Jquery中下来列表selectbox() ，无法发生onchange事件

<script>
function toChangeCity(){
alert("123");
}
</script>

<script>
$(document).ready(function() {
$('#mySle').selectbox();
});
</script>

<select name="mySle" id="mySle" onchange="toChangeCity();" >
<option selected=""value="123">123</option>
<option selected=""value="456">456</option>
<option selected=""value="789">789</option>
</select>

怎样写就可以调用onchange事件了

mark