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Focal CO₂ dialysis in raphe obscurus does not stimulate ventilation but enhances the response to focal CO₂ dialysis in the retrotrapezoid nucleus

¹Department of Physiology, School of Medicine of Ribeirão Preto, University of Sao Paulo, Brazil; and ²Department of Physiology, Dartmouth Medical School, Lebanon, New Hampshire

Submitted 1 February 2008 ; accepted in final form 29 April 2008

Simultaneous inhibition of the retrotrapezoid nucleus (RTN) and raphe obscurus (ROb) decreased the systemic CO₂ response by 51%, an effect greater than inhibition of RTN (–24%) or ROb (0%) alone, suggesting that ROb modulates chemoreception by interaction with the RTN (19). We investigated this interaction further by simultaneous dialysis of artificial cerebrospinal fluid equilibrated with 25% CO₂ in two probes located in or adjacent to the RTN and ROb in conscious adult male rats. Ventilation was measured in a whole body plethysmograph at 30°C. There were four groups (n = 5): 1) probes correctly placed in both RTN and ROb (RTN-ROb); 2) one probe correctly placed in RTN and one incorrectly placed in areas adjacent to ROb (RTN-peri-ROb); 3) one probe correctly placed in ROb and one probe incorrectly placed in areas adjacent to RTN (peri-RTN-ROb); and 4) neither probe correctly placed (peri-RTN-peri-ROb). Focal simultaneous acidification of RTN-ROb significantly increased ventilation (E) up to 22% compared with baseline, with significant increases in both breathing frequency and tidal volume. Focal acidification of RTN-peri-ROb increased E significantly by up to 15% compared with baseline. Focal acidification of ROb and peri-RTN had no significant effect. The simultaneous acidification of regions just outside the RTN and ROb actually decreased E by up to 11%. These results support a modulatory role for the ROb with respect to central chemoreception at the RTN.

control of breathing; serotonin; brain stem

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