Current ideas on central chemoreception by neurons and glial cells in the retrotrapezoid nucleus

Abstract

Central chemoreception is the mechanism by which CO2/pH-sensitive neurons (i.e., chemoreceptors) regulate breathing in response to changes in tissue pH. A region of the brain stem called the retrotrapezoid nucleus (RTN) is thought to be an important site of chemoreception (23), and recent evidence suggests that RTN chemoreception involves two interrelated mechanisms: H+-mediated activation of pH-sensitive neurons (38) and purinergic signaling (19), possibly from pH-sensitive glial cells. A third, potentially important, aspect of RTN chemoreception is the regulation of blood flow, which is an important determinate of tissue pH and consequently chemoreceptor activity. It is well established in vivo that changes in cerebral blood flow can profoundly affect the chemoreflex (2); e.g., limiting blood flow by vasoconstriction acidifies tissue pH and increases the ventilatory response to CO2, whereas vasodilation can wash out metabolically produced CO2 from tissue to increase tissue pH and decrease the stimulus at chemoreceptors. In this review, we will summarize the defining characteristics of pH-sensitive neurons and discuss potential contributions of pH-sensitive glial cells as both a source of purinergic drive to pH-sensitive neurons and a modulator of vasculature tone.

Fig. 1.

Defining characteristics of RTN chemoreceptors in vivo and in vitro. A1: the typical firing rate response of a CO₂/H⁺-sensitive neuron in vivo to changes in end expiratory CO₂; increasing Exp CO₂ increased neuronal activity in a reversible and repeatable manner. A2: average firing rate of CO₂/H⁺-sensitive (n = 26) and CO₂/H⁺-insensitive (n = 39) neurons in vivo under control (4% CO₂) and hypercapnic (10% CO₂) conditions. After recording cells were labeled with biotinamide for later conformation of location, morphology, and neurochemical phenotype. A3: the structure of two CO₂/H⁺-sensitive neurons labeled in vivo (ML, marginal layer). A4: plots the location of 17 CO₂/H⁺-sensitive neurons (black dots) recorded in vivo. Bregma level −11.4 mm (Amb, nucleus ambiguous; FN, facial nucleus; ML, marginal layer; Py, pyramidal tracts). A5: a biotinamide (Alexa Fluor 488 fluorescence)-labeled CO₂/H⁺-sensitive RTN neuron recorded in vivo (top) and the same cell expresses vesicular glutamate transporter-2 (VGLUT2) mRNA (bright-field illumination; bottom). A6: top shows a CO₂/H⁺-sensitive RTN neuron recorded in vivo and labeled with biotinamide (Cy-3, red), and bottom shows that the same cell is immunoreactive for Phox2b (Alexa 488, green), biotinamide with Cy-3 (red); colocalization is shown in yellow. B1: trace of firing rate and bath pH show characteristic responses of a pH-sensitive RTN neuron to pH values ranging from 6.9 to 7.5; these cells are spontaneously active at control pH 7.3, nearly silent at pH 7.5, and robustly active at pH 6.9. B2: average firing rate of pH-sensitive (n = 40) and pH-insensitive (n = 47) neurons recorded in vitro at varying pH. *P <0.01 for effect of pH. B3: structure of three biocytin-filled pH-sensitive neurons that are reminiscent of CO₂/H⁺-sensitive RTN neurons recorded in vivo. B4: composite map shows the location of 11 pH-sensitive neurons (black dots) recorded in vitro (Amb, nucleus ambiguous; FN, facial nucleus; ML, marginal layer; Py, pyramidal tracts). B5: agarose gel of single cell RT-PCR for Phox2b and GAPDH; the chemosensitive RTN neuron expresses Phox2b, but the Purkinje cell does not. This figure is composed of previously published data (38, 39, 59) and presented here with permission from the appropriate journals.

Fig. 2.

Working model of chemoreception by the retrotrapezoid nucleus (RTN). The RTN contains a population of CO₂/H⁺-sensitive neurons that appear to function as respiratory chemoreceptors (see text and Fig. 1 for more details). The RTN also contains a population of pH-sensitive glial cells (12, 16, 53) that may contribute to chemoreception by releasing ATP during hypercapnia. Evidence indicates that ATP can contribute to chemoreception by 1) activating pH-sensitive neurons through a P2Y-receptor-dependent mechanism (arrow 1); 2) inhibiting pH-sensitive neurons by activation of P2X-receptors on interneurons (arrow 2); 3) modulating vascular tone to increase or decrease tissue pH (arrow 3). It is also possible that pH-sensitive neurons influence activity of pH-sensitive glial cells by the release of excitatory neurotransmitters or increased extracellular K⁺ (arrow 4) or regulate vascular tone directly (arrow 5).