Heat-induced cobalt entry: an assay for heat transduction in cultured rat dorsal root ganglion neurons.

A histochemical stain to detect cobalt in cells was used to investigate the ionic basis of heat transduction in mammalian primary afferent neurons. Cultured dorsal root ganglion neurons from the adult rat were exposed to 10-min heat stimuli in an extracellular solution containing cobalt ions. When accumulated intracellular cobalt was precipitated, a subpopulation of neurons was darkly stained. The number of neurons stained depended on the intensity of the heat stimulus, ranging from 1.9% at 22 degrees C to 24.0% at 45 degrees C, a range of temperatures transduced by primary afferent nerve endings in vivo. Results of Trypan Blue exclusion experiments demonstrate that the heat-induced stain is not due to membrane damage, suggesting that heat opens a divalent-permeable ion channel. Agents that block many multivalent cation-permeable channels (lanthanum, ruthenium red and amiloride) did not reduce the number of cells that exhibited heat-induced cobalt staining. Heat-evoked cobalt staining provides an in vitro model for the investigation of the ionic mechanisms of thermal transduction in sensory neurons.