i use this code :
[DllImport("user32.dll")]
public static extern IntPtr SendMessage(IntPtr hWnd,UInt32 Msg,UInt32 wParam,UInt32 lParam);
private void MainForm_Load(object sender,EventArgs e)
{
splitContainer1.Panel1.Focus();
int WM_MOUSEWHEEL = 0x20A;
Message msg = new Message();
msg.Msg = WM_MOUSEWHEEL;
msg.WParam = (IntPtr)786432000;
msg.HWnd = splitContainer1.Panel1.Handle;
SendMessage(msg.HWnd,(UInt32)msg.Msg,(UInt32)msg.WParam,(UInt32)msg.LParam);
}