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1 Neuroscience, Cell Biology and Physiology, Wright State University School of Medicine, Dayton, OH, USA
* To whom correspondence should be addressed. E-mail: robert.putnam{at}wright.edu.
Cardiorespiratory control is mediated in part by central chemosensitive neurons that respond to increased CO2 (hypercapnia). Activation of these neurons is believed to involve hypercapnia-induced decreases in intracellular pH (pHi). All previous measurements of hypercapnia-induced pHi changes in chemosensitive neurons have been made from the soma, but chemosensitive signaling could be initiated in the dendrites of these neurons. In this study, membrane potential (Vm) and pHi were measured simultaneously in chemosensitive locus coeruleus (LC) neurons from neonatal rat brainstem slices using whole-cell pipettes and the pH-sensitive fluorescent dye pyranine. We measured pHi from the soma as well as from primary dendrites to a distance of 160 µm from the edge of the soma. Hypercapnia (15% CO2, pHo 7.00; control: 5% CO2, pHo 7.45) resulted in an acidification of similar magnitude in dendrites and soma (about 0.26 pH unit), but that was faster in the more distal regions of the dendrites. Neither the dendrites nor the soma exhibited pHi recovery during hypercapnia-induced acidification, but both regions contain pH-regulating transporters since they exhibit pHi recovery from an NH4Cl prepulse-induced acidification (at constant pHo 7.45). Exposing a portion of the dendrites to hypercapnic solution did not increase firing rate, but exposing the soma to hypercapnic solution resulted in a near maximal increase in firing rate. These data show that while the pHi response to hypercapnia is similar in the dendrites and soma, somatic exposure to hypercapnia plays a major role in the activation of chemosensitive LC neurons from neonatal rats.
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