Relationships between hypothalamic neuropeptide Y and food intake in the lactating rat.

NPYergic neurons in the hypothalamic arcuate nucleus (ARC) that project to the paraventricular nucleus (PVN) are postulated to regulate food intake and energy balance. This projection is overactive in lactation and is thought to drive hyperphagia in this condition. We have explored further the relationship between hypothalamic NPY and food intake in lactation and tested the hypothesis that hypoinsulinemia is the stimulus to NPY neuronal activity. Compared with nonlactating controls (n = 10), freely fed lactating rats (n = 9) showed significantly increased (p < 0.05) NPY levels in the ARC and medial preoptic area (MPO), but there was no significant increase in whole hypothalamic NPY mRNA levels. Lactating rats (n = 8) that were restricted to control rats' food intake for 3 days showed generally higher hypothalamic NPY levels, with significantly higher concentrations than controls (p < 0.05) in the ARC, MPO, PVN, and lateral hypothalamic area (LHA); NPY mRNA levels were also significantly increased (p < 0.05). Across all three experimental groups, there was a significant inverse correlation between plasma insulin concentration and hypothalamic NPY mRNA levels (r = -0.39, p < 0.01). We conclude that the ARC-PVN projection is overactive in lactation and that this is not a consequence of hyperphagia. Hypoinsulinemia may stimulate these neurons, as it is thought to do in other conditions of energy deficit.