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NERVOUS SYSTEM CELL BIOLOGY

Somatic vs. dendritic responses to hypercapnia in chemosensitive locus coeruleus neurons from neonatal rats

Department of Neuroscience, Cell Biology and Physiology, Wright State University School of Medicine, Dayton, Ohio

Submitted 8 July 2004 ; accepted in final form 8 July 2005

Cardiorespiratory control is mediated in part by central chemosensitive neurons that respond to increased CO₂ (hypercapnia). Activation of these neurons is thought to involve hypercapnia-induced decreases in intracellular pH (pH_i). All previous measurements of hypercapnia-induced pH_i changes in chemosensitive neurons have been obtained from the soma, but chemosensitive signaling could be initiated in the dendrites of these neurons. In this study, membrane potential (V_m) and pH_i were measured simultaneously in chemosensitive locus coeruleus (LC) neurons from neonatal rat brain stem slices using whole cell pipettes and the pH-sensitive fluorescent dye pyranine. We measured pH_i from the soma as well as from primary dendrites to a distance 160 µm from the edge of the soma. Hypercapnia [15% CO₂, external pH (pH_o) 7.00; control, 5% CO₂, pH_o 7.45] resulted in an acidification of similar magnitude in dendrites and soma (0.26 pH unit), but acidification was faster in the more distal regions of the dendrites. Neither the dendrites nor the soma exhibited pH_i recovery during hypercapnia-induced acidification; but both regions contained pH-regulating transporters, because they exhibited pH_i recovery from an NH₄Cl prepulse-induced acidification (at constant pH_o 7.45). Exposure of a portion of the dendrites to hypercapnic solution did not increase the firing rate, but exposing the soma to hypercapnic solution resulted in a near-maximal increase in firing rate. These data show that while the pH_i 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.

acid; brain stem; intracellular pH; pyranine; respiratory control; whole cell

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