Analysis of transient expression of estrogen receptor in the newborn rat brain

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We have found that estrogen receptor (ER) alpha is expressed transiently in the facial nucleus and in the primary auditory cortex of new born rats. However, biological meaning of this expression is still left unexplained. Using TUNEL method, we failed to detect any apoptotic signals in the facial nucleus of fetal to newborn rats between 16 days after conception (FE16) and 11days after birth (PN11). Thus, it is suggested that the cells expressing ER alpha are not those die early in the life. On the other hand, the birth date of the ER alpha positive cells was estimated by giving BrdU to pregnant mothers followed by immunohistochemical detection of ER signals. Colocalization of the BrdU and ER alpha signals was only detected in those samples given BrdU at 11 and 12 days after conception (FE11-12). Thus, motoneurons in the facial nucleus is suspected to attain final cell division around FE11-12 and only a part of them express ER alpha. Since ER signals disappear after PN14, our hypothesis that only immature motoneurons contain ER signals is supported. On the other hand, we examined in the primary auditory cortex of the newborn rats if ER signals were co-expressed with any neuropeptides, such as neuropeptides Y (NPY), gleaning, substance P, cholecystokinine 8 (CCK8), and vasoactive intestinal peptide (VIP) by using double immunohistochemistry method. So far we failed to detect any distinct Colocalization of ER alpha signals with any of these peptides. However, since we detected ER alpha signals in extranuclear loci by electron microscopy in the hippocampal pyramidal neurons, there is a possibility of extra-nuclear action of estradiol through estrogen receptor.