Chronic hypoxia-induced acid-sensitive ion channel expression in chemoafferent neurons contributes to chemoreceptor hypersensitivity

Abstract

Liu X, He L, Dinger B, Fidone SJ. Chronic hypoxia-induced acid-sensitive ion channel expression in chemoafferent neurons contributes to chemoreceptor hypersensitivity. Am J Physiol Lung Cell Mol Physiol 301: L985-L992, 2011. First published September 2, 2011 doi:10.1152/ajplung.00132.2011.-Previously we demonstrated that chronic hypoxia (CH) induces an inflammatory condition characterized by immune cell invasion and increased expression of inflammatory cytokines in rat carotid body. It is well established that chronic inflammatory pain induces the expression of acid-sensitive ion channels (ASIC) in primary sensory neurons, where they contribute to hyperalgesia and allodynia. The present study examines the effect of CH on ASIC expression in petrosal ganglion (PG), which contains chemoafferent neurons that innervate oxygen-sensitive type I cells in the carotid body. Five isoforms of ASIC transcript were increased ϳ1.5-2.5-fold in PG following exposure of rats to 1, 3, or 7 days of hypobaric hypoxia (380 Torr). ASIC transcript was not increased in the sympathetic superior cervical ganglion (SCG). In the PG, CH also increased the expression of channel-interacting PDZ domain protein, a scaffolding protein known to enhance the surface expression and the low pH-induced current density mediated by ASIC3. Western immunoblot analysis showed that CH elevated ASIC3 protein in PG, but not in SCG or the (sensory) nodose ganglion. ASIC3 transcript was likewise elevated in PG neurons cultured in the presence of inflammatory cytokines. Increased ASIC expression was blocked in CH rats concurrently treated with the nonsteroidal anti-inflammatory drug ibuprofen (4 mg·kg Ϫ1 ·day Ϫ1 ). Electrophysiological recording of carotid sinus nerve (CSN) activity in vitro showed that the specific ASIC antagonist A-317567 (100 M) did not significantly alter hypoxiaevoked activity in normal preparations but blocked ϳ50% of the hypoxic response following CH. Likewise, a high concentration of ibuprofen, which is known to block ASIC1a, reduced hypoxia-evoked CSN activity by ϳ50% in CH preparations. Our findings indicate that CH induces inflammation-dependent phenotypic adjustments in chemoafferent neurons. Following CH, ASIC are important participants in chemotransmission between type I cells and chemoafferent nerve terminals, and these proton-gated channels appear to enhance chemoreceptor sensitivity.

Effect of Endothelin Receptor Antagonist Bosentan on Chronic Hypoxia-Induced Inflammation and Chemoafferent Neuron Adaptation in Rat Carotid Body

“…In a recent study (Liu et al, 2011), we showed that CH increases expression of 5 isoforms of ASIC (ASIC1a, ASIC1b, ASIC2a, ASIC2b, and ASIC3) in the petrosal ganglion (PG). This ganglion contains chemoafferent neuron cell bodies, along with baroreceptor neurons, which terminate in the carotid sinus, as well as gustatory and mechanoreceptor neurons innervating the tongue.…”

“…As previously described (Liu et al, 2011), carotid bodies and petrosal ganglia (PG) were harvested from rats anesthetized with a mixture of ketamine plus xylazine. Tissues were rapidly excised and placed in a lucite chamber containing 100% O 2 -equilibrated modified Tyrode solution at 0°-4°C (in mM: NaCl, 112; KCl, 4.7; CaCl 2 , 2.2; MgCl 2 , 1.1; sodium glutamate 42; HEPES buffer, 5; glucose, 5.6; pH = 7.4).…”

“…Conventional quantitative PCR (qPCR) assays have been used to evaluate the effect of bosentan on expression of the inflammatory cytokines, interleukin-1b (IL-1b), tumor necrosis factor-a (TNFa), and interleukin-6 (IL6), as well as the chemokine, monocyte chemoattractant protein-1 (MCP-1). In a recent study, we showed that CH induces inflammation-dependent upregulation of acid-sensitive ion channels (ASIC) in chemoafferent neuron cell bodies located in the petrosal ganglion (PG) of the IXth cranial nerve (Liu et al, 2011). Consequently, we have also evaluated the effect of bosentan treatment on the expression of ASIC in PG.…”

Sustained Exposure to Cytokines and Hypoxia Enhances Excitability of Oxygen-Sensitive Type I Cells in Rat Carotid Body: Correlation with the Expression of HIF-1α Protein and Adrenomedullin

“…Moreover, concurrent treatment with the nonsteroidal anti-inflammatory drug, ibuprofen, or the steroid, dexamethasone, blocked CH-induced inflammation and chemoreceptor adaptation ). Other studies in our laboratory suggest that adaptation involves cytokinemediated changes in the functional properties of the afferent terminals of primary sensory chemoreceptor neurons (Liu et al, 2011a;Liu et al, 2011b). But it is also possible that increased sensitivity to hypoxia occurs in type I cells, which have been shown to express specific cytokine receptors Wang et al, 2006).…”

“…The method has been described in detail previously Liu et al, 2011a). Briefly, in accord with the kit instructions (RNAqueous-Micro, Ambion, Austin, TX), total RNA was extracted from tissue samples pooled from groups of 5 rats for each experimental condition.…”