Tetrodotoxin Serves As An Odorant In Rough-Skinned Newts (Taricha granulosa)
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1
Michigan State University, Department of Zoology, United States
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2
Marine Biological Laboratory, United States
Tetrodotoxin (TTX) is used as a chemical defense against predation by a variety of organisms, including pufferfish and many species of newts, and is a potent blocker of voltage-gated sodium channels. Given its often lethal effects, it is perhaps surprising that some of the organisms that produce TTX have also co-opted it for use in intraspecific communication -- for example, it serves as an attractant pheromone in pufferfish. The mechanisms by which TTX serves as an odorant are unknown. We are examining the behavioral significance and neural mechanisms of TTX detection in male adult rough-skinned newts, Taricha granulosa. Our immunohistochemical data demonstrate that TTX is abundantly present in skin glands in all male newts in the population we are examining, from the Willamette Valley (Oregon). We are using a behavioral assay in which newts are presented with wedges of foam impregnated with TTX or the carrier solution in a round chamber; we record their behavior over a three hour trial. Our preliminary data suggest that newts can detect concentrations of at least 100 nM - 1 µM TTX. Using electro-olfactogram (EOG) recordings, which measure summed generator potentials, we find that TTX evokes robust responses from the olfactory epithelium in concentrations ranging from 1 nM to 1 µM. These responses are similar in shape and time course to those evoked by other odorants, such as mixtures of amino acids. Guanidinium constitutes an important functional group on the TTX molecule, interacting with amino acids around the pore to block the sodium channel. The amino acid arginine also contains a guanidinium group. In a cross-adaptation experiment, we found that bathing the olfactory epithelium with arginine reversibly suppressed EOG responses evoked by TTX, suggesting that the guanidinium group plays a role in olfactory detection of TTX. Do these molecules activate odorant receptors, or a different class of receptors? The canonical odorant transduction pathway involves a G-protein coupled receptor that, when activated, stimulates production of ATP, gating a cation channel that passes both sodium and calcium. The calcium that enters the olfactory receptor neuron then gates a chloride channel, and the resulting efflux of chloride depolarizes the cell. To determine whether TTX activates this pathway, we substituted choline chloride for sodium chloride in our Ringer’s solution. Interestingly, in the presence of choline chloride, responses evoked by TTX are abolished but responses evoked by other odorants are unaffected, demonstrating that EOG responses evoked by TTX depend on external sodium. These results suggest that TTX is transduced using a mechanism other than the classical odorant transduction pathway. Substitution of barium for calcium in the Ringer’s solution diminishes responses evoked by amino acids and possibly by arginine, suggesting that arginine may activate the canonical odorant transduction pathway as well as the pathway that is activated by TTX.
Acknowledgements
This research was supported by a grant from the US National Science Foundation (IOS 0817785) as well as fellowships from the Marine Biological Laboratory. We thank Emma Coddington (Willamette University) for supplying us with newts, and Jake Billhorn for providing them with excellent care.
Keywords:
Amphibians,
Behavior,
evolution,
Olfaction,
Sodium Channels,
TTX
Conference:
Tenth International Congress of Neuroethology, College Park. Maryland USA, United States, 5 Aug - 10 Aug, 2012.
Presentation Type:
Poster Presentation (see alternatives below as well)
Topic:
Sensory: Olfaction and Taste
Citation:
Schroeder
JJ,
Walters
BS,
Davis
T,
Chamness
JR and
Eisthen
HL
(2012). Tetrodotoxin Serves As An Odorant In Rough-Skinned Newts (Taricha granulosa).
Conference Abstract:
Tenth International Congress of Neuroethology.
doi: 10.3389/conf.fnbeh.2012.27.00411
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Received:
02 May 2012;
Published Online:
07 Jul 2012.
*
Correspondence:
Dr. Heather L Eisthen, Michigan State University, Department of Zoology, East Lansing, MI, 48824, United States, eisthen@msu.edu