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Evidence of a novel transduction pathway mediating detection of polyamines by the zebrafish olfactory system
Department of Physiology, University of Utah School of Medicine, Salt Lake City, UT 84108-1297, USA
* Author for correspondence (e-mail: mike.michel{at}m.cc.utah.edu)
Accepted 17 February 2003
To better understand the full extent of the odorant detection capabilities
of fish, we investigated the olfactory sensitivity of zebrafish to a monoamine
and several polyamines using electrophysiological and activity-dependent
labeling techniques. Electro-olfactogram (EOG) recording methods established
the relative stimulatory effectiveness of these odorants as: spermine >>
spermidine
agmatine > glutamine > putrescine
cadaverine
histamine > artificial freshwater. The detection threshold for the potent
polyamines was approximately 1 µmol l1. Cross-adaptation
experiments suggested that multiple receptors are involved in polyamine
detection. Three observations indicated that polyamine signaling may involve a
transduction cascade distinct from those used by either amino acids or bile
salts. Like bile salts and the adenylate cyclase activator forskolin, but
unlike amino acid odorants, polyamines failed to stimulate activity-dependent
labeling of olfactory sensory neurons with the cation channel permeant probe
agmatine, suggesting a signaling pathway different from that used by amino
acid stimuli. Also supporting distinct amino acid and polyamine signaling
pathways is the finding that altering phospholipase C activity with the
inhibitor U-73122 significantly reduced amino acid-evoked responses, but had
little effect on polyamine- (or bile salt-) evoked responses. Altering cyclic
nucleotide-mediated signaling by adenylate cyclase activation with forskolin,
which significantly reduced responses to bile salts, failed to attenuate
polyamine responses, suggesting that polyamines and bile salts do not share a
common transduction cascade. Collectively, these findings suggest that
polyamines are a new class of olfactory stimuli transduced by a
receptor-mediated, second messenger signaling pathway that is distinct from
those used by amino acids or bile salts.
Key words: electro-olfactogram, activity labeling, olfactory receptor neuron, odorant receptor, zebrafish, Danio rerio
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