Immunocytochemical detection of serotonin content in raphe neurons of newborn and young adult rabbits before and after acute hypoxia.

The present immunocytochemical study demonstrates serotonin (5-HT) depletion in the dorsal raphe nucleus (DRN) of 3- and 21-day-old rabbits following exposure to mild (10% ambient partial pressure of oxygen) and severe hypoxia (5% ambient oxygen). Under the mild hypoxic condition, 5-HT immunoreactivity in cells and fibers of the DRN was decreased in 3-day-old as well as 21-day-old rabbits, as indicated by decreased intensity of the staining compared to age-matched controls. Although this decrease was more pronounced in the younger animals, recovery from mild hypoxia was seen in both age groups. Hypoxic effects were more striking in 3-day-old animals under the severe hypoxic condition, indicating a greater depletion of 5-HT than in the mildly hypoxic condition. However, little additional effect on the older age group was seen. Further, a decreased ability of the 3-day-old rabbits to recover following severe hypoxia suggests that protracted effects on the developing serotonergic system occur following severe hypoxia during the neonatal period. This was demonstrated by the long-lasting decrease in the number of stained cells and fibers of the DRN 4-hr after return to normal conditions (21% O2). We conclude that newborns have a decreased rate of 5-HT synthesis and/or metabolic turnover that results in rapid depletion of intracellular stores and protracted time to recover from a hypoxic challenge. Similar effects could occur in human fetuses, newborns or infants following birth trauma, apnea or other events associated with severe hypoxia.