RECEPTIVE PROPERTIES OF PRIMARY AFFERENT-FIBERS FROM RABBIT PLEURA, IN-VITRO

We investigated the physiological properties of mediastinal pleural pr imary afferent units by recording single nerve fibre activity from the phrenic nerve in an in vitro preparation of rabbit tissue. A total of 41 units with conduction velocities in the group III and IV range wer e examined for their responsiveness to mechanical, thermal and chemica l stimuli. Most receptive fields were adjacent to the phrenic nerve-pe ricardiacophrenic artery complex. The thresholds to punctate mechanica l stimulation (von Frey hairs) were widely scattered around a median o f 5.4 mN; all fibres showed slowly adapting responses to mechanical st imulation. Heat sensitivity was observed in 7/41 units (17%), while 17 /41 (41%) of the fibres exhibited a spurious transient excitation to s trong and rapid cooling. Chemosensitivity was scarce with respect to c apsaicin (7/33 (21%) of the units responding) but more common to CO2-s aturated synthetic interstitial fluid (pH 6.1, 5/16 (31%) responding). The most effective stimulus was a mixture of bradykinin, serotonin, h istamine and prostaglandin E-2 ('inflammmatory soup') which evoked sti mulus responses in 27/33 (82%) of the afferent fibres challenged. Sens itization to mechanical stimuli occurred in 5/41 (12%) of the units, f ollowing the application of heat or inflammatory mediators. The rabbit pleura appears as a tissue mainly innervated by multimodal mechano- a nd chemosensitive afferent units.