Neonatal peptides affect developing rats: beta-endorphin alters nociception and opiate receptors,...

The dose-response relationship of neonatal (days 1-7) administration of beta-endorphin (BE) and corticotropin-releasing factor (CRF) on body weight, eye opening, response to thermal pain, and concentrations of plasma and adrenal corticosterone were measured in developing rat pups. At the highest dose (50 micrograms/pup), neonatal CRF reduced body weight, while 10 and 50 micrograms/pup accelerated eye opening. In addition, a reduction in concentration of plasma and adrenal corticosterone was correlated with the dose of neonatal CRF, whereas adrenal weights were not altered. BE produced none of these effects, but 1, 10 and 50 micrograms/pup on days 1-7 significantly reduced baseline latencies in a novel water-bath tail-flick test on day 9. These same animals showed reduced numbers of brain opiate receptors on day 14. The results indicate that peptide administration during the sensitive neonatal period can alter the development of physiological processes that will later be influenced by the peptide.