It is certainly ideal to let your graduate work serve as the basis for your graduate career. You'll be getting pubs, familiarizing yourself with the literature, and making connections within that research area. And if you're focused purely on, say, PTSD, and you go to a faculty job talk and say to ignore your vita, that what you really have your heart set on is substance research, you may get more than a few raised eyebrows. However, that isn't to say that you are marrying your research area when you start grad school. Many (or maybe most) professor's interests drift throughout the course of their career-- I wonder what the percentage is of professors who retire still researching what they focused on in grad school-- i bet it's pretty low. Furthermore, you don't always need to study exactly what your advisor studies in grad school-- there may be some flexibility, as long as it fits within the context of the data that are available (or that you could collect). For example, many people with PTSD also abuse substances. In this example, you could focus on substance as a means of self-medication for PTSD (or something like that), build expertise in both areas, and then shift your work toward substance later on. It would become more difficult as the research areas become more disparate, of course.
