Does sympathetic nerve discharge affect the firing of polymodal C-fibre afferents in humans?

Experimental and clinical studies in animals and humans have indicated that nociceptive nerve fibres can acquire sensitivity to norepinephrine after i njury or chemical sensitization, To evaluate the functional relevance of su ch sensitization, we recorded the activity of single polymodal C-fibre affe rents in healthy human volunteers and investigated whether intense physiolo gical sympathoexcitation could affect their firing properties, This was stu died before and after chemical sensitization of receptive fields by topical application of mustard oil, All afferent C fibres investigated (11 units i n 10 subjects) were mechano-heat-sensitive, and four of seven fibres subjec ted to mustard oil were also chemosensitive. Putative sensitivity to sympat hetic stimulation was investigated during low-frequency (0.25 Hz) electrica l stimulation of the unit receptive held at a threshold intensity sufficien t to evoke an action potential in the afferent fibre after every second to third stimulus, Following a prolonged period of silent rest, sympathoexcita tion was elicited by forced mental arithmetic for 60 s, again followed by a long silent rest period, During stress, sympathetic nerve traffic increase d to 625 +/- 146% of the control level, while firing of the afferent units remained unchanged. There was no sign of sympathetically mediated direct ac tivation of afferent units and no change in the relative amounts of afferen t activations caused by the background electrical stimulation, Results were similar for all units, both before (seven units in six subjects) and after (seven units in seven subjects) chemical sensitization of their cutaneous receptive field. The results suggest that if chemical sensitization of noci ceptive C afferent neurons with mustard oil does induce sensitivity to nora drenaline in humans, it is not sufficient to make C nociceptive fibres resp ond to short-lasting physiological variations in sympathetic outflow.