Christina,

Apoptosis is sort of a hobby of mine.  I do not think you should rely on
the p53 or Bax antibodies you mentioned as markers of apoptosis.  To my
knowledge, there are no good antibodies to detect markers of apoptosis for
rat tissue (that are commercially available). There are some that are
patent pending in pharmaceutical companies that are not generally available
even to academic researchers.
There are some other antibodies that you might investigate further
including the Apo 2.7 antibody developed by Schlossman (sp?) at Harvard and
sold by Immunotech (or Beckman/Coulter or whatever that company is called
now).  It recognizes an unidentified mitochondrial surface protein; but I
am not sure if there is any evidence that it works in rat.
Another possibility you might look into is the LeY antigen. If I remember
correctly, I think it recognizes a carbohydrate antigen in mouse, so it may
work in rat tissue. I think DAKO sells this antibody; but, I am not
surprised that a DAKO rep has not answered. Given the recent bashing of
Hacker and other solicitors, I suspect all the companies are a little
gun-shy.
I agree with Russ Allison that morphology may be the most reliable if you
know what to look for and TUNEL is a good alternative to detect the late
stages of apoptosis; however, proper controls are necessary because of the
possibility of artifacts. There are several variations of the "TUNEL"
reaction that you should look into if you choose to label DNA fragments.
If you are just getting started in detecting apoptosis, my recommendation
would be TUNEL.

Ty Lee
tylee@itis.com