<asp:UpdatePanel ID="UpdatePanel1" runat="server">
                        <ContentTemplate>
                            &nbsp;
                   
                    <asp:DropDownList ID="cmbUnita" runat="server">
                    </asp:DropDownList>
                        </ContentTemplate>
                        <Triggers>
                            <asp:AsyncPostBackTrigger ControlID="cmbPrefettura" EventName="SelectedIndexChanged" />
                        </Triggers>
                    </asp:UpdatePanel>