Leptin regulation of Agrp and Npy mRNA in the rat hypothalamus

Agouti-related protein (AGRP) is synthesized in the same neurones in the ar cuate nucleus as neuropeptide Y (NPY), another potent orexigenic peptide. A GRP antagonizes the action of alpha -melanocyte stimulating hormone, a deri vative of pro-opiomelanocortin (POMC) at the hypothalamic MC4 receptor to i ncrease food intake. Although leptin has been shown to regulate Agrp/Npy an d Pomc-expressing neurones, there are differences with respect to Agrp regu lation in leptin receptor-deficient mice and rats. Unlike the obese leptin receptor-deficient db/db mouse, which exhibits upregulation of Agrp mRNA ex pression in the medial basal hypothalamus (MBH) compared to lean controls, the obese leptin receptor-deficient (fa; Koletsky) rat does not exhibit upr egulation of Agrp expression. To determine whether this represents a genera l difference between leptin receptor-deficient mice and rats, neuropeptide gene expression was analysed in the MBH of lean and obese rats segregating for a different leptin receptor mutation, Lepr(ia) (Zucker). Fasting in lea n rats (+/fa) for 72 h significantly increased Agrp and Npy mRNA expression , and decreased Pomc mRNA expression as detected by a sensitive solution hy bridization/S1 nuclease protection assay. Npy mRNA levels were significantl y increased in fed obese fa/fa compared to lean rats, and further increased in the obese animals after fasting. In contrast, Agrp mRNA levels did not differ between fed lean and fed obese rats, and fasting did not significant ly change Agrp levels in obese rats. To determine whether the change in Agr p expression that occurs with food deprivation in lean rats could be preven ted by leptin replacement, Sprague-Dawley rats were fasted and infused via subcutaneous osmotic micropumps for 48 h with either saline or recombinant mouse leptin. Fasting significantly increased Agrp and Npy, and decreased P omc mRNA levels. Leptin infusion almost completely reversed these changes s uch that there was no significant difference between the levels in the fast ed rats and those that were fed ad libitum. Thus, in fasted lean rats, Agrp and Npy are upregulated in parallel when leptin levels fall and are downre gulated by leptin infusion. By contrast, the absence of a functional leptin receptor results in the upregulation of Npy but not Agrp mRNA.