A Small Population of Tyrosine Hydroxylase‐lmmunoreactive Neurons in the Guinea‐Pig Arcuate Nucleus Contains Progestin Receptor‐lmmunoreactivity

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Abstract
We studied the co-localization of progestin receptor-immunoreactive (PR-IR) cell nuclei and tyrosine hydroxylase-immunoreactive (TH-IR) cell bodies in guinea-pig brain with a double antibody, immunocytochemical technique. Sections were first immunostained for estradiol-induced PR-IR using a peroxidase-antiperoxidase technique with diaminobenzidine as the chromogen followed by alpha-naphthol as the chromogen for TH-IR. We examined the periventricular-preoptic area and the arcuate nucleus, because these two sites are dense in both PR-IR cells and TH-IR cells (cell groups A14 and A12, respectively), and the dorsal hypothalamic Area A13, because this area contains a high density of TH-IR cells, but few PR-IR cells. No co-localization was seen in the periventricular-preoptic area or Area A13. However, a small proportion (5% to 13%) of TH-IR cells in the arcuate nucleus was observed to have PR-IR cell nuclei with the rostral arcuate showing the greatest concentration of co-localized cells. In order to determine if the estradiol pretreatment required to induce PR-IR influenced TH-IR, TH-IR in estradiol-primed guinea-pigs was compared with that of vehicle-injected controls. This treatment did not noticeably influence the amount of TH-IR in the arcuate nucleus. Therefore, the results of these experiments suggest that, although some of the TH-IR neurons in the arcuate nucleus contain PR-IR, this relationship is seen in less than 15% of the TH-IR cells. In many cases, PR-IR neurons were found to have TH-IR varicosities closely associated with their cell bodies.