SQL & PL/SQL

When i try to execute the following PL/SQL in sqlplus it works well but when i run the same program in Toad i get an error not all variables bound error.Can anyone kindly explain me what's the root cause of this problem

VARIABLE g_heading VARCHAR2(2048) ;
declare 
   v_result varchar2(100) := 'This is test data';
  begin
  :g_heading := v_result; 
  dbms_output.put_line(:g_heading);
  end;
    /

Any suggestions are highly appreciated
Thanks in advance!!