Aromatase and estrogen receptor beta expression in the rat olfactory bulb: Neuroestrogen action in the first relay station of the olfactory pathway?

The expression pattern of aromatase (ARO), the enzyme converting androgens to estrogens, was analyzed in the olfactory bulb of adult male rats and was compared with the distribution of estrogen receptor beta (ERbeta), the main estrogen receptor isoform expressed in this brain region. A strong ARO immunolabeling obtained with a specificity tested antibody was observed in juxtaglomerular neurons of the glomerular layer and a weaker immunoreaction was detected in the mitral cell layer of the main olfactory bulb, while the granule cell layer of the main olfactory bulb as well as all layers in the accessory olfactory bulb showed faint immunolabeling. Fluorescence double labeling experiments revealed that ARO detected in juxtaglomerular neurons of the main olfactory bulb colocalized with tyrosine hydroxylase (TH) and glutamic acid decarboxylase 67 (GAD67), while no colocalization between ARO and the calcium binding proteins calretinin (CR) and calbindin (CB) was observed. Furthermore, the TH immunoreactive neurons expressed metabotropic glutamate receptor 1 (mGluR1) too. ERβ immunoreactivity, in contrast to ARO, was detected in all layers of both the main and accessory olfactory bulb. In the glomerular layer of the main olfactory bulb it was expressed in TH and GAD67 containing juxtaglomerular neurons, and it colocalized with CR, CB and even with glial fibrillary acidic protein too. Our morphological findings suggest that ARO expression is a novel feature of dopaminergic/GABAergic juxtaglomerular neurons in the adult rat main olfactory bulb, and raise the possibility that ARO activity may change in function of olfactory input via mGluR1. In situ estrogen production in the olfactory bulb in turn may modulate interglomerular circuits through ERbeta.