Leptin but not neuropeptide Y up-regulated glucagon-like peptide 1 receptor expression in GT1-7 cells and rat hypothalamic slices.

In an attempt to gain better insight into the central effects of glucagon-like peptide (GLP-1), we studied the action of glucose and of regulatory peptides on the expression of its receptor (GLP-1R) in hypothalamic GT1-7 cells and in ventromedial (VMH) and lateral (LH) rat hypothalamus slices. The promoter activity of GLP-1R in transfected GT1-7 cells increased with leptin, whereas neuropeptide Y (NPY) did not modify it. Interestingly, when cells were incubated with both NPY and leptin, NPY blocked the stimulating effect of leptin. The effects of leptin and NPY were also confirmed at messenger RNA levels. In hypothalamic slices, GLP-1R messenger RNA levels increased at higher glucose concentrations in the VMH. In addition, leptin exerted a stimulating effect; and NPY did not modify receptor expression. By contrast, in the LH, the opposite effects were found for those parameters, except at 20 mmol/L glucose. These findings suggest that the stimulating effect of leptin on GLP-1R expression, with no changes in NPY-induced activity, could enhance the anorexic actions generated through this receptor. In addition, the different responses of the VMH and LH may be related to specific functions of these structures, as already known in vivo, highlighting the interest of hypothalamic slices for this kind of study.

Effects of glucose and insulin on glucokinase activity in rat hypothalamus.

In an attempt to study the role of glucokinase (GK) and the effects of glucose and peptides on GK gene expression and on the activity of this enzyme in the hypothalamus, we used two kinds of biological models: hypothalamic GT1-7 cells and rat hypothalamic slices. The expression of the GK gene in GT1-7 cells was reduced by insulin (INS) and was not modified by different glucose concentrations, while GK enzyme activities were significantly reduced by the different peptides. Interestingly, a distinctive pattern of GK activities between the ventromedial hypothalamus (VMH) and lateral hypothalamus (LH) were found, with higher enzyme activities in the VMH as the glucose concentrations rose, while LH enzyme activities decreased at 2.8 and 20 mM glucose, the latter effect being prevented by incubation with INS. These effects were produced only by d-glucose and the modifications found were due to GK, but not to other hexokinases. In addition, GK activities in the VMH and the LH were reduced by glucagon-like peptide 1, leptin, orexin B, INS, and neuropeptide Y (NPY), but this effect was only statistically significant for NPY in LH. Our results indicate that the effects of both glucose and peptides occur on GK enzyme activities rather than on GK gene transcription. Moreover, the effects of glucose and INS on GK activity suggest that in the brain GK behaves in a manner opposite to that in the liver, which might facilitate its role in glucose sensing. Finally, hypothalamic slices seem to offer a good physiological model to discriminate the effects between different areas.