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.
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