Evidence that TGF beta may directly modulate POMC mRNA expression in the female rat arcuate nucleus.

Abstract

The purpose of the present study was to determine whether TGF beta, 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 TGF beta(1), and the relative POMC mRNA expression was assessed by in situ hybridization using a radiolabeled POMC riboprobe. The results showed that 4 x 10(-10) M TGF beta(1) 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 TGF beta 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 TGF beta and the presence of type II receptor for TGF beta, respectively, in POMC neurons. The results indicated that TGF beta receptor I mRNA and TGF beta receptor II protein were expressed in numerous POMC neurons. Regional analysis revealed that the highest proportion of POMC neurons expressing TGF beta receptors was located in the rostral part of the arcuate nucleus. Using dual labeling immunohistochemistry, we also found that Smad2/3 immunoreactivity, a TGF beta(1) downstream signaling molecule, was present in the cytoplasm and nucleus of some POMC (beta-endorphin) neurons. We next examined whether the number of POMC neurons expressing TGF beta-RI mRNA was affected by sex steroids. Quantification of the number of POMC neurons expressing TGF beta receptor I mRNA in ovariectomized, ovariectomized E2-treated, and ovariectomized E2 plus progesterone-treated animals revealed that estrogen treatment decreased the expression of TGF beta 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 TGF beta(1) may directly modulate the activity of POMC neurons through the activation of TGF beta receptors. Therefore, the present study provides additional evidence for the involvement of TGF beta(1) in the regulation of neuroendocrine functions and supports the existence of a glial-to-neurons communication within the arcuate nucleus.