Identification of sex hormone-binding globulin in the human hypothalamus.

Gonadal steroids are known to influence hypothalamic functions through both genomic and non-genomic pathways. Sex hormone-binding globulin (SHBG) may act by a non-genomic mechanism independent of classical steroid receptors. Here we describe the immunocytochemical mapping of SHBG-containing neurons and nerve fibers in the human hypothalamus and infundibulum. Mass spectrometry and Western blot analysis were also used to characterize the biochemical characteristics of SHBG in the hypothalamus and cerebrospinal fluid (CSF) of humans. SHBG-immunoreactive neurons were observed in the supraoptic nucleus, the suprachiasmatic nucleus, the bed nucleus of the stria terminalis, paraventricular nucleus, arcuate nucleus, the perifornical region and the medial preoptic area in human brains. There were SHBG-immunoreactive axons in the median eminence and the infundibulum. A partial colocalization with oxytocin could be observed in the posterior pituitary lobe in consecutive semithin sections. We also found strong immunoreactivity for SHBG in epithelial cells of the choroid plexus and in a portion of the ependymal cells lining the third ventricle. Mass spectrometry showed that affinity-purified SHBG from the hypothalamus and choroid plexus is structurally similar to the SHBG identified in the CSF. The multiple localizations of SHBG suggest neurohypophyseal and neuroendocrine functions. The biochemical data suggest that CSF SHBG is of brain rather than blood origin.

Co-expression of vasopressin and androgen-binding protein in the rat hypothalamus.

In previous studies we have observed the expression of androgen binding protein (ABP) in the rat hypothalamo-neurohypophysial system. With immunocytochemical double staining we found partial co-localization with oxytocin. In the present study we used antibodies to the anti-diuretic hormone arginine vasopressin (AVP) for co-localization with ABP in the rat hypothalamus. Both antigens were seen in the magnocellular paraventricular and supraoptic nuclei. Dense fiber networks with varicosities containing both AVP and ABP immunoreactivity were visible throughout the hypothalamus, the median eminence and in the posterior pituitary lobe. Double immunostaining revealed also co-existence in the parvocellular portion of the paraventricular nucleus and in the suprachiasmatic nucleus. ABP immunoreactive neurons in the preoptic region were devoid of AVP staining, AVP neurons in the bed nucleus of the stria terminalis stained only occasionally for ABP. We conclude that both the magnocellular and the parvocellular hypothalamic vasopressin systems are capable of expressing the steroid binding globulin, which is probably subject to axonal transport, along with the peptide hormone. Intrahypothalamic expression of ABP may be among the mechanisms necessary for rapid actions of steroids on hypothalamic neuroendocrine systems.

Distribution of androgen-binding protein in the rat hypothalamo-neurohypophyseal system, co-localization with oxytocin.

Androgen-binding protein (ABP) is known to be expressed in the male and female rat hypothalamus. In the present study, we observed immunocytochemically ABP in neurons of the magnocellular hypothalamic nuclei, in the preoptic region and in the lateral hypothalamus. Dense fiber networks with varicosities, containing ABP immunofluorescence, were visible throughout the hypothalamus, the median eminence and in the posterior pituitary lobe. Double immunostaining revealed a partial coexistence of ABP-and oxytocin immunoreactivity in a portion of the magnocellular perikarya. ABP was isolated by affinity chromatography from hypothalamus homogenates. Western blots resulted in immunoreactive (IR) bands with an approximate molecular weight of 35 and 50 kDa. Mass spectrometry of these preparations confirmed the presence of ABP, which was almost identical to ABP isolated from rat testis. It is likely that ABP, expressed in magnocellular oxytocinergic neurons, is subject to axonal transport and release in the hypothalamo-neurohypophyseal system.