Sex, diet density and feeding centre gene expression

# Abstract # ## Background ## Research into energy balance and growth has infrequently considered genetic sex, yet there is sexual dimorphism for growth across the animal kingdom. We test the hypothesis that sexual dimorphism for growth and body weight in birds is potentially dependent on hypothalamic agouti-related peptide (AGRP) expression, since previous research indicates hypothalamic AGRP expression is correlated with growth potential. Because growth has been heavily selected in some chicken lines, food restriction is necessary to improve reproductive performance and welfare, but this increases hunger. Dietary dilution has been proposed to ameliorate this undesirable effect. We aimed to distinguish the effects of gut fullness from nutritional feedback on hypothalamic gene expression and its interaction with sex. ## Methods ## Restricted-fed 12-week-old male and female fast growing chickens were released from restriction and fed ad-libitum or a diet diluted with a non-nutritive bulking agent, ispaghula husk, for 2 days. A control group remained on the quantitative restricted diet. The basal hypothalamus was then dissected and gene expression of hypothalamic satiety peptides were measured using real-time PCR. To confirm observed sex difference effects, the experiment was repeated using only ad-libitum and restricted fed broiler breeders and observations made in an unrelated breed of ad-libitum fed male and female chickens. Linear Mixed Models (Genstat 18) were used for statistical analysis with transformation where appropriate. ## Results ## There were pronounced sex differences in a fast growing chicken strain, AGRP (P<0.001) expression was higher in males. In genetically distinct chickens, males had higher AGRP mRNA (P=0.002) expression than females, suggesting sex difference was not restricted to a fast growing strain. AGRP (P<0.001) expression was significantly decreased in ad-libitum-fed birds but was high and indistinguishable between birds on a quantitative versus qualitative restricted diet. Inversely, anorectic expression was significantly higher in ad-libitum fed birds. ## Conclusion ## Expression of orexigenic peptides in the avian hypothalamus are correlated with sex differences in growth and body weight. The differences in gene expression between sexes provides further evidence of AGRP expression being correlated to growth potential. Results also suggest that gut-fill alone does not reduce orexigenic gene expression.