Identification of medial preoptic neurons that concentrate estradiol and project to the midbrain in the rat

Fluorescent dye retrograde tracing was combined with steroid hormone autoradiography to study the midbrain projections of the estrogen‐concentrating neurons in the preoptic region of the rat brain. Microinjections of the dyes DAPI, true blue, or a mixture of DAPI and primuline were made into the ventral tegmental area and into the midbrain central gray of ovariectomized, adrenalectomized 2‐3‐month‐old female rats; 3 or 4 days later these animals were injected with [3H]estradiol; the brains were then processed for autoradiography. After exposures of from 3 to 12 months, the autoradiograms were developed and examined for reduced silver grains under cell nuclei (indicating binding of [3H]estradiol) and retrogradely transported fluorescent dye in the cytoplasm (indicating an efferent projection to the midbrain).