Chem. Senses 28: 339-348,
2003
© Oxford University Press 2003
Olfactory Signal Modulation by Molluscan Cardioexcitatory Tetrapeptide (FMRFamide) in Axolotls (Ambystoma mexicanum)
Department of Zoology, Michigan State University, East Lansing, MI 48824, USA
Correspondence to be sent to: Heather L. Eisthen, Department of Zoology, Michigan State University, 203 Natural Sciences Building, East Lansing, MI 48824, USA. e-mail: eisthen{at}msu.edu
The terminal nerve, which innervates the nasal epithelia of most jawed
vertebrates, is believed to release neuropeptides that modulate activity of
sensory receptor neurons. The terminal nerve usually contains
gonadotropin-releasing hormone as well as at least one other peptide that has
not been characterized, but which bears some structural similarity to
molluscan cardioexcitatory tetrapeptide (FMRFamide) and neuropeptide tyrosine
(NPY). We investigated the effects of FMRFamide on both voltage-gated currents
and odorant responses in the olfactory epithelium of axolotls (Ambystoma
mexicanum), using whole-cell patch clamp and electro-olfactogram (EOG)
recording techniques. In the presence of FMRFamide, the magnitude of a
voltage-gated inward current was dramatically increased, reaching an average
of 136% of the initial (pre-exposure) magnitude in neurons that showed a
response to the peptide. This increase is detectable within 
1–2 min
of exposure to FMRFamide and is sustained for at least 10 min. In EOG
experiments, odorant responses are not affected during FMRFamide application,
but are sometimes increased or decreased during the subsequent wash period. On
average, the largest single EOG response in each trial was detected 25
min after initial FMRFamide application, and ranged from 110 to 147% of
baseline. These results suggest that a compound similar to FMRFamide, if
released from the terminal nerve, may function in peripheral olfactory signal
modulation.
Key words: salamander, terminal nerve, whole-cell patch clamp, electro-olfactogram (EOG), neuropeptide, neuromodulation
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