Department of Biology, Kline Biology Tower, Yale University, New Haven, CT 06520-8103,
To address the question of whether the olfactory system in flies is able to adapt to odorants we tested flies in a modified T-Maze assay. Flies which were pre-exposed to a high dose of isoamyl acetate (IAA) for 30 sec showed a reduced response to a subsequent IAA stimulus: their response index was 20% that of naive flies. Recovery occurred on the order of minutes: after a two minute recovery, response was 75% of that of the naive flies. Cross-adaptation was also observed, in that flies pre-exposed to IAA showed a reduced response to a variety of other odorants in the T-Maze.
In contrast, the mutant transient receptor potential (trp), originally isolated in Drosophila by virtue of a defect in visual system electrophysiology, shows no adaptation in the olfactory T-Maze paradigm under these conditions. The trp gene likely codes for a voltage-independent Ca2 channel. Interestingly, the olfactory adaptation defect in trp301 is dominant.
Olfactory adaptation was shown to occur in the periphery. In the wild-type, the amplitude of electroantennograms (EAGs) was shown to be smaller in pre-exposed animals than in naive animals. Although odorant stimulation of naive trp mutants elicits a normal EAG amplitude, trp mutants were shown to be abnormal in antennal adaptation. Surprisingly, whereas wild-type flies show a recovery to 50% of the naive response at 2-3 min, trp301 recovers faster, showing 50% of the naive response after 1.5 min.
Evidence that this adaptation phenotype maps to trp comes from experiments with two temperature-sensitive alleles, trpCM and trp313. Both alleles exhibit a faster recovery time than wild-type at the restrictive temperature, but a normal recovery at the permissive temperatures (Wild-type recovery is unaffected by these differences in temperature).
The simplest interpretation of these results is that the Ca2 influx which occurs through TRP channels is involved in the adaptation process of the antennal system.