Estradiol-concentrating forebrain and midbrain neurons project directly to the medulla.

The location and number of estradiol (E2)-concentrating neurons afferent to the dorsal medulla were determined by combining retrograde fluorescent tract tracing with steroid hormone autoradiography. Injections of Fluro-Gold were made into the medulla of 80 day old, ovariectomized, and adrenalectomized female rats. After 7 days survival to allow for retrograde transport, females were injected with [3H]estradiol; they were then perfused and their brains processed for steroid hormone autoradiography. Following a 4-12 month exposure period, autoradiograms were developed and microscopically analyzed for the presence of E2-concentrating neurons that project to the medulla. Numerous E2-concentrating neurons were identified that send axons directly to the medulla; the majority were found in the bed nucleus of the stria terminalis, paraventricular nucleus of the hypothalamus, central nucleus of the amygdala, and the central gray. Of the E2-concentrating neurons in the bed nucleus of the stria terminalis, 12.7% also projected to the medulla. E2-concentrating neurons that sent axons to the medulla were also identified in and ventromedial to the lateral parvicellular subdivision in the caudal half of the paraventricular nucleus of the hypothalamus (69.4%). Over one-third of the E2-concentrating neurons found in the central nucleus of the amygdala coursed to the medulla. The central gray was the only mesencephalic brain region that contained E2-concentrating neurons that projected to the medulla (41.9%). The medulla-bound E2-concentrating forebrain and midbrain neurons identified in the present study may influence autonomic tone via direct projections.