Relationship between α-melanocyte-stimulating hormone- and neuropeptide Y-containing neurons in the goldfish hypothalamus

Intracerebroventricular (ICV) injection of α-melanocyte-stimulating hormone (α-MSH) inhibits, whereas ICV injection of neuropeptide Y (NPY) stimulates food intake in the goldfish. However, there is little information about the functional relationship between α-MSH-induced anorexigenic and NPY-induced orexigenic actions in the goldfish. In this study we examined the relationship between α-MSH- and NPY-containing neurons in the goldfish hypothalamus to investigate whether these α-MSH- and NPY-containing neurons have direct mutual inputs. α-MSH- and NPY-like immunoreactivities were distributed throughout the brain, especially in the diencephalon. In particular, α-MSH-containing nerve fibers or endings lay in close apposition to NPY-containing neurons in a specific region of the hypothalamus, the nucleus posterioris periventricularis (NPPv). NPY-containing nerve fibers or endings also lay in close apposition to α-MSH-containing neurons specifically in the interior part of the nucleus lateralis tuberis (NLTi). We also investigated the effect of ICV injection of melanocortin 4 receptor agonist (melanotan II) at 100 pmol/g body weight (BW), which is enough to suppress food intake, or NPY at 10 pmol/g BW, which is enough to enhance food intake, on expression levels of mRNA for NPY or proopiomelanocortin (POMC) in the hypothalamus. ICV injection of melanotan II and NPY induced a significant decrease in the expression levels for NPY and POMC mRNA, respectively. These observations suggest that α-MSH- and NPY-containing neurons share direct mutual inputs in the NPPv and the NLTi of the hypothalamus, and that α-MSH and NPY functionally interact or exhibit mutual inhibition to regulate feeding behavior in the goldfish.