You're already gonna be a DO so don't bother worrying about it. As a med student I used to think that kind of thing mattered a lot, but as time goes by and I get further into my training I see it is less and less important, if at all. Bottom line: be a good doctor. As someone mentioned there may be some old timers w/ bias, but for the most part they are retiring/dying off.
If you like neuro, go for that. Residency is arduous. Better have a solid interest in what you're doing. Anes-based fellowships still like to take anes but a high quality candidate from another field is a boon to a training program, and many good programs know that. I think the trend has been towards accepting more non-anes. Who can say what will be happening in 6+ years? Or if you'll still be interested in pain in 6+ years.