Among the many responses to GH administration is suppression of voluntary feed intake (FI) in some species, attributed to improvement in the efficiency of nutrient utilization and, therefore, reduced need for ingested substrates. Commercial broiler chickens have been genetically selected for generations for rapid growth, realized largely via the major correlated response of increased voluntary feed consumption. Neuropeptide Y (NPY) and monoamines play very important roles in the central regulation of feeding. Preliminary studies from our laboratory suggest that the appetite-suppressive effect of GH may be independent of its actions as a repartitioning agent, and may involve alterations in NPY expression at the pre-translational level. The purpose of this investigation was to explore the dose-response nature of the appetite-suppressive effect of GH in juvenile broilers, and the possible involvement of NPY and monoamines in this process. A GH dose-response study was conducted using 8-week-old female broilers infused i.v. with GH in a pulsatile pattern for 7 days at 0, 10, 50, 100 or 200 microgram/kg body weight per day. Hypothalamic NPY and epinephrine (EP) concentrations decreased in a dose-related manner with GH. At the highest dosage, voluntary FI decreased 19% (P<0.05) and hypothalamic NPY mRNA decreased approximately 50% in the infundibular nuclei and midline region (P<0.0001). In contrast, birds pairfed to the high-GH dosage group did not differ from controls, verifying that changes in NPY and monoamines were not secondary to reduced FI. We conclude that hypothalamic NPY and EP are likely candidates to explore further as mediators of the appetite-suppressive effect of GH.

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