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Male moths are confronted with complex odour mixtures in a natural environment when flying towards a female-emitted sex pheromone source. Whereas synergistic effects of sex pheromones and plant odours have been observed at the behavioural level, most investigations at the peripheral level have shown an inhibition of pheromone responses by plant volatiles, suggesting a potential role of the central nervous system in reshaping the peripheral information. We thus investigated the interactions between sex pheromone and a behaviourally active plant volatile, heptanal, and their effects on responses of neurons in the pheromone-processing centre of the antennal lobe, the macroglomerular complex, in the moth Agrotis ipsilon. Our results show that most of these pheromone-sensitive neurons responded to the plant odour. Most neurons responded to the pheromone with a multiphasic pattern and were anatomically identified as projection neurons. They responded either with excitation or pure inhibition to heptanal, and the response to the mixture pheromone + heptanal was generally weaker than to the pheromone alone, showing a suppressive effect of heptanal. However, these neurons responded with a better resolution to pulsed stimuli. The other neurons with either purely excitatory or inhibitory responses to all three stimuli did not exhibit significant differences in responses between stimuli. Although the suppression of the pheromone responses in AL neurons by the plant odour is counter-intuitive at first glance, the observed better resolution of pulsed stimuli is probably more important than high sensitivity to the localization of a calling female.

In the moth, Agrotis ipsilon, newly mated males cease to be attracted to the female-produced sex pheromone, preventing them from re-mating until the next night, by which time they would have refilled their reproductive glands for a potential new ejaculate. The behavioural plasticity is accompanied by a decrease in neuron sensitivity within the primary olfactory centre, the antennal lobe (AL). However, it was not clear whether the lack of the sexually guided behaviour results from the absence of sex pheromone detection in the ALs, or if they ignore it in spite of detection, or if the sex pheromone itself inhibits attraction behaviour after mating. To test these hypotheses, we performed behavioural tests and intracellular recordings of AL neurons to non-pheromonal odours (flower volatiles), different doses of sex pheromone and their mixtures in virgin and newly mated males. Our results show that, although the behavioural and AL neuron responses to flower volatiles alone were similar between virgin and mated males, the behavioural response of mated males to flower odours was inhibited by adding pheromone doses above the detection threshold of central neurons. Moreover, we show that the sex pheromone becomes inhibitory by differential central processing: below a specific threshold, it is not detected within the AL; above this threshold, it becomes inhibitory, preventing newly mated males from responding even to plant odours. Mated male moths have thus evolved a strategy based on transient odour-selective central processing, which allows them to avoid the risk-taking, energy-consuming search for females and delay re-mating until the next night for a potential new ejaculate.

In the male moth, Agrotis ipsilon, mating induces a transient inhibition of behavioural and central nervous responses to sex pheromone. Newly mated males are not attracted to sex pheromone, and the sensitivity of their antennal lobe (AL) neurons is lower than in virgin males. This rapid transient olfactory inhibition prevents them from re-mating unsuccessfully until they have refilled their sex glands. We hypothesized that this olfactory 'switch off' might be controlled by neuromodulators such as biogenic amines. To test our hypothesis, we studied the effects of octopamine (OA) and serotonin (5-hydroxytryptamine, 5-HT) on the coding properties of pheromone-sensitive AL neurons in virgin and newly mated males. We show that AL neuron sensitivity increased in newly mated males after injection of OA or 5-HT, but only OA treatment affected certain response characteristics of AL neurons in virgin males. Whereas all measured AL neuron response characteristics were different between virgin and newly mated males, amine treatment in newly mated males restored only the latency and spike frequency, but not the duration of excitatory and inhibitory phases, which were initially found in virgin males. Additionally, we investigated the behavioural effects of OA and 5-HT treatments in virgin and mated males. Although OA and 5-HT enhanced the general flight activity of newly mated males, amine treatments did not restore the behavioural pheromone response of mated moths. Altogether, these results show that, although biogenic amines modulate the olfactory system in moths, OA and 5-HT are probably not involved in the post-mating inhibition of responses to sex pheromone in A. ipsilon males.

To find a mating partner, moths rely on pheromone communication. Released in very low amounts, female sex pheromones are used by males to identify and localize females. Depending on the physiological state (i.e. age, reproductive state), the olfactory system of the males of the noctuid moth Agrotis ipsilon is 'switched on or off'. To understand the neural basis of this behavioural plasticity, we performed a detailed characterization of the qualitative, quantitative and temporal aspects of pheromone coding in the primary centre of integration of pheromonal information, the macroglomerular complex (MGC) of the antennal lobe. MGC neurons were intracellularly recorded and stained in sexually mature virgin males. When stimulating antennae of males with the three main components of the female pheromone blend, most of the neurons showed a biphasic excitatory-inhibitory response. Although they showed different preferences, 80% of the neurons responded at least to the main pheromone component (Z-7-dodecenyl acetate). Six stained neurons responding to this component had their dendrites in the largest MGC glomerulus. Changes in the stimulus intensity and duration affected the excitatory phase but not the inhibitory phase properties. The stimulus intensity was shown to be encoded in the firing frequency, the number of spikes and the latency of the excitatory phase, whereas the stimulus duration only changed its duration. We conclude that the inhibitory input provided by local interneurons following the excitatory phase might not contribute directly to the encoding of stimulus characteristics. The data presented will serve as a basis for comparison with those of immature and mated males.

Spatiotemporal odour coding is thought to be linked closely with the specific glomerular anatomy of the primary olfactory centre. In most insects the number of the glomeruli within the antennal lobe is limited to fewer than 100, allowing their individual identification. In the grapevine moth, Lobesia botrana, a map of the antennal lobe glomeruli was reconstructed three-dimensionally, by comparing three different brains in males and females. The map of the antennal lobe of females served then as a basis to identify glomeruli containing dendritic arborisations of 14 physiologically characterised projection neurons. Projection neurons responding to the same plant compound did not always arborise in the same glomerulus and some neurons arborising in the same glomerulus responded to different compounds. Different zones of target glomeruli were, however, identified when pooling all neurons responding to one of two different compounds respectively (alpha-farnesene and nonatriene). All identified glomeruli of specifically responding projection neurons were situated close to the anterior surface of the antennal lobe. One broadly responding projection neuron arborised in a more posteriorly situated glomerulus. A local interneuron responding to only one compound was arborising densely in a neighbouring glomerulus and had sparse branches in all other glomeruli. These results are discussed with respect to plant odour processing and structure-function relations in antennal lobe neurons. The 3D AL atlas will, in the future, also be used to obtain a better understanding of coding mechanisms of grapevine odours in this pest insect.