The orexigenic effect of kyotorphin in chicks involves hypothalamus and brainstem activity and opioid receptors

Kyotorphin (KTP), first isolated in the bovine brain and now having been identified in a variety of species, is known most extensively for its analgesic-like properties. KTP indirectly stimulates opioid receptors by releasing methionine enkephalin (met-enkephalin). Stimulation of opioid receptors is linked to hunger perception. In the present study, we sought to elucidate the effect of KTP on food intake in the neonatal chick. Intracerebroventricular injection of 0.6, 3.0 and 12 nmol KTP increased feeding up to 60 min post-injection. KTP treated chicks increased pecking efficiency and decreased time spent in deep rest, 20 and 30 min following injection, respectively. Gastrointestinal transit rate was not affected by KTP. Blocking mu, delta, and kappa opioid receptors suppressed orexigenic effects of KTP, suggesting that all three types are involved in KTP's stimulatory effect. The lateral hypothalamus (LH) and arcuate nucleus (ARC) of the hypothalamus and the nucleus of the solitary tract (NTS), within the brainstem had increased numbers of c-Fos immunoreactive cells following KTP treatment. In conclusion, KTP caused increased feeding in broiler-type chicks, likely through activation of the LH, ARC, and NTS.