Identification and properties of parietal pleural afferents in rabbits

Abstract

Although pain and dyspnoea are common symptoms in pleural diseases, there are few studies on the sensory innervation of the pleura. Using rabbits, after removal of all muscles in the intercostal space to be studied, we investigated the afferents of the internal intercostal nerve by applying to the internal thoracic wall pieces of gauze soaked in warmed (37 degrees C), buffered saline (mechanical stimulation) or solutions containing lactic acid, inflammatory mediators or capsaicin (chemical stimulation). The afferent conduction velocity ranged from 0.5 to 14 m s(-1). Most units (97%) were activated by mechanical stimulation of the pleura (local positive pressure range = 4.5-8.5 cmH2O) and we found a linear relationship between the discharge rate of afferents and the force applied to the thoracic wall. The majority of mechanosensitive units (70%) also responded to one or several chemical agents. Thus, the afferents were activated by lactic acid (49%) and/or a mixture of inflammatory mediators (50%). Local application of capsaicin elicited an initial increased or decreased background afferent activity in 57% of the afferents, a delayed decrease in firing rate being noted in some units initially activated by capsaicin. Capsaicin blocked the afferent response to a further application of inflammatory mediators but did not affect the mechanosensitive units. Thus, sensory endings connected with thin myelinated and unmyelinated fibres in the internal intercostal nerve detect the mechanical and chemical events of pleural diseases.

Figure 1. Schematic representation of the experimental protocol showing the recording of afferent activities in an internal intercostal nerve after dilacerations of muscles in this space, and the processing of the nerve signal

Distal stimulation of the nerve allowed to evoke either a compound nerve potential (A, B, and C waves correspond to the different fibres with different axonal conduction velocities) or single potentials in discriminated units having a spontaneous background discharge.

Figure 2. Response of pleural afferents to mechanical stimulation

A, examples of single afferent activities activated by the application of a gauze soaked in warmed (37°C), buffered (pH 7.40) isotonic saline against the thoracic pleura, in front of the dissected intercostal spaces. A strain gauge allowed measurement of the force produced during the gauze application. Upper to lower traces are: discharge rate (f_impulses), discriminated unit, raw nerve activity, and force. B, the relationship between force and discharge rate. The lowest force levels (less than 5 g) correspond to simple touchs.

Figure 3. Afferent activities elicited by mechanical or chemical stimulation

Examples of afferent activities elicited by mechanical (B) or chemical stimulation with the 0.04 m lactic acid solution (C). A, conduction velocities of the three discriminated positive units (discriminated units have the same amplitude at the output of the window discriminator). Touching the thoracic pleura with a gauze soaked in warmed, buffered saline allowed us to identify the mechanosensitive unit (1) having the largest amplitude and the fastest conduction velocity (5.4 m s⁻¹). Stimulation with lactic acid elicited activities in small units (2 and 3) having the lowest conduction velocities (0.8 and 0.7 m s⁻¹). f_impulses corresponds to the global counting of discriminated units with positive polarity.

Figure 4. A 30-s application to the thoracic pleura of a gauze soaked in the inflammatory mixture (10⁻⁵m) immediately activated the three discriminated positive units, which continued to fire after the gauze had been removed

f_impulses corresponds to the global counting of discriminated units with positive polarity.

Figure 5. Afferent activities elicited by the inflammatory mixture – block effect of capsaicin

A, the measurement of conduction velocities in the three discriminated negative units. B, mechanical stimulation (touch) of the thoracic pleura activated unit 1 (large potential, high conduction velocity). C, the inflammatory mixture activated new units (2 and 3) whereas the discharge of the mechanosensitive unit had adapted during the first 10 s of gauze application. Then, the afferent nerve discharge displayed a rhythmic pattern in phase with the frequency of the ventilatory pump. Capsaicin applications elicited the same pattern of response (mechano- then chemostimulation) and also suppressed the excitatory effects of the inflammatory mixture. The mechanosensitivity was attenuated but persisted during capsaicin application. f_impulses corresponds to the global counting of discriminated units with negative polarity.