[...]
<asp:Content ID="Content1" ContentPlaceHolderID="pbSideBarCol" Runat="Server">
   <asp:DropDownList 
        AutoPostBack="true"
  ID="regioniFilter"
  Runat="server"
    >
  <asp:ListItem Value="0" Text="-- Filtra per Regione --"></asp:ListItem>
  <asp:ListItem Value="1" Text="Toscana"></asp:ListItem>
  <asp:ListItem Value="2" Text="Lazio"></asp:ListItem>
  [...]
    </asp:DropDownList>
</asp:Content>


<asp:Content ID="Content2" ContentPlaceHolderID="pbContentsPlaceHolder" Runat="Server">
    [...]    
    <asp:SqlDataSource ID="ComuniDataSource" runat="server"
  SelectCommandType="StoredProcedure"
  SelectCommand="ComuniInProvincia_Statistiche"
  ProviderName="<%$AppSettings:DBProvider%>"
  ConnectionString="<%$ConnectionStrings:DBConnString%>"
    >
        <SelectParameters>
      <asp:ControlParameter ControlID="regioniFilter" PropertyName="SelectedValue" Name="?idRegionePar" />  
  </SelectParameters>
    </asp:SqlDataSource>
</asp:Content>