in both membrane preparations: high affinity sites (i.e. Y1-subtype) amount to 80% in frontal cortex and 23% in hippocampus. The remaining sites are of the Y2-subtype. Out of 23 Conus venom preparations, 17 inhibit the binding of [3H]NPY in both membrane preparations, but only two of them (from Conus aulicus and C. pennaceus) do so with high potency (IC50 < 5 micrograms protein/ml). Only one venom preparation (from C. mercator) had weak discriminatory properties (IC50Y2/IC50Y1 = 6). Venom from C. anemone increased the [3H]NPY binding 5-fold and with an IC50 of 15-18 micrograms protein/ml. This binding occurred to the venom itself and was unrelated to the NPY receptors since it was equally potent when displaced by [Leu31, Pro34]NPY, NPY-(18-36), PYY and NPY.

Quantitative receptor autoradiography demonstrates a differential distribution of neuropeptide-Y Y1 and Y2 receptor subtypes in human and rat brain

Quantitative receptor autoradiography was performed on sections of rat and human brain using [125I]peptide YY ([125I]PYY) to measure the anatomical distribution of neuropeptide Y (NPY) receptors. Masking Y1- and Y2-NPY subtypes with the agonists (Leu31,Pro34]NPY and NPY13-36, respectively demonstrated a differential distribution of Y1 and Y2 receptors between human and rat brain. In human brain, the highest density of [125I]PYY binding was found in pyramidal layers (CA4-CA1) of hippocampus, head and tail regions of caudate nucleus, locus coeruleus and substantia nigra. There was moderate [125I]peptide YY binding to NPY receptors in the molecular layers of the hippocampus, frontal and temporal cerebral cortex, especially in superficial layers, anterior amygdala, central grey and inferior colliculus. Low levels of binding were observed in white matter. The selective Y1 receptor agonist, [Leu31,Pro34]NPY did not effectively reduce [125I]PYY binding to any human brain region examined except for approximately 20-40% of the binding sites in the molecular layer of the dentate gyrus, layer IV of the frontal cortex and the radiatum and oriens layers of the hippocampal complex. In contrast, the Y2 agonist, NPY13-36 was effective at reducing [125I]PYY binding in all human brain regions examined. In rat brain, high densities of [125I]PYY binding was measured in cerebral cortex, thalamus and inferior colliculus which was sensitive to [Leu31,Pro34]NPY. In contrast, high densities of the NPY13-36 sensitive binding was found in the hippocampus, striatum and nucleus accumbens. Medium to low densities of NPY13-36 sensitive binding was found in medulla and pons. This data suggests that human brain contains primarily Y2-type NPY receptors with only a few regions expressing Y1-type receptors. No human brain region examined contained solely Y1-type receptors. In contrast to human brain, rat brain contains regions which express only Y1 receptors as well as regions containing only Y2 receptors and regions containing both Y1 and Y2 receptors.

Chronic desipramine treatment reduces regional neuropeptide Y binding to Y2-type receptors in rat brain

Chronic treatment of rats with desipramine and imipramine (5 mg/kg/twice daily/i.p.) for 14 days caused a significant reduction in the binding of [3H]propionyl NPY to membranes prepared from frontal cortex, nucleus accumbens, hypothalamus and hippocampus. There was no change in binding of [3H]propionyl NPY in the parieto-occipital cortex, striatum or amygdala. Scatchard analysis of binding data from frontal cortical and hippocampal membranes showed that [3H]propionyl NPY bound to a single site with a Kd of approximately 0.3 nM. The loss of [3H]propionyl NPY binding in hippocampal and frontal cortical membranes revealed that chronic tricyclic antidepressant treatment produced a reduction in the number of binding sites with no change in the affinity for the ligand. Chronic desipramine treatment did not alter the ability of NPY (0.01-25 microM) to stimulate inositol phosphate accumulation in rat frontal cortical slices as compared to saline-treated animals. The lack of change of NPY-induced inositol phosphate accumulation following chronic desipramine treatment showed that there was no change to Y1 NPY-type receptors which are linked to the hydrolysis of inositol phospholipids. However, the ability of NPY (0.05-0.5 microM) to inhibit forskolin (1 microM) stimulated adenylate cyclase via Y2 NPY-type receptors in rat frontal cortical slices was significantly reduced following chronic desipramine treatment. This finding suggests that the reduction of [3H]proprionyl NPY binding in selective brain regions may be the result of an antidepressant-induced loss of Y2-type NPY receptors which are negatively linked to adenylate cyclase.