Re: Paraform/fluoro

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What fluorescent retrograde tracers is he using?  If he's using DiI, I would 
absolutely recommend sticking with aqueous-based fixatives, preferably 
paraformaldehyde.  Don't use Prefer if it contains alcohols or organic solvents!  
Other fluorescent tracers may be less picky, but I would still advise caution, 
because most neurotracers work because they can't breach the membrane once they are 
in the axon, so you don't want to use anything that can possibly perturb the 
membrane.  That's why people use paraformaldehyde rather than formaldehyde in these 
applications.  The methanol in formalin has the potential to perturb the membrane and 
you can get cross-talk between in axons in the nerve.  In otherwords, the dye crosses 
too easily from axon to axon instead of staying within the axon of interest.

Paraformaldehyde is no different than formaldehyde once it's in solution.  However, 
it will tend to polymerize over time, and precipitate.  Thus, you have to make it 
fresh.  The purpose of the methanol in formalin is to keep the formaldehyde in 
solution.  But it can and does mess up cell membranes.

Personally, I wouldn't ask him to venture into places unknown with unknown additives 
in the commercial fix.  It's just too likely to mess up his experiments.  And some of 
these tracer experiments are quite long and involved.  He should stick with the 

Karen Larison
Institute of Neuroscience
University of Oregon


Date:          Thu, 02 Mar 2000 18:46:52 -0800 (PST)
From:          "P. Emry" <>
Subject:       Paraform/fluoro

Hi all,

I am working with a post doc who wants to inject some "fluorescent
retrograde neurotracers" in to pigs and use paraformaldehyde to fix the
specimens. (Forgive me if I have muddled that up.)

He is being very kind in considering my formaldehyde aversions and 
said he would work after I go home.  He is reluctant to use my
substitute Prefer thinking it may effect the fluorescent signal.

Have any of you worked with Prefer or any other
formaldehyde-paraformaldehyde substitute for that use?  (I have never used
paraform. and have no idea how it differs from formaldehyde.)

I am sure he will want some references if possible.



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