Effects of leptin receptor mutation on Agrp gene expression in fed and fasted lean and obese (LA/N-fa(f)) rats

Agouti-related protein provides an orexigenic signal, probably through inte raction with central melanocortin receptors. Expression of Agrp is markedly increased in the hypothalamus of mice deficient in leptin (Lep(ob)/Lep(ob) ) or its receptor (Lepr(db)/Lepr(db)), suggesting that leptin mediates sign als suppressing Agouti-related protein production. The regulation of Agrp e xpression in the rat hypothalamus has not been reported. We, therefore, ana lyzed the expression of Agrp in the medial basal hypothalamus of lean (+/+, +/fa(f)) and obese leptin receptor-deficient (fa(f)/fa(f)) LA/N rats. Usin g a sensitive solution hybridization/S1 nuclease protection assay, we found no significant difference in Agrp messenger RNA (mRNA) levels (pg/mu g tot al RNA +/- SEM) in obese rats (n = 5), compared with lean controls (n = 5): 0.46 +/- 0.06 us. 0.47 +/- 0.06 (P = 0.9). Similarly, no difference in Agr p expression was found using in situ hybridization or semiquantitative RT-P CR. In contrast to Agrp, Pone mRNA levels were significantly suppressed in the obese, compared with the lean, rats (P = 0.001). Thus, the ratio of Pom e to Agrp mRNA is decreased in the obese rats and may be an important modul ator of food intake. To assess the physiological regulation of Agrp in rats , we examined the effect of food deprivation in lean Sprague Dawley (SD) ra ts. There was a 273% increase in medial basal hypothalamus Agrp mRNA in so rats fasted for 48 h (n = 8), compared with rats fed ad libitum (n = 8): 0. 82 +/- 0.23 us. 0.30 +/- 0.08 (P = 0.0001). Lean LA/N rats (n = 7) fasted f or 48 h also showed a 231% increase in Agrp expression, compared with fed l ean controls (n = 8): 0.74 +/- 0.11 vs. 0.32 +/- 0.03 (P = 0.002), whereas Pome expression was decreased by 32% in fasted animals from the same experi ment (0.34 +/- 0.05 us. 0.50 +/- 0.07; P = 0.03). There were no significant differences in Agrp or Pome mRNA levels between fasted and fed obese LA/N- fa(f) rats. These results suggest that, in the rat, the Agrp response to fa sting may involve leptin-mediated phenomena, but factors in addition to lep tin must also be involved in the regulation of Ag rp gene expression.