Evidence that TGFβ may directly modulate POMC mRNA expression in the female rat arcuate nucleus

Abstract

The purpose of the present study was to determine whether TGFβ, a cytokine secreted by hypothalamic astrocytes, was able to regulate POMC neurons in the arcuate nucleus. In a first set of experiments, mediobasal hypothalamic fragments were exposed to TGFβ1, and the relative POMC mRNA expression was assessed by in situ hybridization using a radiolabeled POMC riboprobe. The results showed that 4 × 10-10 M TGFβ1 was efficient in decreasing significantly the amounts of POMC mRNA (P < 0.01). Interestingly, the decrease of relative POMC mRNA levels was higher in the rostral than in the caudal parts of the arcuate nucleus. In a second set of experiments, we examined the occurrence of TGFβ receptors expression in arcuate POMC neurons. Dual labeling in situ hybridization and in situ hybridization, coupled to immunohistochemical labeling, were performed to examine mRNA expression of the type I serine-threonine kinase receptor for TGFβ and the presence of type II receptor for TGFβ, respectively, in POMC neurons. The results indicated that TGFβ receptor I mRNA and TGFβ receptor II protein were expressed in numerous POMC neurons. Regional analysis revealed that the highest proportion of POMC neurons expressing TGFβ receptors was located in the rostral part of the arcuate nucleus. Using dual labeling immunohistochemistry, we also found that Smad2/3 immunoreactivity, a TGFβ1 downstream signaling molecule, was present in the cytoplasm and nucleus of some POMC (β-endorphin) neurons. We next examined whether the number of POMC neurons expressing TGFβ-RI mRNA was affected by sex steroids. Quantification of the number of POMC neurons expressing TGFβ receptor I mRNA in ovariectomized, ovariectomized E2-treated, and ovariectomized E2 plus progesterone-treated animals revealed that estrogen treatment decreased the expression of TGFβ receptor I mRNA in POMC neurons located in the rostral half of the arcuate nucleus, an effect reversed by progesterone in a subset of the most rostral cells. Taken together, these data reveal that TGFβ1 may directly modulate the activity of POMC neurons through the activation of TGFβ receptors. Therefore, the present study provides additional evidence for the involvement of TGFβ1 in the regulation of neuroendocrine functions and supports the existence of a glial-to-neurons communication within the arcuate nucleus.