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

1. The response of neuroma nerve endings to different stimuli was studied i n a saphenous nerve neuroma preparation in vitro. 2. Electrical activity was recorded from 141 single fibres dissected of sap henous nerve. One-third (27%) displayed spontaneous activity. Based on thei r response to mechanical and chemical stimuli, neuroma nerve fibres were cl assified as mechanosensory fibres (47.5%), mechanically insensitive chemose nsory 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 discha rge (tonic units). Polymodal units were additionally activated by at least one of the following stimuli: acidic solutions; a combination of bradykinin , prostaglandin E-2, serotonin, substance P and histamine (all at 1 mu M) p lus 7 mM KCl (inflammatory soup); 600 mM NaCl and capsaicin. 4. Low pH solutions increased the firing discharge of polymodal endings pro portionally to the proton concentration. The 'inflammatory soup' evoked a f iring response characterized by the absence of tachyphylaxis, which appeare d when its components were applied separately Both stimuli sensitized polym odal fibres to mechanical stimulation. Hypertonic NaCl (600 mM) and capsaic in (3.3 mM) induced a prolonged discharge that outlasted the stimulus durat ion. 5. Mechanically insensitive chemosensory neuroma fibres exhibited responses to chemical stimuli analogous to polymodal fibres. They became mechanicall y sensitive after chemical stimulation. 6. These findings show that neuroma nerve endings in the rat saphenous nerv e neuroma in vitro are functionally heterogeneous and exhibit properties re miniscent of those in intact mechanosensory polymodal and 'silent' nocicept or sensory afferents, including their sensitization by algesic chemicals.