REGIONAL DISTRIBUTION OF PUTATIVE NPY Y(1) RECEPTORS AND NEURONS EXPRESSING Y(1) MESSENGER-RNA IN FOREBRAIN AREAS OF THE RAT CENTRAL-NERVOUS-SYSTEM

Using monoiodinated radioligands of peptide YY (PYY), and the recently introduced neuropeptide Y (NPY) analogue [LeU31,Pro34]NPY, receptor b inding sites of the Y1 and Y2 type were localized in the rat brain by quantitative in vitro autoradiography. The binding specificity and aff inity of both radiolabelled ligands were analysed by ligand binding st udies employing rat brain membrane homogenates from cerebral cortex an d hippocampus. Using in situ hybridization histochemistry, the regiona l distribution and cellular localization of mRNA encoding the Y1 recep tor were investigated in rat brain sections and compared to the distri bution of Y1-specific binding sites. PYY binds to both Y1 and Y2 recep tors, while long C-terminal fragments such as NPY13-36 and NPY16-36 bi nd preferentially to Y2 receptors. [Leu31 Pro34]NPY is a specific agon ist for Y1 receptors. Highest densities of [I-125] PYY binding sites w ere found in the cerebral cortex, the thalamus, the lateral septum, th e hippocampus and the mesencephalic dopaminergic areas. In order to bl ock putative Y2 receptors, a series of [I-125]PYY binding experiments was performed in the presence of NPY 3-36 (1 muM), a Y2 preferring C-t erminal fragment. High densities of binding sites remained present in the cerebral cortex, the thalamus and the medial mammillary nucleus wh en Npy13-36 Was present in the incubation medium. Furthermore, these a reas were highly enriched with [I-125][Leu31,Pro34]NPY binding sites. In contrast, the hippocampal complex had its binding capacity reduced by approximately 50%, while the lateral septum and mesencephalic dopam inergic areas had their binding capacities reduced even further. Linea r regression analysis showed a high degree of correspondence between [ I-125][LeU31,Pro34]NPY binding and that obtained with [I-125]pyy in th e presence of 1 muM Npy13-36, suggesting that the two independent appr oaches to visualizing Y1 binding sites are comparable. In situ hybridi zation histochemistry revealed high levels of Y1 mRNA in the granular cell layer of the hippocampal dentate gyrus, several thalamic nuclei a nd the hypothalamic arcuate nucleus. Moderate levels of Y1 mRNA were s een in the frontoparietal cortex, several thalamic nuclei, the hippoca mpal pyramidal layers, the subiculum, the olfactory tubercle, the clau strum and a number of hypothalamic nuclei. The mesencephalon, the amyg dala and most basal ganglia showed very low levels of hybridization. T he present study further clarifies the anatomical distribution of mult iple NPY binding sites within the central nervous system of the rat, a nd extends earlier suggestions that Y1 and Y2 receptor types are prese nt within the central nervous system.