Estradiol influences oxytocin-immunoreactive brain systems.

The rat brain was examined immunocytochemically for estrogen-dependent changes of oxytocin immunoreactivity at the light microscopical level. Ovariectomized rats were treated with subcutaneous silastic implants with estradiol, or empty implants as controls for 2 days (short term treatment). Another group of rats was injected weekly for 2 months with 1 mg estradiol (long term, high dose treatment). After perfusion fixation serial Vibratome sections were stained with antibodies to oxytocin. In control animals, oxytocin immunoreactive perikarya were found in the magnocellular hypothalamic nuclei. Accessory oxytocin neurons appeared in various hypothalamic sites: immunostained neuronal processes were visible in the preoptic region, the lateral septum, the ventromedial hypothalamus and the median eminence. In short term estradiol treated animals, additional immunoreactive perikarya could be observed in the septohypothalamic nucleus, the lateral subcommissural area, the medial preoptic area, the perifornical region, the zona incerta and the ansa lenticularis. An increased number of immunostained fibers was found in the lateral septum, the preoptic region, the striatum and the amygdala. Animals treated with high doses of estradiol for 2 months showed oxytocin immunostaining only in the paraventricular and supraoptic nuclei and in the median eminence. The distribution of oxytocin immunoreactive neurons in the magnocellular nuclei did not change with changing estradiol levels. Physiological amounts of estrogen given for 2 days increased the number of oxytocinergic neurons visible outside the classical magnocellular nuclei while prolonged, high dose estrogen treatment diminished immunostaining in these oxytocinergic systems.