[Neuroscience] Brain impedance

[Ana.Cervera-Ferri from uv.es]

Hi all, 

I want to stimulate through bipolar teflon-coated steel electrodes in
the tegmentum in rats with pulses of 50-500 uA. I need to know which is
the average brain impedance in vivo to calculate the voltage needed. I
suppose that the contribution of the electrode impedance is low but if I
have to take it into account any advise is welcome.  
Thank you in advance!

Ana