Different responses to repeated applications of zingerone in behavioral studies, recordings from intact and cultured TG neurons, and from VR1 receptors

When applied repetitively to the cornea, capsaicin, the pungent compound in hot pepper, causes an initial eye-wiping response that diminishes upon repeated exposure (tachyphylaxis). This diminution, however, is not observed upon repetitive application of its pungent analogue, zingerone, to the cornea or tongue. In addition, compared with capsaicin, the lingual application of zingerone produces a gustatory response with a shorter latency and duration. Because both the tongue and the cornea are innervated by the trigeminal nerve, and because zingerone and capsaicin are structurally related, it is not evident why the responses to these compounds should give such different behavioral and psychophysical endpoints. We have addressed this issue by measuring the neural responses from rat trigeminal ganglion neurons (TG) to repeated applications of zingerone applied to the cornea, from cultured rat TG neurons, and from cloned capsaicin receptors (VR1) expressed in Xenopus oocytes and then comparing these effects to those evoked by capsaicin. Extracellular recordings from the trigeminal ganglion revealed that the responses to repeated corneal applications of 30 mM zingerone show desensitization. Cultured TG neurons, and oocytes expressing VR1 receptors, were also desensitized by repeated applications of zingerone. Electrophysiological recordings revealed that these two vanilloids could activate the same receptor (VR1), currents in the same neuron, and cross-desensitize. The more rapid onset and shorter duration responses seen with zingerone (compared with capsaicin) provides a rationalization for its more rapid onset and shorter duration gustatory response. We attribute the different behavioral responses to periodic applications of these two agonists to two competing effects: one leading to sensitization, and the other to tachyphylaxis. Which of these dominates depends on the concentration, exposure time, and interstimulus interval. Consequently, whether or not zingerone will exhibit tachyphylaxis depends critically on the experimental conditions. (C) 2000 Elsevier Science Inc. All rights reserved.