Rats normally eat about 85% of their food at night. Lactation increases food intake 3- to 4-fold, but the diurnal pattern of food intake persists. The mechanisms responsible for the diurnal and lactation-induced changes in food intake are still unresolved, hence we have further investigated the possible roles of serum
leptin and hypothalamic expression of
neuropeptide Y (NPY), agouti-related
peptide (AgRP) and
pro-opiomelanocortin (
POMC) in rats. Suppressor of
cytokine signalling-3 (SOCS-3) acts as a feedback inhibitor of
leptin signalling in the hypothalamus, hence changes in expression of SOCS-3 were also investigated. Changes in expression of NPY, AgRP or
POMC alone could not account for the diurnal changes in intake and their alteration by lactation. However, there were increased AgRP
mRNA:
POMC mRNA ratios at night and also during lactation, which were very similar to estimated changes in food intake. Such changes in expression may result in dominance of the orexigenic AgRP
peptide over the appetite-suppressing
POMC-derived
peptides, and so could contribute to the
hyperphagia in these states. Diurnal and lactation-related changes in the AgRP
mRNA:
POMC mRNA ratio and food intake are not due to changes in
leptin alone. However, hypoleptinaemia, possibly through increased expression of NPY, may contribute to the
hyperphagia of lactation. In the dark, expression of SOCS-3 was decreased in non-lactating rats; lactation decreased SOCS-3 expression in both light and dark phases. However, such changes are likely to enhance the ability of
leptin-responsive neurones to transmit the
leptin signal, and so are unlikely to contribute to either the nocturnal increase in appetite or the
hyperphagia of lactation.