Plasma opioids in the first hours of life.

Abstract

Summary: High concentrations of ACTH are present in the neonatal circulation in response to birth stress. Since ACTH and β-lipotropin (βLPH) have a common precursor, and ACTH and β-endorphin (βEP) are released in parallel in stressful situations, we measured plasma levels of βLPH and βEP in the first 24 hours of life. Blood samples were taken from the umbilical cord at birth in 27 neonates, subdivided into four groups. A further blood sample was obtained from the jugular vein after 30 min in nine cases (group A), after 6 hours in six (group B), and after 12 hours in six (group C), and after 24 hours in six (group D). βLPH and βEP were measured by specific radioimmunoassays after silicic acid extraction of the plasma and Sephadex G-75 column chromatography. In cord plasma, the mean (±S.E.) βLPH concentrations (pg/ml) varied between 219.5 ± 84.5 (group D) and 241 ± 43.3 (group B), while those of βEP ranged between 70.2 ± 8.2 (group C) and 54.6 ± 14.4 (group D). The βLPH/βEP ratio on a molar basis ranged from 1.12 ± 0.73 (group A) to 1.37 ± 0.60 (group C). Plasma βLPH concentrations determined at 30 min were 199.1 ± 50.4, and these declined slightly in group C (after 12 hours: 168.2 ± 30.2), but only showed a significant fall after 24 hours (88.5 ± 27.0; P < 0.01 in comparison with the previous groups), reaching normal adult levels (71.7 ± 25.1). The pattern of βEP plasma levels (pg/ml) was very similar to that reported for βLPH. βEP plasma levels only showed a significant decrease after 24 hours (22.7 ± 8.9; P < 0.01) when compared to the 12-hour age group C. βEP concentrations at 30 min (76.1 ± 47.7), 6 hours (68.6 ± 29.3), and 12 hours (51.8 ± 18.1) of life did not differ significantly from those found in mixed cord blood and were constantly higher than normal adult levels [24.5 ± 12.3]. Since the half-life of βLPH and βEP is respectively 45 and 37 min, the present data demonstrate that neonates are able to release βLPH and βEP during the first hours of life. Both these molecules may contribute to help the newborn in the transition from a close dependence on the mother, to an autonomous existence. Speculation: Although the physiologic role of circulating PLPH and PEP has not been fully clarified, the presence of these peptides in neonatal plasma could represent an important analgesic factor in overcoming birth stress. Thus, an abnormal presence of these substances could be related to pathological situations. However, the source, the regulatory mechanisms and the biologic effects of circulating opioids in neonatal life are still unclear and require further investigation.