>< Then we put the tissue (rat or
mouse brain) into 30% sucrose in 0.1 M phosphate buffer (PB) and leave
it for at least overnight (you don't say how long you leave it) or until
the tissue floats. ><

That should read until the tissue sinks. :)

It's much easier to section this material if you have a cryostat that has a temperature control on the specimen holder.  I will echo the suggestion that you try different freezing media, although we have never had any trouble with the OCT compound.  We have only done a couple rodent brain frozen sections though.

If your section is laying flat on your blade/blade holder before picking up, then part of the problem is in your mounting technique.  I recommend starting the pickup at one end of your tissue (at the edge closest to you towards the bottom of the slide) and then learn to move the slide slightly towards you as you lower it down to the blade.  If you just plop the slide down flat on your section, you will introduce wrinkles and bubbles in your section.

Admittedly, I've never totally perfected whole brain sections and am awed by those publications that show flawless sections (but wonder how many it took to get that look!).

Good luck and let us know what works for you!

Teri Johnson
Stowers Institute for Medical Research
Kansas City, MO