The Relationship Between the Altered Ventilatory Response to Hypercapnia and Neurons from the Locus Coeruleus (LC), Nucleus Tractus Solitarius (NTS) and Retrotrapezoid Nucleus (RTN)

We have found that during development, the ventilatory response to hypercapnia (high CO2) changes in a triphasic manner: a declining response from P1–P7, minimal response from P7–P10, and an increasing response from P10–P21. There is also an alteration in the ventilatory response to high CO2 in animals adapted to chronic hypercapnia (CH). CH was studied using two protocols in which animals were exposed to 7.5% CO2 starting either before birth (protocol 1 or Prot 1) or after birth (protocol 3 or Prot 3). It was found that the triphasic developmental response to high CO2 was either shifted to the right (Prot 1) or suppressed (Prot 3) in CH adapted animals. This altered ventilatory response could be due to a change in the properties of individual chemosensitive (CS) neurons. We studied CS neurons from the LC, NTS and RTN in control animals to see if there is a change in the response to high CO2 over development that mimics the change seen at the whole animal level. In neurons from all 3 areas, we found that chemosensitivity is fully developed at birth and did not change from P1–P21. In CH adapted animals, only NTS neurons were studied. It was found that adaptation to CH did not alter their CS response. In summary, we found that in two situations there is an altered ventilatory response to high CO2 that is not due to a change in the properties of individual CS neurons. We hypothesize that this altered ventilatory response may be due to a change in the network properties.[Supported by NIH grant R01 HL56883.]