FOCAL CENTRAL CHEMORECEPTOR SENSITIVITY IN THE RTN STUDIED WITH A CO2DIFFUSION PIPETTE IN-VIVO

We describe and use a CO2 diffusion pipette to produce a quickly rever sible focal acidosis in the retrotrapezoid nucleus region of the rat b rain stem. No tissue injection is made. Instead, artificial cerebrospi nal fluid (aCSF) equilibrated with CO2 circulates within the micropipe tte, providing a source for continued CO2 diffusion into the tissue fr om the pipette tip. Tissue pH electrodes show the acidosis is limited to 500 mu m from the tip. In controls (aCSF equilibrated with air), 1- min pipette perfusions increased tissue pH slightly and decreased phre nic nerve amplitude. In moderate-and high-CO2 groups (aCSF equilibrate d with 50 or 100% CO2), 1-min perfusions significantly decreased tissu e pH and increased phrenic nerve amplitude in a dose-dependent manner. The responses developed and reversed within minutes. Compared with ou r prior use of medullary acetazolamide injections to produce a focal a cidosis, in this approach the acidosis 1) arises and reverses quickly and 2) its intensity can be varied. This allows study of sensitivity a nd mechanism. We conclude from this initial experiment that retrotrape zoid nucleus region chemoreceptors operate within the normal physiolog ical range of CO2-induced tissue pH changes.