Never done it before, but I would lean towards aCSF, I would do the dissections in that medium as well. Do you want your axons alive and functioning? what imaging are you doing? 

I'm agreeing with V V-K below. If you need live tissue, go the electrophysiology route (even oxygenate the solution). If you don't need live tissue, I would fix it according to the type of imaging you need to do. 

Also never done it before, but in electrophysiology I often use Tyrode's or Ringer's  solutions for storing DRG or dorsla horn for several hours.

thanks Robert, Viacheslav -

I want to keep it "live" to do time series imaging of axons

Dear Mary Ellen,

For electrophysiology of optic nerve + tectum, one of the post-docs in lab I work in used aCSF, and these preps are kept alive for hours (apply drugs, wash out drugs, etc.).  I work on fish, so it is a different aCSF.  However, a quick search on PubMed found this paper:

Richardson S(1), Siow B, Batchelor AM, Lythgoe MF, Alexander DC.  A viable isolated tissue system: a tool for detailed MR measurements and controlled perturbation in physiologically stable tissue.  Magn Reson Med. 2013 Jun;69(6):1603-10. doi: 10.1002/mrm.24410. Epub 2012 Jul 20.

They work on rat optic nerve, and they performed electron microscopy of their prep after they had kept for over 3 hours in vitro in order to show the lack of damage.  They give the aCSF recipe they use, and credit this paper for it:

Garthwaite G(1), Brown G, Batchelor AM, Goodwin DA, Garthwaite J.  Mechanisms of ischaemic damage to central white matter axons: a quantitative histological analysis using rat optic nerve.  Neuroscience. 1999;94(4):1219-30.

I know that there are sometimes subtle differences between rats and mice, but I would start with that recipe.

Good Luck!

Regards,

Jill