Neural sites and pathways regulating food intake in birds: A comparative analysis to mammalian systems

Experiments using two retrogradely transported fluorescent dyes (bisbenzimide-true blue, and Evans blue-granular blue) were performed in order to determine whether the same or different populations of neurons of the paraventricular nucleus of the hypothalamus (PVH) project to the pituitary gland, dorsal vagal complex, and spinal cord in the rat. The results suggest that cells projecting to the pituitary gland are concentrated in the magnocellular core of the nucleus, while the descending connections arise primarily from the surrounding parvocellular division. The occurrence of neurons double-labeled with both dyes further indicate that at lease 10-15% of the labeled cells in the parvocellular division send divergent axon collaterals to the dorsal vagal complex and to the spinal cord. Cell counts suggest that at least 1,500 cells in the PVH project to the medulla and/or spinal cord. These results, combined with a cytoarchitectonic analysis, show that the PVH consists of eight distinct subdivisions, three magnocellular and five parvocellular. The lateral hypothalamic area and zona incerta also contain a large number of cells projecting to the dorsomedial medulla and spinal cord; approximately 15% of such cells are the double-labeled following injections of separate tracers into these two regions of the same animal.

Observations that a pancreatic polypeptide-like substance, possibly neuropeptide Y, is present in hypothalamic areas and may coexist with catecholamines prompted evaluation of its role in controlling feeding behavior. Intracerebroventricular administration of 2 or 10 micrograms of human pancreatic polypeptide to ovariectomized rats pretreated with estradiol benzoate plus progesterone significantly increased the number of animals feeding, and total food intake in tests conducted during the light phase of the day. Administration of neuropeptide Y, 2 or 10 micrograms, induced feeding in all rats, and food intake was 3 times greater than that observed after human pancreatic polypeptide injection. These findings imply that neuropeptide Y, or a closely related pancreatic polypeptide-like neuropeptide, plays an important role in neural regulation of feeding behavior.