NEUROPEPTIDE-Y RECEPTOR NUMBERS ARE REDUCED IN THE HYPOTHALAMUS OF STREPTOZOTOCIN-DIABETIC AND FOOD-DEPRIVED RATS - FURTHER EVIDENCE OF INCREASED ACTIVITY OF HYPOTHALAMIC NPY-CONTAINING PATHWAYS

Neuropeptide Y (NPY) injected into the hypothalamus stimulates feeding and affects pituitary secretion. Insulin-deficient diabetes and food deprivation markedly increase hypothalamic NPY and NPY mRNA levels, suggesting increased activity of NPYergic pathways in the hypothalamus, which could account for hyperphagia and neuroendocrine changes in these conditions. To clarify these changes, NPY receptor characteristics were compared amongst rats with 3-weeks' untreated streptozotocin diabetes, insulin-treated normoglycemic diabetics, and non-diabetics, and also in food-deprived (72 h), food-deprived then refed, and in freely fed rats- Hypothalamic tissue homogenates (pooled from 3 rats; n = 9 per group) in Tris/HCl buffer were incubated with 30 pM [I-125]porcine NPY and unlabeled NPY (range, 1 pM to 1 muM) for 1 h. Bound and free fractions were separated by vacuum filtration. Scatchard analysis revealed both high-affinity (K(d) 0.3-0.8 nM) and low-affinity (K(d) 14-40 nM) NPY receptor populations. Compared with nondiabetics. diabetic rats showed significantly reduced numbers (B(max)) of both high-affinity receptors (10 +/- 2 vs. 57 +/- 2 pmol/mg protein; p < 0.001) and low-affinity receptors (113 +/- 25 vs. 544 +/- 48 pmol/mg protein; p < 0.00 1). Insulin treatment partially restored B(max) of both high- and low-affinity receptors (24 +/- 1 and 334 +/- 60 pmol/mg protein, respectively; p < 0.01 vs. both other groups). Food deprivation also reduced B(max) of high-affinity (36 +/- 2 vs. 56 +/- 7 pmol/mg protein in freely fed; p < 0.05) and low-affinity receptors (288 +/- 6 vs. 457 +/- 17 pmol/mg protein; p < 0.05). Refeeding for 48 h after 72-h fasting did not significantly alter B(max) of either high- or low-affinity receptors (35 +/- 3 and 318 +/- 53 pmol/mg protein, respectively; p > 0.05 vs. food-deprived rats; p < 0.05 vs. freely fed rats). Neither diabetes nor food deprivation significantly affected the NPY receptors' affinities. Reduced hypothalamic NPY receptor numbers in diabetes and food deprivation suggest downregylation due to increased NPY release in the hypothalamus. This may mediate food-seeking behavior, hyperphagia, and pituitary dysfunction in diabetes and food deprivation.