Intracellular free calcium responses to protons and capsaicin in cultured trigeminal neurons.

Acidic solutions and capsaicin are selective chemical stimuli for nociceptive neurons. The effect of these stimuli on intracellular calcium concentration was analysed in cultured trigeminal neurons of newborn rabbits. Rapid reductions in pH (from 7.4 to 5) evoked a transient rise in intracellular calcium concentration of 270% on average over the basal level (162.5 +/- 3.5 nM; n = 174) in 86% of the neurons. Maximal responses were found at pH 5.5. Proton-induced transients were diminished or abolished by 20 mM CaCl2, by zero CaCl2 and by 1 microM Ruthenium Red. In response to 1 microM capsaicin, 40% of the cells that were sensitive to protons also increased their intracellular calcium concentration to 218% of control. Capsaicin-induced intracellular calcium concentration rises were composed of an initial peak followed by a second, slower intracellular calcium concentration elevation. The capsaicin response was completely blocked by 1 microM Ruthenium Red, and disappeared in zero calcium, but was augmented in high extracellular calcium. Intracellular calcium concentration responses to capsaicin were still observed in neurons whose response to protons was desensitized by sustained exposure to low pH (pH 6.5). Cells surviving a 10-24h capsaicin (10 microM) treatment, still displayed responses to pH reductions. These results suggest that intracellular calcium concentration rises induced by moderate reductions in pH0 and capsaicin occur through different mechanisms.