Focal central chemoreceptor sensitivity in the RTN studied with a CO2 diffusion pipette in vivo

Li, Aihua, and Eugene E. Nattie. Focal centralchemoreceptor sensitivity in the RTN studied with aCO₂ diffusion pipette in vivo.J. Appl. Physiol. 83(2): 420-428, 1997.We describe and use a CO₂diffusion pipette to produce a quickly reversible focal acidosis in theretrotrapezoid nucleus region of the rat brain stem. No tissueinjection is made. Instead, artificial cerebrospinal fluid (aCSF)equilibrated with CO₂ circulateswithin the micropipette, providing a source for continuedCO₂ diffusion into the tissue fromthe pipette tip. Tissue pH electrodes show the acidosis is limited to500 µm from the tip. In controls (aCSF equilibrated with air), 1-minpipette perfusions increased tissue pH slightly and decreased phrenicnerve amplitude. In moderate- andhigh-CO₂ groups (aCSF equilibratedwith 50 or 100% CO₂), 1-minperfusions significantly decreased tissue pH and increased phrenicnerve amplitude in a dose-dependent manner. The responses developed andreversed within minutes. Compared with our prior use of medullary acetazolamide injections to produce a focal acidosis, in this approachthe acidosis 1) arises and reversesquickly and 2) its intensity can bevaried. This allows study of sensitivity and mechanism. We concludefrom this initial experiment that retrotrapezoid nucleus regionchemoreceptors operate within the normal physiological range ofCO₂-induced tissue pH changes.