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Oxytocin (OT) has an organizational effect within the central nervous system and can have long-lasting effects on the expression of social behavior. OT has recently been implicated in modulating the release of serotonin through activation of receptors in the raphe nuclei. Here we test the hypothesis that OT can have an organizational effect on the serotonergic system. Male prairie voles received an intraperitoneal injection on postnatal day 1 with 3.0 or .3 µg OT, an OT antagonist, or a saline control. Brains were collected on day 21 and immunostained for serotonin. Serotonin axons were quantified in the anterior hypothalamus, cortical amygdala, medial amygdala, paraventricular nucleus of the hypothalamus, and ventromedial hypothalamus. Males treated with 3.0 µg OT displayed significantly higher serotonin axon length densities in the anterior hypothalamus, cortical amygdala, and the ventromedial hypothalamus than control males. These results support the hypothesis that OT has an organizational effect on the serotonin system during the neonatal period, and that these effects are site-specific., (Copyright © 2011 Wiley Periodicals, Inc.)

Background: Previous studies have demonstrated that neonatal manipulation of oxytocin (OT) has effects on the expression of estrogen receptor alpha (ER alpha) and the central production of oxytocin observed in juveniles (at weaning, 21 days of age). The goal of this study was to determine whether the effects of neonatal manipulation of OT last into adulthood, and if the effects differ from those observed during the early postnatal period. On the first day of life, prairie voles (Microtus ochrogaster) received one of three doses of OT (High, 3 microg; Med, 0.3 microg; Low, 0.03 microg), an OT antagonist, or isotonic saline. Another group was handled, but not injected. Then as adults, brains were collected, sectioned, and stained for ER alpha or OT using immunocytochemistry., Results: In females, treatment with OT increased the expression of ER alpha immunoreactivity in the ventral lateral septum (0.03 microg) and the ventromedial nucleus of the hypothalamus and central amygdala (0.3 microg). In males, OT antagonist increased ER alpha expression in the bed nucleus of the stria terminalis. There was no apparent effect of OT on the number of cells producing OT in the paraventricular nucleus of the hypothalamus., Conclusion: The current results suggest that neonatal manipulation of OT has long-term organizational effects on the expression of ER alpha in both males and females. The lack of effect on OT neurons in the paraventricular nucleus suggests that some developmental effects of OT previously observed in weanlings do not persist into adulthood. Developmental effects of OT on ER alpha patterns were sexually dimorphic, dose-dependent, and site-specific.