Octopamine enhances moth olfactory responses to pheromones, but not those to general odorants

Effects of octopamine on responses of olfactory receptor neurons of Bombyx mori males and females, specialized to the reception of pheromone components and general odorants, respectively, were compared. Injections of octopamine had no effect on the transepithelial potential of antennal sensilla trichodea in both sexes. In males, octopamine increased significantly the amplitude of receptor potentials and nerve impulse responses elicited by the pheromone components bombykol and bombykal. However, the responses of homologous female general odorant-sensitive neurons to linalool and benzoic acid were not affected. In control experiments, injection of physiological saline did not increase the responses in any neuron type.     

Inhibitors of sensillar esterase reversibly block the responses of moth pheromone receptor cells

Three volatile alkyl-thio-trifluoro propanones inhibiting the esterase in olfactory sensilla of the silkmoths Antheraea polyphemus and A. pernyi were used to test the hypothesis that enzymatic pheromone degradation is responsible for the decline of the receptor potential after pheromone stimulation. Test stimuli were the pheromone components (E,Z)-6,11-hexadecadienyl acetate, a substrate for the sensillar esterase, and (E,Z)-6,11-hexadecadienal, not degraded by the esterase. Each compound acts on a separate type of receptor cell. In both receptor cell types the trifluoro propanones caused a partially reversible reduction of sensitivity as indicated by smaller receptor potential amplitudes and lower nerve impulse frequencies. Since application of the esterase inhibitors did not prolong the decline of the receptor potential of the acetate cell, the esterase is not responsible for the rapid pheromone deactivation. When the trifluoro propanones were applied after the pheromone at high concentrations, they rapidly inhibited (repolarized) both receptor cell types. Experiments with local application of trifluoro propanones revealed that the inhibitory effect spreads within seconds along the length of the sensillum. The inhibition of the electrophysiological responses might be due to an antagonistic action of the trifluoro propanones at the pheromone-binding sites, either at the receptor molecules or at the pheromone-binding protein. Accepted: 4 February 1998     

Temporal resolution of odour pulses by three types of pheromone receptor cells inAntheraea polyphemus

Summary Temporal response characteristics of three cell types of maleA. polyphemus, each responding to a different pheromone component, have been measured using series of short (20 ms) pheromone pulses. The stimuli were delivered through capillaries 20 m in diameter and applied to single olfactory sensilla trichodea. Two of three cell types sensitive to (E,Z)-6,11-hexadecadienal and (E,Z)-4,9-tetradecadienyl acetate are able to resolve at least 5 stimuli/s whereas the third, responding to the major pheromone component (E,Z)-6,11-hexadecadienyl acetate, is slower, resolving only about 2 stimuli/s. These results suggest that receptor cells are able to respond to pulses of pheromone concentration as they occur downwind from a point source. The time-averaged number of nerve impulses does not seem to be a reliable measure of the amount of pheromone reaching the sensillum. Responses of the cells thus reflect the non-uniform distribution of pheromone in a plume rather than the average concentration.     

Resolution of pheromone pulses in receptor cells of Antheraea polyphemus at different temperatures

The ability of pheromone receptor cells of male Antheraea polyphemus (Saturniidae) to resolve stimulus pulses was determined at different temperatures (8°, 18°, 28°C). The cells were stimulated by repeated 20-ms puffs of the pheromone components (E, Z)-6, 11-hexadecadienyl acetate and (E, Z)-6,11-hexadecadienal. At higher temperatures, higher frequencies of stimulus pulses were resolved by the nerve-impulse response: about 1.25 pulses per second at 8°C, 2.5 pulses/s at 18°C and 5 pulses/s at 28°C. The decreased ability of receptor cells to resolve stimulus pulses at low temperatures may reduce the male moth's chance of reaching the pheromone source. The peak nerve-impulse frequency increased whereas the duration of nerve-impulse responses to single stimulus pulses decreased at higher temperatures. At a given temperature and stimulus intensity the peak nerveimpulse frequency decreased with shorter intervals between the stimulus pulses, but the duration of the responses remained almost constant. The time needed for recovery from adaptation caused by a single stimulus pulse was longer at lower temperatures. The aldehyde receptor cell recovered more quickly than the acetate cell. At low stimulus concentration, the resolution ability of the acetate cell was strongly decreased, whereas in the aldehyde cell it was only slightly impaired.     

Neurons discovered in male Helicoverpa zea antennae that correlate with pheromone-mediated attraction and interspecific antagonism

Responses of single receptor neurons in the antennae of male Helicoverpa zea to sex pheromone components and to behavioral antagonists were recorded using a cut-sensillum extracellular recording technique. Three types of sensilla were identified from sampling 325 male-specific sensilla trichodea located at the lateral edge of antennomeres. The majority of these sensilla (71%) contained a receptor neuron tuned to the principal sex pheromone component (Z)-11-hexadecenal. A second sensillar type (10%) contained a receptor neuron that responded only to (Z)-9-tetradecenal. A third sensillar type (19%) contained a large-spiking neuron tuned to the secondary pheromone component (Z)-9-hexadecenal, but this neuron also could be stimulated to equivalent spike frequencies by the same emitted amounts of (Z)-9-tetradecenal. A smaller-spiking neuron in this sensillar type responded to two compounds known to act only as behavioral antagonists, (Z)-11-hexadecen-1-ol and (Z)-11-hexadecenyl acetate, and to (Z)-9-tetradecenal. Cross-adaptation studies confirmed the presence of one large- and one small-spiking neuron in the third sensillar type. Dose-response studies correlated to collected stimuli amounts showed that the large-spiking neuron in the third sensillar type was equally tuned to (Z)-9-hexadecenal and (Z)-9-tetradecenal, whereas the smaller-spiking neuron was far more sensitive to (Z)-11-hexadecen-1-ol and to (Z)-11-hexadecenyl acetate than to (Z)-9-tetradecenal. Accepted: 29 September 1997     

Octopamine and tyramine modulate pheromone-sensitive olfactory sensilla of the hawkmoth <Emphasis Type="Italic">Manduca sexta</Emphasis> in a time-dependent manner

In moths octopamine improved pheromone-dependent mate search time dependently. In the nocturnal hawkmoth Manduca sexta long-term tip recordings of trichoid sensilla were performed to investigate whether biogenic amines modulate pheromone transduction time dependently. At three Zeitgebertimes octopamine, tyramine and the octopamine antagonist epinastine were applied during non-adapting pheromone-stimulation. At ZT 8-11, during the photophase, when sensilla were adapted, octopamine and to a lesser extent tyramine increased the bombykal-dependent sensillar potential amplitude and initial action potential (AP) frequency. In addition, during the photophase, when sensilla are less able to resolve pheromone pulses, octopamine rendered pheromone responses more phasic and sensitive, and raised the spontaneous AP frequency. During the late scotophase, at ZT 22-1, when the antenna appeared maximally sensitized for pheromone pulse detection and endogenous octopamine levels are high, exogenously applied octopamine was ineffective. Epinastine blocked the pheromone-dependent AP response at ZT 8-11 and slightly affected it at ZT 22-1, while it had no effect on the sensillar potential amplitude. Epinastine decreased the spontaneous AP activity during photophase and scotophase and rendered pheromone responses more tonic in the scotophase. We hypothesize that the presence of octopamine in the antenna is obligatory for the detection of intermittent pheromone pulses at all Zeitgebertimes.     

Biogenic amines modulate olfactory receptor neurons firing activity in Mamestra brassicae

The modulatory effects of the biogenic amines octopamine and serotonin on pheromonal receptor neurons of Mamestra brassicae were investigated. The responses to sex pheromone components of two cells types (A and B) in single male long sensilla trichodea were monitored. Cell types A and B do not respond to the same compound. The response of type A to a pulse of the major sex pheromone component increased 5 min after octopamine injection. Responses of type B to other odorants increased after 30 min. In the absence of any pheromone stimulation the background firing activity of type A increased following octopamine injection. This background activity was used to evaluate the kinetics of octopamine and other biogenic amine effects on olfactory receptor neurons. Octopamine increased this background activity in a concentration- and time-dependent manner. Clonidine, an octopamine agonist, was shown to be more powerful in increasing the background activity of olfactory receptor neurons. The effects of octopamine and clonidine were hypothesized to arise from specific receptor activation as chlorpromazine (an octopamine antagonist) was shown to block the effect of octopamine. Serotonin, a known neuromodulator in most animal species, induced a reversible inhibition of spike firing. Altogether, these results indicate that biogenic amines can modulate the sensitivity of olfactory receptor neurons of moths either directly or by an action on adaptation.     

Antennal response of codling moth males, Cydia pomonella L. (Lepidoptera: Tortricidae), to the geometric isomers of codlemone and codlemone acetate

Single sensillum recordings from Cydia pomonella male antennae showed three different types of receptor neurons. The most abundant type was most sensitive to the main pheromone compound (E,E)-8,10-dodecadienol, while its response to the geometric isomers E,Z, Z,E and Z,Z was comparable to a tenfold lower dose of (E,E)-8,10-dodecadienol. This neuron type also responded to the four behaviorally antagonistic isomers of (Δ,Δ)-8,10-dodecadienyl acetate, among which it was most sensitive to the E,E isomer. Cross-adaptation studies showed that these compounds were all detected by the same receptor neuron type. Receptor neurons specifically tuned to (E,Z) or (Z,Z)-8,10-dodecadienol were not found, although these two compounds are behaviorally active. A second type of receptor neuron responded to all isomers of (Δ,Δ)-8,10-dodecadienyl acetate and was most sensitive to the E,E isomer. This neuron type did not respond to any of the isomers of (Δ,Δ)-8,10-dodecadienol. A third receptor neuron type was highly sensitive to the plant compound α-farnesene. The finding that the receptor neuron type tuned to the main pheromone compound responded even to strong behavioral antagonists aids the interpretation of ongoing behavioral studies for the development of the mating disruption technique in codling moth. Accepted: 3 March 2000     

The contribution of olfactory receptor neurons to the perception of pheromone component ratios in male redbanded leafroller moths

Summary (Z)-11-tetradecenyl acetate (Z-11, 14:AC) must be in a 1009 ratio with (E)-11-tetradecenyl acetate (E-11,14:AC) to produce maximal wing fanning and attraction in male redbanded leafrollers. Earlier electrophysiological studies had indicated that mixtures of these pheromone components elicited responses from olfactory receptor neurons that appeared to differ from those expected on the basis of the responses to the individual components. Here we evaluate whether the behavioral sensitivity to particular ratios of Z- and E-11,14:AC has a correlate in the response properties of olfactory receptor neurons.The stimuli included the ratios of Z- and E-11, 14:AC used in earlier behavioral work plus several different mixtures of the seven components found in the pheromone blend, and equivalent amounts of the individual components. These stimuli were presented over a range of intensities to individual trichoid sensilla on the male antenna. In common with earlier results, the receptor neuron with the larger amplitude action potential responded most strongly to Z-11,14:AC, whereas the companion receptor neuron in the sensillum responded most strongly to E-11,14:AC. In contrast with earlier results, each receptor neuron responded exclusively to its own most effective stimulus, without regard to the presence of any other compound. They failed to respond uniquely to any of the other five compounds in the female pheromone blend, or to any of the tested combinations of these compounds. These minor components also failed to modulate the responses elicited in receptor neurons by appropriate ratios of Z- and E-11,14:AC. Thus, the responses of the two types of olfactory receptor neurons found in trichoid sensilla failed to show an optimum at the pheromone ratio known to elicit peak behavioral activity.Abbreviation RBLR redbanded leafroller moth     

A comparative study of pheromone perception in two species of Noctuid moths

The electrical activity of single olfactory receptor neurons in male soybean looper (SBL) Pseudoplusia includens(Walker) and cabbage looper (CL) Trihoplusia ni(Hübner) moths was evaluated in response to stimulation with fixed amounts of the individual components of their respective pheromone blends. In common with earlier observations in the CL, there are at least two classes of morphologically distinct pheromone sensitive sensilla on the antenna of male SBL, each of which contains two olfactory receptor neurons. In both species, one class of sensilla contains an olfactory receptor neuron sensitive to (Z)-7-dodecen-1-ol acetate (Z-7, 12:AC), the major component in each insect's blend, and a companion receptor neuron which is sensitive to (Z)-7-dodecen-1-ol (Z7,12: OH). In both species the second class of sensilla contains an olfactory receptor neuron which is sensitive to one of the minor components of the pheromone blend. (Z)-5-dodecen-1-ol acetate (Z-5,12:AC) is an effective stimulus in SBL, whereas (Z)-7-tetradecen-1-ol acetate (Z-7,14:AC) is an effective stimulus in CL. However, these two stimulatory compounds have been identified only in the female CL gland; neither has been found in the SBL gland. Thus, in contrast to the CL, which has receptor neurons which are responsive exclusively to conspecific pheromone components, the SBL has a class of receptor neurons which is responsive to a minor component of another species' pheromone blend. Field-trapping assays in which Z-5,12:AC is added to the SBL blend suggest that this single CL component is a powerful inhibitor of male SBL behavioral responses to conspecific pheromone blends. The difference observed in the specificity of the receptor neurons in this second class of sensilla are thus believed to play an integral role in the isolation processes that are maintained between these two species and may well account for the observed behavioral differences in their responses to heterospecific pheromone blends.