Transforming growth factor β1 may directly influence gonadotropin-releasing hormone gene expression in the rat hypothalamus

Abstract

In vitro studies using immortalized GT1 cells suggest that hypothalamic astrocytes employ TGFβ1 to directly regulate the secretion of GnRH, the neurohormone that controls sexual maturation and adult reproductive function. However, whether such astrocyte-GnRH neuron signaling occurs in vivo is not clear. In the present study, we used in situ hybridization and immunohistochemistry to determine whether astrocytes and GnRH neurons express the molecular components necessary to set in motion communication processes involving TGFβ1 signaling. Double-labeling experiments showed that astrocytes in the male rat preoptic region (POA) expressed TGFβ1 mRNA and that GnRH perikarya were often found in close association with TGFβ1 mRNA-expressing cells. In addition, GnRH neuronal cell bodies in the POA expressed both type II TGFβ receptors (TGFβ-RII), which selectively bind TGFβ, and Smad2/3, one of the primary transducers of TGFβ signaling, suggesting that they are fully capable of responding directly to TGFβ1 stimulation. Consistent with this hypothesis, incubation of POA explants with TGFβ1 caused a significant, dose-dependent decrease in GnRH mRNA expression in individual neurons. This effect was observed within 1 h after TGFβ1-treatment and was inhibited by addition of the soluble form of TGFβ-RII to the incubation medium. In contrast, whereas both TGFβ1 and TGFβ-RII mRNAs were abundantly expressed in both glial cells and capillaries in the median eminence, the projection field of GnRH neurons, TGFβ-RII immunoreactivity was mainly restricted to the processes of tanycytes and did not colocalize with GnRH-immunoreactive fibers. This observation supports previous in vivo studies showing that TGFβ1 is unable to directly modulate decapeptide release from GnRH nerve terminals. Thus, astrocyte-derived TGFβ1 may directly influence GnRH expression and/or secretion in vivo by acting on the perikarya, but not the terminals, of GnRH neurons.