Execute workflow rule when a Support rep replies to the existing request

Is there a way to set up a workflow rule so that it executes when one of our support reps replies to a request? For example, a customer submits a request, and we want to notify them when we have replied to the request, but we DON'T want them to get a notification when we have edited the request in some other way (e.g. assigned it, or changed some other detail of the ticket that is not as important).

I couldn't find a criteria that would work for this (in the criteria drop-down).

Hope that's clear enough.

Thanks!