Estrogen concentration by substance p‐immunoreactive neurons in the medial basal hypothalamus of the female rat

We used the combined technique of steroid autoradiography and immunohistochemistry to simultaneously visualize estrogen-concentrating and substance P-immunoreactive (sP-IR) cells. A substantial proportion (26.1%) of sP-IR cells that bound estrogen was found in the anterior two thirds of the arcuate nucleus with a rapidly diminishing number of cases (8.7%) in the posterior extent of this nucleus. Highest proportions (42.9%) of sP-IR cells retaining estrogen were found in the ventrolateral part of the ventromedial hypothalamic nucleus. The ventral premammillary nucleus was observed to have both populations but coexistence was comparatively infrequent (2.4%). Occasional cases of coexistence were also found in the lateral hypothalamic area. These results provide anatomical evidence that a subset of sP-IR neurons are directly regulated by estrogen and support the hypothesis that sP-containing cells participate in the transduction of hormonal information into a CNS circuitry that regulates gonadotropin secretion and sexual behavior.