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In nature, virtually all olfactory stimuli are mixtures of many single odorants. Behavioral experiments repeatedly have demonstrated that an animal's olfactory system is capable of discriminating behaviorally relevant from irrelevant odor mixtures. However, the sensory mechanisms that underlie such discriminative capability remain elusive. The limited anatomical and physiological evidence collected from both insect and vertebrate models that pertains to this topic is scattered in the literature dating back to early 1980s. Thus, a synthesis of this information that includes more recent findings is needed in order to provide a basis for probing the fundamental question from a new angle. In this review, we discuss several proposed models for mixture processing, along with experimental data gathered from both the initial stage of olfactory processing (i.e., antennal lobe in insects or olfactory bulb in vertebrates) and higher areas of the brain, with an emphasis on how the lateral circuits in the antennal lobe or olfactory bulb may contribute to mixture processing. Based on empirical data as well as theoretical modeling, we conclude that odor mixtures may be represented both at the single-neuron level and at the population level. The difference between these two types of processing may reside in the degree of plasticity, with the former being hard-wired and the latter being more subjected to network modulation.

Pheromone-responsive olfactory interneurons were studied to determine the extent to which their physiological and morphological properties complemented the behavior and peripheral olfactory neurobiology observed in hybrid male moths created by interbreeding two species of heliothine moth, Heliothis virescens and Heliothis subflexa. Complete recordings were made from a total of 33 neurons, and 16 projection neurons (PNs) were subsequently stained with a fluorescent dye. Stained PNs tuned to pheromonal odorants had dendritic arborizations restricted to one of four olfactory glomeruli that together constituted the macroglomerular complex (MGC). As in parental males, PNs tuned to (Z)-11-hexadecenal always had an arbor in the cumulus, the largest of the MGC glomeruli. Previous neurophysiological investigations revealed that PNs with dendritic arbors restricted to the dorso-medial glomerulus (DM) of the MGC responded specifically to either (Z)-9-tetradecenal (Z9-14:Ald; H. virescens males) or (Z)-9-hexadecenal (Z9-16:Ald; H. subflexa males). Hybrid males, which responded equally well in wind tunnel tests to blends containing either Z9-14:Ald or Z9-16:Ald, had DM PNs that responded to both odorants. PNs specific for a third compound, (Z)-11-hexadecenol, required by hybrid males for behavioral activity were localized to the antero-medial MGC glomerulus (AM). Thus, neuronal activity across the cumulus, DM and AM glomeruli represented an attractive blend in hybrid males. Neurons tuned to (Z)-11-hexadecenyl acetate and Z9-14:Ald were restricted to a fourth, ventro-medial glomerulus. The across-glomerular pattern of activity associated with attractive pheromone blends was most similar to that of H. subflexa males, signifying a dominant effect of H. subflexa genes. These results indicate that the behavioral phenotype of hybrid males can be linked to underlying central olfactory characteristics., (Copyright 2006 S. Karger AG, Basel.)

Shifts in male preference for qualitatively different pheromone blends appear to have played a fundamental role in the divergence of olfactory communication and evolution of moth species. As an initial step in documenting the genetic complexity underlying such shifts, we characterized the behavioral responses of hybrid male moths created by mating two heliothine moth species, Heliothis subflexa and Heliothis virescens. Between 67 and 96% of hybrid males flew upwind and contacted the pheromone source when presented with a blend consisting of (Z)-11-hexadecenal (Z11-16:Ald), (Z)-9-hexadecenal (Z9-16:Ald), and (Z)-11-hexadecenol (Z11-16:OH) in a 1:0.5:0.1 ratio that has previously been shown to be attractive to H. subflexa males. In addition, an H. virescens blend of Z11-16:Ald and (Z)-9-tetradecenal (Z9-14:Ald) enhanced by the addition of Z11-16:OH (in a 1:0.05:0.1 mixture) was attractive to hybrid males (26-64% source contact), but significantly fewer males reached the odor source compared to the blend containing Z9-16:Ald. A blend in which the dosage of Z9-14:Ald was doubled, however, was equally attractive (75-77% source contact) as the Z9-16:Ald-containing blend. Consecutive presentation of two blends revealed that individual hybrid males responded equally well to blends containing either Z9-14:Ald or Z9-16:Ald. Together these results suggest that in addition to Z11-16:Ald, hybrid males: (1) required either Z9-16:Ald (likeH. subflexamales) or Z9-14:Ald (like H. virescens males); (2) required the presence of Z11-16:OH (H. subflexa dominant); (3) were not adversely affected by the presence of Z11-16:Ac (H. subflexa dominant). The behavioral response phenotype of hybrid males was therefore influenced by genetic factors inherited from both parental species., (Copyright 2006 S. Karger AG, Basel.)

The antennal imaginal disc was transplanted between premetamorphic male larvae of two different Lepidopteran moth species. Following adult eclosion, electrophysiological recordings were made from 33 central olfactory neurons in the antennal lobes of both Helicoverpa zea donor to Heliothis virescens recipient (Z-V) and reciprocal (V-Z) transplants. Under the influence of sensory neuron input derived from the transplanted antennal imaginal disc, most antennal lobe projection neurons (29/33) were classified as belonging to physiological categories encountered previously in donor species males. Furthermore, when stained many of these neurons had dendritic arbors restricted to donor-induced glomerular locations predicted by their physiology. However, some neurons with unexpected physiological profiles were also identified (4/33), but only in V-Z transplants. These profiles help to explain why some V-Z bilateral transplants were able to respond to both pheromone blends in flight tunnel bioassays, an unforeseen result counter to the assumption that a donor antenna develops a normal donor antennal olfactory receptor neuron complement. Stainings of several neurons in V-Z transplant males also revealed unusual morphological features including multiglomerular dendritic arbors and "incorrect" glomerular locations. These results indicate a developmental plasticity in the final dendritic arborization pattern of central olfactory neurons, including an ability to colonize and integrate inputs across topographically novel donor glomeruli, different from those found in the normal recipient antennal lobe., ((c) 2005 Wiley-Liss, Inc.)

At low ambient temperature Helicoverpa zea male moths engage in warm-up behavior prior to taking flight in response to an attractive female pheromone blend. Male H. zea warm up at a faster rate when sensing the attractive pheromone blend compared with unattractive blends or blank controls (Crespo et al. 2012), but the mechanisms involved in this olfactory modulation of the heating rate during preflight warm-up are unknown. Here, we test three possible mechanisms for increasing heat production: 1) increased rate of muscle contraction; 2) reduction in mechanical movement by increased overlap in activation of the antagonistic flight muscles; and 3) increased activation of motor units. To test which mechanisms play a role, we simultaneously recorded electrical activation patterns of the main flight muscles (dorsolongitudinal and dorsoventral muscles), wing movement, and thoracic temperature in moths exposed to both the attractive pheromone blend and a blank control. Results indicate that the main mechanism responsible for the observed increase in thoracic heating rate with pheromone stimulation is the differential activation of motor units during each muscle contraction cycle in both antagonistic flight muscles. This additional activation lengthens the contracted state within each cycle and thus accounts for the greater heat production. Interestingly, the rate of activation (frequency of contraction cycles) of motor units, which is temperature dependent, did not vary between treatments. This result suggests that the activation rate is determined by a temperaturedependent oscillator, which is not affected by the olfactory stimulus, but activation of motor units is modulated during each cycle. [ABSTRACT FROM AUTHOR]

Pheromone-responsive olfactory interneurons were studied to determine the extent to which their physiological and morphological properties complemented the behavior and peripheral olfactory neurobiology observed in hybrid male moths created by interbreeding two species of heliothine moth, Heliothis virescens and Heliothis subflexa. Complete recordings were made from a total of 33 neurons, and 16 projection neurons (PNs) were subsequently stained with a fluorescent dye. Stained PNs tuned to pheromonal odorants had dendritic arborizations restricted to one of four olfactory glomeruli that together constituted the macroglomerular complex (MGC). As in parental males, PNs tuned to (Z)-11-hexadecenal always had an arbor in the cumulus, the largest of the MGC glomeruli. Previous neurophysiological investigations revealed that PNs with dendritic arbors restricted to the dorso-medial glomerulus (DM) of the MGC responded specifically to either (Z)-9-tetradecenal (Z9–14:Ald; H. virescens males) or (Z)-9-hexadecenal (Z9–16:Ald; H. subflexa males). Hybrid males, which responded equally well in wind tunnel tests to blends containing either Z9–14:Ald or Z9–16:Ald, had DM PNs that responded to both odorants. PNs specific for a third compound, (Z)-11-hexadecenol, required by hybrid males for behavioral activity were localized to the antero-medial MGC glomerulus (AM). Thus, neuronal activity across the cumulus, DM and AM glomeruli represented an attractive blend in hybrid males. Neurons tuned to (Z)-11-hexadecenyl acetate and Z9–14:Ald were restricted to a fourth, ventro-medial glomerulus. The across-glomerular pattern of activity associated with attractive pheromone blends was most similar to that of H. subflexa males, signifying a dominant effect of H. subflexa genes. These results indicate that the behavioral phenotype of hybrid males can be linked to underlying central olfactory characteristics. Copyright © 2006 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

Shifts in male preference for qualitatively different pheromone blends appear to have played a fundamental role in the divergence of olfactory communication and evolution of moth species. As an initial step in documenting the genetic complexity underlying such shifts, we characterized the behavioral responses of hybrid male moths created by mating two heliothine moth species, Heliothis subflexa and Heliothis virescens. Between 67 and 96% of hybrid males flew upwind and contacted the pheromone source when presented with a blend consisting of (Z)-11-hexadecenal (Z11–16:Ald), (Z)-9-hexadecenal (Z9–16:Ald), and (Z)-11-hexadecenol (Z11–16:OH) in a 1:0.5:0.1 ratio that has previously been shown to be attractive to H. subflexa males. In addition, an H. virescens blend of Z11–16:Ald and (Z)-9-tetradecenal (Z9–14:Ald) enhanced by the addition of Z11–16:OH (in a 1:0.05:0.1 mixture) was attractive to hybrid males (26–64% source contact), but significantly fewer males reached the odor source compared to the blend containing Z9–16:Ald. A blend in which the dosage of Z9–14:Ald was doubled, however, was equally attractive (75–77% source contact) as the Z9–16:Ald-containing blend. Consecutive presentation of two blends revealed that individual hybrid males responded equally well to blends containing either Z9–14:Ald or Z9–16:Ald. Together these results suggest that in addition to Z11–16:Ald, hybrid males: (1) required either Z9–16:Ald (likeH. subflexamales) or Z9–14:Ald (like H. virescens males); (2) required the presence of Z11–16:OH (H. subflexa dominant); (3) were not adversely affected by the presence of Z11–16:Ac (H. subflexa dominant). The behavioral response phenotype of hybrid males was therefore influenced by genetic factors inherited from both parental species. Copyright © 2006 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

Shows that with naturally intermittent odor stimulation, spike patterns recorded from moth antennal-lobe output neurons varied predictably with the fine-scale temporal dynamics and intensity of the odor. How this finding supports the hypothesis the olfactory circuits compensate for contextual variations in the stimulus pattern with high temporal precision; Observation that the timing of output neuron activity is constantly modulated to reflect ongoing changes in stimulus intensity and dynamics that occur on a millisecond timescale.