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The Case of the Floating Baseline

Mike Nakagawa miken at hpwala.wal.hp.com
Thu Sep 1 13:09:45 EST 1994


In article <33lqr5$ev0 at usenet.INS.CWRU.Edu>, mxl29 at po.CWRU.Edu (Mark
Lyubkin) writes:
|> 
|> Has anyone encountered this peculiar and most frustrating phenomenon while
|> doing extracellular recordings from brain tissue; specifically rat
|> hippocampus CA1 region.  My problem is getting a stable baseline response
|> over time.  Usually the EPSP slowly begins to increase and continues
|> increasing almost linearly even though the stimulating current is kept
|> constant.   
|> 	One possibility I explored was whether the recording microelectrode
|> was leaking NaCl.I haven't rejected that possibility but it seems
unlikely after
|> experimenting with different NaCl concentrations and electrode tip sizes. I
|> also checked to make sure that the recording electrode is not
slipping or moving.
|> 
|> Any thoughts?  I would greatly appreciate it.
|> 

It's been a while since I've done CA1 recordings (at Case at the Applied
Neural Control Lab), but I do recall that when unexpected changes occurred
in the standard control behavior, it was usually time to check the
grounding.  Bad ground electrodes, poor connections, or ground loops are
places to look, if you haven't already.  It seems that you are looking at
the recording electrode for problems, but a poor reference could be just
as responsible for a bad recording (it takes two electrodes to make a
potential difference).

I doubt that a leaking electrode would be responsible for the behavior, if
the problem is in extracellular recordings.  There is a fairly high NaCl
concentration in most artificial CSF solutions, so unless you are
consistently making electrodes that leak *a lot*, the NaCl from inside the
electrode probably wouldn't affect the extracellular concentration very
much, except when using an occasional bad electrode.  (I never found
leakage to be a problem in my experiments.)  If you are taking
intracellular measurements, however, you'd want to change to a high-K+
solution in the microelectrode.

Check with the Applied Neural Control Lab at Case.  I know one grad student
of Dr. Dominique Durand who has *extensive* experience with all kinds of
brains slice recordings.  Problems such as yours are often most easily
solved by having an experienced person look through the set-up, and she
has lots of experience.  Let me know if you want me to pass any info to
her.

Hope I've helped a fellow Case person.

Good luck,

---mike...
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