Neuropeptide Y-like gene expression in the salmon brain increases with fasting

In mammals there is a well-established connection between neuropeptide Y (NPY) and the balance between energy intake and expenditure: NPY stimulates food intake and the hypothalamus shows a dramatic increase in NPY mRNA in response to fasting. The widespread occurrence of NPY in the brains of all vertebrates investigated raises the possibility that NPY may be involved in food intake and energy balance regulation in nonmammalian vertebrates as well. We used in situ hybridization to examine whether brain NPY-like gene expression is involved in energy balance regulation in salmon. A radiolabeled oligonucleotide probe was employed to screen the salmon forebrain and parts of the midbrain for NPY-like mRNA. Distribution of NPY-like gene expression was determined, followed by examination of brains from fed or food-deprived chinook and coho salmon. Regions expressing NPY-like mRNA were the caudoventral telencephalon, preoptic area, thalamus, optic tectum, and caudal hypothalamus. The region showing a difference in NPY-like gene expression between fed and fasted individuals was the preoptic area of the hypothalamus where significantly greater hybridization signal area was found with fasting. Plasma insulin levels were also shown to differ, with fasted animals having significantly lower insulin levels. These results suggest that the role of NPY-like peptides in the regulation of energy balance may have arisen early in vertebrate evolution.