Abstract
We measured frequency response functions between odorants and action potentials in two types of neurons in Drosophila antennal basiconic sensilla. CO2 was used to stimulate ab1C neurons, and the fruit odor ethyl butyrate was used to stimulate ab3A neurons. We also measured frequency response functions for light-induced action potential responses from transgenic flies expressing H134R-channelrhodopsin-2 (ChR2) in the ab1C and ab3A neurons. Frequency response functions for all stimulation methods were well-fitted by a band-pass filter function with two time constants that determined the lower and upper frequency limits of the response. Low frequency time constants were the same in each type of neuron, independent of stimulus method, but varied between neuron types. High frequency time constants were significantly slower with ethyl butyrate stimulation than light or CO2 stimulation. In spite of these quantitative differences, there were strong similarities in the form and frequency ranges of all responses. Since light-activated ChR2 depolarizes neurons directly, rather than through a chemoreceptor mechanism, these data suggest that low frequency dynamic properties of Drosophila olfactory sensilla are dominated by neuron-specific ionic processes during action potential production. In contrast, high frequency dynamics are limited by processes associated with earlier steps in odor transduction, and CO2 is detected more rapidly than fruit odor.
Highlights
Carbon dioxide sensitivity occurs in a variety of insects, including some with major health and agricultural impacts on humans
In Drosophila antennae, one of the four neurons of the largest basiconic sensilla responds to CO2, whereas other neurons in these sensilla respond to fruit odors
The ab1C neurons express two gustatory receptors GR21a and GR63a that together comprise the CO2 receptor [1,2]. These neurons lack the odorant receptors (ORs) and the auxiliary OR83b (Orco) receptors, which are common to all other neurons that mediate odor responses in basiconic sensilla [3,4,5]
Summary
Carbon dioxide sensitivity occurs in a variety of insects, including some with major health and agricultural impacts on humans. The ab1C neurons express two gustatory receptors GR21a and GR63a that together comprise the CO2 receptor [1,2]. These neurons lack the odorant receptors (ORs) and the auxiliary OR83b (Orco) receptors, which are common to all other neurons that mediate odor responses in basiconic sensilla [3,4,5]. Drosophila possess a family of ionotropic chemoreceptor molecules, located in coeloconic sensilla and other antennal structures [5,6], which include acid sensitive receptors responsive to high concentrations of CO2 [7]. CO2 alone may be attractive or repellent under different testing conditions [1,8,9,10] while combination of CO2 with other odors may overcome repulsion or create attraction [8,9,11]
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