Dropdownlist com GridView

Olá!

Estou tentando fazer uma gridview que mostra o conteúdo dependendo de uma dropdown.

Fiz conforme vi na internet, mas na hora de testar nada acontece.

Meu código é esse:

<asp:ScriptManager ID="ScriptManager1" runat="server">

 </asp:ScriptManager>
 <asp:UpdatePanel ID="UpdatePanel1" runat="server">
 <ContentTemplate>
 Tipo de log:<br />
 <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True">
 <asp:ListItem Value="Entrou no sistema">Entrou no sistema</asp:ListItem>
 <asp:ListItem Value="Saiu do sistema">Saiu do sistema</asp:ListItem>
 </asp:DropDownList>
 <br />
 <br />
 <asp:GridView ID="gridusuarios" runat="server" AutoGenerateColumns="False" Visible=False Height="41px" Width="574px" AllowPaging="True" PageSize="15" OnPageIndexChanging="GridViewImgs_PageIndexChanging" DataSourceID="SqlDataSource1">
 <RowStyle BackColor="Transparent" BorderColor="Transparent" ForeColor="#0088AA" HorizontalAlign="Center"
 VerticalAlign="Middle" Font-Bold="False" Font-Names="Verdana" Font-Size="8pt" Font-Underline="False" />
 <HeaderStyle BackColor="#E0E0E0" BorderColor="Transparent" Font-Names="Verdana" HorizontalAlign="Center"
 VerticalAlign="Middle" Font-Bold="False" Font-Size="8pt" />
 </asp:GridView>
 <asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:ConnectionString %>"
 ProviderName="<%$ ConnectionStrings:ConnectionString.ProviderName %>" SelectCommand='SELECT "TXT_DESCRICAO", "DT_DATA", "NUM_LOGIN_FK", "NUM_ID" FROM "LOG" WHERE ("TXT_DESCRICAO" = :TXT_DESCRICAO)'>
 <SelectParameters>
 <asp:ControlParameter ControlID="DropDownList1" Name="TXT_DESCRICAO" PropertyName="SelectedValue"
 Type="String" />
 </SelectParameters>
 </asp:SqlDataSource>
 </ContentTemplate>
 </asp:UpdatePanel>

Por favor me ajudem...

Obrigada. Bjs