Additive effects of lactation and food restriction to increase hypothalamic neuropeptide Y mRNA in rats

Neuropeptide Y (NPY) is the most powerful appetite stimulant known, and rates of synthesis and release in the hypothalamus correlate closely with nutritional status. Pregnancy and lactation provide an excellent model of physiological hyperphagia. In this study the authors measured food intake, plasma glucose, insulin and luteinizing hormone (LH) and hypothalamic NPY mRNA in rats during pregnancy and in early and late lactation. The effect of food restriction (to 80% of control) during lactation was also studied. Pregnancy resulted in a modest increase in daily food intake over non-lactating controls (controls: 15.6 +/- 0.6 g, pregnant: 19.8 +/- 1.1 g, P < 0.01). During lactation food intake increased dramatically to 355% of non-lactating levels by the 12th day. Insulin and glucose levels were unchanged in lactation, except in the food-restricted animals, when insulin levels were reduced to 49.5 +/- 18.4 pmol/l compared with 215 +/- 55 pmol/l (P < 0.01) in lactating, non-restricted animals, and glucose was reduced to 3.7 +/- 0.2 mmol/l compared with 5.1 +/- 0.2 mmol/l in non-restricted lactating animals. Hypothalamic NPY mRNA was unchanged in pregnancy, moderately increased after 5 days lactation (130 +/- 6.2% of control, P < 0.01) and increased further at 14 days lactation (179 +/- 14%, P < 0.001). The greatest changes occurred in the animals who were food-deprived during lactation, when hypothalamic NPY mRNA levels reached 324 +/- 44% (P < 0.001) of non-lactating levels. Increases in hypothalamic NPY synthesis may be partly responsible for the increase in food intake seen in lactation, but unlike in food deprivation, the increase is not related to circulating insulin, suggesting involvement of other regulatory factors.