Background: Sex hormone-binding globulin (SHBG) is a 94-kDa homodimer that binds steroids and is made in the hypothalamus. We have demonstrated that infusions of SHBG into the hypothalami of rats increase their female sexual receptivity except when SHBG is coupled to dihydrotestosterone (DHT) suggesting that SHBG has an active function in behavioral neuroendocrinology. Methods: This study examines the possibility that SHBG is internalized by neuronal and/or non-neuronal brain cells as one possible mode of action using in vitro and in vivo techniques. Results: First, analysis of the uptake of radiolabeled SHBG (125I-SHBG) found 125I-SHBG uptake in HT22 hippocampal cells stably transfected with cDNA for ERβ (HT22-ERβ). The addition of DHT to 125I-SHBG significantly inhibited 125I-SHBG uptake in HT22-ERβ cells but not in HT22-ERα or HT22 wild-type cells. SHBG internalization was specific as it did not occur in either the human neuroblastoma cell line SK-N-SH or the glioma cell line C6. Second, SHBG was labeled with a fluor (Alexa-555TM), and infused into the lateral cerebroventricles of ovariectomized rats. Optimal SHBG uptake was seen 10 min after these infusions. SHBG uptake was seen in specific parts of the choroid plexus and periventricular cells as well as into cells in the paraventricular nucleus, the medial forebrain bundle, and the habenula. Conclusions: These studies suggest that SHBG is internalized by brain cells, which may be affected by the presence of ERβ. The gonadal steroids have numerous effects in brain and the discovery that the steroid-binding protein SHBG is taken up into neurons and brain cells may demand a change in thinking about how steroids are delivered to brain cells to affect neurophysiology.

Keywords:
Sex hormone-binding globulin, Iodination of SHBG, Plasma membrane-associated receptor for SHBG, SHBG immunocytochemistry, SHBG receptors, Brain SHBG, SHBG uptake, ERβ, HT22 cells, Choroids plexus
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