Long time no read :)
What I'm not quite certain about in your question is if you just want to
visualize the cell during the recording (for that Alexa 488 would be
perfect) or if you are planning on fixing the slices for post
In case of the latter, I would recommend using Biocytin. Just add it to
your regular recording solution and patch the cells as you normally
would. Afterwards, fix the slices in paraformaldehyd. After at least 24
hours, you can easily process the cells using an ABC-detection kit and a
streptavidin linked fluorophore. I've previously done this with 200
micron cerebellar slices and it worked beautifully - and quick as well.
The whole post processing took just about two hours including mounting.
I take it your still in the pharmacology department?