A General Odorant Background Affects the Coding of Pheromone Stimulus Intermittency in Specialist Olfactory Receptor Neurones

In nature the aerial trace of pheromone used by male moths to find a female appears as a train of discontinuous pulses separated by gaps among a complex odorant background constituted of plant volatiles. We investigated the effect of such background odor on behavior and coding of temporal parameters of pheromone pulse trains in the pheromone olfactory receptor neurons of Spodoptera littoralis. Effects of linalool background were tested by measuring walking behavior towards a source of pheromone. While velocity and orientation index did drop when linalool was turned on, both parameters recovered back to pre-background values after 40 s with linalool still present. Photo-ionization detector was used to characterize pulse delivery by our stimulator. The photo-ionization detector signal reached 71% of maximum amplitude at 50 ms pulses and followed the stimulus period at repetition rates up to 10 pulses/s. However, at high pulse rates the concentration of the odorant did not return to base level during inter-pulse intervals. Linalool decreased the intensity and shortened the response of receptor neurons to pulses. High contrast (>10 dB) in firing rate between pulses and inter-pulse intervals was observed for 1 and 4 pulses/s, both with and without background. Significantly more neurons followed the 4 pulses/s pattern when delivered over linalool; at the same time the information content was preserved almost to the control values. Rapid recovery of behavior shows that change of perceived intensity is more important than absolute stimulus intensity. While decreasing the response intensity, background odor preserved the temporal parameters of the specific signal

Differential Interactions of Sex Pheromone and Plant Odour in the Olfactory Pathway of a Male Moth

Most animals rely on olfaction to find sexual partners, food or a habitat. The olfactory system faces the challenge of extracting meaningful information from a noisy odorous environment. In most moth species, males respond to sex pheromone emitted by females in an environment with abundant plant volatiles. Plant odours could either facilitate the localization of females (females calling on host plants), mask the female pheromone or they could be neutral without any effect on the pheromone. Here we studied how mixtures of a behaviourally-attractive floral odour, heptanal, and the sex pheromone are encoded at different levels of the olfactory pathway in males of the noctuid moth Agrotis ipsilon. In addition, we asked how interactions between the two odorants change as a function of the males' mating status. We investigated mixture detection in both the pheromone-specific and in the general odorant pathway. We used a) recordings from individual sensilla to study responses of olfactory receptor neurons, b) in vivo calcium imaging with a bath-applied dye to characterize the global input response in the primary olfactory centre, the antennal lobe and c) intracellular recordings of antennal lobe output neurons, projection neurons, in virgin and newly-mated males. Our results show that heptanal reduces pheromone sensitivity at the peripheral and central olfactory level independently of the mating status. Contrarily, heptanal-responding olfactory receptor neurons are not influenced by pheromone in a mixture, although some post-mating modulation occurs at the input of the sexually isomorphic ordinary glomeruli, where general odours are processed within the antennal lobe. The results are discussed in the context of mate localization

Effects of a linalool background on the decay of the firing response to a single pheromone pulse.

<p>The mean firing rate of Ph-ORNs in response to the 1st pulse of a pheromone pulse train at 1 pps in neutral background (dashed line, mean of n = 54 recordings) or linalool background (solid line, n = 45) was calculated. Then, we estimated the exponential decay function in neutral (inset: dashed line) or linalool background (solid line) in normalized scale; black points in the inset: 90% fall decay.</p

Transgenic line for characterizing GABA-receptor expression to study the neural basis of olfaction in the yellow-fever mosquito.

The mosquito Aedes aegypti is an important vector of diseases including dengue, Zika, chikungunya, and yellow fever. Olfaction is a critical modality for mosquitoes enabling them to locate hosts, sources of nectar, and sites for oviposition. GABA is an essential neurotransmitter in olfactory processing in the insect brain, including the primary olfactory center, the antennal lobe. Previous work with Ae. aegypti has suggested that antennal lobe inhibition via GABA may be involved in the processing of odors. However, little is known about GABA receptor expression in the mosquito brain, or how they may be involved in odor attraction. In this context, generating mutants that target the mosquitos olfactory responses, and particularly the GABAergic system, is essential to achieve a better understanding of these diverse processes and olfactory coding in these disease vectors. Here we demonstrate the potential of a transgenic line using the QF2 transcription factor, GABA-B1QF2-ECFP, as a new neurogenetic tool to investigate the neural basis of olfaction in Ae. aegypti. Our results show that the gene insertion has a moderate impact on mosquito fitness. Moreover, the line presented here was crossed with a QUAS reporter line expressing the green fluorescent protein and used to determine the location of the metabotropic GABA-B1 receptor expression. We find high receptor expression in the antennal lobes, especially the cell bodies surrounding the antennal lobes. In the mushroom bodies, receptor expression was high in the Kenyon cells, but had low expression in the mushroom body lobes. Behavioral experiments testing the fruit odor attractants showed that the mutants lost their behavioral attraction. Together, these results show that the GABA-B1QF2-ECFP line provides a new tool to characterize GABAergic systems in the mosquito nervous system

A linalool background alters orientation of male <i>Spodoptera littoralis</i> to pheromone source.

<p>Walking activity and orientation behaviour of male <i>Spodoptera littoralis</i> were measured on a locomotion compensator. Males were submitted to the main pheromone component, Z9E11–14:Ac during 2 min, <b><i>1a:</i></b> a neutral background was maintained for the duration of the test (n = 30). <b><i>1b</i></b>: in addition to pheromone, a linalool background was applied after 1 min (n = 42). Mean upwind speed: average of the distances per second walked by males in direction of the source (solid line) and mean ± SEM (dashed lines). Orientation index: oi = cos (θ) * ρ, where θ is the mean angle of track sample and ρ the length of the mean vector at t_i. Horizontal grey bar = linalool background.</p

Responses of the PID to pulse trains of linalool.

<p>To trace the dynamics of the odorant concentration in air from the stimulation device in the electrophysiological set-up, the PID signal was recorded while linalool was delivered as ten 50 ms pulses at 1, 2, 4, 7 and 10 pps.</p

Background and pulse rate affect the firing response of Ph-ORNs to pulses.

<p>The curves presents the mean firing rates of the Ph-ORNs whose individual responses appear in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0026443#pone-0026443-g003" target="_blank">figure 3</a> responding to pulse trains at 1, 4 and 10 pps in neutral (4a) and in linalool backgrounds (4b).</p

Linalool background improves the information content of the firing activity of a population of Ph-ORNs in response to pulsed pheromone.

<p>The Jensen Shannon divergence (D_JS) was calculated bin by bin (bin size = 20 ms) between control and pheromone pulses in air (n = 57) or between control and pulses in linalool (n = 46). The info/bin value varies from 0 (the probability function of spike distribution in control and stimulated neurones are identical) to 1 (the probability distributions do not overlap). Dashed line: D_JS curve for pheromone pulses in air; solid line: D_JS curve for pheromone pulses in linalool.</p

Monitoring of the odorant in the walking track recording device with a miniature Photo Ionisation Detector.

<p>To check the shape of the odour panache in the locomotion compensator, the probe of the PID was positioned either at the output of the stimulus delivering tube or over the sphere while a flow of air odorized with linalool (10% in mineral oil) was delivered.</p

Behavioural responses of virgin <i>A. ipsilon</i> males to heptanal.

<p>The proportion of males showing an oriented flight towards the stimulus source was highest at a dose of 100 µg heptanal. Numbers in brackets represent the numbers of tested males. Bars with same letters are not statistically different (chi-square-test, p<0.05).</p