Mechanisms of acid-induced activation of airway afferent nerve fibres in guinea-pig

Abstract

The mechanisms underlying the response of airway afferent nerves to low pH were investigated in an isolated guinea-pig airway nerve preparation. Extracellular recordings were made from single jugular or nodose vagal ganglion neurons that projected their sensory fibers into the airways. The airway tissue containing the mechanically sensitive receptive fields was exposed into acidic solutions. Rapid and transient (approximately 3 s) administration of 1 mM citric acid to the receptive field consistently induced action potential discharge in nociceptive C-fibers (41/44) and nodose Adelta fibres (29/30) that are rapidly adapting low threshold mechanosensors (RAR-like fibres). In contrast, citric acid activated only 8/17 high threshold mechanosensitive jugular Adelta fibres. The RAR-like fibres were slightly more sensitive than C-fibres to acidic solutions (pH threshold > 6.7). The RAR-like fibres response to the approximately 3 s acid treatment was not affected by a vanilloid receptor 1 (VR1) antagonist, capsazepine (10 microM), and was rapidly inactivating (action potential discharge terminated before the acid administration was completed). Gradual reduction of pH did not activate the RAR-like fibres even when the pH was reduced to approximately 5.0. The C-fibres responded to the gradual reduction of pH with persistent action potential discharge that was nearly abolished by capsazepine (10 microM) and inhibited by over 70 % with another VR1 antagonist iodo-resiniferatoxin (1 microM). In contrast the C-fibre response to the transient approximately 3 s exposure to pH approximately 5.0 was not affected by the VR1 antagonists. We conclude that activation of guinea-pig airway afferents by low pH is mediated by both slowly and rapidly inactivating mechanisms. We hypothesize that the slowly inactivating mechanism, present in C-fibres but not in RAR-like fibres, is mediated by VR1. The rapidly inactivating mechanism acts independently of VR1, has characteristics similar to acid sensing ion channels (ASICs) and is found in the airway terminals of both C-fibres and RAR-like fibres.

Figure 1. Concentration-response curve of nodose Aδ fibres and jugular C-fibres to citric acid

Citric acid was administered as 500 μl volume in ≈3 s into superfusion over the mechanically sensitive receptive field at 5 min intervals. Each point represents mean ± s.e.m. of at least 5 experiments. * P < 0.05, ** P < 0.001.Inset, representative traces of a nodose Aδ fibre (left) and a jugular C-fibre (right) response to administration of 500 μl of 1 mm citric acid into superfusion over the mechanically sensitive receptive field. Vertical line preceding the burst of action potentials is an artifact caused by pipette manipulation and indicates the beginning of acid administration. Note immediate onset and shorter duration of action potential discharge in nodose Aδ fibre compared with jugular C-fibre.

Figure 2. Percentage of nodose Aδ and jugular C-fibres responding to low bicarbonate buffer, pH = 6.7

Low bicarbonate buffer was administered as a 500 μl volume in ≈3 s into superfusion over the mechanically sensitive receptive field. Inset, representative trace of a nodose Aδ fibre response to low bicarbonate buffer. Vertical line preceding the burst of action potentials is an artifact caused by pipette manipulation and indicates the beginning of low bicarbonate buffer administration.

Figure 3. Effect of 10 μM capsazepine on citric acid-induced response

Response to administration of 500 μl volume of citric acid (1 mm) into superfusate over the mechanically sensitive receptive field was recorded before and after 15 min superfusion with 10 μM capsazepine. Left, response of nodose Aδ fibres was unaffected by capsazepine (n = 5). Right, capsazepine significantly inhibited jugular C-fibres response to 1 mm citric acid (n = 9). * P < 0.05.

Figure 4. Representative traces of a jugular C-fibre (A) and a nodose Aδ fibre (B) response to gradual reduction of pH from 7.4 to ≈5 (left traces) and rapid reduction of pH from 7.4 to ≈5 (right traces) and the effect of capsazepine and iodo-resiniferatoxin on these responses

A, superfusion with phosphate buffer gradually reduced pH at the receptive field from 7.4 to ≈5.0 (see pH trace) and induced persistent action potential discharge of jugular C-fibre (A, left upper trace). After recovery of pH to 7.4 by superfusion with Krebs solution, rapid reduction of the pH to from 7.4 to ≈5.0 by administration of 500 μl of phosphate buffer induced a short burst of action potential discharge (A, left upper trace, note change in chart speed, horizontal 3 s bars indicate phosphate buffer administration). Then, 10 μM capsazepine was added to Krebs solution and tissue was superfused for 15 min. Capsazepine prevented persistent activation of the same jugular C-fibre by subsequent gradual reduction of pH from 7.4 to ≈5.0 (A, left lower trace). In contrast, capsazepine did not affect response to rapid reduction of the pH from 7.4 to ≈5.0 after pH recovery to 7.4 (A, right lower trace). B, gradual reduction of pH from 7.4 to ≈5.0 failed to activate nodose Aδ fibre (B, left) but rapid reduction of pH from 7.4 to ≈5.0 induced burst of action potential discharge (B, right). Beneath the figure is the tabulation of results showing inhibitory effects of capsazepine (10 μM) and iodo-resiniferatoxin (1 μM) on gradual and rapid reduction of pH. Experiments with iodo-resiniferatoxin were performed in the same manner as with capsazepine except that the tissue was superfused with iodo-resiniferatoxin for 30 min. Data are shown as the percentage decrease in the number of action potentials evoked in the absence of the inhibitor. In the absence of inhibitor the C-fibres evoked an average of 192 ± 40 action potentials in response to the gradual reduction of pH, and 34 ± 8 action potentials in response to rapid reduction of pH. * P < 0.05, ** P < 0.01, NS not significant.