American Society for Cell Biology
chemorepellent, Tetrahyema thermophila
Tetrahymena thermophila are free-living, ciliated, eukaryotic organisms that respond to stimuli by moving toward chemoattractants and avoiding chemorepellents. Chemoattractant responses involve faster ciliary beating, which propels the organisms forward more rapidly. Chemorepellent signaling involves ciliary reversal, which disrupts forward swimming, and causes the organism to jerk back and forth, swim in small circles, or spin in an attempt to get away from the repellent. Many food sources, such as proteins, are chemoattractants for Tetrahymena, while a variety of compounds are repellents. Repellents in nature are thought to come from the secretions of predators, or from ruptured organisms, which may serve as “danger” signals. Several hormones involved in human pain signaling have been shown to be chemorepellents in Tetrahymena, including substance P, ACTH, PACAP, VIP, and nociceptin.
We have been studying the response of Tetrahymena to nociceptin, using pharmacological inhibitors in order to elucidate components of the nociceptin signaling pathway. We have found that G-protein inhibitors and a number of mammalian tyrosine kinase inhibitors have no effect on nociceptin avoidance. However, the tyrosine kinase inhibitor, genistein, inhibits avoidance to nociceptin, likely by an unrelated mechanism. Nociceptin avoidance is also inhibited by the calcium chelator, EGTA, and partially inhibited by the ER calcium ATPase inhibitor, thapsigargin. Whole cell electrophysiology experiments in a calcium-containing buffer show that addition of 50 μM nociceptin to the buffer causes a sustained depolarization of approximately 30 mV. This depolarization is nearly eliminated in the presence of EGTA, further supporting the hypothesis that calcium is involved in nociceptin signaling.
J-113397, an inhibitor of the human nociceptin receptor, also inhibits nociceptin avoidance in Tetrahymena, though other nociceptin antagonists we tested had no effect on avoidance. Further experimentation on this organism will give a more complete picture of the signaling pathway, as well as allowing greater comparison between nociceptin avoidance in Tetrahymena and nociceptin signaling in vertebrates.
Braun, N., Lampert, T., Nugent, C., Kuruvilla, H. Nociceptin is a Chemorepellent in Tetrahymena thermophila. Presented at the 51st Annual ASCB Conference, December, 2011.