The inhibition of liver ornithine decarboxylase expression in neonatal rats by maternal separation or CNS β-endorphin is independent of the pituitary

Previously we have shown, in rat pups, that either short-term maternal separation (MS) or central (but not peripheral) administration of β-endorphin (BE) markedly decrease basal levels of ornithine decarboxylase (ODC) activity throughout the body and suppresses liver ODC reponsiveness to injected growth hormone (GH). In this study, hypophysectomized (hypox) pups were used to determine whether the pituitary mediates these effects. Hypophysectomy clearly did not prevent the inhibitory actions of MS or intracisternal (i.c.) BE on liver ODC gene expression. The inability of GH to stimulate ODC activity in hypox animals exposed to MS or given BE i.c. is not due to nutritional deprivation, as glucose supplementation did not reverse the response. The results from these studies demonstrate that the pituitary is not the conduit by which either MS or centrally-administered BE regulates liver ODC activity. Also, they support the hypothesis that BE or an analogous opioid neuropeptide is a prime organizer within the CNS of the adaptive physiological response of neonatal rats to short-term MS. As we have previously shown that autonomic neuronal pathways are not involved in the effects of MS on peripheral tissues, the data obtained suggest that increased activity of this CNS opioid system during MS triggers the release of “neurochemicals” into the bloodstream capable of suppressing growth in the mammalian neonate.