| Three experiments for this thesis were conducted to study the potential mechanisms of early programming feed intake, difference of feed intake between two genotypes of broilers, and zinc in regulating feed intake.The first experiment was conducted to study and compare the dynamics of yolk sac utilization and developmental patterns of serum and hypothalamic factors regulating feed intake in two lines of newly-hatched broilers during 11 days posthatching. AA broilers with fast gaining rate and Beijing-You (BY) broilers with slow gaining rate were used. The results showed that the BY broilers grew in a similar fashion with the AA broilers, but ADFI and ADG of the former were significantly lower (P≤0.0004) than those of the latter. The dynamics of yolk sac utilization were also similar between the two lines with a decrease in an exponential way with age, and had significant negative correlations with ADFI (P≤0.008), but the yolk sacs of BY broilers were utilized faster than those of AA broilers. During 3 days posthatching, leptin and insulin levels in the yolk sacs of both BY and AA broilers increased with age, and leptin and insulin levels in the yolk sacs of the former were higher (P≤0.08) than those of the latter at the same day. Serum insulin levels of BY broilers varied greatly with age, while those of AA broilers kept about 7.5μIU/ml during the postnatal period except day 0. Serum leptin levels of BY broilers decreased with age in a degree, while those of AA broilers at days 1 and 3 were higher than that at day 0 (P≤0.03), and then declined. Hypothalamic leptin levels of BY broilers decreased with age, but those of AA broilers on day 1 was higher than those on day 0 (P = 0.0017), and then decreased. Similar changes of hypothalamic insulin and NPY with age between both lines were observed. Hypothalamic insulin levels were relatively high during 5 days posthatching, and then decreased. Hypothalamic NPY levels increased with age, and reached the highest on days 7. Hypothalamicα-MSH levels of BY broilers were relatively low during 7 days posthatching, and then increased; while those of AA broilers decreased with age till days 5, and then increased. Correlation analysis showed that ADFI had high correlations (R > 0.65, P < 0.12) with serum leptin, hypothalamic insulin, leptin and NPY levels, but there were no correlations (R≤0.50, P > 0.26) between ADFI and serum insulin or hypothalamicα-MSH levels. These results suggested that the dynamics of yolk sac utilization were similar between BY and AA broilers with a decrease in an exponential way with age, and had significant negative correlations with ADFI, but the yolk sacs of BY broilers were utilized faster than those of AA broilers, which was in part due to the higher levels of leptin and insulin in the yolk sacs of BY broilers in comparision with AA broilers. Developmental changes of serum and hypothalamic appetite regulatory factors with age differed in line and parameter; leptin, insulin, NPY andα-MSH might play a role in regulation of feed intake in newly-hatched broilers, but the difference of feed intake between the two lines could not be explained by them.The second exprement was conducted to investigate and compare developmental patterns of hypothalamic LR, IR, NPY and POMC mRNA in the two lines of newly-hatched broilers during two weeks posthatching, trying to find the potential mechanisms of early programming feed intake and differences of feed intake between the two genotypes. The results showed that the similar developmental changes of hypothalamic LR, IR, NPY and POMC mRNA with age were observed between BY and AA broilers. LR, IR and NPY mRNA were highly expressed in hypothalamus at hatch (day 0), and higher than those at the other days (P < 0.06), and then decreased in a degree with no obvious difference (P≥0.148). Hypothalamic POMC mRNA levels were also high at hatch (day 0), and higher than those on days 7 and 14 (P≤0.0515) signifficantly, but no difference was observed among the other ages (P > 0.10). And also there was no difference (P > 0.174) in hypothalamic LR and IR mRNA between both lines during two weeks posthatching. Hypothalamic NPY and POMC mRNA levels of AA broilers at days 7 were significantly higher (P < 0.04) than those of BY broilers at the same age, but this phenomenon was not observed at the other ages (P > 0.10). These results suggested that hypothalamic LR and IR might play a role in early programming feed intake of broilers, and their expressions were down-regulated respectively by leptin and insulin. Also, NPY and POMC might play a role in the development of the appetite regulatory system and early programming of feed intake in newly-hatched broilers. The difference of feed intake between the two lines could not be explained by hypothalamic LR, IR, NPY and POMC mRNA.The third experiment was conducted to investigate the effects of dietary zinc deficiency on appetite regulatory factors of newly-hatched broilers to probe into the potential mechanisms of zinc in regulating feed intake. The results showed that dietary zinc deficiency decreased significantly (P < 0.0001) ADG (about 100%) and feed efficency ratio (about 30%) of the broilers at two weeks posthathing, and influenced ADFI with relation to age. ADFI was not influenced (P≥0.2712) by dietary zinc deficiency during 2 days after hatch, and then significantly decreased (P≤0.0474). Dietary zinc deficiency decreased serum zinc and glucose levels (P < 0.005) significantly, mainly due to zinc deficiency; but serum leptin, insulin and insulin/glucagon moler ratiowere declined, due to both zinc deficiency and the reduce of feed intake. Dietary zinc deficiency did not influenced serum glucagon and hypothalamic zinc concentration (P≥0.1331), but increased hypothalamic NPY and its mRNA levels significantly (P < 0.012). Hypothalamic AMPK activity increased by about 10% in zinc-deficient broilers, but there was not significant difference (P≥0.2483). Theses results suggested that the decrease of feed intake induced by dietary zinc deficiency could not be explained by serum leptin, insulin and glucagon, hypothalamic NPY and AMPK activity, and further studies are needed.In conclusion, leptin and its receptor, insulin and its receptor, NPY, andα-MSH might play a role in regulation of feed intake in newly-hatched broilers, but the mechanisms remained unknown; and also these observed parameters could not explain why there was obvious difference in feed intake between the two lines. And the changes of serum leptin, insulin and glucagon, and hypothalamic NPY and AMPK activity also could not explain why feed intake was reduced by dietary zinc deficiency; thereafter, further studies need to elucidate the mechanisms.
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