Analysis of transient expression of estrogen receptor-alpha in newborn rat primary auditory cortex

In the primary temporal cortex (Te1) of newborn rats, we detected transient expression of alpha-type estrogen receptor (ER alpha). Since they have a pyramidal-like shape, they are considered neurons. By immunohistochemistry we found that they were absolutely devoid of glial fibrillary acidic protein but some of them contained calretinin, a calcium binding protein. It is already known that neurons in layer V of the Te1 extend their projections to the contralateral side of the Te1, the ipsilateral inferior colliculus, or the ipsilateral medial geniculate nucleus. Thus, we applied a retrograde track tracer into those regions of newborn rats and examined the possible colocalization of ER alpha signals and the tracer in the same cells. So far no clear colocalization of both signals has been detected in cells in the Te1. Thus, the cells expressing ER alpha transiently are not projecting to the assumed regions, at least at the newborn age examined in the present experiment. The possibility exists that (1) they are not projection neurons but local interneurons, (2) even though they are projection neurons, they did not have any synaptic contacts with their target region(s), (3) they may die after they are attached to the target neurons. Further analyses are needed to clarify the biological roles of ER alpha expressed transiently in these neurons. On the other hand, no ER beta cells were detected in the same region of the brain under the same condition. Thus, this finding was limited to the ER alpha.