Estrogen influences on oxytocin mRNA expression in preoptic and anterior hypothalamic regions studied by in situ hybridization.

Possible estrogen influences on oxytocin mRNA expression were studied in preoptic and anterior hypothalamic regions as might be important for behavioral as well as neuroendocrine controls. In situ hybridization for oxytocin mRNA determination was supported by immunocytochemical identification and was compared with vasopressin mRNA in situ hybridization. With these techniques, oxytocin-expressing neurons were identified in medial preoptic, anterior commissural, periventricular, paraventricular, supraoptic, and perifornical nuclei as well as the bed nucleus of the stria terminalis and intersupraopticoparaventricular (internuclear) islands. Distribution and number of oxytocin mRNA-containing neurons and oxytocin mRNA levels were compared between ovariectomized control rats given cholesterol implants and ovariectomized rats given short-term (2 days) or long-term (2 months) estradiol treatment (10% estradiol, subcutaneous silastic implants). Effectiveness of long-term estrogen treatment was confirmed behaviorally. While there was a trend in several cell groups for a larger number of oxytocin-mRNA-containing neurons to be observed following 2 days of estrogen treatment, this was not statistically significant. Moreover, additional oxytocin-mRNA containing cell groups were not seen after short or long estradiol treatment. With computer-aided analysis, mean pixels per oxytocin-mRNA expressing neuron (reflecting oxytocin mRNA content) were compared between groups: In the supraoptic nucleus and the anterior commissural nucleus, these were increased both by 2 days and 2 months of estradiol treatment. These differences may be important in modulating female reproductive behavior. Present findings also suggest that estradiol can affect the oxytocinergic system via an indirect route since the cell groups influenced here by estradiol do not contain estrogen receptors. Oxytocinergic neurons may serve as a good system to compare direct transcriptional with indirect effects of hormones.