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RESEARCH Harald Tichy

Infrared and temperature reception in blood-sucking bugs

Blood-sucking insects go through certain stages as they seek for hosts and they use multiple senses of smell, CO2 and radiant heat. But how insects detect and transduce infrared radiation into nerve impulses still remains controversial. Non-thermal and thermal mechanisms have been claimed to account for infrared detection. This question may be answered either from a biophysical perspective, considering possible mechanisms, or just empirically, based on recording the activity of receptor cells involved in infrared detection. From the empirical point of view, effects are called non-thermal if during exposure to radiant heat no significant change in temperature is measured in the infrared sense organ or if the temperature increase caused by a warm air stream does not show effects similar to those induced by infrared radiation.

Among blood-feeding bugs, Triatominae are widely distributed in Central and South American, and some species are important vectors of Chagas disease. Rhodnius prolixus uses heat as the major cue for short range orientation to a warm blooded host and antennal sense organs are responsible for guiding the bugs. In cooperation with Prof. Claudio Lazzari (University of Tours, France) we study whether R. prolixus possesses sense organs sensitive to pure infrared radiation and/or to variation in ambient temperature. The specialization of a sense organ for radiant or convective heat is perhaps a matter of degree rather than kind. Determining the most efficient mode of heat transfer will establish the “adequate” stimulus of the sense organ. We identified morphologically distinct types of sense organs on the bug’s antennae which respond to both infrared radiation and warm air. The results of our studies gain insights into the functioning of infrared sense organs and the role of infrared radiation in host location; it also allows estimating the distance at which the bug’s infrared sense is able to detect the warm blooded host. Correlation between structural features of the sense organs and their sensitivity for infrared radiation and convection will indicate their efficiency for the transfer of the one or the other mode of heat. We assume structure-based mechanisms that optimize the transfer of radiant or convective heat.

A                                                 B

 

Extracellular recorded activity from a warm cell and a cold cell of a peg-in-pit sensillum and a warm cell of a tapered hair on the antennae of the bloodsucking bug to slowly oscillating changes in IR and T. A: responses of a pair of warm and cold cells of a peg-in-pit sensillum during one IR oscillation period of 600 s duration. a time course of instantaneous radiation power (RP). b time course of instantaneous impulse frequency (F) of warm cell and cold cell. c expanded views of original recorded action potentials. Large-amplitude action potentials are from warm cell, small-amplitude action potentials from the cold cell. d template window showing the template boundaries of the spike waveforms from warm and cold cells. B: responses of a warm cell of a tapered hair during one T oscillation period of 600 s duration. a time course of T. b time course of instantaneous impulse frequency of warm cell. c expanded views of original recorded action potentials from the warm cell. d template window showing the template boundaries of the spike waveforms from the warm cell.

 

The figure illustrates the effect of still air (Aa) and of an air stream at different constant temperatures (B: 28°C; C: 23°C) flowing over the bug’s antenna on the impulse frequency of the warm cell in the peg-in-pit sensillum to constant-amplitude oscillations in the power of infrared radiation (range: 0 to 5 mW/cm²). The same oscillating changes in radiation power produces strong responses in still air (Aa, upper trace: time course of the power of infrared radiation; lower trace: time course of impulse frequency, bin width 5 s), medium responses when an air stream at 28°C crosses the antenna (Ba) and low responses during presentation of an air stream at 23°C (Ca). Thus, the transfer of radiant heat is high in still air and low during forced convection occurs; in the latter case, the responses to infrared radiation are stronger the higher the  temperature of the air stream. In both still air and moving air, the warm-cell’s responses to oscillations in infrared radiation depend simultaneously on instantaneous radiant power and its rate of change. This double dependence is estimated by plotting impulse frequency as function of both parameters. Regression planes [F = yo + a RP + b (ΔRP/Δt); where F is the impulse frequency and yo is the intercept of the regression plane with the F axis reflecting the height of the regression plane] were utilized to determine the differential sensitivity for instantaneous radiation power (a-slope) and its rate of change (b-slope). Impulse frequency increases linearly with raising instantaneous radiation power and its rate of change; but sensitivity for both parameters is stronger in still air (Ab) than during forced convection, with higher sensitivity values at the higher ambient temperatures (Bb, Cb). This dependence indicates that the process underlying infrared reception in the bug’s warm cell is thermal. It also means, however, that the bug’s body temperature – which is the same by a poikilothermic insect than ambient temperature – affects sensitivity for infrared stimulation. One aim of the experiments is to describe the temperature range that governs the warm-cell’s sensitivity for infrared radiation. F impulse frequency, RP radiation power, R2, coefficient of determination; the number of points per plot was > 60.

 


ON and OFF responses to changes in food odor concentration

Olfaction did not arise and evolve to just “smell” food, but rather to act and interact with the habitat. Thus it might be misleading to study olfaction without its natural coupling to the gradual expanding odor plume that is moving slowly from the food odor source. In order to mimic such slow and continuous changes in odor concentration at various rates we invented a dilution flow olfactometer that allows us to employ both fluctuating and creeping changes in food odor concentration. This form of stimulation was critical for the discovery of the ON and OFF olfactory cells on the cockroach’s antennae which provide excitatory responses for both increments and decrements in food odor concentration. This arrangement suggests that, if the cockroach moves to the food odor source, it uses the ON olfactory cells to detect the food odor and the OFF olfactory cells in order to detect that it gets lost from the odor signal. The parallel ON and OFF olfactory cells perform the olfactory equivalent of temporal contrast enhancement and promote the detection of slight changes in food odor concentration. It follows that the ON and OFF responses, while optimized to signal fluctuations in odor concentration, will also be able to signal the durations of the odor stimulus and the odorless gap between two odor stimuli by the duration of the discharge of one or the other olfactory cell. The direct transduction of the OFF durations into excitatory responses enables the detection of temporal concentration patterns without explicate knowledge of the time elapsed between two odor pulses.

During slowly oscillating changes in the concentration of the odor of lemon oil, the cockroach’s ON and OFF olfactory cells adapt to the actual odor concentration and the rate at which concentration changes. When odor concentration oscillates rapidly with brief periods, adaptation improves the gain for instantaneous odor concentration and reduces the gain for the rate of concentration change. Conversely, when odor concentration oscillates slowly with long periods, adaptation increases the gain for the rate of change at the expense of instantaneous concentration. Without this gain control the ON and OFF olfactory cells would, at brief oscillation periods, soon reach their saturation level and become insensitive to further concentration increments and decrements. At long oscillation periods, on the other hand, the cue would simply be that the discharge begins to change. Because of the high gain for the rate of change, the cockroach will receive creeping changes in odor concentration, even if they persist in one direction. Gain control permits a high degree of precision at small rates when it counts most, without sacrificing the range of detection and without extending the measuring scale.

The ON and OFF odor detection is the most significant division among olfactory features extracted by the peripheral olfactory system. Still, separate ON and OFF channels do not help with temporal contrast enhancement because each channel signals exactly what it smells without caring about what is going on in its antagonistic pair. To accomplish such a phenomenon, one has to postulate that ON and OFF channels talk to each other - opposite signals may enhance each other whereas similar or equal signals may weaken each other. It is also conceivable that at some stage the ON and OFF channels are combined thereby providing ON/OFF responses to both the onset and the termination of the odor stimulus. Since odor increments and odor decrements do not occur physically at the same time in the same place, the convergence of the ON and OFF channels may not loose information about the rate of concentration change. Some ON/OFF neurons may be sensitive to rapid concentration changes but insensitive to slow concentration changes, others may be sensitive to slow concentration changes but insensitive to rapid concentration changes. Thus it is tempting to reject the hypotheses that olfactory performance is limited to sampling ON responses alone.

The intent of our present experiments is to simultaneously examine the activity from multiple neurons in the cockroach’s antennal lobe to food odor stimulation by means of 16-channel silicon based microprobe arrays.

Antagonistic responses of the ON and OFF olfactory cells to oscillating changes in the concentration of the odour of lemon oil; oscillation periods are 6 s in A and 60 s in B: a, time course of odour concentration; b, simultaneously recorded impulses of an ON and OFF olfactory cell. The OFF olfactory cell displays larger impulse amplitudes than the ON olfactory cell. c and d, responses of the ON and OFF olfactory cells represented in raster plots.


Humidity transduction models


Humidity influences the survival of insects mainly by affecting their water content. If humidity can be kept within certain limits, exposure to dry or humid conditions may not be harmful. Insects are capable of maintaining a stable water balance by searching for a suitable environment. Humidity choice responses depend on the existence of hygroreceptive sensilla, as has been demonstrated by experimentation in several species. Insect hygroreceptors associate as antagonistic pairs of a moist cell and a dry cell together with a cold cell in small cuticular sensilla on the antennae. The mechanisms by which the atmospheric humidity stimulates the hygroreceptive cells remain elusive.

Three models for humidity transduction have been proposed. In these models, hygroreceptors are proposed to operate either as 1) mechanical hygrometers in which activity is initiated by swelling and shrinking of hygroscopic sensillum structures, 2) evaporimeters in which the rate of evaporation of water due to the dryness of the air leads to quantitative changes in the lymph concentration, and 3) psychrometers in which the degree of cooling during evaporation of water is used to measure the humidity (or the dryness) of the air. These models pose some intriguing questions as to the adequate stimulus. If it is assumed that temperature does not affect the hygroreceptors per se, it follows that hygroreceptors possessing these different transduction mechanisms would respond in different ways when tested with humidity changes at different temperatures. Hygroreceptors acting as mechanical hygrometers would respond to the relative humidity of the air (i.e., the ratio of the actual vapor pressure to the saturation water vapor pressure) independently of the ambient temperature. Evaporation rate detectors would respond to the saturation deficit of the air (i.e., the difference between the actual vapor pressure and the saturation water vapor pressure). In psychrometers, hygroreceptors are functioning as wet-bulb and dry-bulb thermometers which determine the temperature depression due to the cooling effect of water evaporating from the sensillum surface.

We tested the adequacy of the three models on the cockroach’s moist and dry cells by determining whether the specific predictions about the temperature-dependence of the humidity responses are indeed observed. While in previous studies stimulation consisted of rapid step-like humidity changes, we developed a delivery system changed for changing humidity slowly and continuously up and down in a sinusoidal fashion. The low rates of change made it possible to measure instantaneous humidity values based on UV-absorption and to assign these values to the hygroreceptive sensillum. Testing constant amplitude oscillations in vapor pressure at different temperatures and expressing the humidity stimulus as oscillations in the relative humidity, in the saturation deficit, or in the wet and dry bulb temperature, enable determining which of these humidity parameters adequate explains the moist and dry cell responses.

The results of our studies do not verify the mechanical hygrometer model or the evaporation rate detector model. The temperature dependence of the moist cell’s humidity responses could not be attributed to relative humidity or to saturation deficit, respectively. However, the close relationships of the moist cell’s response with the wet-bulb temperature and the dry cell’s response with the dry-bulb temperature fit excellently into the psychrometer model. Thus, the hygroreceptors respond to evaporation and the resulting cooling due to the wetness or dryness of the air. The drier the ambient air (absolutely) and the higher the temperature, the greater the evaporative temperature depression and the power to desiccate.

Delivery system for sinusoidal humidity modulation. Compressed air is divided at 1 into two air streams to be set at different water vapor pressure values. Each stream is further split into two substreams (at 2) and their flow rates are adjusted by needle valves (V) and monitored continuously by flow meters (F) before passing through a tank of water at constant depth and temperature (42 °C). One substream bubbles out through many holes in a polyethylene tubing in a tank of ion-exchange water at constant 42 °C. Temperature is controlled by thermostat 1 (T1). The second substream is conducted through the spiral tube in the same tank but remains dry when it is also warmed to 42 °C. After emerging from the tank, the two substreams are combined in a single stream (at 3) variable in water vapor content from dry to almost saturated. Homogeneity of mixture is enhanced by a 2-l series connected vessel. Water-jacket insulation is shown. The temperature of the two air streams is then set at given temperature levels by driving them through a thermostatically controlled heat exchanger (T2). After passing through electrical proportional valves (eV1, eV2), the air streams are combined to a single stream (at 4). The vapor pressure of this stream is sinusoidally modulated by mixing the two streams in a ratio determined by the proportional valves by means of the output sequencer function of the data acquisition software (Spike2; Cambridge Electronic Design, CED, Cambridge, UK). By 180° phase shifting of the control voltages of the electrical proportional valves, the flow rate of the combined air stream is held constant. The antenna is placed at the outlet of the stimulus air stream, the recording or different electrode (DE) inserted at the base of the hygroreceptive sensillum and the reference or indifferent electrode (IE) into the tip of the antenna. Humidity stimulation is measured by a UV-absorption hygrometer (H).

 

 

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