Ontogeny of the beta-endorphin response to stress in the rat: role of the pituitary and the hypothalamus

Abstract

Neonatal rats show a period of diminished adrenocorticotropin responsiveness to stress during the first 2 weeks of life. To test whether beta-endorphin-like peptides (beta-EPLPs) follow the same pattern of hyporesponsiveness to stress during this period, we examined the ontogeny of the beta-EPLPs response to two different types of stressors (ether vapors and cold) during the early postnatal period. The content of beta-EPLPs was estimated in the serum, the pituitary gland and the hypothalamus prior to and 5 min following exposure to stressful stimuli. Furthermore, to determine the relationship between the responsiveness of beta-EPLPs to stress and that of the hypothalamic-pituitary-adrenal axis in the developing rat, the content of hypothalamic corticotropin-releasing factor (CRF) and serum corticosterone was estimated prior to and following stress. Results indicated that stress induced an increase in the serum corticosterone levels at all ages tested (days 1-22), however, the stress-induced elevations of serum corticosterone were significantly greater on days 1 and 22 than on days 3-14. Significant stress-induced elevations of serum immunoreactive beta-endorphin (ir-beta-EP) were observed on days 14 and 22 of life, while changes on days 1, 3, 8 and 10 were either nonexistent or not statistically significant. Gel filtration analysis revealed that the increases in serum ir-beta-EP following stress on days 14 and 22 resulted primarily from increases in the beta-lipotropin component with lesser increases in the beta-endorphin component. Pituitary content of beta-EPLPs was not affected by stress before day 10, but was markedly reduced in the 10- and 14-day-old rats, following stress. A similar, although not statistically significant decrease was observed in the pituitary content of beta-EPLPs of the 22-day-old rats after exposure to stress. Furthermore, exposure to cold stress in the 14-day-old rats induced more pronounced changes in the serum ir-beta-EP and corticosterone levels as well as in the pituitary ir-beta-EP content than it did with ether stress. Despite variations in serum corticosterone as well as serum and pituitary content of beta-EPLPs, no changes in the hypothalamic ir-beta-EP content were seen in rats after subjection to stress, while small, not statistically significant reductions in the hypothalamic CRF content were observed at 5 min after the onset of stress in the 14-and 22-day-old rats. Thus, during the first 2 weeks of life neonatal rats exhibit a reduced capacity to secrete beta-EPLPs in response to stress.(ABSTRACT TRUNCATED AT 400 WORDS)