[Neuroscience] On cell recording? What goes in the pipette?

[Bill]

Hi,

I want to do some on cell recording, to see if a drug modulates the
firing rate of a spontaneously active population on neurons. What
should I fill the electrode with? Regular aCSF will get too basic
without CO2, 0.9% NaCl will blow away K+ etc from the extracellular
space when I come to seal onto the cell. What about regular aCSF with
10mM HEPES and pHed; along the lines of 130 NaCl, 2.5 KCl, 2 CaCl2, 1
MgCl2, 10 HEPES; 1.25 NaH 2PO4, 25 glucose; pH was adjusted using NaOH
(an aCSF recipe I've seen used when people want to alter the pH of
slices on the whole).

Thoughts? 

Thanks.