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Responses of nerve fibres of the rat saphenous nerve neuroma to mechanical and chemical stimulation: an in vitro study

Luis Rivera, Juana Gallar, Miguel Angel Pozo and Carlos Belmonte

1. The response of neuroma nerve endings to different stimuli was studied in a saphenous nerve neuroma preparation in vitro.

2. Electrical activity was recorded from 141 single fibres dissected of saphenous nerve. One-third (27 %) displayed spontaneous activity. Based on their response to mechanical and chemical stimuli, neuroma nerve fibres were classified as mechanosensory fibres (47·5 %), mechanically insensitive chemosensory fibres (17·0 %), polymodal nociceptor fibres (28·4 %) and unresponsive fibres (7·1 %).

3. Mechanosensory and polymodal neuroma endings responded to von Frey hair stimulation either with a few impulses (phasic units) or a sustained discharge (tonic units). Polymodal units were additionally activated by at least one of the following stimuli: acidic solutions; a combination of bradykinin, prostaglandin E₂, serotonin, substance P and histamine (all at 1 µM) plus 7 mM KCl (inflammatory soup); 600 mM NaCl and capsaicin.

4. Low pH solutions increased the firing discharge of polymodal endings proportionally to the proton concentration. The 'inflammatory soup' evoked a firing response characterized by the absence of tachyphylaxis, which appeared when its components were applied separately. Both stimuli sensitized polymodal fibres to mechanical stimulation. Hypertonic NaCl (600 mM) and capsaicin (3·3 mM) induced a prolonged discharge that outlasted the stimulus duration.

5. Mechanically insensitive chemosensory neuroma fibres exhibited responses to chemical stimuli analogous to polymodal fibres. They became mechanically sensitive after chemical stimulation.

6. These findings show that neuroma nerve endings in the rat saphenous nerve neuroma in vitro are functionally heterogeneous and exhibit properties reminiscent of those in intact mechanosensory, polymodal and 'silent' nociceptor sensory afferents, including their sensitization by algesic chemicals.

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