We have recently identified
prolactin (PRL)-releasing
peptides (PrRPs) and their stimulating effects on feeding behavior in chicks. To investigate further metabolic functions of PrRP, the present study was performed to clarify whether intracerebroventricular (ICV) injection of
PrRP31, an active form of PrRP in chicks, affects heat production (HP), respiratory quotient (RQ) and plasma concentrations of metabolic fuels in chicks. The ICV injection of
PrRP31 (94 and 375 pmol) did not affect HP but significantly lowered RQ. The change in RQ implies that
PrRP31 shifted the utility of metabolic fuels in the body. This idea was confirmed by subsequent results in which ICV injection of
PrRP31 significantly reduced
glucose but increased non-
esterified fatty acid concentrations in plasma. These shifts in blood metabolic fuels would not be through the increased plasma
insulin, because the ICV injection of
PrRP31 significantly decreased plasma
insulin concentration. On the other hand, ICV injection of another orexigenic
peptide,
neuropeptide Y (NPY) also induced the
insulin release and the metabolic effects were similar to those of
PrRP31. Because ICV injection of
PrRP31 increased NPY
mRNA in the diencephalon, the NPY may mediate the metabolic functions of
PrRP31. In summary, the present study suggests that central
PrRP31 shifts the utilities of peripheral energy sources, which is not via
hyperinsulinemia but via the diencephalon.