Confocal imaging reveals activity-dependent intracellular Ca2+ transients in nociceptive human C fibres

Unmyelinated nociceptive fibres are a key element in the human nociceptive system, however, it is very difficult to investigate such fibres in vivo in more detail. An alternate approach are studies on isolated human nerves. H ere we describe that confocal Ca2+ imaging reveals new information about th e physiology of human nociceptive C fibres. Confocal images at two emission wavelengths were collected from regions with unmyelinated nerve fibres wit hin segments of biopsied human sural nerves stained with the Ca2+-sensitive fluorescent dyes Calcium Green-1 and Fura Red. Short trains of supramaxima l electrical stimuli applied to one end of the nerve as well as bath applic ation of capsaicin resulted in an increase in the free intracellular Ca2+ c oncentration. Intracellular Ca2+ transients were seen at action potential f requencies above 1 Hz. They were absent in Ca2+-free bathing solution and r educed during bath application of cadmium. This indicates an extracellular source of the activity-dependent rise in [Ca2+](i). Furthermore, Ca2+ trans ients were also observed during elevation of the extracellular K+ concentra tion or during short trains of calcium action potentials. Such 'Ca2+ spikes ' were elicited by a combination of tetrodotoxin and potassium channel bloc kers. These data suggest the presence of voltage-dependent Ca2+ channels in the membrane of nociceptive human nerve fibres. (C) 1999 International Ass ociation for the Study of Pain. Published by Elsevier Science B.V.