The purpose of this study is to compare the effect of a single administration of a DPP-IV inhibitor (vildagliptin: Galvus ®) versus no treatment over two populations of diabetic patients: without diabetic autonomic neuropathy (NA, i.e. the control group) and with diabetic autonomic neuropathy (i.e. the neuropathy group). The investigators hypothesize that the therapeutic efficacy of DPP-IV inhibitors is partly mediated by the autonomic nervous system. This hypothesis will be validated if a lower glycemic response to DPP-IV inhibitor treatment is observed for the neuropathy group compared to control.