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

Sex pheromone-plant odour interactions in the olfactory pathway of virgin and mated <i>A. ipsilon</i> males.

<p>Whereas pheromone sensitivity decreases drastically in AL output neurons after mating, heptanal sensitivity seems to increase already at the AL input level. Synergistic behavioural responses to odour mixtures in virgin males are correlated with enhanced antennal lobe responses. Likewise inhibitory behavioural responses to mixtures of pheromone and plant odour in mated males match inhibitory interactions within ordinary glomeruli of the antennal lobe. Pheromone reception and antennal lobe processing, on the other hand are inhibited by heptanal, independently of mating state. This might serve to improve temporal resolution of discontinuous stimuli, which are common in a natural environment. AL: antennal lobe; hep: heptanal; MGC: macroglomerular complex; mix: heptanal/pheromone mixture; OG: ordinary glomeruli; ORN: olfactory receptor neuron; phe: pheromone. Size of disks indicates response strength. Dash means no response. Numbers refer to previously published data: (1) Barrozo et al., 2011 <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0033159#pone.0033159-Barrozo2" target="_blank">[26]</a> (2) This paper. (3) Gadenne et al., 2001 <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0033159#pone.0033159-Gadenne1" target="_blank">[25]</a>. (4) Barrozo et al., 2010 <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0033159#pone.0033159-Barrozo1" target="_blank">[21]</a>.</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

Odour-evoked calcium signals in the antennal lobe.

<p><b>A</b>) Example of an anatomical staining of a right antennal lobe (AL) with the outline of the entire AL and MGC. Two activity maps obtained in response to heptanal (10 µg) (hep10) and to the pheromone blend (10 ng) (phe10) are shown with the outline of the AL. Numbers next to dots indicate the position of the nine analysed ordinary glomeruli (1–9), as well as three analysed locations within the MGC, for which activity was pooled. <b>B</b>) Activity signals obtained in a mated male stimulated with four doses of heptanal (hep) (1–1000 µg), four presentations of pheromone (phe) at 10 ng, and the respective pheromone/heptanal mixtures (mix). All maps are scaled to the same minimum/maximum.</p

Heptanal-sensitive ORNs and OG calcium-evoked responses in virgin and mated males.

<p><b>A</b>) Typical recording showing an excitatory response to heptanal (100 µg), no response to the pheromone (10 ng) and the solvent (mineral oil), and excitation to the pheromone/heptanal mixture in a virgin male. The grey bar indicates the duration of the stimulus (0.5 s). <b>B</b>) Mean spike frequency of Hep-ORNs to pheromone (10 ng), heptanal at different doses, and their mixture in virgin (n = 13) and mated (n = 13) males. Hep-ORNs show dose-dependent response to heptanal, but no response to the pheromone and solvent. The addition of pheromone in the mixture does not modify the response of Hep-ORNs to heptanal at any dose tested. No differences were detected between virgin and mated males. <b>C</b>) Time course of odour-evoked calcium activity in the OG. The grey bar indicates the duration of the stimulus (1 s). <b>D</b>) Mean calcium responses in the OG to pheromone (10 ng), heptanal at different doses, and their mixture in virgin (n = 9) and mated (n = 8) males. Stimulation with pheromone induced no response. Heptanal-induced responses increased with the dose and were significantly higher in mated than in virgin males, although it was not different from mixture responses. Hep: heptanal; mix: pheromone/heptanal mixture; phe: pheromone; sol: solvent.</p

Responses of AL PNs within the MGC of virgin and mated males.

<p><b>A</b>) Typical responses of a pheromone-sensitive PN in a virgin male, showing an excitatory response to pheromone (1 ng), no response to heptanal (100 µg) and the solvent (hexane), and a reduced firing rate during excitation to the pheromone/heptanal mixture. The grey bar indicates the duration of the stimulus (0.2 s). <b>B</b>) Spike frequency of PNs during the excitatory period to the pheromone (1 ng in virgin and 10 ng in mated males), heptanal (100 µg) and the pheromone/heptanal mixture in virgin (n = 17 neurons) and mated (n = 15 neurons) males. Spike frequencies of PNs do not differ between heptanal and solvent. Hep: heptanal; mix: pheromone/heptanal mixture; phe: pheromone; sol: solvent.</p