- Nociceptive polycationic peptides,
- Tetrahymena thermophila
Chemorepellents are compounds which cause a cell to move away from the source of the repellent, or down a concentration gradient of the compound. In ciliates such as Tetrahymena thermophila and Paramecium tetraurelia, this reorientation is accomplished by ciliary reversal, resulting in jerky or backward swimming which is known as an “avoidance reaction”. This reaction can easily be seen under a simple dissection microscope, allowing for easy characterization of ciliate behavior in different compounds.
A number of compounds are known chemorepellents in Tetrahymena thermophila, including ATP and GTP which have a negative overall charge, and polycations such as lysozyme and pituitary adenylate cyclase activating polypeptide (PACAP) which have multiple positive charges. The physiological significance of these chemorepellents is uncertain. Nucleotide triphosphates, such as ATP and GTP, have been postulated to serve as a warning signal to other cells that cellular lysis has occurred, analogous to “blood in the water”. Polycations such as lysozyme are thought to be similar to naturally occurring secretions of the organisms’ predators, allowing some organisms to escape by sensing the polycation gradient.
Since we have previously found that polycationic peptides such as lysozyme and PACAP are chemorepellents in Tetrahymena which appear to signal through a G-protein linked receptor, we decided to explore whether human nociceptive peptides would also cause Tetrahymena to exhibit avoidance through a similar mechanism. The peptides we tested: ACTH 1-24, PTH, substance P, and bradykinin all carry a net positive charge at a pH of 7.0 and are associated with G-protein linked receptors in humans. Our hypothesis was that the efficacy of the compound in terms of causing avoidance would be linked to its charge. In addition, we hypothesized that these compounds would all work through a single receptor, the previously characterized lysozyme/PACAP receptor (Mace et al., 2000; Hassenzahl et al., 2001).