Capsaicin-induced ion fluxes in dorsal root ganglion cells in culture

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Abstract
Capsaicin is a pungent pain-producing compound found in plants of the capsicum family; it exerts excitatory, desensitizing, and toxic effects on a subset of sensory neurons, including the polymodal nociceptor population. We have carried out a quantitative study of capsaicin- induced fluxes of sodium, guanidine, calcium, rubidium, and chloride ions in cultures of neonatal and adult rat DRG neurons, in conjunction with the use of a histochemical stain that identifies capsaicin- sensitive neurons by means of cobalt uptake. Those cells that take up cobalt in a capsaicin-dependent manner (EC50 = 0.2 microM) represent about 50% of the total neuronal population derived from neonatal DRGs on short-term culture. Overnight treatment of cultures with 2 microM capsaicin leads to the loss of the cobalt-staining subpopulation. The capsaicin-insensitive neurons contain immunoreactive neurofilament epitopes that are present in fewer than 10% of capsaicin-sensitive neurons. This observation provides indirect evidence that the sensitive cells correspond to the small, dark B-type neurons, which are negative for neurofilament immunoreactivity in vivo. A capsaicin-dependent calcium uptake (EC50 = 0.2 microM), as measured by 45Ca incorporation, is shown by a DRG neuronal subpopulation that, like the cobalt-staining population of DRG neurons, is lost after overnight capsaicin treatment (2 microM). Capsaicin application leads to the accumulation of millimolar levels of calcium within a few minutes. Cadmium and other divalent cations block capsaicin-induced calcium uptake, but little or no inhibition is seen with organic calcium channel antagonists. Mitochondria, rather than the endoplasmic reticulum, are the probable destination of the internalized calcium, because ruthenium red inhibits calcium uptake (IC50 = 0.05 microM), whereas methylxanthines are inactive. The subset of sensory neurons that takes up calcium also releases 86Rb when exposed to capsaicin (EC50 = 0.06 microM). No efflux of 36Cl ions could be induced by capsaicin. These cells also show a capsaicin-induced uptake of 22Na or 14C guanidine (EC50 = 0.06 microM). In contrast, chick DRG cells in culture showed no capsaicin-induced calcium or cobalt uptake. Primary cultures of rat superior cervical ganglion neurons and Schwann cells, and a number of neuronal cell lines, also failed to respond to capsaicin, as judged by the calcium, cobalt, or guanidine uptake assays.