Carolina E. Reisenman

The neurobiology of insect olfaction: sensory processing in a comparative context.

The simplicity and accessibility of the olfactory systems of insects underlie a body of research essential to understanding not only olfactory function but also general principles of sensory processing. As insect olfactory neurobiology takes advantage of a variety of species separated by millions of years of evolution, the field naturally has yielded some conflicting results. Far from impeding progress, the varieties of insect olfactory systems reflect the various natural histories, adaptations to specific environments, and the roles olfaction plays in the life of the species studied. We review current findings in insect olfactory neurobiology, with special attention to differences among species. We begin by describing the olfactory environments and olfactory-based behaviors of insects, as these form the context in which neurobiological findings are interpreted. Next, we review recent work describing changes in olfactory systems as adaptations to new environments or behaviors promoting speciation. We proceed to discuss variations on the basic anatomy of the antennal (olfactory) lobe of the brain and higher-order olfactory centers. Finally, we describe features of olfactory information processing including gain control, transformation between input and output by operations such as broadening and sharpening of tuning curves, the role of spiking synchrony in the antennal lobe, and the encoding of temporal features of encounters with an odor plume. In each section, we draw connections between particular features of the olfactory neurobiology of a species and the animals life history. We propose that this perspective is beneficial for insect olfactory neurobiology in particular and sensory neurobiology in general.

Enantioselectivity of Projection Neurons Innervating Identified Olfactory Glomeruli

Projection neurons (PNs) with arborizations in the sexually dimorphic “lateral large female glomerulus” (latLFG) in the antennal lobe (AL) of the moth Manduca sexta previously were shown to respond preferentially to antennal stimulation with (±)linalool, a volatile compound commonly emitted by plants. In the present study, using intracellular recording and staining techniques, we examined the responsiveness of latLFG-PNs to the enantiomers, (+)linalool and (–)linalool and found that (1) latLFG-PNs are more responsive to antennal stimulation with (+)linalool than with (–)linalool, (2) PNs with arborizations in a glomerulus adjacent to the latLFG are preferentially responsive to (–)linalool, and (3) PNs with arborizations confined to other glomeruli near the latLFG are equally responsive to both enantiomers of linalool. Structure-activity studies showed that the hydroxyl group in this tertiary terpene alcohol is the key feature of the molecule determining the response of enantioselective PNs to linalool. In contrast, the responses of non-enantioselective PNs are less dependent on the alcoholic functionality of linalool. Our findings show that PNs innervating a uniquely identifiable glomerulus respond preferentially to a particular enantiomer of an odor substance. Moreover, PNs with arborizations in a glomerulus adjacent to the latLFG, although less sensitive than latLFG-PNs to linalool, respond preferentially to the opposite enantiomer, demonstrating that information about stimulus-absolute configuration can be encoded in different olfactory glomeruli.

Circadian control of photonegative sensitivity in the haematophagous bug Triatoma infestans

Abstract The negative phototactic response of Triatoma infestans (Hemiptera) was studied in an arena, half of which was kept dark and the other half illuminated with different light intensitites. For each intensity we measured the time the insects spent in the dark half, the time to reach the side opposite to that where they were released, and the number of passages through the middle line of the arena. T. infestans displayed a photonegative behaviour that was enhanced by high light intensities. Bugs maintained in 12:12 light-dark cycles responded differently to the same illumination levels when tested in their photophase and scotophase: sensitivity to light was higher during the latter. Bugs entrained to light-dark cycles, kept afterwards either in constant darkness or in constant light, and tested in their subjective night and day, showed the same responses as bugs from the light-dark group tested in their corresponding photophase and scotophase. Thus, phototactic sensitivity is under endogenous control. The behaviour shown by T. infestans may be understood as being composed of at least two different drives: an exploratory one, and a negative phototactic response that is under endogenous control and is particularly sensitive to light during the scotophase, when activity peaks occur.

Antagonistic effects of floral scent in an insect–plant interaction

In southwestern USA, the jimsonweed Datura wrightii and the nocturnal moth Manduca sexta form a pollinator–plant and herbivore–plant association. Because the floral scent is probably important in mediating this interaction, we investigated the floral volatiles that might attract M. sexta for feeding and oviposition. We found that flower volatiles increase oviposition and include small amounts of both enantiomers of linalool, a common component of the scent of hawkmoth-pollinated flowers. Because (+)-linalool is processed in a female-specific glomerulus in the primary olfactory centre of M. sexta, we hypothesized that the enantiomers of linalool differentially modulate feeding and oviposition. Using a synthetic mixture that mimics the D. wrightii floral scent, we found that the presence of linalool was not necessary to evoke feeding and that mixtures containing (+)- and/or (−)-linalool were equally effective in mediating this behaviour. By contrast, females oviposited more on plants emitting (+)-linalool (alone or in mixtures) over control plants, while plants emitting (−)-linalool (alone or in mixtures) were less preferred than control plants. Together with our previous investigations, these results show that linalool has differential effects in feeding and oviposition through two neural pathways: one that is sexually isomorphic and non-enantioselective, and another that is female-specific and enantioselective.

Infection of Kissing Bugs with Trypanosoma cruzi, Tucson, Arizona, USA

A survey of triatomine insects found that 41.5% were infected with the causative agent of Chagas disease.

The effect of cumulative experience on the use of elemental and configural visual discrimination strategies in honeybees

We addressed the question of whether the amount of individual experience determines the use of elemental or configural visual discrimination strategies in free-flying honeybees Apis mellifera. We trained bees to fly into a Y-maze to collect sucrose solution on a rewarded stimulus presented in one of the arms of the maze. Stimuli were colour disks, violet (V), green (G) or yellow (Y), which were of equal psychophysical salience for honeybees. Training followed an A+, BC+ design, followed by an AC versus BC test. Training consisted of 6 (3 A+ and 3 BC+), 20 (10 A+ and 10 BC+) or 40 (20 A+ and 20 BC+) acquisition trials. Elemental models of compound processing predict a preference for the non-trained stimulus AC while configural models predict a preference for the trained stimulus BC. Our results show that increasing the number of acquisition trials results in a change of the internal representation of stimuli. After six training trials, bees favoured an elemental strategy and preferred AC to BC during the tests. Generally, increasing the number of training trials resulted in an increase of the choice of BC. Thus, short training favoured processing of the compound as the sum of its elements (elemental account) while long training favoured its processing as being different from the sum of its elements (configural account). Additionally, we observed that the change in stimulus processing was also influenced by stimulus similarity. Colour perceptual similarity favoured configural processing with increasing experience.

Chemosensory Selectivity of Output Neurons Innervating an Identified, Sexually Isomorphic Olfactory Glomerulus

The antennal lobe (AL) of insects, like the olfactory bulb of vertebrates, is characterized by discrete modules of synaptic neuropil called glomeruli. In some insects (e.g., moths and cockroaches), a few glomeruli are sexually dimorphic and function in labeled lines for processing of sensory information about sex pheromones. Controversy still exists, however, about whether projection (output) neurons (PNs) of glomeruli in the main AL are also narrowly tuned. We examined this critical issue in the AL of the moth Manduca sexta. We used intracellular recording and staining techniques to investigate the chemosensory tuning of PNs innervating an identifiable, sexually isomorphic glomerulus, G35, in the main AL. We found that the morphological features and chemosensory tuning of G35-PNs were nearly identical in females and males. G35-PNs responded to low concentrations of the plant-derived volatile compound cis-3-hexenyl acetate (c3HA), but the sensitivity threshold of female PNs was lower than that of male PNs. The propionate and butyrate homologs of c3HA could evoke excitatory responses but only at moderate-to-high concentrations. Other plant volatiles did not evoke responses from G35-PNs. Moreover, PNs innervating glomeruli near G35 (in females) showed little or no response to c3HA. Female G35-PNs were hyperpolarized by (±)linalool, a compound that excites PNs in an adjacent glomerulus, thus providing evidence for lateral-inhibitory interactions between glomeruli. Our results show that PNs arborizing in an identified glomerulus in the main olfactory pathway are morphologically and physiologically equivalent in both sexes and have characteristic, limited molecular receptive ranges that are highly conserved across individuals.

Local interneuron diversity in the primary olfactory center of the moth Manduca sexta

Local interneurons (LNs) play important roles in shaping and modulating the activity of output neurons in primary olfactory centers. Here, we studied the morphological characteristics, odor responses, and neurotransmitter content of LNs in the antennal lobe (AL, the insect primary olfactory center) of the moth Manduca sexta. We found that most LNs are broadly tuned, with all LNs responding to at least one odorant. 70% of the odorants evoked a response, and 22% of the neurons responded to all the odorants tested. Some LNs showed excitatory (35%) or inhibitory (33%) responses only, while 33% of the neurons showed both excitatory and inhibitory responses, depending on the odorant. LNs that only showed inhibitory responses were the most responsive, with 78% of the odorants evoking a response. Neurons were morphologically diverse, with most LNs innervating almost all glomeruli and others innervating restricted portions of the AL. 61 and 39% of LNs were identified as GABA-immunoreactive (GABA-ir) and non-GABA-ir, respectively. We found no correlations between odor responses and GABA-ir, neither between morphology and GABA-ir. These results show that, as observed in other insects, LNs are diverse, which likely determines the complexity of the inhibitory network that regulates AL output.

Triatoma infestans can be captured under natural climatic conditions using yeast-baited traps

The effectiveness of a trap for triatomines baited with yeast cultures has been previously demonstrated in laboratory assays. We report results from assays testing yeast-traps for Triatoma infestans performed under natural climatic conditions. All assays were conducted at experimental chicken-coops colonised by bugs, situated at an endemic area for Chagas disease in Argentina. Two different models of traps were tested, for use either on the floor, or attached to walls. The results obtained clearly demonstrate that yeast-baited traps are effective for capturing triatomine bugs. Floor traps, when baited, captured significantly more bugs than their controls (t-test, P = 0.0008, k = 10). In addition, wall traps were also more effective, when compared to their controls (t-test, P = 0.011, k = 10). The maximum capture by a single yeast-trap was 39 bugs in one night, while for the same period the maximum capture by a control-trap was two bugs. Traps captured significantly more bugs at nights when maximum temperature was above 18 degrees C (t-test, P = 0.0002). Results indicate that yeast traps are capable of capturing significant numbers of free insects in one night. The potential use of yeast- traps for the detection of triatomine bugs is discussed.

Neuroethology of Oviposition Behavior in the Moth Manduca sexta

Olfactory cues play decisive roles in the lives of most insect species, providing information about biologically relevant resources, such as food, mates, and oviposition sites. The nocturnal moth Manduca sexta feeds on floral nectar from a variety of plants (and thus serves as a pollinator), but females oviposit almost exclusively on solanaceous plants, which they recognize on the basis of olfactory cues. Plants, however, respond to herbivory by releasing blends of volatiles that attract natural enemies of herbivores. Thus, oviposition behavior probably results from the sensory evaluation not only of attractive host plant volatiles but also of repellent volatiles that indicate the acceptability or inappropriateness, respectively, of host plants for the females’ offspring. Here we describe results from chemical–ecological, neurophysiological, and behavioral experiments aimed at understanding the neural mechanisms that control oviposition behavior in M. sexta.