Immunoreactive Thromboxane Synthase is Measurable in Ovine Fetal Hypothalamus as Early as 86 Days' Gestation

Thromboxane A₂ (T×A₂) augments hypothalamus-pituitary-adrenal axis activity in both fetal and adult animals. We have proposed that T×A₂ acts as a neuromodulator within the brain to stimulate the release of corticotropin releasing hormone (CRH) or arginine vasopressin (AVP) into the hypophyseal-portal blood[1]. We performed the present experiments to identify immunoreactive thromboxane synthase (T×S) within fetal brain regions and to quantify developmental changes in the T×S immunoreactivity measurable within those regions. We found that immunoreactive T×S was present in fetal hypothalamus, pituitary, brainstem, and lung. In fetal hypothalamus, we found immunoreactive T×S in three identifiable molecular weights, approximately 65, 42, and 35 kD. In fetal pituitary and lung, we found the 65 and 35 kD forms, and in the brainstem we found only the 35 kD form. In fetal pituitary, there was a clear ontogenetic change in T×S immunoreactivity. The 42 kD T×S immunoreactivity was not present in the youngest fetal sheep studied (86–90 days' gestation), but was expressed in the other age groups (125–128, 135–139, 141-term, and postnatal ages). The other molecular weight forms appeared to increase in the older fetuses, but the changes were not significant. In the hypothalamus, all three forms of T×S were measurable at all ages, and there was no signficant change in relative abundance. We conclude that immunoreactive T×S is present in the fetal brain throughout the last half of fetal gestation, but that the significance of multiple molecular weight forms is not clear.