Increased CO2 (hypercapnia) is a major stimulus for increased respiration and blood pressure. This pathway for cardiorespiratory control involves specialized central neurons, called chemosensitive neurons, that sense hypercapnia, but the cellular mechanisms that transduce hypercapnia into an increased neuronal firing rate are not well understood. Our work will involve the study of individual neurons from at least two chemosensitive brainstem areas (locus coeruleus and either nucleus tractus solitarius or ventrolateral medulla) and at least one nonchemosensitive area (either inferior olive or hypoglossal nucleus) using slices from neonatal rat brains. Simultaneous measurements of neuronal membrane potential (Vm) and intracellular pH (pHi) will be achieved using perforated patch recordings or whole cell recordings (WCR) combined with pH- sensitive fluorescent dyes and fluorescence imaging microscopy.
Our first aim i s to identify the signal pathways that transduce hypercapnia into an increased firing rate. It will consist of 4 separate aims: i) study the roles of molecular CO2, external pH (pHO) and pHi as the proximate signal of chemoreception by exposing neurons to solutions that vary in each of these parameters; ii) examine the phenomenon of """"""""washout"""""""", whereby the Vm response to hypercapnia is lost during WCR measurements, to see if additional signal molecules (e.g. Cai, polyamines or carbonic anhydrase) are also involved in chemoreception; iii) study the """"""""hypoxia paradox"""""""" (hypoxia-induced acidification does not appear to increase firing rate in chemosensitive neurons) to see if it is due to the lack of additional signal molecules; and iv) study the changes in the Vm and pHi response to hypercapnia in chemosensitive neurons from rats with reduced chemosensitivity (induced by chronic exposure to hypercapnia).
Our second aim i s to study the effects of the signals, identified in Aim 1, on various K+ channels and determine the role of each of these channels in modifying the shape of the action potential and neuronal firing rate. Three K+ channels will be studied: i) inward rectifying K+ channels, important in determining the slope of the interspike depolarization and thereby the firing rate of the neuron; ii) Ca2+-activated K+ channels, important in determining the shape of the action potential and the magnitude of the after hyperpolarization; and iii) TWIK-related acid sensitive K+ channels (TASK), important in determining the resting Vm. This work should indicate the precise nature of the proximate signal of chemosensitivity, elucidate the way in which hypercapnic stimuli affect various K+ channels and give insight into how these effects are integrated to result in the final neuronal response. Further, by comparing the findings in neurons from 2 chemosensitive areas, our findings should help clarify why there are numerous chemosensitive regions in the brainstem. These studies will contribute to our understanding of respiratory diseases thought to be due in part to central chemoreceptor dysfunction, such as sudden infant death syndrome (SIDS) and central alveolar hypoventilation syndromes.

Agency
National Institute of Health (NIH)
Institute
National Heart, Lung, and Blood Institute (NHLBI)
Type
Research Project (R01)
Project #
5R01HL056683-07
Application #
6642230
Study Section
Respiratory and Applied Physiology Study Section (RAP)
Program Officer
Twery, Michael
Project Start
1997-07-01
Project End
2005-07-31
Budget Start
2003-08-01
Budget End
2004-07-31
Support Year
7
Fiscal Year
2003
Total Cost
$243,100
Indirect Cost
Name
Wright State University
Department
Physiology
Type
Schools of Medicine
DUNS #
047814256
City
Dayton
State
OH
Country
United States
Zip Code
45435
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Imber, Ann N; Putnam, Robert W (2012) Postnatal development and activation of L-type Ca2+ currents in locus ceruleus neurons: implications for a role for Ca2+ in central chemosensitivity. J Appl Physiol 112:1715-26
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Erlichman, Joseph S; Leiter, J C; Gourine, Alexander V (2010) ATP, glia and central respiratory control. Respir Physiol Neurobiol 173:305-11
Erlichman, Joseph S; Leiter, J C (2010) Glia modulation of the extracellular milieu as a factor in central CO2 chemosensitivity and respiratory control. J Appl Physiol 108:1803-11

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