Distribution of Progestin-Concentrating Cells in Rat Brain: Colocalization of [3H]ORG.2058, a Synthetic Progestin, and Antibodies to Tyrosine Hydroxylase in Hypothalamus by Combined Autoradiography and Immunocytochemistry*

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Abstract
The anatomical distribution of progestin-concentrating cells in rat brain was investigated by thaw-mount autoradiography. Fifteen 23-day-old ovariectomized and adrenalectomized female Holtzman rats were injected sc with 5 .mu.g 17.beta.-estradiol in oil/100 g BW daily for 4 days. On the fifth day, all animals were injected iv with 1 .mu.g/100 g BW [3H]ORG.2058, a synthetic progestin. The animals were killed after 30 or 60 min, and the brains were frozen and processed for autoradiography. Nuclear concentration of radioactivity was found in certain cells of the forebrain and midbrain and was prevented by prior injection (15 min) of excess unlabeled ORG.2058. In the preoptic-septal region, progestin-labeled neurons were observed in the nucleus (n.) septi lateralis, n. interstitialis striae terminalis, n. preopticus medialis, n. preopticus lateralis, n. preopticus periventricularis, and n. preopticus suprachiasmaticus. In the hypothalamus, labeled neurons were seen in n. periventricularis hypothalami, n. arcuatus hypothalamus, n. ventromedialis hypothalami, n. dorsomedialis hypothalami, n. premammillaris ventralis, n. premammillaris dorsalis, and lateral hypothalamus. In the extrahypothalamic region, a few labeled cells were found in the central, medial, and cortical nuclei of the amygdala, the organum subforniculare, the lateral geniculate nucleus, the parietal and entorhinal cortex, and the central gray of the midbrain. Some of the progestin-concentrating cells were characterized with combined autoradiography and immunocytochemistry. Tyrosine hydroxylase-containing cells of the arcuate nucleus and the hypothalamic periventricular nucleus (group A12) showed nuclear concentration of radioactivity. Tyrosine hydroxylase cells in groups A11, A13, and A14; the substantia nigra (group A9); and the ventral tegmental area (group A10) did not show nuclear concentration of [3H]ORG.2058. The autoradiographic study demonstrate specific progestin-binding sites and nuclei of neurons in select areas of the rat brain. The results of combined autoradiography and immunocytochemistry suggest, for the first time, a direct action of progestin on tuberoinfundibular dopaminergic neurons.