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

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Abstract
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 (V̇e) up to 22% compared with baseline, with significant increases in both breathing frequency and tidal volume. Focal acidification of RTN-peri-ROb increased V̇e 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 V̇e by up to 11%. These results support a modulatory role for the ROb with respect to central chemoreception at the RTN.

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