Nevermind, I found the answer in the documentation:
This property defines the initial sorting criterion for the discussion. This is the syntax for this property: FieldName1 [SortDirection], FieldName1 [SortDirection] SortDirection is either ASC (ascending order) or DESC (descending order). Please note that the FieldName and the SortDirection are case sensitive. If the SortDirection is not specified, ascending order is assumed. For example, if you want to display the discussions from the newest to the oldest, you can specify the following sort expression: Created DESC