EXPRESSION AND NEUROPEPTIDERGIC CHARACTERIZATION OF ESTROGEN-RECEPTORS (ER-ALPHA AND ER-BETA) THROUGHOUT THE RAT-BRAIN - ANATOMICAL EVIDENCE OF DISTINCT ROLES OF EACH SUBTYPE

The recent cloning of a second estrogen receptor (ER) provided a new t ool to investigate and clarify how estrogens are capable of communicat ing with the brain and influence gene expression and neural function. The purpose of the present study was to define the neuroanatomical org anization of each receptor subtype using a side-by-side approach and t o characterize the cellular population(s) expressing the ER beta trans cript in the endocrine hypothalamus using immunohistochemistry combine d with in situ hybridization. Axonal transport inhibition was accompli shed to cause neuropeptide accumulation into the cytoplasm and thus fa cilitate the detection of all positive luteinizing hormone-releasing h ormone (LHRH), corticotropin-releasing factor (CRF), vasopressin (AVP) , oxytocin (OT), gastrin-related peptide (GRP), and enkephalin (ENK) n eurons. The genes encoding either ER alpha or -beta were expressed in numerous limbic-associated structures, and fine differences were found in terms of intensity and positive signal. Such phenomenon is best re presented by the bed nucleus of the stria terminalis (BnST) and preopt ic area/ anterior hypothalamus, where the expression pattern of both t ranscripts differed across subnuclei. The novel ER was also found to b e expressed quite exclusively in other hypothalamic nuclei, including the supraoptic (SON) and selective compartments (magnocellular and aut onomic divisions) of the paraventricular nucleus (PVN). A high percent age of the ER beta-expressing neurons located in the ventro- and dorso medial PVN are of OT type; 40% of the OT-ir cells forming the medial m agnocellular and ventromedial parvocellular PVN showed a clear hybridi zation signal for ER beta mRNA, whereas a lower percentage (15-20%) of OT neurons were positive in the caudal parvocellular PVN and no doubl e-labeled cells were found in the rostral PVN and other regions of the brain with the exception of the SON. Very few AVP-ir neurons expressi ng ER beta transcript were found throughout the rat brain, although th e medial PVN displayed some scattered double-labeled cells (<5%). Quit e interestingly, the large majority of the ERP-positive cells in the c audal PVN were colocalized within CRF-ir perikarya. Indeed, more than 60-80% of the CRF-containing cells located in the caudolateral divisio n of the parvocellular PVN exhibited a positive hybridization signal f or ER beta mRNA, whereas very few (<5%) neuroendocrine CRF-ir parvocel lular neurons of the medial PVN expressed the gene encoding ER beta. A small percentage of ER beta-expressing cells in the dorsocaudal and v entromedial zones of the parvocellular PVN were also ENK positive. The ventral zone of the medial parvocellular PVN also displayed GRP-ir ne urons, but no convincing hybridization signal for ER beta was detected in this neuronal population. Finally, as previously described for the gene encoding the classic ER, LHRH neurons of both intact and colchic ine-pretreated animals did not express the novel estrogen receptor. Th is study shows a differential pattern of expression of both receptors in the brain of intact rats and that ER beta is expressed at various l evels in distinct neuropeptidergic populations, including OT, CRF, and ENK. The influence of estrogen in mediating genomic and neuronal resp onses may therefore take place within these specific cellular groups i n the brains of cycling as well as intact male mammals. (C) 1998 John Wiley & Sons, Inc.